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International Conference of Solution Processed Semiconductor Solar Cells - Oxford 2014

International Conference of Solution Processed Semiconductor Solar Cells, Oxford, United Kingdom, 10th to 12 th September 2014

edited by

Henry Snaith

Department of Physics, University of Oxford, United Kingdom

You are invited to participate in the International Conference Solution processed Semiconductor Solar Cells, which will be held from 10 to 12 September 2014, in Oxford, United Kingdom.

The Solution Processed Semiconductor Solar Cell 2014 conference will bring together leading scientists with expertise in both inorganic and organic solar cells, who have brought their skills to bear upon hybrid and inorganic solution processed photovolatics. Specifically focused technologies are, semiconductor sensitized, solution processed thin film such as CZTS, quantum dot solar cells and newly emerged perovskite solar cells. We expect a significant fraction of the conference will be dedicated to perovskite solar cells, but strongly encourage submission of abstracts for the other device types, since we want to encourage fruitful cross fertilization from between the fields.

Topics include but are not limited to, new materials synthesis and characterisation, fundamental understanding through spectroscopic and optoelectronic investigations, photonic and plasmonic control and enhancement, device optimisation and long term stability investigations. We warmly invite you to submit contributing abstracts to what promises to be an enlightening conference.

International Conference of Solution Processed Semiconductor Solar Cells (SSSC14)

Oxford, United Kingdom

Scientific Committee

Henry Snaith Iván Mora-Seró Sam Stranks

Department of Physics, University of Oxford, United Kingdom University Jaume I, Spain Department of Physics, University of Oxford, United Kingdom

International Conference of Solution Processed Semiconductor Solar Cells (SSSC14)

Oxford, United Kingdom

Invited Speakers

Juan Bisquert Universitat Jaume I, Spain Lioz Etgar The Hebrew University of Jerusalem, Israel Feliciano Giustino University of Oxford, United Kingdom Gary Hodes Weizmann Institute of Science, Israel Seigo Ito University of Hyogo, Japan Alex Jen University of Washington, United States Mercouri Kanatzidis Northwestern University, United States Gerasimos Konstantatos ICFO, Spain David Mitzi Tsutomu Miyasaka

Duke University, USA Toin University of Yokohama, Japan

Arthur Nozik National Renewable Energy Laboratory, United States Emilio Palomares Institute of Chemical Research of Catalonia, Spain Nam-Gyu Park Sungkyunkwan University, Korea Annamaria Petrozza Center for Nanoscience and Technology, Italy Yang Yang University of California, United States Arie Zaban Bar-Ilan University, Israel


Thursday 11th September 8.40 - 9.10 Development of High-Performance Solution-Processed Chalcopyrite/Kesterite Films for Photovoltaic Application

David Mitzi IBM TJ Watson Research Center, PO Box 218 10598 Yorktown Heights, New York

Thursday 11th

September 9.40 - 10.10 Unique Quantization Effects in Quantum Dots and Quantum Dot Solar Cells for PV and Solar Fuels

Arthur Nozika, b

, Matt Beardb, Joey Luther

b, Mark Hanna

b, Justin Johnson

b, Octavi Semonin

b

a, University of Colorado, Boulder, 215 UCB University of Colorado, Boulder, 00309, US b, NREL, 1617 Cole Blvd, Golden, 80409, US

In quantum dots (QDs), quantum rods (QRs) and unique molecular chromophores that undergo singlet fission (SF) the relaxation pathways of photoexcited states can be modified to produce efficient multiple exciton generation (MEG) from single photons . We have observed efficient MEG in PbSe, PbS, PbTe, and Si QDs and efficient SF in molecules that satisfy specific requirements for their excited state energy levels. We have studied MEG in close-packed QD arrays where the QDs are electronically coupled in the films and thus exhibit good transport while still maintaining quantization and MEG. We have developed simple, all-inorganic QD solar cells that produce large short-circuit photocurrents and respectable power conversion efficiencies via both nanocrystalline Schottky junctions and nanocrystalline p-n junctions. These solar cells also showed for the first time external quantum yields (QYs) for photocurrent that exceed 100% in the photon energy regions of the solar spectrum where MEG is possible (i.e., energy conservation is satisfied); the photocurrent internal QYs from MEG as a function of photon energy match those determined via time-resolved spectroscopy and the results settle controversy concerning MEG. Recent analyses of the dramatic effects of solar concentration combined with MEG on the conversion efficiency of solar cells will also be discussed. The properties required for nanocrystals and SF molecules to achieve the high solar conversion efficiencies predicted by theory will be presented. Regarding production of solar fuels, all viable systems must have the following features: two photosystems arranged either in a Z-scheme analogous to biological photosynthesis, or two tandem p-n junctions connected in series where sufficient photopotential (1.23 V + overvoltage for H2O splitting) is generated to drive the redox reactions; strong absorption of solar photons; efficient separation of the photogenerated e-h pairs, efficient transport to and collection of the separated carriers at electrocatalytic surfaces; low overvoltages; appropriate alignment of the redox potentials in the photoelectrodes with those of the fuel-producing reactions; and resistance to dark-and photo-corrosion achieving long-term photostability. Cells with buried junctions in a tandem p-n configuration or a Z-scheme can achieve these requirements.

Thursday 11th

September 10.10 - 10.25 Reversible Photo-induced Halide Segregation in Mixed-halide Hybrid Perovskites for Photovoltaics

Eric Hokea, Eva Unger

a, Hemamala Karunadasa

b, Michael McGehee

a

a, Material Science and Engineering, Stanford University, 476 Lomita Mall, Stanford, CA, 94305, US b, Chemistry, Stanford University, 333 Campus Dr, Stanford, CA 94305, US

Mixed-halide hybrid perovskites such as CH3NH3Pb(BrxI1-x)3 are a promising family of photovoltaic absorber materials that have achieved power conversion efficiencies of over 17%. By varying the halide composition, the optical bandgap can be tuned over the range 1.6-2.3 eV, making this family of materials a suitable candidate for both single-junction solar cells as well as the large bandgap absorber of a tandem solar cell. However, reports of mixed CH3NH3Pb(BrxI1-x)3 devices with higher bromine content have so far not been able to achieve the increase in open circuit voltage that may be expected from the larger bandgap of these materials. We observe photo-induced halide segregation in bromine-rich (0.2<x<1) CH3NH3Pb(BrxI1-x)3 and other mixed-halide perovskites as evidenced by the appearance of intense photoluminescence and absorption features from a new iodide-rich phase upon continuous illumination and the disappearance of these features with time in the dark. We suggest that photoexcitation may induce halide migration, resulting in iodide-rich domains that act as traps and pin the open circuit voltage at a lower energy. The kinetics of this process have a similar temperature


dependence to the hysteretic behavior in planar CH3NH3PbI3-xClx solar cells which is suggestive of a prominent role of halide migration in perovskite photovoltaic hysteresis. These observations are reminiscent of photo-initiated halide migration in lead halides and other metal halides, which has been proposed to occur via a halide-vacancy diffusion mechanism from surface sites. This suggests that improved control of the perovskite stoichiometry, crystallinity and surface passivation are potential strategies towards reducing halide migratory effects and improving the stability of halide perovskite optoelectronic devices.

Thursday 11th

September 10.25 - 10.40 Optical Properties of Perovskite-Based Solar Cells

James Balla, Maximilian Hoerantner

b, Samuel Stranks

b, Sven Huettner

c, Mingzhen Liu

b, Wei Zhang

b,

Ullrich Steinerc, Henry Snaith

b

a, Istituto Italiano di Italiano, Centre for Nanoscience and Tecnology, Via Giovanni Pascoli 70/3, Milano, 20133, IT b, University of Oxford, Clarendon Laboratory, Parks Road, Oxford, OX1 3PU, UK c, University of Cambridge, Cavendish Laboratory, JJ Thomson Avenue, Cambridge, CB3 0HE, UK

Perovskite light-harvesters have rapidly emerged to the forefront of photovoltaics research exhibiting high power-conversion efficiencies and the promise of low-cost fabrication in devices. Despite recent progress in describing the internal mechanisms for photocurrent generation in perovskite solar cells, a full understanding of the device operation still requires an optical analysis of the device stack. This provides an additional platform for maximising the power-conversion efficiency through a precise determination of parasitic losses caused by optical coherence and absorption in non-photoactive layers. Here we present a transfer-matrix model for the charge-generation profile under sunlight in state-of-the-art perovskite-based planar-heterojunction solar cells using experimental refractive index data from spectroscopic ellipsometry. Excellent agreement between the model and experiment is obtained and reveals some important features of the device operation. In particular, we find the optimum thickness for photocurrent generation, an accurate derivation of the wavelength dependence of the internal quantum efficiency from the external quantum efficiency, and the dependence of the photocurrent on incidence angle. In the latter case, we find that perovskite-based solar cells can compete favourably with conventional technologies not only under AM1.5 illumination but also throughout the day in real-world applications. This work provides important insights into device optimisation which should lead to further improvements in device efficiency.

Thursday 11th

September 12.40 - 12.55 Chloride Assisted Perovskite Solar Cells Prepared Uusing Solution-processed Techniques with Efficiency Over 17%

Ludmila Cojocarua, Satoshi Uchida

b, Ajay Kumar Jena

c, Tsutomu Miyasaka

c, Takaya Kubo

a, Hiroshi

Segawaa

a, The University of Tokyo, Research Center for Advanced Science and Technology (RCAST), Komaba 4-6-1, Meguro-ku, Tokyo, 153-8904, Japan b, The University of Tokyo, Komaba Organization for Educational Excellence (KOMEX), Komaba 3-8-1, Meguro-ku, Tokyo, 153-8902, Japan c, Toin University of Yokohama, Graduate School of Engineering, Kurogane-cho 1614, Aoba-ku, Yokohama, 225-8503, Japan

Organometallic halide perovskite compounds have recently emerged as a new class of excellent light absorbers with good carrier-transport property and thus, have demonstrated exceptional progress in solar cells performance. These compounds exhibit very high efficiency in simple planar configurations. As these perovskite structures are hygroscopic, atmospheric conditions significantly influence the crystallization of these structures and hence, change the cell performance drastically. Here, we report the effect of humid and dry atmospheric conditions on surface coverage and morphology of perovskite (CH3NH3PbI3-xClx) films prepared on FTO/TiO2 compact layer. The performance and stability of these perovskite sensitized solid-state planar solar cells using spiro-OMeTAD as hole transport materials have been studied. Our samples, which are solution processed planar thin-film architectures (FTO/compact TiO2/perovskite/HTM/Au) showed remarkable photovoltaic performance with excellent stability. The


cells stored in dry conditions (humidity lower than 1%) without encapsulation exhibited long-term stability. The solar cells fabricated under optimal condition achieved a PCE of 17.3%, which we believe to be the highest value reported to date for a simple planar heterojunction structure. Moreover, the content of the Cl in a final perovskite film have also been discussed.

Thursday 11th

September 11.10 - 11.40 Extremely Slow Photoconductivity Response of CH3NH3PbI3 Perovskite

R. Gottesmana, E. Haltzi

a, L. Gouda

a, S. Tirosh

a, E. Moscon

a, b, F. De Angelis

b, A. Zaban

a

a, 1Department of Chemistry, Center for Nanotechnology & Advanced Materials, Bar Ilan University, Ramat-Gan, Ramat-Gan, 52900, IL b, 2Computational Laboratory for Hybrid/Organic Photovoltaics (CLHYO), CNR-ISTM, Via Elce di Sotto 8, I-06123, Perugia, IT

Advanced characterization of perovskite solar cells is already in progress yielding highly significant data. However, studies of isolated perovskite thin films under the working conditions of solar cells are still scarce. Presented here are photoconductivity measurements of CH3NH3PbI3 deposited between two dielectric-protected Au electrodes spaced ~2000 nm apart. The photo response of the CH3NH3PbI3 which is subjected to a dc bias, involves two time constants one of which is extremely long, lasting for several seconds. Similar time scale is observed upon transformation back to dark. Our findings seem to clarify the origin of the well-known hysteresis in perovskite solar cells.

Thursday 11th

September 11.40 - 12.10 Highly Efficient Hole Conductor Free Perovskite Based Solar Cells

Lioz Etgar Hebrew University, Edmond J. Safra Campus Givat Ram, Jerusalem, 91904, IL

Perovskite is a promising light harvester for use in photovoltaic solar cells. In recent years, the power conversion efficiency of perovskite solar cells has been dramatically increased, making them a competitive source of renewable energy.This work will discuss several topics related to perovskite based solar cells: An in-depth study on two-step deposition, separating the perovskite deposition into two precursors. The effects of spin velocity, annealing temperature, dipping time and methylammonium iodide concentration on the photovoltaic performance are studied. Various concentrations of methylammonium iodide and methylammonium bromide are studied in hole conductor free perovskite solar cells, which reveal that any composition of the hybrid CH3NH3PbInBr3−n can conduct holes. Kelvin probe force microscopy is used to measure cross-sections of hole conductor free CH3NH3PbI3 perovskite solar cells. The work function change are measured at the interfaces between the CH3NH3PbI3 perovskites and a metal oxide, nanocrystalline TiO2 and Al2O3, respectively. The findings from this research are critical for the understanding and further improvement of perovskite based solar cells, and are valid for cells with a hole transport material.

Thursday 11th

September 12.10 - 12.25 The Critical Role of Interfacial Dynamics for the Stability of Organic Photovoltaic Devices

Michele De Bastiania, Valerio D'Innocenzo

a, Samuel Stranks

b, Henry Snaith

b, Annamaria Petrozza

a

a, Istituto Italiano di Tecnologia, via pascoli 70/3, Milan, 20100, IT b, University of Oxford, Parks Road, Oxford, OX1 3PU, UK

In this work we have fabricated organo-lead tri-halide perovskite thin films crystallized in situ on substrates of different natures (e.g. porosity, wettability) and investigated their photoluminescence properties. We observe that the crystallization time and thin film structure are strongly influenced by the chemical nature, the porosity of the substrate, the nature of the precursors and how they are deposited. Moreover we find that the mesoporous scaffold and the crystals’ size can tune the emissive properties of the semiconducting compound both in terms of spectral region and dynamics. Perovskite crystallites (both tri-iodide and mix halides based crystals) grown in the nanometre size porous scaffold present shorter-living and blue-shifted emission with respect to those which are free to grow without any constraints. In particular, a sirugical control of CH3NH3PbI3 crystals growth enables us to observe an


emissive lifetime close to 200 ns. We finally show what the implications on carriers’ diffusion lengths are.

Thursday 11th

September 12.25 - 12.40 Designing Nanostructures and Interfaces for New Generation Solar Cells

Zonlong Zhua, b

, Shihe Yanga, b

a, William Mong Institute of Nano Science and Technology, Hong Kong University of Science and Technology, Hong Kong, HK b, Department of Chemistry, Hong Kong University of Science and Technology, Hong Kong, HK

Nanostructures and interfaces are keys to the new generation photovoltaics, such as perovskite solar cell. One example is the interface between perovskite and semiconductor oxide (eg. TiO2) in perovskite solar cells. By inserting an ultrathin graphene quantum dots (GQDs) layer in between, we have recently succeeded in improving the perovskite solar cell efficiency from 8.81% to 10.15%. By combining with transient absorption measurements, we have found that the improved cell efficiency can be attributed

to the much faster electron extraction time with the GQDs (～100 ps) than without the GQDs (~300 ps).

This highlights the GQDs as a super fast electron tunnel, which may also find use in other optoelectronic devices.On the other hand, these perovskite solar cells have so far largely relied on small-molecule hole transport materials (HTMs) such as spiro-OMeTAD, which commonly suffer from high cost and low mobility. Herein, we demonstrate polyfluorene derived polymers, which contains fluorine and arylamine groups, can indeed outperform spiro-OMeTAD as efficient hole-conducting materials for perovskite solar cells, which achieved a 10.92 % power conversion efficiency. By Photoluminescence (PL) and impedance spectroscopy technique, we have uncovered the higher electrical conductivity and lower series resistance than spiro-OMeTAD, which naturally explain the significantly higher fill factor, photocurrent and open-circuit voltage of the TFB-derived cells than with spiro-MeOTAD. Our systematic results will expedite the design of new HTMs, especially the design of high efficiency and low cost HTMs, for hybrid solar cells.

Thursday 11th

September 12.55 - 13.10 Effect Of anion on Metal-organic halide Perovskite Film Formation and the Performance of Plannar Heterojunction Devices

Wei Zhang, Henry Snaith University of Oxford, Clarendon Laboratory, Parks Road, Oxford, OX13PU, UK, Oxford, GB

Perovskite solar cells, which represent the promise of future generation photovoltaic technology with the lowest cost and highest efficiency, have evoked widespread scientific and industrial interest. Through rational device architecture design, materials interface engineering as well as processing technique optimization, a recorded efficiency around 18% has been attained, showing great potential for commercialization to compete with traditional silicon solar cells. Although the device performance of perovskite solar cells improves unprecedently fast in last two years, the basic properties of metal-organic halide perovskite, MAPbX3 (X=Cl, Br, I), such as the role of cation and anion, for example, are still not well understood. Most of research focuses on the perovskite band gap tuning by changing the ratio of either anions (Br to I) or cations (FA to MA). However, up to date, the effect of anion in percursor solution on the perovskite crystal growth and film formation has not been well studied yet, which is highly likely to correlate with the device performance. In addition, there is a long debate on the existence and role of Cl in mixed-halide perovskite and the results from varied groups employing different characterization techniques are quite controversial. Fully understanding of these questions is critically important for the advancement of perovskite solar cell technology in the next few years. In this work, the effect of anion was systematically studied and we found that anion in precursor solution has great influence on the perovskite crystal growth and film formation. By materials engineering, both film morphology and processing time are greatly improved, which leads to enhanced performance in plannar heterojunction devices.

Thursday 11th

September 14.30 - 15.00 Understanding Perovskites: New Materials for High Efficiency All-solid-state Solar Cells


Mercouri Kanatzidis Department of Chemistry, Northwestern University, Department of Chemistry, Evanston, 60208, US

Liquid dye-sensitized solar cells (DSCs) are now rapidly giving way to solid-state devices. The removal of organic liquid electrolytes containing the I−/I3− redox couple, was achieved using perovskite materials. Using a novel inorganic material, CsSnI3, we have created new all-solid-state solar cells TiO2 based devices free of liquid electrolyte. This results set off an avalanche of new reports worldwide on the application of Pb-based perovskites in all solid state cells with rapid and spectacular advances in power conversion efficiency. Perovskite materials however are complex and they present challenges in stability, handling, processing etc. How much of this chemistry do we understand? Is it important to understand the chemistry at a fundamental level in order to make further progress? For example, CH3NH3PbI3 and CsSnI3 are unusual materials that undergo complex displacive and reconstructive phase transitions that affect their physical behavior. Tin based materials have even more potential in this field because they can replace the lead based systems, for which there are significant environmental toxicity concerns. The synthesis and chemistry of ASnI3 and APb1-xSnxI3 (A = CH3NH3+, HC(NH2)2+, etc) compounds will be presented. These materials consist of a 3D network and behave as medium band gap semiconductors (Eg = 1.20-1.40 eV) with octahedral metal coordination. The mobilities of carriers in these materials appear to be very high, and when doped the compounds also display significant electrical conductivity. This combination of properties renders the materials ideal for solar cell and other applications. Moreover, CH3NH3SnI3 exhibits intense room temperature photoluminescence at ~ 970nm and HC(NH2)2SnI3 at ~ 900 nm, a property that can be utilized in luminescent solar concentrators. All three properties strongly depend on the synthetic method and are very sensitive to Sn4+ doping level. Results on solar cells constructed from these materials will be presented.

Thursday 11th

September 15.00 - 15.30 Material, Interface, and Process Engineering of High Performance Perovskite Solar Cells

Alex Jen Department of Materials Science & Engineering University of Washington, Seattle, WA 98195, USA

In this talk, an integrated approach of combining material design, interface, and process engineering will be discussed to demonstrate significantly improved performance of hybrid perovskite photovoltaic cells. Advances in controlled synthesis, processing, and tuning of the properties of peroskites have enabled significantly enhanced performance of organic-inorganic hybrid solar cells. The performance of these hybrid solar cells is strongly dependent on their efficiency in harvesting light, charge dissociation, transport, and collection at the metal/organic/metal oxide or the metal/perovskite/metal oxide interfaces. Therefore, it is very critical to tailor the molecular compositions of perovskite precursors and the interfaces with charge-transporting layer or device substrates to control the crystallization kinetics of perovskies to improve their quality and surface coverage. In addition, the stability of these hybrid devices can also be dramatically improved through simple surface functionalization to reduce device hysteresis during operation.

Thursday 11

th September 15.30 - 15.45

Lead-Free Organic-Inorganic Tin Halide Perovskites for Photovoltaic Applications

Nakita K. Noela, Samuel D. Stranks

a, Antonio Abate

a, Christian Wehrenfennig

a, Aditya Sadhanala

b, Laura

M. Herza, Michael B. Johnston

a, Henry J. Snaith

a

a, University of Oxford, Clarendon Laboratory, Parks Road, Oxford, GB b, University of Cambridge, Cavendish Laboratory, Thomson Ave, Cambridge CB3 0HE, GB

Within the last few years, lead-based organo-metal halide perovskite solar cells have become the biggest contenders in the drive to provide a cheap, clean, renewable source of energy, rapidly climbing to efficiencies of over 16%. One of the biggest concerns about the use of this material is the toxicity of lead, prompting researchers to search for a lead free alternative. One of the most obvious alternatives to lead is tin, which like lead, is also a group 14 metal. However, the stability of tin in its 2+ oxidation state has proved to be an overwhelming challenge. In this work we have synthesised and characterised the tin perovskite CH3NH3SnI3 by encapsulating it under inert atmosphere. This material shows an


absorption onset at approximately 1000nm, and photoluminescence at 950 nm corresponding to a band gap of approximately 1.25 eV. Here we report the first completely lead-free, CH3NH3SnI3 perovskite solar cell, reaching efficiencies of >6% under 1 sun illumination. Remarkably, we achieve voltages of up to 0.9V, suggesting that this material has intrinsically low voltage losses as compared to its lead analogue and even crystalline silicon. Further investigation and complete stabilisation of this material should allow a much greater increase in performance leading to efficiencies of >20%.

Thursday 11th

September 15.45 - 16.00 Charge Separation and Recombination Dynamics in Sn/Pb Halide Perovskite Solar Cell: Uncovering the Bottleneck of the Photovoltaic Efficiency

Qing Shena, e

, Yuhei Ogomib, e

, Syota Tsukamotob, Kosei Fujiwara

b, Witoon Yindeesuk

a, Koki Sato

c, kenji

Katayamac, Taro Toyoda

a, e, Kenji Yoshino

d, e, Shuzi Hayase

b, e

a, The University of Electro-Communications, 1-5-1 Chofugaoka, Chofu, Tokyo, 182, JP b, Kyushu Institute of Technology, Kitakyushu 808-0196, JP c, Chuo University, Tokyo 112-8551, Japan, JP d, Miyazaki Unversity, Miyazaki 889-2192, JP b, CREST, JST, Saitama 332-0012, JP

Organometal trihalide perovskite-based solid-state hybrid solar cells have attracted unexpected increasing interest because of the high efficiency (the record power conversion efficiency has been reported to be over 17%) and low cost for preparation. The high efficiency was thought to mainly originate from the strong optical absorption over a broader range (up to 800 nm for Pb perovskite ) and longer lifetimes of photoexcited charge carriers (in the order of 10 ns – 100 ns) of the organometal trihalide perovskite absorbers. Recently, Hayase and coworkers have succeeded in harvesting energy in the NIR region by using Sn/Pb cocktail halide based perovskite materials covering up to 1060 nm and an efficiency of 4.18 % was achieved. To improve the photovoltaic performance of Sn/Pb halide based perovskite solar cells, charge separation and recombination dynamics are key factors and should be understood deeply. In this paper, we have studied and clarified charge separation and recombination dynamics of Sn/Pb halide based perovskite solar cell using transient absorption (TA) techniques. CH3NH3Sn0.5Pb 0.5 I3 was deposited onto mesoporous TiO2 substrates using one step method and P3HT was used as a hole transport material. We found that ultrafast charge separation in a time scale of 1 ps was observed at both the TiO2/CH3NH3Sn0.5Pb0.5I3 and CH3NH3Sn0.5Pb0.5I3 /P3HT interfaces. On the other hand, charge recombination at TiO2/CH3NH3Sn0.5Pb0.5I3 and CH3NH3Sn0.5Pb0.5I3/P3HT interfaces occurred in a time scale of 10 μs and 16 ps, respectively. Our results indicate that the bottleneck of photovoltaic efficiency in Sn/Pb cocktail halide based perovskite solar cell is the recombination rather than charge separation and the efficiency can be improved by suppressing the recombination (especially that at CH3NH3Sn0.5 Pb0.5I3/P3HT interface) through appropriate surface passivation and interfacial engineering.

Thursday 11th

September 16.30 - 17.00 Engineering the Electronic Properties of CQDs from the Atomic to Suprananocrystalline Level for Solar Cell Applications

Alexandros Stavrinadis, Arup K. Rath, Pelayo Garcia de Arquer, Gerasimos Konstantatos ICFO-The Institute of Photonic Sciences, Av. Carl Friedrich Gauss, 3 8860 Castelldefels (Barcelona), Spain

Making use of the colloidal character of quantum dots has allowed us to investigate two new photovoltaic-technology driven concepts: PbS CQDs based bulk nano-heterojunctions (BNH) and homojunctions. The first relies on blending the p-type PbS dots with n-type ZnO nanocrystals (NC) for creating compound photoactive layers. The resulting BNH devices show significantly improved figures of merit (Jsc=16 mA/cm2, Voc=0.64 V, FF=50 % and PCE=5.2 % under 1 sun) compared to simple bilayer (BL) devices (Jsc=12.07 mA/cm2, Voc=0.51 V, FF=43 %, PCE=2.64 %). The PCE of the BNH is further increased to 7% under low lighting conditions (10% sun). The improved performance of the BNH device is attributed to: a) improved separation and extraction of photogenerated carriers in the BNH layer due the increased p-n interface and respective conduction channels within it, b) remote passivation of the PbS CQDs' mid-gap trap states by electrons from ZnO NCs located close to the PbS CQDs. The latter hypothesis is supported by a number of physical properties: i) the BNH devices when in dark have significantly higher shunt resistance compared to BLs, ii) temperature (T) depended Voc measurements


indicate that for T approaching 0 K the extrapolated Voc of the BNH devices is close to the PbS CQDs´ band gap, iii) the ideality factor of the BNH device is 1. Photoluminescence (PL) measurements further confirm that ZnO NCs located within 30nm from the PbS dots enhance the emission of the later.

The second work presented describes an electronic doping approach for transforming p-type PbS CQDs to n-type. It relies on elemental doping of the dots with a trivalent cation which when replaces Pb2+ in the PbS structure, it provides extra electrons for raising the Fermi level of the material. This concept was recently demonstrated for the case of Bi3+ as the dopant, and allowed for the creation of Bi:PbS/PbS bilayer homojunctions working as air-stable solar cells with PCE=2.7 % under 1 sun illumination. The optoelectronic properties of the doped material and homojunctions depend on the Bi/Pb molar ratio (studied in the 0.1-4 % range) and suggest that while doping raises electron concentration, this process is limited by the formation of doping-induced traps. Continuing on this work we also investigate Sb, In, Sn as dopants. Systematic characterization of the materials and devices with increasing doping density has revealed the following: a) only Sb resembles the case of Bi for yielding an electron accepting material suitable for the formation of a homojunction, yet Sb is a less efficient dopant, ii) the aforementioned functionality is accompanied by PL quenching and crystal structure distortion iii) the degree of physical incorporation of the dopant in the CQD product is highly element depended and highest for the cases of effective electronic doping.

Thursday 11


CdTe Nanocrystals in Ink-Based Photovoltaics: A Study of Grain Growth and Device Architecture

Ryan Crispa, Matthew Panthani

b, Dmitri Talapin

c, Joseph Luther

d

a, NREL, 16253 Denver West Parkway, Golden, 80401, US b, University of Chicago, Chicago, IL, US

We present on the use of cadmium telluride (CdTe) nanocrystal colloids as a solution-processable “ink” for large-grain CdTe absorber layers in solar cells. After layer-by-layer processing into thin film devices, the resulting grain structure and solar cell performance are found to depend on the initial nanocrystal size, shape, and crystal structure. Inks composed of predominantly wurtzite tetrapod-shaped nanocrystals exhibit better device performance compared to inks composed of irregular faceted nanocrystals or spherical zincblende nanocrystals despite the fact that the final sintered film has a zincblende crystal structure. Five different working device architectures were investigated. Surprisingly, we find the indium tin oxide/CdTe/zinc oxide structure leads to our best performing device architecture (with efficiency >11%) compared to others including two structures with a cadmium sulfide (CdS) n-type layer typically used in high efficiency sublimation-grown CdTe solar cells. Moreover, devices without CdS have improved response at short wavelengths due to improved transmission of high energy light through the window layer.

Thursday 11th

September 17.15 - 17.30 A Quantum Dot Toolkit for Improving Thin Film Photovoltaic Performance

Miri Kazesa, Sophia Buhbut

b, Stella Itzhakov

a, Ohr Lahad

a, Arie Zaban

b, Dan Oron

a

a, The Weizmann Institute of Science, Department of physics of complex systems, Rehovot, 76100, IL b, Bar Ilan University, Department of chemistry, Ramat-Gan, 52900, IL

Beyond the more common application of quantum dots (QDs) as broadband tunable absorbers in photovoltaics (PV), QDs also exhibit many interesting photo physical processes that may serve useful for the design and optimization of various types of solar cells.A mismatched band offset is one of the fundamental origins for photovoltage loss in photovoltaic devices. Here, we show that such over potentials between the absorber and the selective contacts can be reduced by exploiting the photo induced dipole (PID) phenomenon. This is achieved by using type-II QDs on top of another semiconductor serving as a sensitizer and a TiO2 electron conductor. The type-II QDs offer an efficient spatial charge separation that allows for the accumulation of negative charges in the TiO2 and positive charges in the QDs cores across the sensitizer layer that serves here also as a dielectric layer, thus creating a significant photodipole. The generated PID, negatively shift the TiO2 conduction band in respect to the electrolyte, significantly increasing the solar cell open circuit voltage (Voc).Here we investigate the PID effect in detail by using a flat TiO2/CdSe electrode configuration cell, which has several clear advantages. First, the PID effect can be “turned on” (and thus quantified in a direct


manner) at will by controlling the excitation color. Second, it enables to quantify the charge density per QD. Further, to better understand the dynamics of the system, we also use ZnSe:Te/CdS (Te doped) QDs which have a deeper confined hole state. The effect was studied by charge extraction measurements combined with transient photovoltage. The Te doped ZnSe/CdS QDs give rise to a nearly 150mV increase in the open circuit voltage of the cell, leading to a maximal Voc of nearly 800mV despite the use of a polysulfide electrolyte.With a limited choice of hole conducting materials, the flexibility of the PID concept introduces a new design strategy toward the development of additional kinds of high voltage PV cells such as solid state QDSSCs, oxide PVs, organic solar cells (OPV), hetrojunction cells, and perovskite based solar cells. In addition, implementing similar concepts may enable other efficiency improvements such as by use of the QDs in an intermediate band solar cell.

Thursday 11th

September 17.30 - 17.45 Post-Synthesis Assembly for High Efficiency Quantum Dot Sensitized Cells

Xinhua Zhong East China University of Science and Technology, 130 Meilong Rd, Shanghai, 200237, CN

Albeit high-quality QDs can be feasibly obtained relying on the well-developed organometallic high-temperature synthetic route, it is still a great challenge to immobilize the pre-prepared QDs on mesoporous oxide film electrodes with a high surface coverage. A postsynthesis assembly approach, ex situ ligand exchange route, has been developed for the effective deposition of QDs sensitizers (including CdSe, CdS/CdSe, CdSexTe1-x, and type-II CdTe/CdSe QDs) on TiO2 mesoporous film and Power Conversion Eefficiencies (PCE) of 5.42%, 5.32%, 6.36% and 6.76% are obtained for corresponding cell devices, respectively.

1-4 Linker molecules mercaptopropionic acid (MPA) capped water-soluble QDs were

prepared via ex situ ligand exchange form the initial oil-soluble QDs and then immobilized on TiO2 film electrodes with coverage of 34% by immersion the electrode into MPA-capped QD aqueous dispersions. The uniformity of QD deposition throughout the film thickness is confirmed by the elemental mapping technique and also by the nearly constant Cd/Ti ratio within the cross-section of the film. The type-II CdTe/CdSe QD based sensitized solar cells show a record efficiency of 6.76% under simulated AM 1.5 G, full 1 sun illumination was obtained, which is the highest reported to date for liquid junction QDSC. This result situates the sensitized technology in the same range of efficiencies reported with other QD solar cell configurations. Furthermore, a PCE up to 7.04%, and a certified PCE of 6.66% has been obtained on the Cd-, Pb-free “green” CuInS2 QD based QDSC.

5 These encouraging PCE values indicate the promising

future for the QDSC with simple configuration.

Thursday 11th

September 17.45 - 18.00 Effective Atomic-ligand Passivation of Cu2O Nanoparticles Through Solid-state Treatment with Mercaptopropionic acid

Hamed Azimia, Susanne Kuhri

b, Andres Osvet

a, Gebhard Matt

a, Laraib S. Khanzada

a, Mario Lemmer

a,

Norman A. Luechingerc, Mats I. Larsson

d, Eitan Zeira

d, Dirk M. Guldi

b, Christoph J. Brabec

a

a, institute Materials for Electronics and Energy Technology, Martensstrasse 7, R.372, Erlangen, Germany, D-91058, DE b, Department of Chemistry and Pharmacy and Interdisciplinary Center for Molecular Materials, Friedrich-Alexander-University Erlangen-Nuremberg, , 91058 Erlangen, Germany, DE c, Nanograde Llc., Laubisruetistrasse 50, 8712 Staefa, Switzerland., Switzerland d, OneSun Inc., , PO BOX 1399, Sausalito, CA 94966, USA, USA

In colloidal nanoparticles (NPs) devices, trap state densities at their surface exert a profound impact on the rate of charge carrier recombination and, consequently, on the deterioration of the device performance. Here, we report on the successful application of an atomic-ligand strategy to effectively passivate the surface of cuprite (Cu2O) NPs. Cu2O NPs were prepared by means of a novel synthetic route based on flame spray pyrolysis. FTIR, XRD, XPS and HRTEM measurements corroborate the formation of cubic cuprite Cu2O nanocrystals, excluding the possible presence of undesired CuO or Cu phases. Most importantly, steady-state emission and transient absorption assays document that surface passivation results in substantial changes in the intensity of emissive excitonic states – centered at copper and oxygen vacancies – and in the lifetime of free excitons near the band edge. To shed light onto ultrafast processes in Cu2O nanocrystals additional pump probe experiments on the femtosecond


and nanosecond timescales were carried out. Two discernible species were observed. On one hand, an ultra-fast component (~ps) that relates to the free excitons; on the other hand, a long-lived component (~µs) that originates from the defects / trap states. Such knowledge about structure property relationship is decisive for the performance optimization of all oxide colloidal NPs devices.

Thursday 11th

September 18.00 - 18.15 Colloidal AgBiS2 Nanocrystals. New Non-Toxic Material for Solution Processed Solar Cells

María Bernechea, Nichole C. Miller, Gerasimos Konstantatos Institute of Photonic Sciences ICFO, Avda. Carl Friedrich Gauss 3, Castelldefels, 08860, ES

Great progress has been seen over the last years in the field of solution processed solar cells based on inorganic semiconductors. Recent efficiencies have reached 8.6 % in the area of quantum dots solar cells, or 17.9 % for solar cells based on perovskites. However, the future expansion of these cells could be limited due to the presence of toxic elements, more specifically lead (Pb). Efforts are thus required in seeking for earth-abundant, and non-toxic materials.In this context we will present our work based on a new colloidal nanocrystalline material based on AgBiS2. Very few reports exist on the properties of this compound and its utilization in solar cells has yet to be reported.We have developed a synthesis to obtain this material in colloidal solution, which is stable for months. This material possesses a tuneable by size/stoichiometry band gap in the favourable region of 1.0-1.4 eV and very high absorption coefficient: 10

5–10

3 cm

-1 from 900 nm to 400 nm. We have shown that by modifying the synthetic

conditions we can tune the size and stoichiometry of the nanocrystals, whereas the effect of the ligand passivation schemes reflect upon the carrier concentration of solid state films as measured by UPS. The afore-mentioned parameters also have significant impact on solar cell device efficiencies that that are in excess of 2%.

Friday 12th

September 8.30 - 9.00 Perovskite Photovoltaic Cell Design for High Voltage and Non-hysteretic Performance

Tsutomu Miyasaka, Ajay Jena, Ayumi Ishii Graduate School of Engineering, Toin University of Yokohama, 1614 Kurogane-cho, Aoba, Yokohama, 2258503, JP

High efficiency of the organo-lead-halide perovskite cells is backed by high open-circuit voltage, 1.0-1.1V, in addition to strong light absorption due to the hybrid crystalline structure. High voltage is realized by minimised thermal loss in the charge transfers across solid-solid hetero junctions. For the narrow band gap tri-iodide perovskite, voltage loss of the cells using spiro-MeOTAD hole transport material ranges from 0.4 to 0.55 eV with respect to the band gap of 1.55 eV (800 nm). Here, relatively high voltage (Voc >1.05V) is obtained with use of Al2O3 mesoporous scaffold. Mesoporous scaffolds also help to minimize the hysteresis of I-V characteristic, especially in combination with a Cl-doped tri-iodide perovskite coated on a thin TiO2 compact layer. Our group has been focusing the perovskite PV development on two subjects. One is how to achieve higher voltage for narrow gap perovskite cells, and the other is how to exclude the hysteresis in the I-V characteristics to ensure stable cell performance. We found that combination of uniform and very thin TiO2 compact layers (thickness <10 nm) and mesoporous TiO2 scaffolds for Cl-doped tri-iodide perovskite based cells gives I-V characteristics that has excellent responsivity under high-speed scanning and causes no hysteresis, realising perfect matching of efficiencies for forward and reverse scan. Use of Al2O3 scaffold in the same conditions however tends to cause hysteresis depending on the scaffold thickness. Cell stability was highly affected by the quality and density of the compact layer and interfacial structures of metal oxide and perovskite, in which Cl doping improves the quality of perovskite including crystalline orientation. For high voltage generation, one of the best cell structure we found is use of a well-oriented crystalline hole transport material in junction with an oriented high quality perovskite layer. A thin crystalline film of perylene is an efficient hole conductor for this purpose. Self-organised formation of perylene on the surface of perovskite was influenced its orientation by the orientation of underlying perovskite. The fully crystalline perovskite-perylene hybrid cell is capable of Voc exceeding 1.2V maintaining sufficiently high conversion efficiency of 4%. The voltage loss of this cell, <0.35 eV, is one of


the smallest value ever achieved by solid state thin photovoltaic cells, and can be compared to GaAs photovoltaic cell which is capable of high Voc up to 1.12V with respect to band gap of 1.42 eV.

Friday 12th

September 9.00 - 9.30 High Efficiency Perovskite Solar Cells via Oxide Nanoengineering

Nam-Gyu Park School of Chemical Engineering, Department of Energy Science, Sungkyunkwan University, Suwon 440-746, Korea

Perovskite solar cell based on lead iodide light harvester is an emerging photovoltaic technology due to extremely low cost and superb photovoltaic performance. In this talk, technologies for perovskite solar cells with power conversion efficiency (PCE) approaching 17% are presented. The first version of long-term durable perovskite solar cell was developed by Park’s group in 2012, which demonstrated a PCE of 9.7%. In 2013, PCE of 15% was achieved by Gratzel and Snaith groups. However, the average PCE was as low as 12%. Photovoltaic performance was found to be significantly influenced by perovskite preparation procedure. Two-step coating method was superior to single-step deposition procedure. For the two-step coating case, photovoltaic parameters were dependent on crystal size of methylammonium lead iodide. Single-step deposition showed low PCE of about 8-9%, on the other hands, two-step method exhibited much higher PCE with average value of more than 16% and best value of 17%, along with small standard deviation. Substitution of formamidinium for methylammonium led to lower band gap and higher absorption coefficient, which consequently resulted in PCE of 16% and average PCE of 15.5% with better photostability and no I-V hysteresis.

Friday 12


CH3NH3PbI3 Perovskite Solar Cells using CuSCN Inorganic Hole Transmitting Layer

Seigo Ito University of Hyogo, 2167 Shosha, Himeji, 671, JP

Despite the rapid increase in efficiency of the hybrid organic-inorganic methylammonium lead halide perovskites (CH3NH3PbI3, X = Cl-, Br-, I-) solar cells, the hole transporting material (HTM) used were mainly limited to organic compounds, the start-of-the-art 2,2’,7,7’-tetrakis(N,N-di-p-methoxyphenylamine)-9,9’-spirobifluorene (spiro-MeOTAD) and conducting polymers. Compared to organic HTMs, inorganic p-type semiconductors appear to be an ideal choice from the point of view of high mobility, stability ease of synthesis and low cost. Here, we report that combining the perovskite CH3NH3PbI3 with CuSCN as p-type HTM lead to solar cells with very high power conversion efficiency (12.4%) under full sun illumination. Moreover, to improve the stability, Sb2S3 layers were inserted at the interface between TiO2 and CH3NH3PbI3 perovskite to be CH3NH3PbI3 solar cells using inorganic hole transporting material (CuSCN). The CH3NH3PbI3 layer was spin-coated by one-drop method on nanocrystalline TiO2 layer. During the light exposure test without encapsulation, the CH3NH3PbI3 solar cells without Sb2S3 deteriorated to zero efficiency in 12 h and were completely changed from black to yellow, because the perovskite CH3NH3PbI3 was changed to hexagonal PbI2. With Sb2S3, on the other hand, the CH3NH3PbI3 solar cells became stable against light exposure without encapsulation, which didn’t change the crystal structure and the wavelength edges of absorption and IPCE. Therefore, it was considered that the degradation can occur at the interface between TiO2 and CH3NH3PbI3.

Friday 12th

September 10.00 - 10.15 Organo-metal Halide Perovskite Solar Cells with Inorganic Hole Selective Contacts

Sudam Chavhana, Ivet Kosta

a, Oscar Miguel

a, Hans Grande

a, Victoria Gonzalez-Pedro

b, Eva Barea

b, Ivan

Mora-Serob, Ramon Tena-Zaera

a

a, CIDETEC, Parque Tecnol, San Sebasti, 20009, ES b, Universitat Jaume I, 12071 Castello, Spain, ES

The extremely rapid evolution of the perovskite solar cells during the last 2 years makes them a very appealing cost- and performance-competitive photovoltaics emerging technology. The state of the art power conversion efficiencies (15-18 %) reached by using different electron transporting materials


(TiO2, ZnO, (6,6)-phenyl C61-butyric acid methyl ester –PCBM-), perovskite variants (CH3NH3PbI3-xClx, Y= Br, Cl) and device architectures (e.g. mesoscopic and planar-based junction) evidence the large versatility and wide room for further progress in the perovskite photovoltaics technology. Nevertheless, the spiro-OMeTAD or conducting polymers are commonly used as hole transporting materials (HTMs). However, the use of solution-processed inorganic HTM appears to be an appealing alternative in terms of cost and device robustness. Among the few reports on inorganic HTM, CuSCN appears to be one of the most promising candidates (i.e. devices with power conversion efficiencies of 12.4 %). A piece of work on the use of CuSCN as HTM in perovskite solar cells with different architectures, including perovskite films processed by different techniques, will be presented here. A comparative analysis of the solar cell performance with respect to reference devices with spiro-OMeTAD and without HTM will be shown in order to emphasize the CuSCN role and influence in the perovskite solar cell performance. A correlation between the differences in the device performance and properties of the HTM as well as the perovskite/HTM interface will be introduced. In particular, some advanced processing strategies to improve the perovskite/CuSCN interface will be proposed. A critical analysis about the influence of pinholes in the perovskite film and consequent TiO2/CuSCN local interfaces will be also shown. All in all, an overview about the potentiality of CuSCN as hole selective contact in perovskite photovoltaics technology and crucial information for its successful integration will be given.

Friday 12th

September 10.15 - 10.30 Solution Deposition-conversion of Perovskite Solar Cells

Pablo Docampo LMU Munich, Butenandtstraße 11, Munich, 81377, DE

Solar cells based on alkyl ammonium halogen plumbate perovskites have recently garnered a large amount of interest in the photovoltaics community. Previously reported record-breaking devices incorporate a solution processed planar heterojunction with the MAPbI3 sandwiched between n and p-type charge collection layers. Here, the focus will be on the fabrication protocols developed for the deposition of planar heterojunction solar cells via a solution deposition-conversion technique on TiO2 coated FTO substrates and with state-of-the-art Spiro-OMeTAD as the hole transporter. Our results show that the processing conditions of the conversion solution have a deep influence on the resulting perovskite structures, from extended charge diffusion lengths to other morphological effects. We have studied the prepared structures not only via structural analysis, but also through time-resolved photoluminescence measurements. In this way we have pinpointed the origin of the limits to power conversion efficiency, mainly short diffusion lengths of the photoexcited species. By tuning the deposition conditions, we have obtained solar cells exhibiting power conversion efficiencies of up to 15% for the typical MAPbI3 perovskite and approaching 7% for wide bandgap perovskite absorbers.

Friday 12th

September 11.00 - 11.30 CH3NH3PbBr3 for high voltage PV cells

Gary Hodes, Eran Edri, Saar Kirmayer, Michael Kulbak, Yaron Tidhar, Boris Rybtchinski, David Cahen Weizmann Institute of Science, Herzl St. 2, Rehovot, 76100, IL

Among the many desirable properties of the hybrid halide perovskite semiconductors, the one that interests us the most in our search for a viable high voltage cell is the high values of VOC relative to the semiconductor bandgap that can be obtained. For this reason, we have concentrated on CH3NH3PbBr3 with a bandgap of 2.3 eV. We have considered two main factors in maximizing the VOC of cells based on this perovskite: The nature of the selective charge extraction or nucleation layers and the nature of the perovskite itself. The most important selective contact for attaining high VOC in our case is the hole conductor. The VOC was found to scale with the depth of the hole conductor HOMO level in most cases. The nature of the CH3NH3PbBr3 was also important. This material tends to form large crystals that do not cover the substrate well. Addition of small amounts of Cl (through PbCl2) to the Br perovskite was found to improve the cells, as was well-known for the corresponding iodide perovskite, given values of VOC >1.5 V and with improved photocurrents and efficiencies compared to the pure Br perovskite. An in-depth study of the effect of Cl showed that the CH3NH3Pb(Br,Cl)3 formed smaller and better covering crystals than the pure bromide due to nucleation of the perovskite on the relatively insoluble (compared to the corresponding higher halides) PbCl2. More recent efforts are directed towards lowering the


bandgap of the CH3NH3Pb(Br,Cl)3 to the range of 1.8 - 2 eV, which would be more ideal for use as the high voltage part of a tandem cell.

Friday 12th

September 11.30 - 12.00 Organo-lead Tri- Halides Perovskites for Highly Efficient Solar Cells: Photophysical versus Structural Properties

Annamaria Petrozza Center for Nano Science and Technology of Italia , via Pascoli 70/3 20133 Milano

Here we present a comprehensive picture of the main photophysical properties of methyl ammonium lead tri-iodide perovskite (CH3NH3PbI3) and its Chlorine-doped counterpart ( Cl-doped CH3NH3PbI3) with a particular focus on the structure-optical properties relationship, emphasizing the role of the interfaces from a molecular to mesoscopic level. By combining optical, vibrational, and structural investigations we investigate time scale and dynamics of carriers thermalization, state filling effect, band gap renormalization, exciton formation, and ionization and we correlate them to the thin film morphology.

Friday 12


Photo-induced Interfacial Charge Recombination in Double Heterojunction Solar Cells

Emilio Palomares a, ICIQ, Avda. Països Catalans,16, Tarragona, 43007, ES b, ICREA, Passeig Lluis Companys, 23, Barcelona, E-08010, ES

In one hand, in my lecture, I will present our latest results on the use of Transient Photovoltage (TPV) and Laser Transient Absorption Spectroscopy ( L-TAS) measurements to study the interfacial recombination charge transfer reactions occuring in double heterojunction solar cells under working conditions when using as hole transport material spiro-OMETAD and other semiconductor polymers. The implications of HOMO ( Highest Occupied Molecular Orbital) energy levels, in the semiconductor materials used as hole conductors, charge regeneration and charge recombination will be also discussed. Moreover, we will discuss the use of the Time Correlated Single Photon Counting (TCSPC) technique to adress the efficiency of inorganic/organic solar cells. Last but not least, results on the the use of novel hole transport materials for MAPbI3-xClx for efficient solar cells with the structure FTO/dTiO2/mTiO2/HTM/Metal.On the other hand, I will show results of efficient small molecule based orgnaic solar cells ( smOSC) with efficiencies close to 7% and how photo-induced techniques such as Charge Extraction ( CE), TPV, and Transient Photo-Current ( TPC), which have been widely used to study Dye Sensitized Solar Cells, can also be used to study smOSC.

Friday 12th

September 12.30 - 12.45 Efficient Photoluminescence and Cean Photo-physical Properties of Mixed-Halide Organolead Perovskite Semiconductors

Aditya Sadhanalaa, Felix Deschler

a, Sian Dutton

a, Tudor Thomas

a, Fabian Hanusch

c, Henry J. Snaith

b,

Pablo Docampoc, Richard H. Friend

a

a, University of Cambridge, Optoelectronics group, Cavendish Laboratory, University of Cambridge, Cambridge, GB b, University of Oxford, Clarendon Laboratory, Department of Physics, University of Oxford, Oxford OX1 3PU, UK, GB c, University of Munich , Department of Chemistry and Center for NanoScience (CeNS), University of Munich (LMU), Butenandtstr. 11 , Munich, 81377, Germany

Organometallic mixed halide perovskite-based solar cells have shown a breakthrough in power conversion efficiency. Solution-processed devices with power conversion efficiencies of 10-12% were reported in 2012, and have more recently exceeded 15% in devices processed by evaporation and sequential deposition. Recently new perovskite types have been developed by modifying the chemical composition of the materials.We investigate the photo-physical properties of mixed-halide lead perovskite films with varying bromide-iodide ratios from pure iodide to pure bromide which were


prepared by spincoating from solution. We measure the absorption onset with high sensitivity using photo-thermal deflection spectroscopy. We additionally study the recombination of photo-excited states by transient photoluminescence spectroscopy and correlate our spectroscopic findings with results from X-ray diffraction measurements. We find that our sample fabrication protocol yields materials with single optical transitions and surprisingly clean photo-physical properties.

Friday 12th

September 12.45 - 13.00 Ultrafast Charge Generation, High and Microsecond-long Balanced Mobilities and Slow Recombination in Organometal Halide Perovskite Solar Cell Materials

Carlito Jr Ponsecaa, Tom Savenije

b, Mohamed Abdellah

a, Kaibo Zheng

a, Arkady Yartsev

a, Torbjorn

Paschera, Tobias Harlang

a, Pavel Chabera

a, Tonu Pullerits

a, Andrey Stepanov

c, Jean-Pierre Wolf

c, Villy

Sundstroma

a, Division of Chemical Physics, Lund University, Getingevagen 60, Lund, 22241, Sweden b, Department of Chemical Engineering, Delft University of Technology, 2628 BL Delft, The Netherlands c, GAP-Biophotonics, University of Geneva, 22, Chemin de Pinchat, 1211 Geneva 4, Switzerland

Organometal halide perovskite based solar cells have recently been reported to be highly efficient, giving an overall power conversion efficiency of up to 15%. However, much of the fundamental photophysical properties underlying this performance have remained unknown. Here, we apply photoluminescence, transient absorption, time resolved terahertz and microwave conductivity measurements to determine the timescales of generation and recombination of charge carriers as well as their transport properties in solution-processed CH3NH3PbI3 perovskite materials. We found that electron-hole pairs are generated almost instantaneously after photoexcitation and dissociate in 2 ps forming highly mobile charges (25 cm

2V

-1s

-1) in the neat perovskite and in perovskite/alumina blends;

almost balanced electron and hole mobilities (μe= 12.5 cm2V

-1s

-1, μh = 7.5 cm

2V

-1s

-1) remain very high up

to the microsecond time scale. When the perovskite is introduced into a TiO2 mesoporous structure, electron injection from perovskite to the metal oxide is efficient in less than a picosecond but the lower intrinsic electron mobility of TiO2 (μ < 0.1 cm

2V

-1s

-1) leads to unbalanced charge transport. Microwave

conductivity measurements showed that the decay of mobile charges is very slow, lasting up to tens of microseconds. These results unravels the remarkable intrinsic properties of CH3NH3PbI3 perovskite material if used as intrinsic light absorber and charge transport layer. Moreover, finding a metal oxide with higher electron mobility may further increase the performance of this class of solar cells.

Friday 12th

September 13.00 - 13.15 Electro and Photoluminescence Analysis of Dye Solar Cells and Mesoporous Perovskite Solar Cells

Simone Mastroiannia, b

, Katrine Flarup Jensena, Welmoed Veurman

a, Henning Brandt

a, Andreas Hinsch

a

a, Fraunhofer Institute for Solar Energy Systems , Heidenhofstraße 2, Freiburg im Breisgau, 79110, DE b, Freiburg Materials Research Centre (FMF), Stefan-Meier Str. 21, 79104 Freiburg, DE

At the present state of Dye Solar Cells literature, Electroluminescence (EL) was only investigated on lab scale devices with the purpose to study ageing effects. EL is here systematically applied to study the origin of signal emission through spectral and time resolved analyses and for detecting spatial variations on DSCs. The analyses on DSCs are further improved with Light Beam Induced Current (LBIC) measurements and absorbance imaging (Figure 1). We scaled up this technique from a 2.5 cm

2 cell to a

10 x 10 cm2 module and to a 25 x 25 cm

2 section of a 6000 cm

2 glass frit sealed module. We present a

detailed analysis showing the effects caused by I3- diffusion limitations through time resolved studies

and spatial differences on EL maps attributed to non-uniformity of dye coverage and to different electron-electrolyte recombination rates at the photoelectrode.More fundamental investigations by EL and photoluminescence (PL) are also performed on Perovskite Solar Cells (PSCs) devices to follow charge transport and recombination processes which we assume to be a direct measure for the perovskite crystal grow, its morphology and the electronic properties of the contacting layers as function of solution processed device fabrication. Our results from investigations on 0.2cm

2 perovskite (CH3NH3PbI3)

solar cells shows that PL in particular is sensitive to the bulk excitons in the absorber layer, whereas EL (as in organic LEDs) is most sensitive to electron and hole injection rates at the contact layers and to the diffusion limitation of the charges in the absorber layer. First EL and PL maps are also shown for complete PSCs (Figure 2) together with LBIC and SEM images.


Friday 12th

September 14.30 - 15.00 Dynamic Processes in Perovskite Solar Cells

Juan Bisquert, Ivan Mora-Sero Universitat Jaume I, Avda Sos Baynat, Castello, 12071, Spain

Organometal halide perovskite-based solar cells have recently realized large conversion efficiency over 17% showing great promise for a new large scale cost-competitive photovoltaic technology. The proficient operation of the CPbX3 perovskite solar cell, where C is an organic cation, has been accomplished by many different approaches, and it points to a robust photovoltaic operation mechanism that so far has not yet been fully understood. We report the behaviour of lead halogenate perovskite solar cell probed by a number of dynamic techniques including impedance spectroscopy and time transient dynamics. We compare a number of compositions and morphologies and we show the general characteristics of the observed processes. The perovskite solar cell shows recombination resistance and chemical capacitance, but new and interesting phenomena govern the solar cell behaviour in the long time scale, that influence the solar cell performance in phenomena as hysteresis or time dependent luminescence. These results indicate the need for detailed studies to relate structural, ionic and electronic behaviours in the perovskite solar cells.

Friday 12th

September 15.00 - 15.30 Rational Design of Novel Absorbers for Solution-processed Solar Cells: Advantages and Pitfalls

Feliciano Giustino University of Oxford, Department of Materials, University of Oxford, Oxford OX1 3PH, GB

Recent years have witnessed tremendous progress in the development of new materials for solution-processed solar cells. The impressive pace of experimental R&D is gradually reshaping the role and mission of materials modelling in this area. For example ever more often theorists are asked to screen broad materials libraries in order to identify promising new compounds. In this talk I will try to provide a fair assessment of where we stand in the computational prediction of new absorbers. I will start by discussing a recent attempt at designing novel perovskites. Here we wanted to address the following simple question: can we design perovskites with a given optical gap? In order to answer this question we used a hybrid rational design approach combining extensive first-principles calculations with a simple mathematical model of the metal-halide network. Using this approach we found that the gap correlates with the largest metal-halide-metal bond angle. With this information at hand we performed an extensive set of first-principles calculations, and established that the bond angles can be controlled via the size of the organic cations. This study led us to identify several new cations which would allow tuning the gap from the infrared to the visible. In a second example I will describe our work on the design of semiconductor sensitizers with optimal work function. The starting point of our investigation was stibnite (antimony sulphide), a mineral of the tetradymite family which was successfully used as a dye-replacement in sensitized solar cells. Stibnite is accompanied by three isostructural compounds, obtained by replacing sulphur by selenium and/or antimony by bismuth. Using first-principles calculations we predicted that, while bismuthinite (bismuth sulphide) should not inject electrons into titania, antimonselite (antimony selenide) should be an efficient sensitizer. These predictions were recently confirmed by experiments. Finally I will briefly discuss an example of the pitfalls of rational design. In the context of dye-sensitized solar cells significant efforts are devoted to design new dyes with minimal loss-in-potential, by tuning the frontier orbitals of the dyes in isolation. Here, by combining electronic structure calculations of titania/dye interfaces with atomically-resolved STM measurements, we established that the dye adsorption geometry plays a crucial role in the energy-level alignment. This finding implies that the optimization of dyes in isolation is bound to be ineffective. This example highlights the limitations of rational design, and stresses the importance of understanding the fundamental mechanisms underlying the operation of solution-processed solar cells.

Friday 12th

September 15.30 - 15.45 Optoelectronic Properties of Hybrid Perovskites: a Change of Paradigm

Jacky Evena, Laurent Pedesseau

a, Mikael Kepenekian

b, Claudine Katan

b


a, INSA Rennes FOTON UMR6082, 20 avenue des buttes coesmes, rennes, 35708, FR b, CNRS, Institut des Sciences Chimiques de Rennes UMR6226, 35042 Rennes, FR

3D hybrid perovskites are currently superstar materials for photovoltaics and attract increasing attention of the scientific community. Recently, they also revealed attractive photoluminescence, as it is already well known for 2D layered hybrids. In the quest for a deeper understanding of such appealing properties, theoreticians have recently taken up the challenge. Our scientific approach starts from the identification of a multifaceted change of paradigm related to the recent advances in this field and is based on concepts of solid-state physics originally developed for conventional semiconductors. We will show how the broad light-harvesting abilities and attractive transport properties of 3D metal-halide hybrid perovskites can be related to the multi-bandgap and multi-valley nature of their band structure. Channels in reciprocal space facilitate carrier redistribution after optical excitation. Extensive analysis of older experimental data, as well as comparison to 2D layered hybrids and conventional semiconductor heterostructures, allow clarifying the nature of the photoexcited species. From dielectric responses over a wide frequency range we show that the Wannier-like exciton evidenced at low temperature becomes almost entirely screened at room temperature. Besides the screening by optical phonons, analogy with the reorientation dynamics of CN- in alkali-cyanides and comparison to all-inorganic perovskites suggests further screening due to collective rotational motion of the organic cations. This picture is consistent with the recent experimental findings.As a conclusion, the change of paradigm from DSSC to a new class of semiconductor solar cells is most probably associated at room temperature to free carriers and Bloch states of the inorganic lattice, rather than excitons. The collective rotations of the organic cations are proposed as a fundamental screening mechanism in 3D hybrids.

Friday 12th

September 15.45 - 16.00 The Role of the Methylammonium Cations on the Electronic and Photovoltaic Properties of the CH3NH3PbI3 Perovskite

Claudio Quarti, Edoardo Mosconi, Filippo De Angelis Consiglio Nazionale delle Ricerche (CNR), via elce di sotto 8, Perugia, IT

In the recent years, methylammonium lead-halide perovskites based solar cells have shown an impressive improvement in their performances but, in spite of these results, many open questions about the basics of these materials remain to be addressed. Peculiarity of these hybrid perovskites is the presence of methylammonium cations (MA), having a permanent dipole, that are free to rotate inside the cubo-octahedral PbI cage. The orientation of the MA cations within the perovskite is thus expected to play an important role on the dielectric properties of this class of materials. In this regard, ferroelectricity has been recently observed in the tetragonal phase of CH3NH3PbI3. In light of the potential impact of the MA cations on the properties of the hybrid lead-halide perovskites, we present here a systematic investigation on the interplay between the arrangement of the organic cations and the electronic properties of the CH3NH3PbI3 perovskite. Various configurations for the orientation of the MA cations are studied by first-principles electronic structure calculations, augmented by Car-Parrinello molecular dynamics simulations. In particular, we consider here two kind of configurations, i) with a net alignment of the MA cations, which are expected to show ferroelectric properties, and ii) without a net alignment of the cations, which are likely to be antiferroelectric. The configurations with a net alignment of the MA cations are a few kT more stable than the configurations with isotropic MA orientations. The analysis of the band structure of oriented and not-oriented configurations highlights that the degree of alignment of the MA cations affect the magnitude of the Rashba splitting. Moreover, configurations with oriented MA cations show a bending of the frontier orbitals toward the surface, which helps the charge separation and injection.Thus, the orientation of the MA cations in the CH3NH3PbI3 perovskite affects significantly the electronic and dielectric properties of this material and it is expected to play an important role in the mechanisms at the basis of the photovoltaic performances of hybrid lead-halide perovskites.

Friday 12th

September 16.00 - 16.15 Hybrid Halide Perovskites: Modelling Crystal Dynamics and Devices

Jarvist Frost, Federico Brivio, Keith Butler, Aron Walsh Bath University, Dept of Chemistry, University of Bath, Bath, GB

International Conference of Solution Processed Semiconductor Solar Cells - Oxford 2014 High efficiency hybrid halide perovskite solar cells have been developed faster than the

understanding of the device physics. Here we use electronic structure methods to understand the unique features of this system.

We look at the interaction and dynamics of the organic cation, the role of its polarisation in creating molecular ferroelectric domains within the active device, and the interaction of these domains with both excitons and polarons. Molecular dynamics of the material is used to understand the microscopic motion of the cations and provide inspiration for a physical model of the interaction. Electronic structure calculations provide parameters (elastic strain, and dipole strength) for an on-lattice Monte Carlo simulation of ferroelectric domains, capable of accessing far larger length scales and longer time scales than ab-initio molecular dynamics.We find that the built in field of the solar cell at short circuit is capable of modifying the local electric potential structure of the solar cell, as the domain boundary between twinned molecular dynamics responds slowly to applied electric field. This we connect to the observed hysterisis in these materials, as the built in field at short circuit generates electrostatic traps within the perovskite. The inhomogenious electric potential & other values derived from electronic structure calculations are used as inputs into a Monte Carlo model of polaron transport and recombination, to understand how the small scale structure relates to device operation.

Friday 12


Recent Advances in Perovskite-based Solar Cells

Henry Snaith Oxford University, Clarendon Lab, Parks Road, Oxford

Friday 12th

September 17.15 - 17.30 An Easy-to-fabricate Low-temperature TiO2 Electron Collection Layer for High Efficiency Planar Heterojunction Perovskite Solar Cells

Bert Conings, Linny Baeten, Tanya Jacobs, Rafael Dera, Jan D'Haen, Jean Manca, Hans-Gerd Boyen Hasselt University - Institute for Materials Research, Materials Physics, Wetenschapspark 1, Diepenbeek, 3590, BE

Organometal trihalide perovskite solar cells arguably represent the most auspicious new photovoltaic technology so far, as they possess an astonishing combination of properties. The impressive and brisk advances achieved so far bring forth highly efficient and solution processable solar cells, holding great promise to grow into a mature technology that is ready to be embedded on a large scale. However, the vast majority of state-of-the-art perovskite solar cells contains a dense TiO2 electron collection layer that requires a high temperature treatment (> 450°C), which obstructs the road towards roll-to-roll processing on flexible foils that can withstand no more than ~ 150°C. Furthermore, this high temperature treatment leads to an overall increased energy payback time and cumulative energy demand for this emerging photovoltaic technology. Here we present the implementation of an alternative TiO2 layer formed from a easily prepared nanoparticle dispersion, with annealing needs well within reach of roll-to-roll processing, making this technology also appealing from the energy payback aspect. Chemical and morphological analysis allows to understand and tune the processing conditions of the TiO2 layer, finally resulting in an efficiency up to 13.6% for a planar heterojunction solar cell within an ITO/TiO2/CH3NH3PbI2Cl/poly(3-hexylthiophene)/Ag architecture.

Friday 12th

September 17.30 - 17.45 Enhancing the Performance of Planar Heterojunction Perovskite Solar Cells Via the Addition of Functionalized Silica Nanoparticles

Matthew Carniea, Cécile Charbonneau

a, Matthew Davies

a, Brian O' Regan

b, David Worsley

a, Trystan

Watsona

a, Swansea University, Sêr Solar - College of Engineering, Swansea University, Baglan Bay Innovation Centre, Baglan Energy Park, Port Talbot, Port Talbot SA12 7AX, GB b, Imperial College London, Dept. of Chemistry, Imperial College London, GB

All low temperature manufactured organic-inorganic trihalide perovskite solar cells are now a reality, meaning that perovskite photovoltaics could have significantly lower embodied energy than more


established photovoltaic technologies. To date, some of the most efficient solution processed perovskite solar cells feature a pre-deposited Al2O3 scaffold. We have previously shown that additions of alumina nanoparticles (up to 5 wt.%), to the perovskite precursor solution yield improvements in the device performance of planar heterojunction perovskite cells. It is thought that this improvement is achieved by the nanoparticles influencing perovskite crystallization and subsequent morphology, giving rise to better substrate surface coverage and less pinholes when compared to planar heterojunction devices - manufactured under the same processing conditions and without nanoparticles. This is evidenced by an increase in VOC with increasing nanoparticle loading and SEM images of the crystalized films showing greater substrate coverage. In this work, we have substituted the alumina nanoparticles with 3-aminopropyl (3-oxobutanoic acid) functionalized silica nanoparticles (f-SiO2). We observe performance enhancements in planar heterojunction (PHJ) devices made with up to 0.75 wt. % f -SiO2 nanoparticles present in the precursor solution, yielding power conversion efficiencies (PCE) of up to 12.4 %, compared to the maximum PCE of 10.5 % in the equivalent PHJ devices made without f -SiO2 nanoparticles. The f -SiO2 functional group features a terminal carboxylic acid group, which appears to bind to the compact TiO2 layer inside a pinhole void. The performance enhancement arises in part from an average increase to VOC by up to 50 mV when the nanoparticles are present in the precursor solution and is attributed to substrate passivation by f -SiO2 nanoparticles binding to the compact TiO2 layer within pinholes formed in the perovskite film during processing.

Friday 12th

September 17.45 - 18.00 Cathode Interface Engineering in High Efficiency Low TemperaturePerovskite Solar Cells

Weiming Qiua, b

, Jef Poortmansa, c

, Ludo Froyenb, Paul Heremans

a, c, David Cheyns

a

a, Imec, Kapeldreef 75, Heverlee, 3001, BE b, Department of Metallurgy and Materials Engineering, KULeuven, Kasteelpark Arenberg 44, Heverlee,3001, BE c, Department of Electrical Engineering, KULeuven, Kasteelpark Arenberg 10,Heverlee, 3001, BE

Methylammonium lead halide perovskite based solar cells have attracted tremendous research interest in the past few years, with the power conversion efficiency (PCE) skyrocketed from the first reported value of 3.8% to record values close to 20%.The strong and broad light absorption, high carrier mobility, very long electron and hole diffusion length, and low exciton binding energy make them the ideal materials for the next generation solar cells. In order to effectively utilize all these advantages, layer stack engineering is crucial. The interface between the active layer and the electrodes, which determines the effectiveness of photo-induced hole and electron extraction, plays an important role in those for perovskite solar cells. In this work, high efficiency low temperature solution processed CH3NH3IxCl3-x based perovskite solar cells were fabricated using a device structure of ITO/PEDOT:PSS/Perovskite/PC60BM/ZnO nanoparticles (NPs)/Ag. The ZnO NPs were synthesized according to a low temperature sol-gel method, which opens the opportunity to keep all processing steps below 100°C. The ZnO NP layer was processed by spin-coating its dispersion in 1-butanol for three times, which gave the best performance. Compared to ITO/PEDOT:PSS/Perovskite/PC60BM/Al devices, the incorporation of ZnO NPs greatly boosts the open-circuit voltage (Voc) and fill-factor (FF) by virtue of the improved cathode interface. Our best devices show Voc of 1.03 V, short-circuit current density (Jsc) of 21.65 mA/cm2, and FF of 63.3%, resulting in 13.8% PCE.On top of the increased shunt resistance, the new stack layout shows a high device yield. Statistics on over hundred small scale devices demonstrate over 98% functional solar cells, and a low standard deviation on PCE. We attribute this to the improved topology of the complete stack before the deposition of the metal top contact. Initial lifetime tests in nitrogen environment show an interesting trend: an increase in PCE is observed during the first days, followed by a slow decrease in the following weeks.

Friday 12th

September 18.00 - 18.15 Spray-Deposited Planar Heterojunction Perovskite Solar Cells

Alex Barrowsa, David Lidzey

a, Andrew Pearson

a, Chan Kwak

b, Alan Dunbar

b

a, Department of Physics and Astronomy, University Of Sheffield, Department oHicks Building, Hounsfield Road, Sheffield, S3 7RH, GB b, Department of Chemical and Biological Engineering, University of Sheffield, Sir Robert Hadfield Building, Mappin St, Sheffield, S1 3JD, GB

International Conference of Solution Processed Semiconductor Solar Cells - Oxford 2014 Organometal halide perovskite solar cells have recently become a hot topic within the research

community due to the rapid rise in their efficiency, with champion devices now reaching power conversion efficiencies of over 15%. The solution processable nature of these materials is of great appeal since it should allow the use of low cost roll-to-roll manufacturing techniques on large area, flexible substrates. To date, however, there have been very few reports on perovskite layers fabricated using roll-to-roll compatible techniques, and those which there are have typically focused on vacuum deposition rather than cheap solution processing routes such as printing or spray-coating. We have used ultra-sonic spray-coating as a deposition method for the perovskite layer of planar heterojunction solar cells, successfully demonstrating the fabrication of efficient devices by a solution processable roll-to-roll compatible technique. The formation of a continuous layer of perovskite, free from significant pinholes, has previously been found to be important for the fabrication of efficient photovoltaic devices. To this end we have identified and optimised a number of parameters which influence the morphology of the photoactive active layer of our devices, which are cast from a solution of methylammonium iodide and lead chloride. Casting solvent volatility plays a crucial role in film formation, and we have found that spray-coating from the solvent DMF (boiling point 153°C) produces a far more continuous film than when the solvent DMSO (boiling point 189°C) is used. We find that films which dry either too slowly or too rapidly form more incomplete layers, and thus substrate temperature during film deposition is also a critical parameter. In addition, the temperature of the subsequent thermal annealing treatment – used to crystallise the deposited materials into the mixed halide perovskite – also has a significant effect on film morphology. Optimisation of these parameters in the fabrication of photovoltaic devices with the architecture ITO/PEDOT:PSS/CH3NH3PbI3-xClx/PCBM/Ca/Al leads to average power conversion efficiencies of 7.8% and a champion efficiency of 11.1%. Further work is currently underway to investigate and optimise further parameters including the nitrogen pressure used in the spraying process and the concentration of the perovskite precursor solution.

Friday 12th

September 18.15 - 18.30 Energy Levels of Organic Hole Transport Materials for Perovskite PV and DSSC

Kentaro Kawata, Koichi Tamaki, Tomohisa Goto, Kazuhiro Kato Merck Ltd. Japan, 4084 Nakatsu, Aikawamachi, Aikogun, Kanagawa, JAPAN, 243, JP

Our study focuses on materials development for Perovskite-based photovoltaics with special emphasis on the hole transport material (HTM) because we believe that it is the bottleneck in the overall carrier transport dynamics; a key to further improve the power conversion efficiency (PCE). Here, the energy level alignment is an important aspect. However, there seems to be no standard way to quantify the energy level of HTM’s. We will illustrate how energy levels are experimentally determined and its implication for materials development.We introduced a high throughput materials screening method to simulate electronic properties of HTM’s in order to accelerate our development. The simulation was based on Conductor-like Screening Method (COSMO) where Density Functional Theory (DFT) calculation is carried out taking into account surrounding molecules/ions to simulate condensed and disordered phase like amorphous materials. We could also demonstrate in our own measurement a high efficiency of a Perovskite photovoltaic cell. We will highlight practical approaches to advance the material development for Perovskite photovoltaics.

Posters session Solar Cells Sensitized by Ternary Nanocrystals: Influence of Linkers and Surface Functionalisation on the Charge Transfer

Dmitry Aldakova, Muhammad T. Sajjad

b, Ifor D. W. Samuel

b, Peter Reiss

a

a, INAC/SPrAM, CEA-Grenoble, 17 rue des Martyrs, Grenoble, 38054, FR b, Organic Semiconductor Centre, School of Physics and Astronomy, University of St. Andrews, North Haugh, St Andrews, Fife, KY16 9SS, United Kingdom, GB

Quantum dot sensitized solar cells represent one of the most promising types of semiconductor nanocrystals photovoltaics. They are based on a nanostructured material with a thin layer of deposited nanocrystals that efficiently absorb light. The resulting cell is simple and not expensive because it uses only low-cost materials and processing steps by accessible wet methods. In the proposed talk we alternatively use subtrates of TiO2 or arrays of ZnO nanowires as nanostructured electrodes. For the


sensitization we use ternary nanocrystals attached to the semiconductor in a specific manner using bifunctional linkers. Ternary nanocrystals such as for example CuInS2, CuInSe2 not only have an advantage of not containing toxic metals such as Cd or Pb, but also boast very high absorption coefficients, and good air stability. Bifunctional linkers allow for grafting of nanocrystals onto the nanostructured semiconductors in a highly controlled and homogeneous manner compared to the methods of non-specific atachment such as CBD, SILAR or physisorption. Linker molecules, which are typically developed for the ubiquitous TiO2, are often not compatible with ZnO nanowires as they are very sensitive to pH and thus can be corroded under acidic or basic conditions. To circumvent this problem we have synthesized a new family of phosphonic acid based linkers capable of efficiently binding to ZnO without damaging it. In this talk we will discuss various aspects of self-assembly of different nanocrystals on TiO2 and ZnO. We will present their optical characterisation along with an XPS and structural studies. A special attention will be given to time-resolved photophysical investigation of the assemblies. More precisely, the influence of the size and nature of the linkers on the rate and efficiency of the charge transfer will be discussed. Also, various surface treatments of the nanocrystals such as ligand exchange, cationic exchange or passivating shell formation are studied. Solar cells based on such ternary quantum dots assembled on TiO2 or ZnO were fabricated and characterized, the influence of the above mentioned attachment and surface-related parameters on their photovoltaic properties are studied, the best cells resulting so far in an efficiency of 1.3%.

Posters session The Effect of pH of the New Acceptor PAMAM Dendritic Material on the Organic Bilayer Solar Cells Performance

Thamraa Alshahrania, Mohammed Mabrook

a, Alaa El-Betany

a, Hongyun Tai

a, Ibrahim Alrougy

a, Neil

McKeowna

Bangor University, 6 penchwintan terrace, Bangor, SA

Converting sun light to electricity using solar harvesting devices is one of the solutions to reduce the world dependence on the fossil fuel. Due to the importance of harvesting solar energy, the continuous development of solar cells is one of the most important developments in the conversion of solar energy. Recently, organic solar cells show many advantages over inorganic devices such as lightweight, flexibility, low cost and variety materials synthesis with different structures However, organic solar cells efficiency is still below 10% and it’s affected by many factors such as the choice of materials and fabrication techniques. Dendrimers are a new class of polymeric materials that are composed of highly branched, well-defined and nature monodisperse macromolecules, which can support the charge transport and film morphology. Multifunctional Polyamidoamine (PAMAM) dendrimers as flexible light harvesting antennae with high efficiency electron transfer was used as the acceptor while Poly (3- hexylthiophene) (P3HT) was used as donor due to their low band-gap and efficiency in organic photovoltaic applications. This work investigate the effect of pH of the new PAMAM dendritic wedge (G0.5) salt on the organic bilayer solar cells in order to improve their performance and morphology. The structure with G0.5 salt at neutral pH 9.02 level resulted in much improved surface morphology and enhances the charge mobility. It was also observed that the organic hetrojunction structure shows high open circuit voltage of about 0.85 V, a short circuit current of 1 mA/cm2 and power conversion efficiency of 1.1%. The results show that at neutral pH 9.2 the PAMAM G0.5 salt exhibit a major peripheral distribution where in the low pH <5 the surface shrink.

Posters session Topographic Characterisation of Perovskite Films to Enable Scale Up Technologies

Jenny Bakera, Joel Troughton

b, Pete Greenwood

a, Tim Claypole

a, Trystan Watson

b, David Gethin

a

a, WCPC, Singleton Park, Swansea, GB b, SPECIFIC, Baglan Bay Innovation Centre, Baglan Energy Park Central Avenue Baglan, Port Talbot United kingdom SA12 7AX, , GB.

In 2009 a lead iodide based perovskite was shown to be a sensitiser for a dye sensitised solar cell1

replacing the dye to make a 3% efficient device. From this work the perovskite cell was developed, using a solid state hole conductor and with the electron transport through the perovskite rather than through a semi-conducting scaffold

2. The research on these cells since 2012 has seen an increase in the verified


efficiency from 13% to 17.9% in just one year. Whilst impressive, most of these results have been on very small scale devices (<0.1cm

2) and use spin coating to deposit the perovskite film, either onto a

mesoporous scaffold or as a planar heterojunction. In order to realise the commercial potential of these devices larger scale printing methods need to be employed such as slot die coating. When coating large areas non-destructive methods of quickly characterising films are required in order to assess uniformity. White light interferometry is a non-contact technique which is much quicker than AFM profiling, it does not require the vacuum environment of an SEM and has few limitations on substrate size. As such it can, within seconds, produce a 3D profile of mixed halide (CH3NH3PbI3-xClx) perovskite film. This work examines the uniformity of surface profile over coated large coated areas and compares it with the profile of spin coated films of 1cm

2.

Posters session Enhancing the Open Circuit Voltage of Perovskite Solar Cells

Andreas Binek, Pablo Docampo, Thomas Bein Ludwig-Maximilians-University Munich, research group Bein, Butenandtstr. 5-13, Munich, 81377, DE

Perovskite-based solar cells have recently exceeded the power conversion efficiencies exhibited by some established photovoltaics concepts, such as dye-sensitized, organic and amorphous silicon solar cells. At present, the devices are limited by losses in the open circuit voltage due to the misalignment of the energy levels at the interface between the perovskite and the charge extraction layers. Particularly in wide bandgap perovskite absorbers, which could be potentially useful in tandem applications with a c-Si or CIGS bottom cell, losses up to 0.9 eV can be observed. For this type of application, high temperature steps must be avoided, which precludes the use of mesoporous TiO2 films. Here, we have focused on planar heterojunction devices via a two-step deposition technique of the perovskite absorber. However, this protocol results in a rough film of the perovskite, with some crystals piercing through the hole conductor layer. To minimize these voltage losses in high bandgap hybrid perovskite structures, we have used novel polymers, which form conformal layers over the perovskite film. The materials and interfaces were characterized via X-ray photo emission spectroscopy (XPS) and ultraviolet photo emission spectroscopy (UPS).

Posters session Nanoscale Characterisation of Perovskite-based Solar Cells

Stefania Cacovicha, Giorgio Divitini

a, Xiaoyu Peng

a, Francisco de la Peña

a, Michael Saliba

b, Henry Snaith

b,

Caterina Ducatia

a, Department of Materials Science and Metallurgy, University of Cambridge, 27 Charles Babbage Road , Cambridge CB3 0FS, UK b, Clarendon Laboratory, Department of Physics, University of Oxford, Parks Road, Oxford OX1 3PU, UK

Converting solar energy into electricity provides a much-needed solution to the energy crisis the world is facing today. Over the last 12 months perovskite solar cells, emerging from the field of DSSC, have exhibited a rapidly rising evolution, with efficiency surpassing 15%. Moreover, in order to enhance light harvesting, the addition of core-shell nanoparticles has been investigated in several thin film devices. The morphology of the active layer plays a key role in the understanding the working principles, and optimising the operation of thin films devices. Analytical transmission electron microscopy (TEM) represents a powerful tool that allows probing nanoscale morphology and crystal structure, as well as local elemental composition and spatially-resolved information on the electronic energy levels. In this work cross sectional lamellar specimens of perovskite - based solar cells were prepared from full photovoltaic devices using a focused ion beam milling approach. The sections were analysed by high annular dark field imaging in scanning TEM mode to determine the morphology of the device and by energy dispersive X-ray spectroscopy (EDX) to obtain compositional maps at high spatial resolution. Spectrum images were analysed using principal component analysis and blind source separation to optimise signal-to-noise ratio, thus obtaining high quality maps while limiting the electron dose on the specimen. This novel quantification procedure also naturally separates different compounds without introducing operator bias, and is particularly effective in the presence of complex compounds, such as those developed during solution-based perovskite synthesis.


Posters session Electron Transfer Dynamics in SILAR based QD Electrodes

Hai Wanga, b

, Mischa Bonna, Enrique Canovas

a

a, Max Planck Insitute for Polymer Research, Ackermannweg 10, , Mainz (Germany), 55128, DE b, Graduate School Material Science in Mainz, University of Mainz, Mainz, DE

The direct nucleation of QD nanocrystals onto mesoporous oxide matrices by successive ionic layer adsorption and reaction (SILAR) represents a promising route towards the development of novel architectures for solar energy conversion. This low-cost solution process approach has shown considerable potential for controlling nanocrystal size (as evidenced by a red shift of the absorption threshold as a function of dipping steps) and providing high sensitizer loadings per oxide unit area; features that have allowed the development of sensitized solar cells with efficiencies above 5%. Despite the potential of SILAR systems, little research effort has been devoted to unravel the donor-acceptor electron transfer dynamics, a key aspect for optimum photovoltaic performance. In this work, photo-induced electron transfer (ET) processes in PbS/SnO2 SILAR based electrodes are investigated using optical pump-THz probe (OPTP) spectroscopy. The ET dynamics are correlated with QD structure determined using HRTEM analysis. ET rates are found to be independent of QD size, dielectric environment and temperature, suggesting Fermi level pinning at the QD/oxide interface and tunnelling as the ET related mechanism. On the other hand, ET rates in SILAR based QD sensitized oxides are found to be highly dependent on photo-excitation energy. This observation provides unambiguous proof that hot electron transfer processes take place at the intimate QD/oxide interface. The implications of these results for solar cell design are discussed.

Posters session Inorganic-Organic Hybrid Solar Cells Using Bismuth Sulfide Nanowire Array/Silver Sulfide Core-Shell Heterojunction

Yiming Caoa, Gerasimos Konstantatos

a, Andrew MacLachlan

b

a, ICFO-The Institute of Photonic Sciences, ICFO – The Institute of Photonic Sciences, Mediterranean Technology Park, Av. Carl Friedrich Gauss 3 , Castelldefels (Barcelona), 8860, ES b, Centre for Plastic Electronics and Department of Chemistry, Imperial College London, South Kensington Campus, Exhibition Road, SW7 2AZ, U.K.

Semiconductor nanowires are one of the essential building blocks for photovoltaic cells with a view to allowing short distances for carrier transport within the nanowire and interpenetrating heterojunction at the nanoscale for charge generation following light absorption. In this work, we demonstrated near infrared sensitive, non-toxic bismuth sulfide nanowire arrays for solution-processed inorganic-organic hybrid photovoltaic cells. The bismuth sulfide nanowire arrays were grown on a transparent conducting substrate from mild aqueous chemistry. By decorating the nanowire with silver sulfide, the performance of hybrid solar cells based on spiro-OMeTAD organic hole transporter remarkably boosted. Silver sulfide coats the nanowire homogeneously via successive ionic layer adsorption reaction, evolving exotic core-shell nanostructure with controlled shell thickness, which not only extends the photon response of the hybrid photovoltaic cells to infrared region, but also forms type-II heterojunction with bismuth sulfide to tune interfacial charge transfer. In this regard, the hybrid solar cells achieved a power conversion efficiency of 2.5 % under simulated air mass 1.5 global illumination conditions, and the external quantum efficiency of the cells peaked at 70 % even with silver sulfide shell thickness of over 200 nm, which is attributed to the efficient charge generation at bismuth sulfide nanowire/silver sulfide interface as evidenced by transient absorption measurements, and to the rapid charge collection within the bismuth sulfide nanowire. Our study will pave the path to applications of non-toxic metal sulfide heterojunctions in solar cells and solar fuels.

Posters session Low-temperature Deposition of TiO2 Compact Layers for Metal-organic Perovskite Solar Cells on Plastic

Francesco Di Giacomoa, Andrea Capasso

b, Valerio Zardetto

c, Alessandra D'Epifanio

d, Maria Luisa Grilli

b,

Fabio Matteoccia, Stefano Razza

a, Nicola Lisi

b, Aldo Di Carlo

a, Wilhelmus M.M. Kessels

c, Mariadriana

Creatorec, Silvia Licoccia

d, Thomas M. Brown

a


a, Centre for Hybrid and Organic Solar Energy (CHOSE), Department of Electronic Engineering, University of Rome - Tor Vergata, Via del Politecnico 1, 00133, Rome, Italy b, Department of Materials and New Technologies, ENEA, Casaccia Research Centre, Via Anguillarese 301, 00123, Rome, Italy c, Department of Applied Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands d, Department of Chemical Science and Technologies, University of Rome - Tor Vergata, Via della Ricerca Scientifica 1, 00133, Rome, Italy

Organic-inorganic perovskites demonstrated to be a potentially disruptive material for photovoltaics. Power conversion efficiencies (PCE) in the order of 18% and 10% have been recently reached on glass and plastic substrates, respectively[1, 2]. Plastic solar cells are attractive in many technological areas, but their fabrication poses more strict constrains in terms of processing temperatures. In perovskite-based cell, an electron-transporting layer on top of the cathode appears necessary to correctly collect the charges at the electrode without inducing recombination. A TiO2 compact layer is usually deposited on glass electrodes by spray pyrolysis to this end. However, such process implies temperatures exceeding 400°C which are unsuitable for plastic substrates. Here we present a comparison of low-temperature TiO2 compact layers on PET/ITO/TiO2/CH3NH3PbI3-xClx/Spiro-O-MeTAD/Au structures (with and without mesoporous TiO2). The TiO2 compact layers were deposited by two techniques under 150°C: sputtering and plasma enhanced atomic layer deposition (PE-ALD).The planar architecture exhibited a very high VOC (970 mV for 40 nm of sputtered TiO2 and 880 mV for 11 nm of PE-ALD TiO2) but with current densities below 1.5 mA cm

-2. When a UV-sintered TiO2 mesoporous scaffold was used

to guide the perovskite growth and increase the charge injection, an increase of the PCE of one order of magnitude was obtained (from 0.3 to 5.8% for sputtering and from 0.8 to 7.4 for PE-ALD). As a further proof of the importance of the compact TiO2, mesostructured devices without compact layer were realized; as expected, the high recombination rate between the perovskite and the ITO limited the VOC below 50 mV. In summary, three new low-temperature TiO2 processing techniques (i.e., sputtering, PE-ALD, and UV-sintering) were successfully adopted to fabricate efficient flexible metal-organic perovskite solar cells with efficiency up to 7.4%.

Posters session Ferroelectric Property and Hysteresis in Perovskite Solar Cells

Hsin-Wei Chena, Nobuya Sakai

a, Ajay Kumar Jena

a, Kuan-Lin Wu

a, Akihiro Kojima

b, Masashi Ikegami

a,

Tsutomu Miyasakaa

a, Toin University of Yokohama, Toin University of Yokohama, 1614 Kurogane-cho, Aoba-ku Yokohama, 225-8503 Japan, Yokohama, 1614, JP b, Peccell Technologies, Inc., 1614 Kurogane-cho, Aoba, Yokohama, Kanagawa 225-8502, JP

Although there has been a rapid progress in efficiency of perovskite-based solar cells, hysteresis involved in the current-voltage performance is not yet completely understood. Owing to the complex structure of the cells, it is not easy to attribute this hysteresis behavior to any of the components, such as bulk of perovskite, interfaces at HTM and TiO2 mesoporpus and compact layer. In this work, we have made a thorough investigation on hysteresis in solid state perovskite solar cells and found a strong correlation between the intrinsic ferroelectric property of perovskite and hysteresis in the cells. Solid state perovskite cells of different architecture, planar heterojunction and mesoporous (TiO2 and Al2O3) structures with and without hole transport material (HTM) were fabricated. Current-voltage (I-V) measurements were conducted on these cells after having biased them at different voltages in the dark. The results revealed the link between I-V hysteresis and ferroelectric property of CH3NH3PbI3-xClx perovskite, the latter being affected by the bias application. A greater magnitude of hysteresis in case of planar heterojunction and Al2O3 mesostrcutre cells in comparison to TiO2 mesostructure cells evidently indicated the significance of bulk property of perovskite in origin of hysteresis. It could be related to polarizability that influences charge carrier separation and transport processes in the perovskite film. In transient photocurrent dynamics, all devices showed fast photocurrent response under light switching between on and off. This indicates that electrons and holes efficiently collected in all types of the cells. In addition, we observed that photocurrent in the cells increased almost linearly with bias voltage under 1 sun illumination, which suggests light-induced polarization of perovskite. This polarization process in


the perovskite film alters the electron collection in the cell during the backward and forward scan, resulting in generation of hysteresis in the I-V characteristics.

Posters session Colourful Organolead Halide Perovskites for Possible Building-Integrated Photovoltaics

Matthew Daviesa, b

, Matt Carnieb, Peter J. Holliman

a, Arthur Connell

a, Peter Douglas

c, Trystan Watson

b,

Cecile Charbonneaub, Joel Troughton

b, David Worsley

b

a, The School of Chemistry, Bangor University, Bangor, LL57 2UW, GB b, SPECIFIC, College of Engineering,, Swansea University, Baglan Bay Innovation Centre, Central Avenue, Baglan, Port Talbot, SA12 7AZ, GB c, Chemistry Group, College of Engineering, Swansea University, Singleton Park Swansea, SA2 8PP, GB

Organometallic halide perovskites based photovoltaic (PV) devices are showing revolutionary efficiencies for such a potentially low-cost and low-embodied energy technology with efficiencies exceeding 15% and low temperature manufacturing processes.Thus perovskite based PV, with incremental increases in efficiencies and subject to stability, could compete with thin film technologies that require vacuum deposition and expensive non-trivial processing. Aesthetically pleasing coloured organometal halide perovskite cells are also possible though modifying the halide(s) used.

Here we

report the characteristics of a series of methylamine lead halide perovskites with different halides and halide combinations. We have also investigated the effect of lengthening the alkyl chain with the rationale that this may circumvent any possible stability problems towards moisture/humidity. As one might expect, changing the alkyl chain and/or the halide(s) alters the crystal structure and band gap of the perovskite which results in vivid and colourful solar cells; a characteristic which is seen as one of the main benefits of DSC. The band gaps of the synthesised perovskites range from 1.5 – 2.3 eV highlighting the controllability of the optical properties of the organolead halide perovskites. X-ray diffraction and scanning electron microscopy (SEM) with elemental mapping via energy-dispersive x-ray (EDX) analysis has been used to characterise perovskites on sensitised thin films. The smaller ionic radii chloride and bromide halides result in perovskites with larger band gaps and a cubic crystal structure, the iodide based organolead perovskites have a tetragonal crystal structure and have a near-optimum band gap of 1.5 eV. The intermediate band gap iodide/bromide mixtures show a mixed phase crystal structure. Optical microscopy highlights the cubic to tetragonal, through a mixed phase, crystal morphology which correlates well with the XRD data. All the perovskites studied are photovoltaic with efficiencies between 0.5 – 8 % when made under normal laboratory conditions. Coloured perovskite devices made with a transparent conducting laminate back contact which can be specifically used in building integrated PV are detailed. PV device performance and stability is evaluated in relation to the crystal and physical properties of the perovskite and suitability for use in photovoltaic devices is discussed.

Posters session Optical Tunability and Disorder Induced Effects in 2D-layered and 3D Organic-inorganic Perovskite Pseudobinary alloys

Emmanuelle Deleportea, Gaëtan Lanty

a, Khaoula Jemli

a, Yi Wei

a, Jean-Sébastien Lauret

a, Joël Leymarie

b,

Jacky Evenc

a, LAC, Ecole Normale Supérieure de Cachan, 61, avenue du Président Wilson, Cachan, 94235, FR b, LASMEA, Université Balise Pascal, Clermont-Ferrand II, Les Cézeaux, 63177 Aubière, FR c, FOTON, INSA, Université Européenne de Bretagne, 35 708 Rennes, FR

Organic-inorganic hybrid semiconductors based on metal halide units have attracted attention due to their interesting structural and optical properties. In particular, attention has been focused on the 2D-layered organic-inorganic perovskite semiconductors (R-NH3)2PbX4 with R an organic group and X the halide (Cl-, Br-, I-) because of their application in emitting optical devices and recently on 3D perovskites such as CH3NH3PbX3 which reveal particularly convenient for the photovoltaïc applications. Among several advantages, these materials are remarkable due to their excitonic properties: for example the 2D perovskites form self-assembled ordered quantum well structures, in which the excitons are strongly confined in the very thin inorganic wells PbX62-, resulting in a very large exciton binding energy of a few hundred of meV. Understanding the nature of the excitons in perovskites is crucial to optimize the opto-


electronic devices, the emitting ones requiring strong excitons at room temperature as well as the absorbing ones where the exciton has to be separated to generate free carriers.

We focus here our attention on 2D mixed perovskites (C6H5-C2H4-NH3)2PbZ4(1-x)Y4x, and 3D mixed perovskites CH3NH3PbZ3-xYx where Z, Y = I, Br or Cl. Firstly, we show that 2D and 3D mixed perovskites afford similar continuous optical tunability at room temperature. Secondly, studying experimentally the disorder induced effects on the optical properties, we demonstrate that they can be considered as pseudobinary alloys, exactly like Ga1-xAlxAs, Cd1-xHgxTe inorganic semiconductors. We develop a theoretical analysis which allows to describe the influence of alloying on the excitonic properties of perovskite molecular crystals. Despite a large binding energy of several 100 meV, the 2D excitons present a Wannier character rather than a Frenkel character. The small inhomogeneous broadening reported in 3D hybrid alloyed compounds is similarly consistent with delocalized electron hole pairs at room temperature rather than strongly localized excitons. Posters session Two Stage Solar Electricity Cells

Catherine Kari Derow Oxford Brookes University, Oxford OX3 0BP, GB

I propose usage of the energy from the sun in a two-stage process. First, benefiting from the given life-based abilities of organisms to harness solar energy to create fuel, and second, employing this fuel to feed microbial electricity-producing cells. 1. Benefiting from the given life-based abilities of organisms to harness solar energy to create fuel: Life forms which can harness light energy in photosynthetic processes are a given. Plants and diverse microbes are able to harness solar energy to create fuel : carbon dioxide + water + light energy -> carbohydrate. The input of energy allows the creation of greater order from more chaotic matter. In turn this more ordered matter can be employed to release energy when it is returned to less ordered forms. This is in accordance with the Laws of Thermodynamics. The matter produced from photosynthetic processes could be employed to produce electricity. 2. Employing matter produced via photosynthesis to feed microbial electricity-producing cells. Matter produced via photosynthesis and waste from such matter could be employed as nutrition, possibly after eco-friendly processing e.g., fermentation, to produce media, to feed microbes in electricity-producing cells. Some microbes, e.g., Bacteroidia, produce negativity based upon their metabolic processes which allow them to live. In electricity-generating cells, of appropriate scale, such microbes could be fed media produced from waste, based on matter produced from photosynthesis perhaps destined for recycling, in such a cell the negativity could transfer to an anode. This would promote the flow of current from such anode(s) into an electricity requiring system, feeding electricity needs from lighting, heating, cooling, and washing, to transport. In a smooth controlled release programme to minimise energy waste. Cells could be designed for maximum electricity generation and minimum waste, perhaps involving larger cells for some applications and perhaps many small cells combined to provide energy for other applications. Thus harnessing given photosynthetic processes could fuel electricity-producing cells based on given microbial processes, in order to generate electricity to help meet world energy needs and energy security for all. Posters session Cation Replacement in Lead Trihalide Perovskites: Broad Bandgap Tunability for Highly Efficient Planar Heterojunction Solar Cells

Giles Eperon University of Oxford, Clarendon Laboratory, Parks Rd, Oxford, GB

Perovksite-based solar cells have attracted significant recent interest, with power conversion efficiencies in excess of 15% already superceding a number of established thin-film solar cell technologies. Most work has focused on a methylammonium lead trihalide perovksites, with a bandgaps of ~1.55eV and greater. Here, we explore the effect of replacing the methylammonium cation in this perovskite, and show that with the slightly larger formamidinium cation, we can synthesise formamidinium lead trihalide perovskites with a bandgap tunable between 1.48 and 2.23 eV. We take the 1.48 eV-bandgap perovskite as most suited for single junction solar cells, and demonstrate long-range electron and hole diffusion lengths in this material, making it suitable for planar heterojunction


solar cells. We fabricate such devices, and due to the reduced bandgap we achieve high short-circuit currents of >23mAcm-2, resulting in power conversion efficiencies of up to 14.2%, the highest efficiency yet for solution processed planar heterojunction perovskite solar cells. Formamidinium lead triiodide is hence promising as a new candidate for this class of solar cell. Furthermore, we report on recent work involving replacement of the methylammonium cation with other small cations of varying sizes, and demonstrate impressive control over the bandgap whilst retaining the important 3D lead triiodide matrix. We show results from first-principles computational models that enable us to intelligently select the optimum cation for a particular bandgap of perovskite, including even narrower bandgap perovskites optimum for the process of solar energy conversion. We additionally demonstrate the good agreement of experimental and simulated results.

Posters session Electronic and Optical Properties of Metal-organic Perovskites

Marina Filip, Feliciano Giustino University of Oxford - Department of Materials, Parks Road, Rex Richards Building, Oxford, GB

Mesoscopic solar cells based on organic-inorganic metal halide perovskites have shown great progress reaching nearly 18% certified efficiency (NREL), showing the fastest growth among solution-processed photovoltaic technologies to date. Methylammonium lead-iodide is at the center of these developments as the key material in perovskite solar cells exhibiting an optimum combination of electronic and optical properties: a direct band gap of 1.5 eV and relatively high electron and hole mobilities. In this context, the target of 20% efficiency for these solar cells appears to be within reach. The electronic properties of this class of materials are highly dependent on the structural properties of the perovskite lattice. We analyze this dependence in a systematic computational study within the local density approximation (LDA) to density functional theory (DFT). We show that the structural features may be manipulated by changing the cationic components at the center of the inorganic cuboctahedral cavity and develop a tight binding model to rationalize these trends.

Posters session Inverted Architecture Organometallic Perovskite Solar Cells Using Copper Thiocyanate and Nikel Oxide as Hole Conductors

Soham Ghosha, Anand S. Subbiah

a, Ansuman Halder

a, Neha Mahuli

b, Gary Hodes

c, Shaibal K. Sarkar

a

a, Department of Energy Science and Engineering, Indian Institute of Technology Bombay, IIT Bombay, Powai, Mumbai, 400076, India b, Center for Research in Nanotechnology and Science, Indian Institute of Technology Bombay, IIT Bombay, Powai, Mumbai, 400076, India c, Department of Materials and Interface, Weizmann Institute of Science, Weizmann Institute of Science, 76100 Rehovot, Israel

Organometallic perovskite materials have become one of the most attractive research domains in photovoltaics in the last few years. Organometallic halides like MAIPbI3 and MAIPbI3-xClx exhibit promising conversion efficiency of >15% that may lead to an economically viable photovoltaic technology in near future. As a performance bottleneck, the organic polymers used for hole conduction can be identified. Here we present two alternative inorganic hole transporting materials, NiO and CuSCN under reverse architecture in MAIPbI3-xClx based perovskite solar cells. Electrodeposited CuSCN and NiO were used as hole transporting layers. The device performances reveal an average efficiency of 4.5% and 7.3% using CuSCN and NiO respectively. This technology never challenges the existing ones but rather showed a possibilistic scenario for further developments.

Posters session Aqueous Synthesis of Antimony Sulfide at Room Temperature for Sensitized Solar Cells

Karl Goedela, Aditya Sadhanala

a, Bart Roose

a, Ullrich Steiner

a, Sandeep Pathak

b

a, University of Cambridge, Cavendish Laboratory, J J Thomson Avenue, Cambridge, CB3 0HE, GB b, University of Oxford, Clarendon Laboratory, Parks Road, Oxford, OX1 3PU, GB

International Conference of Solution Processed Semiconductor Solar Cells - Oxford 2014 We report on a new aqueous synthesis route of antimony sulfide (Sb2S3) at room temperature via

chemical bath deposition for the use as absorber layers in Sb2S3 sensitized solar cells. The most-commonly applied method is the aqueous chemical bath deposition using antimony chloride and sodium thiosulfate as precursors at low temperatures. This technique however is circumstantial due to its mutiple-step cooling protocol. For large-scale application the necessity of cooling is costly and energy-intensive. Our method is able to decelerate the reaction at ambient conditions and we are able to fabricate sensitized mesoporous-TiO2 solar cells, which offer on average higher photovoltaic efficiency compared to the devices using the standard Sb2S3 synthesis method. Via PDS measurements we show that this is achieved by reducing the sub-bandgap trap-states. The Sb2S3 films are further characterized using UV-vis, XPS and XRD spectroscopy. Our method thereby can contribute to the development of low-cost Sb2S3 solar cells with high efficiencies.

Posters session Excitons versus Free Charges: a Photophysical Picture of Organo Lead Halide Perovskite

Giulia Grancinia, Valerio D'Innocenzo

a, c, Ajay Ram Srimath Kandada

a, Marcelo J. P. Alcocer

c, Samuel D.

Stranksb, Michael Lee

b, Guglielmo Lanzani

a, c, Henry Snaith

b, Annamaria Petrozza

a

a, Center for Nano Science and Technology @Polimi, Istituto Italiano di Tecnologia, via Giovanni Pascoli 70/3, 20133, Milan, Italy, IT b, University of Oxford, Clarendon Laboratory, Parks Road, Oxford, OX1 3PU, United Kingdom, UK c, Dipartimento di Fisica, Politecnico di Milano, Piazza L. da Vinci, 32, 20133 Milano, Italy, Italy

Very recently, a new class of material based on hybrid mixed halide perovskite emerged as a promising solution for photovoltaics. Used first as light antenna in the “dye-sensitized” configuration it has been recently demonstrated that they can also operate extremely well (with η > 16%) in planar heterojunction architecture. The variety of configurations in which they have been used along with the high performances demonstrated, cast the doubt on the fundamental physics governing the photovoltaic mechanism, in particular regarding the nature of the elementary photo-excitations, i.e. whether free charges or bound excitons are generated and their dynamics with a special attention to exciton formation and charge generation. With this aim we address the photophysical properties of both CH3NH3PbI3 and the CH3NH3PbI3-xClx. By temperature dependent linear absorption measurements we estimate the exciton binding energy to be about 50 meV. Using this value we model the ionization processes using Saha equation leading to an evaluation of the branching ratio between free carriers and excitons. We find that at equilibrium there is a predominant fraction of free charges under photovoltaic operating regime. Finally, since non-equilibrium dynamics can also affect the device operation, we investigate the photoinduced dynamics at ultrafast timescale by temperature-dependent ultrafast transient absorption spectroscopy. The results demonstrate that the operation mechanism of this material at room temperature is fundamentally governed by free charges and not excitonic species.

Posters session Slot-die Coating of Organolead Halide Perovskite for Roll-to-roll Photovoltaics

Peter, C Greenwooda, b

, Antony Lewisa, Joel Troughton

a, Jenny Baker

b, Trystan, M Watson

a, David, T

Gethinb, David, A Worsley

a

a, Specific - College of Engineering - Swansea University, Baglan Bay Innovation Centre, Central Avenue, Baglan, SA12 7AX, United Kingdom b, Welsh Centre for Printing and Coating - Swansea University, Swansea, SA2 8PP, United Kingdom

Organolead halide perovskite photovoltaic devices produced from sub-micron films of methylamonium lead iodide are making significant progress towards delivering high performance low-cost solution-processed photovoltaics. Since 2012 the efficiency of these devices have increased considerably with certified conversion efficiencies of 17.9% reported this year. Conventionally these devices are produced by the spin-coating method where an excess of the precursor solution is applied to the device substrate which is then rotated at high speed to produce an even film. This batch-processing technique although highly reproducible is wasteful and incompatible with high-volume continuous manufacturing. If solution processed perovskite photovoltaic devices are to realise their potential, scalable manufacturing methods based on industrial printing and coating processes are essential. An alternative to the conventional spin-coating method is slot-die coating. This roll-to-roll compatible


process pumps a coating formulation into a machined die positioned above the moving substrate. A controlled volume of liquid exits the die through a precise slot to produce a continuous wet film on the substrate as it passes beneath the coating head. Here, slot-die coating is demonstrated as a successful candidate for the deposition of methylamonium lead iodide solutions that can be crystallised to form sub-micron films of organolead halide perovskite. Films produced by the slot-die coating process and the spin-coating method show strong similarities when examined using a scanning-electron microscope and stylus profilometer. This suggests the suitability of slot-die coating as a candidate process for the large scale manufacture of perovskite photovoltaics. Work is ongoing to optimise a device architecture that will enable the production of photovoltaic devices using films produced by this slot-die coating method.

Posters session Influence of the Organic Cation on the Structure of Hybrid Organic Inorganic Perovskites

Enrico Greul, Thomas Bein, Pablo Docampo LMU, Butenandtstr. 11, Haus E, Munich, 81377, DE

Hybrid organic perovskite photovoltaics have recently become the focus of a large number of research groups because of the high device efficiencies (>15 %) which can be achieved with this type of material.The high abundance of its components, simple processability, high extinction coefficients and extremely long lifetimes of photoexcited charge carriers make this technology a strong candidate for applications in large area photovoltaics. Nevertheless, the fundamental properties of the materials are still under investigation, particularly the exact role of the individual components of the hybrid perovskite structure and their interactions with each other are not well known. Here, we study a range of organic cations that were infiltrated into the lead iodide network by different preparation techniques and their influence on the resulting hybrid halide perovskite. The properties of the resulting materials were characterized by powder X-ray diffractometry (PXRD), UV-Vis spectroscopy and photoluminescence (PL) spectroscopy.

Posters session Highly Stable Hole-transporting Layer for Perovskite Solar Cells Based on Functionalized Single-walled Carbon Nanotubes

Severin Habisreutinger, Tomas Leijtens, Giles Eperon, Samuel Stranks, Robin Nicholas, Henry Snaith University of Oxford, Clarendon Laboratory, Oxford, UK

Within only four years, the power conversion-efficiencies of solar cells using metal-organic halide materials have skyrocketed in an unprecedented manner, reaching up to 17.9% (as of May, 2014). This unprecedented rise in efficiencies spurs the notion that this technology will in the foreseeable future make the transition into a commercial setting. However, one significant weakness of the perovskite absorbers is their vulnerability to heat and moisture. The hygroscopic nature of some of the material constituents causes a rapid decomposition of the material and thus strongly impacts the stability of photovoltaic devices. In the most common architecture the top layer of a device acts as hole-transporting layer, which selectively transfers photogenerated holes from the perovskite to the top electrode. As the top layer, its secondary function is therefore to protect the sensitive absorber from external stressors such as moisture. Thus far, the best performances have always been achieved using the amorphous hole transporter spiro-OMeTAD (2,2,7,7-tetrakis (N,N-dimethoxyphenylamine)-9,9-spirobifluorene). Yet, due to its intrinsically low hole mobility, it requires reactive doping which is commonly done with Li-TFSI. This dopant strongly undermines the overall stability aspect of spiro-OMeTAD because its lithium constituents are highly reactive and likely to interact with the perovskite structure, and, more importantly, its hygroscopic nature leads to an increased moisture insertion through the spiro-OMeTAD layer. In this work, we demonstrate a completely new hole-transporting structure that is composed of two layers. The first layer is a dense, interconnected network of highly conductive, polymer-functionalized single-walled carbon nanotubes, whereas the second layer is a dense, electronically inert and thermally stable polymer matrix. Charge extraction is exclusively mediated by the carbon nanotubes, whereas the polymer matrix fills gaps and holes in the nanotube network thus preventing shunting due to direct contact between electrode and perovskite. More importantly, however, the polymer matrix is key to giving this structure a high degree of stability against both heat and moisture. We show that employing this hole-transporting double-layer structure on


perovskite solar cells can achieve efficiencies of more than 15% while simultaneously achieving an unprecedented degree of protection against degradation, even allowing direct contact of an operational device with water. Allowing for both high efficiency as well as stability, this hole-transporting structure may prove to be a crucial development for perovskite solar cells for becoming a viable alternative to conventional photovoltaics.

Posters session Efficient Wide Bandgap Perovskite Solar Cells

Fabian Hanusch, Pablo Docampo, Andreas Binek, Bein Thomas University of Munich, Butenandtstr 11, Munich, 81377, DE

Organic inorganic hybrid perovskite solar cells have recently become a strong candidate for photovoltaic applications, already achieving power conversion efficiencies similar to other established thin film technologies. Their facile solution processing also makes them potential candidates as partners for conventional solar cells in a tandem device architecture. However, the bandgap of the usual perovskite structure based on methylammonium lead iodide is too narrow to be optimally used in tandem solar cells. The bandgap of this structure can be widened by exchanging some or all of the iodide ions by bromide. However, this type of material requires a mesoporous titania scaffold, which has been sintered at high temperatures to perform efficiently.

[1] This eliminates them as potential

candidates for a tandem configuration with conventional inorganic solar cells which are not stable at the required temperatures. In this context, we present highly efficient wide bandgap solar cells with a planar heterojunction layout. We show that this system exhibits very slow photoluminescence decay dynamics, and therefore charge diffusion lengths approaching the micron scale and power conversion efficiencies approaching 7 %.

Posters session Fast, Energy Efficient Solution Processing of Dye-sensitized Solar Cells

Peter Hollimana, Arthur Connell

a, Eurig Jones

a, Leo Furnell

a, Matthew Davies

a, Laurie Peter

b

a, Bangor University, School of Chemistry, Bangor, LL57 2UW, GB b, Bath University, Department of Chemistry, Bath University, Claverton Down, Bath BA2 7AY, GB

To substantially scale PV, continuous, roll-to-roll (R2R) processing is an important technology option. In this context, the scaled manufacturing of dye sensitized solar cells (DSC) still requires optimization of current R2R roll process limitations (e.g. time and temperature) to maximize throughput and yield and to reduce costs. In addition, DSC power conversion efficiencies need to increase to compete on LCOE versus p-n junction PV. This paper will present our recent developments in this context:-

1. Structure-activity relationships of new dyes focusing on the dye-oxide interface 2. Low T sintering of binder-containing colloids for mesoporous photo-electrodes 3. 2-D mapping of co-sensitized DSC devices 4. In situ studies of device lifetime The paper will also consider how the unique selling points (USPs) of different PV technologies can

affect device use. For DSC, USPs includes colour and transparency. Posters session Domain Size Control of Solution-Processed Perovskite Thin Films via Colloidal Monolayer Lithography

Maximilian Hörantner, Wei Zhang, Michael Saliba, Konrad Wojciechowski, Henry Snaith University of Oxford, 64 Weldon Road, Oxford, GB

Organic-inorganic metal halide perovskites have led to remarkable advancements in low-cost solar cells with power conversion efficiencies (PCEs) already exceeding 17%. High device performance can be achieved in a number of different architectures, but depends heavily on the quality of the perovskite film formation as the interface dynamics between charge transport layers and the perovskite. Here, we control perovskite crystal domains size by guiding the growth through highly ordered porous macro-patterned metal oxide arrays, which are fabricated via colloidal monolayer lithography. The organic-inorganic perovskite material fills the macro-pores remarkably well leading to fully controlled domain


size with tuneable film thickness. We fabricate semi-transparent perovskite solar cells through the structuring of the photoactive material, which leads to enhanced open-circuit voltage and fill factor resulting in increased power conversion efficiencies of up to 9.2% at relatively high average visible transmittance of active layer of around 45%. The controlled macro morphology of perovskite films opens up a wide range of possible applications ranging from charge transport optimization to optical enhancements and photonic structuring.

Posters session Synthesis of CdSe/CdS and CdS/CdSe Core-shell Particles in One Step Solution Srocesses for Hybrid Solar Cell Applications

C. Selene Coria-Monroya, Mérida Sotelo-lerma

b, Claudia Martínez-Alonso

a, Paola Moreno-Romero

a,

Carlos A. Rodríguez-Castañedaa, Hailin Hu

a

a, Instituto de Energías Renovables, Universidad Nacional Autónoma de México, Priv. Xochicalco S/N, Temixco, 62330, MX b, Universidad de Sonora, Hermosillo, Sonora, MX

Nanoparticles of cadmium selenide (CdSe) and sulfide (CdS) have being showed as good electron acceptors for hybrid solar cells. They can be prepared from aqueous solutions with cadmium, selenium or sulfur sources, sodium citrate as cadmium ion complex agent and an alkaline solution to adjust the pH value. It is observed that the reaction kinetics of CdSe and CdS depend both on the hydroxide source (NH4OH or KOH) and pH values. CdSe precipitated more at lower pH value and CdS, at higher one. These phenomena were carefully used to achieve the formation of core/shell particles of CdSe and CdS in one-step solution process with either of two as core or shell. The appropriate pH values have been selected at each stage of synthesis for each alkaline source. X-ray diffraction patterns, diffuse optical reflectance spectra and scanning electron micrographs of the core/shell products were analyzed. Photovoltaic performance of heterojunctions with core/shell particles and poly(3-hexylthiophene) (P3HT) was studied and compared with that of CdSe- or CdS-P3HT. The hydroxide source used for CdSe and CdS formation influenced on the photovoltaic parameters of the corresponding solar cells. It is concluded that very thin layer or shell of CdS was formed on CdSe core with either of two alkaline sources at pH 10-11. The formation of CdSe shell on CdS core, on the other hand, was possible only in selenium shell solution with pH lower than 9.

Posters session Designing Efficient Counter Electrodes for Quantum-Dot-Sensitized Solar Cells

Jin-Song Hua, Yan Jiang

a, Li-Jun Wan

a

CAS Key Laboratory of Molecular Nanostructure and Nanotechnology, Institute of Chemistry, Chinese Academy of Sciences, 2 North First Street, Zhongguancun, Beijing, 100190, China

Quantum-dot-sensitized solar cell (QDSSC) has been considered as an alternative to new generation photovoltaics, but it still presents very low conversion efficiency. Besides the continuous effort on improving photoanodes and electrolytes, the focused investigation on charge transfer at interfaces and rational design for counter electrodes are recently receiving much attention. Rationally designing nanostructures aiming at solving the critical issues on counter electrodes would be possible to shed light on how to break through the conversion efficiency record of QDSSCs and push this field to move forward. In this presentation, the effort in this group on designing new nanostructures as efficient counter electrodes for QDSSCs will be discussed. For example, tunnel junction arrays configured with degenerate n-type tin-doped indium oxide nanowire (ITO) core and degenerate p-type Cu2S nanocrystal shell (ITO@Cu2S) was designed and fabricated as new efficient counter electrode for QDSSCs. ITO nanowire array core provided a three dimensional conductive network. ITO core and Cu2S nanocrystal shell formed effective tunnel junctions with carrier transport path shorter than 100 nm. It was found that sheet resistance (Rh) was not only dependent on the electron conductivity of the substrate but also related to the semiconductor-electrode interface between CTO and catalysts. The high-quality tunnel junctions resulted in the considerable decrease in Rh of the device, and facilitated electron transfer from CTO to Cu2S. Moreover, the chemical inert nature of ITO made this type of counter electrode stable in liquid electrolyte with no intrinsic issue like copper dissolution in state-of-the-art Cu/Cu2S counter electrode. Compared with planar structures, the three-dimensional nanowire


arrays presented higher surface area and easy accessibility of electrolyte, leading to higher catalytic activity of counter electrode as evidenced by apparently decreased charge transfer resistance. As a result, the power conversion efficiency of QDSSCs with the designed ITO@Cu2S nanowire counter electrodes increased by 84.5% and 33.5% compared to that with Au and Cu2S counter electrodes, respectively. Furthermore, the influence of the interface of ITO/Cu2S on the conversion efficiency of QDSSCs was further investigated by using different synthetic route and the following post-treatment to fabricate the ITO@Cu2S nanowire counter electrodes.

Posters session Predicting the Performance of Quantum Dot Solar Cells with Density Functional Theory: how to Design a Device with More than 9% efficiency

Jon M. Azpiroza, Ivan Infante

a

Universidad del Pais Vasco, Kimika Fakultatea, Euskal Herriko Unibertsitatea, UPV/EHU, Donostia- San Sebastian, 20080, ES

In 2013, Zhong et al. demonstrated that a panchromatic type II core-shell CdTe/CdSe device is capable to absorb light in the NIR leading to photocurrents of up to 19 mA/cm

2. With a Voc of 0.61 V and

a fill factor of 0.57, this device is to date the most performing sensitized quantum dot solar cell (QDSSC) with a 6.76% power conversion efficiency (PCE). Despite this important result, this PCE, however, still lags behind perovskite solar cells, which until now holds a record of 15.4%. Unlike perovskites, quantum dots present a higher tunability of the band gap, which can favor the absorption of a larger fraction of the solar emission spectrum, and therefore the generation of larger photocurrents, potentially leading to higher cell efficiencies. In this work, we investigated the possibility to deduce the performances of a QDSSC using first principle calculations, in particular with Density Functional Theory (DFT). We designed a computational protocol that allows calculating with good accuracy the absorption spectra, the IPCE, the photocurrent and, finally, the efficiency of a QD-based solar architecture. Unlike other approaches, we rely minimally on experimental parameters, rendering this protocol very robust. After successfully benchmarking this model on known QDSSC, we decided to design new panchromatic core-shell structures that might be good candidates to achieve higher efficiencies. We found out that in the same experimental conditions found for the CdTe/CdSe device, the CdTe/HgSe core-shell architecture could deliver photocurrents higher than 25 mA/cm

2, and provide efficiencies greater than 9%. In addition, this

newly designed protocol permits to analyze in-silico the effect of other elements present in QDSSC, like, for example, type of ions, ligands, size of the QD, to estimate the outcome they might have on the spectroscopic properties of these materials and, therefore, their efficiencies.

Posters session Characterization of Flexible Perovskite Solar Cells: Toward Power Solution for Wearable Electronic Devices

Byeong Jo Kima, Dong Hoe Kim

b, Gill Sang Han

a, Hyun Suk Jung

a

a, School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon, Korea b, Department of Materials Science and Engineering, Seoul National University, Seoul 151-744, Korea

In this work, 1000-fold bending durable perovskite solar cell is demonstrated with a high energy conversion efficiency, 12.2%, by formation of TiOx compact-electron-collection nanolayer. We emphasize here that this is the first time to realize such a high i) bending durability, and ii) energy conversion efficiency, in flexible perovskite solar cells. Perovskite solar cells, based on state-of-the-art inorganic-organic halide perovskite materials, are one of the most promising device for realizing wearable power source, on account of its high energy conversion efficiency, and economic fabrication process such as roll-to-roll printing. Therefore, recently, the main concern of research in this field has been moved to fabricating a highly flexible device, while preserving the high efficiency. The key process to achieve such kind of highly efficient & flexible perovskite solar cells is oxide film deposition process to make compact-electron-collection layers at a low temperature. In this regard, we report on highly efficient and bending durable perovskite solar cells on a cheap polyethylene naphthalate (PEN) substrate, with an TiOx compact-electron-collection layer that is fabricated at below 80 oC using plasma enhanced atomic layer deposition method. We observed i) extremely fast charge injection to the TiOx layer, yielding the high efficiency of 12.2%, and ii) fairly stable energy conversion efficiency up to 1 mm


of bending radius, and iii) constant device performance up to 1000 bending cycles with 10 mm of bending radius. In addition, this study demonstrates that in our device the only obstacle to enhancing the bending durability is the commercial transparent conducting oxide film (tin-doped indium oxide, in this work), evidenced by no crack formation over 1000 cycles of bending test in ITO-free devices we made. Therefore, our process is promising to achieve a perovskite solar cell close to a practical wearable power source, having higher efficiency and bending durability, by adopting newly developed transparent conducting materials, such as poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) or nanostructured metal web layers.

Posters session Spin-orbit Coupling and Rashba Effect in Hybrid Perovskite Heterostructures

Mikaël Kepenekiana, Laurent Pedesseau

b, Claudine Katan

a, Jacky Even

b

a, CNRS, Université de Rennes 1, Campus de Beaulieu, Rennes, 35000, FR b, FOTON, Université de Bretagne and CNRS, Rennes, FR

Metal-halide hybrid perovskites with 3D and 2D inorganic frameworks are currently of great interest for low-cost manufacture of solar cells and light-emitting devices, respectively. Obviously, the design of functional devices requires a thorough understanding of underlying physical mechanism that may be gained from concepts and tools of solid-state physics. In this work, we show that the prerequisite for a realistic modelling of such hybrids are twofold: both spin-orbit coupling (SOC) and the proper space group should be taken into account. In fact, Pb-based perovskites are the most studied but, despite the relativistic effects expected for lead, the giant effect of SOC on the conduction band has only been evidenced recently. It happens that DFT band gaps may fortuitously match experimental ones because SOC and many body effects are large and act in opposite directions. This was recently confirmed by self-consistent GW-SOC calculations. SOC is also mandatory to describe accurately metal substitution, interfaces, intensity and polarization of optical absorption, effective masses or to accurately implement empirical approaches. Moreover, 3D and 2D hybrids undergo phase transitions and symmetry breaking. From DFT and k.p perturbation approaches, we show that tetragonal or orthorhombic strain lead to band gap shifts and, due to large SOC for lead-based materials, electronic states involved in optical absorption are little perturbed by local distortions of the lattice. Next, spin-splitting subsequent to lack of centrosymmetry combined with SOC may lead to Rashba-Dresselhaus effects. This gives the opportunity to manipulate the spin with potential applications in spintronics. Rashba and Dresselhaus effects were originally defined in würtzite and zinc-blende semiconductors bulk-structures, respectively. We show that interplay of SOC and ferroelectric distortions for α (P4mm space group) and β (I4cm space group) phases of CH3NH3MI3, M=Pb,Sn , induces such spin-splittings. This was recently confirmed by GW+SOC and tight-binding approaches. We further evidence that similar Rashba-type spin-splitting occurs in 2D hybrid perovskites. This affords complementary routes for material engineering based on Rashba-Dresselhaus effects.

Posters session Optoelectronic Properties of Methylammonium Lead Halide Perovskites: Complementary ab initio GW Simulations and Ellipsometry Modelling.

Aurelien Leguya, Pooya Azarhoosh

b, Mariano Campoy-Quiles

c, ChunHung Law

a, Aron Walsh

d, Brian

O'Regana, Jenny Nelson

a, Mark van Shilfgaarde

b, Piers Barnes

a

a, Imperial College London, London SW7 2AZ, United Kingdom, UK b, King's College London, Strand, London WC2R 2LS, UK c, ICMAB, Campus de la UAB 08193 Bellaterra, Spain, 08193 Bellaterra, Barcelona, Spain, Spain d, University of Bath, Claverton Down, Bath, North East Somerset BA2 7AY, UK

Hybrid perovskite materials CH3NH3PbI3 (MAPI) and CH3NH3PbI3-xClx (MAPIC) are used as optically active components in high efficiency solution processed solar cells. Despite the tremendous interest focusing on these semiconductors, a range of significantly different possible energy band diagrams have been suggested and the optical constants of the material have not yet been reported. In this work, we solve these issues with a complementary study ellipsometry modelling and the highest level of ab initio quantum chemical calculations available for crystal structures: relativistic quasi-particle self-consistent GW simulations with no adjustable parameters. Ellipsometry looks at the change of polarization state of


a reflected light beam which can be modelled to yield information about the optical properties of a material. An ensemble of critical points of the joint density of states (four in this case) can be fitted to precisely derive the index of refraction and extinction coefficient of both MAPI and MAPIC.The nature of the critical points can also be used infer detailed information about the energy band structure of MAPI and MAPIC. We show excellent agreement between the optical constants of MAPI calculated from ellipsometry and the ab initio band structure calculations, the clear first experimental validation of ab-initio calculations on this material. The very close match emphasises the superiority of the GW approximation over density-functional based approaches and the importance of accounting for spin-orbit coupling which contributes 1 eV to the band gap. The combined results of ellipsometry modelling and quantum chemical calculations enable us to highlight the following interesting features which could have implications for device preparation: The bandgap edge is found to be anisotropic (2D). This means that absorption occurs only in two directions <100> and <010> but not in the third <001> around the first critical point of the joint density of states. If oriented crystals of MAPI can be grown, the active layer used in solar cells can become significantly thinner! The valence band maximum and conduction band minimum are non-parabolic. Their relative flatness is due to spin-orbit degeneration and suggests a high effective mass of the charge carriers. This suggests that mobilities could be further enhanced by doping the material.

Posters session The Dynamics and Structure of CH3NH3 Iions in Methyl Ammonium Lead Halide Perovskites

Aurelien Leguya, Jarvist Frost

b, Victoria Garcia-Sakai

c, Winfried Kochelmann

c, Joao Cabral

a, ChunHung

Lawa, Aron Walsh

b, Brian O'Regan

a, Jenny Nelson

a, Piers Barnes

a

a, Imperial College London, London SW7 2AZ, United Kingdom, UK b, University of Bath, Claverton Down, Bath, North East Somerset BA2 7AY, UK c, Science and Technology Facilities Council, Rutherford Appleton Laboratory, Didcot, Oxfordshire, OX11 0QX, UK

Hybrid perovskite materials CH3NH3PbI3 (MAPI) and CH3NH3PbI3-xClx (MAPIC) are used as optically active components in high efficiency solution processed solar cells. Within the perovskite crystal structure the methyl ammonium (MA) ions are caged between lead halide octahedra. The MA ions are electrical dipoles which have the potential to contribute to ferroelectric properties of the material. These ions are also speculated to play a critical role in the stability and hysteresis of MAPI and MAPIC photovoltaic devices. We present neutron diffraction and quasi-inelastic neutron scattering (QENS) data to examine the structure and behaviour of the MA ions in MAPI and MAPIC for the temperature range 7 – 380 K. Neutron diffraction allows the crystal positions of hydrogen nuclei to be determined which cannot be easily achieved using X-ray diffraction. Two crystalline phase transitions are observed at 150-170 K and ~ 330-350 K. QENS focusses on the dynamic motion of hydrogen nuclei within the structure. The data could be consistent with the rotation of the hydrogen ions around the C-N axis. A second rotational mode is observed above 140 K which can be attributed to reorientation of the C-N axis with respect to the crystal. Activation energies for these rotational movements are estimated, and the residence times in the possible orientations are obtained. The inferred active fraction of rotating MA is analysed. The proportion of CH3-rotors undergoing reorientation around the C-N axis increases linearly with temperature, which could be consistent with the reported H-bonds between MA and the halides of the inorganic moiety. The fraction experiencing reorientations of the C-N axis itself is independent of temperature, thus pointing at steric hindrance due to the extreme softness of the material at atomic level. Different geometries of reorientation are compared corresponding to hops between different possible lowest energy configurations. Molecular dynamics simulations are used to determine the most likely geometry. Finally we speculate on the possible consequences of these reorientations for the material properties. In particular, we use Ising-type simulations to show how the different possible MA arrangements could contribute to ferro- or antiferro-electric properties, and the possible consequences for the charge transport characteristics of the material. We examine whether realignment of the MA ion domains under varying electric fields could contribute to the hysteresis observed in the current-voltage curves of MAPI and MAPIC solar cells.


Posters session Evaluating the Role of Charge Trapping and Doping in an Organometal Trihalide Perovskite: Charge Transport, Emissivity, and Photovoltaic Performance

Tomas Leijtensa, Samuel Stranks

b, Giles Eperon

c, Erik Johansson

d, Ian McPherson

e, Henry Snaith

f

Instituto Italiano de Tecnologia, University of Oxford, 84 Percy Street, Oxford, IT

Here, an overview will be provided of our recent findings of charge trapping and doping in CH3NH3PbI3-xClx perovskite films. We focus on the two structures that are most promising for solar cell applications: mesosuperstructured perovskite films where the perovskite is infiltrated into a mesoporous Al2O3 scaffold, and neat perovskite films such as those used in planar heterojunction solar cells. We probe the presence of sub gap electron acceptor states, or traps, in both neat and mesostructured perovskite films via a combination of X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry (CV) measurements. We use these finding to complement photoconductivity, photoluminescence, mobility, and solar cell performance measurements to develop a clear picture of the role of charge trapping and doping in the two types of perovskite structures. We find that the mesoporous scaffold n-dopes the perovskite and leads to a passivation of a high density of trap states, raising the photovoltage of the solar cells as compared to the neat planar heterojunction solar cells whose behavior is dominated by a high density of trap states. We link this effect to the crystal size in the perovskite and surface effects at the oxide interfaces. Still, neat perovskite films are far superior to the mesosuperstructured films in terms of charge transport (long range mobilities in excess of 20 cm2 V-1 s-1), making the more promising for optoelectronic applications if alternative methods can be found to passivate the trap sites and dope the material.This work gives new insights into dominant loss pathways in solution processed organometal trihalide perovskite solar cells, and can be used to direct future work towards overcoming these losses. The extremely high long range mobility of the neat perovskite films also demonstrates the potential of this class of materials in other applications such as solution processed transistors.

Posters session Towards Vapour Deposited Perovskite Solar Cells

Mingzhen Liu, Michael Johnston, Henry Snaith University of Oxford, Clarendon Laboratory, Parks Road, Oxford, GB

In the past two years, organometal halide perovskites such as CH3NH3PbX3(X = Cl, Br, I) have shown great potential as a photovoltaic material that achieves over 15% solar to electrical power conversion efficiency (PCE) in nanostructured devices or in planar heterojunction structures. The generation of high efficiency perovskite solar cells, by vapour deposition, is likely to greatly shorten the timeline to large-scale manufacturing of the first generation of perovskite solar cells. In this pioneering work, perovskite solar cells synthesized by dual-source evaporation have shown optimal efficiency and uniformity in film formation. After pushing the PCE of vapour-deposited perovskite solar cells to over 15%, we further performed a series of fundamental studies to compare the photophysics and charge mechanism of perovskite film produced between by solution-process and vapour deposition. Our results suggest that the charge mechanism is heavily influenced by the fabrication process, which alters the crystallization and film morphology. Next, we sought to further probe the superiority of vapour deposition as a technique to develop perovskite films by establishing control over the crystallization and morphology of the perovskite film by vapour deposition and studying the outcome. This outcome confers greater reproducibility to device performance and more importantly, establishes the principle that the properties of the perovskite film are sensitive to changes in the fabrication process, which is closely correlated to the resulting film thickness and morphology.

Posters session External Load Dependent Degradation Behaviors of Planar Heterojunction Perovskite Solar cells

Chang-Qi Maa, Lianping Zhang

a, Feng Tang

b, Na Wu

a, Liwei Chen

b

a, Printable Electronics Research Center, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, 398 Ruoshui Road, SEID, SIP, Suzhou, 215123, CN


b, International Laboratory, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, 398 Ruoshui Road, SEID, SIP, Suzhou, 215123, CN

Solution-Processed organic-inorganic lead halide based perovskite solar cells have achieved tremendous progresses in the last few years, and power conversion efficiency (PCE) of more than 16% has been reported in the literature, showing great application aspect in the next future. [1-3] Stability of this type solar cell was then considered as the next key issue that needs to be solved before the industrialization. In this presentation, we will report our latest results on the degradation behaviors of planar heterojunction perovskite solar cells with a structure of ITO/PEDOT:PSS/CH3NH3PbIxCl3-x/PC61BM/Al. The solar cells were continuously illuminated with a white light, and the device performance was checked periodically with a special set-up that can automatically attach an external load to the circuit as required after I-V sweeping. Results indicate that degradation process of perovskite solar cells is highly depended on the attached external loads. When the solar cell was illuminated at open circuit, Voc increases slightly with the increase of testing time, whereas Jsc and FF decline under the continuous illumination, yielding a total decay of PCE. Such a decay process was slowed down when an external load was attached, and the decay rate was found to be negatively correlated to the external load. A detail discussion on the relationship between the degradation process and the external load, and a general decay mechanism will be given in this presentation.

Posters session Development of New Materials for Perovskite Solar Cells

Tngli Maa, b

, Huawei Zhoua, Yantao Shi

a

a, Dalian University of Technology, 2 linggong Rd. Dalian, China b, Kyushu Insititute of Technology, 2-4 Hibikino, Wakamatsu-ku, Kitakyusyu, 808, Japan

As one of the key components in perovskite solar cells, HTM separates photo-excited electron-hole pairs and transports the holes to the external circuit. Thus photovoltaic performance of perovskite solar cells is highly dependent on the properties of HTM, in which spiro-OMeTAD is currently widely used as the matrix for hole transportation. Adding different kinds of functional additives to spiro-MeOTAD can improve the properties of HTM and ensure the high efficiency perovskite solar cells. In this paper, we use a stable ionic liquid N-butyl-N’-(4-pyridylheptyl)imidazoliumbis (trifluoromethane) sulfonimide (BuPyIm-TFSI) as a dual-functional additive to simultaneously improve the electrical property of HTM and suppress charge combination in perovskite solar cells. BuPyIm-TFSI improved the conductivity of HTM and reduced the dark current of the solar cell. The PCE greatly increased from 3.83% (with no additive in HTM) to 7.91%, and these rates are comparable to those in conventional HTM containing lithium salt and TBP. Moreover, using this dual-functional additive can effectively simplify the components of HTM and help reduce the production cost. We also used the carbon to replace the Au electrode, the results will also be presented.

Posters session Photoinduced Charge Recombination Studies in MAPbICl/Polymer Solar Cells.

José Manuel Marín-Beloquia, Emilio Palomares

a, b

a, ICIQ, Paisos Catalans, 16. Tarragona 43007, Spain b, ICREA, Passeig Lluís Companys, 23 Barcelona, E-08010, Spain

Perovskite solar cells have attracted much attention in the last couple of years as they present a cheap and efficient alternative to usual commercial solar cells. We present herein the use of photo-induced time-resolved techniques to study charge recombination kinetics in complete CH3NH3PbClxI3-x (MAPbICl) solar cells. The work includes the use of: Charge Extraction (CE), Transient Photo-Voltage (TPV), Transient Photo-Current (TPC) and Laser Transient Absorption Spectroscopy (L-TAS). The MAPbICl devices were fabricated with the following architecture FTO/d-TiO2/mp-TiO2/CH3NH3PbClxI3-x/HTM/Au, where low-bandgap polymers semiconductor polymers have been used as hole transport materials (HTM). The influence of chemical additives such as Lithium Bis(Trifluoromethanesulfonyl) Imide (LiTFSI) and tert-Butyl Pyridine (tBuPyr), materials processing and illumination conditions are correlated with the device efficiency. In overall, the key losses for the device efficiency are identified and discussed to


understand the charge transfer reactions in order to improve the solar cell efficiency with the aim to reach the maximum theoretical solar to energy conversion efficiency.

Posters session Lead Precursors for the Preparation of Perovskites Solar Cells and Stability Studies

Fadi Kamal Aldibajaa, Elena Mas-Marzá

a, Ivan Mora-Seró

a, Juan Bisquert

a, b

a, Department of Physics, Universitat Jaume I, Casellón, 12071, Spain b, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia

Perovskites Solar Cells (PSCs) have attracted an enormous attention due to its impressive light to power conversion efficiency, easy manufacturing with low temperature processes and simple layer deposition by solution methods. Organo-metal lead halide Perovskites (ABX3) for Solar Cells are normally prepared by the reaction of PbI2 with CH3NH3I. Nowadays, several research groups have focused their efforts on the modifications of these Perovskites by the introduction of different organic cations, anions, as well as replacement of lead by other less toxic metal such as tin. In this communication, we report the use of different non-halide lead precursors for the preparation of these Perovskites as well as the corresponding Solar Cells. In addition to this, studies of the stability of the Perovskites and the Solar Cells devices will also be presented.

Posters session Functions of Porous TiO2 Electrodes on CH3NH3PbI3 Perovskite Solar Cells: Enhancement of Perovskite Crystal Transformation and Prohibition of Short Circuit

Gai Mizutaa, Govindhasamy Murugadoss

b, Soichiro Tanaka

c, Hitoshi Nishino

d, Seigo Ito

e

a, Department of Electrical Engineering and Computer Sciences, University of Hyogo, 2167, Shosha, Himeji-shi, 671, JP b, Energy Technology Laboratories, Osaka Gas Co, 6-19-9 Konohana-Ku, Osaka 554-0051, Japan, JP

In order to analyse the crystal transformation from hexagonal PbI2 to CH3NH3PbI3 by sequential (two-steps) deposition process, the perovskite CH3NH3PbI3 layers were deposited on flat and/or porous TiO2 layers. Although the narrower pores using small nanoparticle prohibited the effective transformation, the porous-TiO2 matrix can help the crystal transformation of PbI2 to CH3NH3PbI3 by sequential two-steps deposition. The resulting PbI2 crystal in porous TiO2 electrodes didn’t deteriorate the photovoltaic effects. Moreover, it is confirmed that the porous TiO2 electrode had a function to prohibit the short circuit between working and counter electrodes in perovskite solar cells. Using flat-TiO2 substrate, the PbI2 precursor layer can’t be completely converted to CH3NH3PbI3 crystal by the two-steps method in spite of thin PbI2 layer (200 nm). On the other hand, the PbI2 precursor in porous TiO2 layer (d = 90 nm) with 1-μm thickness can be completely converted to CH3NH3PbI3 by the two-steps method. Therefore, it was considered that the enhancement of crystal transformation from PbI2 to CH3NH3PbI3 in the porous TiO2 layer using the two-steps method was attributed to the interface between TiO2 and PbI2 in the porous TiO2 substrate.

Posters session CH3NH3PbI(3-x)(BF4)x : A molecular Ion Substituted Hybrid Perovskite

Satyawan Nagane, Umesh Bansode, Onkar Game, Satishchandra Ogale National Chemical Laboratory, Dr. Homi Bhaba Road, Pashan, Pune, 411008, IN

Organometal halide perovskites are considered a paradigm shift in the field of third generation solar cells. Various strategies are underway to further engineer the perovskite constituents to improve their properties and thereby enhance the corresponding solar cell efficiency. Here we report an interesting strategy to partially incorporate F

- ion within the perovskite structure in the form of BF4

- molecular ion.

Importantly, I- and BF4

- both have nearly same ionic radius which makes the incorporation of BF4

-

feasible in the structure. The incorporation of BF4- within the perovskite structure is supported by the

elemental mapping and ATR-FTIR studies. The synthesized hybrid perovskite CH3NH3PbI(3-x)(BF4)x shows sharp absorption onset at 760 nm and near band edge luminescence at 766 nm. The BF4

- molecular ion


substituted hybrid perovskite also shows one order of magnitude enhancement in the low frequency electrical conductivity as compared to the conventional perovskite (CH3NH3PbI3). When tested in a photo-sensor device architecture the modified perovskite shows over four orders of magnitude photosensitivity under AM1.5 radiation.

Posters session Electronic Structure of Graphene-Polymer Interfaces for Organic Photovoltaics

Keian Noori, Feliciano Giustino University of Oxford, Department of Materials, Parks Road, Oxford, OX1 3PH, GB

The use of graphene as an acceptor material represents an interesting alternative to fullerene derivatives and CNTs in bulk heterojunction OPV devices. Although excellent progress has been made in this direction, the efficiency of devices made with solution-processable functionalized graphene (SPFGraphene) remain low (~1.1 %) [1]. In this context, several important questions remain unanswered, including what is the maximum achievable open-circuit voltage (VOC) and what are the effects of the oxygen functional groups on VOC. In this work we address these questions using a first-principles approach, combining DFT with hybrid functionals, and considering large interface models including over 700 atoms. We study the atomic structure and energetics of ideal graphene/P3HT interfaces and determine a maximum ideal VOC of ~0.7-0.9 eV, in excellent agreement with reduced (annealed) SPFGraphene/P3HT devices. Additionally, we find that the presence of oxygen functional groups can raise the VOC of the interface by several tenths of an eV (in good agreement with the corresponding non-annealed devices), owing to a distortion of the polymer layer and a simultaneous rise in the energy of the highest occupied graphene state. Our results indicate that, while annealed devices already operate at or near their theorectical maximum VOC, the dispersity of the polymer matrix and the density of functional groups on reduced graphene oxide can be used to control the interfacial energy-level alignment so as to increase the open-circuit voltage above 1 V. Taken together our present findings represent the first step towards the rational design of graphene/polymer bulk heterojunction solar cells at the nanoscale.

Posters session Enhanced Photoluminescence and Solar Cell Performance via Lewis Base Passivation of Organic-Inorganic Lead Halide Perovskites

Nakita Noela, Antonio Abate

a, Samuel Stranks

a, Elizabeth Parrott

a, Victor Burlakov

b, Alain Goriely

b, Henry

Snaitha

a, Clarendon Laboratory, Department of Physics, University of Oxford, Parks Road, Oxford, OX1 3PU, UK b, Mathematical Institute, University of Oxford, Andrew Wiles Building, Radcliffe Observatory Quarter, Woodstock Road, Oxford, OX2 6GG, UK

Organic-inorganic metal halide perovskites have recently emerged as a top contender to be used as an absorber material in highly efficient, low cost photovoltaic devices. Solution processed semiconductors tend to have a high density of defect states and exhibit a large degree of electronic disorder. Perovskites appear to go against this trend and despite relatively little knowledge of the impact of electronic defects, solar-to-electrical power conversion efficiencies of up to 17.9 % have been achieved. Here, through treatment of the crystal surfaces with the Lewis bases thiophene and pyridine, we demonstrate electronic passivation of under-coordinated Pb atoms within the crystal. This significantly inhibits non-radiative electron-hole recombination within the CH3NH3PbI3-xClx perovskite films and we achieve photoluminescence lifetimes which are enhanced by nearly an order of magnitude, up to 2 µs. Through this passivation, we achieve power conversion efficiencies for solution processed planar heterojunction solar cells enhanced from 13% to 15.3% and 16.5 % for the untreated, thiophene and pyridine treated solar cells respectively.

Posters session Recombination Dynamics in Perovskite Solar Cells

Andreas Paulkea, Thomas JK Brenner

a, Sam Stranks

b, Henry Snaith

b, Dieter Neher

a


a, University of Potsdam, Institute for Physics , Karl-Liebknecht Strasse 24-25, Office: 2.28.1.021, Potsdam, 14476, DE b, Clarendon Laboratory, Parks Road, Oxford, OX1 3PU, GB

We apply Time-Delayed-Collection-Field (TDCF) experiments on working perovskite (MAPbI3-xClx) solar cells with different device architectures. After excitation with a wavelength-tuneable nanosecond optical pulse the photogenerated charges are subsequently extracted by a voltage pulse with a minimal delay of 10ns. By varying the delay we can probe the temporal evolution of the photogenerated charges in the device and quantify recombination losses. Most important is here the dependence of the carrier dynamics on the optical pulse fluence and intensity of the applied background illumination. From this results we can conclude on the predominant recombination mechanism in perovskite solar cells, e.g. bimolecular recombination. We show results on mesoporous-TiO2/Perovskite/Spiro and planar PEDOT:PSS/Perovskite/PCBM devices, were we found significant differences in the carrier dynamics. For the mesoporous TiO2 system we found exceptionally long carrier lifetimes on the order of 100µs. We are also planning to do these experiments on some more device architectures and alos at low temperatures.

Posters session Quantum and Dielectric Confinement of Charge Carriers in Hybrid Perovskite Heterostructures

Laurent Pedesseau, Mikael Kepenekian, Claudine Katan, Jacky Even a, Université Européenne de Bretagne, INSA, FOTON, UMR 6082, 35708 Rennes, France, FR b, CNRS, Institut des Sciences Chimiques de Rennes, UMR 6226, 35042 Rennes, France, FR

Quantum and dielectric confinements are known to induce dramatic changes of physical properties. For conventional semiconductors, modelling and understanding of quantum confinement are well developed, while significant dielectric confinement rarely occurs. Moreover, models developed to investigate dielectric confinement are often limited to abrupt interfaces. In this work, both these effects are investigated in layered Hybrid Organic Perovskites (HOP). First, concepts of effective mass and quantum well are carefully analyzed. For ultrathin layers, the effective mass model fails to understand quantitatively the quantum confinement effect. Our findings suggest that absence of superlattice coupling and importance of non-parabolicity effects prevents the use of simple empirical models based on effective masses and envelope function approximations. We present an alternative approach where 2D HOP are treated as composite materials and introduce a first principles based procedure to calculate band offsets. This is the first quantitative evaluation of the type-I quantum well concept introduced for such hybrids by Mitzi. Next, we introduce a new method to investigate dielectric confinement in 2D HOP, beyond the standard approximation based on dielectric constant profiles with abrupt interfaces. We show that dielectric self-energy profiles significantly contribute to the confinement of both monoelectronic and excitonic states. These approaches may be extended to 3D HOP-based heterostructures and are relevant for other classes of layered materials. Noteworthy, dielectric effects are also crucial in 3D HOP. In fact, screening of the exciton by collective rotations of organic cations has recently been suggested.

Posters session Synthesis, Recrystallization Studies of Organic and Inorganic tin Halide Perovskites

Lekha Peedikakkandya, Parag Bhargava

b

a, Indian Institute of Technology Bombay, Centre for Research in Nanotechnology and Science, IIT Bombay , Powai, Mumbai-400076, India b, Indian Institute of Technology Bombay, Department of Metallurgical Engineering and Materials Science , IIT Bombay , Powai, Mumbai-400076., India

Organic and inorganic metal halide perovskites are of interest for their application in high efficiency solar cells. Tin halide perovskites have been reported as light harvesting and high conducting hole transport material for opto-electronic and solar devices. They are an environment friendly alternate to the Pb halide perovskite solar cells, but stability and reactivity of organic and inorganic tin(II) halide


perovskites is still not fully understood. In this work we present detailed study on solid state and solution synthesis, characterization and stability study of organic and inorganic tin halides. Recrystallization and grain orientation of Sn and Pb halide perovskites in different polar organic solvents was also studied. As synthesised materials were characterised using X-ray diffraction (XRD), Raman spectroscopy, absorption and emission spectroscopy, transmission electron microscopy (TEM), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Their application as light absorbing material and as hole transport materials in sensitized solar cells were also investigated. Application of CsSnI3/CsSnBrxI3-x as an alternative for iodide and sulphide electrolytes in CdS-QDSSC was also tested.

Posters session Effect of TiO2 Matrices Sensitized with Sb2S3 on Hybrid Solar Cells

Mauricio Solis de la Fuente, Oscar A. Jamarillo, Marina Rincón, P.K. Nair Instituto de Energias Renovables - UNAM, Privada Xochicalco S/N, Col. Centro, Temixco, Morelos, 62580, MX

Hybrid solar cells have opened the path for novel designs aimed to increase the efficiency of solution processed semiconductor solar cells. The combination of conductive polymers with oxide matrices was quickly dominated by architectures based on poly-3-hexyl-thiophene (P3HT) and TiO2. Promising results were obtained with the structure TiO2/ Sb2S3/P3HT, motivating the present contribution. We compare alternative methods to obtain this architecture, and report its impact on the performance of TiO2 based hybrid solar cells. For that purpose, different TiO2 matrices were fabricated by screen printing (nanoparticles) and solvothermal method (1D nanostructures), and sensitized with Sb2S3 by chemical bath deposition. Optimization of film thickness, mass ratio, annealing treatments, was followed by TEM and SEM techniques. The performance of the Sb2S3-sensitized TiO2 matrices/P3HT hybrid solar cells was investigated using electrochemical impedance spectroscopy (EIS). Important differences in light-harvesting and charge-carrier transport are anticipated due to the different morphologies and surface chemistry of the TiO2 matrices.

Posters session Film Formation in Perovskite Solar Cells

Michael Salibaa, Kwan Tan

b, David Moore

b, Wei Zhang

a, Ulrich Wiesner

b, Henry Snaith

a

a, University of Oxford, Parks Road, Oxford, GB b, Cornell University, 214 Bard Hall, Ithaca, New York 14850, USA

Structure control in solution-processed hybrid perovskites is crucial to design and fabricate highly efficient solar cells. Here, we utilize in situ grazing incidence wide-angle X-ray scattering (GIWAXS) and scanning electron microscopy to investigate the structural evolution and film morphologies of methylammonium lead mixed halide perovskite during thermal annealing. We show that the material evolution can be characterized by three distinct structures: a crystalline precursor structure not described previously, a 3D perovskite structure, and a mixture of compounds resulting from degradation. Finally, we demonstrate how understanding the processing parameters provides the foundation needed for optimal perovskite film morphology and coverage.Based on this, we investigated the photovoltaic device performance with planar heterojunction architectures under different annealing conditions. We observed that a short rapid thermal annealing at 130 °C leads to the growth of large micron-sized textured perovskite domains and improved short circuit currents and power conversion efficiencies up to 13.5% for planar heterojunction perovskite solar cells.This work highlights the criticality of controlling the thin film crystallization mechanism of hybrid perovskite materials and offers a simple pathway for further enhancements in perovskite solar cells.

Posters session Charge Carrier Dynamics in MeNH3PbI3

Tom Savenijea, Carlito Ponseca

b, Lucas Kunneman

a, Mohamed Abdellah

b, Kaibo Zheng

b, Yuxi Tian

b, Ivan

Scheblykinb, Tonu Pullerits

b, Arkady Yartsev

b, Villy Sundström

b


a, Delft University of Technology, , The Netherlands b, Department of Chemical Physics, Lund University, Lund, Sweden

Organometal halide perovskites have recently attracted enormous attention since MeNH3PbX3 can be successfully applied as photoactive material in photovoltaic devices, yielding solar cells with an efficiency exceeding 15%. Surprisingly, the exact mechanism how charges are generated and transported so well is unclear. In this work, we investigated the charge carrier generation, mobility and recombination in MeNH3PbI3. For MeNH3PbI3 deposited on Al2O3 we observed fast formation of microsecond lived charge carriers on pulsed laser excitation in the visible at 300 K using time resolved photoconductivity measurements with terahertz radiation or microwaves as probe. At low laser fluences a maximum charge carrier mobility of about 5 cm

2/Vs, yielding charge carrier diffusion lengths well

above 5 μm are found. At higher laser intensities higher order recombination processes become operative lowering the mobile charge carrier population rapidly with a rate constant of, γ= 13×10

-10

cm3s

-1. Reducing the temperature results in increasing charge carrier mobilities following a T

-1.6

dependence, which we attribute to a reduction in phonon scattering (Σµ = 16 cm2/Vs at 165K). Despite

the fact that Sµ increases on lowering the temperature, γ diminishes with a factor six implying that charge recombination in MeNH3PbI3 is temperature activated. For MeNH3PbI3 deposited on charge carrier specific electrodes, such as TiO2 and PCBM lower signal sizes are found which are explained by the rapid collection of one type of carrier by the electrode. Those collected carriers have lower mobilities leading to a reduction of signal size and, in addition to different decay kinetics. The results underline the importance of the perovskite crystal structure, the exciton binding energy and the activation energy for recombination as key factors in optimizing new perovskite materials.

Posters session Planar Heterojunction CH3NH3PbIxBr3-x Perovskite Solar Cells by Sequential Process

Rui Sheng, Qingshan Ma, Sanghun Woo, Anita Ho-Baillie, Shujuan Huang, Xiaojing Hao, Martin Green Green UNSW School of Photovoltaic & Renewable Energy Engineering, The University of New South Wales School of Photovoltaic and Renewable Energy Engineering, Sydney, 2052, AU

Great attention has recently been drawn to developing CH3NH3PbIxBr3-x perovskite solar cells because its bandgap can be continuously tuned from 1.63 to 2.3eV by chemically manage the composition of halide[1]. This makes CH3NH3PbIxBr3-x perovskite solar cells especially attractive for tandem application. However, solution processed deposition will results unreacted compounds which are defects in the active layer, and in turn, this will lead to low efficiency of the device. In this work we aim to develop high efficiency CH3NH3PbIxBr3-x solar cells by sequential process. For the fabrication of perovskite solar cells, a better crystallinity and high purity film (less unreacted compounds-- MAX and PbX, X=I and Br) may be achieved by the sequential process compared to one-step solution process. In this study, PbX is deposited either via thermal evaporation or via spin coating, the CH3NH3PbIxBr3-x film is then formed by dipping the PbX coated sample into the MAX solution. Cells based on CH3NH3PbIxBr3-x and CH3NH3PbIxCl3-x perovskite will be characterised and compared. Their crystallinity and composition will be studied by XRD and EDX respectively. Their optical properties will be investigated by ellipsometry and absorption measurements. These results will be presented and discussed at the conference.

Posters session Solution Processed Electron Blocking Layer Toward Large Area PbS Quantum Dot Solar Modules

Hyung Cheoul Shim, Jihoon Jang, Hyekyoung Choi, Sohee Jeong KIMM (Korea Institute of Machinery and Materials), 156, Gajeongbuk-Ro, Yuseong-Gu, Daejeon, 305, KR

Semiconducting nanocrystal quantum dot (QD) have attracted a lot of attention as a next-generation solar cell material due to possibility of band gap engineering and increasing the power conversion efficiency (PCE) beyond the conventional Shockley-Queisser (SQ) limit utilizing multiple exciton generation (MEG) processes. In recent years, moreover, research related to the various architecture of the solar cell and the surface treatment techniques of QD is reported the development rate of the PCE


has also sharply increased. Especially, colloidal QD sample allow the solution process that provide flexibility in engineering design to reduce the manufacturing cost than silicon (Si) solar cells existing.However, aside from low PCE than Si based solar cells, it is still difficult to industrialization of QD solar cells. That is, all solution process that can contribute to the reduction of process costs can be lowered power generation bids to offset the low PCE is still impossible. For typical example, most of the layers comprising QD solar cell except QD layer usually have been prepared by vapor deposition techniques. In particular, some buffer layers such as lithium fluoride or molybdenum oxide have sometimes been employed for well charge separation and transportation in conjunction of QD solar cell with electron or hole accepting electrode, it is not easy to be applied the solution process to form a buffer layer unlike other transparent conductive oxide (TCO) layer on glass substrate. Therefore, it is essential to secure the proper buffer layers which is not only compatible with both QD and carrier accepting electrode but also enabling all solution process that provide in the true meaning of low cost with large area QD solar cell. In this report we demonstrate all solution processed lead sulfide (PbS) QDs solar cell modules having a layered by layered architecture with very large active area. The solution processable p3HT layer was adopted as buffer for electron blocking from QD to silver electrode. In addition, soft polymeric P3HT layer shows good contact with other layers, and also protecting QD layer during coating process of PEDOT:PSS that contains sulfonic acid can lead the degradation of QDs. Flexible solar modules based on these structures exhibit promising power conversion efficiency under AM 1.5 conditions

Posters session New Concepts for Highly-efficient Perovskite Solar Cells

Thomas Stergiopoulos, Wei Zhang, Henry Snaith Oxford Universty, Department of Physics, Clarendon Laboratory, Parks Road, Oxford, GB

A novel type of thin film photovoltaic device, known as perovskite solar cell, has entered the field during the last 2 years based on a solution-processable layered semiconductor as the absorber, which combines a range of unique properties such as large optical absorption cross section, low exciton binding energy and high crystallinity. Quite notably, this absorber was successfully incorporated in different solar cell configurations (sensitized solar cells, mesosuperstructured solar cells, planar heterojunction solar cells and hole conductor-free solar cells) providing maximum overall power conversion efficiencies ranging from around 11% up to more than 17.0% under 1 sun AM1.5G solar illumination. However, to make a big step towards commercialization, improved efficiencies of more than 20% (surpassing those of CIGS cells), accompanied with stability performance and absence of non-toxic elements (such as lead), should be attained. To this end, we here present some novel concepts for highly efficient perovskite solar cells employing either lead-free perovskites or by judicious engineering of the perovskite/electron acceptor interface.

Posters session Photoluminescence from Organometal Halide Perovskites: Excitons, Free Charge Carriers and Mid-Gap Electronic States

Samuel Stranksa, Victor Burlakov

b, Tomas Leijtens

a, James Ball

a, Alain Goriely

b, Henry Snaith

a

a, University of Oxford, Clarendon Laboratory, Parks Road, Oxford, GB b, University of Oxford, Mathematical Institute, OCCAM, Woodstock Road, Oxford, GB

Organic-inorganic perovskites are attracting increasing attention for their use in high-performance solar cells. Nevertheless, detailed understanding of charge generation, interplay of excitons and free charge carriers, and recombination pathways, crucial for further device improvement, are still incomplete. In this work we present a very generic yet analytically solvable model describing both equilibrium properties of free charge carriers and excitons in the presence of electronic sub-gap trap states, and their kinetics after photo-excitation, in the perovskite CH3NH3PbI3-xClx. At low fluences the charge trapping pathways limit the photoluminescence quantum efficiency whereas at high fluences the traps are predominantly filled and recombination of the photo-generated species is dominated by efficient radiative bimolecular processes. The model is able to reproduce the time-resolved photoluminescence decays and photoluminescence quantum efficiencies, which we show approach 100% at low temperatures and at high fluences. The results from the model strongly indicate that the


trap concentration increases with increasing temperature, suggesting an intrinsic origin of trap states. Our work provides an understanding of how to further enhance the material performance for high-efficiency perovskite solar cells and light-emitting diodes.

Posters session Lead-Halide Perovskite Solar Cell Fabricate by CH3NH3I-Dripping on PbI2-CH3NH3I Precursor Layer

Soichiro Tanakaa, Seigo Ito

a, Hitoshi Nishino

b

a, Department of Electric Engineering and Computer Science, Graduate School of Engineering, University of Hyogo, 2167 Shosha, Himeji, Hyogo, 671, JP b, Energy Technology Laboratories, Osaka Gas Co., Ltd., 6-19-9 Konohana-Ku, Osaka 554-0051, JP

Recently, the organic/inorganic hybrid solar cells using organic hole conductors and CH3NH3PbX3 (X = halides) on nanocrystalline-TiO2 layers have received a lot of attention due to the applicable conversion efficiency (15%). Because of the organic hole conductor, however, the material price can be extremely high. In this paper, therefore, we have fabricated solar cells using only inorganic (no double bond) printed materials: TiO2, CuSCN and CH3NH3PbI3. In this paper, a new deposition method of CH3NH3PbI3 layer has been investigated. Several structures of inorganic printed solar cells have been fabricated: <glass/FTO/bl-TiO2/ nanocrystalline-TiO2/CH3NH3PbI3/CuSCN/Au> and <glass/FTO/bl-TiO2/CH3NH3PbI3/CuSCN/Au>. In one spin coating procedure, a precursor solution of PbI2 (1.3 M) and MAI (50mg/mL) in DMF and DMSO (9:1 volume ratio) and a solution of MAI (dissolved in 2-propanol, 10mg/mL) were coated on TiO2 electrodes, successively. The resulting layers were annealed at 100 ˚C for 15 minute to be solar cells. SEM surface images of 2-steps process lead-halide perovskite (a) and dripping process lead-halide perovskite (b). Current-voltage characteristics of the heterojunction solar cells with (black and red) and without (green) mp-TiO2 measured under 100mWcm

-2. Red line shows 2-

steps process lead-halide perovskite. Red and Green lines show dripping process lead-halide perovskite. The best photovoltaic characteristics were performed by a printed solar cells of <glass/FTO/bl-TiO2/ nanocrystalline-TiO2 /CH3NH3PbI3(dripping)/ CuSCN/Au>, which were 18.41 mA cm

-2

of the short-circuit current density, 0.97 V of the open-circuit voltage, of 0.64 of the fill factors, and 11.3% of the conversion efficiency (Fig. 1). Since the conversion efficiency of perovskite solar cells on flat-TiO2 by 2-steps method was almost zero (data will be shown in conference), the<glass/FTO/bl-TiO2 /CH3NH3PbI3(dripping)/ CuSCN/Au> cell performed 7.0% conversion efficiency. The lead-halide perovskite crystal diameter by dripping process became larger than that by 2-steps process, which may be the significance for the photovoltaic effects.

Posters session Solution Processed Antimony Selenide-based Solar Cells: Further Insights into the Chemical Composition and Photovoltage Limitations

T. Tuyen Ngoa, David Pickup

b, Sudam Chavhan

a, Ivet Kosta

a, Celia Rogero

b, Enrique Ortega

b, Oscar

Miguela, Hans Grande

a, Ramon Tena

a

a, CIDETEC, Parque Tecnol, San Sebasti, 20009, ES b, Material Physics Center, 20018 San Sebastian, ES

After pioneering studies by Hodes’s and Larramona’s groups on Sb2S3-based semiconductor sensitized solar cells, there is growing photovoltaic interest in the VA-VIA group compound semiconductors. Very recently, we proposed–in parallel with Il Seok’s and Tang’s groups- solution processed Sb2Se3 as an innovative light harvester for semiconductor sensitized solar cells. Thanks to the well-suited bandgap (i.e. ~ 1.2 eV) and the efficient collection of electrical charge carriers, photogenerated from the NIR photons, short circuit photocurrents higher than 22 mA/cm2 have been reached in solution processed Sb2Se3-based single-junction devices. However, the open circuit voltage remains unexpectedly modest (i.e. around 300 mV), limiting the power conversion efficiency of the solar cells. In particular, TiO2/Sb2Se3/CuSCN planar heterojunction solar cells based on electrodeposited Sb2Se3 films show a power conversion efficiency of 2.1 % (Jsc ~ 18 mA/cm2 and Voc ~ 302 mV) [5]. As is the case for solution processed Sb2S3 and Sb2S3 in general, the chemical nature and accurate stoichiometry of the electrodeposited Sb2Se3 films isstill unclear. In addition to the TiO2/Sb2Se3/CuSCN solar cell preparation and characterization, a detailed characterization of the Sb2Se3 films by spectroscopic techniques (X-ray and Ultraviolet Photoemission Spectroscopies, XPS and UPS) will be


presented here. The XPS results indicate a significant Sb2O3 content and non-stoichiometric Sb2Se3 phases in the films. The work function and valence band maximum values of the films, determined by UPS studies, will also be given. Furthermore, a tentative band diagram for the TiO2/Sb2Se3/CuSCN heterostructure will be proposed in order to provide further insights into the reported limitations of the photovoltage of Sb2Se3-based solar cells.

Posters session Influence of TiO2 Crystal Orientation on the Adsorption of CdSe Quantum Dots Studied by the Photoacoustic and Photoelectron Yield Spectroscopy

Taro Toyodaa, b

, Witoon Yindeesuka, Keita Kamiyama

c, Shuzi Hayase

d, Qing Shen

a, b

a, The University of Electro-Communications, 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan b, CREST, Japan Science and Technology Agency (JST), Kawaguchi, Saitama 332-0012, Japan c, Bunkoukeiki, Co. Ltd., Hachioji, Tokyo 192-0033, Japan d, Kyushu Institute of Technology, Kitakyushu, Fukuoka 808-0196, Japan

An intense effort aimed at third-generation solar cells is being undertaken. Semiconductor quantum dots (QDs) have been studied for their light harvesting capability as sensitizers. Despite the potential advantages, a major breakthrough in conversion efficiency of QD-sensitized solar cells (QDSCs) that equals or exceeds dye-sensitized solar cells (DSSCs) has yet to be reported. Fundamental understanding of the surface chemistry of semiconductor QD adsorption is lacking, and this deficiency needs to be addressed. We describe the adsorption and growth of CdSe QDs on single crystals of rutile TiO2 with different crystal orientations. We used AFM to characterize the morphology of the QDs and photoacoustic (PA) spectroscopy for the optical absorption. Photoelectron yield (PY) spectroscopy was applied to characterize the valence band maximum (VBM) of the single crystal TiO2. The AFM images and the absorbance measurements showed that the number of CdSe QDs grown on the (111) surface was larger than those grown on the (110) and (001) surfaces. The adsorption becomes linearly proportional to the adsorption time. However, the rate of adsorption is different for each crystal orientation. The crystals grow higher on (111) surfaces than on (110) and (001) surfaces. The position of VBM for (111) surface is higher than those for the (110) and (001) surfaces. Hence, the growth of CdSe QDs on (111) surfaces is more active than on the other orientations. The increase in the average diameter of CdSe QDs with adsorption time is independent of the crystal orientation of TiO2. Although the growth rate of CdSe QDs on (001) surfaces is lower than other surfaces, the crystal quality is better on the former.

Posters session Ultra-fast Post-annealing of Oraganolead Halide Perovskite in Solar Cells

Joel Troughtona, Matthew Carnie

a, Matthew Davies

b, Katherine Hooper

a, David Worsley

a, Trystan

Watsona

a, SPECIFIC - Swansea University, SPECIFIC, Baglan Bay Innovation Centre, Central Avenue, Baglan, Port Talbot, SA12 7AX, GB b, School of Chemistry, Bangor University, Bangor University, Bangor, Gwynedd, Wales, LL57 2UW, GB

Recent advances in organometal halide perovskite solar cells have seen lab device power conversion efficiencies (PCEs) exceed 15%. Whilst high PCEs are essential in order to prove the commercial viability of these devices, their ease of processing must also be addressed. Currently, organometal halide perovskite cells may undergo an annealing treatment following the active layer deposition from solution. This process typically involves heating the device to 100°C for 45 minutes: A potential bottleneck in large-scale production.

In this work, we present a novel method for quickly annealing the active layer in organometal halide perovskite solar cells in as little as 2.5 seconds. Instead of conventional annealing in a hot air oven, near infrared (NIR) lamps are used to rapidly heat the cell substrate and drive perovskite crystallisation. This heating technique has already been demonstrated for use in dye-sensitised solar cells and other applications. Through the use of this method, we have produced a co-deposited perovskite-Al2O3 device with PCE in excess of 10.0% annealed in just 2.85 seconds; this is compared to an otherwise identical device yielding a PCE of 10.5% annealed in an oven at 100°C for 45 minutes. Optimal annealing parameters were obtained using UV-Visible-NIR spectrophotometry as well as through RGB image


analysis of the perovskite film. The work also explores the effect of NIR annealing on perovskite crystal structure.

Posters session Work Function Engineering of Graphene Nanoribbons: A first-principles Study

George Volonakis, Feliciano Giustino Department of Materials, University of Oxford, Parks Road, Oxford, OX1 3PH, GB

Graphene and its derivatives are well known for having remarkable properties such as a very high charge carrier mobility, mechanical flexibility, optical transparency and moreover being compatible with low-cost and efficient solution-based processes. Consequently, over the past few years various graphene derivatives have been examined as possible alternatives for different layers of solar cell (SC) devices. However, the use of graphene in SC layers (e.g. the hole or electron transport layer) still poses some challenges. In all these applications it is important to be able to tune the graphene work function in such a way that the optimum energy level alignment of the SC’s layers is achieved. In this work, we investigate strategies for engineering the work function of graphene derivatives by employing first-principles density functional theory calculations. Here we focus on functionalized graphene nanoribbons, and we consider the case where different species attach to the edges of pristine and oxidized graphene ribbons. After identifying the most stable adsorption geometries we calculate the effects of these functional groups on the work function. Our results reveal that the work function is very sensitive to the chemical and structural properties of the functional groups. Our calculated work function shifts are consistent with recent UPS measurements. In order to rationalize the calculated trends and gain a deeper understanding of the underlying atomic scale mechanisms, we develop a simple electrostatic model, which captures the essential physics at play.

Posters session Flexible Monograin Membrane Photovoltaic Modules from CZTS - A Roadmap to Production

Christoph Waldaufa, Axel Neisser

a, Timo Holopainen

b, Kaia Ernits

b, Dieter Meissner

c

a, crystalsol GmbH, Simmeringer Hauptstrasse 24, Wien, 1110, AT b, crystalsol OÜ, Akademia Tee 15a, 12618 Tallinn, EST c, Tallinn University of Technology, Ehitajate tee 5, Tallinn 19086, EST

Reaching cost competitiveness for photovoltaic solar modules has been and still is the major driving force of ongoing R&D efforts in the solar industry. crystalsol has been developing a novel design for PV module and a production technology based on the monograin membrane approach that combines the advantages of high throughput, low cost deposition techniques mainly from the printing industry with the versatility of a flexible, light weight, thin film solar module. The module design comprises a monolayer of crystal grains embedded in a fully flexible polymer membrane. The main feature of the monograin membrane technology is the separation of the synthesis of the semiconductor material from its large area deposition. The light absorbing material, the kesterite compound semiconductor Cu2ZnSn(S,Se)4, is produced in a simple annealing process from elemental and binary precursor materials in a bulk process without the need for a vacuum step. Large area deposition for module production is realized by embedding the powder into a thin polymer layer – the monograin membrane. This two step approach allows the utilization of optimized single crystal growth conditions to obtained the best possible absorber material while at the same time producing modules in a non-complex, cost-effective and high throughput roll-to-roll printing process. Monolithic integration of solar cells is realized by patterning the front and the back contact layers that cover the membrane and by embedding conductive channels into the membrane, leading to a high degree of flexibility in module size, shape and cell layout even for small production batches. Crystalsol has setup two laboratory production lines for powder synthesis and membrane manufacturing respectively. Current development efforts focus on scale up of the powder production process and the installation and ramp up of a pilot line for module manufacturing. Product qualification of prototype modules of up to 20 × 20 cm2 has recently been started and field tests are under way. Good product stability has already been verified by first results from accelerated life time testing.The contribution will introduce the process flow of the crystalsol roll-to-roll production process, discuss the inherent technological advantages of the monograin membrane technology and report on the current status and achievements. It will address typical scale up issues in


PV module manufacturing and give an outlook onto future development of the monograin membrane technology.

Posters session Interplay Between QD Stoichiometry and Electron Transfer Efficiency in SILAR based QD Sensitized Oxides

Hai Wanga, b

, Irene Barcelóc, Mischa Bonn

a, Enrique Cánovas

a

a, Max Planck Institute for Polymer Research, Ackermannweg 10, Mainz, 55128, Germany b, Graduate School Material Science in Mainz, University of Mainz, Mainz, Germany, Germany c, Institut Universitari d'Electroquímica i Departament de Química Física, Universitat d'Alacant, Apartat 99, E-03080 Alacant, Spain

The successive ionic layer adsorption and reaction (SILAR) method represents a promising low-cost solution process approach to develop QD sensitized oxides to be exploited for solar energy conversion. The QD sensitization of a mesoporous oxide matrix by SILAR is achieved by repeated cycles of 4 successive dipping steps of an oxide film into beakers containing: (i) a cation solution (ii) pure solvent to remove the excess of unbound cations, (iii) an anion solution and (iv) pure solvent to remove the excess of unbound anions. This process – from (i) to (iv) – is termed one SILAR cycle. Complete SILAR cycles provide QDs which are necessarily anion rich, while half cycle treatments (ending with step (ii)) allow generating QDs that are cation rich; This atomic control on QD surface chemistry provides a test bed system for exploring how QD stoichiometry affects ET dynamics. Exploiting optical pump-THz probe (OPTP) measurements, we study the effect of QD stoichiometry on the ET efficiency on PbS QDs directly nucleated by SILAR onto oxide matrices. QD to oxide electron transfer efficiency is maximized in lead rich quantum dots (terminated by half SILAR cycle). This effect can be traced to atomic surface passivation of QDs provided by lead cations - in perfect agreement with theoretical calculations1. The passivation efficiency (PE) effect is found to be QD size dependent (absolute QD sizes are obtained using HRTEM), particularly PE is found to increase linearly with QD surface area (a manifestation of the kinetic competition between electron trapping at the QD surface and ET to the oxide). Finally, we demonstrate that the improvement in ET efficiency for lead rich QDs (monitored by OPTP on QD/oxide electrodes) is directly and quantitatively correlated to the increase of photocurrent in QD sensitized photovoltaic devices, highlighting the relevance of these results for solar cell optimization.

Posters session Perovskite Processing: a Thermal Evaluation

Alice Williams, Trystan Watson, David Worsley SPECIFIC, Swansea University, SPECIFIC, Baglan Bay Innovation & Knowledge Centre, Central Avenue, Baglan Energy Park, Baglan, Port Talbot, GB

Hybrid organic / inorganic perovskites such as methylammonium lead tri-halides (MAPbX3-nYn: X, Y = halogen, n = 0-3) are materials of substantial interest as the light-harvester in photovoltaic devices. There is currently some debate about the effect of processing upon the structure and composition of the resulting material, including suggestion, in the case of the mixed halide MAPbI3-nCln, that there is only one halogen (iodine) present in the resulting material. We have used thermal analysis, spectroscopy and ‘hyphenated’ techniques, which facilitate evolved gas analysis, to understand changes occurring during material processing. Samples of MAPbI3-nCln were prepared by processing aliquots of 40 % precursor solution (in DMF) at different cure temperature / time combinations. TGA and DSC were used to monitor mass loss and heat flow isothermally (during the cure process) and on a temperature ramp (after the material had formed). The hyphenated techniques TGA-GCMS and DTA-FTIR were used to analyse any volatiles released during each process. TGA-FTIR and TGA-GCMS showed only solvent evolution during the cure step. Post-cure TGA-FTIR analysis of material prepared at 100 °C (15 minutes) and 30 °C (240 minutes) shows that the resulting material still contains residual solvent, which is released at increasing temperature; it is possible that a small amount of solvent becomes incorporated within the perovskite matrix, requiring bond cleavage (and consequently increased energy) for its release. Post-cure Differential Scanning Calorimetry (DSC) shows differences in thermodynamic properties. A sample cured at 100 °C for 80 minutes shows no features over the temperature range 20 – 120°C but a sample cured at 30 °C shows overlapping features around 80 °C; these are not present on


subsequent scans. As solvent is demonstrably present in the processed sample this is not surprising; however, the number and nature of the features is interesting. They could be attributed to simple release of solvent or something more complex: if the incorporated solvent acted as a plasticizer, the material may have formed in an amorphous state; the thermodynamic features could therefore include a phase change. DSC also demonstrates that the material resulting from the MAPbI3-nCln precursor is not simply MAPbI3. It is well documented that MAPbI3 undergoes a defined, reversible, tetragonal – cubic phase change around 55 °C, which is easy to replicate using the MAPbI3 / DMF precursor; however, this phase change not present in materials produced from MAPbI3-nCln / DMF; this shows that the material resulting from the mixed halide is not simply MAPbI3.

Posters session Modification of Electron Collection Interface for Highly Efficient Heterojunction Perovskite Solar Cells

Konrad Wojciechowskia, Sam Stranks

a, Antonio Abate

a, Golnaz Sadoughi

a, Asitya Sadhanala

b, Chang-Zhi

Lic, Alex K.-Y. Jen

c, Henry Snaith

a

a, University of Oxford, Clarendon Laboratory, Parks Road, Oxford, OX1 3PU, GB b, University of Cambridge, Cavendish Laboratory, J. J. Thomson Avenue, Cambridge, CB3 0HE, GB c, University of Washington, Department of Materials Science & Engineering, Seattle, Washington 98195, US

Organic−inorganic halide perovskites, such as CH3NH3PbX3 (X = I−,Br−,Cl−), have emerged as an attractive material for fabrication of low cost solar cells, which can provide a high efficiency alternative to established, more costly technologies. Over the last 2 years there has been an exceptional rise in power conversion efficiencies, demonstrating the outstanding potential of these perovskite materials. However, in most device architectures, a current-voltage response displays anomalous hysteresis, the phenomenon that is one of the key issues which needs to be resolved for further development of this technology. Here we present a significant decrease in hysteretic anomalies through the engineering of the electron collecting interface and an enhancement of photovoltaic performance of perovskite solar cells. We provide an insight into the physical processes occurring at the interfaces, which can influence charge collection and overall device operation. By modifying the n-type contact with a self-assembled fullerene monolayer, which displays a two-fold passivation effect, we achieve up to 17.4 % power conversion efficiency and stable power output reaching 15.7 % under constant 0.81 V forward bias for planar heterojunction perovskite cells.

Posters session Improved Efficiency of Perovskite Solar Cells Through the p-type Metal Complexes doped HTM.

Kuan-Lin Wu, Tsutomu Miyasaka Toin University of Yokohama, Kurogane-cho, Aoba, Yokohama, Kanagawa, 225, Japan

A recent study of organic-inorganic hybrid perovskite as light harvesters and hole transport materials has revolutionized the emerging photovoltaic technology. In the past three years, the performance of perovskite solar cells has improved rapidly to reach efficiencies as high as 16%. So far, perovskite solar cells have mainly focused on 2,2’,7,7’-tetrakis(N,N-di-p-methoxyphenylamine)-9,9’-spirobifluorene (spiro-MeOTAD) as HTM. However, spiro-MeOTAD suffers from low conductivity and hole mobility. The widespread application in organic electronic devices has an important strategy to alter the charge-transport properties of organic semiconductor using chemical doped. To overcome these problems additional charge carrier are generated by doping, a common technique to tune the electrical properties. Herein we report on the use of a novel class of metal complexes as p-type dopants for spiro-MeOTAD and their application in Perovskite solar cells. Moreover, we discuss the influence of doping level on photovoltaic performance using the electrochemical impedance spectroscopy and achieve the high efficiency with spiro-MeOTAD-based Perovskite solar cells.

Posters session Studying the Voltage and Energy Loss in Perovskite Solar Cells

Jizhong Yaoa, Piers Barnes

a, Thomas Kirchartz

b, Jenny Nelson

a


a, Centre for Plastic Electronics and Department of Physics, Imperial College London, London, GB b, IEK-5 Photovoltaics , Forschungzentrum Julich, Germany, Germany

We examine the voltage and energy loss in methyl-ammonium lead-halide perovskite solar cells in comparison to crystalline silicon (c-Si) and polymer:fullerene systems. The contributions to voltage and energy loss can be interpreted either in terms of a balance of generation and recombination events or in terms of energy levels. Using the principles of detailed balance, combined with electroluminescence spectroscopy (EL) and sub-bandgap quantum efficiency measurements, we derive the theoretical upper limit of the open-circuit voltage (Voc,rad), when only radiative recombination occurs. The voltage difference ΔVoc,nr between the actual Voc and Voc,rad is attributed to non-radiative, non-geminate recombination. We show that pervoskite solar cells have a slightly larger non-radiative voltage loss (0.28 V) than c-Si (0.22 V) but smaller than the best organic system (0.35 V). The voltage drop between optical bandgap (Eopt/q) and Voc,rad for perovskite and c-Si are similar, at about 0.26 V for both, whilst the organic devices investigated exhibit a much larger voltage drop due to the need for a heterojunction to separate excitons. The voltage loss can, alternatively, be divided into components representing geminate and non-geminate recombination. The results show that recombination losses in perovskite devices are dominated by non-geminate processes in contrast to c-Si and organic solar cells which show a higher proportion of geminate loss. The understanding of different factors contributing to recombination and limiting Voc in the different material systems can be used in future research to minimize energetic losses and increase stability in perovskite solar cells.

Posters session CdS-CdSe Co-sensitized ZnO Nanorods Synthesis and its Photoelectrochemical Characterization

Kamila Zarębska, Magdalena Skompska Warsaw University, Faculty of Chemistry, Laboratory of Electrochemistry, Pasteura, Warsaw, 02-093, PL

Solar cells based on nanostructural ZnO with semiconductor sensitizer (semiconductor-sensitized solar cell (SSSC)) consist of environmental friendly components and provide a wide range of light absorption, good stability and low preparation costs. Substitution of a dye (typically used in Graetzel DSSCs) with a n-type semiconductor in the form of quantum dots (QDs) allow avoiding the problem of ZnO instability in the presence of acidic groups of the dyes. Semiconductor QDs offer also other significant advantages over dyes, such as higher extinction coefficient and easily tuned band gap from infrared to ultraviolet by a control of QD size and composition. This work is focused on preparation and characterization of SSSCs based on ZnO nanorods covered with CdS and CdSe quantum dots by SILAR (successive ionic layer adsorption and reaction) technique. It is shown that quantity and quality of deposited semiconductor layer have a great influence on efficiency of SSSC. The most promising results obtained for multilayer system in which both semiconductor are deposited in order ZnO/CdS/CdSe, which is effect of electronic band structure. The application of two layers system leads to broadening the absorption spectral region. In addition CdS interlayer between ZnO nanorods and CdSe QDs passivates the ZnO suppressing the charge carrier recombination and in effect, the power conversion efficiency of the cell is increased. The photoelectrochemical responses of the ZnO/CdS/CdSe systems were studied in aqueous solution of Na2SO3 as the hole scavenger. The studies were focused on the relationship between the number of SILAR cycles and performance of the device.