Für meine Familie Till min familj500212/... · 2012-03-01 · Für meine Familie Till min familj . List of Papers This thesis is based on the following papers and manuscripts, which

“Planet Noah” – Eva Unger (1997)

Für meine Familie Till min familj

List of Papers

This thesis is based on the following papers and manuscripts, which are re-ferred to in the text by their Roman numerals. Reprints were made with per-mission from the publisher.

I Bilayer Hybrid Solar Cells based on Triphenylamine-

Thienylenevinylene Dye and TiO2 Eva L. Unger, Emilie Ripaud, Philippe Leriche, Antonio Cravino, Jean Roncali, Erik M. J. Johansson, Anders Hagfeldt, Gerrit Boschloo, Journal of Physical Chemistry C, 114, 11659-11664 (2010)

II Contribution from a hole-conducting dye to the photocur-rent in solid-state dye sensitized solar cells Eva L. Unger, Ana Morandeira, Mats Persson, Burkhard Zietz, Emilie Ripaud, Philippe Leriche, Jean Roncali, Anders Hagfeldt, Gerrit Boschloo, Physical Chemistry Chemical Physics, 13, 20172-20177 (2011)

III Spray-pyrolyzed vs spin-cast titania films as acceptor in small molecular bilayer hybrid solar cells Eva L. Unger, Francesca Spadavecchia, Kazuteru Nonomura, Pål Palmgren, Giuseppe Cappeletti, Anders Hagfeldt, Erik M. J. Johansson, Gerrit Boschloo Submitted Manuscript

IV Excitation energy dependent charge separation at small-molecular semiconductor/TiO2 interface Eva L. Unger, Tomas Edvinsson, Joe Roy-Mayhew, Håkan Rensmo, Anders Hagfeldt, Erik M. J. Johansson, Gerrit Bosch-loo Submitted Manuscript

V In Manuscript: Hole transporting dye as light harvesting an-tenna in dye sensitized TiO2 hybrid solar cells Eva L. Unger, Lei Yang, Burkhard Zietz, Anders Hagfeldt, Erik M. J. Johansson, Gerrit Boschloo

Comments on my own Contribution For all papers and manuscripts that are part of this thesis I had the main re-sponsibility and carried out most of the experimental work, data analysis and writing of the manuscripts. Synthesis of the compounds TDCV-TPA (paper I) and TVT-TPA (paper II) was carried out by Philippe Leriche and Emilie Ripaud from the University of Angers, France. Photoelectron spectroscopy measurements were carried out by Erik Johansson, I took part in the data analysis. The DFT-calculations presented in paper IV were carried out by Tomas Edvinsson. The AFM images presented in paper III were taken by Francesca Spadaveccia under supervision of Pål Palmgren. The AFM meas-urements in paper IV were carried out by Joe Roy-Mayhew.

List of papers to which I have contributed during my PhD-studies which are not part of this thesis:

Efficient infiltration of low molecular weight polymer in nanoporous TiO2

Erik M. J. Johansson, Sulena Pradhan, Ergang Wang, Eva L. Unger, Anders Hagfeldt, Mats R. Andersson Chemical Physics Letters, 502, 225 (2011). Comparing spiro-OMeTAD and P3HT hole conductors in efficient solid state dye sensitized solar cells Lei Yang, Ute B. Cappel, Eva L. Unger, Martin Karlsson, Karl Martin Karlsson, Erik Gabrielsson, Licheng Sun, Gerrit Bosch-loo, Anders Hagfeldt, Erik M. J. Johansson Physical Chemistry Chemical Physics, 14, 779 (2011)

Table of Contents

1 Photons ...................................................................................................13 1.1 The sun – our star ...........................................................................13 1.2 Aim and outline of this thesis .........................................................15

2 Solar cells ...............................................................................................17 2.1 Light absorption & charge separation ............................................17

2.1.1 Inorganic solar cells.............................................................19 2.1.2 Organic solar cells ...............................................................19 2.1.3 Dye-sensitized solar cells ....................................................20

2.2 Hybrid solar cells............................................................................21 2.2.1 Bilayer HSCs .......................................................................21 2.2.2 Nanostructured HSCs ..........................................................22 2.2.3 Hybrids among HSCs ..........................................................23

2.3 Solar cell device performance ........................................................24 2.3.1 Current-voltage measurements ............................................24 2.3.2 Photocurrent density and external quantum efficiency .......26

3 Excitons ..................................................................................................29 3.1 Light harvesting..............................................................................29

3.1.1 Exciton formation ................................................................29 3.1.2 Light absorption...................................................................31 3.1.3 Injection dyes.......................................................................32 3.1.4 Hole transporting dyes.........................................................34

3.2 Exciton diffusion and energy transfer ............................................35 3.2.1 Energy transfer mechanisms................................................36 3.2.2 Spectral overlap & photoluminescence quantum yield .......37 3.2.3 The exciton diffusion length................................................39 3.2.4 The energy transfer efficiency.............................................41

4 Electrons & Holes ..................................................................................43 4.1 Charge carrier separation................................................................43

4.1.1 Photo-induced charge transfer .............................................43 4.1.2 Geminate electron-hole pair separation GS ........................44 4.1.3 Energy level alignment........................................................45 4.1.4 Charge separation in steady-state ........................................47 4.1.5 Transient absorption spectroscopy ......................................48

4.2 Charge collection............................................................................50 4.2.1 Charge transport & recombination ......................................50 4.2.2 Morphology .........................................................................51

5 Results for XDCS-1................................................................................53 5.1 Dye multilayers on TiO2.................................................................53 5.2 Exciton diffusion in TDCV-TPA...................................................54 5.3 Interfacial exciton separation..........................................................57 5.4 Interface modification ....................................................................58 5.5 Interface morphology .....................................................................59

6 Results for XDSC-2................................................................................62 6.1 Matching dyes ................................................................................62 6.2 Solar cell devices ............................................................................64 6.3 Energy transfer from HTD to ID ....................................................66 6.4 Injection, regeneration and pore infiltration ...................................67

7 Conclusion..............................................................................................68

8 Materials & Methods..............................................................................70 8.1 Materials used and sample preparation ..........................................70 8.2 Characterization methods ...............................................................73

8.2.1 Microscopic & Profilometric methods ................................73 8.2.2 UV-vis, IR & Photoluminescence spectroscopy .................73 8.2.3 Electrochemical methods.....................................................74 8.2.4 Transient absorption spectroscopy ......................................75 8.2.5 Photoelectron Spectroscopy ................................................76 8.2.6 Solar cell device characterization ........................................76

Sammanfattning på svenska..........................................................................78

Acknowledgement ........................................................................................81

References.....................................................................................................83

Abbreviations

Physical variables and other abbreviations

( )A (difference in) absorbance

A area

A(e-,E) (electron, energy) acceptor

AFM atomic force microscope

AM (1.5 G) air mass density (1.5 global)

BHJ bulk hetero junction

c concentration

C capacitance

CE counter electrode

CV cyclic voltammetry

d dye-layer thickness

D(e-or E) electron or energy donor

DPV differential pulse voltammetry

(s)DSC (solid-state) dye-sensitized solar cell

E energy

E0-0 electronic transition from ground-state to excited state

EA electron affinity

E(X)B (exciton) binding energy

EC conduction band edge potential

EF,redox redox Fermi energy

qEF,(n or p) (quasi) Fermi level (of electrons or holes)

EV valence band edge potential

EQE external quantum efficiency

ET energy transfer

Fc ferrocene

FF fill factor

FTO fluorine doped tin oxide

HOMO highest occupied molecular orbital

HSC hybrid solar cell

HTD hole transporting dye

Io, IT, IR intensity of incident, transmitted and reflected light

IP ionization potential

ICT internal charge transfer

ID injection dye

IQE internal quantum efficiency

J current density

j0 dark saturation current density

Jph photocurrent (density)

JSC short-circuit current

LXD (ED) exciton diffusion length

LHE light harvesting efficiency

LUMO lowest unoccupied molecular orbital

NC conduction band density of states

NHE normal hydrogen electrode

OSC, OPV organic solar cell, organic photovoltaics

Pin, Pspec irradiance, spectral irradiance

PES photoelectron spectroscopy

PIA photo-induced absorption

PL photoluminescence

R reflectance

R(S or Sh) (series or shunt) resistance

SEM scanning electron microscope

t TiO2 layer thickness

T transmission or temperature

TAS transient absorption spectroscopy

V voltage/potential

Vfb flat band potential

Va applied potential

VOC open-circuit voltage

WE working electrode

absorption coefficient

extinction coefficient

r relative dielectric constant

wavelength

(charge carrier) mobility

frequency

(solar cell) power conversion efficiency

CT charge transfer,

CC charge collection efficiency

XD exciton diffusion efficiency

ET energy transfer efficiency

(spec) (spectral) photon flux

CS charge separation (efficiency)

PL photoluminescence quantum yield

Constants

c = 2.998·108 ms-1 speed of light (in vacuum)

e = 1.602·10-19 C elementary charge

NA = 6.022·1023 mol-1 Avogadro’s constant

h = 6.626·10-34 Js = 4.136·10-15 eVs Planck’s constant

kB = 1.381·10-23 Ws/K = 8.617·10-5 eV/K Boltzmann’s constant

0 = 8.854·10-12 Fm-1 vacuum permittivity

Chemical compounds (short names & chemical name)

AcAc acetylacetone

CBz chlorobenzene

cDCA cheno-deoxycholic acid

DCM dichloromethane a.k.a. methylene chloride

EtOH ethanol

Li-TFSI lithium bis(trifluoromethanesulfonyl)imide

MeCN acetonitrile

MPN methoxypropionitrile

NO2-bzac para-nitrobenzoic acid

P3HT poly(3-hexyl-thiophene)

PEDOT:PSS poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate)

PMMA poly(methyl methacrylate)

spiro-MeOTAD 2,2’,7,7’-tetrakis(N,N-di-methoxyphenyl-amine)-9,9’-spirobifluorene

tBP 4-tertbutylpyridine

TTIP Ti(IV)-tetraisopropoxylate

D5L0A1 4-(diphenylamino)phenyl-cyanoacrylic acid (Figure 3.3)

D5L0A3 4-(diphenylamino)phenyl-rhodanine-3-acetic acid (Figure 3.3)

D9 5-(4-(Bis(4-methoxyphenylamino)styryl)thiophen-2-yl)-2-cyanoacrylic Acid (Figure 3.3)

SQ02 5-carboxy-2-[[3-[(2,3-dihydro-1,1-dimethyl-3-ethyl-1H-benzo[e]indol-2-ylidene)methyl]-2-hydroxy-4-oxo-2-cyclobuten-1-ylidene]methyl]-3,3-dimethyl-1-octyl-3H-indolium (Figure 3.4)

SQ 5-carboxy-2-[[3’-[(5’’-ethyloxycarbonyl-3’,3-dibutyl-1’’-pentyl-H-indol-2’’-ylidene) methyl]-2’-hydroxy-4’-oxo-2-cyclobuten-1’-ylidene]methyl]-3,3-trimethyl-1-methyl-3H-indolium (Figure 3.4)

TDCV-TPA tris[4-(dicyano-vinyl-2-thienyl)phenyl]amine (Figure 3.6)

TP-TPA tris[4-(phenyl-thiophen-2-yl)phenyl]amine (Figure 3.6)

TVT-TPA tris[4-(2-thienyl-vinyl-2-thienyl)phenyl]amine (Figure 3.6)

13

1 Photons

1.1 The sun – our star The sun – the star at the center of our solar system - is the primary energy source for our planet. Since prehistoric times the significance of the sun for life on earth has been recognized. The energy radiated from the sun towards our planet supports life by driving photosynthesis as well as climate and weather phenomena.1

Light has wave-like properties described by the wavelength and fre-quency . At the same time, light can be described as a stream of particles – photons.2 The energy E of a photon is given by Planck’s constant h multi-plied by the frequency :

1240( )

( )

c h cE h h E eV

e nm (1.1)

Outside the earths atmosphere the irradiance Pin,3 which is the power density

of the incident sunlight, is 1353 W/m2. The spectral irradiance Pspec( ) out-side the earth’s atmosphere is the AM0 spectrum shown in Figure 1.1 (right) which resembles the black body radiation at 6000 K.2 The Pspec( ) is related to Pin and the spectral photon flux spec( ) via:4

( ) ( )in spec spec

h cP P d d (1.2)

At sea level, the solar spectrum is decreased due to scattering and absorption in the earth’s atmosphere. Solar spectra depend on the air mass (AM) the solar radiation passes through which depends on the angle between the incident solar radiation and the zenith at the point of measurement. The AM1.5 spectrum corresponds to an angle of 48o and is a widely used stan-dard spectrum for the characterization of solar cells. The characteristic fringes in the spectrum are due the absorption of radiation by carbon dioxide (CO2), water (H2O) and ozone (O3). According to equation (1.2), integration over Pspec gives Pin which is approximately 1000 W/m2 for the AM1.5 spec-trum.

14

Figure 1.1.: (left) Illustration of the dependency of solar energy incident on planet earth. (right) Spectral irradiance Pspec in dependency of wavelength

and/or energy E for the black-body-radiation (dashed), AM0 (red) and AM1.5 (black) spectrum.

Photosynthetic processes have taken place since 3 billion years on our planet. In photosynthesis light energy drives chemical reactions through which solar energy is converted and stored in chemical bonds.1 Some natural photosynthetic systems have developed light harvesting antennas to capture incident sunlight more efficiently and funnel the absorbed excitation energy to a reaction centre (RC in Figure 1.2).1, 5-7 There, the energy drives the photo-electrochemical reactions that convert water (H2O) and carbon-dioxide (CO2) into oxygen (O2) and carbo-hydrates (Cm(H2O)n).

8 This is schematically shown in Figure 1.2 (left). After a plants life cycle and under certain conditions, botanical matter can be converted into fossil fuels.

RCDe- Ae-e- e-2 H2OO2 4 H+

CO2

Cm(H2O)n

ET

LH-antenna

ID TiO2e-

HTD

ET

e-

+

-

V


CO2

Cm(H2O)n

ET

LH-antenna

ID TiO2e-

HTD

ET

e-

+

-

V


CO2

Cm(H2O)n

ET

LH-antenna

ID TiO2e-

HTD

ET

e-

+

-

V

Figure 1.2: Conceptual picture showing light harvesting and energy trans-fer to a reaction center (RC) in photosynthetic systems (left) in comparison with the concept behind the solar cell devices investigated herein with a hole transporting dye (HTD) acting as antenna to an injection dye (ID) bound to TiO2 acting as electron acceptor (right).

15

The conversion of biological matter into fossil fuels is a process that takes millions of years. Power generation today is to 72%9 based on burning fossil fuels which can be considered to reverse the reactions taking place in photo-synthesis, depicted in Figure 1.2 (left). The combustion of fossil fuels gener-ates CO2 and consumes O2. The effect of the increased CO2 emission on the atmosphere and climate of our planet has received much media attention during recent years10 which has increased public awareness. Burning up the planets fossil fuel resources will reverse the effect that 3 billion years of photosynthesis had on the atmosphere of our planet.11

Finding alternative ways of converting energy is therefore a necessity to secure the existence of our own species on this planet.12 Considering the constant flux of solar energy incident on the planet the direct conversion of sunlight into electricity is an appealing alternative. This is what solar cells do.

1.2 Aim and outline of this thesis In this thesis solar cell devices that can be categorized as hybrid solar cells (HSC) have been investigated. Conceptually, these type of solar cell devices are related both to dye-sensitized solar cells (DSCs) and organic solar cells (OSCs). An introduction to solar cells and overview over different solar cell technologies is given in chapter 2.

The device architectures investigated herein employ bulk dye layers for light harvesting mimicking the light harvesting antennas in photosynthetic systems (Figure 1.2). Light absorption (LHE) in bulk dye layers or organic semiconductors leads to the formation of excitons.13, 14 Excitons are bound electron-hole pairs that can migrate through the dye layer via exciton diffu-sion ( XD) or energy transfer ( ET). These processes and their experimental investigation are discussed in chapter 3. The devices investigated herein are therefore referred to as excitonic dye solar cells (XDSCs).

At interfaces excitons can become separated into free charge carriers: electrons and holes. Therefore, the bulk dye layer need to function as hole transporting medium (HTM) as well as light harvesting medium and will be referred to as hole-transporting dyes (HTDs). The physical prerequisites and experimental methods for the investigation of charge separation ( CT, GS) and transport ( CC) is the topic of chapter 4.

The first 4 chapters provide background information and description of experimental techniques to characterize solar cell devices, light absorption, exciton diffusion, charge separation and transport in XDSCs.

16

Figure 1.3: Roadmap for the theory part of this thesis

Two different device architectures of XDSCs have been investigated in the course of this thesis.

In the first approach a hole transporting dye (HTD) was used as bulk-light harvesting material in conjunction with the inorganic wide-bandgap semi-conductor titanium dioxide (TiO2), schematically depicted in Figure 1.4. In these excitonic dye solar cells light is harvested in one dye component only. This device architecture will therefore be referred to as XDSC-1. The results for XDSC-1 are presented in papers I, III and IV and are summarized in chapter 5.

In the second device architecture (Figure 1.5) two dyes are used: an inter-facial dye (ID) bound chemically to the TiO2 surface and a HTD. Accord-ingly, these types of devices will be referred to as XDSC-2 and are sche-matically shown in Figure 1.5. The results on this system are summarized in chapter 6 and presented in the papers II and V.

PEDO

T:PS

S

Back

con

tact

HTDFTO

TiO2

EC

LHE

XD

CC

CS

PEDO

T:PS

S

Back

con

tact

HTDFTO

TiO2

EC

LHE

XD

CCCC

CS

FTO

TiO2

PEDO

T:PS

S

Back

con

tact

ID HTD

EC CS

LHE

XD

ET

CC

FTO

TiO2

PEDO

T:PS

S

Back

con

tact

ID HTD

EC CSCS

LHE

XDXD

ET

CCCC

Figure 1.4: XDSC-1 Figure 1.5: XDSC-2 (chapter 5, papers I, III & IV) (chapter 6, papers II & V)

After concluding remarks in chapter 7 a section describing the materials preparation and instrumentation used in this work is given in chapter 8.

17

2 Solar cells

2.1 Light absorption & charge separation Photovoltaic devices, or solar cells, convert sunlight into electricity. On a black-box level one can describe a solar cell as a device that absorbs sun light and converts the absorbed energy into electrical power (voltage x cur-rent) between two contact points (Figure 2.1).

Photons in...

Current & Voltage out.-

- -- - - -

+ + + ++ +

--

++

-

+

1) Lightabsorption

2) Chargeseparation

Photons in...

Current & Voltage out.-

- -- - - -

+ + + ++ +

--

++

-

+

1) Lightabsorption

2) Chargeseparation

Figure 2.1: Black-box picture of the two processes occurring in photo-voltaic devices: Light absorption & Charge separation.

The fraction of light absorbed in the active components of a photovoltaic device is the light harvesting efficiency (LHE). When determining the LHE of a solar cell device the light reflected at the front-contact of the solar cell device (R) and absorbed by layers that do not contribute to the photocurrent should be accounted for.15 The processes occurring in the device that lead to the formation of separated charge carriers are summarized in the internal quantum efficiency (IQE a.k.a. APCE) of the solar cell device. The product of LHE and IQE is referred to as the external quantum efficiency (EQE a.k.a. IPCE). The EQE is the fraction of photons incident on the device that are converted into electrons flowing in an external electric circuit. Accounting for the reflectance of the front contact R, the EQE can be expressed as:

( ) (1 ( )) ( ) ( , )EQE ,V R LHE IQE V (2.1)

The power conversion efficiency of a solar cell device is defined as the maximum electric power output Pmax with respect to the power of the inci-dent light Pin (1.2) which will be specified in equation (2.3).

18

Some historically important names and dates16-23 that lead to the develop-ment of modern solar cells are summarized graphically in Figure 2.2.

1839 BecquerelDiscovers a photovoltaic effectbetween Pt plates immersed in

electrolyte solution

1877 Adams & DaySe between Pt-wires:

First solid state Schottky junction1887 Moser

Dye-sensitized Photoeffect

1921 EinsteinNobel prize for the explanation of the

photovoltaic effect

1883 FrittsFirst thin film solar cell with molten Se between two metal contacts.

1927 Grondahl & GeigerCu/Cu20 photovoltaic cells38 publications in 2 years

1930’s SchottkySchottky junction

1946 Ohl U.S. Patent 2,402,662”Light sensitive devices”1959 Kallman & Pope

Fist organic solar cell

1954 Chapin & Fuller

Si PN-junction (6%)1960’s GerischerWork on interfacial electron transfer1986 Tang

Organic PN junction(1%) 1991 O’Regan & Grätzel

Grätzel-type dye sensitized nanostructured solar cell (7%)

1954 ReynoldsCdS solar cells (6%)

1995 Heeger et al.Bulk hetero junction OSC

1958 Vanguard ISolar powered satellite

1983 Boing10 % CIGS

The dawn of photovoltaics(EU2012)

2012

GaAs28.3 %

Thin Film-CdTe 16.7% -CIGS 19.6%-CZTSSe 10.1%

DSC11%OSC

10%

concentratormulti-

junction43.5%

Photosynthesis1-2 %

Si: 20-25 %amorphous 12.5 %







photovoltaic effect













1983 Boing10 % CIGS


2012

GaAs28.3 %


DSC11%OSC

10%OSC10%

concentratormulti-

junction43.5%

Photosynthesis1-2 %








photovoltaic effect













1983 Boing10 % CIGS


2012

GaAs28.3 %


DSC11%OSC

10%

concentratormulti-

junction43.5%

Photosynthesis1-2 %








photovoltaic effect













1983 Boing10 % CIGS


2012

GaAs28.3 %


DSC11%OSC

10%OSC10%

concentratormulti-

junction43.5%

Photosynthesis1-2 %


Figure 2.2: “The dawn of photovoltaics”. Short summary of historically important names and dates in solar cell research and efficiencies of differ-ent types of solar cell devices according to Greens efficiency table (ver39).24

There are many different types of solar cell devices. These differ from each other in the materials used for light harvesting and the processes through which absorbed photons are converted into free electrons and holes. Current record efficiencies for a selection of different types of lab-scale solar cells are included in Figure 2.2.24

Solar cells employing inorganic semiconductors for light harvesting can be categorized as inorganic solar cells (ISCs).25 Solar cells employing or-ganic compounds for light harvesting are organic solar cells (OSCs) and dye-sensitized solar cells (DSCs). The similarities and differences of ISCs, OSCs and DSCs are compared in Figure 2.3 and these types of technologies will be briefly introduced in the following sections. This overview does not reflect upon the technological importance of the respective device technology. Em-phasis is placed on OSC and DSC as these technologies are relevant for the type of HSC-devices investigated herein.

19

2.1.1 Inorganic solar cells Silicon solar cells are sometimes referred to as 1st generation solar cells which reflects the technological importance of these devices. The impulse for the technological development was the breakthrough of a 6% efficient silicon p-n-junction reported by Chapin, Fuller and Pearson21 in 1954. Since then improvements in process technology has lead to device efficiencies of 20%-25% in lab-scale devices.24 The absorption coefficient of silicon is rela-tively low and a layer of 150 m of material is needed for sufficient light harvesting. This requires the manufacturing of high purity grade silicon which is costly and energy intensive.

Also in 1954, Reynolds et al. reported a 6% efficient CdS solar cell.23 CdS belongs to a group of solar cell technologies that are now classified as 2nd generation solar cell. These are technologies that use inorganic semi-conductors with higher absorption coefficients ( ) compared to silicon. Therfore, less material is required and these type of devices are referred to as thin film solar cells. This category includes amorphous Si, CdS, CdTe, Cu-InSe2 (CIS), CuInGaSe2 (CIGS)26, 27and CuZnSnS/Se (CZTS).28 For these type of solar cells efficiencies approaching 20 % have been achieved in lab-scale devices.24

Both in silicon and thin-film inorganic solar cells charge separation is considered to be driven by an electric field established at the junction be-tween the p-type and n-type inorganic semiconductors, sketched in Figure 2.3 (left).29 However, the action and importance of diffusion currents in in-organic solar cells should not be neglected.11

-

+

n-typeinorganic

p-typeinorganic

+

-

eVO

C

-

+

n-type organic(Acceptor)

eVO

C

-

+

-

+

-

+

”p-type”

eVO

C

-

+

n-type wide-bandgap semi-conductor

electrolyte

ISC OSC DSC

Exciton

p-type organic(Donor)

GCT

Greg

qEF,n

qEF,p

qEF,n

qEF,p

qEF,n

qEF,p

-

dye

+

GinjEA

IP

- cb

Eredox

+

-

cb

vb

Exciton

Exciton

-

+

n-typeinorganic

p-typeinorganic

+

-

eVO

C

-

+

n-type organic(Acceptor)

eVO

C

-

+

-

+

-

+

”p-type”

eVO

C

-

+

n-type wide-bandgap semi-conductor

electrolyte

ISC OSC DSC

Exciton

p-type organic(Donor)

GCT

Greg

qEF,n

qEF,p

qEF,n

qEF,p

qEF,n

qEF,p

-

dye

+

GinjEA

IP

- cb

Eredox

+

-

cb

vb

Exciton

Exciton

Figure 2.3: Comparison of an inorganic solar cell (ISC), an organic solar cell (OSC) and a dye-sensitized solar cell (DSC). Adapted from ref.30

2.1.2 Organic solar cells In the 1960’s the advances in silicon solar cells inspired research on devices utilizing organic dyes and pigments for solar energy harvesting31, 32 which are referred to as organic solar cells OSCs. In 1986, Tang18 reported an OSC comprising the junction of an n-type and a p-type organic semiconductor with a power conversion efficiency of 1% which is schematically shown in

20

Figure 2.3 (middle). Light harvested in the bulk of the organic semi-conducting materials leads to the formation of Frenkel-type excitons which will be discussed in chapter 3. To separate these excitons into free electrons and holes (positive charge carriers) a thermodynamic offset at the junction between the n-type and p-type material is required. As excitons need to be transferred to the vicinity of an interface to become charge-separated, the exciton diffusion length LXD becomes a limiting factor to the device perform-ance of OSCs.

The need to overcome the exciton diffusion bottleneck led to the devel-opment of bulk-heterojunction (BHJ) OSCs.19 In this approach the organic n- and p-type materials are deposited as a mixture and the device efficiency depends on the phase-separation and domain size formed after co-deposition of the two materials.

A detailed description of the development and the mechanism of charge separation in OSC can be found elsewhere.30, 33-35 According to a press re-lease by the company Mitsubishi Chemicals BHJ organic solar cells recently achieved power conversion efficiencies of 9.2%.36 Organic tandem cells using small-molecular organic materials have recently reached efficiencies of 8.3 %.37

2.1.3 Dye-sensitized solar cells Dye-sensitized solar cells (DSCs) have their origin in the discovery by Moser in 188717 that dye-sensitization of halogenated silver plates leads to an increased photocurrent. However, it was not until the 1970’s that the un-derlying mechanism for current generation in these type of devices was es-tablished.38, 39 The field of DSC gained much interest when O’Regan and Grätzel published a power conversion efficiency of 7% for these devices.20 The leap in efficiency came from the incorporation of meso-porous TiO2 as electrode material which challenged the paradigm of that time that a high interfacial surface area needs to be avoided in solar cell devices.

In DSCs, light is harvested in a dye chemically bound to the surface of a meso-porous wide-bandgap semiconductor such as titanium dioxide (TiO2). Excited dye molecules inject electrons into the TiO2 conduction band, sketched in Figure 2.3 (right). Subsequently, the oxidized dye molecules need to be regenerated which is accomplished either by a liquid electrolyte20,

40, 41 or a solid-state hole transporting material.42, 43 A thorough description of the conversion mechanism in and materials development for DSC can be found elsewhere.44-46 DSCs are an active research area with 2-3 articles pub-lished every day.44 The recent record for DSCs is above 12%.47

21

2.2 Hybrid solar cells The type of solar cells investigated in this thesis can be classified as hybrid solar cells (HSCs). Conceptually, these devices are related to both OSC and DSC. The commercial potential of OSCs, DSCs and HSCs is in low-cost, light-weight and portable solar cells for consumer electronics. HSCs com-bine inorganic and organic semi-conducting materials.48, 49 This definition encompasses a wide range of solar cell devices with varying material-combinations and device architectures. A more detailed summary and review about different types of HSCs can be found in recent review articles48-53 and book chapters54, 55 and the literature cited therein.

As the devices investigated herein are examples of HSCs using a wide-bandgap semiconductor (TiO2), HSCs utilizing a small-bandgap inorganic component (e.g. Si,56 CdS,57 CdSe,58 CdTe,59 CuInS2,

60 PbSe,61) are not dis-cussed in the context of this work. Wide-bandgap semiconductors such as TiO2 and ZnO only absorb solar radiation with wavelengths shorter than ~400 nm. In these HSCs the inorganic component is involved in the interfa-cial charge separation and charge transport but does not contribute signifi-cantly to the light harvesting efficiency (LHE). Similar to OSCs, photons are often harvested in the bulk of an organic component leading to the formation of strongly-bound excitons13 which will be further discussed in chapter 3. Exciton separation into free charge carriers is the subject of chapter 4.

2.2.1 Bilayer HSCs The simplest possible device architecture are bilayer HSCs (Figure 2.4, left). This geometry is very useful to investigate new material combinations and to determine parameters like the exciton diffusion length LXD

* (section 3.2.3, paper I & III) or the interfacial charge transfer efficiency (section 4.1, paper IV). Efficiencies for bilayer wide-bandgap metal oxide/organic semiconduc-tor solar cell devices are typically in the order of 0.1-0.6%.6, 62-75 Reported HSCs often employ polymers as the organic compound but there are some examples for HSCs incorporating small molecular semiconductors such as merocyanines62, 63 or porphyrins6, 72, 73, 76. In this thesis work, bilayer devices using small-molecular triphenyl-amine based compounds in conjunction with TiO2 were investigated. The results on bilayer devices are presented in paper I and paper III. The hybrid solar cells comprising the small molecular semiconductor TDCV-TPA in conjunction with TiO2 investigated herein gave high conversion efficiencies compared to other devices of this architec-ture (paper III).

* Within this thesis exciton diffusion processes will be indicated with X to distinguish from energy transfer process in general (ET).

22

organic

inorganic

Bilayer HSC

A) Infiltration B) Blend

substratesubstratesubstrate

organicorganic

inorganic

Nanostructured HSC

organic

inorganic

Bilayer HSC

A) Infiltration B) Blend

substratesubstratesubstrate

organicorganic

inorganic

Nanostructured HSC Figure 2.4: Different device architectures of hybrid solar cells. Nanostruc-tured HSC can be made following an infiltration or a blend-approach. Adapted from Bouclé et al.50

2.2.2 Nanostructured HSCs To overcome exciton diffusion limitations nanostructures HSCs are being investigated. In principle, nanostructured HSCs can be categorized according to two different preparation approaches, illustrated in Figure 2.4.51 The or-ganic material can be infiltrated into a nanostructured inorganic matrix (Fig-ure 2.4, middle). This is a common approach for the preparation of e.g. solid-state DSC42, 77 and nanostructured HSC.78, 79 Alternatively, the organic and inorganic component can be co-deposited in a blend in analogy to the preparation of organic BHJ solar cells (Figure 2.4, right).80-84 Following the BHJ-approach, efficiencies over 2% have been achieved in ZnO/polymer BHJ devices.83For devices utilizing TiO2 the infiltration approach might be preferable as high processing temperatures are necessary to form an inter-connected crystalline TiO2 network.85

The charge separation efficiency in HSCs is determined by the energetic alignment and electronic coupling at the organic/inorganic hetero-interface. Often HSCs have lower conversion efficiencies compared to OSCs with similar device architecture. The charge separation efficiency at the hybrid interface can be affected by modifying the inorganic/organic interface.70, 86, 87

In addition, the organic and inorganic phase need to form bi-continuous percolating pathways for charge transfer to the electron- and hole-selective contacts of the device.88 This motivates the preparation of ordered nanostruc-tures to achieve efficient exciton harvesting and charge transport. Ordered nanostructures might possibly also aid the infiltration of the organic semi-conductor into the inorganic network. Among the nanostructures investi-gated for HSC applications are TiO2 nanotube arrays89 and ZnO nanorods.90

Solid-state dye sensitized solar cells (sDSCs)42, 91 can be classified as nanostructured HSCs prepared using an infiltration approach.42, 49 Spiro-MeOTAD is the most commonly used hole transporting material (HTM) since it was first introduced in DSCs in 1998 by Bach et al..42 Recently an efficiency of 7.2% for a sDSC has been reported by Burschka et al.92 who used a cobalt-complex as dopant for spiro-MeOTAD. As in liquid-DSC light

23

is harvested in a dye-monolayer chemisorbed to the meso-porous metal ox-ide. Limited infiltration of the solid HTM into the porous metal oxide and a faster charge carrier recombination rate lead to efficiency-optimized sDSCs that only comprise a 2 m thick meso-porous TiO2 layer.93 The LHE of these devices is therefore below 100%.

2.2.3 Hybrids among HSCs There are now more and more examples reported in literature that com-

bine aspects of different hybrid solar cells like e.g. utilizing a dye-sensitized meso-porous TiO2 in combination with a light-absorbing hole transporting medium such as poly-3-hexylthiophene (P3HT).78, 89, 94-101 Device efficien-cies were found to be increased significantly by introducing additives like Li-TFSI and/or tBP.89, 96, 99, 101 These additives were found to influence the energetics at the hetero-interface which can increase the charge separation efficiency.101

Potentially, both the dye at the interface, herein referred to as ID, and the light absorbing hole-transporting medium, herein referred to as HTD, can contribute to the photocurrent in these devices. Efficiencies above or close to 3 % have been achieved using this type of device architecture for HSCs.96, 98-

100, 102 Employing an ID in a HSC device can enhance the interfacial proper-ties at the organic/inorganic junction due to its amphiphilic properties. IDs could also act as electron acceptors (Ae-)†, prevent interfacial recombination and/or act as an energy acceptors (AE) for excitation energy absorbed by the HTD.97 The XDSC-2 devices investigated herein can be considered exam-ples for this approach.

An interesting approach to utilize the volume of the hole transporting ma-terial for light harvesting is the utilization of energy-relay dyes: Fluoropho-res are dissolved in the electrolyte102-104 of liquid DSC, dispersed in the solid state hole transporting medium,102, 105-107 or embedded in the metal oxide matrix.108 Absorbed excitation energy can be transferred to the ID via a För-ster resonant energy transfer (FRET)109 which gives rise to a panchromatic spectral response of the device.110

† Within this summary electron acceptors and donors will be denoted as Ae-/De- while energy acceptors and donors will have a capital “E” as an indicator (AE/DE).

24

2.3 Solar cell device performance

2.3.1 Current-voltage measurements Charge separation requires a built-in asymmetry in the solar cell devices that drives electrons and holes in opposite directions, schematically shown in Figure 2.1. In the dark, solar cells exhibit the characteristic current density – voltage (J-V) curve of a diode represented by the equivalent circuit shown in Figure 2.5. Solar cells can be described by the Shockley equation (2.2) which describes the current density J extracted from a solar cell as a function of applied potential Va:

0

1( ) exp 1

1 / /a s a

a phs sh B sh

V JR A VJ V J j

R R nk T e R A (2.2)

where A is the area of the devices and e, kB and T are the elementary charge, Boltzmann constant and temperature, respectively. The dark satura-tion current density j0 is the leakage current through the diode in the absence of illumination. The ideality factor n is related to the type of recombination processes in the device. In organic and dye-sensitized solar cells n is thought to be related to both exciton and charge carrier recombination.30 The series resistance RS is determined by the conductivity of the materials comprising the junction an outer electric circuit and should be minimized. The shunt resistance RSh is associated with charge carrier recombination and should be maximized. The effect of RS and RSh on the J-V-curve is illustrated in Figure 2.5 (middle). Under illumination the whole J-V curve is shifted by the amount of photo-generated current density Jph, illustrated in Figure 2.5 (right).

1 0 -1

-20

-15

-10

-5~V/(R S

A)

~j0+V/(R

ShA) ~n

Jph

ln (J

/ A

cm-2)

V (V)

VOC

~j0

-0.4 0.0 0.4 0.8

-0.4

0.0

0.4

0.8

VOC

JSC P

max

J (m

A cm

-2)

V (V)

A = (VJ)max

Jph

j

V

RS

RSh

j0, n

Jph

1 0 -1

-20

-15

-10

-5~V/(R S

A)

~j0+V/(R

ShA) ~n

Jph

ln (J

/ A

cm-2)

V (V)

VOC

~j0

-0.4 0.0 0.4 0.8

-0.4

0.0

0.4

0.8

VOC

JSC P

max

J (m

A cm

-2)

V (V)

A = (VJ)max

Jph

j

V

RS

RSh

j0, n

Jph

Figure 2.5 (left) Equivalent circuit representing solar cell diodes. (middle) Tafel-plot of a current density-voltage (J-V) curve in the dark and under il-lumination illustrating: n, j0, RS and RSh. (right) (J-V) curve in the dark and under illumination illustrating the short circuit current JSC, open circuit voltage VOC, maximum power point Pmax.

25

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6

-15

-10

-5

16 cyc

+ dy

e

log(

j / A

cm

-2)

V (V)

no dye

2 cyc

0 2 4 6 8 10 12 14 16

10-8

10-7

10-6

10-5

10-4

j 0 (A c

m-2)

# spray cycles

dye layerTiO2 layer (varied)

TiO2 layer (varied)

PEDOT:PSS PEDOT:PSS

FTOFTO

graphite graphite

dye layerTiO2 layer (varied)

TiO2 layer (varied)

PEDOT:PSS PEDOT:PSS

FTOFTO

graphite graphite

no dye + dye

Figure 2.6: (left) Tafel plot of dark current-voltage measurements on bi-layer devices prepared with spray-pyrolyzed titanium dioxide layers of dif-ferent thickness (indicated in figure) with and without TDCV-TPA. (right) Dark current exchange density in dependency of the number of spray cycles of the titanium dioxide blocking layers.

Analysis of the dark J-V behavior of solar cells gives valuable information for the optimization of individual layers in the device architecture. In paper I (ESI) we investigated the dependency of the device performance (ESI) on the thickness of a spray-pyrolyzed TiO2 layer. Besides acting as electron accepting component at the inorganic/organic interface, the TiO2 layer also fulfills a crucial role in blocking recombination pathways to the conducting substrate. The j0 decreases and the RSh increases with the amount of spray cycles shown in Figure 2.6.

The dye layer was found to additionally decrease j0 which shows that the organic semiconductor can act as an insulating layer between the electron and hole selective contact. This effect is also discussed in paper III where we discuss the differences in the electronic junction formed in bilayer solar cell devices employing TiO2 layers prepared by spray-pyrolysis and spin-casting. The power conversion efficiency ( ), of a solar cell device is determined by measuring the J-V-curve of the device under illumination with a calibrated light source (AM 1.5, 1000 Wm-2) as shown Figure 2.5 (right). This allows the determination of the open circuit voltage (VOC) and short circuit current (JSC) from the intercept with x and y-axis, respectively. The maximum power output of the solar cell device Pmax is the maximum product of J and V. The ratio of Pmax over the power density Pin of the incident radiation is the of the solar cell device:

max SC OC

in in

P J V FF

P P (2.3)

where FF is the fill factor which is a parameter describing the ratio between the maximum power point (Pmax) and the product of VOC and JSC. Disregarding the effect of RS and RSh, equation (2.2) can be solved analyti-cally and the VOC can be estimated from the JSC and j0:

26

0

ln 1 SCOC

kT JV n

e j (2.4)

Equation (2.4) shows that the VOC is a function of the ideality factor n. It is slightly ambiguous which processes determine n in OSC and DSC. We found an increase in n with increasing absorber layer thickness, presented in paper III.

The maximum voltage of a single junction solar cell corresponds to the bandgap of the light harvesting material. However, the necessity of a ther-modynamic driving force to achieve charge separation reduces the maximum voltage that can be generated from the solar cell device.35

In OSC this is discussed in terms of the exciton binding energy EXB that needs to be overcome to generate separated charge carriers. EXB is usually considered to be in the order of 0.3 – 0.5 eV.30, 111 In DSC this corresponds to the driving forces necessary for electron injection Ginj and hole regenera-tion Greg (DSC, Figure 2.3). A Ginj of > 0.1 eV is considered necessary to achieve a high charge injection efficiency. For DSC based on the iodide/tri-iodide redox couple (Eredox = 0.35 V vs NHE) the Greg for the regeneration of the commonly used Ruthenium-sensitizers is about 0.5 – 0.6 eV.112 In total, the charge separation in DSC thus coarsely requires the reduction of the initial energy inherent in the excitons by about 0.8 eV.

The maximum VOC in OSC and DSC corresponds to the quasi-Fermi lev-els at the electron and hole selective contacts. In DSC this corresponds to the EF in TiO2 with respect to the oxidation potential of the redox electrolyte. In OSC the VOC is considered to be limited by the energy difference between the ionization potential (IP) of the donor and the electron affinity (EA) of the acceptor,30 sketched in Figure 2.3.

2.3.2 Photocurrent density and external quantum efficiency As discussed in the beginning of this chapter, the external quantum effi-ciency EQE of a solar cell device depends on the properties of the material used for light harvesting and is a wavelength dependent quantity.

The current generated in a photovoltaic device (J) can be calculated by in-tegrating the EQE with respect to the AM1.5 photon flux spectrum spec( ) as expressed in equation (2.5).30

1 5

( ) ( ) ( )spec

AM .

J V EQE ,V e d (2.5)

As the EQE is commonly determined under short circuit conditions, the in-tegrated EQE spectra should correspond to JSC. In Figure 2.7 the theoretical JSC calculated from (2.6) is shown assuming that all photons above the band-gap energy Eg give rise to an EQE of 90%.

27

Solar cells utilizing a bulk organic absorber for light harvesting are limited by the spectral LHE of the material (section 3.1), by the exciton diffusion length LXD ( XD, section 3.2), the interfacial charge separation efficiency ( CS, section 4.1) and the charge collection efficiency ( CC, section 4.2).

500 1000 1500 2000 25000

1

2

3

4

5

0

20

40

60

43 2 1

J SC(m

A c

m-2)

(1014

s-1cm

-2)

(nm)

h (eV)

Figure 2.7: Spectral photon flux ( ) of the AM1.5 spectrum and short cir-cuit current dependent on the band gap of the absorber Eg assuming an EQE of 90% for all absorbed photons > Eg.

In paper I we derived an expression for the EQE of the XDSC-1 devices investigated herein that was slightly refined in paper IV by considering the regeneration efficiency of holes created at the interface that was referred to as the geminate separation efficiency ( GS):

54

XD CT GS CCEQE LHE (2.6)

As will be discussed in chapter 3, energy transfer processes in disordered organic materials are often assumed to occur via a hopping of excitons be-tween adjacent molecules. This can be described by exciton diffusion which will be referred to as XD in this summary to distinguish this process from energy transfer processes in general, denoted with ET.

As the XDSC-2 devices comprise both an interface dye (ID) and hole-transporting dye (HTD), both dyes can contribute to the photocurrent. In paper II, we derived an expression for the overall EQE taking both contribu-tions into account:

( )ID ID HTD HTD ET IDEQE LHE IQE LHE IQE IQE (2.7)

In this system, the HTD can both inject directly into TiO2 transfer energy to the ID similar to the mechanism in DSCs comprising energy relay dyes.102,

107, 113 For the latter, the EQEHTD depends on the IQEID. This is illustrated in Figure 2.8. If the spectral response from the ID and HTD are well separated the ratio between IQEHTD and IQEID indicates whether the EQE response of

28

the HTD can originate solely from an energy transfer mechanism or if direct injection must be considered.

Insufficient regeneration of holes created at the organic/inorganic hetero-interface can become apparent in low IQE values. In some cases, comparison with e.g. a different hole-regenerating medium can give an indication whether hole-regeneration is a limiting factor to the device performance. In paper II we therefore compared the performance of bilayer hybrid solar cell devices using both the hole-transporting dye TVT-TPA and spiro-MeOTAD.

CB

ID

EX

B

CB

ID

EX

B

IQEID

HTDHTD

CB

ID

EX

B

HTD

ETTiO2 TiO2 TiO2

IQEHTDET to IDdirect inj.

Depends on IQE of ID!

CB

ID

EX

B

CB

ID

EX

B

IQEID

HTDHTD

CB

ID

EX

B

HTD

ETTiO2 TiO2 TiO2

IQEHTDET to IDdirect inj.

Depends on IQE of ID! Figure 2.8: Charge separation in ID-HTD-devices. (left) After injection, the injection dye (ID) needs to become regenerated by the hole transporting dye (HTD). The HTD can either contribute by direct injection (middle) or en-ergy transfer (right) to the EQE.

It is useful to differentiate between the individual processes to identify the bottlenecks of solar energy conversion in the solar cell devices.

In the following two chapters the processes of formation and transfer of excitons in the organic absorber (chapter 3) and interfacial dissociation of excitons into separated charge carriers (chapter 4) will be discussed.

29

3 Excitons

3.1 Light harvesting

3.1.1 Exciton formation The conversion of photon energy into electricity requires that the absorbed photon energy is intermittently retained in the absorbing material. This re-quires materials which possess an occupied lower energy electronic level which is occupied and an unoccupied higher-energy level. To avoid that the absorbed energy is entirely converted into heat the energy levels should be separated by an energetic gap. In addition, the energetic difference between these energy levels should overlap with photon energies in the visible range of the electromagnetic spectrum shown in Figure 1.1. This prerequisite is fulfilled by semiconductors and dyes. There are a couple of similarities be-tween inorganic semiconductors and dyes which are summarized in Figure 3.1 and Table 3.1.

In this section the differences between light absorption in inorganic semi-conductors and dyes are addressed. In DSC and HSC a mixed terminology for the description of energy levels is used as these device contain both or-ganic and inorganic compounds. Absorbed photon energy larger than the bandgap (semiconductor) or E0-0 transition of the inorganic semiconductor or organic material is lost through thermalization and relaxation processes on the pico-second (10-12 s) time-scale. From the relaxed excited state excitons can decay back to the ground state through radiative (photoluminescence) or non-radiative processes (Figure 3.1) which occur on the nanosecond time-scale (10-9 s)

In insulators and semiconductors the quantum mechanical state compris-ing an electron in a higher energy level and an electron vacancy – a hole - in a lower energy level is referred to as exciton.114 The electron and hole exert a coulomb force on each other, referred to as the exciton binding energy EXB.

2

04XB Coulombr

eE V

r (3.1)

Excitons are considered to be quasi-particles with no net charge which thus transport energy without transporting net charge.115 The EXB depends on rela-tive distance r between electron and hole and the relative dielectric constant

30

r of the material in which the exciton is formed. Inorganic semiconductors consist of a large number of atoms giving rise to energy bands comprising a manifold of electronic states leading to the delocalization of photo-generated electrons and holes. In addition, these materials have high r. Consequently the exciton binding energy in inorganic semiconductors is typically low (0.01 – 0.1 eV) and these types of excitons are referred to as Wannier-Mott excitons.116 The wide-bandgap semiconductor TiO2, used as inorganic com-ponent in this work has an r or 48 (anatase) and absorbs light below ~400 nm. This material is thus transparent for most of the light in the visible re-gion.

In organic semiconductors, insulators and dyes typically have low r and excitons are often localized on the same molecule or polymer chain segment. Due to the low r and small distance r, the EXB of excitons created in organic materials is typically in the order of 0.1 – 1 eV which are referred to as Frenkel excitons.117

HOMO

S2

LHE

knr

PL10

-8s10 -12s S1

E0-0

e-

h+

Exciton

PL10 -12s

VB

CB

10-8s

EgEF

EA IP

Vacuum level

Ecb

Evb

LHEe-

h+

Exciton

S0

LUMO

E

knr

HOMO

S2

LHE

knr

PL10

-8s10 -12s S1

E0-0

e-

h+

Exciton

PL10 -12s

VB

CB

10-8s

EgEF

EA IP

Vacuum level

Ecb

Evb

LHEe-

h+

Exciton

S0

LUMO

E

knr

Figure 3.1: Comparison of light absorption in inorganic semiconductors (left) and organic compounds such as dye molecules (right)

Table 3.1: Analogies & differences between (inorganic) semiconductors and dyes

Molecules Semiconductors

lower energy state (occupied states)

highest occupied molecular orbital (HOMO)

valence band (vb)

higher energy states (unoccupied states)

lowest unoccupied molecular orbital (LUMO)

conduction band (cb)

Energy gap E0-0, Eg , Eopt Band gap, Eg

Type of excitons Frenkel excitons Wannier-Mott excitons

Exciton binding energy EXB 1-0.1 eV 0.01 eV

31

3.1.2 Light absorption Sunlight is composed of electromagnetic radiation with different wavelength corresponding to a photon energy according to equation (1.1). In the UV and visible part of the spectrum (Figure 1.1) absorbed light can be converted into electronic transitions while in the IR, molecular vibration and rotations are excited. Electronic transitions lead to the formation of excitons.

Absorption of light in matter follows Lambert-Beer’s law (equation 3.2) which describes the exponential decay of light with the initial intensity I0 in a material with the absorption coefficient ( ) as a function of the thickness d, sketched in Figure 3.2 (left). In chemistry, this law is usually expressed in its decadic form and as a function of the extinction coefficient ( ) of a spe-cies in solution with the concentration c where dcuvette denotes the thickness of the cuvette the measurement is performed in.

( ) ( )0 0 10film cuvette

d cdI I e I (3.2)

In absorption spectroscopy, the transmittance T (=I/I0) of a sample is meas-ured which is often expressed as the absorbance, defined as the decadic loga-rithm of the transmittance T (=I/I0) (section 8.2.2).

0 10film

cuvette

dIA lg c d

I ln (3.3)

An important parameter for solar energy conversion, directly related to the absorbance is the light harvesting efficiency (LHE), also referred to as ab-sorptance or number of absorbed photons given by:

1 1 10 1 10 1A cd dLHE T e (3.4)

The light harvesting efficiency describes the fraction of photons harvested as a function of wavelength (or energy). The absorption coefficient ( ) of a solid sample reflects on the probability for an electronic transition to occur at that particular wavelength in the mate-rial. Practically, this is an important parameter as it allows e.g. the determi-nation of the thickness of a light harvesting material, e.g. the dye layer thickness d, from the sample absorbance. This parameter is also needed to model the exciton diffusion length LXD.

The precise determination of ( ) of the HTDs investigated herein was not trivial as this demands the precise measurement of the sample thickness on the nanometer height scale. In the course of this thesis work we tested different methods for the determination of the dye layer thickness d for the HTD TDCV-TPA. In paper I we estimated this parameter from the attenua-tion of the titanium photoelectron spectroscopy signal, in paper III we em-ployed DekTak profilometry and in paper IV we determined d by AFM pro-filometry. The latter two methods gave comparable results. In Figure 3.2 the

32

absorption coefficient spectrum of the small molecular semiconductor TDCV-TPA (structure shown in Figure 3.6) is shown. The absorption coef-ficients at absorption maximum ( ,max) determined by DekTak profilometry (section 8.2.1), were 3.2 107 m-1 for TDCV-TPA (paper III), 1.1 107 m-1 for TVT-TPA and 1.9 107 m-1 for TP-TPA (paper V). For comparison, the

,max of P3HT is in the order of 1.7 107 m-1.118, 119 A property directly related to is the light penetration depth 1/ which is

the thickness of the material where approximately 67% of the incident light has been absorbed. In dye molecules electronic transitions occur between discrete energy levels giving rise to an absorption spectrum with maxima and minima (Figure 3.2, right).

0 20 40 60 80 1000

20

40

60

80

100

d (nm)

I (%

I0)

1/

sample

I = I0e-ax

0

1

2

3

4300 400 500 600 700

Light penetration depth

(107 m

-1) Absorption coefficient

4 3.5 3 2.5 20

50

100

150

(nm)

1/(n

m)

h (eV) Figure 3.2: (left) Light absorption following Lambert Beer’s law in a sam-ple with the absorption coefficient in dependency of sample thickness d. (right) Wavelength & energy dependency of the absorption coefficient and light penetration depth 1/ in a solid film of the hole transporting dye TDCV-TPA (shown in Figure 3.6).

Compared to inorganic semiconductors (e.g. silicon), organic semiconduc-tors have large ( ). This means that less material is needed to achieve the same light harvesting efficiency. On the other hand, light absorption in or-ganic compounds leads to the formation of Frenkel-type excitons which have a stronger binding energy and therefore need an energetic offset to become charge-separated. This will be discussed further in chapter 4.

3.1.3 Injection dyes In dye sensitized solar cells, the dye chemisorbed to the surface of the wide-bandgap semiconductor via a group that can establish a chemical bond to TiO2 which is referred to as an anchor group. Many different types and families of dyes for the sensitization of TiO2 have been investigated. Among those are a family of Ruthenium-bipyridyl metal-organic complexes, like N3 120 and N719,121 that exhibit high power conversion efficiencies in DSCs.

33

As an alternative, organic sensitizers have been developed and investigated in DSC.122 44, 123 A successful design principle of organic sensitizers for DSC are the so-called donor-linker-acceptor dyes. Upon light absorption, the elec-tron density is vectorially redistributed from the donor to the acceptor part of the molecule which is beneficial for charge separation in DSC.124-126 Triphenylamines. Among the dyes investigated were a couple of triphenyl-amine-based dyes, developed in the group of Licheng Sun for the sensitiza-tion of TiO2 in DSC. In Figure 3.3:, the chemical structure of the dyes D5L0A1,125 D5L0A3,127 and D9128 are shown. D5L0A3 was the ID studied in the devices described in paper V.

N

NS

O

S

COOH

NCN

COOH

N

S

O

O

NCCOOH

D5L0A1 D5L0A3 D9

N

NS

O

S

COOH

NCN

COOH

N

S

O

O

NCCOOH

D5L0A1 D5L0A3 D9

Figure 3.3: Triphenyl-amine-based sensitizers investigated as injection dyes (IDs) in this thesis.

Squaraines are an interesting family of sensitizers for DSC as they can be tuned to absorb in the red part of the electromagnetic spectrum. Highly effi-cient squaraine-dyes have been developed for DSC.126, 129-131 Squaraines have also been used as small bandgap and small molecular semiconductors in OSC.132, 133

In this work, the two squaraine compounds shown in Figure 3.4 have been employed as IDs. Due the characteristic spectrum and absorption in the red it was easy to distinguish the contribution of SQ to the EQE from the HTD-contribution (paper II). SQ utilized as HTD in bilayer HSC-devices comprising spray-pyrolyzed TiO2 gave conversion efficiency of 0.1%.134

Squaraines have the tendency to form H-aggregates when adsorbed to the surface of meso-porous TiO2 electrodes.126 This is apparent in a band in the absorption spectrum around 600 nm which was observed both for SQ and the dye SQ02, shown in Figure 3.5 (left). The aggregates have a lower quan-tum efficiency for electron injection into TiO2. To prevent aggregates co-adsorbers such as cheno-deoxycholic acid (cDCA) are added in excess to the dye solution during sensitization.130

34

NCH

CH

O

O

N+

OH

OO

O

NCH

CH

O

O

N+

OH

O

SQ SQ02

NCH

CH

O

O

N+

OH

OO

O

NCH

CH

O

O

N+

OH

O

SQ SQ02

Figure 3.4: Squaraine sensitizers investigated as ID in this work

In paper II, we discuss that the photoluminescence signal of SQ adsorbed to meso-porous ZrO2 depends on the degree of aggregation. The same effect was observed for SQ02 with different amounts of added co-adsorber cDCA. The band around 600 nm in Figure 3.5 (left) is associated with the formation of SQ02 H-aggregates. The photoluminescence, corrected for the difference in LHE increased by two orders of magnitude with increasing cDCA concen-tration, shown in Figure 3.5 (right). This effect complicated the energy trans-fer measurements from HTD to ID, discussed in paper II.

500 600 700 8000.0

0.2

0.4

0.6

0.8

1.0

2010

0 m

M1

mM

2 m

M5 A no

rm

(nm)

40 m

M

650 700 750 800 8500

2

4

6

8

700 8000.0

0.1

0.2

0 mM1 mM

2 mM

5 mM

10 mM

20 mM

PL

(cou

nts/

107 )

(nm)

40 mM

Figure 3.5: (left) Normalized absorption spectra of SQ02 adsorbed to ZrO2 from 0.2 mM solution concentration with different added amounts of cDCA. (right) Photoluminescence spectra of the samples: A strong dependency of the photoluminescence on the degree of SQ02-aggregation was observed.

3.1.4 Hole transporting dyes In this work, organic hole-transporting materials which could act as bulk absorbers in HSCs were investigated. We focused our search on small-molecular compounds, like spiro-MeOTAD, as these materials would have to be infiltrated into the nano-porous metal oxide electrodes (Figure 2.4).

We tested different types of small molecular compounds and dyes in this devices architecture134 and found a class of triphenylamine (TPA) based molecules that exhibited promising results in solar cell devices, shown in Figure 3.6. All these compounds have been previously investigated as small molecular semiconductors in opto-electronic devices.135-137 The TPA-unit is a

35

common functionality in hole transporting materials in opto-electronic de-vices.43, 136, 138 The TPA centre introduces a certain 3D geometry that im-pedes specific molecular organization. A long-range order in bulk light ab-sorbing materials can result in anisotropic exciton and charge transport.136 Albeit the disorder of TPA based materials, high hole-mobilities have been measured in some of these compounds.139

The compounds TDCV-TPA and TVT-TPA were developed in the group of Roncali.135, 140 TDCV-TPA gave power conversion efficiency of 1.85 % in optimized organic bilayer solar cells using C60 as an acceptor.141 Results on hybrid solar cell devices with this compound are presented in paper I, paper III and paper V. The dye TVT-TPA gave = 0.41 % in a bulk heterojunction with PCBM.135 This compound was used in combination with the ID SQ (Figure 3.4) in paper II. The compound TP-TPA was devel-oped in the group of Shirota137 for optoelectronic devices. This compound was used as HTD in combination with the ID D5L0A3 (Figure 3.3) in paper V.

S

NCCN

N

S CN

CN

S

CN

NC

TDCV-TPA

N

S

S

S

S

S

S

TVT-TPA TP-TPA

S

N

S

S

S

NCCN

N

S CN

CN

S

CN

NC

TDCV-TPA

N

S

S

S

S

S

S

TVT-TPA TP-TPA

S

N

S

S

Figure 3.6: Structure of the molecular semiconductors used as hole trans-porting dyes in this work.

When adsorbing molecules on a surface130, 142, 143 or depositing them as a solid film,5, 144, 145 the optical and electronic properties can change with re-spect to the properties of molecules in solution due to a different chemical and physical environment in the solid film and specific intermolecular inter-actions. The HTDs investigated herein exhibited and unspecific spectral broadening in solid state films compared to solution spectra as discussed in papers II, IV and V.

3.2 Exciton diffusion and energy transfer Excitons generated in an organic absorber need to be transferred to an inter-face with a suitable energetic offset to become separated into free charge carriers. Exciton transfer can occur via different types of energy transfer mechanisms.

36

3.2.1 Energy transfer mechanisms There are different mechanisms for the energy transfer (ET) between an ex-cited molecule acting as energy donor (DE

*) and an energy acceptor (AE). DE and AE can either be molecules of the same or different compounds.

The most commonly referred mechanisms are the Förster resonant energy transfer (FRET)146 and Dexter energy transfer (DET).147 FRET was derived for energy transfer between two point-dipoles often described as the ex-change of a virtual photon (indicated with a double arrow connected by a spring in Figure 3.7). Depending on the spectral overlap between donor and acceptor (JDA) FRET can be efficient at distances < 10 nm. DET is described as a concurrent double electron exchange between the donor and acceptor HOMO and LUMO, respectively. This requires orbital overlap and is thus active between adjacent molecules. In samples with high optical density a third mechanism which involves emission and re-absorption of a photon is probable as well often referred to as “trivial” mechanism for energy transfer (TET). These mechanisms are schematically compared in Figure 3.7.

h

FRET:

DET:

TET:

ET: DE* + AE DE + AE*

hh

FRET:

DET:

TET:

ET: DE* + AE DE + AE*

Figure 3.7: Comparison of different energy transfer (ET) mechanisms: Förster resonant energy transfer (FRET), Dexter energy transfer (DET) and trivial mechanism (TET).

While applicable in e.g. DSC comprising energy relay dyes102, 107, 148 the FRET mechanism is not necessarily valid to describe energy transfer in de-vices comprising polymers and dye molecules in a condensed state. 100, 113, 146 In aggregated dye and polymer layers intermolecular distances are small, molecular orbitals can overlap. In addition these samples often have high optical densities. Therefore, other ET processes like DET and TET cannot be neglected.

Exciton motion through an aggregated molecular material can be de-scribed via a (incoherent) hopping mechanism from molecule to molecule or by a coherent movement of a delocalized excitation through the material.149 Strong intermolecular electronic coupling VDA and a long-range order in the

37

molecular material (crystallinity) promotes the delocalization of excited states over several molecular units. Exciton transport can in this case be very efficient with exciton diffusion length on the order of several 10s of nanome-ters, which cannot be described by a Förster-type point-to-point energy transfer mechanism.5, 146 For distinction in this work, energy transfer proc-esses in general are indicated with ET while exciton diffusion processes are indicated with XD.

In aggregated molecular materials that exhibit disorder rather than long-range order the electronic coupling VDA is considered to be low and energy transfer to occur from localized excitations on molecules in the vibrationally relaxed state via incoherent hopping to neighboring molecules, depicted in Figure 3.8. The hopping rate kXT of a localized excitation between accessible states can be described by:150, 151

21( )XT DA DAk V J E

h (3.5)

AE

DE*

ET

ET

XD

TiO2

TiO2

cb TiO2

EX

B

HTDID

long range ET

XD

ID

HTDAE

DE*

AE

DE*

ET

ET

XD

TiO2

TiO2

cb TiO2

EX

B

HTDID

long range ET

XD

ID

HTDAE

DE*

Figure 3.8: Schematic picture of exciton diffusion in a molecular aggre-gated system via a hopping mechanism.

3.2.2 Spectral overlap & photoluminescence quantum yield Equations derived for energy transfer rate often contain a factor comprising the spectral overlap (integral) of the energy donor (DE) and acceptor (AE), like JDA in equation (3.5). In this case, the overlap integral is a function of the normalized emission factor FD(E) of an energy donor DE and the normal-ized absorption factor AA(E) of an energy acceptor AE:151

( ) ( )DA D AJ F E A E dE (3.6)

3 1

3 1

( ) ( )with ( ) and ( )

( ) ( )D A

E PL E E EF E A E

E PL E dE E E dE

For energy transfer processes between dye molecules of the same compound the spectral self-overlap integral should determine kXT. The spectral overlap is, however, very difficult to evaluate for a dye or organic semiconductor in

38

the solid state as the absorption and emission spectra are affected by the physical state of the material. We comment on the changes in the spectral properties of the compounds between the (solid) solution spectra and the solid films in paper II, paper IV and paper V.

In Figure 3.9 the AA and FD for the dye TDCV-TPA measured in a PMMA matrix and of a solid film were evaluated according to equation (3.6). We find a progressive red-shift for the photoluminescence of TDCV-TPA in PMMA with increasing molar concentration. The absorption spec-trum was not affected and the absorbance of the investigated samples was below 0.1 to avoid TET effects.152 The red-shift in the PL can be caused by spectral diffusion145 due to excitation energy transfer between molecules within the film to lower-energy sites.153 Due to this red-shift of the photolu-minescence the JDA apparently decreases by an order of magnitude between spectra of the PMMA (1 mol) and the solid film shown in Figure 3.9. How-ever, as this effect is caused by exciton diffusion the decreased JDA for the solid film has no significance for kXT (eq3.5). On the contrary: spectral diffu-sion could be considered an indication for exciton diffusion within a mate-rial.

0

1

2

3

0

1

2

3

3.5 3.0 2.5 2.0 1.50

1

2

3

0

1

2

3

50m

ol

1 mol

J = 0.037 eV-1

film

@PMMA

J = 0.35 eV-1

1m

ol

AA

F D *

AA(e

V-2)

F D a

nd A

A(e

V-1)

E (eV)

FD

Figure 3.9: The spectral overlap (filled) of TDCV-TPA is substantially smaller for a solid film compared to a dispersion in a PMMA matrix. For higher concentrations of TDCV-TPA in PMMA ( mol/gPMMA) the emis-sion was red-shifted.

A factor that determines the kET in FRET is the photoluminescence quantum yield ( PL). This is the amount of photons absorbed by a sample converted into emitted photons and can be determined by measuring the photolumines-cence of a sample (PLS) with respect to a reference sample (PLref) of known photoluminescence quantum yield ( PL,ref ).

154

2

2

S ref SPL PL,ref

S refref

PL LHE n

LHE nPL (3.7)

The measurement needs to be corrected for differences in the light harvest-ing efficiencies of sample and reference as well as differences in the refrac-

39

tive indices n of the solvents the measurements are carried out in. The PL depends on the physical properties and environment of a compound and reflects the degree of deactivated PL through non-radiative processes.

For the samples shown in Figure 3.9 the PL was found decreased by 85 % for thin film of TDCV-TPA on glass and even more for thicker dye layers using the photoluminescence of a dilute sample in PMMA (1 mol/g) as reference ( PL,ref). The PL can be affected a lot by environmental factors. The PL of a HTD as a solid film is much reduced compared to the PL of the compound in a solid solution (e.g. PMMA matrix as in paper IV & paper V). As the decrease in PL can be caused by exciton diffusion, the PL measured for a solid film does not reflect on the PL of donor molecules in the film that might undergo energy transfer to an acceptor via a FRET. This illus-trates that the formalism for energy transfer derived by Förster cannot be directly applied to describe energy transfer in solid state materials.

3.2.3 The exciton diffusion length In XDSC-1 devices we assumed exciton diffusion as the underlying mecha-nism for energy transfer between molecules, depicted in Figure 3.8. An im-portant parameter for excitonic solar cells is the exciton diffusion length LXD which is the distance an exciton can diffuse during its limited lifetime X defined as:

XD ED X XL L D (3.8)

Exciton diffusion to a charge-separating interface is a bottleneck of solar energy conversion in excitonic solar cells.155 The LXD in molecular materials without long-range order is typically in the order of 10 nm.119, 156, 157

There are several methods to determine the exciton diffusion length which involve both the fit of time-resolved methods5, 153, 157 and steady-state meth-ods.153, 158 Excitons reaching the charge-separating interface can be investi-gated by probing charge carriers generated following interfacial charge-separation. This can be determined from the generated photocurrent meas-ured e.g. in EQE measurements158-161 or by an increase in micro-wave-conductivity measured for electrons injected into e.g. TiO2.

73, 119 Alterna-tively the surface photovoltage162 or the quenching of the photolumines-cence5, 157, 158, 163 can be used to probe the number of excitons reaching the hetero-interface.

In the simplest configuration to determine the exciton diffusion length is by investigating bilayer samples with varying thickness of the light harvest-ing material on the quenching substrate. More advanced models to determine the exciton diffusion length in a 3D geometry have been reported in litera-ture.83 Alternatively to varying the thickness of the light harvester, the spac-ing between exciton quenching sites can be varied.164

40

There are cases where exciton diffusion is found to be anisotropic due to the specific inter-molecular interaction and long-range organization.6, 151, 165 It might therefore be necessary to control the morphology of the absorber layer. This can be achieved by molecular self assembly. Ideally the molecu-lar orientation is in such direction that excitons are transferred towards the charge-separating interface. In disordered organic materials the LED is often quite small.158

Sometimes conflicting result for LXD for one and the same material can be found in literature. The experimental values depend on the experimental conditions. The possibility of (Förster) resonant energy transfer (FRET) to the quencher (as observed for e.g. P3HT and C60) can give rise to a coarse over-estimation of the LXD.156, 163, 166 At the same time, FRET can be utilized for increased exciton harvesting as excitons can be collected further away from the charge separating interface.153, 167 The LXD can also be underesti-mated in cases, where the CT is much lower than 100% and the exciton dis-sociation becomes limited by CT (capture limited).5, 153

LXD from EQE-measurements. The exciton diffusion length, LXD, can be derived from external quantum efficiency (EQE) measurements following the exciton diffusion model introduced by Ghosh and Feng.168 This model is based on solving the diffusion equation for excitons created in a bulk ab-sorber layer following a generation profile G(x).

2

2

( )( )

dn x Dd n nG x

dt dx (3.9)

Expressions for G(x) taking the effect of reflecting contacts and interference into account have been derived.169 As the graphite back contacts employed in the device architecture were not very reflective a simple generation profile following Lambert Beer’s law (equation 2.1) was used. Solutions of (eq3.8) presented in literature were used for the evaluation of the fraction of excitons reaching the interface119, 161, 170 for different boundary conditions summarized in the supporting information of paper I. Kroeze et al.119, 171 treated the inter-facial charge transfer efficiency CT as an independent parameter. In paper I we raised the question whether the non-unity CT is a parameter that would need to be taken into account in the diffusion equation. There are some ap-proaches that take a limited interfacial exciton separation into account by introducing a capture rate K or similar.153 5

LXD from PL-quenching. Alternatively, the LXD was determined from

steady-state photoluminescence, PL, quenching experiments.70, 153, 157 The PL of a sample on a quenching substrate (PLS) is compared to the PL on a non-quenching substrate (PLref) expressed in (3.10) and schematically shown in

41

Figure 3.10. In these measurements the PL signal should be corrected for the relative difference in the number of absorbed photons (= LHE).

1S ref

quenchingSref

PL d LHEPL

LHEPL d (3.10)

quencher

dye/polymerd

h

h

hv’ hv

’

PLrefPLS

substratequencher

dye/polymerd

h

h

hv’ hv

’

PLrefPLS

substrate

Figure 3.10: Schematic picture depicting the principle of PL quenching measurements.

3.2.4 The energy transfer efficiency In the XDSC-2 devices the HTD acts as a light harvesting antenna and hole regeneration medium to the injection dye (ID). The possibility for long-range energy transfer rather than pure exciton diffusion can give rise to an in-creased exciton harvesting efficiency in these type of devices.156, 163, 166, 167 Devices combining conducting polymers with IDs is an interesting merge between solid state DSC and polymer/titania HSC.78, 89, 94-101, 172-175

In these devices we did not distinguish between exciton diffusion and other energy transfer processes. Instead all mechanisms leading to an energy transfer from the HTD to the ID are summarized in the energy transfer effi-ciency ET. The contribution of the HTD to the EQE via energy transfer can be described with a similar formalism as derived for the energy-relay-dye system:102, 148

, ,HTD ET HTD ET ID ETEQE LHE IQE (3.11)

This is included in equation (2.7) derived for contribution from the HTD to the photocurrent in XDSC-2 via energy transfer.

Qualitatively, the quenching of the photoluminescence of the polymer or HTD by the ID can indicate the possibility for energy transfer. These meas-urements are often carried out in solution or a matrix like PMMA.100 The Förster model for energy transfer needs adaptation to treat energy transfer between an assembly of donor and acceptor molecules in these type of de-vices. As ET occurs to a layer of ID’s rather than single acceptor molecules the ET can be very efficient.148

42

In paper II we estimated the excitation energy transfer from the HTD to the ID from steady-state photoluminescence measurements in a sample geome-try that would resemble the device architecture. Instead of TiO2 meso-porous ZrO2 was used as photoluminescence quenching via electron injection should be less probably on ZrO2 due to the higher lying conduction band. Steady state photoluminescence quenching measurements were carried out using meso-porous ZrO2 sensitized with an ID and the HTD deposited by spin-casting. Reference samples comprising the ID or HTD were only meas-ured under the same conditions and corrected for the difference in LHE (equation 3.9). The energy transfer was evaluated by comparing the emission spectra as well as the excitation spectra of the sample comprising both dyes with reference samples. Emission spectra are recorded by exciting the sam-ples at a fixed excitation wavelength and measuring the photoluminescence at different emission wavelengths. This is illustrated in Figure 3.11 (top). Alternatively, excitation spectra can be measured where the emission at a fixed wavelength is monitored while scanning through different excitation wavelengths shown in Figure 3.11 (bottom). From the ratio between direct excitation of ID and indirect excitation via the HTD the energy transfer effi-ciency ET ( XT in paper II) from the HTD to ID can be estimated.

0

1

2

3

400 500 600 700 800

0

2

4

x106

De

tec

tio

n

PL

(cou

nts)

Ex

cita

tio

n

Detection

ET

Excitation

x108

PL

(cou

nts)

(nm)

N

CH

CH

O

O

N+

O

OO

O

N

S

S

S

S

S

S

ET

h·

SQ

TVT-TPA

h· ’

N

CH

CH

O

O

N+

O

OO

O

N

S

S

S

S

S

S

ET

h·

SQ

TVT-TPA

h· ’

Figure 3.11: (left) Comparison of the LHE spectra and the photolumines-cence emission (middle) and excitation spectra (bottom) for samples com-prising ID, HTD or both. (right) Schematic representation of the principle behind these measurements: The emission of the HTD is quenched because of energy transfer (ET) to the ID. The signal of the ID increases as a conse-quence.

43

4 Electrons & Holes

4.1 Charge carrier separation As established in chapter 2, the second step in the conversion of solar energy into electricity is the conversion of excitons into separated charge carriers. In the previous chapter it was discussed that the exciton binding energy in or-ganic semiconductors and dyes is large. Charge separation in solar cell de-vices comprising organic light harvesting materials requires therefore an energetic offset.13, 14

The interfacial charge separation mechanism in HSCs can be expected to be governed by analogous factors as the charge separation efficiency CS in organic solar cells35 and dye sensitized solar cells.38, 112 The interfacial charge separation efficiency is a function of the interfacial charge transfer35 or charge injection efficiency44 ( CT or inj), a factor describing the effi-ciency of geminate electron-hole pair separation54 or regeneration of holes created at the interface44 ( GS or reg) and the collection efficiency of sepa-rated electrons and holes ( CC) which will be discussed in section 4.2.

CS CT GS CC inj reg CC

in OSC in DSC

(4.1)

4.1.1 Photo-induced charge transfer The first step in interfacial charge-separation is a photo-induced electron transfer step between an electron donor De- and electron acceptor Ae- For an organic molecule at an inorganic semiconductor junction this can be treated with formalism derived by Gerischer.38 In this the interfacial charge transfer rate kCT at semiconductor electrodes is expressed as a function of the overlap between the populated excited states of the donor WD(E) and the density of accessible, unoccupied states DOSA(E) on the electron acceptor given in equation (4.2) and illustrated in Figure 4.1.

( ) ( ) ( ) ( )CT A Dk E E DOS E W E dE (4.2)

The term (E) is the transfer frequency which is proportional to the elec-tronic coupling VDA

2 between the De- and Ae- The electronic coupling VDA depends on the distance, orientation and orbital overlap between De- and Ae- and will thus be determined by the specific interaction at the or-

44

ganic/inorganic heterojunction. In the case of the HTDs investigated herein, the lack of a chemical functionality that promotes the formation of a chemi-cal bond to TiO2 made it difficult to presume a preferential molecular orien-tation at the hetero-interface.

In paper IV we tried to draw conclusions for the specific interaction and preferential orientation of the hole transporting dye TDCV-TPA at TiO2 interfaces from shifts observed in PES measurements and IR-measurements (ESI, paper IV). This information in combination with images on the elec-tron density distribution in the excited state acquired from time-dependent DFT-calculations allows the discussion of possible surface interactions and conformations.

At the investigated TDCV-TPA/TiO2 junction we observed an excitation-energy dependent IQE and photoluminescence quenching. In analogy with other examples in literature176, 177 this effect could have been caused due to “hot injection” from higher electronic or vibronic transitions.178 We therefore investigated the energetic alignment at this hetero-interface in detail.

DOS

Dye*

WD

semicond.

Dye

semicond.

E0-0

EcbHTM

Greg

Ginj

Evb

EE

HOMO

LUMO

HTMEF EF

vb

cb D*

D

D+

DOS

Dye*

WD

semicond.

Dye

semicond.

E0-0

EcbHTM

Greg

Ginj

Evb

EE

HOMO

LUMO

HTMEF EF

vb

cb D*

D

D+

Figure 4.1: Electron injection from an excited dye molecule to a semicon-ductor followed by regeneration by a hole transporting medium (HTM) in a Gerischer Diagram representation (left) and a simplified diagram depicting energy levels or band-edges in a DSC-type device (right).

4.1.2 Geminate electron-hole pair separation GS In OSC the formation of coulombically bound geminate electron-hole pairs after a charge-transfer event at a hetero-junction is considered to determine the charge separation efficiency in these solar cell devices.35 These types of interactions might also play a role in the charge carrier generation process in HSC and should be treated as a separate parameter such as the geminate separation efficiency ( GS).

54 The formation of ground-state CT-complexes has also been observed at the hybrid P3HT/TiO2 interface.179

45

As there is a manifold of states available both for the photo-generated elec-trons in TiO2 and holes in the organic layer, charge separation is an entropic favorable process.35 Once the coulomb potential of electron and hole is over-come the gradients in the electrochemical potential in the respective phase promotes charge carrier drift and diffusion away from the interface.13, 14 In this respect using a hole transporting or redox relay, like in the DSC, with a positive driving force for regeneration of holes created at the interface is beneficial for rapid charge carrier separation.

In paper IV, we concluded that the observed excitation energy dependent IQE in paper IV could be caused by a more efficient regeneration of “higher energy” holes created at the hetero-junction. Unfortunately, in the XDSC-1 comprising the HTD only we could not distinguish between oxidized HTD directly adjacent to the hetero interface and “holes” located in the bulk of the material due to their identical spectroscopic signature.

4.1.3 Energy level alignment To evaluate the thermodynamic driving forces for interfacial charge separa-tion energy level diagrams as shown in Figure 4.1 are invoked. For the proc-ess at the hetero-interface in the hybrid solar cell devices investigated in this work, the excited state of an ID or HTD relative to accessible electronic states in the TiO2 conduction band are of interest. The derivation of energy level diagrams is not trivial.

The excited state of the organic compound is usually determined with re-spect to the HOMO energy level. The latter is often derived from the stan-dard oxidation potentials determined by electrochemical methods carried out in a solution environment. While relevant for e.g. liquid DSC112 these values have limited significance for solid state hetero-junctions. Preferably, energy levels should be measured under similar physical conditions as present in the device by employing experimental methods like photoelectron-spectroscopy (PES).180-183 This allows the determination of molecular energy levels with respect to e.g. the Fermi level in TiO2 and accounts for shifts in energy levels due to the specific interaction between the components like molecular band bending, charge transfer and molecular re-orientation.180, 184, 185 However, the absence of oxygen in high vacuum can have an effect on the surface compo-sition of metal-oxides186 which also can affect the interfacial energy level alignment.180

TiO2 is an intrinsic n-type semiconductor with a high relative dielectric constant and electron mobility. This makes TiO2 an interesting compound as electro-selective and conducting component in dye-sensitized solar cells44, 187 and hybrid solar cells.50, 51 For interfacial charge transfer it is the density of accessible states in the TiO2 which are relevant. These can be experimentally determined using X-ray absorption spectroscopy188 or theoretical calcula-tions.189

46

The conduction band edge EC can be estimated from the flatband potential Vfb using (eq4.3) which can be measured by capacitance-voltage (C-V) measurements on e.g. a semiconductor/liquid junction.190

B Dcb cb fb

C

k T NE / e V V ln

e N (4.3)

In paper III we determined the Vfb for spray-pyrolyzed TiO2 to be -0.5 V vs NHE from C-V measurements of the TiO2 films on FTO substrates in con-tact with an inert electrolyte. The Vfb was determined from the Mott-Schottky relation:191

2

2 20

2 Bappl fb

D r

A k TV V

C N R e e (4.4)

where 0 is the vacuum permittivity, kB the Boltzmann constant, T the tem-perature, A the geometric area of the substrate. There are many pitfalls when performing Mott-Schottky measurements.192 We found that the sub-strates had to be free from any defects and it was not possible to determine the Vfb of porous semiconductor layers where the conducting substrate is in partial contact with the electrolyte.193 In paper III the results from Mott-Schottky measurements performed on spin-cast TiO2 films led us to con-clude that these films were not pinhole free. Differences in the substrate roughness R191, 194 could account for different slopes found for the Mott-Schottky response associated with the FTO substrates which could have been misinterpreted as differences in doping density ND.

For spray-pyrolyzed TiO2 the Vfb was determined to be -0.5 V vs NHE (at pH 7) in close agreement with values reported elsewhere.195, 196 The doping density of the spray-pyrolyzed TiO2 was determined to be 1.3 1017 cm-3. Assuming a density of states NC of 1019 cm-3 the conduction band Ecb is cal-culated to be 112 mV more negative than the Vfb using equation (4.3). In papers I, II, III and V we estimated the energetic alignment at the or-ganic/inorganic hetero-junctions from electrochemical measurements carried out in a solution environment and optical measurements on solid films or dyes adsorbed to a meso-porous metal oxide substrate. In paper IV we inves-tigated the energy level alignment at the hetero interface between the HTD TDCV-TPA and TiO2 in more detail using a combination of PES, UV-Vis and DFT-calculations. The HOMO energy level measured with PES was more than 0.5 eV lower in energy compared to the redox potential measured in solution (see discussion in paper IV). In addition, we observed a shift in the signal for a sub-monolayer of TDCV-TPA compared to the bulk dye layer which indicated a specific interaction and orientation if dyes at the hetero-interface.

47

4.1.4 Charge separation in steady-state Excitons can become charge separated via oxidative electron transfer to a material with a higher electron affinity (EA), denoted as electron acceptor (Figure 4.2, left). Alternatively, excitons can be quenched by a reductive electron transfer from a material with a higher ionization potential (IP) acting as a hole acceptor.55

In paper I we found the photoluminescence of TDCV-TPA quenched by PEDOT:PSS. To investigate whether excitons become separated into free charge carriers at the PEDOT:PSS rather than the TiO2 interface we per-formed EQE-measurements both from the front-side and back-side of the device using transparent back-contacts, illustrated in Figure 4.2 (left). For illumination from the PEDOT:PSS side (back-side) of the device we ob-served an EQE response (Figure 4.2, right) which did not resemble the LHE spectrum. Instead we found minima in the EQE spectrum where the LHE spectrum exhibited maxima. This is referred to as an antibatic response63 as the organic layers far from the charge separating interface act as a filter for the incident light. This proved that in the investigated devices, charge sepa-ration occurs at the TDCV-TPA/TiO2 interface.

h+,inj

e-,inj

Electron contacte.g. TiO2

HTD Hole contact e.g. PEDOT:PSS

Front-sideillumination

Exciton diffusion

h+,inj

e-,inj

Electron contacte.g. TiO2

HTD Hole contact e.g. PEDOT:PSS

Front-sideilluminationFront-side

illuminationExciton diffusion

400 500 600 7000

1

2

3

4

5

6

EQE bssimulated back-side

illumination

EQ

E (

%)

(nm)

front-side

illumination

simu

lated E

QE

fs

Figure 4.2 (left) Illustration of front-side and back side illumination of a so-lar cell device. (right) Experimental results (paper I) proving that charge separation occurs at the TiO2/TDCV-TPA interface.

In solar cell devices using a bulk organic absorber for light harvesting the exciton diffusion length LXD has an impact on the internal quantum effi-ciency (IQE). Under certain conditions, e.g. for dye layers thin enough to neglect limitations arising from exciton diffusion and charge carrier mobil-ity, the interfacial charge separation efficiency CS is reflected in the IQE of the solar cell device. This is shown in Figure 4.3 (left) and is discussed in paper IV.

(1 )CS

EQEIQE R

LHE (4.5)

48

0

50

100

b

a1-

PL S

/PL re

f (%)

5 nm

(nm)

IQE

(%) 2 nm

400 500 6000

50

100

unmodified

with NO2-bzac

0

5

10

0

10

20

30

40

400 500 600 7000

20

40

SQ

LHE

(%)

EQ

E (%

)

TVT-TPA

16 %

IQE

(%)

(nm)

30 %

Figure 4.3: (left) Interfacial charge separation at the TiO2/TDCV-TPA in-terface investigated by the IQE (a) and wavelength dependent PL-quenching (b). (right) From EQE/LHE=IQE the relative conversion effi-ciency of light harvested in the ID and HTD can be estimated determined for XDSC-2 type devices.

In the devices comprising both ID and HTD it is of interest to compare the IQE of each constituent dye as shown in Figure 4.3 (right). In the XDSC-2 devices the HTD can contribute to the photocurrent both via electron injec-tion and energy transfer. For the latter, the EQEHTD depends on the IQEID as described by equation (2.7). Comparison of IQEHTD and IQEID gives a first indication whether conversion exclusively via energy transfer is possible. In the example shown in Figure 4.3 (right, bottom) the IQEHTD is larger than the IQEID which means that direct or mediated electron transfer from the HTD to TiO2 gives an important contribution to the photocurrent.

The photoluminescence of a light harvesting material in presence and ab-sence of a quenching interface can reflect on the interfacial charge separation efficiency CS (see section 3.2.3). We used this method as an alternative to the IQE measurements to investigate CS in paper IV. Also in these meas-urements we found an excitation-energy dependence of the photolumines-cence quenching for TDCV-TPA on TiO2 substrates with respect to plain glass substrates, shown in Figure 4.3 (left, b). The shape and trend in the data closely resembled the excitation-energy dependence observed in the IQE (Figure 4.3 left, top) which is another indication of wave-length de-pendent electron-transfer at this hetero-interface. Interestingly, TiO2 sub-strates modified with nitro-benzoic acid (NO2-bzac) exhibited no wave-length-dependence and an overall more efficient PL-quenching.

4.1.5 Transient absorption spectroscopy The term transient indicates measurement techniques in which short-lived species are generated through a perturbation of the system by an external

49

stimulus. In transient absorption spectroscopy the evolution of species gen-erated following e.g. a photo-induced process is investigated.

Injection from a photo-excited sensitizer into TiO2 can be a very fast process occurring on the femto-second time scale.44 On a qualitative level the photo-induced charge transfer from a dye to TiO2 can be investigated using slower transient pump-probe spectroscopic techniques such as photo-induced absorption (PIA) and LASER flash-photolysis (LFP) spectroscopy. Following electron injection dye molecules become oxidized giving rise to absorption of the transiently generated species (ATS). As there are thus less molecules in the ground state to absorb light a ground-state bleach (-AGS) appears at the absorption maximum of the dye.197 In systems like DSC spec-tral features caused by a Stark effect after photo-induced charge separation can obscure the transient spectra.198, 199 The difference in the absorption spec-trum A is thus:

GS TS StarkA A A A (4.6)

For the identification of short-lived species their spectral signatures need to be obtained using complementary methods like spectro-electrochemistry. In this method the absorption spectrum is measured while oxidizing or reducing the compound electrochemically.

Monochromator

D

Currentamplifier

Lock-in amplifierHz

Pump

Probe

-+=

Monochromator

D

Currentamplifier

Lock-in amplifierHz

Pump

Probe

-+=

-2

0

2

400 500 600 700 800 900-4-2024 D5L0A3

+ TPTPA

blu

e L

ED

D5L

0A3

AD5L0A3

(nm)

LA

SE

Rg

reen

ATPTPA

A (1

0-5)

A (1

0-5)

Figure 4.4: (left) Schematic diagram of the photo-induced absorption (PIA) experimental setup, adapted from ref200.(right) PIA-spectrum of TP-TPA on TiO2 using a blue LED or a green LASER as excitation-pump light source. (bottom) PIA spectra of ID D5L0A3 on TiO2 with and without the HTD TPTPA.

In photo-induced absorption (PIA) (experimental setup sketched in Figure 4.4, left) the absorption spectrum perturbed by an additional excitation pulse from a light source (LED or LASER) is measured with respect to the spec-trum without the additional perturbation. In the transient spectra A species such as the oxidized sensitizer dye in DSC after photo-induced electron transfer to TiO2 can be observed.200, 201

50

Transient absorption measurements were also carried out using a nano-second LASER flash-photolysis setup. In this time-resolved spectroscopic method a nanosecond LASER pulse which can be tuned over a wide wave-length range is used as the pump light source. These measurements were carried out in paper V to investigate the regeneration of the ID D5L0A3 by the HTD TP-TPA.

In Figure 4.4 (right) examples for PIA spectra measured for the dye TP-TPA on TiO2 for excitation with the blue LED and green LASER is shown. For the latter no signal from oxidized TP-TPA due to photo-induced elec-tron transfer to TiO2 is observed. The graph below shows an example for a PIA-spectrum of an ID (D5L0A3) on TiO2 with and without HTD. The de-crease in the D5L0A3 bleach and increase in oxidized TP-TPA signal indi-cates regeneration of holes on D5L0A3 created after photo-induced electron transfer to TiO2.

4.2 Charge collection

4.2.1 Charge transport & recombination Once electrons and holes have become separated at the hetero-interface they have to be transported to their respective electrodes. These transport proc-esses depends on the electron ( e-) and hole-mobility ( h+) in the respective phase. As the Fermi-level in e.g. an n-type semiconductor like TiO2 is very close to the conduction band, the h+ is very low and the material can thus be used as an electron-transporting and selective contact. In organic semicon-ductors charge carriers affect the local geometry through their electric field giving rise to polarons that move through the material between molecules or chain segments through hopping.202

The charge collection efficiency ( CC) is thus affected by e-, h and by the recombination rate (krec) of separated electrons and holes. In chapter 2 we defined the voltage of a solar cell device to be determined by the quasi-Fermi levels of electrons and holes in their respective phases. Recombination occurring on a comparatively long time scale can be investigated by follow-ing the decay of the open circuit voltage VOC after switching off an illumina-tion source. In Figure 4.5 an example for the VOC decay of bilayer HSCs built with spray-pyrolyzed and spin-cast TiO2, discussed in paper III, is shown. Due to insufficient blocking of the FTO contact, the VOC decay oc-curs on a faster time-scale for the spin-cast TiO2 devices.

Recombination depends on the different interfaces between components in the solar cell junction. An increase in interfacial area can give rise to a higher photocurrent due to more charge-separation sites but can also increase the number of sites where recombination can occur. Electrical shorts be-

51

tween e.g. the hole and electron contacts and the energy level alignment at the electric contacts also affect the recombination dynamics of the device.

krec

CC

,h+

,e- HTD

TiO2

HTD

0.001 0.01 0.1 1 100.0

0.2

0.4

0.6

0.8

VO

C (V

)t (s)

EF,TiO2

EF,HTD

,e-

,h+

rec

VOC

krec

CC

,h+

,e- HTD

TiO2

HTD

0.001 0.01 0.1 1 100.0

0.2

0.4

0.6

0.8

VO

C (V

)t (s)

EF,TiO2

EF,HTD

,e-

,h+

rec

VOC

Figure 4.5: (left) Recombination in HSC can be monitored by following the time-evolution of the VOC after switching off illumination (middle). (right) Charge carrier diffusion (dashed) and recombination (arrow) in nanostruc-tured hybrid solar cells.

4.2.2 Morphology The preparation of organic/inorganic HSCs with a well-defined morphology and optimized dimensions for exciton harvesting and charge carrier transport in the organic and inorganic phase is a major scope in HSC-research.53, 54, 89,

90, 203-205 When preparing nanostructured HSCs following the infiltration approach

(Figure 2.4) the pore-filling fraction becomes a limiting factor to the device performance.78, 206-211 A high pore filling fraction can facilitate the separation of geminate electron-hole pairs.208

Preparation strategies to infiltrate a nanostructure inorganic semiconduc-tor with an organic semiconductor include spin-casting,209 melt infiltra-tion212, dip-coating68 and in-situ polymerization.213, 214 Pore-filling has been investigated by means of PIA-spectroscopy,206, 210 photoelectron-spectroscopy (PES)-depth-profiling,207 dissolving the hole transporting me-dium and measuring the solution concentration208 and investigation of a cross-section with energy-filtered transmission electron microscopy.78 Even in partially filled pores hole-regeneration and transport might still be effi-cient if the pore walls are covered or if hole transport via the sensitizer dye in sDSCs compensates for insufficient contact to the hole transporting me-dium.206, 208

The morphology of nanostructured HSCs can be investigated using meth-ods like scanning electron microscopy (SEM) or atomic force microscopy (AFM). These methods were employed to characterize morphological differ-ences between spray-pyrolyzed and spin-cast TiO2 substrates in paper III. The porosity of a material can be investigated with gas adsorption, SEM, transmission electron microscopy (TEM) and TEM tomography.215 The latter

52

has been used to visualize the domain size in ZnO/P3HT HSC devices pre-pared following the BHJ-blend-approach.83

Figure 4.6 shows the cross-section SEM image of a nanostructured HSC device comprising a ~ 450 nm thick meso-porous TiO2 film that was pre-pared by spin-casting a diluted TiO2 nanoparticle paste (described in section 8.1) onto FTO substrates that were covered with a dense layer of TiO2 using spray-pyrolysis (section 8.1). The HTD TDCV-TPA was deposited via spin-casting from a 20 mM solution and the devices were contacted with PE-DOT:PSS and graphite as described in section 8.1. The individual layers can be distinguished in the SEM cross-section image shown below.

spray-pyrolyzed TiO2

FTO

meso-porous TiO2

TDCV-TPAPEDOT:PSS

graphite


FTO

meso-porous TiO2

TDCV-TPAPEDOT:PSS

graphite

Figure 4.6: (left) SEM cross section image of a nanostructured HSC com-prising a ~ 450 nm thick meso-porous TiO2 layer on spray-pyrolyzed TiO2 infiltrated with TDCV-TPA and contacted with PEDOT:PSS and graphite powder.

AFM-measurements can provide valuable information on the surface topog-raphy of a sample. In Figure 4.7 the 3D-AFM images generated using the Nanotec WScM 5.0 software216 for the spin-cast and spray-pyrolyzed sam-ples compared in paper III are shown. There is an obvious difference in sur-face morphology of the two substrates which led to distinctively differences in the device performance.

Figure 4.7: 3D-AFM images comparing the surface topography of spray-pyrolyzed TiO2 (left) and spin-cast TiO2 (right) on FTO substrates.

53

5 Results for XDCS-1

This chapter summarizes the results obtained for TDCV-TPA/TiO2 hybrid solar cell device are subject of:

paper I (TDCV-TPA dye multilayers on TiO2) paper III (Comparison of spray-pyrolyzed and spin-cast TiO2) paper IV (Interfacial energetics at the TDCV-TPA/TiO2 interface)

This chapter also contains unpublished data on interface modification and nanostructured TDCV-TPA/TiO2 HSC.

5.1 Dye multilayers on TiO2 The first project within this thesis work was the investigation of dye multi-layers on dense titanium dioxide films reported in paper I. We found that a spray-pyrolyzed TiO2 layer prepared at least in 4 spray cycles (Figure 2.6) was necessary to avoid electrical short-circuits in the devices (paper I).

In Figure 5.1 (left) the current-voltage (J-V) curves of TDCV-TPA/spray-pyrolyzed TiO2 bilayer hybrid solar cells with varying dye layer thickness d under 1 sun (AM 1.5, 1000 Wm-2) is shown. The graph on the right shows the corresponding spectral response of the external quantum efficiency, EQE. The data presented in Figure 5.1 is for a series of bilayer hybrid solar cells built with spray-pyrolyzed TiO2 and varying dye layer thickness d. This data is in part discussed in paper III and was used to derive the IQE data discussed in paper IV.

The same trends with increasing d was observed for the devices reported in paper I and presented in Figure 5.1 but the maximum efficiency of the devices in paper I was 0.27 % while for the devices in Figure 5.1 attained up to = 0.47%. This could be attributed to differences in the dye batches of TDCV-TPA used for the preparation of these devices.

As discussed in paper I, the EQE and thus the photocurrent of the devices increases with d until limited by the exciton diffusion length LXD which will be discussed in more detail in the next section. For dye layers exceeding 30 nm a reduction in JSC was observed which can be explained by limited hole-transport through the TDCV-TPA layer.

The fill factor decreases with increasing d as hole transport in the TDCV-TPA layer becomes a limiting factor, an effect which has been observed in other hybrid devices as well.217 The increase in current at positive applied

54

potentials Va indicates that for thicker dye layers a higher voltage is needed to establish the same interfacial electric field necessary for exciton dissocia-tion and/or hole transport through the dye layer.65

A striking effect is the increase in open circuit voltage VOC with increas-ing d reaching a saturation value around 0.95 V for d exceeding 20 nm (pa-per I & paper III). This corresponds approximately to the difference in quasi-Fermi level between electrons in titania and holes at the hole-collecting contact with PEDOT:PSS. As we investigated in paper IV the energy alignment at the heterojunction shifts to higher binding and the in-creasing VOC is thus not due to a molecular band-bending effect218. Instead, as discussed in paper III, for incomplete dye coverage the VOC is influenced by the PEDOT:PSS/TiO2 contact.

0.0 0.5 1.00.0

0.5

1.0

1.5

71 nm

30 nm

17 nm

15 nm

9 nm

5 nm

J (m

A c

m-2)

V (V)

2.5 nm

400 500 600 700

0

5

10

15

71 nm

30 nm

17 nm15 nm9 nm

5 nmEQ

E (%

)

(nm)

2.5 nm

Figure 5.1: (left) Current-voltage, J-V, curves in dependency of dye layer thickness d (indicated in figure) for bilayer HSC on spray-pyrolyzed TiO2 layers. (right) Spectral response of the external quantum efficiency, EQE, for devices with different dye layer thickness d (indicated in figure).

5.2 Exciton diffusion in TDCV-TPA As discussed in section 3.2.3, exciton diffusion is an energy transfer process of vibrationally relaxed excited dye molecules transferring their excitation energy to nearby molecules. Both in paper I and paper III we determined the exciton diffusion length (LXD) in TDCV-TPA by analyzing the EQE in de-pendency of d using the solutions of equation (3.8) derived by Kroeze et al.119 which are included in the electronic supporting information (ESI) of paper I for different illumination directions and different boundary condi-tions. The EQE in dependency of d and the fits to the experimental data are shown in Figure 5.2 (left). The values for LXD and the interfacial charge transfer efficiency CT are summarized in table 5.1.

55

For the devices analyzed in paper III the LXD was longer. This can be attrib-uted to the difference in the absorption coefficient determined for TDCV-TPA in the two studies. In paper I, was determined from PES (table 5.1) and also estimated from the solution extinction coefficient reported else-where (see ESI). In paper III was determined from DekTak Profilometry (table 5.1) and in paper IV from AFM-profilometry. The latter two methods gave very similar results. This illustrates that the uncertainty in LXD depends on the method utilized for the determination of the dye layer thickness.

Table 5.1: Absorption coefficients and exciton diffusion length LXD and interfacial charge separa-tion efficiency CT determined for different boundary conditions presented in paper I and paper III.

boundary cond. (m-1) LXD, 520 nm CT,520nm LXD, 380 nm CT,380nm

non-quenching 5.5 107 m-1 a 3.5 nm 36% paper I

quenching 1.7 nm 67%

non-quenching 3.2 107 m-1 b 6.5 nm 52% 16 nmc 73 %a paper III & Fig. 5.2

quenching 3.3 nm 100% 8.2 nmc 100 %a

a determined from Ti2p-PES-signal attenuation. b determined by DekTak Profilometry. c determined from the analysis of the EQE at 380 nm using 380nm = 1.72 107 m-1 (Figure 5.2).

0 20 40 600

5

10

15

@ 380 nm

@ 520 nm

EQ

E (%

)

d (nm) 0 10 20 30 40 50

0

25

50

75

100

K = 0.5 nm -1

K = 0.07 nm -1

K = 0.05 nm-1

K = 0.005 nm-1

K = 0.0005 nm-1

unmodified with NO2-bzac

1 - P

L S/P

L ref

d (nm)

LX

D=

15

nm

Figure 5.2: (left) Exciton diffusion model fit to d-dependence of the EQE distinguished between the higher energy (380 nm) and lower energy (520 nm) transition. (right) Photoluminescence quenching in dependency of dye layer thickness d both for unmodified and nitro-benzoic acid (NO2-bzac) modified TDCV-TPA/TiO2 interfaces.

The observed EQE spectra with increasing d for bilayer TDCV-TPA/TiO2 HTD exhibit an interesting effect for d > 20 nm. The response at the red-edge of the EQE spectra decreases which can be attributed to an inner filter effect as low-energy excitons have a lower probability of reaching the charge separating interface. Meanwhile, the EQE at higher excitation energy in-creases further indicating a longer LXD of higher-energy excitons. In Figure 5.2 (left) the d-dependency of the EQE both for the higher energy transition

56

at 380 nm and for the lower energy transition at 520 nm is shown. The fit in Figure 5.2 (left) of the experimental data to the exciton diffusion model de-rived from equation (3.9) suggests both a longer exciton diffusion length LXD (16 nm) and a higher interfacial charge separation efficiency CT (73 %) for excitons generated at a higher excitation energy. This indicates that excitons, generated at a higher energy, have a higher probability to reach the charge separating interface.149, 150

In paper IV we discuss that the higher energy electronic transition is a -* transition while the lower energy transition has ICT character. The differ-

ence in nature of the electronic transition and differences in electronic cou-pling with nearby neighbors might have an influence on the kET. Maybe even long-range energy transfer processes from higher-energy excitons to lower-energy excitons is possible. There are still open questions when it comes to a fundamental understanding of the nature of excitons in molecular aggregated compounds and the energy transfer mechanisms governing their migration.

As discussed in 3.2.3 an alternative approach to determining the LXD is by comparing the photoluminescence of the bulk light harvesting material on a quenching substrate (e.g. TiO2, PLS) with the PLref on a non-quenching refer-ence (e.g. glass) in dependency of the dye layer thickness d. Results from these experiments are shown in Figure 5.2 (right). For plain TiO2 substrates even for very thin layers the quenching was always < 40 % (see paper I-ESI and paper IV). This is not a singularity for this particular hybrid interface as non-unity quenching has also been observed for e.g. P3HT/TiO2.

70, 167 Using nitro-benzoic acid as an interface modifier 70, 86, 219, 220 gives rise to a

more efficient photoluminescence quenching with CT approaching 100% for thin dye layers, shown in Figure 5.2 (right). Moreover, we did not observe the wavelength-dependency of the photoluminescence quenching discussed for an unmodified TiO2 substrate in paper IV and shown in Figure 4.3. In solar cell devices built with nitro-benzoic acid we did not observe a positive effect on the device performance which will be discussed in section 5.4. This exemplifies that photoluminescence quenching does not reflect on the cur-rent generated via charge separation at a hetero-junction. Interface modifiers might also promote other PL-deactivation pathways or recombination of separated charge carriers.

The interfacial exciton separation efficiency should have an impact on the LXD which can be accounted for by introducing an exciton capture constant K.5, 153 Using the equation derived by Holzhey et al.153 the data shown in Figure 5.2 (right) can be modeled with a common value for LXD of 15 nm for both the unmodified TiO2 substrate and the nitro-benzoic acid modified sub-strate but different K. It is yet unclear in how far a limited exciton separation efficiency at the hetero-interface affects the LXD

5, 153 or if this parameter can be treated as an independent parameter.119

57

5.3 Interfacial exciton separation In excitonic solar cells the charge separation efficiency is determined by the energetic alignment at the hetero-interface. The energy alignment and spe-cific interaction between the first layers of the organic and inorganic compo-nent at the interface determine thus the overall conversion efficiency of the device.

When deriving the IQE from the EQE data shown in Figure 5.1 (right) we observed that even for very thin layers of TDCV-TPA the IQE is less than unity (paper I and IV). Add to this for the TDCV-TPA/TiO2 junction the IQE exhibits an energy dependency indicating a higher charge separation efficiency for excitation at lower wavelength (paper IV). From the energy level diagram derived in paper I from electrochemical measurements it was apparent, that there was a low thermodynamic driving force for electron transfer from excited TDCV-TPA into the TiO2 conduction band (Figure 5.3, left). The hypothesis of paper IV was, that the wavelength-dependent IQE was caused by a more efficient electron transfer from higher electronic and vibronic states in the molecule.176-178

Through a combination of photoelectron spectroscopy (PES) and UV-vis spectroscopy of dye layers in different thickness domains on TiO2 in combi-nation with quantum chemical calculations we derived a more refined energy level diagram shown in (Figure 5.3, right). From this analysis the higher energy transition was not found much more favorable with respect to the Fermi level in TiO2. We therefore speculated, that a more favorable regen-eration of holes created at the hetero-interface subsequent to a - * transi-tion could be an explanation for the observed wavelength-dependent IQE. A more efficient transport of holes away from the hetero interface can also explain the higher EQE for thicker d at lower wavelength discussed in the previous section.

TDCV-TPA does not possess chemical functionalities, like carboxylic acid groups, that have been found to form chemical bonds with TiO2. We deduced from a combination of PES-data, IR spectroscopy and DFT-calculations that TDCV-TPA interacts with TiO2 via its peripheral CN-groups. In the calculated LUMO energy levels the electron density is situ-ated predominantly on the peripheral CN-functionalities (paper IV). An in-teraction or electronic coupling between TDCV-TPA and the TiO2 surface via the CN-groups should facilitate interfacial charge transfer.

58

Figure 5.3: Comparison of the energy level diagrams derive from electro-chemical measurements in solution (adapted from paper I) and PES-measurements (adapted from paper IV).

5.4 Interface modification Molecules or treatments affecting the energetic alignment at the hybrid inter-face have been found to affect the device performance.70, 219, 220 The para-substituted nitro-benzoic acid (shown in Figure 5.4, left, inset) was found to increase the photocurrent in TiO2/P3HT bilayer hybrid solar cells.70 Interface modifying agents can influence the hetero-interface through different effects. These molecules can affect the energetic offset at the hetero-interface and mediate both charge injection and recombination. They can also improve the wetting at the organic-inorganic hetero-interface. Even though the TDCV-TPA photoluminescence was more efficiently quenched at nitro-benzoic acid modified TiO2 substrates (Figure 5.2, right) we did not observe an in-crease in photocurrent for nitro-benzoic acid modified TDCV-TPA/TiO2 hybrid solar cells. Instead we found a decrease in short-circuit current (Figure 5.4, left) and a much faster VOC-decay (Figure 5.4, right). We inter-pret these findings with an increase in interfacial recombination upon intro-duction of nitro-benzoic acid at the TDCV-TPA/TiO2 interface. Possibly, the interface modifier also impedes the specific interaction between TDCV-TPA and TiO2 discussed in paper IV.

The deviation of the experimental outcome from our hypothesis shows, that the organic-inorganic hetero-interface in hybrid solar cells is governed by the specific interactions between the organic and inorganic component.

4

3

2

1

0

-1

DOSTiO2 WD

10 Å 100 Å1 Å

HOMO-1

E B (e

V vs

E F,T

iO2)

EF,TiO2

-*

ICTHOMO

7.5

7.0

6.5

6.0

5.5

5.0

4.5

4.0

3.5

3.0

gra

ph

ite

PE

DO

T:

PS

STDCV-TPA

TiO

2

F:S

nO

2

e-

h+EF(h+)

EF(e-)

VOC,max

4

3

2

1

0

-1

DOSTiO2 WD

10 Å 100 Å1 Å

HOMO-1

E B (e

V vs

E F,T

iO2)

EF,TiO2

-*

ICTHOMO

4

3

2

1

0

-1

DOSTiO2 WD

10 Å 100 Å1 Å

HOMO-1

E B (e

V vs

E F,T

iO2)

EF,TiO2

-*

ICTHOMO

7.5

7.0

6.5

6.0

5.5

5.0

4.5

4.0

3.5

3.0

gra

ph

ite

PE

DO

T:

PS

STDCV-TPA

TiO

2

F:S

nO

2

e-

h+EF(h+)

EF(e-)

VOC,max

7.5

7.0

6.5

6.0

5.5

5.0

4.5

4.0

3.5

3.0

gra

ph

ite

PE

DO

T:

PS

STDCV-TPA

TiO

2

F:S

nO

2

e-e-

h+h+EF(h+)

EF(e-)

VOC,max

59

Figure 5.4: (left) Current-voltage of unmodified and nitro-benzoic acid modified TDCV-TPA/TiO2 bilayer hybrid solar cells. (right) Open-circuit voltage decay showing a much faster recombination for the nitro-benzoic acid modified devices.

5.5 Interface morphology By increasing the interfacial area for charge separation, the device perform-ance of hybrid solar cells can be increased. The morphology of the nanos-tructured metal oxide affects the exciton harvesting efficiency, the charge collection efficiency and charge carrier recombination.

In paper III, we compared two different approaches for the preparation of TiO2 substrates for bilayer HSCs, described in literature: spray-pyrolysis and spin-casting. The preparation procedures are shown in Figure 8.1 and Figure 8.2. Devices built with these two different TiO2 substrates exhibited distinct differences in the device performance and spectral response of the EQE. This could have either been explained by a difference in the conduction band energy or a different nano-morphology of the prepared TiO2 substrates. We found that both preparation procedures result in anatase TiO2 with similar optical and electronic properties but with distinctively different nano-morphologies. The spin-casting route lead to nano-porous TiO2 films which explained the increased EQE at absorption maximum. In paper III we also discuss the significance of the TiO2 layer as a blocking layer in this hybrid solar cell architecture. Device efficiencies of 0.47 for spray-pyrolyzed TiO2 and 0.6 % for spin-cast TiO2 were achieved which compares favorable with other bilayer hybrid solar cell devices.6, 62-75

Scanning-electron microscope images both from the surface of the sub-strates (top) and cross-sections of spin-cast and spray-pyrolyzed TiO2 sub-strates are included in Figure 5.5. The differences in surface topography of the different TiO2 samples are shown, as an example for AFM images, in Figure 4.7.

N

O OH

OO

60

Hybrid solar cells comprising higher surface area TiO2 were built by deposit-ing a meso-porous TiO2 layer from a dilute TiO2-nanoparticle paste onto FTO substrates covered with a dense TiO2 layer prepared by spray-pyrolysis. SEM top view and cross-section images are included in Figure 5.5. In Figure 5.6 we compare the device performance of the bilayer HSC, prepared with spray-pyrolyzed TiO2 and spin-cast TiO2, to HSC with increasing interfacial area due to the addition of a meso-porous TiO2 layer. In Table 5.2 the device performance data of the devices shown in Figure 5.6

FTO spray-TiO2 spin-TiO2 TiO2 np.~ 50 nm

TiO2 np.~ 250 nm

spray-TiO2 spin-TiO2

TiO2 np.~ 50 nm

TiO2 np.~ 250 nm

TiO2 np.~ 450 nm

Top-viewSEM

SEMcross-

sections


TiO2 np.~ 250 nm


TiO2 np.~ 250 nm

spray-TiO2 spin-TiO2

TiO2 np.~ 50 nm

TiO2 np.~ 250 nm

TiO2 np.~ 450 nm

Top-viewSEM

SEMcross-

sections

Figure 5.5: (top) SEM images at a magnification of 200k of the top surface of different TiO2 substrates used in hybrid solar cells with TDCV-TPA. (bottom) SEM-cross-section at a magnification of 300k of different TiO2 substrates used investingated in hybrid solar cell devices. The arrows indi-cate the samples associated with each other.

Table 5.2: Device performance of hybrid solar cells shown in Figure 5.6.

TiO2 substrate VOC (V)

JSC

(mA cm-2) FF

(%) JSC

a

(mA cm-2) IQE b

(%)

spray-pyrolyzed TiO2 0.95 1.05 0.32 0.31 1.11 15

spin-cast TiO2 0.97 1.88 0.33 0.60 1.62 17

~50 nm TiO2 0.92 1.91 0.40 0.70 2.05 19

~250 nm TiO2 0.8 2.28 0.45 0.83 2.95 30

~450 nm TiO2 0.85 3.17 0.58 1.56 3.77 41

a determined from the integrated EQE according to equation (2.5) b determined from the EQE/LHE of the solar cell samples.

A continuous increase in device performance can be observed, summarized in Table 5.2, with our champion device achieving 1.56 % conversion effi-ciency under 1 sun illumination. This device has among the highest effi-ciency for hybrid solar cell devices prepared by an infiltration approach and without additional interface dyes.65, 66, 90, 221, 222 Our device efficiencies have

61

not yet surpassed the efficiencies reported for optimized OSC devices using TDCV-TPA in combination with C60 (1.85%).223 Comparing the IQE of the devices, included in Table 5.2, an increased exciton harvesting efficiency due to the higher interfacial area becomes apparent in an increasing IQE. The integrated EQE was found to be higher compared to the JSC in the cur-rent-voltage measurements which can be rationalized with lower light inten-sities in the experimental EQE setup. We found a non-linear dependency of the photocurrent on the light intensity with higher relative photocurrents at lower intensity illumination levels.

0.0 0.5 1.00

1

2

3


spin-cast flat TiO2

~50nm np

~250nm np

J (m

A c

m-2)

V (V)

~450nm np

400 500 600 7000

10

20

30

EQ

E (%

)

(nm)

spray-pyrolyzed

spin

-cas

t

~50nm

np

~250nm

np

~

450nm

np

Figure 5.6: (left) Current density - voltage (J-V) curves of TDCV-TPA/TiO2

HSC prepared with the TiO2 substrates shown in Figure 5.5. (right) Spec-tral response of the external quantum efficiency (EQE) of the same samples.

62

6 Results for XDSC-2

This chapter summarizes work carried out on devices comprising both a hole transporting dye (HTD) and an interfacial sensitizer (ID) and is based on results presented in:

paper II (Contribution of HTD TVT-TPA in sDSC) paper V (Light-harvesting antenna TP-TPA in ID/HTD HSCs)

6.1 Matching dyes In the XDSC-2 devices both the ID and HTD can contribute to the photocur-rent. Below, the different ID’s and HTD’s investigated in this work are summarized in the diagram below. The HOMO energy levels were estimated from the redox potentials measured electrochemically and summarized in the Tables 6.1 and 6.2. The excited state energy levels were derived from the HOMO by subtracting the optical transitions in the IDs or HTDs, summa-rized in Tables 6.1 and 6.2. This energy level diagram represents the ap-proximate energetic alignment of the IDs and HTDs with respect to the nor-mal hydrogen electrode (NHE). The Fermi-level of TiO2 is at about -0.5 V vs NHE at neutral pH (see paper III) is indicated in the diagram.

1.5

1.0

0.5

0.0

-0.5

-1.0

-1.5

HTDs

SQ

02

SQ

HY

103D

9

D5L

0A3

E (V

vs N

HE)

D5L

0A1

IDs

TD

CV

-TP

A

TP

-TP

A

TP-

TP

A

Figure 6.1: Approximate energy levels of all IDs and HTDs investigated and presented in this work. The HOMO and excited state energy levels were estimated from the solution redox potentials and optical bandgap.

63

Table 6.1 Optical and electrochemical properties of HTDs investigated herein HTD Amax

(nm)

PLmax

(nm)

EA/PL/Eg

(eV)

A,maxc

(m-1)

Eox

(V vs Fc)

ED*

(V vsNHE)

TDCV-TPA 512, 497a 687, 581a 2.1, 2.3 a 3.2 107 0.63, 1.18 -0.84

TVT-TPA 427, 423a 579, 479a 2.5, 2.7 a 1.1 107 0.34, 0.62 -1.53

TP-TPA 406, 392a 504, 456a 2.8, 2.9 1.9 107 0.38, 0.69 -1.78

spiro-MeOTAD 366 424 3.03 - - -

a Solid solution in PMMA. b with respect to the PL in PMMA, c determined by DekTak profilometry

Table 6.2 Optical and electrochemical properties of IDs investigated herein

a on ZrO2 b in PMMA c in chlorobenzene d PL quenched by aggregation e setting EF,TiO2 = -0.5 vs NHE

As TiO2 was used as the inorganic component, compounds designed as DSC-sensitizers with a high IQE are suitable to be used as ID in the XDSC-2 devices discussed herein. The excited state needs to have a sufficient ther-modynamic driving force for efficient interfacial charge transfer.

More importantly, compounds to be used as HTD need to be found for XDSC-2 which need to be adequate hole transporting media. In addition, this compound functions as a regenerative medium for the interfacial ID and needs to have a thermodynamically positive driving force for the regenera-tion of holes created at the interface. The possibility for direct photo-induced electron transfer to TiO2 can be an advantage as this allows the HTD to con-tribute to the photocurrent independently of the internal quantum efficiency of the ID as described in equation (2.7).

The dye TDCV-TPA was found to have a more positive oxidation poten-tial compared to all IDs investigated herein. Furthermore, we found the ex-cited state of the compound to be very close to the TiO2 conduction band (paper IV). An additional ID at this junction (e.g. D5L0A1) was found to decrease the EQE from this HTD. These two properties make this compound unsuitable to be used in conjunction with an interface dye.

TVT-TPA and TP-TPA also have oxidation potentials that are relatively low. This limits the possibilities for IDs that can be used in combination with

ID A,max

(nm)

PL,max

(nm)

EA/PL,norm

(eV)

Eox

(V vs Fc)

(M-1 cm-1)

ED*

(V vs NHE) e

D5L0A1 412a, 421b 561 a, 503 b 2.59a, 2.70b 0.61 36 000125 -1.35

D5L0A3 472a, 466b 652 a, 532 b 2.42a, 2.50b 0.65 44 000127 -1.14

D9 462125 632125 2.32128 0.29128 33 000128, 224 -1.40

HY103 635a 659a 1.92a 0.43225 66 000225 -0.86

SQ (JJ3) 649a, 653c 697a,d, 663c 1.80a, 1.85b 0.22 220 000 -0.95

SQ02 652 a, 660a 690a,d, 662b 1.85a, 1.89b 0.22 319 000126 -1.00

64

these HTDs. In paper II we found that both the ID SQ and the HTD TVT-TPA contribute to the photocurrent but the relatively low IQE of SQ might have been due to an insufficient driving force for regeneration from the HTD to the ID.

For XDSC-2 a large spectral overlap between the HTD-emission and the ID-absorption facilitates long-range energy transfer from the HTD to the ID as discussed in section 3.2.1. This can give rise to an improved exciton har-vesting and apparently longer exciton diffusion length.156, 167 To a first ap-proximation, HTDs with a larger bandgap than the ID should favor energy transfer from the HTD to the ID.

Both in paper II and paper V we compare the spectral overlap therefore both for liquid or solid (PMMA) solutions of the compounds and for solid state samples where the dyes are in a comparable physical and chemical environment as in the solar cells. In paper II we compared the spectral over-lap of the emission of the HTD TVT-TPA with the absorption spectrum of the ID SQ(JJ3) both for the spectra measured in solution and as solid films or adsorbed to ZrO2. In the solid state the spectral overlap apparently in-creases. In paper V we compare the spectral overlap of TP-TPA with the ID D5L0A3 both for the compounds dispersed in a solid PMMA matrix and as adsorbed or spin-cast film of ZrO2. In a solid PMMA matrix the spectral overlap is much higher compared to the solid state spectra. It is unclear, whether the solution of solid state spectral overlap is the most relevant for energy transfer in the investigated device architecture.

6.2 Solar cell devices Hybrid solar cell devices with different combinations of HTD’s and ID’s were built during the course of this work. Some of the other HTD’s that were investigated are summarized elsewhere.134

A compound that was found interesting to be used as HTD in combina-tion with various ID’s was the dye TVT-TPA. In Figure 6.2 (left) solar cell devices using TVT-TPA (Figure 3.6) and different ID’s built with flat spray-pyrolyzed TiO2 substrates are compared. One can clearly identify the individual contributions of the HTD and ID except for the ID D5L0A1 which absorbs in the same spectral range as TVT-TPA. As the absorption features of TVT-TPA and SQ were spectrally well separated these two compounds were chosen for a more thorough investigation that we published as a proof of principle in paper II. Therein, we also compared the device performance of SQ/TVT-TPA to devices built with spiro-MeOTAD as hole transporting medium. The spectral response of the EQE exhibited a signal dye to the contribution of spiro-MeOTAD to the photocurrent. Due to this additional signal, and a larger IQEID for the spiro-MeOTAD/SQ, the device-performance was found comparable to the TVT-TPA/SQ devices. The

65

higher IQEID was attributed to a higher regeneration efficiency of SQ by spiro-MeOTAD compared to TVT-TPA. This finding highlights the neces-sity of efficient regeneration of ID by the HTD. Spiro-MeOTAD derivatives, modified to extend their absorbance further into the visible part of the elec-tromagnetic spectrum could be an interesting family of compounds to use as HTD.105

400 500 600 7000

2

4

6

TV

T-T

PA

D5L

0A1

EQ

E (%

)

(nm)

SQ

(JJ3)

D9

D5L

0A3

400 500 600 7000

5

10

15

20

D5L

0A1

TP

-TP

A

SQ

02

HY

103

EQ

E (%

)

(nm)

D5L

0A3

Figure 6.2(left) Hybrid bilayer solar cell devices comprising TVT-TPA and various interface dyes (indicated in figure). (right) Nanostructured hybrid solar cells using TP-TPA as a HTD and different ID’s.

In continuation of this project we also tested the commercially available compound TP-TPA as HTD which has similar spectral and electronic prop-erties as TVT-TPA. In continuation of the work presented in paper II we decided to focus more on the regeneration of ID by the HTD. As both TP-TPA and TVT-TPA have low oxidation potentials the choice for the ID’s was limited. In Figure 6.2 (right) the spectral response of the EQE for solar cell devices using TP-TPA as HTD and various IDs (indicated in the Figure) is shown. These devices comprised a ~ 200 nm thick meso-porous TiO2 layer on top of the FTO-substrates covered by a dense TiO2 layer. Again, one can clearly distinguish between the respective contributions from the ID’s and HTD. In case of the D5L0A1/TP-TPA the spectral response of HTD and ID overlap but the overall EQE at a wavelength of 400 nm is much higher compared to the device comprising the HTD only.

In paper V we summarize the work carried out on the investigation of TP-TPA in combination with D5L0A3. TP-TPA is a compound developed for OSC and OLED and is usually deposited via evaporation.137 This compound was difficult to dissolve in concentration higher than 20 mM. When infiltrat-ing solid state hole-transporting materials like e.g. spiro-MeOTAD into meso-porous films solution concentrations in the order of 100 mM are used, which is crucial to achieve a high pore filling fraction.209

66

In paper II, we derived an expression for the external quantum efficiency of XDSC-2 given in equation (2.7). The EQE can be separated into the contri-bution from the ID and HTD. As the HTD can contribute to the photocurrent both via energy transfer and via electron transfer the photo-conversion mechanism of these devices becomes difficult to distinguish. For the energy transfer mechanism the EQEHTD(ET) depends on the IQEID. By comparing the IQEHTD and IQEID it can be already determined whether the EQEHTD can originate exclusively from an ET to the ID – otherwise electron transfer needs to occur. For the ID/HTD combination in paper II, the IQEHTD was larger than the IQEID (illustrated in Figure 4.3, right) which indicated, that, in this case, the HTD TVT-TPA contributes to the photocurrent via direct injection.

6.3 Energy transfer from HTD to ID In paper II we estimated the energy transfer efficiency ( ET, denoted XT in paper II) for samples where the ID was adsorbed to a meso-porous ZrO2 film and the HTD was spin-cast on top. These samples were thus comparable to the geometry and physical properties of the XDSC-2 devices. The measure-ments are described in section 3.2.4 and the data attained for the SQ/TVT-TPA combination of ZrO2 can be found in paper II. From the excitation spectra, the energy transfer efficiency ( ET) from the HTD to ID was esti-mated to be 26 %.

In the measurement on ZrO2 we found a large dependency of the squaraine photoluminescence on the degree of aggregation, discussed in section 3.1.3. Upon addition of the HTD a decrease in aggregation was ob-served. The HTD can act as a co-adsorber preventing squaraine aggregates and at the same time can contribute to the photocurrent.

In paper V we compared photoluminescence quenching measurements of the TP-TPA/D5L0A3 couple both in PMMA and on ZrO2. In both cases, the TP-TPA emission was quenched in vicinity of the ID. For the ZrO2 samples it was unclear whether the increased signal in the red was due to emission from D5L0A3. Even in these samples aggregation of ID might have an im-pact on the photoluminescence and we were not yet able to estimate the ET for this ID/HTD combination.

In XDSC-2 devices all energy transfer mechanisms introduced in Figure 3.7 are possible. Exciton diffusion within the HTD layer might occur prior to a long-range energy transfer to an ID. We note, that for these measurements the degree of photoluminescence quenching also depends on the degree of infiltration of the HTD into the meso-porous structure. Photoluminescence from non-infiltrated HTD on top of the film or in an overstanding layer is likely not to become quenched by the ID.

67

6.4 Injection, regeneration and pore infiltration Injection and regeneration can in part be derived from the IQE of the ID and HTD, respectively. Qualitatively, the electron transfer from an ID or HTD to TiO2 can be investigated using transient spectroscopy. With these methods also the regeneration of photo-oxidized ID by the HTD can be investigated as described in section 4.1.5. In the nanosecond TAS measurements con-ducted in paper V we found that the signal of oxidized D5L0A3 decreased upon addition of TP-TPA simultaneously to the appearance of a signal that could be assigned to oxidized TP-TPA.

As for the energy transfer also the regeneration of the ID by the HTD de-pends on the pore infiltration. A high pore-filling fraction is important for efficient ID regeneration and hole transport to the hole-selective contact.208 To investigate the effect of pore infiltration we prepared some samples by evaporating a layer of TP-TPA onto ~450 nm thick meso-porous TiO2 lay-ers which resulted in a smooth dye layer of 150 nm thickness clearly visible in a scanning electron microscope image of a cross section of the sample (Figure 6.3 a). One of the samples (Figure 6.3 b) was heated to 220oC where the dye layer started to melt. This is an alternative way to infiltrate organic compounds into porous metal oxides.212 TP-TPA was found to infiltrate the pores of the TiO2 film. The absorbance of the samples was found similar but the photoluminescence of the heat-treated sample was quenched by 71 % which we interpret as a proof for successful infiltration of TP-TPA into the meso-porous metal oxide matrix.66 For efficient hybrid solar cells devices, a pore-filling fraction of 100% should be aspired208 but this is not trivial to achieve. In spin-casting the solvent molecules might prevent 100% pore-filling. Melting requires elevated temperatures which not all IDs might with-stand.212

a b c

400 500 600 7000

1

2

3

4

5

melted

PL

(cou

nts/

107 )

(nm)

-71%

evaporated

a b c

400 500 600 7000

1

2

3

4

5

melted

PL

(cou

nts/

107 )

(nm)

-71%

evaporated

Figure 6.3 a) SEM-cross section image showing a~ 150 nm thick evapo-rated TPTPA layer on ~450 nm meso-porous TiO2. b) Evaporated layer melt-infiltrated into TiO2 c) Comparison of sample photoluminescence

68

7 Conclusion

During the past 5 years, hybrid solar cells (HSCs) have been established as a new research area combining and uniting aspects of dye-sensitized solar cells (DSCs) and organic solar cells (OSCs). In this work we investigated hybrid solar cells starting from the objective to find small-molecular light harvesting hole-conductors to be used in conjunction with titanium dioxide in bilayer and nanostructured hybrid solar cells. The devices investigated herein can be considered to be sDSC where the “transparent” hole conduc-tors such as spiro-MeOTAD is replaced by a hole transporting dye (HTD).

The three triphenyl-amine based compounds shown in Figure 3.6 gave promising results in the intended device architecture. Two different device architectures were investigated: XDSC-1 that use a HTD in direct conjunc-tion with TiO2 and XDSC-2 that used a combination of an interface dye (ID) and a HTD.

In direct conjunction with flat or porous TiO2 the HTD TDCV-TPA gave rise to comparatively high solar cell efficiencies for this type of device archi-tecture (paper III, chapter 5). The exciton diffusion length in TDCV-TPA (5.6 nm, assuming a non-quenching PEDOT:PSS contact, paper III) was found to be dependent on the experimental conditions and to be influenced by the interfacial charge separation efficiency. The observed dependence of the LXD on the interfacial charge separation efficiency and excitation energy should be further investigated. Another interesting questions is in how far the LXD depends on the molecular order in HTD-materials.76, 149, 151

In the investigation of the interfacial energy alignment we found the low-est energy excited state of TDCV-TPA to be slightly below the TiO2 Fermi level. Due to this low driving force for electron injection and the very posi-tive oxidation potential this compound was not suitable to use as HTD in conjunction with an interface dye (ID). In hybrid solar cells the energetic alignment at the inorganic/organic interface determines the charge separation efficiency of these devices (paper IV). Factors influencing and improving the energetic alignment at the hetero interface need to be identified and modified to improve the power conversion efficiencies of HSC devices.70, 86

Solid state dye-sensitized solar cells using a light absorption hole trans-porting dye (HTD) were realized using the TPA-compounds TVT-TPA (paper II) and TP-TPA (paper V). For TVT-TPA we observed excitation energy transfer to the ID (SQ) but concluded that direct injection from the HTD accounts for a large fraction of the HTD response. In the solid state

69

devices investigated herein it was not trivial to identify the specific energy transfer mechanisms and we consider that different types of energy transfer might occur in the devices. Further work is needed to investigate and distin-guish between different types of energy transfer mechanisms in these type of devices.

The two investigated architectures (XDSC-1 and XDSC-2) are interesting systems. Comparable devices have received much attention in hybrid solar cell research.78, 89, 94-101 Finding suitable light harvesting materials and simul-taneously replace the commonly used spiro-MeOTAD in solid state dye sen-sitized solar cells is a challenge.43, 226 In direct comparison with devices us-ing spiro-MeOTAD as hole transporting medium, the investigated HTD’s seemed to regenerate the ID less efficient. The search and investigation of more light harvesting small-molecular hole-transporting semiconductors is an important research task for these type of solar cell devices.

One strives after interpenetrating organic and inorganic phases to achieve a pore size in the order of the exciton diffusion length at the same time as getting efficient charge carrier transport in the respective phase. Morphology control is thus an essential aspect of HSC research. The fabrication of HSC following the “infiltration approach” (Figure 2.4) of organic semiconductors into the meso-porous scaffold of the inorganic component will require alter-native ways to the commonly used spin-casting approach to achieve a high pore filling fraction. Alternative methods for the infiltration of a hole-transporting media into a meso-porous metal oxide electrode, like melt-ing.212, 227 dip-coating68 and in-situ polymerization,213, 214 need to be further investigated.

Scientific work of academic weight and interest in an applied research area, like DSC or HSC, should ideally both investigate fundamental aspects of device operation and simultaneously prove the significance of the experi-mental findings by demonstrating high conversion efficiencies. Combining these two aspects requires an interdisciplinary approach and is more efficient when conducted in a team-effort.

70

8 Materials & Methods

8.1 Materials used and sample preparation Conducting glass: Glass, coated with layer of transparent electrical conduc-ing (TEC) fluorine-doped tin oxide (FTO) were used as support and front contact for solar cell devices discussed herein. The sheet-resistance was ap-proximately 15 /square and was purchased from Pilkington.

Injection dyes: The triphenyl-amine based injection dyes were developed and synthesized in the group of Licheng Sun at KTH, Stockholm. The squaraine dye SQ (JJ3) used in paper II was received as a gift from Dr. Jian-jun He (now AnaSpec Inc.) who previously worked in the Hagfeldt group. The dye SQ02 was purchased from Solaronix (Switzerland). Hole transporting dyes: The dyes TDCV-TPA and TVT-TPA were devel-oped and synthesized in the group of Jean Roncali in Angers, France. The TDCV-TPA batch used for the studies in paper III and paper IV was pur-chased from Sigma-Aldrich. TP-TPA was purchased from Lumitech (China). Spray-pyrolysis (of TiO2): Dense layers of TiO2 were prepared by spray-pyrolysis. A TiO2 precursor was prepared under nitrogen by adding 2.4 ml Ti(IV)-tetraisopropoxylate (Aldrich, TTIP) dropwise to 3.6 ml acetylacetone (Fluka, AcAc)228 resulting in a bright yellow liquid containing diisopropoxy-Ti(IV)-bis(acetoacetonate) (Ti(acac)2OiPr2) which could be stored in a sealed flask at 8 oC until further use. Prior to deposition the precursor was diluted 1:9 with ethanol (Kemityl) to give a solution of ~1.2 M.

FTO-glass substrates were heated to 450oC on a high temperature Titan hotplate (PZ 28-3TD with TR 28-3T temperature controller, Harry Gerstig-keit GmbH) covered by a Robax-glass plate to avoid direct deposition of TiO2 onto the hotplate. The precursor was deposited using a commercial airbrush as spray nozzle (Clas Olsons or Biltema) equipped with an adapter for vertical-spraying (Figure 8.1, step 1). The nozzle was moved at ~5cm/s and a distance of ca. 6 cm over the substrate surface. One spray-cycle consti-tutes a single movement of the spray-nozzle over the substrates. TiO2 film formation occurs by reaction of the organo-titanate with atmospheric oxygen

71

at the hot substrate surface (Figure 8.1, step 2). In this deposition method the metal oxide film grows with ~ 10 nm per spray cycle.

1) Spray 3) Crystal &film growth

2) Pyrolysis

Ti(OC3H7)2(acac)2TiO2

450 ºC 450 ºC 450 ºC

TiO2

O2

org

.

1) Spray 3) Crystal &film growth

2) Pyrolysis

Ti(OC3H7)2(acac)2TiO2

450 ºC 450 ºC 450 ºC

TiO2

O2

org

.

Figure 8.1: Illustration of reaction steps during spray-pyrolysis.

Spin-casting: Thin films of a material can be deposited homogeneously onto susbtrates by depositing the material solution onto a substrate, followed by spinning at high speed during which the solvent evaporates. In this work, a Chemat Technology KW-4A spin-coater was used. The spin-speed (500 – 8000 rpm) and time (3 – 80 sec) can be varied which has an influence on the thickness of the deposited films. Spin-cast TiO2: Alternatively, flat TiO2 films can be prepared by using a spin-casting approach.70 A precursor, TTIP is diluted 1:6 (volume ratio) with and ethanol mixture (containing small quantities of water and HCl). A volume of 20 l cm-2 substrate surface was deposited followed spin-casting at 2000 rpm for 30 sec (Figure 8.2, step 1). The substrates were kept at 120 °C overnight and then calcined at 450 °C.

Titanium dioxide formation in this preparation procedure takes place dur-ing the blending of the precursor and storage at 120 oC (Figure 8.2, step 2). In this step TiOx crystallites are formed through hydrolysis of the TTIP-precursor followed by water and alkoxide condensation reactions.

2) Gelation 3) Sintering1) Spin-casting

Ti(OH)4

Ti(OC3H7)4 Ti(OH) xOy TiO2

20-120 ºC 120 ºC 450 ºC


Ti(OH)4


20-120 ºC 120 ºC 450 ºC


Ti(OH)4


20-120 ºC 120 ºC 450 ºC Figure 8.2: Reaction steps during sol-gel spin-casting preparation TiO2.

Meso-porous TiO2: Meso-porous layers of TiO2 on glass or FTO-glass sub-strates were prepared by doctor-blading or spin-casting a paste of TiO2 nano-particles (Dyesol, DSC 18NR-T). To achieve thinner layer thicknesses the paste was diluted with -terpeneol and iso-propanol. The samples were sin-tered using a standard sintering procedure (180 oC, 10min; 320oC, 10 min; 390 oC, 10min; 500 oC, 30 min) on a PZ 28-3TD hotplate with a program-mable temperature controller (Harry Gerstigkeit GmbH).

72

Meso-porous ZrO2: Similar to TiO2, meso-porous films of ZiO2 can be prepared. While having similar morphological properties and surface chem-istry, the conduction band of ZrO2 is ~ 1eV higher compared to the TiO2 conduction band. For the ZrO2 films used in this work a dispersion of ZrO2 nanoparticles, prepared by Leif Häggmann according to a recipe from Brian O’Regan,229 was used. Meso-porous ZrO2 films where prepared by spin-casting and sintering procedure analogously to the titania films. Sensitization with ID: Dyes containing anchoring groups were left to chemisorb to the TiO2 substrates from EtOH solution (c between 0.1 mM and 0.5 mM) for up to 12 hours. Co-adsorber: For some sensitizing dyes employed in DSC it was found crucial to employ co-adsorbers. The co-adsorber cheno-deoxycholic acid (short: cDCA, Fluka) was added to the dye-bath of squaraine sensitizers (SQ and SQ02) to avoid aggregation. HTD spin-casting: The HTD’s were dissolved in dicholoromethane (DCM), chlorobenzene (CBz) or tetrahydrofurane (THF) and deposited by spin-casting. The solubility limit was usually around 20 mM (lower for TP-TPA). Thinner layers were deposited by diluting the solution. The spin-casting speed influences the thickness of the deposited layer. Deposition was usually carried out at 4000 rpm by dropping the HTD-solution onto the spinning substrates. Back contact: The back contact of solar cell devices was established using a spin-cast layer of PEDOT:PSS (aqueous dispersion, Aldrich). Solvent resi-dues were removed in vacuum (2.5 10-5 mbar). A square hole (punch, Fisher, 0.19 cm2) or round hole (one-hole punch, 0.24 cm2) was punched out of a piece of double-sided tape (Stokvistapes, Clas Olsons), mounted onto the sample and filled with graphite powder (Aldrich). Electric contact was estab-lished by pressing a second FTO-glass substrate against the graphite-filled tape. Silver paint was applied to establish contact points to the solar cell device. Solar cell samples: The schematic picture below summarizes the prepara-tion steps involved in the manufacturing of the solar cell samples investi-gated herein. Steps indicated with dotted arrows were optional. In paper I, III & IV samples were prepared directly onto the dense TiO2 substrates.

73

FTO-glassGraphite powder

Tape with hole

FTO-substrate

Dense TiO2

Meso-porous TiO2

Interface dye

Hole transp.dye

FTO-glassGraphite powder

Tape with hole

FTO-substrate

Dense TiO2

Meso-porous TiO2

Interface dye

Hole transp.dye

Figure 8.3: Preparation route for solar cell samples

8.2 Characterization methods

8.2.1 Microscopic & Profilometric methods Scanning electron microscopy (SEM): SEM-images were acquired on a Zeiss Leo1550 SEM situated in the clean-room facility of the Ångström laboratory at Uppsala University. For cross-section measurements the sam-ples were scribed with a diamond-tip from the glass-side and cracked prior to SEM-imaging in a 90o sample holder. To some extend the infiltration of organic material into a porous inorganic material can be investigated in this manner. Atomic force microscopy (AFM): AFM measurements were carried out using a NanoScope III AFM situated in the Physics department at the Ång-ström Laboratory, Uppsala University together with Francesca Spadavec-chia. The images were analyzed using the Nanotec WScM 5.0 software.216 Profilometry: In profilometric methods a cantilever is scanned across the surface of a sample registering height differences. The Sloan DekTak pro-filometer in the Hagfeldt group at Uppsala University is adequate to measure the height of meso-porous TiO2 films above a thickness of 500 nm. Better sensitivity can be achieved on the DekTak 150 step-profilometer situated at KTH, Stockholm.

8.2.2 UV-vis, IR & Photoluminescence spectroscopy UV-visible (UV-vis) spectroscopy was carried out on an Ocean Optics HR2000 fiberoptic spectrometer which measures the whole sample spectrum at once. Samples can be measured either in a horizontal or vertical setup.

74

Alternatively, UV-Vis spectra were measured using a Cary 5000 spec-trometer. The spectral resolution of this instrument is considerable higher. This instrument can be equipped with an integrating sphere to measure the diffuse reflectance R and transmission T which is important for scattering samples such as meso-porous TiO2 films and bulk dye layers. Scattered light is re-reflected inside the integrating sphere until detected, illustrated in Figure 8.4. For solid samples the absorbance was calculated using:

0

T

R

IA log

I I (8.1)

I0 IIT

IR

Detector

Sample

I0 IIT

IR

Detector

Sample

I0

IT or IR

Integrating sphereT-setup R-setup

I0

IT or IR

I0

IT or IR

Integrating sphereT-setup R-setup

Figure 8.4 (left) Influence of reflection and transmission on optical meas-urements performed on solid samples. (right) Measuring sample transmit-tance and reflectance with an integrating sphere. Adapted from ref. 171

Infrared (IR) spectroscopy: FT-IR spectra were recorded using a Nicolet Avatar 370 DTGS IR-spectrometer equipped with a Smart MIRacleTM acces-sory. This equipment is particularly suitable to measure on solid samples. As certain functional groups can have distinct IR signals this method is a valuable tool for structural identification of chemical compounds. IR spectra are commonly recorded in wavenumbers (= 1/ ). Steady-state photoluminescence (PL): These measurements were carried out on a Fluorolog Jobin Yvon spectro-fluorometer in the Chemical Physics group, Uppsala University. This setup is equipped with two detectors per-pendicular to the direction of sample excitation. Solid samples were meas-ured at a 30 – 60 o angle with respect to the excitation beam (RA-mode) or at a 0 – 20o angle using the front-face mode of the Fluorolog.

8.2.3 Electrochemical methods All electrochemical measurements presented herein were carried out on a CH-Instruments 660 potentiostat. Measurements were either performed on compounds in solution, adsorbed to meso-porous TiO2 electrodes on con-ducting FTO-substrates (ID characterization) or spin-cast dye layers on FTO-substrates (HTD characterization. Ferrocene was used as an internal

75

reference as this compound has a relatively constant redox potential of 0.63 V vs NHE under various experimental conditions.230

Electrochemical measurements were carried out in MeCN or DCM solu-tion containing a supporting electrolyte (TBAPF6). For measurements on films the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethyl-sulfonyl)-imide purchased from io-li-tec was used. The electrochemical setup consists of a working electrode (WE), counter electrode (CE, usually a Pt wire was used as CE) and a reference electrode (RE, usually quasi-Ag/Ag+ RE or Ag/AgCl). Cyclic voltammetry (CV): is a convenient electrochemical method to de-termine the oxidation and reduction potentials of chemical compounds. The voltage at the WE is swept linearly between a higher and lower set potential at a certain scan rate (V/sec) while the current is monitored. At potentials more positive (more negative) than the oxidation (reduction) potential of the analyte a negative (positive) current is detected at the WE due to the oxida-tion (reduction) of the analyte in solution. Differential pulse voltammetry (DPV): In this method a voltage pulse with the width V, superimposed on a staircase voltammogram, is applied to the electrochemical. The current is measured both right before and towards the end of the voltage pulse which avoids the effect of charging currents. Plot-ting the difference in current as a function of applied potential, oxidation (reduction) potentials are derived from the peaks at Vp + V/2.231 Mott-Schottky: The capacitance at a semiconductor – liquid or semiconduc-tor-metal junction the junction capacitance in dependency of the applied potential Va is derived from the junction impedance. For a non-reactive cir-cuit the capacitance C can be calculated from the imaginary part Z’’ of the impedance at high frequency .196

1

2C

Z'' (8.2)

From the plot of C-2 vs Va (eq 4.4) the flat-band potential Vfb and doping density ND of the semiconductor can be determined from the x-intercept and slope, respectively.

8.2.4 Transient absorption spectroscopy Spectro-electrochemistry combines spectroscopic methods such as UV-Vis spectroscopy with electrochemical methods. This method is a valuable aid for the interpretation of transient absorption spectroscopy data. This method was utilized to acquire the UV-visible spectra of oxidized or reduced species which are transiently generated during electron transfer reactions. The UV-

76

Vis-spectra are recorded during or after an electrochemical potential scan or step to oxidation or reduction potentials of the compound of interest.

Photo-induced absorption (PIA) measurements were carried out in an ex-perimental setup comprising a 20 W tungsten-halogen lamp as probe-light and a frequency modulated pump-light source. The latter was either a blue LED (Luxeon Star, max 460 nm), a green laser (Lasermate GML 532-100FLE, max 532 nm) or a red laser (Coherent Model #31-1050, max 635 nm) modulated by a HP 33120A function generator. Transmitted light was analyzed by a monochromator (Acton Research Corporation SP-150) in combination with a silicon photodiode. For a more detailed description of the PIA setup see the thesis of Ute Cappel.200 Nanosecond transient absorption spectroscopy (ns-TAS) was carried out on a LP920 laser flash photolysis spectrometer from Edinburgh instruments. A frequency tripled Nd:YAG laser (Continuum Surelight II) in combination with an OPO (Continuum Surelight) was used to generate laser pump-light pulses of tunable monochromatic wavelength. A Xe arc-lamp was used as probe light and spectrally resolved measurements were recorded by using a CCD-camera. The time evolution of the signals at a particular wavelength was recorded using a photomultiplier tube (PMT).

8.2.5 Photoelectron Spectroscopy In photoelectron spectroscopy (PES) samples are ionized by high-energy radiation with a known energy h . From the kinetic energy (Ekin) of the emitted electrons and the work function of the material , the binding energy EB of the detected electrons can be determined.

B kinE h E (8.3)

Information about the chemical composition of a sample and the chemical and electronic states of the elements composing the sample can be acquired. All PES measurements presented in this work were carried out by Erik Johansson. For the measurements either the X-ray photoelectron spectrometer (XPS) at the physics department at Uppsala University and the experimental endstation at BeamLine I411 at the Synchrotron facility MAX-lab in Lund, Sweden were used.232, 233

8.2.6 Solar cell device characterization Current density – voltage (J-V) measurements were carried out under simulated AM1.5 solar irradiation (Newport solar simulator model 91160) calibrated (e.g. to 1000 Wm-2) with a silicon diode as a reference. A com-

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puter-controlled source meter (Keithley 2400) was used for the measure-ments. External quantum efficiency (EQE) measurements were carried out using a xenon lamp (ASB-XE-175) as light source in combination with a com-puter-controlled monochromator (AB301-T). For devices with relatively low current outputs EQE measurements wer carried out measuring the voltage drop over a resistor with a Keithley multimeter (Model 2700). For higher current outputs measurements could be carried out using a potentiostat (PAR 273).

The external quantum efficiency EQE (a. k. a. incident photon to current conversion efficiency, IPCE) can be calculated from the current density J, generated by a solar cell under monochromatic irradiation Pspec:

1240SC SC SC

spec spec spec

J ( ) h c J ( ) J ( )EQE( )

( ) e P ( ) P ( ) (8.4)

Photovoltage decay: In an illuminated solar cell under equilibrium open-circuit conditions, charge carrier separation rate at the hetero interface equals the recombination rate. When switching off the illumination source a new equilibrium will be established through the recombination of charge carriers. This process can be monitored following the decay of the open circuit volt-age (VOC). This method is integrated in the “toolbox” experimental setup in which software-controlled white LED and a DAQ multimeter are coupled to allow for various different measurements being performed in the same ex-perimental setup.

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Sammanfattning på svenska

Figur 1: Solenergi driver den naturliga fotosyntesen och kan omvandlas till ström med hjälp av slceller.

Utan solen… ... skulle vår planet vara ett kallt och obeboligt ställe. Livet skulle helt enkelt aldrig ha utvecklats utan denna motor som har försörjt vår planet med energi sen första början. Bland annat driver solens kraft fotosyntesen som är en process som har pågått i mer än tre miljarder år.

I fotosyntesen omvandlas vatten (H2O) och koldioxid (CO2) från luften till syre (O2) och de kol-väte-molekyler som bygger upp själva växterna. Under fotosyntesprocessen bildas de ämnen som lägger grunden till vår när-ing, bland annat kolhydrater, proteiner, fetter, vitaminer och syrgas. Under de miljarder år där fotosyntes har pågått har vissa av växterna efter deras död hamnat under betingelser där de omvandlats till fossila bränslen.

Dessa använder vi oss av idag för att driva våra bilar, värma våra hus med och tillverka t.ex. plast. De senaste åren har man dock blivit medveten om att den ökade emissionen av CO2, som frisläpps genom uppbränning av fossila bränslen, påverkar jordens klimat. Dessutom kommer det inte att finnas fossi-la bränslen i all evighet och vi måste komma på andra sätt att täcka mänsklig-hetens energi-konsumption.

Solceller… ...omvandlar solljus direkt till elektricitet och är därför en mycket intressant teknologi för att omvandla solens energi. Det pågår därför mycket forskning och industriell tillverkning av solceller världen runt. I nuläget finns det många olika solcellsteknologier. De vanligaste solcellerna är baserade på kisel men det finns också många andra slags. Vid Uppsala universitet finns det till exempel två forskningsaktiviteter kring solceller. På fasta tillstånds fysik forskar man på så kallade CIGS solceller.

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Figur 2: Färgämnes sensitiserade solceller kan lätt byggas själv. Här visas infärgning av porös titanoxid med hibiskus-te och spän-ningsmätning på färdiga solceller. Materialet som visas ingår i skol-cells-lådorna‡ som man kan bygga solceller med på skol-laborationer

På fysikalisk kemi och fysik forskar man på färgämnes-sensitiserade solcel-ler. Färgämnes-sensitiserade solceller består av ledande glas, nanostrukture-rad titandioxid, ett färgämne som sätts fast på ytan och en elektrolytvätska.

Först tillverkas det ett lager av nanoporös titandioxid på det ledande gla-set. Någonting som är ”nano” har dimensioner på en längdskala som bara är några miljaonstel delar av en millimeter – det är alltså väldigt små dimensio-ner vi pratar om och man kan inte ser dem med det blotta ögat. Med hjälp av ett elektronmikroskop kan man dock zooma in tills man kan se de pyttesmå partiklar och porer som det porösa titandioxidlagret består av. Detta visas i Figur 3 på högre sidan.

Figur 3: Färgämnes-sensitiserade solceller består av ett färgämne, ke-miskt bunden till porös titandioxid (elektron-mikroskop bild till höger). De exciterade färgämnerna injicerar elektroner in i titandioxiden och behöver sedan regeneras av t.ex. en elektrolyt vätska.

‡ http://www.teknat.uu.se/ViewPage.action?siteNodeId=35884&languageId=3&contentId=-1

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I dessa solceller är det färgämnen som fångar in solljus. Färgämnen exciteras och släpper sedan elektroner till det nano-porösa titandioxidlagret. Man kan använda ganska så vardagliga ämnen som t.ex. blåbär eller rödvin för att färga in solcellerna. Efter att färgämnet har släppt en elektron (en minus-laddning) till titandioxiden blir den plus-laddad själv och behöver få tillbaka en elektron för att neutraliseras.

För detta använder man oftast en elektrolyt (en vätska som innehåller jo-ner – som i ett bilbatteri) men det görs ansträngningar att ersätta vätskan med ett fast ämne och man kalla dessa solceller sedan för fasta tillstånds färgäm-nes-sensitiserade solceller.91

Excitoniska färgämnes solceller… ... har varit mitt forskningsprojekt under mina doktorandstudier. Syfet

med projektet var att hitta ledande färgämnen som man skulle kunna använ-da som ett tjockt färgämneslager i stället för ett tunnt monolager på titandi-oxid. På detta vis blir solcellerna lite lika organiska solceller där ljuset ab-sorberas i ett tjockt färgämneslager och skapar så kallade excitoner. Dessa excitoner måste sedan flytta på sig mot gränsytan till titanoxiden för att där kunna bryta upp dessa exciterade tillstånd till fria laddningsbärare (elektro-ner och hål). Elektronerna rör sig sedan genom titanoxiden medan hålen måste kunna röra sig genom färgämneslagret. Till en början kan man alltså fånga in mer ljus eftersom man har ett tjockt färgämneslager men solcellerna blir sedan begränsade av att excitonerna inte kan röra sig hur långt som helst genom materialet. Den längden en exciton kan förflytta sig – excitondiffu-sionslängden (LXD) – är en av de parametrar som jag har studerat. Sedan har jag också studerat ett system där man använder både ett färgämne vid titan-oxidgränsytan och ett ledande fastfärgämne i ett tjockt lager. Eftersom två olika färgämnen används i dessa solceller absorberas ljus i olika områden av solspektrat absorberas. I dessa solceller kan det tjocka färgämneslagret fånga in solljus och sedan föra över denna energi till färgämnet vid titandioxidytan. På det viset är solcellerna jag har forskat på mycket lika de ljus-infångande antenner som fotosyntetiska organismer använder sig av för att fånga solljus (Figur 1.2 i avhandlingen).

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Acknowledgement

I want to thank everyone who has supported me during the last 5 years – they haven’t been the easiest.

My PhD project was financed by the Swedish Research Council (veten-

skapsrådet). I also want to use this opportunity to thank the Anna-Maria-Lundins stipendiefond from Smålands nation, the Liljewalchs stipendie-fond, Rektors resebidrag från Wallenbergstiftelsen and the Royal Acad-emy of Science for travel grants that allowed me to attend conferences.

A big “thank you” goes to my supervisors: Gerrit, for giving me the op-

portunity to work on this fantastic research project with you, for your scien-tific input and the critical questions. Anders, for always being motivational and supportive and your support for skolceller. Erik, for all the PES-measurements, for your well-reflected comments and for bringing more HSC-activities to the group. I also want to thank Laila, for handling receipts and other administrative matters and Ulrika, for sorting things out.

I cordially thank all current and past members of the Hagfeldt group for making the past years so enjoyable: Leif, for always being helpful, keeping the lab running and the storage room well stocked. Martin, for all the nice discussions and for sharing the writing-the-thesis-experience (you don’t dis-turb me that much). Kazu, for discussions on science and philosophy. Marcelo, for being supportive and lunch-running. Libby & Sandra, for sharing office and Lei for sharing office and lots of cookies. Susanna, for always being helpful. Nick, for being such a genuine personality. Peter, for helping to invade Sweden. Ute, for paving the way for other PhD students. Tannia & Maria, for all the nice chats – there is not enough dancing in the Hagfeldt group without you. Alex, for the saddest smiley-face. Halina, for-good vibes. Jarl, for being the coolest guy ever during his defence. All the short-time visitors that make the Hagfeldt group such a vibrant and diverse working environment.

I thank my collaborators: Jean, you taught me a great deal about how to write scientific papers. Emilie & Philippe, thank you for the synthesis of TPA’s and nice collaboration. Antonio, for giving me an introduction to OSCs. Ana and our shared project student Mats, it has been great working with you. Burkhard, I have learned a lot from you. Francesca, for a fun project together and for visiting Uppsala during the beach-volleyball season.

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Pål for assistance with the AFM. Joe, for AFM-profilometry and for smores. Tomas, your enthusiasm is contagious. Håkan, on can learn a lot from you in just 2 hours.

I want to thank: all the nice people working in the CMD: the Lars Kloo group, Licheng Sun group and Håkan Rensmo group. Especially Martin, Daniel, Erik and Haining for giving me dyes to play around with.

To all my friends at Fotomol: Measuring fluorescence was just an ex-cuse to come round and chat. The physical chemists at BMC, especially Ma-lin and Anna.

It was always nice to visit the CIGS group – especially when we wanted to see some real solar cells and for Friday beers. Special thanks to Uwe for guided tours to the roof, Timo for coffee and complaints, Adam for snow-boarding and smiles, Per-Oskar for a flashy defence. Jon and Kerstin for being very good friends and former neighbors - the sweets really helped getting through the worst.

It was a pleasure to work with all fellow lab-teachers - sharing the bur-den and the passion. Olle, it was fun teaching chemistry for biologists with you.

I thank my motivated project students: Stefan, my Erasmus-student from Marburg. David, for inspiring skolceller. Katta and Julia for being a great summer worker. Paul, for hunting efficiencies.

To all the nice and inspiring people that I met and met again through summer schools (Quantsol), conferences and outreach-activities: I hope I’ll see you again. I also want to thank the TekNat staff and Energihuset for the collaboration on skolceller.

I may have thanked you before but want to thank you once more (for proof-reading and giving feedback on this thesis): Martin, Burkhard, Jon, Kristofer, Miia, Peter, Erik, Anders and Gerrit.

I want to thank my friends and family for all the support during recent years and at the same time excuse myself for my absent-mindedness lately.

Ich möchte meinen Freunde und meiner Familie für die tatkräftige Unter-stützung in den letzten Jahren danken. Mama für die lieben Besuche. Papa, dafür dass er mich immer machen lässt, was ich will. Joachim und Peter, die nettesten Brüder die es gibt. Tine, für all die Jahre die wir uns schon kennen. Inga, für ein halbwegs synchrones Leben. Moana, für gro artige Geschichten, Besuche und Reisen. Tack, Martin (sötis).

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References

1. L. O. Björn, Photobiology - The science of life and light, 2nd edn., Springer, New York, 2008.

2. D. A. McQuarrie and J. D. Simon, Physical Chemistry: A molecular ap-proach, University Science Books, Sausalito, 1997.

3. K. Ångström, Annalen der Physik, 1900, 3, 720-732. 4. S. Bowden and C. Honsberg, vol. 2012. 5. B. A. Gregg, J. Sprague and M. W. Peterson, The Journal of Physical

Chemistry B, 1997, 101, 5362. 6. A. Huijser, B. M. J. M. Suijkerbuijk, R. J. M. Klein Gebbink, T. J. Savenije

and Siebbeles, Journal of the American Chemical Society, 2008, 130, 2485. 7. G. Calzaferri and K. Lutkouskaya, Photochemical & Photobiological Sci-

ences, 2008, 7. 8. N. A. Campbell and J. B. Reece, Biology, 8th edn., Benjamin Cummings,

2007. 9. I. E. A. IEA, Key World Energy Statistics, www.iea.org, 2010. 10. D. Guggenheim, 2006, p. 100 min. 11. P. Würfel, Physik der Solarzellen, 2. Auflage edn., Spektrum Akademi-

scher Verlag, Heidelberg Berlin, 2000. 12. G. Carlin, HBO, USA, 1992, p. 7:39. 13. B. A. Gregg, J. Phys. Chem. B, 2003, 107, 4688-4698. 14. B. A. Gregg, MRS Bulletin, 2005, 30, 20-22. 15. G. F. Burkhard, E. T. Hoke and M. D. McGehee, Advanced Materials,

2010, 22, 3293. 16. A.-E. Becquerel, Comptes Rendus Acad. Sci., 1839, 9, 561-567. 17. J. Moser, Akademischer Anzeiger Nr. XVI, 1887. 18. C. W. Tang, Applied Physics Letters, 1986, 48, 183-185. 19. G. Yu, J. Gao, J. C. Hummelen, F. Wudl and A. J. Heeger, Science, 1995,

270, 1789-1791. 20. B. O'Regan and M. Grätzel, Nature, 1991, 353, 737. 21. D. M. Chapin, C. S. Fuller and G. L. Pearson, J. Appl. Phys., 1954, 25, 676. 22. L. Fraas and L. Partain, Solar Cells and their Application, John Wiley and

sons, 2010. 23. D. C. Reynolds, G. Leies, L. L. Antes and R. E. Marburger, Physical Re-

view, 1954, 96, 533. 24. M. A. Green, K. Emery, Y. Hishikawa, W. Warta and E. D. Dunlop, Pro-

gress in Photovoltaics: Research and Applications, 2012, 20, 12. 25. B. A. Gregg and M. C. Hanna, Journal of Applied Physics, 2003, 93, 3605-

3614. 26. A. Hultqvist, PhD thesis, Uppsala University, 2010.

84

27. P.-O. Westin, PhD Thesis, Uppsala University, 2011. 28. J. J. Scragg, Springer Berlin Heidelberg, 2011. 29. M. A. Green, Solar cells: operating principles, technology, and system

applications, Prentice Hall, 1982. 30. B. Kippelen and J.-L. Bredas, Energy & Environmental Science, 2009, 2,

251. 31. H. Meier, W. Albrecht and U. Tschirwitz, Angewandte Chemie Interna-

tional Edition in English, 1972, 11, 1051. 32. H. Meier, Angewandte Chemie International Edition in English, 1965, 4,

619. 33. J. Nelson, Current Opinion in Solid State and Materials Science, 2002, 6,

87. 34. H. Hoppe and N. S. Sariciftci, J. Mater. Res., 2004, 19, 1924-1945. 35. T. M. Clarke and J. R. Durrant, Chemical Reviews, 2010. 36. R. F. Service, Science, 2011, 332, 293. 37. J. Meiss, T. Menke, K. Leo, C. Uhrich, W.-M. Gnehr, S. Sonntag, M. Pfeif-

fer and M. Riede, Applied Physics Letters, 2011, 99, 043301. 38. H. Gerischer, F. Willig, F. Schäfer, H. Gerischer, F. Willig, H. Meier, H.

Jahnke, M. Schönborn and G. Zimmermann, in Physical and Chemical Ap-plications of Dyestuffs, Springer Berlin / Heidelberg, 1976, vol. 61, p. 31.

39. R. Memming, Faraday Discussions of the Chemical Society, 1974, 58, 261. 40. S. M. Feldt, E. A. Gibson, E. Gabrielsson, L. Sun, G. Boschloo and A.

Hagfeldt, Journal of the American Chemical Society, 2010, null. 41. T. Daeneke, T.-H. Kwon, A. B. Holmes, N. W. Duffy, U. Bach and L.

Spiccia, Nature Chemistry, 2010, 3, 211. 42. U. Bach, D. Lupo, P. Comte, J. E. Moser, F. Weissortel, J. Salbeck, H.

Spreitzer and M. Grätzel, Nature, 1998, 395, 583. 43. M. Thelakkat, Macromolecular Materials and Engineering, 2002, 287,

442-461. 44. A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo and H. Pettersson, Chemical

Reviews, 2010, 110, 6595. 45. M. Grätzel, Journal of Photochemistry and Photobiology C: Photochemis-

try Reviews, 2003, 4, 145. 46. T. Marinado, PhD thesis, Kungliga Tekniska Högskolan, 2009. 47. A. Yella, H.-W. Lee, H. N. Tsao, C. Yi, A. K. Chandiran, M. K. Nazeerud-

din, E. W.-G. Diau, C.-Y. Yeh, S. M. Zakeeruddin and M. Grätzel, Science, 2011, 334, 629-634.

48. E. Arici, N. S. Sariciftci and D. Meissner, Encyclopedia of Nanoscience and Nanotechnology, 2004, 3, 929-944.

49. S. Günes and N. S. Sariciftci, Inorganica Chimica Acta, 2008, 361, 581. 50. J. Bouclé and J. Ackermann, Polymer International, 2011. 51. J. Bouclé, P. Ravirajan and J. Nelson, Journal of Materials Chemistry,

2007, 17, 3141-3153. 52. A. A. D. T. Adikaari, D. M. N. M. Dissanayake and S. R. P. Silva, Selected

Topics in Quantum Electronics, IEEE Journal of, 2011, 16, 1595. 53. J. Weickert, R. B. Dunbar, H. C. Hesse, W. Wiedemann and L. Schmidt-

Mende, Advanced Materials, 2011, n/a.

85

54. V. Gowrishankar, B. E. Hardin and M. D. McGehee, in Organic Photovol-taics: Materials, Device Physics and Manufacturing Technologies, eds. C. J. Brabec, V. Dyakonov and U. Scherf, WILEY-VCH Verlag GmbH, Weinheim, 2008, pp. 329-356.

55. N. C. Greenham, in Organic Photovoltaics: Materials, Device Physics and Manufacturing Technologies, eds. C. J. Brabec, V. Dyakonov and U. Scherf, WILEY-VCH Verlag GmbH, Weinheim, 2008, pp. 329-356.

56. C.-Y. Liu, Z. C. Holman and U. R. Kortshagen, Nano Letters, 2008, 9, 449. 57. C. Xia, N. Wang and X. Kim, Electrochimica Acta, 2011, 56, 9504. 58. W. U. Huynh, J. J. Dittmer and A. P. Alivisatos, Science, 2002, 295, 2425-

2427. 59. Y. Kang, N.-G. Park and D. Kim, Hybrid solar cells with vertically aligned

CdTe nanorods and a conjugated polymer, AIP, 2005. 60. E. Arici, N. S. Sariciftci and D. Meissner, Advanced Functional Materials,

2003, 13, 165. 61. J. J. Choi, Y.-F. Lim, M. E. B. Santiago-Berrios, M. Oh, B.-R. Hyun, L.

Sun, A. C. Bartnik, A. Goedhart, G. G. Malliaras, H. D. Abruña, F. W. Wise and T. Hanrath, Nano Letters, 2009, 9, 3749.

62. T. Skotheim, J. M. Yang, J. Otvos and M. P. Klein, The Journal of Chemi-cal Physics, 1982, 77, 6144-6150.

63. T. Skotheim, J. M. Yang, J. Otvos and M. P. Klein, The Journal of Chemi-cal Physics, 1982, 77, 6151-6161.

64. T. J. Savenije, J. M. Warman and A. Goossens, Chemical Physics Letters, 1998, 287, 148.

65. A. J. Breeze, Z. Schlesinger, S. A. Carter and P. J. Brock, Physical Review B, 2001, 64, 125205.

66. K. M. Coakley and M. D. McGehee, Applied Physics Letters, 2003, 83, 3380-3382.

67. P. A. v. Hal, M. M. Wienk, J. M. Kroon, W. J. H. Verhees, L. H. Slooff, W. J. H. v. Gennip, P. Jonkheijm and R. A. J. Janssen, Advanced Materials, 2003, 15, 118-121.

68. P. Ravirajan, S. A. Haque, J. R. Durrant, D. Poplavskyy, D. D. C. Bradley and J. Nelson, Journal of Applied Physics, 2004, 95, 1473-1480.

69. W. A. Daoud and M. L. Turner, Reactive and Functional Polymers, 2006, 66, 13.

70. C. Goh, S. R. Scully and M. D. McGehee, Journal of Applied Physics, 2007, 101, 114503.

71. D. C. Olson, S. E. Shaheen, M. S. White, W. J. Mitchell, M. F. A. M. van Hest, R. T. Collins and D. S. Ginley, Advanced Functional Materials, 2007, 17, 264-269.

72. T. J. Savenije, R. B. M. Koehorst and T. J. Schaafsma, Chemical Physics Letters, 1995, 244, 363.

73. J. E. Kroeze, T. J. Savenije, L. P. Candeias, J. M. Warman and L. D. A. Siebbeles, Solar Energy Materials and Solar Cells, 2005, 85, 189.

74. J. A. Ayllon and M. Lira-Cantu, Applied Physics A, 2009, 95, 249-255. 75. M. Lira-Cantu and F. C. Krebs, Solar Energy Materials and Solar Cells,

2006, 90, 2076.

86

76. L. D. A. Siebbeles, A. Huijser and T. J. Savenije, Journal of Materials Chemistry, 2009, 19, 6067.

77. H. J. Snaith and M. Grätzel, Applied Physics Letters, 2006, 89, 262114. 78. A. Abrusci, I. K. Ding, M. Al-Hashimi, T. Segal-Peretz, M. D. McGehee,

M. Heeney, G. L. Frey and H. J. Snaith, Energy & Environmental Science, 2011, 4, 3051.

79. S.-S. Kim, J. Jo, C. Chun, J.-C. Hong and D.-Y. Kim, Journal of Photo-chemistry and Photobiology A: Chemistry, 2007, 188, 364.

80. W. J. E. Beek, L. H. Slooff, M. M. Wienk, J. M. Kroon and R. A. J. Janssen, Advanced Functional Materials, 2005, 15, 1703-1707.

81. W. J. E. Beek, M. M. Wienk and R. A. J. Janssen, Adv. Mater., 2004, 16, 1009.

82. W. J. E. Beek and R. A. J. Janssen, in Hybrid Nanocomposites for Nanotechnology, 2009, p. 1.

83. S. D. Oosterhout, M. M. Wienk, S. S. van Bavel, R. Thiedmann, L. Jan Anton Koster, J. Gilot, J. Loos, V. Schmidt and R. A. J. Janssen, Nature Materials, 2009, 8, 818.

84. W. J. E. Beek, M. M. Wienk and R. A. J. Janssen, in Organic Photovol-taics: Materials, Device Physics and Manufacturing Technologies, eds. C. J. Brabec, V. Dyakonov and U. Scherf, WILEY-VCH Verlag GmbH, Weinheim, 2008, pp. 329-356.

85. A. C. Arango, S. A. Carter and P. J. Brock, Applied Physics Letters, 1999, 74, 1698-1700.

86. J. Weickert, F. Auras, T. Bein and L. Schmidt-Mende, The Journal of Physical Chemistry C, 2011, 115, 15081.

87. L. Yun-Yue, L. Yi-Ying, C. Liuwen, W. Jih-Jen and C. Chun-Wei, Applied Physics Letters, 2009, 94, 063308.

88. M. D. McGehee, Nat Photon, 2009, 3, 250. 89. G. K. Mor, S. Kim, M. Paulose, O. K. Varghese, K. Shankar, J. Basham

and C. A. Grimes, Nano Letters, 2009, 9, 4250. 90. D. C. Olson, J. Piris, R. T. Collins, S. E. Shaheen and D. S. Ginley, Thin

Solid Films, 2006, 496, 26. 91. M. Karlsson, PhD thesis, Uppsala University, 2012. 92. J. Burschka, A. Dualeh, F. Kessler, E. Baranoff, N.-L. Cevey-Ha, C. Yi, M.

K. Nazeeruddin and M. Grätzel, Journal of the American Chemical Society, 2011, 133, 18042.

93. H. J. Snaith, A. J. Moule, C. Klein, K. Meerholz, R. H. Friend and M. Grätzel, Nano Letters, 2007, 7, 3372.

94. M. Antoniadou, E. Stathatos, N. Boukos, A. Stefopoulos, J. Kallitsis, F. Krebs, C. and P. Lianos, Nanotechnology, 2009, 20, 495201.

95. D. Gebeyehu, C. J. Brabec, N. S. Sariciftci, D. Vangeneugden, R. Kie-booms, D. Vanderzande, F. Kienberger and H. Schindler, Synthetic Metals, 2001, 125, 279.

96. R. Zhu, C.-Y. Jiang, B. Liu and S. Ramakrishna, Advanced Materials, 2009, 21, 994.

97. H. J. Lee, H. C. Leventis, S. A. Haque, T. Torres, M. Grätzel and M. K. Nazeeruddin, Journal of Power Sources, 2011, 196, 596.

87

98. K.-J. Jiang, K. Manseki, Y.-H. Yu, N. Masaki, K. Suzuki, Y.-l. Song and S. Yanagida, Advanced Functional Materials, 2009, 19, 2481.

99. L. Yang, U. B. Cappel, E. L. Unger, M. Karlsson, K. M. Karlsson, E. Gab-rielsson, L. Sun, G. Boschloo, A. Hagfeldt and E. M. J. Johansson, Physical Chemistry Chemical Physics, 2012, 14, 779.

100. S.-J. Moon, E. Baranoff, S. M. Zakeeruddin, C.-Y. Yeh, E. W.-G. Diau, M. Grätzel and K. Sivula, Chemical Communications, 2011, 47, 8244.

101. A. Abrusci, R. S. Santosh Kumar, M. Al-Hashimi, M. Heeney, A. Petrozza and H. J. Snaith, Advanced Functional Materials, 2011, 21, 2571.

102. B. E. Hardin, E. T. Hoke, P. B. Armstrong, J.-H. Yum, P. Comte, T. Torres, J. M. J. Frechet, M. K. Nazeeruddin, M. Grätzel and M. D. McGehee, Nat Photon, 2009, 3, 406.

103. J.-H. Yum, B. E. Hardin, E. T. Hoke, E. Baranoff, S. M. Zakeeruddin, M. K. Nazeeruddin, T. Torres, M. D. McGehee and M. Grätzel, ChemPhy-sChem, 2011, 12, 657.

104. B. E. Hardin, J.-H. Yum, E. T. Hoke, Y. C. Jun, P. Pechy, T. Torres, M. L. Brongersma, M. K. Nazeeruddin, M. Grätzel and M. D. McGehee, Nano Letters, 2010, 10, 3077.

105. K. Driscoll, J. Fang, N. Humphry-Baker, T. s. Torres, W. T. S. Huck, H. J. Snaith and R. H. Friend, Nano Letters, 2010, 10, 4981.

106. G. K. Mor, J. Basham, M. Paulose, S. Kim, O. K. Varghese, A. Vaish, S. Yoriya and C. A. Grimes, Nano Letters, 2010, 10, 2387.

107. J. H. Yum, B. Hardin, S. J. Moon, E. Baranoff, F. Nüesch, M. McGehee, M. Grätzel and M. Nazeeruddin, Angewandte Chemie, 2009, 121, 9441.

108. S. Buhbut, S. Itzhakov, E. Tauber, M. Shalom, I. Hod, T. Geiger, Y. Garini, D. Oron and A. Zaban, ACS Nano, 2010, 4, 1293.

109. J. I. Basham, G. K. Mor and C. A. Grimes, ACS Nano, 2010, 4, 1253. 110. J.-H. Yum, E. Baranoff, S. Wenger, M. K. Nazeeruddin and M. Gratzel,

Energy & Environmental Science, 2011, 4, 842. 111. B. P. Rand, D. P. Burk and S. R. Forrest, Physical Review B, 2007, 75,

115327. 112. S. Ardo and G. J. Meyer, Chemical Society Reviews, 2009, 38, 115. 113. N. Humphry-Baker, K. Driscoll, A. Rao, T. Torres, H. J. Snaith and R. H.

Friend, Nano Letters, 2012. 114. J. Frenkel, Physical Review, 1931, 37, 17. 115. W. Y. Liang, Phys. Educ., 1970, 5, 226. 116. G. H. Wannier, Physical Review, 1937, 52, 191. 117. V. May and O. Kühn, Charge and Energy Transfer Dynamics in Molecular

Systems, 1st Edition edn., Wiley-VCH, Berlin, 2000. 118. P. E. Shaw, A. Ruseckas and I. D. W. Samuel, Advanced Materials, 2008,

20, 3516. 119. J. E. Kroeze, T. J. Savenije, M. J. W. Vermeulen and J. M. Warman, The

Journal of Physical Chemistry B, 2003, 107, 7696-7705. 120. M. K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry-Baker, E. Mueller, P.

Liska, N. Vlachopoulos and M. Graetzel, Journal of the American Chemi-cal Society, 1993, 115, 6382.

88

121. M. Nazeeruddin, S. M. Zakeeruddin, N. Humphry-Baker, M. Jirousek, P. Liska, N. Vlachopoulos, V. Shklover, C. H. Fischer and M. Grätzel, Inorg. Chem., 1999, 38, 6298-6305.

122. A. Mishra, M. K. R. Fischer and P. Bäuerle, Angewandte Chemie Interna-tional Edition, 2009, 48, 2474.

123. D. P. Hagberg, PhD Thesis, Summary, Royal Institute of Technology - KTH, 2009.

124. D. P. Hagberg, T. Edvinsson, T. Marinado, G. Boschloo, A. Hagfeldt and L. Sun, Chem. Commun., 2006, 2245-2247.

125. D. P. Hagberg, T. Marinado, K. M. Karlsson, K. Nonomura, P. Qin, G. Boschloo, T. Brinck, A. Hagfeldt and L. Sun, The Journal of Organic Chemistry, 2007, 72, 9550.

126. T. Geiger, S. Kuster, J.-H. Yum, S.-J. Moon, M. K. Nazeeruddin, M. Grätzel and F. Nüesch, Advanced Functional Materials, 2009, 19, 2720.

127. T. Marinado, D. P. Hagberg, M. Hedlund, T. Edvinsson, E. M. J. Johans-son, G. Boschloo, H. Rensmo, T. Brinck, L. Sun and A. Hagfeldt, Physical Chemistry Chemical Physics, 2009, 11, 133.

128. D. P. Hagberg, J.-H. Yum, H. Lee, F. De Angelis, T. Marinado, K. M. Karlsson, R. Humphry-Baker, L. Sun, A. Hagfeldt, M. Grätzel and M. K. Nazeeruddin, Journal of the American Chemical Society, 2008, 130, 6259.

129. J.-H. Yum, P. Walter, S. Huber, D. Rentsch, T. Geiger, F. Nüesch, F. De Angelis, M. Grätzel and M. K. Nazeeruddin, Journal of the American Chemical Society, 2007, 129, 10320.

130. J. H. Yum, J. S. Moon, R. Humphry-Baker, P. Walter, T. Geiger, F. Nüesch, M. Grätzel and M. Nazeeruddin, Nanotechnology, 2008, 19, 424005.

131. Y. Shi, R. B. M. Hill, J.-H. Yum, A. Dualeh, S. Barlow, M. Grätzel, S. R. Marder and M. K. Nazeeruddin, Angewandte Chemie International Edition, 2011, 50, 6619.

132. G. Brocks and A. Tol, The Journal of Physical Chemistry, 1996, 100, 1838. 133. U. Mayerhöffer, K. Deing, K. Gruß, H. Braunschweig, K. Meerholz and F.

Würthner, Angewandte Chemie International Edition, 2009, 48. 134. M. Persson, Uppsala University, 2010. 135. S. Roquet, A. Cravino, P. Leriche, O. Alévêque, P. Frère and J. Roncali,

Journal of the American Chemical Society, 2006, 128, 3459-3466. 136. J. Roncali, P. Leriche and A. Cravino, Advanced Materials, 2007, 19, 2045-

2060. 137. H. Kageyama, H. Ohishi, M. Tanaka, Y. Ohmori and Y. Shirota, Advanced

Functional Materials, 2009, 19, 3948. 138. Y. J. Shirota, J. Mater. Chem., 2000, 10, 1-25. 139. H. Ohishi, M. Tanaka, H. Kageyama and Y. Shirota, Chemistry Letters,

2004, 33, 1266. 140. A. Cravino, S. Roquet, P. Leriche, O. Alévêque, P. Frère and J. Roncali,

Chem. Commun., 2006, 1416-1418. 141. A. Cravino, P. Leriche, O. Alévêque, S. Roquet and J. Roncali, Advanced

Materials, 2006, 18, 3033-3037.

89

142. A. Ehret, L. Stuhl and M. T. Spitler, The Journal of Physical Chemistry B, 2001, 105, 9960.

143. F. Nüesch, J. E. Moser, V. Shklover and M. Grätzel, J Am Chem Soc, 1996, 118, 5420-5431.

144. N. Banerji, S. Cowan, E. Vauthey and A. J. Heeger, The Journal of Physi-cal Chemistry C, 2011, 115, 9726.

145. R. Karpicz, S. Puzinas, S. Krotkus, K. Kazlauskas, S. Jursenas, J. V. Grazulevicius, S. Grigalevicius and V. Gulbinas, The Journal of Chemical Physics, 2011, 134, 204508.

146. T. H. Förster, Discuss. Faraday Soc., 1959, 27, 7-17. 147. D. L. Dexter and W. R. Heller, Physical Review, 1953, 91, 273-278. 148. E. T. Hoke, B. E. Hardin and M. D. McGehee, Opt. Express, 2010, 18,

3893. 149. E. Hennebicq, G. Pourtois, G. D. Scholes, L. M. Herz, D. M. Russell, C.

Silva, S. Setayesh, A. C. Grimsdale, K. Müllen, J.-L. Brédas and D. Bel-jonne, Journal of the American Chemical Society, 2005, 127, 4744.

150. K. Kawata, V. M. Burlakov, M. J. Carey, H. E. Assender, G. A. D. Briggs, A. Ruseckas and I. D. W. Samuel, Solar Energy Materials and Solar Cells, 2005, 87, 715.

151. A. Huijser, T. J. Savenije, A. Shalav and L. D. A. Siebbeles, Journal of Applied Physics, 2008, 104, 034505.

152. J. R. Lakowicz, Principles of Fluorescence Spectroscopy, Third Edition edn., Springer, New York, Singapore, 2006.

153. A. Holzhey, C. Uhrich, E. Brier, E. Reinhold, P. Bauerle, K. Leo and M. Hoffmann, Journal of Applied Physics, 2008, 104, 064510.

154. J. Stampfl, S. Tasch, G. Leising and U. Scherf, Synthetic Metals, 1995, 71, 2125.

155. S. R. Forrest, MRS Bulletin, 2005, 30, 28-32. 156. Y. Liu, M. A. Summers, C. Edder, J. M. J. Fréchet and M. D. McGehee,

Advanced Materials, 2005, 17, 2960-2964. 157. D. E. Markov, E. Amsterdam, P. W. M. Blom, A. B. Sieval and J. C.

Hummelen, The Journal of Physical Chemistry A, 2005, 109, 5266. 158. H. R. Kerp, H. Donker, R. B. M. Koehorst, T. J. Schaafsma and E. E. van

Faassen, Chemical Physics Letters, 1998, 298, 302. 159. M. G. Harrison, J. Grüner and G. C. W. Spencer, Physical Review B, 1997,

55, 7831. 160. Y. Terao, H. Sasabe and C. Adachi, Correlation of hole mobility, exciton

diffusion length, and solar cell characteristics in phthalocyanine/fullerene organic solar cells, AIP, 2007.

161. S. Södergren, A. Hagfeldt, J. Olsson and S.-E. Lindquist, Journal of Physi-cal Chemistry, 1994, 98, 5552-5556.

162. J. Tousek, J. Tousková, I. Krivka, P. Pavlacková, D. Výprachtický and V. Cimrová, Organic Electronics, 11, 50.

163. S. R. Scully and M. D. McGehee, Journal of Applied Physics, 2006, 100, 034907.

90

164. Z. Sun, Y. Cheng, M. Lechmann, J. Li, J. Li, J. Wu, A. C. Grimsdale, K. Müllen, H.-J. Butt and J. S. Gutmann, Physical Chemistry Chemical Phys-ics, 2009, 11, 1604-1609.

165. R. R. Lunt, J. B. Benziger and S. R. Forrest, Advanced Materials, 2010, 22, 1233-1236.

166. A. Liu, M. A. Summers, S. R. Scully and M. D. McGehee, J. Appl. Phys., 2006, 99, 093521.

167. S. R. Scully, P. B. Armstrong, C. Edder, J. M. J. Fréchet and M. D. McGe-hee, Advanced Materials, 2007, 19, 2961.

168. A. Ghosh, K. and T. Feng, Journal of Applied Physics, 1978, 49, 5982-5989.

169. L. A. A. Pettersson, L. S. Roman and O. Inganas, Journal of Applied Phys-ics, 1999, 86, 487-496.

170. P. Peumanns, Y. Aharon and S. Forrest, R., Journal of Applied Physics, 2003, 93, 3693-3723.

171. J. E. Kroeze, PhD Thesis, Technische Universiteit, 2004. 172. M. Wang and X. Wang, Solar Energy Materials and Solar Cells, 2008, 92,

766. 173. M. Wang and X. Wang, Solar Energy Materials and Solar Cells, 2007, 91,

1782. 174. N. Kudo, S. Honda, Y. Shimazaki, H. Ohkita, S. Ito and H. Benten, Applied

Physics Letters, 2007, 90, 183513. 175. N. Kudo, Y. Shimazaki, H. Ohkita, M. Ohoka and S. Ito, Solar Energy

Materials and Solar Cells, 2007, 91, 1243. 176. W. A. Nevin and G. A. Chamberlain, 1991, 69, 4324-4332. 177. S. Ferrere and B. A. Gregg, Journal of the American Chemical Society,

1998, 120, 843. 178. G. Benkö, J. Kallioinen, J. E. I. Korppi-Tommola, A. P. Yartsev and V.

Sundström, Journal of the American Chemical Society, 2001, 124, 489. 179. I. Haeldermans, K. Vandewal, W. D. Oosterbaan, A. Gadisa, J. D. Haen,

M. K. V. Bael, J. V. Manca and J. Mullens, Applied Physics Letters, 2008, 93, 223302.

180. R. J. Davis, M. T. Lloyd, S. R. Ferreira, M. J. Bruzek, S. E. Watkins, L. Lindell, P. Sehati, M. Fahlman, J. E. Anthony and J. W. P. Hsu, Journal of Materials Chemistry, 2011, 21, 1721.

181. U. B. Cappel, A. L. Smeigh, S. Plogmaker, E. M. J. Johansson, H. Rensmo, L. Hammarström, A. Hagfeldt and G. Boschloo, The Journal of Physical Chemistry C, 2011, 115, 4345.

182. M. Hahlin, PhD thesis, Uppsala University, 2010. 183. E. M. J. Johansson, PhDThesis, Uppsala University, 2006. 184. S. Braun, W. Osikowicz, Y. Wang and W. R. Salaneck, Organic Electron-

ics, 2007, 8, 14-20. 185. B. Bröker, O. T. Hofmann, G. M. Rangger, P. Frank, R. P. Blum, R.

Rieger, L. Venema, A. Vollmer, K. Müllen, J. P. Rabe, A. Winkler, P. Ru-dolf, E. Zojer and N. Koch, Physical Review Letters, 2010, 104, 246805.

186. M. Lira-Cantu, K. Norrman, J. W. Andreasen, N. Casan-Pastor and F. C. Krebs, Journal of The Electrochemical Society, 2007, 154, B508-B513.

91

187. A. Hagfeldt and M. Grätzel, Accounts of Chemical Research, 2000, 33, 269.

188. A. Sandell, B. Sanyal, L. E. Walle, P. Uvdal and A. Borg, Journal of Elec-tron Spectroscopy and Related Phenomena, 2010, 183, 107.

189. R. Asahi, Y. Taga, W. Mannstadt and A. J. Freeman, Physical Review B, 2000, 61, 7459.

190. R. Memming, in Semiconductor Electrochemistry, Wiley-VCH Verlag GmbH, 2007, p. 22.

191. P. J. Cameron and L. M. Peter, The Journal of Physical Chemistry B, 2003, 107, 14394.

192. F. Cardon and W. P. Gomes, J. Phys. D: Appl. Phys., 1978, 11, L63. 193. F. Fabregat-Santiago, G. Garcia-Belmonte, J. Bisquert, P. Bogdanoff and

A. Zaban, Journal of The Electrochemical Society, 2003, 150, E293-E298. 194. M. E. Özsan, D. R. Johnson, M. Sadeghi, D. Sivapathasundaram, G. Good-

let, M. J. Furlong, L. M. Peter and A. A. Shingleton, Journal of Materials Science: Materials in Electronics, 1996, 7, 119.

195. G. Boschloo, PhD thesis, Technische Universiteit Delft, 1996. 196. G. K. Boschloo, A. Goossens and J. Schoonman, J. Electrochem. Soc.,

1997, 144, 1311-1317. 197. S. Karlsson, PhD thesis, Uppsala University, 2010. 198. U. B. Cappel, S. M. Feldt, J. Schöneboom, A. Hagfeldt and G. Boschloo,

Journal of the American Chemical Society, 2010, 132, 9096. 199. S. Ardo, Y. Sun, F. N. Castellano and G. J. Meyer, The Journal of Physical

Chemistry B, 2010, 114, 14596. 200. U. B. Cappel, PhD Thesis, Uppsala University, 2011. 201. G. Boschloo and A. Hagfeldt, Inorganica Chimica Acta, 2008, 361, 729. 202. V. Coropceanu, J. Cornil, D. A. da Silva Filho, Y. Olivier, R. Silbey and J.-

L. Brédas, Chemical Reviews, 2007, 107, 926. 203. P. Docampo, S. Guldin, M. Stefik, P. Tiwana, M. C. Orilall, S. Hüttner, H.

Sai, U. Wiesner, U. Steiner and H. J. Snaith, Advanced Functional Materi-als, 2010, 20, 1787.

204. P. Ravirajan, A. M. Peiro, M. K. Nazeeruddin, M. Grätzel, D. D. C. Brad-ley, J. R. Durrant and J. Nelson, The Journal of Physical Chemistry B, 2006, 110, 7635.

205. M. D. McGehee, MRS Bulletin, 2009, 34, 95-100. 206. U. B. Cappel, E. A. Gibson, A. Hagfeldt and G. Boschloo, The Journal of

Physical Chemistry C, 2009, 113, 6275. 207. I. K. Ding, N. Tétreault, J. Brillet, B. E. Hardin, E. H. Smith, S. J. Rosen-

thal, F. Sauvage, M. Grätzel and M. D. McGehee, Advanced Functional Materials, 2009, 19, 2431.

208. J. Melas-Kyriazi, I. K. Ding, A. Marchioro, A. Punzi, B. E. Hardin, G. F. Burkhard, N. Tétreault, M. Grätzel, J.-E. Moser and M. D. McGehee, Ad-vanced Energy Materials, 2011, 1, 407.

209. H. J. Snaith, R. Humphry-Baker, P. Chen, I. Cesar, S. M. Zakeeruddin and M. Grätzel, Nanotechnology, 2008, 19, 424003.

210. C. Olson, D. Veldman, K. Bakker and F. Lenzmann, Int. J. Photoenergy, 2011, 2011.

92

211. K. M. Coakley, Y. Liu, M. D. McGehee, K. L. Frindell and G. D. Stucky, Advanced Functional Materials, 2003, 13, 301.

212. K. Fredin, E. M. J. Johansson, T. Blom, M. Hedlund, S. Plogmaker, H. Siegbahn, K. Leifer and H. k. Rensmo, Synthetic Metals, 2009, 159, 166.

213. P. Atienzar, T. Ishwara, M. Horie, J. R. Durrant and J. Nelson, Journal of Materials Chemistry, 2009, 19, 5377.

214. S. Lu, S.-S. Sun, X. Jiang, J. Mao, T. Li and K. Wan, Journal of Materials Science: Materials in Electronics, 2010, 21, 682.

215. M. Klingstedt, PhD thesis, Stockholm University, 2012. 216. I. Horcas, R. Fernandez, J. M. Gomez-Rodriguez, J. Colchero, J. Colchero,

J. Gomez-Herrero and A. M. Baro, Review of Scientific Instruments, 2007, 78, 013705.

217. A. C. Arango, L. R. Johnson, V. N. Bliznyuk, Z. Schlesinger, S. A. Carter and H. H. Hörhold, Advanced Materials, 2000, 12, 1689-1692.

218. E. M. J. Johansson, M. Odelius, P. G. Karlsson, H. Siegbahn, A. Sandell and H. Rensmo, The Journal of Chemical Physics, 2008, 128, 184709.

219. J. Krüger, U. Bach and M. Grätzel, Advanced Materials, 2000, 12, 447-451.

220. E. M. J. Johansson, R. Schölin, H. Siegbahn, A. Hagfeldt and H. Rensmo, Chemical Physics Letters, 2011, 515, 146.

221. P. Ravirajan, D. D. C. Bradley, J. Nelson, S. A. Haque, J. R. Durrant, H. J. P. Smit and J. M. Kroon, Applied Physics Letters, 2005, 86, 143101.

222. D. A. R. Barkhouse, H. E. Bishop, B. M. Henry, G. R. Webster, P. L. Burn and H. E. Assender, Organic Electronics, 2010, 11, 649.

223. A. Cravino, P. Schilinsky and C. J. Brabec, Advanced Functional Materi-als, 2007, 17, 3906-3910.

224. K. M. Karlsson, PhD thesis, KTH Stockholm, 2011. 225. Y. Hao, X. Yang, J. Cong, H. Tian, A. Hagfeldt and L. Sun, Chemical

Communications, 2009, 4031-4033. 226. C. S. Karthikeyan and M. Thelakkat, Inorganica Chimica Acta, 2008, 361,

635. 227. K. R. Haridas, J. Ostrauskaite, M. Thelakkat, M. Heim, R. Bilke and D.

Haarer, Synthetic Metals, 2001, 121, 1573. 228. L. Kavan and M. Grätzel, Electrochimica Acta, 1995, 40, 643. 229. B. O'Regan, is thanked for the recipy for the ZrO2 nanoparticle dispersion. 230. V. V. Pavlishchuk and A. W. Addison, Inorganica Chimica Acta, 2000,

298, 97. 231. A. J. Bard and L. R. Faulkner, Electrochemical Methods: Fundamentals

and Applications, John Wiley & Sons, Inc., Weinheim, New York, 2001. 232. M. Bässler, J. O. Forsell, O. Björneholm, R. Feifel, M. Jurvansuu, S. Ak-

sela, S. Sundin, S. L. Sorensen, R. Nyholm, A. Ausmees and S. Svensson, Journal of Electron Spectroscopy and Related Phenomena, 1999, 101-103, 953.

93

233. S. Svensson, J. O. Forsell, H. Siegbahn, A. Ausmees, G. Bray, S. Söder-gren, S. Sundin, S. J. Osborne, S. Aksela, E. Nommiste, J. Jauhiainen, M. Jurvansuu, J. Karvonen, P. Barta, W. R. Salaneck, A. Evaldsson, M. Lögdlund and A. Fahlman, Review of Scientific Instruments, 1996, 67, 2149-2156.

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Für meine Familie Till min familj500212/... · 2012-03-01 · Für meine Familie Till min familj . List of Papers This thesis is based on the following papers and manuscripts, which