Ultrafast Photoinduced Processes in Core and Core–Shell ...lup.lub.lu.se/search/ws/files/5516980/5051929.pdf · Mohamed Mohamed Ahmed Ahmed Ahmed AbdellahAbdellahAbdellah Doctoral

LUND UNIVERSITY

PO Box 117221 00 Lund+46 46-222 00 00

Ultrafast Photoinduced Processes in Core and Core–Shell Quantum Dots for Solar CellApplications “Tiny Crystals for Big Applications”

Qenawy, Mohamed

2015

Link to publication

Citation for published version (APA):Qenawy, M. (2015). Ultrafast Photoinduced Processes in Core and Core–Shell Quantum Dots for Solar CellApplications “Tiny Crystals for Big Applications”. Division of Chemical Physics, Department of Chemistry, LundUniversity.

Total number of authors:1

General rightsUnless other specific re-use rights are stated the following general rights apply:Copyright and moral rights for the publications made accessible in the public portal are retained by the authorsand/or other copyright owners and it is a condition of accessing publications that users recognise and abide by thelegal requirements associated with these rights. • Users may download and print one copy of any publication from the public portal for the purpose of private studyor research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal

Read more about Creative commons licenses: https://creativecommons.org/licenses/Take down policyIf you believe that this document breaches copyright please contact us providing details, and we will removeaccess to the work immediately and investigate your claim.

https://portal.research.lu.se/portal/en/publications/ultrafast-photoinduced-processes-in-core-and-coreshell-quantum-dots-for-solar-cell-applications-tiny-crystals-for-big-applications(e8429083-a838-485c-9fbb-42d5a2f35bec).html

Ultrafast Ultrafast Ultrafast Ultrafast Photoinduced Photoinduced Photoinduced Photoinduced ProcessesProcessesProcessesProcesses in in in in Core Core Core Core and and and and CoreCoreCoreCore––––Shell Quantum Dots Shell Quantum Dots Shell Quantum Dots Shell Quantum Dots for for for for Solar Cell Solar Cell Solar Cell Solar Cell

ApplicationApplicationApplicationApplicationssss

““““Tiny Tiny Tiny Tiny Crystals Crystals Crystals Crystals for for for for Big ApplicationsBig ApplicationsBig ApplicationsBig Applications””””

Mohamed Mohamed Mohamed Mohamed Ahmed Ahmed Ahmed Ahmed AbdellahAbdellahAbdellahAbdellah

Doctoral Doctoral Doctoral Doctoral DissertationDissertationDissertationDissertation

Faculty opponent

Professor Prashant V. KamatPrashant V. KamatPrashant V. KamatPrashant V. Kamat, Professor of Science

Radiation Laboratory and Department of Chemical & Bimolecular Engineering,

University of Notre Dame, Indiana, United States

Organization: Lund University,: Lund University,: Lund University,: Lund University, Chemistry Department,Chemistry Department,Chemistry Department,Chemistry Department, Division of Chemical Physics, P.O. Box 124 SEDivision of Chemical Physics, P.O. Box 124 SEDivision of Chemical Physics, P.O. Box 124 SEDivision of Chemical Physics, P.O. Box 124 SE----221 00, 221 00, 221 00, 221 00, Lund, SwedenLund, SwedenLund, SwedenLund, Sweden

Document name: Doctoral Dissertation: Doctoral Dissertation: Doctoral Dissertation: Doctoral Dissertation

Date of issue: 2015: 2015: 2015: 2015----02020202----16161616

Author: : : : Mohamed Ahmed AbdellahMohamed Ahmed AbdellahMohamed Ahmed AbdellahMohamed Ahmed Abdellah QenawyQenawyQenawyQenawy

Title: Ultrafast Ultrafast Ultrafast Ultrafast Photoinduced Photoinduced Photoinduced Photoinduced ProcessesProcessesProcessesProcesses in in in in Core Core Core Core and and and and CoreCoreCoreCore––––Shell Quantum Dots Shell Quantum Dots Shell Quantum Dots Shell Quantum Dots for for for for Solar Cell ApplicationSolar Cell ApplicationSolar Cell ApplicationSolar Cell Applicationssss

““““Tiny Tiny Tiny Tiny Crystals Crystals Crystals Crystals for for for for Big ApplicationsBig ApplicationsBig ApplicationsBig Applications””””

Abstract The balance between our demands of energy and the energy we are consuming is not in equilibrium anymore. Therefore, the search for other energy resources is indispensable. Sustainable energy sources offer the alternative to the fossil fuels. Within many types of sustainable energy sources, solar energy offers more than the total global energy consumption. Solar cells are the smart conversion tools to harvest the incident photons and create electricity out of these photons. Solar cells have passed through different generations where two important factors control the solar cells market. The solar cell efficiency and price play the cornerstones in the solar cell marketing. Third generation of solar cells aims to maximize the price–performance equation by using cheap materials without compromising efficiency. Nanomaterials have emerged as the promising building blocks to harvest the solar light in the third generation of the solar cells. Among them, quantum dots (QDs) can be used as viable candidate due to their superb features such as high extinction coefficient, a tunable absorption edge, and the possibility to generate and collect multiple excitons by using single, high energy photon. Both the behaviors of photoexcited electrons and holes determine the overall efficiency of QD based solar cells. This thesis presents a systematic study of the ultrafast photoinduced charge dynamics in QD solar cell materials including the charge transfer, exciton migration, carrier trapping and their influence on real solar cell performance. The materials investigated start with conventional neat core CdSe QDs and extend to gradient Cd1-xSe1-yZnxSy core–shell (CS) QDs. The latter are used to obtain improved optical and device performance. The electron injection from CdSe into ZnO nanowires were first observed to be very fast (few ps). This fast electron injection encourages us to study the possibility to inject multiple electrons from a QD under high excitation conditions. We revealed that a competition between electron injection and Auger recombination occurs.

Compared with electrons, the photoinduced holes are more likely to be trapped. However, such trap states sometimes can be radiative with long lifetime up to tens of microseconds in oleic acid capped CdSe QDs. In this scenario, the hole injection in p-type QD solar cells are proved to be less efficient (<10%) compared with electron injection in n-type counterparts. It is highly affected by the surface trapping sites induced by the linker exchange process. The hole injection can then be improved by passivating the surface trap sites using core shell structures. Besides electron or hole injection, exciton migration can also occur via Förster resonant energy transfer (FRET). We found that FRET between QDs would enable to make use of the absorption of light by the indirectly attached QDs in QD-sensitized metal oxide (MO) anodes. In well-organized multi-sized QD mixtures, the energy transfer is even more pronounced. We experimentally observed the FRET process in randomly arranged multi-sized QD assembly and tandem stacked QD layers by using time-resolved and steady-state spectroscopies. Theoretical simulations where dipole distribution model was introduced for coupling calculations complies well with the experimental results. In order to minimize the effect of surface defects and improve the photostability of QD solar cells, we investigated the core–shell QD system where the surface trapping of carriers can be well passivated by shell materials with enhanced optical properties and device performance. Herein, a wider band gap semiconductor is employed as a shield shell around the active core in gradient growth, known as gradient Cd1-xSe1-yZnxSy CSQDs. Such QDs offer higher photostability, higher fluorescence quantum yield, and less interfacial defects than the conventional step-like CSQDs. We first characterized the gradient CSQDs using steady-state optical spectroscopy and HR-TEM images in order to determine their dimensions and to evaluate the shell thickness. Then XRD and EDX were used to characterize the chemical composition and the crystal structures.

The photodynamic of these CSQDs in photovoltaic systems was also studied. We first found that the electron injection from the active core to n-type MO showed relatively larger exponential shell thickness dependence compared with step-like CSQDs. We established that the highest electron injection efficiency (~ 80%) can be found with shell thickness up to 1.3 nm. Such shell also allows high surface passivation providing optimal conditions for charge collection in solar cells. Finally, we integrated our knowledge about the electron and hole behaviors to explain the solar cell performances according to the core–shell structure. We confirmed that the hole trapping is the critical factor for QD-sensitized solar cell efficiency. The trapping can be well repaired by using optimal core–shell structure.

Key words: Quantum dots, coreQuantum dots, coreQuantum dots, coreQuantum dots, core––––shell, electron injection, hole injection, hole trapping, exciton migration, solar shell, electron injection, hole injection, hole trapping, exciton migration, solar shell, electron injection, hole injection, hole trapping, exciton migration, solar shell, electron injection, hole injection, hole trapping, exciton migration, solar cells, ultrafast dynamics, timecells, ultrafast dynamics, timecells, ultrafast dynamics, timecells, ultrafast dynamics, time----resolved spectroscopy.resolved spectroscopy.resolved spectroscopy.resolved spectroscopy. Classification system and/or index terms (if any) Language: English: English: English: English

Supplementary bibliographical information ISBN: ISSN and key title Price Recipient’s notes Number of pages: Security classification

I, the undersigned, being the copyright owner of the abstract of the above-mentioned dissertation, hereby grant to all reference sources permission to publish and disseminate the abstract of the above-mentioned dissertation.

Signature: Date: 2015-02-16

Ultrafast Photoinduced Ultrafast Photoinduced Ultrafast Photoinduced Ultrafast Photoinduced ProcessesProcessesProcessesProcesses in in in in Core and CoreCore and CoreCore and CoreCore and Core––––Shell Quantum Dots Shell Quantum Dots Shell Quantum Dots Shell Quantum Dots

for Solar Cell for Solar Cell for Solar Cell for Solar Cell ApplicationApplicationApplicationApplicationssss

“Tiny “Tiny “Tiny “Tiny Crystals Crystals Crystals Crystals for for for for Big ApplicationsBig ApplicationsBig ApplicationsBig Applications””””

Mohamed Mohamed Mohamed Mohamed Ahmed Ahmed Ahmed Ahmed AbdellahAbdellahAbdellahAbdellah

Doctoral DissertationDoctoral DissertationDoctoral DissertationDoctoral Dissertation

Academic thesis, which by due permission of the Faculty of Science, Lund University, will be publicly defended on Friday 13th of March, 2015 at 09:15 a.m. in lecture hall C, at the Center of Chemistry and Chemical Engineering, Getingevägen 60, Lund, for the degree of Doctor of Philosophy in Science.

Cover picture: Quantum dots for solar energy harvesting by Maiy A. ElQuantum dots for solar energy harvesting by Maiy A. ElQuantum dots for solar energy harvesting by Maiy A. ElQuantum dots for solar energy harvesting by Maiy A. El----wakeelwakeelwakeelwakeel....

Mohamed AbdellahMohamed AbdellahMohamed AbdellahMohamed Abdellah

Full name: Mohamed Ahmed Abdellah QenawyMohamed Ahmed Abdellah QenawyMohamed Ahmed Abdellah QenawyMohamed Ahmed Abdellah Qenawy

Copyright © Mohamed Abdellah© Mohamed Abdellah© Mohamed Abdellah© Mohamed Abdellah

Department of Chemical Physics, Faculty of Science, Lund University, Sweden

ISBN 978978978978----91919191----7422742274227422----388388388388----0000 Printed in Sweden by Media-Tryck, Lund University Lund 2015

5اآليه سورة الضحى “And your Lord (ALLAH) is going to give you (Prophet Muhammed), and you will be satisfied” Surat Al-Duhaa 5

Dedicated to,

Soul of my mother

Soul of my father-in-law

Dalia

Eyad, Yahia and Dareen

Contents

AbstractAbstractAbstractAbstract.......................................................................................................... i AcknowledgementsAcknowledgementsAcknowledgementsAcknowledgements........................................................................................ iii List of papersList of papersList of papersList of papers................................................................................................. vi Contribution reportContribution reportContribution reportContribution report...................................................................................... viii Abbreviations and symbolsAbbreviations and symbolsAbbreviations and symbolsAbbreviations and symbols........................................................................... xi Chapter 1: IntroductionChapter 1: IntroductionChapter 1: IntroductionChapter 1: Introduction............................................................................... 1 1.1 Sustainable energy sources.................................................................... 1 1.2 Quantum Dot (QD)............................................................................ 2 1.3 Quantum Dot (QD) properties........................................................... 4 1.4 Core–shell quantum dots (CSQDs)…………………………………. 5 1.5 Quantum dots applications.................................................................. 6 1.6 Quantum dots toxicity......................................................................... 7 Chapter 2: Experimental methodsChapter 2: Experimental methodsChapter 2: Experimental methodsChapter 2: Experimental methods................................................................ 9 2.1 Materials.............................................................................................. 9 2.2 Synthesis of neat CdSe QDs………………………………………… 9 2.3 Synthesis of gradient Cd1-xZnxSe1-ySy CSQDs…...................………… 10 2.4 QDs surface ligand exchange……………………....………………… 10 2.5 ZnO NWs and NiO mesoporous film preparation……….………….. 11 2.6 QDs characterization………………………………………….……... 11 2.7 Solar cell device fabrication………………………………………….. 12 2.8 Transient absorption spectroscopy (TA)…………………………...… 12 2.9 Time-resolved PL spectroscopy (TRPL)……………………………... 13 2.10 Time-resolved terahertz spectroscopy (THz)……………………….. 14 2.11 X-ray diffraction (XRD)………………………………….………… 14 Chapter 3: Exciton dynamics in QDsChapter 3: Exciton dynamics in QDsChapter 3: Exciton dynamics in QDsChapter 3: Exciton dynamics in QDs…………………………….………. 15 3.1 Motivation…………………………………………………………... 15 3.2 Steady-state absorption and emission…………………….………….. 15 3.3: Exciton dynamics…………………………………………………… 17 3.4 Paper I “Electron injection from QDs to MO”……………………… 20 3.5 Paper II “Multiple exciton harvesting”………………………………. 22 3.6 Paper III “Ultra-long-lived trapped holes in QDs”…………………... 25 3.7 Paper IV “Hole injection vs hole trapping”………………………….. 27 3.8 Paper V “Energy transfer in CdSe-ZnO NWs system”………………. 30 3.9 Paper VI “Energy funneling through multi-sized QD films”………... 33

Chapter 4: Photoinduced dynamics in coreChapter 4: Photoinduced dynamics in coreChapter 4: Photoinduced dynamics in coreChapter 4: Photoinduced dynamics in core––––shell QDsshell QDsshell QDsshell QDs………………...... 37 4.1 Motivation…………………………………………………………... 37 4.2 Classification of core–shell QDs……………………………………... 37 4.3 Important aspects for preparing the gradient type I

CSQDs……………………………………………..………….……. 39

4.4 Characterization of type I gradient Cd1-xZnxSe1-ySy CSQDs………..… 41 4.5 Paper VII “Electron injection from gradient Cd1-xZnxSe1-ySy CSQDs to

MO”…………………………………………………………….…... 45

4.6 Paper VIII “hole trapping vs quantum dot sensitized solar cell performance”……………………………….………………………...

48

Chapter 5: Conclusions and future workChapter 5: Conclusions and future workChapter 5: Conclusions and future workChapter 5: Conclusions and future work………………………..………... 51 Svensk sammanfattningSvensk sammanfattningSvensk sammanfattningSvensk sammanfattning……………………………………………………. 54

Arabic summary (Arabic summary (Arabic summary (Arabic summary (الملخص العربىالملخص العربىالملخص العربىالملخص العربى))))………………………………….….……… 57

References….References….References….References….……………………………………………………..………... 60

Ultrafast Photoinduced Processes in Core and Core–Shell Quantum Dots for Solar cell Applications

i

Abstract

The balance between our demands of energy and the energy we are consuming is not

in equilibrium anymore. Therefore, the search for other energy resources is

indispensable. Sustainable energy sources offer the alternative to the fossil fuels. Within

many types of sustainable energy sources, solar energy offers more than the total global

energy consumption. Solar cells are the smart conversion tools to harvest the incident

photons and create electricity out of these photons. Solar cells have passed through

different generations where two important factors control the solar cells market. The

solar cell efficiency and price play the cornerstones in the solar cell marketing. Third

generation of solar cells aims to maximize the price–performance equation by using

cheap materials without compromising efficiency. Nanomaterials have emerged as the

promising building blocks to harvest the solar light in the third generation of the solar

cells. Among them, quantum dots (QDs) can be used as viable candidate due to their

superb features such as high extinction coefficient, a tunable absorption edge, and the

possibility to generate and collect multiple excitons by using single, high energy

photon. Both the behaviors of photoexcited electrons and holes determine the overall

efficiency of QD based solar cells. This thesis presents a systematic study of the ultrafast

photoinduced charge dynamics in QD solar cell materials including the charge transfer,

exciton migration, carrier trapping and their influence on real solar cell performance.

The materials investigated start with conventional neat core CdSe QDs and extend to

gradient Cd1-xSe1-yZnxSy core–shell (CS) QDs. The latter are used to obtain improved

optical and device performance. The electron injection from CdSe into ZnO nanowires

were first observed to be very fast (few ps). This fast electron injection encourages us to

study the possibility to inject multiple electrons from a QD under high excitation

conditions. We revealed that a competition between electron injection and Auger

recombination occurs.

Compared with electrons, the photoinduced holes are more likely to be trapped.

However, such trap states sometimes can be radiative with long lifetime up to tens of

microseconds in oleic acid capped CdSe QDs. In this scenario, the hole injection in p-

type QD solar cells are proved to be less efficient (<10%) compared with electron

injection in n-type counterparts. It is highly affected by the surface trapping sites

induced by the linker exchange process. The hole injection can then be improved by

passivating the surface trap sites using core shell structures. Besides electron or hole


ii

injection, exciton migration can also occur via Förster resonant energy transfer (FRET).

We found that FRET between QDs would enable to make use of the absorption of

light by the indirectly attached QDs in QD-sensitized metal oxide (MO) anodes. In

well-organized multi-sized QD mixtures, the energy transfer is even more pronounced.

We experimentally observed the FRET process in randomly arranged multi-sized QD

assembly and tandem stacked QD layers by using time-resolved and steady-state

spectroscopies. Theoretical simulations where dipole distribution model was

introduced for coupling calculations complies well with the experimental results. In

order to minimize the effect of surface defects and improve the photostability of QD

solar cells, we investigated the core–shell QD system where the surface trapping of

carriers can be well passivated by shell materials with enhanced optical properties and

device performance. Herein, a wider band gap semiconductor is employed as a shield

shell around the active core in gradient growth, known as gradient Cd1-xSe1-yZnxSy

CSQDs. Such QDs offer higher photostability, higher fluorescence quantum yield, and

less interfacial defects than the conventional step-like CSQDs. We first characterized

the gradient CSQDs using steady-state optical spectroscopy and HR-TEM images in

order to determine their dimensions and to evaluate the shell thickness. Then XRD

and EDX were used to characterize the chemical composition and the crystal structures.

The photodynamic of these CSQDs in photovoltaic systems was also studied. We

first found that the electron injection from the active core to n-type MO showed

relatively larger exponential shell thickness dependence compared with step-like

CSQDs. We established that the highest electron injection efficiency (~ 80%) can be

found with shell thickness up to 1.3 nm. Such shell also allows high surface passivation

providing optimal conditions for charge collection in solar cells. Finally, we integrated

our knowledge about the electron and hole behaviors to explain the solar cell

performances according to the core–shell structure. We confirmed that the hole

trapping is the critical factor for QD-sensitized solar cell efficiency. The trapping can

be well repaired by using optimal core–shell structure.

Acknowledgements

iii

Acknowledgements

‘‘A person who is not thankful to his benefactor is not thankful to ALLAHA person who is not thankful to his benefactor is not thankful to ALLAHA person who is not thankful to his benefactor is not thankful to ALLAHA person who is not thankful to his benefactor is not thankful to ALLAH’’, Prophet MuhammedProphet MuhammedProphet MuhammedProphet Muhammed

First of all, praise is to ALLAHALLAHALLAHALLAH, the most Merciful for his gifts.

Second of all, I would like to apologize in advance if somehow I forgot to express my

gratitude to any one that gave a hand in order to make this work real. Anyway, I will

try to do my best to give everyone his right in the acknowledgment. Since 29th of

November 2010, I started as a PhD student in Chemical Physics department. That was

my dream once I arrived to Sweden therefore I would like to express my sincere

gratitude to these fantastic people. In particular to:

My main supervisor Prof. Tõnu PulleritsTõnu PulleritsTõnu PulleritsTõnu Pullerits first for accepting me as a guest PhD then

for the opportunity he gave me to continue my PhD studies in Chemical Physics

department, for introducing me to the quantum dots in solar cell application project,

for his continuous and innumerable help, support, discussion, science and

encouragement. Honestly if I will talk about TõnuTõnuTõnuTõnu, I think I need more than one thesis

to express how he was helpful and supportive in everything. The head of the

department Prof. Villy SundströmVilly SundströmVilly SundströmVilly Sundström for giving me the chance to continue my PhD

studies in this amazing research environment.

My co-supervisors, Dr. Karel ŽídekKarel ŽídekKarel ŽídekKarel Žídek (Dearest boss) and Dr. Kaibo Zheng Kaibo Zheng Kaibo Zheng Kaibo Zheng

(Kaiboscopy, Kaiboskite, etc). Actually I do not know what to say about these smart

guys. I want to say without you this work will stay in dark. To have a nice friend that

is awesome. To have a nice co-supervisor that is luck. For me I had both in one. KaiboKaiboKaiboKaibo

and KarelKarelKarelKarel you taught me a lot and if I think how to pay back, no letters or words or

sentences, are enough to pay back your help and support. Dr. Khadga J. KarkiKhadga J. KarkiKhadga J. KarkiKhadga J. Karki thanks

a lot for all the nice work we had together in Lund or in (Oregon) USA and for useful

and interesting discussion.

Chemical Physics amazing leaders, ArkadyArkadyArkadyArkady, IvanIvanIvanIvan, DonatasDonatasDonatasDonatas, TorbjörnTorbjörnTorbjörnTorbjörn and EbbeEbbeEbbeEbbe

thanks for all the time we discuss about science or life related issues. It was a great time

to play football, tennis table, floor ball, ice skiing, and ice skating. Pullerits’s Pullerits’s Pullerits’s Pullerits’s group

current and former members, AtsunoriAtsunoriAtsunoriAtsunori, Junshen Junshen Junshen Junshen (brother), Nils Nils Nils Nils (nilspedia), EvaEvaEvaEva,

SeshaSeshaSeshaSesha, FadiFadiFadiFadi, ThorstenThorstenThorstenThorsten, RebeccaRebeccaRebeccaRebecca, MehrnazMehrnazMehrnazMehrnaz, SumeraSumeraSumeraSumera, MadhuriMadhuriMadhuriMadhuri, Abdelrazik MousaAbdelrazik MousaAbdelrazik MousaAbdelrazik Mousa,

and Abdallah AbdelwahabAbdallah AbdelwahabAbdallah AbdelwahabAbdallah Abdelwahab I could not have done this work without the former

members experience and the current members collaboration. Therefore, thanks a lot.

Acknowledgements

iv

Chemical Physics department current and former members, AliceAliceAliceAlice (tobeb), AbomaAbomaAbomaAboma

(Obama), ErikErikErikErik, AmalAmalAmalAmal (Reeeeta), AhmedAhmedAhmedAhmed, TobiasTobiasTobiasTobias, KasperKasperKasperKasper, MainakMainakMainakMainak, DavidDavidDavidDavid, CarlitoCarlitoCarlitoCarlito,

AhiburAhiburAhiburAhibur, ErlingErlingErlingErling, YuxiYuxiYuxiYuxi, DanielaDanielaDanielaDaniela, JensJensJensJens, WeiWeiWeiWei, TomasTomasTomasTomas, MartinMartinMartinMartin, MatthiasMatthiasMatthiasMatthias, DimaliDimaliDimaliDimali,

KamalKamalKamalKamal, SamirSamirSamirSamir, RafaelRafaelRafaelRafael, JakubJakubJakubJakub, LottaLottaLottaLotta, KarinKarinKarinKarin, HelenaHelenaHelenaHelena, YingyotYingyotYingyotYingyot, and SunnySunnySunnySunny thanks for

the nice and funny time we had in the lab to break the daily boring routine.

Our department unknown soldiers Maria LevinMaria LevinMaria LevinMaria Levin, Thomas BjorkmanThomas BjorkmanThomas BjorkmanThomas Bjorkman, AnnAnnAnnAnn----ChristinChristinChristinChristin

(Anki), and Katarina FredrikssonKatarina FredrikssonKatarina FredrikssonKatarina Fredriksson. I cannot imagine my work without you, you were

always efficient and helpful. Thank you very much. Incoherent football team members

in Victoriastadion, RafaelRafaelRafaelRafael, DonatasDonatasDonatasDonatas, AbomaAbomaAbomaAboma, MatthiasMatthiasMatthiasMatthias, MartinMartinMartinMartin, and GiulianoGiulianoGiulianoGiuliano. Even

thought, we did not win the tournament but we had very nice games. Thank you guys

for such a nice time. Weekly floor ball game members, AbomaAbomaAbomaAboma, PavelPavelPavelPavel, KarelKarelKarelKarel, TomasTomasTomasTomas,

DavidDavidDavidDavid, MartinMartinMartinMartin, ArkadyArkadyArkadyArkady, TorbjörnTorbjörnTorbjörnTorbjörn, EbbeEbbeEbbeEbbe, FadiFadiFadiFadi, and AddisAddisAddisAddis. I know we all suffer from

AbomaAbomaAbomaAboma’s playing style but I think we all enjoyed it. Thanks. My fantastic officemate.

PavelPavelPavelPavel, you are amazing officemate. I was lucky to share you the office during my PhD

studies. Thanks a lot for all the time we spent discussing or talking about science or

other stuff. To RamiRamiRamiRami for revising the Arabic summery. Thanks. GunillaGunillaGunillaGunilla, thank you

very much for your help and efforts during printing my thesis.

To our collaborators at Lund University, Dr. Maria E. MissingMaria E. MissingMaria E. MissingMaria E. Missing and Prof. Reine Reine Reine Reine

WallenbergWallenbergWallenbergWallenberg for HR-TEM images for our QDs. Without your clear images I would

not have been able to do the science I did. To our collaborators at Max-Lab, Dr. Qiushi Qiushi Qiushi Qiushi

ZhuZhuZhuZhu and Dr. Sophie CantonSophie CantonSophie CantonSophie Canton. Really I do not know what to say in order to thank you.

For the long measurements here in Lund Synchrotron or in Argonne National Lab in

USA. Therefore, as long as the time we spent together I thank both of you. QiushiQiushiQiushiQiushi,

you are super nice and you know the way to push me very kindly for more samples.

SophieSophieSophieSophie, please keep watermelon for me for the next beam time. Thanks a lot. To our

collaborators at Technical University of Denmark, Nan ZhuNan ZhuNan ZhuNan Zhu and Prof. Qijin ChiQijin ChiQijin ChiQijin Chi for

the AFM measurements. Thanks for your time and your efforts.

Egyptian Friends in Lund and Uppsala, Ahmed MostafaAhmed MostafaAhmed MostafaAhmed Mostafa, Ahmed FawzyAhmed FawzyAhmed FawzyAhmed Fawzy, Haitham Haitham Haitham Haitham

ElElElEl----taliawytaliawytaliawytaliawy, Wael AwadWael AwadWael AwadWael Awad, Tarek SelimTarek SelimTarek SelimTarek Selim, Abdallah ShokryAbdallah ShokryAbdallah ShokryAbdallah Shokry, Mohamed EssamMohamed EssamMohamed EssamMohamed Essam,

Mohamed MabroukMohamed MabroukMohamed MabroukMohamed Mabrouk, Ibrahim AbdelwahabIbrahim AbdelwahabIbrahim AbdelwahabIbrahim Abdelwahab, Mostafa Mostafa Mostafa Mostafa FekryFekryFekryFekry, Tarik DishishaTarik DishishaTarik DishishaTarik Dishisha,

AbdulrahmanAbdulrahmanAbdulrahmanAbdulrahman, Wael MobarkWael MobarkWael MobarkWael Mobark, Dr. Mohamed IbrahimMohamed IbrahimMohamed IbrahimMohamed Ibrahim, Mohamed IsmailMohamed IsmailMohamed IsmailMohamed Ismail, Yasser Yasser Yasser Yasser

GaberGaberGaberGaber, Mahmoud SayedMahmoud SayedMahmoud SayedMahmoud Sayed, Mohamed HagresMohamed HagresMohamed HagresMohamed Hagres, Mohamed RedaMohamed RedaMohamed RedaMohamed Reda, Nagwa ElnwishyNagwa ElnwishyNagwa ElnwishyNagwa Elnwishy

and Ahmed Elzohry.Ahmed Elzohry.Ahmed Elzohry.Ahmed Elzohry. Good feeling to have friends that bring home to you abroad

therefore, thank you very much guys for the time we spent together playing football,

eating, indoor cooking, and outdoor barbecue.

Acknowledgements

v

My former supervisors in Egypt, Prof. Hasan SalmanHasan SalmanHasan SalmanHasan Salman, and Dr. Mahmmoud SayedMahmmoud SayedMahmmoud SayedMahmmoud Sayed

Chemistry and Physics Departments, respectively at Qena Faculty of Science, South

Valley University, Egypt for their continuous encouragement during my PhD studies

in Sweden. Thank you very much. My colleagues at the Chemistry Department at

Qena Heba NassarHeba NassarHeba NassarHeba Nassar, Hisham ElsoghierHisham ElsoghierHisham ElsoghierHisham Elsoghier, Haytham FawzyHaytham FawzyHaytham FawzyHaytham Fawzy, Ahmed EissaAhmed EissaAhmed EissaAhmed Eissa, and Nada Nada Nada Nada

NabilNabilNabilNabil, for keeping the teaching responsibilities running in our department at Qena

during my study. All the best for you guys.

The funding agencies, the Knut and Alice Wallenberg FoundationKnut and Alice Wallenberg FoundationKnut and Alice Wallenberg FoundationKnut and Alice Wallenberg Foundation, the Swedish Swedish Swedish Swedish

Energy AgencyEnergy AgencyEnergy AgencyEnergy Agency, the Swedish Research CouncilSwedish Research CouncilSwedish Research CouncilSwedish Research Council, and Ministry Ministry Ministry Ministry of Higher Education of Higher Education of Higher Education of Higher Education

and Scientific Research Egypt and Erasmusand Scientific Research Egypt and Erasmusand Scientific Research Egypt and Erasmusand Scientific Research Egypt and Erasmus (FFEEBB program). Thanks a lot for

funding our work.

The souls of my Mother Safaa AbdelrehimSafaa AbdelrehimSafaa AbdelrehimSafaa Abdelrehim, my father-in-law Dr. Mohamed TalaatMohamed TalaatMohamed TalaatMohamed Talaat,

my Master supervisor Dr. Yossif EbeadYossif EbeadYossif EbeadYossif Ebead, my colleagues Prof. NasrNasrNasrNasr MansourMansourMansourMansour and Dr.

Ayman KhalafAyman KhalafAyman KhalafAyman Khalaf, and my student Aya TarekAya TarekAya TarekAya Tarek. All of them except my father-in-law passed

away during my study. I am dedicating this work for all of you and I hope ALLAHALLAHALLAHALLAH,

the Almighty, bless all of you.

Last but definitely not least, to DaliaDaliaDaliaDalia, my wife, really there are no words in any

dictionary could explain how much I should be thankful for you. But if I said that she

took care of my active three kids during my PhD studies, you can imagine how much

I should be thankful. To my mother-in-law Dr. HHHHanan Elmaghrabyanan Elmaghrabyanan Elmaghrabyanan Elmaghraby, just to let you

know you are like my mom. Thanks a lot for taking care of DaliaDaliaDaliaDalia and the kids as well

and for your continuous sincere prayers for me. To Ahmed TalaatAhmed TalaatAhmed TalaatAhmed Talaat, thank you very

much and all the best. EaydEaydEaydEayd, YahiaYahiaYahiaYahia, and DareenDareenDareenDareen, sorry guys to be away from you

during my PhD studies. Please forgive me. Thanks for the time we spent together

during my vacation. You may not understand me now but EyadEyadEyadEyad, I know you love

chemistry, go ahead please. YahiaYahiaYahiaYahia, I know you love laser and I hope you can be better

than me. DareenDareenDareenDareen, my queen, be a way from both chemistry and Laser and focus on

what you like. Thanks guys. To my father Ahmed AbdellahAhmed AbdellahAhmed AbdellahAhmed Abdellah, brother MahmoudMahmoudMahmoudMahmoud, and

sisters ShymaaShymaaShymaaShymaa and EsraaEsraaEsraaEsraa, thanks a lot. To the rest of my family thanks a lot.

Mohamed AbdellahMohamed AbdellahMohamed AbdellahMohamed Abdellah Lund 2015Lund 2015Lund 2015Lund 2015


vi

List of Papers

This thesis is based on the following pThis thesis is based on the following pThis thesis is based on the following pThis thesis is based on the following papersapersapersapers::::

I. Karel Žídek, Kaibo Zheng, Carlito S. Ponseca, Jr., Maria E. Messing, Reine Wallenberg, Pavel Chábera, Mohamed AbdellahMohamed AbdellahMohamed AbdellahMohamed Abdellah, Villy Sundström, and Tõnu Pullerits

“Electron Transfer in QuantumElectron Transfer in QuantumElectron Transfer in QuantumElectron Transfer in Quantum----DotDotDotDot----Sensitized ZnO Nanowires: Sensitized ZnO Nanowires: Sensitized ZnO Nanowires: Sensitized ZnO Nanowires: Ultrafast TimeUltrafast TimeUltrafast TimeUltrafast Time----Resolved Absorption and Terahertz StudyResolved Absorption and Terahertz StudyResolved Absorption and Terahertz StudyResolved Absorption and Terahertz Study”

Journal of American Chemical Society, 134, (2012201220122012), 12110−12117.

II. Karel Žídek, Kaibo Zheng, Mohamed AbdellahMohamed AbdellahMohamed AbdellahMohamed Abdellah, Nils Lenngren, Pavel Chábera, and Tõnu Pullerits

“Ultrafast Dynamics of Multiple Exciton Harvesting in the Ultrafast Dynamics of Multiple Exciton Harvesting in the Ultrafast Dynamics of Multiple Exciton Harvesting in the Ultrafast Dynamics of Multiple Exciton Harvesting in the CdSeCdSeCdSeCdSe−ZnO System: Electron Injection −ZnO System: Electron Injection −ZnO System: Electron Injection −ZnO System: Electron Injection versusversusversusversus Auger RecombinationAuger RecombinationAuger RecombinationAuger Recombination”

Nano Letters, 12, (2012201220122012), 6393−6399.

III. Mohamed AbdellahMohamed AbdellahMohamed AbdellahMohamed Abdellah****, Khadga J. Karki*, Nils Lenngren, Kaibo Zheng, Torbjörn Pascher, Arkady Yartsev, and Tõnu Pullerits

*Contributed equally to this work

“Ultra Ultra Ultra Ultra LongLongLongLong----Lived Radiative Trap States in CdSe Quantum DotsLived Radiative Trap States in CdSe Quantum DotsLived Radiative Trap States in CdSe Quantum DotsLived Radiative Trap States in CdSe Quantum Dots”

Journal of Physical Chemistry C, 118, (2014201420142014), 21682−21686.

IV. Kaibo Zheng*, Karel Žídek*, Mohamed AbdellahMohamed AbdellahMohamed AbdellahMohamed Abdellah****, Wei Zhang, Pavel Chábera, Nils Lenngren, Arkady Yartsev, and Tõnu Pullerits

*Contributed equally to this work

“Ultrafast Ultrafast Ultrafast Ultrafast Charge Transfer Charge Transfer Charge Transfer Charge Transfer from CdSe from CdSe from CdSe from CdSe Quantum Dots Quantum Dots Quantum Dots Quantum Dots to pto pto pto p----Type Type Type Type NiO: NiO: NiO: NiO: Hole Injection Hole Injection Hole Injection Hole Injection vs.vs.vs.vs. Hole TrappingHole TrappingHole TrappingHole Trapping”


V. Kaibo Zheng, Karel Žídek, Mohamed AbdellahMohamed AbdellahMohamed AbdellahMohamed Abdellah, Magne Torbjörnsson, Pavel Chábera, Shuyan Shao, Fengling Zhang, and Tõnu Pullerits

“Fast Monolayer Adsorption and Slow Energy Transfer in CdSe Fast Monolayer Adsorption and Slow Energy Transfer in CdSe Fast Monolayer Adsorption and Slow Energy Transfer in CdSe Fast Monolayer Adsorption and Slow Energy Transfer in CdSe Quantum Dot Sensitized ZnO NanowiresQuantum Dot Sensitized ZnO NanowiresQuantum Dot Sensitized ZnO NanowiresQuantum Dot Sensitized ZnO Nanowires”

Journal of Physical Chemistry A, 117, (2013201320132013), 5919−5925.


vii

VI. Kaibo Zheng, Karel Žídek, Mohamed AbdellahMohamed AbdellahMohamed AbdellahMohamed Abdellah, Nan Zhu, Pavel Chábera, Nils Lenngren, Qijin Chi, and Tõnu Pullerits

“Directed Energy Transfer in Films of CdSe Quantum Dots: Beyond Directed Energy Transfer in Films of CdSe Quantum Dots: Beyond Directed Energy Transfer in Films of CdSe Quantum Dots: Beyond Directed Energy Transfer in Films of CdSe Quantum Dots: Beyond the Point Dipole Approximationthe Point Dipole Approximationthe Point Dipole Approximationthe Point Dipole Approximation”

Journal of American Chemical Society, 136, (2014201420142014), 6259−6268.

VII. Mohamed AbdellahMohamed AbdellahMohamed AbdellahMohamed Abdellah, Karel Žídek, Kaibo Zheng, Pavel Chábera, Maria E. Messing, and Tõnu Pullerits

“BalancingBalancingBalancingBalancing Electron Transfer Electron Transfer Electron Transfer Electron Transfer and and and and Surface Passivation Surface Passivation Surface Passivation Surface Passivation inininin Gradient Gradient Gradient Gradient CdSe/ZnS CdSe/ZnS CdSe/ZnS CdSe/ZnS CoreCoreCoreCore----Shell Quantum Dots Attached Shell Quantum Dots Attached Shell Quantum Dots Attached Shell Quantum Dots Attached to ZnOto ZnOto ZnOto ZnO”

Journal of Physical Chemistry Letters, 4, (2013201320132013), 1760−1765.

VIII. Mohamed AbdellahMohamed AbdellahMohamed AbdellahMohamed Abdellah, Rebecca Marschan, Karel Žídek, Maria E. Messing, Abdallah Abdelwahab, Pavel Chábera, Kaibo Zheng, and Tõnu Pullerits

“Hole Trapping: The Critical Factor for Quantum Dot Sensitized Hole Trapping: The Critical Factor for Quantum Dot Sensitized Hole Trapping: The Critical Factor for Quantum Dot Sensitized Hole Trapping: The Critical Factor for Quantum Dot Sensitized Solar Cell PerformanceSolar Cell PerformanceSolar Cell PerformanceSolar Cell Performance”


Reprints were made with permission from the respective publishers.


viii

Contribution report

The project was initiated by Prof. Tõnu PulleritsProf. Tõnu PulleritsProf. Tõnu PulleritsProf. Tõnu Pullerits.

Without the fantastic co-authors none of these papers would be

available.

I. Within the team, I prepared and characterized the samples, also I helped

in experimental data collection, and I was involved in the paper writing.

II. Within the team, I prepared and characterized the samples, also I helped


III. With the co-authors, Khadga and I are equally contributing to this work

by designing and preforming the experiments, data analysis and writing

the manuscript. I prepared and characterized the studied samples.

IV. With the co-authors, Kaibo, Karel and I are equally contributing to this

work by designing and preforming the experiments, data analysis and

writing the manuscript. I prepared and characterized the studied samples.

V. With the co-authors, I contributed to this work by preparing and

characterizing the studied samples, also I put hand in the experimental

data analysis and writing the manuscript.

VI. Within the team, I prepared and characterized the samples, also I helped


VII. With the co-authors, I prepared and characterized the samples, I designed

and preformed the experiments, I analyzed the data, and I wrote the first

draft of the manuscript.

VIII. With the co-authors, I prepared and characterized the samples, I designed

and preformed the experiments, I analyzed the data, and I wrote the first

draft of the manuscript.


ix

I am a coI am a coI am a coI am a co----author of the following papers which are notauthor of the following papers which are notauthor of the following papers which are notauthor of the following papers which are not included in included in included in included in

this thesisthis thesisthis thesisthis thesis::::

I. Karel Žídek, Kaibo Zheng, Pavel Chábera, Mohamed AbdellahMohamed AbdellahMohamed AbdellahMohamed Abdellah, and Tõnu Pullerits

“Quantum Quantum Quantum Quantum Dot Photodegradation Dot Photodegradation Dot Photodegradation Dot Photodegradation due to CdSedue to CdSedue to CdSedue to CdSe----ZnO ZnO ZnO ZnO Charge Charge Charge Charge TransferTransferTransferTransfer: Transient : Transient : Transient : Transient Absorption StudyAbsorption StudyAbsorption StudyAbsorption Study”

Applied Physics Letters, 100, (2012201220122012), 243111-4.

II. Kaibo Zheng, Karel Žídek, Mohamed Mohamed Mohamed Mohamed AbdellahAbdellahAbdellahAbdellah, Pavel Chábera, Mahmoud S. Abd El-sadek, and Tõnu Pullerits

“Effect of Effect of Effect of Effect of Metal Oxide Morphology Metal Oxide Morphology Metal Oxide Morphology Metal Oxide Morphology on on on on Electron Injection Electron Injection Electron Injection Electron Injection from CdSe from CdSe from CdSe from CdSe Quantum Dots Quantum Dots Quantum Dots Quantum Dots to ZnOto ZnOto ZnOto ZnO”

Applied Physics Letters, 102, (2013201320132013), 163119-5.

III. Khadga J. Karki, Fei Ma, Kaibo Zheng, Karel Žídek, Abdelrazek Mousa, Mohamed AbdellahMohamed AbdellahMohamed AbdellahMohamed Abdellah, Maria E. Messing, Reine Wallenberg, Arkady Yartsev, and Tõnu Pullerits

“Multiple Multiple Multiple Multiple Exciton Generation Exciton Generation Exciton Generation Exciton Generation in in in in NanoNanoNanoNano----Crystals RevisitedCrystals RevisitedCrystals RevisitedCrystals Revisited: : : : Consistent Consistent Consistent Consistent Calculation Calculation Calculation Calculation of the of the of the of the Yield Based Yield Based Yield Based Yield Based on on on on PumpPumpPumpPump----Probe Probe Probe Probe SpectroscopySpectroscopySpectroscopySpectroscopy”

Scientific Reports, 3, (2013201320132013), DOI: 10.1038/srep02287.

IV. Nils Lenngren, Tommy Garting, Kaibo Zheng, Mohamed AbdellahMohamed AbdellahMohamed AbdellahMohamed Abdellah, Noëlle Lascoux, Fei Ma, Arkady Yartsev, Karel Žídek, and Tõnu Pullerits

“Multiexciton Absorption Cross Sections of CdSe Quantum Dots Multiexciton Absorption Cross Sections of CdSe Quantum Dots Multiexciton Absorption Cross Sections of CdSe Quantum Dots Multiexciton Absorption Cross Sections of CdSe Quantum Dots Determined by Ultrafast SpectrDetermined by Ultrafast SpectrDetermined by Ultrafast SpectrDetermined by Ultrafast Spectroscopyoscopyoscopyoscopy”

Journal of Physical Chemistry Letters, 4, (2013201320132013), 3330–3336.

V. Thorsten Hansen, Karel Žídek, Kaibo Zheng, Mohamed AbdellahMohamed AbdellahMohamed AbdellahMohamed Abdellah, Pavel Chábera, Petter Persson, and Tõnu Pullerits

“Orbital Topology Controlling Charge Injection in QuantumOrbital Topology Controlling Charge Injection in QuantumOrbital Topology Controlling Charge Injection in QuantumOrbital Topology Controlling Charge Injection in Quantum----DotDotDotDot----Sensitized Sensitized Sensitized Sensitized Solar CellsSolar CellsSolar CellsSolar Cells”


VI. Carlito S. Ponseca, Jr., Tom J. Savenije, Mohamed AbdellahMohamed AbdellahMohamed AbdellahMohamed Abdellah, Kaibo Zheng, Arkady Yartsev, Torbjörn Pascher, Tobias Harlang, Pavel Chábera, Tõnu Pullerits, Andrey Stepanov, Jean-Pierre Wolf, and Villy Sundström


x

“Organometel Halide Perovskite Solar Cell Materials Rationalized: Organometel Halide Perovskite Solar Cell Materials Rationalized: Organometel Halide Perovskite Solar Cell Materials Rationalized: Organometel Halide Perovskite Solar Cell Materials Rationalized: Ultrafast Charge Generation, High and MicrosecondUltrafast Charge Generation, High and MicrosecondUltrafast Charge Generation, High and MicrosecondUltrafast Charge Generation, High and Microsecond----Long Balanced Long Balanced Long Balanced Long Balanced Mobilities, and Slow RecombinationMobilities, and Slow RecombinationMobilities, and Slow RecombinationMobilities, and Slow Recombination”

Journal of American Chemical Society, 136, (2014201420142014), 5189–5192.

VII. Tom J. Savenije, Carlito S. Ponseca, Lucas Kunneman, Mohamed Mohamed Mohamed Mohamed AbdellahAbdellahAbdellahAbdellah, Kaibo Zheng, Qiushi Zhu, Sophie E. Canton, Ivan G. Scheblykin, Tõnu Pullerits, Arkady Yartsev, and Villy Sundström

“Thermally Activated Exciton Dissociation and Recombination Thermally Activated Exciton Dissociation and Recombination Thermally Activated Exciton Dissociation and Recombination Thermally Activated Exciton Dissociation and Recombination Control theControl theControl theControl the Carrier Dynamics in Organometal Halide PerovskiteCarrier Dynamics in Organometal Halide PerovskiteCarrier Dynamics in Organometal Halide PerovskiteCarrier Dynamics in Organometal Halide Perovskite”


VIII. Karel Žídek, Kaibo Zheng, Mohamed AbdellahMohamed AbdellahMohamed AbdellahMohamed Abdellah, Pavel Chábera, Tõnu Pullerits, and Masanori Tachyia

“Simultaneous Creation and Recovery of Trap States on Quantum Simultaneous Creation and Recovery of Trap States on Quantum Simultaneous Creation and Recovery of Trap States on Quantum Simultaneous Creation and Recovery of Trap States on Quantum Dots in a Photoirradiated CdSeDots in a Photoirradiated CdSeDots in a Photoirradiated CdSeDots in a Photoirradiated CdSe−ZnO System−ZnO System−ZnO System−ZnO System”

Journal of Physical Chemistry C, 118, (2014201420142014), 27567–27573.

IX. Karel Žídek, Mohamed AbdellahMohamed AbdellahMohamed AbdellahMohamed Abdellah, Kaibo Zheng, and Tõnu Pullerits

“Electron Relaxation in the CdElectron Relaxation in the CdElectron Relaxation in the CdElectron Relaxation in the CdSe Quantum DotsSe Quantum DotsSe Quantum DotsSe Quantum Dots----ZnO Composite: ZnO Composite: ZnO Composite: ZnO Composite: Prospects for Photovoltaic ApplicationsProspects for Photovoltaic ApplicationsProspects for Photovoltaic ApplicationsProspects for Photovoltaic Applications”

Scientific Reports, 4, (2014201420142014), DOI: 10.1038/srep07244.


xi

Abbreviations and symbols

PVsPVsPVsPVs Photovoltaics 1111ststststG PVsG PVsG PVsG PVs First generation photovoltaics 2222ndndndndG PVsG PVsG PVsG PVs Second generation photovoltaics 3333ededededGPVsGPVsGPVsGPVs Third generation photovoltaics DSSCsDSSCsDSSCsDSSCs Dye-sensitized solar cells BHJBHJBHJBHJ Bulk heterojunction QDSCsQDSCsQDSCsQDSCs Quantum dots solar cells QDSSCsQDSSCsQDSSCsQDSSCs Quantum dots-sensitized solar cells OSCsOSCsOSCsOSCs Organic solar cells NCsNCsNCsNCs Nanocrystals QDsQDsQDsQDs Quantum dots VBVBVBVB Valence band VBMVBMVBMVBM Valence band maximum CBCBCBCB Conduction band CBMCBMCBMCBM Conduction band minimum UVUVUVUV Ultra violet VisVisVisVis Visible PLPLPLPL Photoluminescence FWHMFWHMFWHMFWHM Full width at half maximum QYQYQYQY Quantum yield EBEEBEEBEEBE Exciton binding energy MEGMEGMEGMEG Multiple exciton generation SCsSCsSCsSCs Solar cells MOMOMOMO Metal oxide CSQDsCSQDsCSQDsCSQDs Core–shell quantum dots CSSCSSCSSCSS Core–shell–shell TOPOTOPOTOPOTOPO Tri-n-octylphosphine oxide OAOAOAOA Oleic acid LEDLEDLEDLED Light-emitting-diodes TVsTVsTVsTVs televisions NWsNWsNWsNWs Nanowires NPsNPsNPsNPs Nanoparticles TRTRTRTR Time-resolved TATATATA Transient absorption PDPDPDPD Photo diode EDXEDXEDXEDX Energy-dispersive X-ray spectroscopy


xii

⟨�⟩ mean number of excited electron-hole pairs per QD XRDXRDXRDXRD X-ray diffraction TOPTOPTOPTOP Tri-n-octylphosphine MPAMPAMPAMPA Mercaptopropionic acid TMAOHTMAOHTMAOHTMAOH Tetramethylammonium hydroxide ODEODEODEODE 1-octadecane HMTAHMTAHMTAHMTA Hexamethylenetetramine SEMSEMSEMSEM Scanning electron microscopy HRHRHRHR----TEMTEMTEMTEM High resolution transmission electron microscopy AFMAFMAFMAFM Atomic force microscopy FTOFTOFTOFTO Fluorine-doped SnO2 TCSPCTCSPCTCSPCTCSPC Time-correlated single photon counting nsnsnsns Nanosecond pspspsps Picosecond fsfsfsfs Femtosecond nmnmnmnm Nanometer eVeVeVeV Electron volt AugRAugRAugRAugR Auger recombination rrrr Particle radius EEEEg,eff.g,eff.g,eff.g,eff. Effective band gap EEEEgggg((((∞)))) Bulk band gap mmmmeeee Electron effective mass mmmmhhhh Hole effective mass ε Relative permittivity DOSDOSDOSDOS Density of states αBBBB Bohr exciton radius ε0000 Free vacuum permittivity μ Reduced mass KEKEKEKE Kinetic energy τRRRR Radiative lifetimes τnonRnonRnonRnonR Nonradiative lifetimes τObObObOb Observed lifetime THzTHzTHzTHz Terahertz spectroscopy KKKK injinjinjinj Electron injection rate ΔGGGG Free energy change ΔEEEEelelelel Electron energy difference ΔEEEEhhhh Hole energy difference ΔEEEECCCC Coulombic energy change CTSCTSCTSCTS Charge transfer state PLEPLEPLEPLE Photoluminescence excitation


xiii

PEEPEEPEEPEE Photoinduced emission enhancement λ Reorganizational energy IAIAIAIA----QDsQDsQDsQDs Indirect attached QDs to MO RRRR The fraction of IA-QDs DADADADA----QDsQDsQDsQDs Direct attached QDs to MO FRETFRETFRETFRET Fӧrster resonance energy transfer IPCEIPCEIPCEIPCE Incident photon-to-electron conversion efficiency kkkkETETETET Energy transfer rate RRRRDADADADA Donor–acceptor distance 1/1/1/1/RRRR6666 Distance term LLLLDDDD Diffusion length BBBB Dispersion factor β Distance exponential dependence factor ηinjinjinjinj Electron injection efficiency ηEC Electron collection efficiency ηhhhh Hole collection efficiency

Introduction

1

Chapter 1: Chapter 1: Chapter 1: Chapter 1: IntroductionIntroductionIntroductionIntroduction

1.1 Sustainable energy source1.1 Sustainable energy source1.1 Sustainable energy source1.1 Sustainable energy sourcessss::::

In the modern society, energy is the engine of economic development. More than 85%

of the global energy consumption comes from the fossil fuels such as oil, natural gas,

and coal. During 2013, the world consumed about 4200 million tons of oil, 3100

million tons of oil equivalent of natural gas, and 3900 million tons of oil equivalent of

coal according to BP statistical review report.1 Burning these fossil fuels as an energy

source has a huge deteriorating impact on the environment.2 Due to the shortage and

pollution from the fossil energy sources, governments and scientists in many countries

have been making effort to find an alternative energy sources “sustainable energy

sources”. There are several types of sustainable energy sources such as nuclear,

hydroelectric, wind, geothermal, and solar (photovoltaics).3

Harvesting the energy from Sun has many advantages. Compared to nuclear power

plants, solar sources (see figure 1.1A) are technically facile and safe. Moreover, they are

free of gas pollution and territory limitation, as long as solar radiation is sufficient in

that area. It is reported that 1% of the solar power is more than enough for the total

global energy consumption if it could be converted into electricity or fuel.4 Over the

past years, conversion of light into the electrical energy has been explored using

different materials. The first generation of photovoltaics (1stG) includes semiconductor

solar cells mainly based on Si material with high efficiency. The second generation

(2ndG) of solar cells was aimed to reduce the high costs of the first generation solar cells

through the utilization of thin film technology with the challenge of increasing the thin

film absorption to compensate for the reduced thickness in the photoactive layers. The

2ndG PVs are considered as low cost/low efficiency devices compared to 1stG

counterparts. The third generation (3edG) of PVs should combine the cheap material

processing typical for the 2ndG with the high conversion efficiency of the first

generation. The 3rdG is based mainly on nanostructured materials with three major

types that have dominated research in recent years (see figure 1.1B): (a) dye-sensitized

solar cells (DSSCs), (b) bulk heterojunction (BHJ) PV cells or organic PV cells, and (c)

quantum dot solar cells (QDSCs). The later considered as “The Next Big Thing” in

photovoltaics.5

Introduction

2

Figure 1.1: A) Solar radiation spectrum outside of the atmosphere (yellow area)

compared to the sea-level radiation (red area)6, and B) reported efficiency of

quantum dot solar cells (QDSCs, black) compared to dye sensitized solar cells

(DSSCs, red) and various types of organic solar cells (OSCs, blue)7.

1.2 Quantum Dot (QD): 1.2 Quantum Dot (QD): 1.2 Quantum Dot (QD): 1.2 Quantum Dot (QD):

Quantum dot (QD) is a semiconductor nanocrystal (NC) with a size of 2-10 nm. In

the bulk semiconductor the charge carriers can be considered as being free particles

with a modified mass (the so-called effective mass). When the size of material

approaches the exciton Bohr radius (typically several nanometers), the picture changes

into the “particle-in-box” problem. The term “dot” in quantum dot refers to

confinement of electrons and holes in all three dimensions. This is an extension of

quantum “wires” and quantum “wells” that confine excitons in one and two

dimensions, respectively (see figure 1.2A).

In semiconductors, exciton (electron-hole pair) is created after the absorption of a

photon with energy above the semiconductor band gap energy. Band gap is the

minimum energy difference between the highest occupied state in the valence band

(VBM) and the lowest occupied state in the conduction band (CBM). Depending on

the electron momentum at the CBM and VBM, the band gap can be either direct or

indirect. Most of the semiconductors have direct band gap where the CBM and VBM

occur at the same value of electron momentum (CdSe, CdTe, and many others). On

the other hand some semiconductors, such as Si, have the indirect band gap where the

CBM and VBM occur at different values of electron/hole momenta.

Introduction

3

Figure 1.2: A) Quantum well (2D), quantum wire (1D) and quantum dot (0D)8,

and B) quantum dot size related to the conduction band and valence band.

Even though the initial theory behind the exciton has existed since the 1930’s9-11 it

was not until the 1980’s that researchers began to focus on the effect of quantum

confinement on excitons in semiconductor materials.12 In the 1970’s, advances in

microfabrication led to quantum confinement in two-dimensional wells13, eight years

later was reported the first demonstration of one-dimensional wires.14 Finally, about 30

years ago in the former Soviet Union, Aleksey Ekimov12 and Alexander Efros15 explained

the spectral differences of what appeared to be tiny particles of variable sizes. One to

two years later, Louis Brus16 in the United States and Arnim Henglein17 in Germany

discovered striking color change of II-VI semiconductor nanoparticles grown as

aqueous colloidal suspensions.

Introduction

4

1.3 Quantum Dot (QD) properties:1.3 Quantum Dot (QD) properties:1.3 Quantum Dot (QD) properties:1.3 Quantum Dot (QD) properties:

Due to the quantum confinement effect, the energy levels are quantized depending on

the QD’s size.18 As the NC’s size decrease (smaller than exciton radius), the quantum

confinement becomes more dominant, and consequently the QD properties will

change. As a result of the quantum confinement effect, the band gap becomes wider

(see figure 1.2 B) and QDs therefore absorb at higher energies. Also the oscillator

strength is concentrated into a few transitions as a result of changes in the density of

electronic states from the bulk size to the QD size. The QDs’ photoluminescence (PL)

depends on the QD’s size as well, following the band gap changes19 (see figure 1.3A).

An interesting feature of semiconductor QDs is the symmetric and relatively narrow

emission peak redshifted with respect to the corresponding absorption peak. The red

shift of the emission is known as Stokes shift. Stokes shift is the difference between

positions of the band maxima of the absorption and emission spectra in wavelength or

frequency unit. This difference comes from the fact that, after the excitation, the QDs

tend to relax by emitting photons with some energy losing as dissipation of thermal

phonons in a crystal lattice. PL wavelength depends on the QD material and size, as

stated before (see figure 1.2 B). The PL from QDs usually feature a high quantum yield

(QY ≈ 0.2–0.9).20 Moreover, QDs excel in high molar extinction coefficients, better

photostability compared with common standard fluorophores, as well as the high

resistance to photo- and chemical degradation.21-23

In QDs, the excitons (electron-hole pairs) occupy discrete energy states similar to

electrons of a single atom and that is why the QDs are sometimes referred to as the

“artificial atoms”18 (see figure 1.3 A). After excitation with sufficient energy photon, the

QDs’ electrons will be excited from the valence band (VB) to the conduction band

(CB) leaving the positive quasi-particles holes in the VB. Holes can be considered as a

mobile positive charge in the VB. Then both the electrons and holes rapidly lose their

energy and jump to levels near the bottom of their conduction band (CBM) and the

top of their valence band (VBM), respectively (see figure 1.3 A). As the electrons relax

across the band gap to recombine with the holes, the energy released by recombination

from the excited state to the ground state can be calculated as the sum of the electron

and hole confinement energy, the exciton binding energy (EBE), and the band gap

energy.24 In QDs, a photon with energy twice higher than the band gap can create

multiple excitons, this phenomenon is called “multiple exciton generation” (MEG) (see

Introduction

5

figure 1.3 B).25-26 The same phenomenon exists also in bulk semiconductors. Efficient

formation of more than one electron-hole pair out of one incident high energy photon

is potentially very important from solar cell application point of view. Recently, several

studies have proven the successful utilization of MEG to enhance the quantum

efficiency of QD-sensitized solar cells (SCs) based on QD-metal oxide (MO) system.27-

28

Figure 1.3: A) Excitation, relaxation (cooling) and electron-hole recombination in

QD, and B) multiple exciton generation in QD 1) excitation with high energy

photon, 2) the electron relax to the CBM releasing its energy to process 3, and 3)

creating another exciton by using the energy of process 2).

1.4 Core1.4 Core1.4 Core1.4 Core––––shell quantum dots (CSQDs): shell quantum dots (CSQDs): shell quantum dots (CSQDs): shell quantum dots (CSQDs):

As the crystal becomes smaller, the proportion of the atoms on the surface increases,

which has an impact on the nanoparticle properties. In general, during the synthesis of

QDs, some atoms on the crystal facet are incompletely bonded within the crystal lattice

which leads to one or more of “dangling bonds”.29 Most nanocrystals are highly faceted,

what leads to an array of dangling bonds with a band energy level similar to the crystal

itself.30-31 These states could work as a trap states if their energy is located within the

QD band gap resulting in: i) reducing the overlap between the electron and hole, and

ii) increasing the nonradiative depopulation pathways of the excited states. One way

that minimize the surface states, is by passivating the nanocrystals surface using organic

ligands (capping agents) like tri-n-octylphosphine oxide (TOPO) or oleic acid (OA).

Capping agents adsorb to the nanocrystal surface through dative ligand–metal bonds

between the ligand active group and the metal atoms on the nanocrystal surface.

{

CBM

VBM

{CB

VB

QDQDQDQD

hν Electron-hole recombination

AAAA

hν>2Eg

1

2

Eg3

BBBB

MEG in QDMEG in QDMEG in QDMEG in QD

Introduction

6

Nonpolar end group ligands solubilize the nanocrystals in nonpolar solvents and the

same holds for polar end group ligands in case of polar solvent.29

Another more effective way to passivate the surface defect states is to coat the

semiconductor nanocrystal with an inorganic shell with higher band gap thereby

insulating the excitons in the core from the surface states. In other words, core–shell

quantum dots (CSQDs) can be broadly defined as comprising core (inner

semiconductor) and a shell (outer semiconductor). In solid state devices, this process is

quite simple, as overgrowth of an inorganic layer with a wider band gap can be readily

achieved with complete surface passivation. However, the task is more challenging in

preparing colloidal CSQDs such as CdSe–ZnS and CdSe–CdS CSQDs with sufficient

and stable PL. CSQDs can be categorized based on the band gaps of both the core and

the shell materials into many types, each useful for different applications (see chapter 4

for more details).32

1.5 Quantum dots applications: 1.5 Quantum dots applications: 1.5 Quantum dots applications: 1.5 Quantum dots applications:

Both neat core and CSQDs are gradually attracting more and more attention, since

these nanocrystals have emerged at the frontier of material science and many other

fields, such as biomedicals, pharmaceuticals, optics, and catalysis. In the next

subsections, some of the QDs important applications will be listed.

1.5.11.5.11.5.11.5.1 QDs for biomedical applications:QDs for biomedical applications:QDs for biomedical applications:QDs for biomedical applications: QDs are mainly used for controlled drug

delivery33, for bioimaging34, cell labeling35, as biosensors35, and in tissue

engineering applications.36

1.5.21.5.21.5.21.5.2 QDs for light emitting diodes (LED), electricity inQDs for light emitting diodes (LED), electricity inQDs for light emitting diodes (LED), electricity inQDs for light emitting diodes (LED), electricity in––––light out: light out: light out: light out: one

strikingly beautiful aspect of colloidal QDs is the way their emission colors

change with their size (band gap). This allows QDs to be used as ultra-efficient

light-emitting-diodes. Recently, Sony Corporation of Tokyo announced that

using QDs for flat-screen televisions (TVs) will transmit more richly colored

images than other TVs on the market.37

1.5.31.5.31.5.31.5.3 QDs for solar cells (SQDs for solar cells (SQDs for solar cells (SQDs for solar cells (SCs), light inCs), light inCs), light inCs), light in––––electricity out:electricity out:electricity out:electricity out: “No signs of abating”.38

Colloidal QDs films form the core of solar cells devices. Figure 1.5 shows the

principle of operation of three types of solar cells that employ semiconductor

QDs as light harvesters.39 In these types, the QDs film absorb the light to

produce electron-hole pair, which must then be separated and extracted at the

opposite electrodes.38 Since, the first report on infrared colloidal QD solar cells

Introduction

7

on 200540 much progress has been made towards technologically relevant

efficiencies. Efforts have been made to improve the efficiency of QD based

solar cells leading to enhance the efficiency from 2% in 200841 to around 3%

in 2009-201042-43, 4.6% in 2011 for metal/QDs device44, and 6% in 2011 as

well by optimizing the device architecture and its constituent materials.45

Recently, by engineering the QDs band alignment both the performance and

the stability of the QD (semiconductor bulk heterojunction) solar cells were

improved (certified efficiency ~ 8.6%).46

Figure 1.5: Schematic diagram of QDs based solar cells5 i) semiconductor

heterojunction SCs, ii) polymer/QDs hybrid SCs, and iii) QDs-sensitized SCs.

1.6 Quantum dots toxicity: 1.6 Quantum dots toxicity: 1.6 Quantum dots toxicity: 1.6 Quantum dots toxicity:

QD toxicity has been the major roadblock for using these materials in clinical research

and applications. Along with composition, the physical and surface characteristics (size,

shape, surface charge, and capping agent) of the QDs play the major role in the toxicity.

In 2006, Nawrot et al. have reviewed current understanding of the impact of cadmium

(Cd) on human health.47 Current understanding states that environmental exposure to

Cd increases total mortality in a continuous fashion, without any evidence of critical

dosage. They have noted that tubular kidney starts at urinary Cd concentrations

between 0.5 and 2 μg urinary Cd per g creatinine. In addition, recent investigation

focusing on bone effects showed increased risk of osteoporosis even at urinary Cd as

low as 1 μg Cd per g creatinine.48 For the pulmonary toxicity, which refers to the side

effect on the lungs developed by interaction with nanoparticles. This is important for

work place exposures resulting from preparation of large quantities of nanomaterials as

powders that can be dispersed to the air. Fortunately, most colloidal QDs like those

used in this thesis do not form dry powders. Even upon solvent evaporation they

typically remain waxy due to the large amount of organic molecules encapsulating them

Introduction

8

(capping agent). In general working with QDs is safe provided that one shows care and

respect starting from the synthesis until using them for different application and final

disposal.

Experimental methods

9

Chapter 2: Chapter 2: Chapter 2: Chapter 2: Experimental methodsExperimental methodsExperimental methodsExperimental methods

In this chapter, the preparation of different samples and the experimental methods used

in this thesis are presented.

2.1 Materials:2.1 Materials:2.1 Materials:2.1 Materials:

All chemicals used in the synthesis were purchased from Sigma–Aldrich without further

purification. Cadmium oxide (99.5%), selenium (>99.5%), oleic acid (OA, >99%),

and trioctylphospine (TOP, >90%) were used to prepare neat CdSe quantum dots

(QDs). In addition, zinc acetate (99%) and sulfur (99%) for gradient Cd1-xZnxSe1-ySy

core–shell QDs. Mercaptopropionic acid (MPA, >95%) and tetramethylammonium

hydroxide (TMAOH, 25% wt. in methanol) were used to exchange the ligand of QDs

for sensitization on metal oxide (MO). Solvents used in the QD synthesis and ligand

exchange included 1-octadecane (ODE, 90%), toluene (anhydrous, 99.8%), methanol

(anhydrous, 99.8%), acetone (HPLC, >99.8%), ethanol (99.7%). Zinc acetate

(>99%), ethylamine (>99.5%) and 2-methoxyethanol (anhydrous, 99.8%) were used

to prepare ZnO seed layer for nanowires synthesis, and the precursors for ZnO NWs

growth included zinc nitrate hexahydrate (>99%) and hexamethylenetetramine

(HMTA, >99%). ZnO and TiO2 nanoparticles (NPs) were purchased from Sigma–

Aldrich with average particle size (<35 nm and 15-20 nm, respectively). For p-type

NiO mesoporous films, anhydrous NiCl2 and polyethyleneoxide–polypropyleneoxide–

polyethyleneoxide triblock co-polymers (F108) were purchased from Sigma–Aldrich

without further purification as well.

2.22.22.22.2 Synthesis of Synthesis of Synthesis of Synthesis of neat neat neat neat CdSe QDsCdSe QDsCdSe QDsCdSe QDs::::

In a three-neck flask, a 70 ml octadecene ODE solution containing 5.54 g OA and

0.51 g CdO was first heated to 180 °C to form a clear solution under N2 atmosphere.

1.3 mmol Se powder and 0.5 g trioctylphosphine (TOP) were dissolved in 10 ml ODE

by stirring for 1 hour under N2 atmosphere as well. The obtained TOP-Se solution was

quickly injected in the three-neck flask.49 During the injection of TOP-Se solution, the

Cd2+ precursor solution was heated up to 180, 200, 220, 240, 280 and 320 °C to obtain

different sizes of QDs. After 2 minutes reaction, the flask was removed from the heater

and cooled down to room temperature. Finally, methanol and acetone were added to


10

precipitate the QDs, which were consequently redissolved in toluene. The purification

procedures were repeated twice in order to remove unreacted materials before using the

QDs for any measurements.

2.32.32.32.3 Synthesis ofSynthesis ofSynthesis ofSynthesis of gradient gradient gradient gradient CdCdCdCd1111----xxxxZnZnZnZnxxxxSeSeSeSe1111----yyyySSSSyyyy CSCSCSCSQDs:QDs:QDs:QDs:

Cd1-xZnxSe1-ySy gradient CSQDs were prepared via single-step hot injection method.

Typically, 0.4 mmol (0.05 g) of CdO, 4.0 mmol (0.88 g) of zinc acetate, 17.6 mmol

of oleic acid and 20 ml of 1-octadecene (ODE) were placed in a round flask. The

mixture then heated to 150 °C, degased for 20 min, filled with N2 gas, and further

heated to 310 °C to obtain a clear solution of Cd- and Zn-oleate. At this temperature

(injection temperature), 0.4 mmol (0.03 g) of Se powder, and 4.0 mmol (0.13 g) of

sulfur powder both dissolved in 3.0 ml TOP under N2 gas were quickly injected into

the cationic oleate reaction flask. After the injection the growth temperature was

adjusted to 300 °C for different shell growth time 5sec, 1, 3, 5, 10, and 15 min where

part of the reaction mixture was cooled down using ice bath to eliminate the QD’s

growth. As prepared QDs were purified using 1:3 chloroform-acetone mixture twice

and finally the purified QDs were redispersed in hexane.20

2.42.42.42.4 QDs surface QDs surface QDs surface QDs surface ligand ligand ligand ligand exchangeexchangeexchangeexchange::::

Attachment of the QDs to the MO was carried out by exchanging the capping agent

of as-obtained oleic acid-capped QDs with MPA linker. MPA is a bifunctional

molecule to link the QD to the MO via its two terminals (-SH and –COOH).

Therefore, 1.0 ml oleic acid-capped QDs with the concentration of 0.4 mM was mixed

with 0.1 ml MPA and 1.0 ml acetone. The mixture was stirred for 30 min. Afterwards

0.5 ml of ethanol was added to the mixture, which was then stirred another 15 min.

The resulting reactants were centrifuged at 3500 rpm for 5 min and the supernatant

was discarded. The resulting precipitates were redissolved in ethanol. Finally, TMAOH

was added drop wisely, so that a totally clear solution containing MPA-capped QDs

was formed. In order to prepare MO NPs film, purchased ZnO NPs, and TiO2 NPs

suspension was doctor-bladed on a glass substrate then annealed at 450 °C for 30 min

and then dried at room temperature. Then the MO films were immersed into MPA-

capped QDs ethanol solution in the dark. Afterwards, the films were washed

thoroughly with ethanol and dried using N2 gas. In paper VI, the linker exchange was

carried out on the substrate directly. Where, two sizes of QDs were mixed with 1:1


11

volume ratio, and then spin-coated onto a glass substrate at a spinning rate of 1000

rpm for 30 s. Afterwards 10% mercaptopropionic acid (MPA) in methanol was spin-

coated with the same condition to exchange the oleic acid capping agent of the as-

obtained QDs. Then the film was rinsed using methanol and toluene to remove

excessive QDs and MPA. This deposition process is repeated for several times to form

a thick layer of QDs.

2.52.52.52.5 ZnO NWs and NiO mZnO NWs and NiO mZnO NWs and NiO mZnO NWs and NiO mesoporous filmesoporous filmesoporous filmesoporous filmssss preparationpreparationpreparationpreparation::::

The synthesis of n-type ZnO NWs followed the widely known hydrothermal method.50

Briefly, 50 mM solution of zinc acetate dihydrate (Zn(CH3COO)2∙2H2O) and

ethanolamine in 2-methoxyethanol was spin-coated onto a fused silica substrate for 3

times and then sintered at 450 °C for 30 min in ambient to form a thin seed layer. The

hydrothermal growth of highly oriented ZnO NWs on the seeded substrate was carried

out at 92.5 °C in aqueous solution containing 20 mM HMTA and zinc nitrate. After

the growth, the sample was carefully rinsed in deionized water and dried at 100 °C. For

p-type NiO MO, by dissolving anhydrous NiCl2 (1.0 g) and polyethyleneoxide–

polypropyleneoxide–polyethyleneoxide triblock co-polymers F108 (1.0 g) into a

mixture of Milli-Q water (12.0 g) and ethanol (24.0 g). The solution was kept in room

temperature for 1 day for stabilization, and then centrifuged. The obtained supernatant

solution was doctor-bladed on a glass substrate and dried at room temperature. The

dried film was then calcined in air at 400 °C for 0.5 h.

2.62.62.62.6 QDs QDs QDs QDs characterizationcharacterizationcharacterizationcharacterization::::

Steady-state absorption and emission spectra were measured using an Agilent 845x

spectrophotometer and Spex 1681 spectrometer, respectively. The photoluminescence

quantum yield (PL QY) of the QDs was measured and estimated by comparing the

QDs’ PL intensities with that of standard rhodamine 6G dye at the same optical density

(0.07).51 The morphologies of QDs studied in this thesis were characterized via high

resolution analytical transmission electron microscopy (HR-TEM, JEOL 3000F

microscope equipped with an Oxford SDD x-ray analyzer) in order to obtain the

average size of the QDs. Chemical composition of CSQDs was estimated from the

energy-dispersive X-ray spectroscopy (EDX) spectra corresponding to the HR-TEM

images. Here TEM is operated in scanning transmission electron microscopy (STEM)

mode probing the information of the local chemical composition of the sample. The


12

morphology of multi-sized QD (paper VI) assemblies was imaged in the tapping mode

by an atomic force microscopy (AFM) (AFM 5500 System, Agilent Technologies).

2.72.72.72.7 Solar cell device fabrication:Solar cell device fabrication:Solar cell device fabrication:Solar cell device fabrication:

In paper VIII to fabricate the photoanode for solar cells, TiO2 paste (Ti-Nanoxide HT)

was coated on the surface of fluorine-doped SnO2 (FTO) by doctor-blading technique.

After annealing at 450 °C for one hour, the obtained TiO2 mesoporous film is suitable

for QD sensitization. The sensitization time was varied for different QDs to keep the

same optical density. Finally, freshly prepared photoanode was scratched to area ~ 0.25

cm2. In order to complete the solar cell, 1.0 M Na2S/1.0 M S solution used as an

electrolyte and FTO glass coated with 200 Å Platinum was used as a cathode.52

2.2.2.2.8888 Transient absorption spectroscopy (TA):Transient absorption spectroscopy (TA):Transient absorption spectroscopy (TA):Transient absorption spectroscopy (TA):

Transient absorption measurements were carried out on a femtosecond laser setup

based on the MaiTai-pumped Spitfire Pro XP (Spectra Physics) with the central output

wavelength of 800 nm and 1 kHz repetition rate delivering ~80 fs pulses. The beam

was split into two parts: one for pumping a collinear optical parametric amplifier

(TOPAS-C, Light Conversion) to generate the pump beam (470 nm), and the second

one was either focused onto a CaF2 crystal or a thin sapphire plate to generate a white

light continuum (for transient spectra measurements), or was used for pumping a

second TOPAS-C to generate narrow-band probe pulses at certain wavelength

depending on the purpose of the measurement (for kinetics measurements). The delay

between the pump and probe pulses was introduced by a delay line. The mutual

polarization between pump and probe beams was set to the magic angle (54.7°) by

placing a Berek compensator in the pump beam. Generally, we used thin-film samples

of QDs on MO (OD ~ 0.05) and colloidal samples in the 1-mm quartz cell (OD

between 0.2 ~0.4). During measurements the film samples were kept in N2 atmosphere

to avoid possible photodegradation (see figure 2.1).


13

Figure 2.1 Schematic picture of the fs TA setup with single wavelength probe and/or

white light (visible) probe. In this setup laser light (800 nm) is emitted and split into

two parts; one for pump (green line through Topas 2) and the other for probe (red

line for white light or blue line through Topas 1). Every second pulse of the pump

light is blocked by the chopper (the pump intensity was controlled by using PD1).

The pump extra path can be used to increase the traveled distance of the pump (used

for single wavelength kinetics experiments). For white light probe (TA spectra), the

beam is first directed on the delay-stage to increase the distance that the probe light

travels compared to the pump beam. Secondly, the white light is generated using a

CaF2 crystal or a thin sapphire plate. For single wavelength kinetics, the probe light

is converted in Topas 1 then directed on the delay stage. For both cases (white light

probe or single wavelength probe), the probe light is then split into a probe and

reference. The probe is overlapped with the pump on the sample and then directed

to the detector (monochromator for white light or PD2 for single wavelength). The

reference bypasses the sample and is directed to the PD3. The difference spectrum

can be created by collecting alternating spectra with pump on and pump off.

2.2.2.2.9999 TimeTimeTimeTime----resolved PL spectroscopy (TRPL):resolved PL spectroscopy (TRPL):resolved PL spectroscopy (TRPL):resolved PL spectroscopy (TRPL):

2.9.1 TimeTimeTimeTime----correlated single photon countingcorrelated single photon countingcorrelated single photon countingcorrelated single photon counting (TCSPC)(TCSPC)(TCSPC)(TCSPC): Time-resolved PL

measurements were performed using a time-correlated single-photon counting device

(PicoQuant). A pulsed diode laser at different repetition rates was used to excite the


14

sample at 438 nm. The pulse duration of the laser was about 200 ps. The PL from the

sample was filtered using long-pass filters from 520 nm to pick up only band edge

emission. The emitted photons were focused onto a fast avalanche photodiode (SPAD,

Micro Photon Device). The response time of the photodiode was <50 ps. The

excitation photon flux was controlled using neutral filters with different optical density.

2.9.2 StStStStrrrreak cameraeak cameraeak cameraeak camera: here the laser source for the time-resolved photoluminescence

setup is a titanium:sapphire passively mode-locked femtosecond laser (Spectra-Physics,

Tsunami), emitting at 820 nm with 80 MHz repetition rate and 150 fs pulse length.

In order to exclude the possible photocharging process in QDs, we also measured decay

kinetics of QDs with different pump repetition rates (80 MHz to 1.6 MHz) tuned by

a pulse picker keeping the pulse energy unchanged. The laser pulses were frequency-

doubled to 410 nm by a second-harmonic generator (Photop technologies, Tripler TP-

2000B). Time-resolved photoluminescence spectra were detected in a picosecond

streak camera (C6860, Hamamatsu, time resolution < 1 ps) coupled to a Chromex

spectrograph, triggered by the Ti:sapphire laser. A long-pass filter from 490 nm was

used in front of the spectrograph to cut off the scattering from the excitation pulses.

2.2.2.2.10101010 TimeTimeTimeTime----resolved terahertz spectroscopy (THz):resolved terahertz spectroscopy (THz):resolved terahertz spectroscopy (THz):resolved terahertz spectroscopy (THz):

The same laser system used in TA measurements was utilized for THz spectroscopy.

The laser beam was split into three. The first beam generates THz radiation by optical

rectification in a 1 mm thick (110) ZnTe crystal. The second beam was used in another

(110) ZnTe crystal for detection of THz pulses via electro-optical sampling. The third

beam seeds an OPA (Topas) generating 529 nm pulses used for sample photoexcitation

with a fluence of 6.3×1014 photons/cm2/pulse (mean number of excited electron-hole

pairs per QD, ⟨�⟩ = 1.2). To avoid absorption of THz radiation due to water vapor

and possible photodegradation of the sample, the THz apparatus was placed in a pure

nitrogen atmosphere.

2.2.2.2.11111111 XXXX----ray ray ray ray diffractiondiffractiondiffractiondiffraction (XRD)(XRD)(XRD)(XRD)::::

XRD experiments were carried out at the Beamline I711, in MAX IV Laboratory. The

energy of the incident X-ray is 12.5 keV (wavelength 0.99 Å). For the sample

preparation, 0.5 mm glass capillaries with the colloidal CSQDs solutions were gently

heated to fully evaporate the solvent. The XRD patterns were measured at room

temperature, in the atmosphere.

Exciton dynamics in QDs

15

Chapter 3: Chapter 3: Chapter 3: Chapter 3: Exciton dynamics in QDsExciton dynamics in QDsExciton dynamics in QDsExciton dynamics in QDs

3.1 Motivation3.1 Motivation3.1 Motivation3.1 Motivation::::

A better understanding of exciton dynamics in nanomaterials is thus important both

fundamentally and technologically53

In order to fully understand the dynamics of charge transfer from QD to metal oxide

(MO) in solar cell system, which is the main aim of the thesis, steady-state optical

properties of QD and the exciton dynamics within QD need to be discussed first. In

this chapter, we will start with the steady-state optical properties (absorption and

emission) of QDs. The ultrafast exciton dynamics within QD will be covered. We

continue with describing electron injection to metal oxide (MO)54, multiple electron

injection vs Auger recombination (AugR) to MO55, hole trapping56 by trap states and

hole injection57 to p-type MO. Finally energy transfer within CdSe QDs/MO system 58 and multi-sized CdSe QDs assembles 59 will be discussed.

3.2 3.2 3.2 3.2 SteadySteadySteadySteady----statestatestatestate absorption and emission: absorption and emission: absorption and emission: absorption and emission:

As we discussed in chapter 1, semiconductor optical properties are determined by the

electronic structure of the semiconductor. In QDs, the size (quantum confinement)

has a great effect on both optical absorption and photoluminescence (PL). As the size

of QDs decreases, blue shift in both optical absorption and PL occurs. This can be

accounted for using the particle-in-sphere model for a sphere with radius r:60

��,.� � = ��∞� + ��

��

+ ��

� − �.� �

�� , (1)

where, Eg(∞) is the bulk band gap, me and mh are the electron and hole effective masses

respectively, and ε is the relative permittivity of the semiconductor.

In eq. 1 the second term on the right hand side shows that the effective band gap is

inversely proportional to the square of the particle’s radius r and thus increases as the

size decreases, while the third term on the right hand side, due to electron-hole

Columbic attraction results in a decrease in the band gap with increasing particle radius.

However, for small particles (small r) the second term is dominating and thus the

effective band gap increases with decreasing r.53


16

Quantum confinement becomes pronounced when the QD’s size is comparable or

smaller than the Bohr exciton radius (αB):

! = �"��

�#� , (2)

In eq. 2 ε and ε0 denote the relative permittivity of the semiconductor and the

permittivity of the vacuum, respectively, μ is the electron and hole reduced mass

$$� �$ + $��⁄ , and e is the electron charge. For example the Bohr radius of bulk

CdSe is ~ 5.6 nm.24 CdSe NCs with sizes comparable to this radius then exhibit a

noticeable blue-shifts in both absorption and PL spectra compared to the bulk spectra.24

Figure 3.1 A shows the absorption spectra of CdSe QDs with different sizes.56

The energy band of QDs would be split into discrete states due to the quantum

confinement both in CB and VB. In the absorption spectra, such discrete states would

induce pronounced exciton peaks. In particular, clear exciton peak would be observed

near the absorption band edge due to the electronic transition from the VBM to the

excitonic state that is located just below the CBM. The difference between the band gap

and the excitonic state energy emerges due to the electron-hole interaction and is

referred to as exciton-binding energy (EBE).24 The recorded EBE value of CdSe QDs,

for instance, is size dependent and can reach 1.0 eV for the 2.8 nm QDs size.24

Apart from absorption spectroscopy, we can also utilize photoluminescence (PL, see

Figure 3.1 B) to probe the excitonic states near the band edge or within the band gap

(i.e defect/trap states). For a typical NC, PL spectrum contain emission from both band

edge and trap states.56 The trap states which are located within the band gap form a

red-shifted PL band compared to the band edge emission (because of the reduced

transition energy between these states and the opposite band).61 The trap emission is

usually very broad because of various trap states with different properties exist. In

practice it is possible to passivate the surface trap states of QDs using organic capping

agent like oleic acid (OA),57 tri-n-octylphosphine oxide (TOPO),62 by surface

modification techniques63 or by growing a semiconductor shell around the core (core–

shell structure).64 This will result in enhanced band edge emission (discussed more in

chapter 4). Some other QDs were found to be weekly luminescent or even non-

luminescent at room temperature like PbI2 and CuS. This property is mainly due to

the indirect band gap65 character of these materials and the high density of internal

and/or trap states which act as PL quenchers.53


17

Figure 3.1: A) Absorption spectra of CdSe QDs with different sizes in toluene56, B)

emission (PL) spectra from different sizes of CdSe QDs in toluene56, and C)

schematic illustrating the optical band edge absorption and the exciton binding

energy (EBE).

3.3: 3.3: 3.3: 3.3: Exciton dynamics:Exciton dynamics:Exciton dynamics:Exciton dynamics:

In QDs, absorption of a photon with energy larger than the QD band gap results in

excitation of an electron in the CB and a hole in the VB. The carriers are bound to

each other by Coulombic interaction and form an exciton due to large EBE. Initially,

within few tens of fs, an electronic dephasing takes place. Electronic dephasing is

considered as a dynamics process in which the energy is conserved while the phase

memory of the quantum state is lost. This process is considered as the fastest event that

excitons experience in semiconductors.66 For instance in CdSe QDs, the electronic

dephasing times at 15 K has been reported to be 85 fs based on three pulse photon

echo measurements.67 After the excitation with high energy photon, both electron and

hole have excessive kinetic energy (KE). Dissipation of this energy due to interaction

between electrons/holes and phonons on time scale of hundreds of fs is resulting in

AAAA BBBB

{ }VBVBVBVB

CBCBCBCB

{

Band gapOptical absorption

Exciton binding energy (EBE)

CCCC

400 450 500 550 600 650 700

0.2

0.4

0.6

0.8

1.0

1.2

1.4

No

rm.

Ab

s.

Wavelength (nm)

2.3 nm

2.6 nm

4.4 nm

7.0 nm

450 500 550 600 650 700 750 800 850

0.0

0.2

0.4

0.6

0.8

1.0

No

rm

. P

L

Wavelength (nm)

2.3 nm

2.6 nm

4.4 nm

7.0 nm


18

intraband relaxation of the electron to the CBM and the hole to VBM. This intraband

relaxation is followed by radiative (emitting a photon)68 or nonradiative (producing

heat) recombination.69

Figure 3.2: Dynamic processes in QD: 1)

electron and/or hole relaxation through

electron-phonon coupling, 2) electron

trapping into trap states and the same may

occur for the hole (hole trap states), 3)

radiative and nonradiative band edge

electron-hole recombination, and 4) radiative

and nonradiative trapped electron-hole

recombination.

In semiconductor NCs, dynamic processes are generally more complicated compared

to the bulk semiconductor due to the presence of many band gap states. These states

are often referred to as surface states or trap states. Depending on the application of

interest for the semiconductor NCs, these surface states or trap states can be unwanted

or useful. For example, in the catalytic application, these states are useful70 where

electron or hole trapping will prevent the recombination and subsequent redox reaction

may occur using the trapped carriers. In contrast, for photovoltaics or luminescence

application these states are usually undesirable.71

In the QDs which are free from trap states, the electron and hole will primarily

recombine radiatively which leads to high PL quantum yield (PL QY), and exciton

lifetime on the order of a few to a few tens of ns (for direct band gap QD).54-55 On the

other hand, in the QDs with trap states, trapping process of electrons and/or holes by

these states becomes significant and occurs on time scale of a few ps to a few tens of ps

(see figure 3.2). As a consequence of high density of trap states, the band edge PL

intensity and the PL QY are very low.57 In QDs solar cells, especially QDs-sensitized

solar cells (QDSSCs), the trap states are usually induced by surface dangling bonds

which are always far from the interface to the acceptor and thus prevent the charge

transfer processes. Therefore, reduction of such trap states is very important for

photovoltaics application. Here, the charge separation plays a crucial role while the

trapping of the charge carriers is a strong competitive to the charge transfer to MO.


19

More details about the charge (electron/hole) injection will be discussed in the

following sections.

In some cases, depending on the density and energy of the trap states, the trapped

carriers (electron and/or hole) can relax to deeper traps within the band gap. The

lifetime of the charge carrier in deep trap states can be as long as tens of ns72 to μs or

even longer.56 The experimentally observed lifetime of a given state τOb depends on

radiative and nonradiative lifetimes, τR and τnonR, respectively. For an ideal crystal where

the PL QY is high (~ 100%), τnonR is very long leading to τOb = τR. In reality, this is

seldom case where the observed lifetime has contributions from both radiative and

nonradiative lifetimes as follows:53

1 '()* = 1 '+* + 1 ',-,+* , (3)

./ 01 = '() '+* , (4)

In the rest of this chapter, the main objectives of papers I-IV will be presented in the

following order: Papers I and II contain work on the electron injection from CdSe QDs

to ZnO (MO).).).). Paper III analyses the hole trapping process and its long-lived

component, whereas the paper IV studies the hole injection from QDs to p-type MO.

In addition, exciton (electron-hole pair) transfer is presented in papers V and VI. As a

result of our work on neat CdSe QDs, a surface modification is necessary. Therefore,

chapter 4 mainly focus on Cd1-xZnxSe1-ySy gradient CSQDs and how controlling the

shell thickness improves the QD’s properties for solar cell applications.


20

3333.4.4.4.4 Paper I “Electron injection from QDs to MO”Paper I “Electron injection from QDs to MO”Paper I “Electron injection from QDs to MO”Paper I “Electron injection from QDs to MO”: : : :

Studies of electron injection from QDs to MOs by using standard techniques (pump-

probe in the visible range, time-resolved PL) bring an ambiguity in interpretation. A

depopulation of QDs can be observed, however, without a possibility to distinguish

electron trapping from electron injection. Reported time scales of electron transfer

therefore varied from picoseconds up to nanoseconds, even for similar QD-MO

systems,54 and various interpretations of the observed kinetics can be found. In this

paper, ultrafast electron injection from CdSe QDs to ZnO NWs is observed to take

place on the picosecond time scale (3–12 ps) depending on the QD’s size by using

ultrafast time-resolved absorption technique (see figure 3.3 A). The electron injection

to the MO was further proved by terahertz (THz) spectroscopy technique where the

conductivity spectrum and rising signal confirms that the electrons arrive in the MO

in similar timescale as they leave the QDs (see figure 3.3 B).

Marcus theory of electron transfer: on 1992 Rudolph Marcus got the Nobel Prize in

chemistry for his theory of electron transfer.73 Tvrdy et al. used Marcus theory to

describe the electron injection rate from CdSe QDs to MOs as a function of the QD

sizes.74 In many-state Marcus model, where the electron is transferred from a discrete

molecule-like state in QD to a continuum of states in MO, the transfer rate can be

expressed as:

∫+∞

∞−

+∆+−= )

4

)(exp(

4

1)()(

2 22

kT

EG

kTEHEk

et λλ

πλρ

πh

, (5)

where, ρ(Ε) is the MO density of states (DOS), |34��|� is the electronic coupling

matrix element, 5 is the reorganizational energy, k is Boltzmann constant, T is the

temperature, and Δ7 is the free energy change. The free energy change ΔG can be

expressed as:

Δ7 = Δ�8 + Δ�9, (6)

where, ΔEel is the difference between the electron initial energy state (QD’s CB) and

the electron final state (MO’s CB) and the Coulombic term ΔEC arises from electron

and hole interaction. In other words, the electron energy difference acts as a driving

force for the electron transfer process where the Coulombic interaction energy acts

against the electron transfer process. Figure 3.3 C demonstrates that the electron


21

injection rates with different driving forces induced by different QD sizes can be well

fitted by Marcus theory.

Figure 3.3: A) Normalized TA kinetics of neat QDs (dotted lines) and QDs-

sensitized ZnO NWs (solid lines) for different size of CdSe QDs, B) TA kinetics

compared to THz kinetics of (2.9 nm) QDs-ZnO NW, C) electron transfer rate (ket)

dependence on the free energy change (ΔG) described by Marcus theory, and D)

schematic illustrating the CTS model of the electron injection.

In addition to the fast injection (~ 6 ps) a slower kinetic component of about ~100

ps is clearly present in both TA and THz signal. This indicates the two-step electron

transfer via charge transfer state (CTS). Such model has been proposed previously in

dye-ZnO systems.75-76 In the CTS model, a fast electron injection is followed by a

slower process where the CTS dissociates and electrons are released to the NW bulk

(see figure 3.3 D). The ultrafast electron injection (~ ps) has important implications

for exploiting the multiple exciton generation (MEG) in QD-sensitized SCs. In this

context, the injection process has to compete with other fast loss channels, like Auger

recombination (AugR).


22

3333....5555 Paper IPaper IPaper IPaper IIIII ““““Multiple exciMultiple exciMultiple exciMultiple exciton harvestington harvestington harvestington harvesting”: ”: ”: ”:

Performance–cost balancing is a crucial factor for solar cell expansion as a sustainable

source of energy. For example the single-junction SCs efficiency is restricted to be

below the Shockley–Queisser limit of about 32%.77 The Shockley–Queisser limit

originates from the fact that only photons with energy higher than or equal to the band

gap can be used to create excitons. Photons with energy less than the band gap will be

not absorbed and photons with energy higher than the band gap will create excitons

with excess energy which will be lossed as heat. QDs have a unique property which can

break the Shockley–Queisser limit via the so-called “multiple exciton generation”

(MEG). Instead of thermal relaxation, the excess energy can be used to excite one or

more electron-hole pairs. The harvesting of multiple electrons out of one excited QD

would increase the efficiency of SCs. Efficient MEGs in various materials of QDs and

QD–MO systems have been reported.27-28, 78-79 On the other hand, utilization of MEG

is limited by AugR where the recombination energy of one electron-hole pair is

transferred to electron or hole in the second electron-hole pair, which relaxes rapidly

via thermal relaxation.

In this paper, we study the electron injection under multiexciton conditions (high

excitation intensity). In paper I, we showed that the electron injection is very fast within

few to few tens of picoseconds for one electron-hole pair per QD (low excitation

intensity). Under multiexciton condition, a solid foundation for distinguishing the

competition between AugR and electron injection was revealed. Electron injection rates

from QDs to MOs strongly depend on the distance (shell thickness in this case)

between QDs and MOs. Three samples representing fast (sample F, no shell),

intermediate (sample I, thin shell), and slow (sample S, thick shell) electron injection

rate were chosen in this study (see figure 3.4 A).

A typical sign for multiple excitons in QDs is the rapid AugR55 leading to a kinetic

component which is significantly shorter than the single electron-hole pair

recombination on the nanosecond time scale.54 AugR contribution was determined

from the difference between TA decay kinetics under low and high excitation

conditions in the absence of the MO. Introducing the MO under the same excitation

conditions will affect the kinetics by the fast electron injection process. Thus for the

studied samples, AugR-electron injection lifetime ratio is 0.01, 1.2, and 4.5 for samples

F, I, and S, respectively.55


23

Figure 3.4 B shows the effect of MO on the dynamics under high excitation intensity.

For sample S, AugR is more dominant where TA dynamics under different excitation

intensities are the same for attached and unattached QDs. Sample I presents a

combination of electron injection and AugR. At low excitation intensity, a fast TA

decay is due to electron injection. By increasing the intensity TA decay becomes faster

due to AugR. For instance, the mean lifetime changes from 60 ps at low excitation

(electron injection only, kinj) to 35 ps at high excitation (electron injection and AugR,

ktot) with AugR lifetime ~ 80 ps (the same in the absence of ZnO NWs ~ 73 ps).

Figure 3.4: A) Normalized TA kinetics of pure QDs (dotted lines) and sensitized

ZnO NWs (solid lines) for samples F, I, and S, B) normalized TA kinetics for

samples F, I, and S attached to ZnO NWs under different excitation intensity (all

excitation intensities are in μJ/cm2), C) schematic illustration of the 2nd electron

injection vs AugR, and D) electron injection rate dependence on QD size (orange

squares) fitted by Marcus theory (solid orange line) adopted from paper I; Auger

biexciton recombination rates (our results, dark cyan circles) and from ref 80 (dark

cyan triangles) fitted by D-p function (p = 4.9, dark cyan line).

Finally, the competition between electron injection and AugR is presented in sample

F. As the excitation intensity increases the TA decay becomes slower till excitation

intensity reaches ~ 700 μJ/cm2, then, TA signal decays faster. The slow decay (~ 50 ps)

can be understood as a competition between the second electron injection and trion

decay (positive trion is a QD contain two holes and one electron). Comparison of the

slow decay lifetime (~ 50 ps) and the reported trion lifetimes (~ 140 ps)81, indicates


24

presence of an efficient injection of the second electron to MO. At higher intensity, the

TA dynamics becomes faster (see figure 3.4 B) due to AugR from more than doubly

excited QDs.

It is also essential to evaluate the size dependence of AugR rate. By the same methods,

AugR contribution can be extracted for different QD sizes. Like pervious results80, 82-83,

AugR lifetime becomes significantly shorter for smaller QDs. The obtained QD size

dependence of AugR rates can be well fitted by a D-p function (p = 4.9) which is in line

with previously reported dependences (see figure 3.4 D).84 We demonstrate that, the

multiple exciton harvesting efficiency changes significantly with QDs’ size. Already

within the narrow range of QD sizes (2–4 nm) the efficiency of the first exciton

harvesting can vary from 30% to 70% for this particular system (QD-linker-ZnO

NWs).


25

3333....6666 Paper IPaper IPaper IPaper IIIIIIIII ““““UltraUltraUltraUltra----longlonglonglong----livedlivedlivedlived trapped holes in QDstrapped holes in QDstrapped holes in QDstrapped holes in QDs”:”:”:”:

After the formation of excitons, not all the carriers (electrons and holes) undergo a

prompt recombination. Some of them get trapped by defects and/or surface states. This

affects the dynamics of excitons85, reduces the PL yield86, and hinders the charge

transport.87 Recent studies have shown that the charge trapping is not always

detrimental to QDs application, and in some systems the trapped states can be used to

broaden the PL spectrum for potential application in white light-emitting-diodes.88-89

In this paper, we investigate the PL from the trapped states in oleic acid capped CdSe

QDs with various sizes (2.3-7.0 nm). The PL from the trapped states (due to localized

carriers) can be observed with red-shift to the PL from the band edge recombination

(due to delocalized carriers). In our investigation we found that the smaller the size, the

larger is the contribution of the red-shifted bands.90 We attribute this to the increase in

the surface-to-volume ratio with the decreasing size of the QDs, and therefore we

associate such red-shifted PL bands to surface trap states. We also found that this red-

shifted emission consists of at least two bands contributing to the surface states related

PL (see figure 3.6 A, PL1 and PL2), which indicates that more than one type of surface

states are radiative.

Furthermore, our photoluminescence excitation (PLE) measurements show that the

excitons above the band edge can get localized (trapped) at the surface and contribute

much more to the surface PL than the excitons excited at the band edge (see figure 3.6

B). This means that the trapping process is more efficient when the QDs are excited

above the band edge.91-92 Time-resolved PL (TRPL) helps us to understand the

dynamics of these trap states. Typically, localized carriers at shallow traps can delocalize

again at room temperature and contribute to the band edge PL, while the carriers at

deep traps are difficult to delocalize with lifetime longer than 2.5 μs (time window for

TRPL) in room temperature (see figure 3.6 C). Using ultrasensitive femtosecond

pump–nanosecond probe setup, a very long-lived component (> tens of μs) has been

observed. The photobleach signal suggests that the electrons remain delocalized and do

not recombine with the holes within few tens of ns (single exciton life time in CdSe

QDs), which means that the holes remain trapped on the microsecond time scale (see

figure 3.6 D). Our results add more evidence that the majority of the trapped carriers

in CdSe QDs are holes.


26

Figure 3.6: A) Normalized PL from 2.3 nm CdSe QDs with red-shifted emission

from the trapped hole states (PL1 and PL2); inset: hole multi-trap states model, B)

PLE spectrum of 2.3 nm CdSe at different emission wavelengths, C) band edge (blue

line), trap-state emission (black line) decay obtained by using a sharp edge filter, and

the fit (red lines), and D) TA kinetics of CdSe QDs at different pump intensities;

inset: TA decay in early time scale (5-20 μs).


27

3333....7777 Paper IPaper IPaper IPaper IVVVV ““““Hole injectionHole injectionHole injectionHole injection vsvsvsvs hole trappinghole trappinghole trappinghole trapping”:”:”:”:

The aim of this work is to evaluate the hole injection from QDs to p-type MO (namely,

NiO). Hole injection and hole trapping have similar spectroscopic signatures and it is

a challenge to separate them from each other. Furthermore the trapping process directly

interferes with injection hindering the efficient charge transfer.56 In addition, we

studied the dependence of hole injection on QD sizes and proved that the passivation

of hole traps by core–shell structure is beneficial for efficient hole injection.

In this work, we mainly used TRPL to analyze the hole dynamics in QDs because

other methods such as TA signals in visible spectral region is mainly sensitive to the

electrons in QDs’ CB. One difficulty to identify the hole injection process in QD-NiO

system is the competition from hole trapping induced by linker molecule exchange

from oleic acid (OA) to short chain thiol-based linker (e.g mercaptopropionic acid,

MPA).93 By comparing the TA and PL kinetics between OA-capped QDs and MPA-

capped QDs, we can prove the hole trapping process within 4 ps (see figure 3.7 A).94

However, the major fast hole trapping (~4 ps) induced by ligand dangling bond can be

significantly reduced by photoirradiation (known as photoinduced emission

enhancement “PEE”). This effect is commonly explained by surface passivation

induced by the chemical change of QD’s surface during the irradiation process.95

In this scenario, we can start to investigate the hole injection in QD-NiO system

after such “photoirradiation passivation” which would exclude the fast quenching

process by surface traps. The situation is different for QDs attached to NiO where we

used a global fitting of TA spectra in QD-NiO resulting in two components. The long-

lived component (~ 6 ns) can be assigned to exciton recombination whereas the fast

component (~ 200 ps) (see figure 3.7 B) can be explained by employing a charged-QD

model. Here, the hole injection from QD to MO will leave a charged-QD behind. If

negatively charged, the QD’s absorption edge will show a red shift on the scale of a few

meV.96 In our case, the difference between original and 10 meV shifted absorption

spectrum gives the TA fast component spectrum (see figure 3.7 B inset). This provides

an evidence for the charged QD model and at the same time directs towards the hole

injection to NiO, because the hole trapping itself does not lead to such behavior.

This fast component (~ 200 ps) is analogous to what we found in TRPL

measurements. The multiexponential fitting of PL kinetics of QD-NiO exhibits an

additional fast decay component (~ 380 ps) compared to those of neat QDs samples.


28

Therefore, it can be assigned to hole injection from QDs to NiO (see figure 3.7 C).

Similar to electron injection to ZnO NWs in paper I, we used Marcus theory (eq. 5)

to study the hole injection rates dependence on the QD’s size. We have modified the

free energy change term (ΔG) calculation (see eq. 7) to account for the hole properties:

∆7 = ∆�� + ∆�9, (7)

where ΔEh stands for the hole energy difference between the initial (QD’s VB) and the

final energy (MO’s VB) states. ΔEh has the same value since the VBM for CdSe is

independent of the QD’s size when attached to MO.97 The second term ΔEC is the

Coulombic interaction between the electron and hole.

Figure 3.7: A) TA kinetics at the band edge absorption and PL decay of OA-and

MPA-capped CdSe QDs on glass, B) TA spectral components extracted from TA

spectra; inset: steady-state absorption (cyan line) and change in absorption after a

shift of the spectrum by 10 meV (blue line), C) PL decay for CdSe QDs on glass;

inset: PL decay for CdSe QDs with different size attached to NiO, D) hole injection

rate vs driving force of different QDs sizes, and E) schematic illustration of hole

trapping vs hole injection to MO.

Figure 3.7 D shows the hole injection rate dependence on the QD’s size according

to eq. 5. We found that the hole injection rate for CdSe-NiO system is more than an

order of magnitude slower than the electron injection rate for CdSe-ZnO system (paper

I). This can be explained by the hole effective mass being larger than the electron

effective mass and the MO’s permittivity being higher for ZnO than NiO. However,


29

the most critical factor limiting the hole injection efficiency is the hole trapping which

is strongly competing with the hole injection. We found that growing the shell (1.3 nm

thickness) around the active core will passivate the hole trap states. Consequently, the

hole injection efficiency was enhanced from less than 10% to 50% for core only and

core–shell QDs, respectively.


30

3333....8 Paper V8 Paper V8 Paper V8 Paper V ““““Energy transfer in CdSeEnergy transfer in CdSeEnergy transfer in CdSeEnergy transfer in CdSe----ZnO NWs ZnO NWs ZnO NWs ZnO NWs

systemsystemsystemsystem”:”:”:”:

One of the important requirements for efficient photoinduced charge separation is the

direct contact between the light absorber and the electron acceptor. However,

multilayers of QDs absorber can boost the solar cell performance as has been reported

for ligand-free QDs.98 Although many models are used to explain the charge collection

from indirectly attached QDs (IA-QDs). In this work we attribute it to the energy

transfer mediated charge collection. Here, the photodynamics in multilayer MPA-

capped CdSe QDs sensitized ZnO NWs with different sensitization time were studied.

Analogous to previous reports,99 in situ steady-state absorption spectroscopy was used

here to quantify the adsorbed amount of QDs on the MO surface. Adsorption can be

described as a combination of a Langmuir-type submonolayer formation and adding

further layers of indirectly attached QDs (aggregation) (see figure 3.8 A). The fraction

of IA-QDs (R) can be calculated as follows:

; = <.=�> ?<.=�> + <.@=,�>A* , (8)

where, <.=�> and <.@=,�> are the concentration of IA-QDs and submonolayer

adsorbed QDs, respectively.

The TA decay kinetics for sensitized ZnO NWs by CdSe QDs show similar decay

components for the electron injection (~10 ps) and CTS dissociation (~100 ps) (paper

I). However, the nanosecond component (~ 3.6 ns) varies with different QDs

sensitization time (see figure 3.8 B). Based on the correlation between the component

amplitude (A3 from the fitting) and the IA-QDs fraction R (calculated from eq. 8) (see

inset of figure 3.8 B) we assign this component to the excitation energy transfer from

the indirectly attached QDs. In addition, the continuous enhancement of the incident

photon-to-electron conversion efficiency indicates that the absorbed photons by IA-

QDs also yield charge carriers. Energy transfer (exciton migration) is the most plausible

mechanism for our system, where the exciton migrates from the IA-QDs to the QDs

adjacent to the MO. Exciton migration from the higher energy (smaller) QDs within

the size distribution of the QDs sample can be confirmed from the steady-state PL

where a pronounced red shift (40 meV) and narrowing of the emission spectrum can

be found in the film of QDs. In contrast to that, no such narrowing occurred in QD

solution (see figure 3.8 C).


31

Figure 3.8: A) Experimental and theoretical fit of the absorption difference data due

to submonolayer adsorption (DA-QD, red line) and QD aggregation (IA-QD, blue

line), B) TA kinetics of QD sensitized ZnO NWs at different sensitization times;

inset: the fraction of IA-QDs (R, black line, calculated from eq. 8), the amplitude of

TA long-lived component (derived from the three exponential fit, A3), C) steady-

state PL (solid lines) and absorption (dashed lines) of QDs in solution and spin

coated on glass, and D) schematic illustration of the exciton migration from IA-QD

to DA-QD followed by electron injection via CTS model.

Another evidence for the FRET in our sample can be found in TA decay kinetics for

neat QDs on glass, which shows a biexponential character with a conventional radiative

exciton recombination (~ 10 ns) and a faster (~ 660 ps) component corresponding to

the energy transfer from small QDs to larger ones. Finally we can evaluate the energy

transfer time constant from the IA-QDs to the DA-QDs as:

1 'BC = 1 ⟨'⟩ − 1 'DEF⁄⁄⁄ , (9)

where, ⟨'⟩ is the TA lifetime of IA-QDs (3.6 ns), and 'DEF is the exciton lifetime of

QDs without any transfer process. We obtain the energy transfer time constant 'BC

from IA-QDs around 5 ns (see figure 3.8 D). The energy transfer time is significantly


32

longer than electron injection time. In order to improve this, a well-designed QD

tandem layers with different sizes to provide an ideal spectral overlap for efficient FRET

is indispensable.


33

3333....9 9 9 9 Paper VIPaper VIPaper VIPaper VI ““““EnergyEnergyEnergyEnergy funneling through multifunneling through multifunneling through multifunneling through multi----sized sized sized sized

QDQDQDQD filmfilmfilmfilmssss”:”:”:”:

After we have proven the exciton migration from IA-QDs to DA-QDs due to the QDs

size distribution in QDs-ZnO NWs system, we turn to study the directed exciton

migration from small to large QDs in more organized multi-sized QD mixtures.

Exciton migration study is also essential for other QD’s applications as light emitters,

lasers and solar cells.100-101 In the development of such devices, QDs are considered as

densely packed to form a thin film with strong absorption and emission. Indeed, FRET

is an important mechanism due to the electronic coupling between the closely packed

QDs.

Here, we report the investigation of FRET dynamics in densely packed multi-sized

CdSe QDs (we used three different QDs: QDs-A, 2.3 nm, QDs-B, 3.7 nm, and QDs-

C, 6.7 nm) based on TA spectroscopy and theoretical modeling. Firstly, steady-state

PL spectroscopy for multi-sized CdSe QDs film (A+B) shows a typical indication of

FRET: a significant increase of the red emission on the expense of the blue emission

due to exciton migration from small QDs to large QDs (see figure 3.9 A). However,

steady-state PL spectra do not provide a full proof of FRET, therefore, we used TA

measurements to prove the FRET presence and to reveal its time scale. TA spectra at

different delays (t) for singly-sized QDs (ΔαA(t), ΔαB(t), ΔαC(t)) and for different

combinations of QDs (A+B, A+C, B+C) (ΔαA+B(t)) at the same delays were recorded,

then fitted using the sum of TA spectra for singly-sized QDs at the same delays:

∆ GH!�I� = JG�I�∆ G�I� + J!�I�∆ !�I�, (10)

The ratio of single-color spectrum (kA(t)/kA(0) for QD-A, and kB(t)/kB(0) for QD-B)

corresponds to the FRET-related change of the excited state population in A and B

relative to their initial population. This ratio gives us valuable information about FRET

dynamics, if this ratio stays the same at different delays for A and B (for example, A+B

mixture in solution) this means no FRET dynamic within A and B mixture (see figure

3.9 B and C). On contrary, for the same mixture in the film form, a significant decrease

of the ratio of QD-A accompanied by increase of the ratio of QD-B with similar

kinetics can be observed. The same effect takes place for the other pair combinations

(see figure 3.9 D). For all possible QD pair combinations, the FRET times were

estimated based on the analysis to be 1-3 ns for different combinations.


34

Figure 3.9: A) Steady-state PL spectra of QD-A and QD-B individually and a

mixture of QDs-(A+B) on glass, B) TA spectra of QD-A, QD-B, and QD-C

individually in solution and a mixture of QDs-(A+B+C) in solution as well, C),ratio

of single-color spectrum as a function of pump–probe delay times of QDs-(A+B+C)

in solution, and D) ratio of single-color spectrum as a function of pump–probe delay

times of QDs with different combinations on glass.

The estimated values did not follow the (1/;EGL ) dependence according to the

standard FRET rate expression (eq. 11).

JBC = 1 'BC⁄ = ;EGL MN ��

ℏ PE� PG

�Q�R�* , (11)

where, RDA is the donor-acceptor distance, μD and μA are the donor and acceptor dipole

moments, respectively, K2 refers to the orientation factor of the dipole–dipole

interaction, n is the medium’s refractive index, and R is the spectral overlap between

the normalized donor emission and acceptor absorption line shapes. Apparently the

simple dipole–dipole description of FRET is not sufficient because of the distance

between donor and acceptor is smaller than the QD dimensions.

We took into account the dipole moment distribution over the whole QD using

Bessel function to describe the exciton wavefunction density in spherical QDs. The

Measured data

Fitting result

QDs-A

QDs-B

QDs-C

500 550 600 650 700-0.025

-0.020

-0.015

-0.010

-0.005

0.000

1 10 100 1000 100000.0

0.5

1.0

1.5

2.0

∆∆ ∆∆ A

bs.

Wavelength (nm)

QDs-A QDs-B QDs-C

Rati

o o

f si

ngle

colo

r

spec

tru

m

Delay (ps)

BBBB

CCCC0.0

0.5

1.0

1.5

2.0

0.0

0.5

1.0

1.5

1 10 100 1000 100000.0

0.5

1.0

1.5

Rati

o t

o s

ingle

colo

r s

pectr

um

QDs-B

1.5 ns

QDs-A

2 ns

QDs-B

QDs-C

QDs-A

Delay (ps)

QDs-C1.5 ns

DDDD500 525 550 575 600 625 650 675

0.0

5.0x106

1.0x107

1.5x107

2.0x107

QDs-A

QDs-B

PL

in

ten

sity

fo

r s

ing

le c

olo

r

Wavelength (nm)

0

1x104

2x104

3x104

4x104

5x104

6x104

PL

in

ten

sity

fo

r m

ixed

co

lors

QDs (A+B) on glassAAAA


35

function drops quickly with increasing the distance from the QD’s center, but still the

dipole distribution influences the interaction of two close QDs (see figure 3.10 A, lower

inset). In order to incorporate the dipole distribution into the FRET theory, we have

modified the “1 ;EGL⁄ ” term (distance term). Namely, we numerically integrate the

dipole–dipole interaction over the volume of donor and acceptor. Our calculations

show that for distances less than 10 nm between donor and acceptor (here, for our

combinations the center-to-center distances range 3-5 nm) the point–dipole

underestimates the resulting FRET rate but for larger distances the point–dipole

approximation is sufficient (see figure 3.10 A). For closely staked QDs, the correction

of the distance term based on Bessel distribution can be more than 50% (see figure

3.10 A, upper inset).

In reality, the QD–QD distances given by the film morphology play also an

important role in FRET rate. In the previous reports often the ideally stacked QD layers

were used to calculate the FRET rate. We have used atomic force microscopy (AFM)

and found a big deviation from the ideal model due to the fabrication procedure. The

QDs tend to aggregate into clusters after ligand exchange.102 We compared our

experimental FRET rates to the calculated rates based on our modifications. As the

distance between QD and the closest neighbor is not a single value, but a distribution,

our calculations result in a distribution of FRET rates around mean value on the

nanosecond time scale. For all the combinations, the mean FRET rates agree with the

FRET rates obtained experimentally (see figure 3.10 B).

Finally, we study the directed energy funneling between tandem layers of QDs with

different sizes. We use steady-state PL for different combinations of QD monolayers

where QD-C is used as the exciton final acceptor and QD-A is the donor. In between

the A and C monolayers we deposit one or two layer(s) of QD-B. In both cases (A+B+C)

and (A+2B+C) (see figure 3.10 C), about 90% of the excitons from QD-A is taken off

due to fast FRET compared to the exciton life time in QD-A. The extra QD-B layer

did not affect the exciton migration from QD-A but it decreases the number of excitons

transferred to the QD-C by 20%. This pronounced effect of the extra QD-B layer on

the exciton migration stimulated us to extract the exciton diffusion length LD for B-C

QDs combination with different number of QD-B layers. We estimate the diffusion

length to be 9.0 nm. For a single layer of QD-B, 95% of excitons transferred to QD-

C (FRET lifetime 3.0 ns, exciton lifetime 28 ns). On the other hand, for many QD-B


36

layers (LD ~ two layers of QD-B), many excitons cannot reach B/C interface and

recombine before the final destination (QD-C) (see figure 3.10 D).

Figure 3.10: A) Distance term of FRET for δ-function approximation (1 ;EGL⁄ ,

black points) and Bessel function model (red points); lower inset: the corresponding

density of dipole moments in the models; upper inset: correction ratio obtained via

Bessel function, B) left panels: experimental ratio of the excitons transferred to donor

(blue lines) compared to the theoretical prediction (red lines), and right panels:

distribution of FRET lifetimes for each pair combination, C) PL spectra of ordered

QDs films, and D)PL ratio between QD-B on glass and QD-B with top layer of

quencher (QD-C) as a function of the number of QD-B layers in order to extract

the exciton diffusion length LD.

500 550 600 650 700 7500

2000

4000

6000

8000

10000

12000

C

2×B

PL

In

ten

sity

Wavelength (nm)

C

2×B

A

A only

0 5 10 15 20 25 30 350.0

0.2

0.4

0.6

0.8

1~5 layer

Number of QD layers

PL

rati

o (

I qu

en

ch

ed/I

non

qu

ench

ed)

Length (nm)

QDs-B

Glass

QDs-C1 layer

θ

LD= 9 nm

4.7 nm

QD B

1 2 3 4 5 6 7 8

DDDDCCCC

AAAA

10 1001E-12

1E-10

1E-8

1E-6

1E-4

-1.0 -0.5 0.0 0.5 1.00

1

Bessel

func.

Bessel function

Dis

tan

ce t

erm

(n

m-6

)

Center-to-center distance (nm)

δδδδ-function

distance (nm)

0 5 10 15

1.00

1.25

Corr. rati

o

Center-to-center D

en

sity

of

dip

ole

mo

men

t

r/R

0.0

0.2

0.4

0.01 0.1 1 10

0.0

0.2

0.4

0.0

0.2

0.4

Rati

o o

f tr

an

sfer

red

e ex

cito

ns

Delay (ns)

experiment

theory

0 1 2 3 4 50.00

0.03

FRET time (ns)

0.00

0.03

Am

pli

tud

e

0.00

0.01

0.02

A-B

A-C

B-C

BBBB

Photoinduced dynamics in core–shell QDs

37

Chapter Chapter Chapter Chapter 4444: : : : Photoinduced dynamics in Photoinduced dynamics in Photoinduced dynamics in Photoinduced dynamics in corecorecorecore––––shell QDsshell QDsshell QDsshell QDs

4.1 Motivation4.1 Motivation4.1 Motivation4.1 Motivation::::

Understanding the fundamental science and the synthetic methodologies of the nano-

heterostructures provides a wealth of new materials, devices, and applications103

Nano-heterostructure crystals (such as, semiconductor-core–semiconductor-shell

QDs) feature highly diverse physical and chemical properties, as we will discuss later.103

Great flexibility in tuning the optical and electronic properties can be achieved by

integrating the nanomaterials into the nano-heterostructures (for example, core–shell

quantum dots, CSQDs). Such structures often lead to significantly improved device

performance. Since 1990, CSQDs have been actively investigated in order to overcome

the surface photodegradation of core-only nanocrystals.103 Coating a semiconductor

QDs (CdSe QDs) by another semiconductor (such as ZnS) can enhance the chemical,

thermal, and photochemical stability compared to the neat QDs. In the CSQDs, the

semiconductor shell is grown on the semiconductor core. The shell improves the

surface passivation of the core by separating the surface dangling bonds from the core

material with an energy barrier in-between. Optical and electronic properties can be

tuned easily by engineering the band alignment between core and shell materials and

consequently it opens the door for these materials to be useful to a wide variety of

fields.104-105

4.2 4.2 4.2 4.2 Classification of coreClassification of coreClassification of coreClassification of core––––shell QDs:shell QDs:shell QDs:shell QDs:

Depending on the VB and CB of the core and shell semiconductors, CSQDs can be

classified into three main categories. Type I CSQDs, reverse type I CSQDs, and type

II CSQDs.

In the type I CSQDs, a shell with a wider band gap than the core is grown around

the core, aiming to improve the properties of the active core (see figure 4.1). For

instance, the shell can passivate the surface traps (usually induced by dangling bonds)

on the core’s surface with energy barriers. This leads to the enhancement of the PL QY


38

by reducing the non-radiative recombination from the traps (e.g., CdSe–ZnS CSQDs).

CSQDs usually show red-shifted excitonic peaks compared with the core materials due

to the partial leakage of the exciton wavefunctions from the core into the shell.106

Figure 4.1: Different types of CSQDs compare to neat core depending on the band

gaps energy of both core and shell materials.

In reverse type I CSQDs, a wider band gap core is coated with a narrower band gap

shell. Thus, the carriers are partially or completely induced and confined in the shell

which improves the charge extraction and injection (e.g., CdS–CdSe and ZnSe–CdSe).

The excitonic peak can be red-shifted by increasing the shell thickness. In order to

improve the photostability and the PL QY properties, growing a second shell with

wider band gap than the first one (core–shell–shell structure, CSS) is needed.106

In the type II CSQDs, either the CBM or the VBM of the core is located within the

band gap of the shell. Consequently, the resulted effective band gap is always smaller

than each one of the constituting core and shell materials (see figure 4.1). This

alignment leads to spatial separation of the electrons and holes into the core or the shell

depending on their band alignment. Manipulating the shell thickness of this type leads

to tuning the emission color which is of the main interest in such system (e.g., CdTe–

CdSe and CdSe–ZnTe).106 The PL lifetime for this type of CSQDs is much longer

compared to type I CSQDs due to the lower wavefunctions overlap of the electrons

and the holes. Again growing a second shell with wider band gap (CSS) is needed here

to improve the photostability and the PL QY. Quasi-type II CSQDs denote a structure,


39

where one of the carriers is delocalized over the CSQD volume and the other carrier is

mainly confined within the core only.103

Among all above-mentioned types of CSQDs, the transition from the core to the

shell materials is sharp, resulting in step-like band alignment.107 In such case,

continuously growth of shell in type I CSQDs would greatly reduce the PL QY.107 This

is due to the lattice mismatch between the core and the shell layers, the nonuniform

growth of the shell, and the formation of grain boundaries between small clusters in

the shell layer.20, 64 The lattice mismatch refers to the situation where two materials

featuring different lattice constants are brought together with interface between them.

All these effects will induce defects acting as carrier traps and hence reducing the PL

QY.

On the contrary to step-like CSQDs, one can also form so-called the gradient type I the gradient type I the gradient type I the gradient type I

CSQDsCSQDsCSQDsCSQDs (see figure 4.1), where the composition changes smoothly in the radial

direction and resulting in a gradual band change from the core to the shell. In this case,

the above-mentioned shell induced defects can be minimized, and the PL QY can be

enhanced. In particular, in gradient CSQDs the soft confinement of electrons and holes

can reduce the unwanted AugR rate compared with step-like CSQDs which have sharp

steps of potential change.29, 83, 108 In the following section we will explain the general

aspects of designing the gradient type I CSQDs (Cd1-xZnxSe1-ySy) which are used in this

thesis in order to improve the core properties for solar cell applications.

4444....3333 Important aspects for preparing the gradient type I Important aspects for preparing the gradient type I Important aspects for preparing the gradient type I Important aspects for preparing the gradient type I CSCSCSCSQDs:QDs:QDs:QDs:

In the following section we will discuss the preparation of gradient Cd1-xZnxSe1-ySy

CSQDs of type I in details. In general for step-like CSQDs, cores with a known

diameter were used as a seed to grow the shell around in a two-steps synthesis procedure

(core synthesis followed by purification process and then growing the shell over the

core). This two-steps synthesis route might create interfacial defects between core and

shell due to the lattice mismatch. However, in order to prepare the gradient type I

CSQDs, we employed the single-step synthesis procedure where the core and shell

materials are introduced at the same time during the growth. The gradient structure is

greatly determined by the following important factors during the synthesis: the

composition of the core and shell materials, the shell precursor’s materials, the chemical


40

reactivity of the different precursors, the reaction temperature, and the shell thickness

control.

We will start by choosing the core and shell materials, where the lattice mismatch

between them plays an important role. For CSQDs, the smaller the lattice mismatch

the lower the density of core–shell interface defect states would occur. For the type I

step–like CSQDs (sharp core–shell interface), the PL QY is increasing with the shell

thickness up to ~ 0.7 nm (~ 2.4 monolayers) and then is decreasing due to the formed

defects due to the lattice mismatch between the core (CdSe) and the shell (ZnS).107, 109-

110 On the other hand, the PL QY of the gradient CSQDs increases gradually with the

shell thickness up to ~ 1.6 nm due to the successive relaxation of the core–shell interface

strain via chemical composition gradient.20 For the thicker shell (> 1.6 nm), the QY

does not increase any more due to lattice imperfections within the shell.109-110

The selection of the shell precursor’s materials is another important issue to be taken

into account to avoid side reactions and unwanted products. In addition, some

precursors require special precautions especially when they are highly toxic and/or

pyrophoric chemicals. For instance ZnS, is one of the most used shell materials for

numerous II–VI and III–V semiconductor NCs.106 The ZnS shell can be grown based

on using diethylzinc and hexamethyldisilathiane as Zn and S precursors, respectively.

However, these two chemicals are pyrophoric and toxic, respectively. Therefore,

alternative precursors were employed in our gradient CSQDs - namely zinc

carboxylates and elemental sulfur which are more suitable for large scale CSQDs

production.106

In the single-step synthesis of the gradient Cd1-xZnxSe1-ySy QDs, both Cd2+ and Zn2+

add together with oleic acid in presence of non-coordinating solvent (namely, 1-

octadecene). Then after the formation of Cd-oleate and Zn-oleate, a mixture of the

anionic precursors (Se2- and S2-) in TOP is quickly injected into the cationic mixture.

Thus, the chemical reactivity difference between the cationic (Cd2+ and Zn2+) and the

anionic (Se2- and S2-), indeed, is very important. In our CSQDs, even if the initial

concentration of the Zn2+ cation was 10 times higher than Cd2+ (in precursor solution),

after 5 sec of the reaction the amount of reacted Cd2+ was 20 times higher than Zn2+

(determined using EDX, see the following sections). This shows that the chemical

reactivity of Cd2+ is much higher than Zn2+. In the same manner, the chemical reactivity

of Se2- is much higher than S2-. Due to both the initial concentration difference and the

chemical reactivity difference between Cd2+(Se2-) and Zn2+(S2-), the gradient shell is


41

formed after 5 sec when the concentrations of Cd2+ and Se2- decreased drastically while

the concentrations of Zn2+ and S are still high to form the gradient shell.

Choosing the proper injection and growth temperatures is very important for

successful CSQDs synthesis. In order to avoid the shell material nucleation, both

injection and growth temperatures were chosen to be below the ZnS nucleation

temperature. Finally, controlling the shell thickness is very important and it was the

keystone of our study. We controlled the shell thickness by cooling the reaction mixture

suddenly after the desired growth time by using an ice bath.

4444....4444 Characterization of type I Characterization of type I Characterization of type I Characterization of type I gradient gradient gradient gradient CdCdCdCd1111----xxxxZnZnZnZnxxxxSeSeSeSe1111----yyyySSSSyyyy CSQDsCSQDsCSQDsCSQDs::::

Most of the basic techniques used to characterize the neat core QDs can be used also

for the CSQDs characterization. In the following sections we will present the different

techniques used to characterize the gradient CSQDs. Starting with steady-state optical

spectroscopy techniques and high resolution transmission electron microscopy (HR-

TEM) in order to determine the core size and to estimate the shell thickness for

different CSQDs. Then we conducted X-ray based spectroscopies in order to

characterize the crystalline and composition such as, X-ray diffraction (XRD) and

energy-dispersive X-ray spectroscopy (EDX).

4.4.1 Core size determination and shell thickness estimation for

gradient CSQDs:

The dimensions of our CSQDs (core sizes and shell thickness) were obtained via HR-

TEM. The results were verified by the steady-state optical spectroscopies using the

known size dependences of the absorption spectra (see figure 4.2 A). The HR-TEM

images are show in figure 4.2 B. By careful analysis of the CSQDs HR-TEM images

and subtracting the core size (3.0 nm) we determined the shell thickness for different

growth time to be 0.6, 1.3, 1.6, 1.9, and 2.3 nm for 1, 3, 5, 10, and 15 min. Figure

4.2 A, presents the steady-state UV-Vis absorption and PL for core (sample (i)) and

CSQDs with different thicknesses (ii, iii, and iv for 0.6, 1.3, and 2.3 nm shell

thicknesses, respectively). The excitonic peak is well resolved for the core which can be

used to estimate the QD size according to the previous reports.111 It revealed the size of

3.0 nm for our core QDs consistent with the TEM observations. Such excitonic peaks


42

can also be shown in the CSQDs with different shell thicknesses up to ~ 1.3 nm (see

figure 4.2 A, red lines). For QDs with the thickest shell (> 1.3 nm), there is no

pronounced excitonic peak (see figure 4.1 A) due to the carrier confinement reduction

by extension of the electron and hole wavefunctions into the shell material.112 For all

the samples, the PL spectrum consists of a single band (FWHM < 33 nm) indicating

relatively narrow QD size distribution (see figure 4.2 A, black lines).

Figure 4.2: A) Steady-state UV-Vis (red lines) and PL (black lines) of the core (i) and

CSQDs of different shell thicknesses (ii, iii, and iv for 0.6, 1.3, and 2.3 nm shell

thicknesses, respectively), B) HR-TEM for the same samples as A).

4.4.2 Structural characterization of gradient CSQDs”:

In order to better understand the optical properties of our gradient CSQDs, one needs

to get insight into the exact structures of the QDs. Particularly, we have studied the

chemical composition and crystalline structures for our QDs with different shell

thickness. We employed EDX and synchrotron-based XRD to estimate the

composition and the crystal structure. Figure 4.3 A shows the XRD patterns for the

neat core and the gradient Cd1-xZnxSe1-ySy CSQDs of different shell thicknesses. We

call the pure CdSe QDs as neat cores, and we use them as references. The neat core is

compared to CSQDs with six different shell thicknesses, ∼“0”, 0.6 1.3, 1.6, 1.9, and

2.3 nm obtained by growth times of 5 s, 1, 3, 5, 10 and 15 min, respectively. For the

CSQDs of 5 s growth time, it has intermediate properties between the neat core and

the CSQDs with a 0.6 nm shell thickness, therefore we call it “0” nm CSQDs. We also

show the XRD spectra of the CdSe and ZnS zinc blende structures. The XRD patterns

of our CSQDs were analyzed in terms of these two structures where the core is more


43

like CdSe phase and the thickest shell is approaching the ZnS phase. The fitting results

provide the lattice parameters for both phases, where one lattice constant is close to the

core (CdSe) and the other is close to the shell (ZnS) (see figure 4.3 B). A clear decrease

in the core unit cell constant as the shell grows up to 1.3 nm is observed. This can be

attributed to the compressive strain of the shell on the core. For CSQDs with thicker

shell thickness than 1.3 nm, the strain in the core is released. The reason behind this

phenomena is not clear, but it may relate to the defect sites which are induced by the

growth of the shell.

Figure 4.3: A) XRD patterns of neat core and CSQDs of different shell thicknesses

compared to zinc blende CdSe113 and ZnS114, B) lattice constants of CdSe core

(black) and ZnS shell (red) as a function of the shell thickness, the gray and red areas

present the lattice constants of bulk CdSe and ZnS, respectively, and C) EDX

average element ratio for different growth time for Cd/(Cd+Zn), dark yellow line

and Se/(Se+S), orange line.

EDX spectra were used to evaluate the chemical composition for the different

CSQDs (see figure 4.3 C). EDX revealed that due to the difference in chemical

reactivity between Cd (Se) precursor and Zn (S) precursor, at the early stage of the

reaction (5 sec) Cd-rich (Cd1-xZnxSe1-ySy) core (small band gap) is formed first, followed

0.6 nm

2.3 nm

1.9 nm

1.6 nm

1.3 nm

"0" nm

Inte

nsi

ty

Bare Core

10 20 30 40 50

2θθθθ

422

331

400

31

1220

200

111

Bulk (Zinc-Blende) CdSe

Bulk (Zinc-blende) ZnS

331

400

3112

20

200

111

Core "0" 0.6 1.3 1.6 1.9 2.3 5.2

5.4

5.6

5.8

6.0

6.2

6.4

Core

Shell

Un

it c

ell

pa

ram

ete

r (

An

gst

rom

)

Shell thickness (nm)

BBBB

0 150 300 450 600 750 9000.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Average e

lem

ent

rati

o

Growth time (sec)

"0" nm

1.3 nm shell thickness




AAAA

CCCC


44

by an alloyed shell over the core. Later the Zn-rich (ZnS) shell (wide band gap) will

externally coat the core.20


45

4444....5555 Paper VIIPaper VIIPaper VIIPaper VII “Electron “Electron “Electron “Electron injection from injection from injection from injection from gradient gradient gradient gradient

CdCdCdCd1111----xxxxZnZnZnZnxxxxSeSeSeSe1111----yyyySSSSyyyy CSQDs to MO”CSQDs to MO”CSQDs to MO”CSQDs to MO”::::

The overall device efficiency of a QD-sensitized solar cells is determined by many

factors such as light harvesting, charge separation, and charge transport.110 If one

utilizes CSQDs in solar cells, the enhanced photostability and the reduction of charge

recombination by surface trapping is expected, but the existence of shell may also

hinder the charge transfer process from QDs compared to neat QDs. Thus those effects

should be overviewed comprehensively during the evaluation of CSQDs in solar cell

applications. In this paper we studied the dependence of both the electron injection

and the trap passivation on the shell thickness in gradient Cd1-xZnxSe1-ySy CSQDs/ZnO

NPs system. We managed to estimate an optimal condition for the best overall electron

collection efficiency.

Figure 4.4: A) TA kinetics of gradient CSQDs of different shell thicknesses (ii, iii,

and iv for 0.6, 1.3, and 2.3 nm shell thicknesses, respectively) compared to core QDs

(i) (red circles are from experiment and red lines are fitted data for QDs in solution;

black circles and black lines represent the experimental and fitted data for QDs

attached to ZnO NPs), B) mean decay rate dependence on the shell thickness for

QDs in solution (red), QDs attached to ZnO NPs (black), and reported values for

step-like CSQDs of the same materials (blue)110, C) electron injection efficiency and

PL QY as a function of shell thickness, and D) energetic scheme for electron

tunneling through the shell followed by electron injection to ZnO NPs for step-like

CSQDs (upper) and gradient CSQDs (lower).


46

TA decay was used to study the electron dynamics in QDs, as we discussed

previously. The TA kinetics of unattached CSQDs were fitted using a biexponential

function resulting in a minor subnanosecond and a dominant nanosecond components

(A1 = 20–45%, τ1 = 0.1–0.3 ns; A2 = 55–80%, τ2 = 12–18 ns) (see figure 4.4 A, red

lines). The observed multiexponential decay has the same character as previously

reported decays of single exciton in CdSe QDs affected by fluctuating rates of

nonradiative processes (QDs charging, change in surroundings, etc.).115-116 The decay

of neat CSQDs was then compared to the CSQD-MO case. Clearly, the attachment

of the QDs to the MO affects the TA decay kinetics due to the electron injection from

QDs to MO (see figure 4.4 A, black lines). The experimental data could not be

reproduced by a simple multi-exponential function and therefore we employed a

stretched-exponential function:

∆T�I� = ∆T�0� × exp Z−?�I '[B⁄ �A!\, (12)

The function has been previously used to describe the dynamics in disordered

nanomaterials and electron injection in QD-MO systems.117-118 Here, '[B is the decay

time and B is the dispersion factor (0 < B < 1) which reflects the disorder degree for the

studied system. It should be noted that we use B instead of commonly used β to avoid

confusion with the later used β factor. The thicker the shell, the more the disorder in

the system for gradient CSQDs can occur due to different composition and thickness

might vary from QD to another. By using the fitting data, we calculated the mean

lifetime of the stretched-exponential function:

⟨'⟩9[DE]^_,( = �'[B `⁄ � ∗ Γ�1 `⁄ �, (13)

where c stands for the Gamma function.

The calculated ⟨'⟩9[DE]^_,( shows a clear dependence on the shell thickness

ranging from 6 ps for core to 15 ns for 2.3 shell thickness. Then the electron injection

rate was calculated for each shell thickness by comparing ⟨'⟩9[DE]^_,( and ⟨'⟩9[DE]

as follows:

JBC�d� = ?1 ⟨'⟩9[DE]^_,(⁄ A − ?1 ⟨'⟩9[DE]⁄ A, 14

Due to the uncertainty in stretched-exponential fit, the TA mean decay time of the

thickest shell layers becomes slower after the attachment. Therefore, we consider the

injection rate for 1.9 and 2.3 nm shell thickness as ~ 0. In Figure 4.4 B we compare


47

the mean decay rates as a function of the shell thickness for reported step-like CdSe–

ZnS CSQDs110 (blue line) and for the gradient Cd1-xZnxSe1-ySy CSQDs (black). Both

CSQD structures follow the expected exponential dependence eq. 15. This dependence

is a result of the distance that electron has to tunnel through the shell in order to be

injected to ZnO NPs.

JBC�d� ∝ ?f^ghA, (15)

The typical i values for step-like CSQDs are reported to be 0.33-0.35 Å-1 depending

on the shell material.110 However, for the gradient CSQDs a larger i value (0.51±0.05

Å-1) was obtained. In the step-like CSQDs as the shell thickness increases, only the

energy barrier width increases (see figure 4.4 D, upper panel). In case of the gradient

CSQDs both the barriers’ width and height increase as the shell thickness increase (see

figure 4.4 D, lower panel) which explains the stronger shell thickness dependence of

the injection rates.

For the solar cell applications, the electron injection efficiency is the important

parameter. We can calculate the efficiency based on the experimental data as follows:

jk,l = JBC �JBC + J+�⁄ , (16)

Even if the electron injection rate drops quickly with the shell thickness, the electron

injection efficiency stays almost unaffected for the shell thickness up to 1.2 nm due to

the long recombination lifetimes in QDs (see figure 4.4 C, black line). Moreover we

have studied surface passivation of QDs by measuring the PL QY for each shell

thickness (unattached QDs). For 1.2 nm shell thickness the PL QY is about two times

higher than the core QDs (see figure 4.4 C, blue line). By combining the information

about electron injection and surface passivation we obtain the optimal shell thickness

~ 1.2 nm, which acts as a shield around the core against surface degradation and surface

trap states, at the same time keeping high electron injection efficiency > 80%.


48

4444....6666 Paper Paper Paper Paper VIIIVIIIVIIIVIII “hole trapping “hole trapping “hole trapping “hole trapping vsvsvsvs.... quantum dot quantum dot quantum dot quantum dot

sensitized solar cell performance”sensitized solar cell performance”sensitized solar cell performance”sensitized solar cell performance”::::

In general, solar cell efficiency depends on the behavior of both photoexcited electrons

and holes.119 In this paper, we confirm the role of the shell thickness in the real CSQDs

solar cell device within certain shell thickness range. We prove that this happens by

decreasing the surface defect states which acts as hole traps (see paper IV). Moreover,

we combined our knowledge about the electron (paper VII) and hole behavior to

explain the dependence of solar cell efficiency on QD shell thickness.121

Figure 4.5: A) PL decay of MPA-CSQDs with different shell thickness compared to

MPA-CdSe QDs up to 1.5 ns, B) PL decay of the early time (up to 75 ps) for the

neat core and different CSQDs, C) I-V characteristics for the core CdSe QDs

compared to the CSQDs with different shell thickness (all are compared to the dark

current, gray area), D) electron collection efficiency (e collection eff., blue line),

electron injection efficiency (e Inj. eff., black line, according to ref.120), hole

scavenging efficiency (h scavenging eff., red line), and combined efficiency (pink

line) as a function of shell thickness, and E) scheme for electron and hole tunneling

through the shell followed injection to MO and S2-/SX2- electrolyte, respectively.

Linker exchange is necessary to attach the QDs to the MO. However, the exchange

introduces numerous surface defects acting as trapping centers resulting in drastic drop

in PL QY for neat core after the linker exchange.57, 122 We have studied the effect of

linker exchange on the CSQDs, where the effect of traps is diminished by the shell


49

layer. Figures 4.5 A and B represent the PL decay kinetics for CSQDs before

attachment to MO. By using the multiexponential function to fit the decay of the

studied QDs, we revealed two fast decay components (9–18 ps and ~ 120 ps) for the

neat core, “0” nm and 0.6 nm shell thickness samples. These fast components are

related to defect states of different origins.56-57 Independent of the origin of these traps,

the fast components were gradually diminished with increasing the shell thickness and

they have been completely vanished by the 1.3 nm thick shell, when the QD’s surface

is almost completely passivated.

In order to study the passivation effect on real device performance, we fabricated

several sets of solar cell devices. Figure 4.5 C shows the current–voltage (I–V)

characteristics of the gradient CSQDs-based devices compared to neat core QDs-based

device. The measurements were done under a simulated AM1.0G solar irradiation

(light intensity of 100 mW/cm2). The solar cell efficiency increases with QD shell

thickness reaching the maximum value at 1.3 nm shell thickness and then starts to

decrease. Furthermore, we have evaluated the cells’ performance by calculating the

electron collection efficiency (ECE). ECE is the ratio between the extracted electrons

out of the device and the number of photons absorbed by the device:

jB9 = 8mn�-,]/n=)]-�)h o�-n-,]/n , (17)

The electron collection efficiency increases with the shell thickness reaching 6% for

1.3 nm shell thickness. Then for thicker shell the electron collection efficiency decreases

again. We can compare the observed values with electron and hole injection efficiencies.

For the thin shell thickness the electron injection efficiency (see paper VII) is very high

and drops exponentially with increasing the shell thickness as the core/MO distance

increase. Consequently, the electron collection efficiency drops with increasing the shell

thickness. Thus the low value of jB9 for the thin shell is related to the hole trapping

process.

In order to quantify the hole collection efficiency we have assumed that the radiative

dipole of the excited electron–hole pair where the hole is trapped is negligible. We

managed to quantify also the hole collection efficiency (untrapped holes). These

untrapped holes can be efficiently scavenged by the electrolyte, thereby j� is the hole

collection efficiency. We quantify j� by evaluating the proportion of the untrapped

holes as a ratio of the PL intensity at the 1.5 ns delay and PL amplitude in the PL

dynamics in Figure 4.5 D as follows:


50

j� = pqrs.t uv^pwx�y�xzpqr"^pwx�y�xz

, (18)

where, It=1.5 ns is the PL intensity at 1.5 ns, It=0 is the PL amplitude, and Iprezero is the

intensity before time zero which is due to long PL lifetime (~ tens of ns) compare to

the time window. The time 1.5 ns was chosen as a delay long enough to allow complete

hole trapping yet fast enough that exciton recombination does not play a prominent

role. Total charge collection efficiency is the product of electron and hole injection

efficiencies. It agrees very well with the charge collection efficiency obtained from real

solar cell performance measurements (see figure 4.5 D pink line).

Figure 4.5 D summarizes the photodynamics of the gradient CSQDs in real solar

cell device. Here, the neat CdSe QDs without any shell protection feature high electron

injection efficiency (~100%), while most of the holes (> 80%) cannot be collected due

to the hole trapping process. Thus, the trapped holes are not able to be injected to the

electrolyte probably due to insufficient driving force.57 Passivation of the hole traps

using core–shell structures provides an efficient way to enhance the hole collection.

However, shell is also an energy barrier preventing the electron/hole transfer from the

core. Fortunately, Cd1-xZnxSe1-ySy CSQDs with shell thickness up to 1.3 nm can offer

electron injection efficiency up to 80% and at the same time an efficient passivation

for the hole traps. Also, based on the charge carrier photodynamics we calculated the

charge carrier (electron and hole) injection efficiency and used these efficiencies to

predict the charge collection efficiency. The result is in good agreement with the

obtained values (see figure 4.5 D pink line). Figure 4.5 E illustrates the 1.3 nm shell

thickness device with efficient electron and hole injection.

Conclusions and future work

51

Chapter 5:Chapter 5:Chapter 5:Chapter 5: ConclusionsConclusionsConclusionsConclusions and future and future and future and future workworkworkwork

5.1 Conclusions5.1 Conclusions5.1 Conclusions5.1 Conclusions::::

We believe that an important way to improve the application of QDs in solar cell fields,

is to understand the photoinduced dynamics in such systems. The thesis presents the

fundamental science of the ultrafast photoinduced dynamics in CdSe QDs and

gradient Cd1-xZnxSe1-ySy CSQDs. We covered, almost all, the possible carrier’s

dynamics (electron injection, hole trapping/injection, and exciton migration) in these

QDs between the folds of this thesis.

We observed directly, by using the combination of time-resolved absorption and

THz spectroscopy, the ultrafast electron injection from neat CdSe QDs to the n-type

MO namely, ZnO NWs. We proved that the electron injection-driving force

dependence can be described by the well-known Marcus theory for charge transfer.

Such ultrafast electron injection process encouraged us to study the multiple electron

injection under high excitation intensity condition. Multiple electron transfer is a

prerequisite for successful utilization of multiple exciton generation for photovoltaics.

We observed a competition between the electron injection process and AugR

(multiexcitons quenching) process under these conditions. Indeed, we found that the

second electron injection is possible, however the multiple exciton harvesting can be

efficient only within a narrow range of QD sizes for the CdSe-MPA/ZnO NWs system.

Defect and/or surface states play an important role in the charge collection in QDs

solar cells. We determined the lifetime of the deep trapped holes for OA capped-CdSe

QDs to be around few tens of microsecond at room temperature. Moreover, we

revealed that at least two different types of surface states (traps) contribute to the surface

PL. Excitons with energy higher than the band gap have higher chance to get trapped

by surface states than the band gap excitons. When utilizing QDs in p-type solar cells,

we managed to distinguish between the hole trapping and hole injection processes in

QDs/NiO mesoporous film system. We found that the hole trapping process is more

dominant especially after linker exchange process. Passivating of the hole trap states

using core–shell structure is found to enhance the hole injection efficiency from < 10%


52

to 50%. Similar to electron injection process, hole injection from CdSe QDs to NiO

followed the Marcus theory for charge transfer.

Besides the charge transfer, exciton migration may also occur in QD solar cell system.

Understanding the Förster resonance energy transfer (FRET) in QDs system is

therefore of fundamental and technological significance. In conventional QDs-

sensitized solar cell structures, we found the energy transfer between QDs in QD-

MPA/ZnO NWs system is ~ 5 ns. We also studied the FRET in densely packed multi-

sized CdSe QDs films. We revealed that point–dipole approximation for FRET is

insufficient in this case. We suggested to use a model taking into account the

distribution of the electronic transition densities in the QDs and using the real film

morphology (center-to-center distance) revealed by AFM images. By taking these two

aspects into account we were able to fully describe the FRET kinetics in our system.

Conventional neat core QDs usually suffer from surface defects as charge traps in

solar cell application. This can be well solved by using core-shell structure to passivate

the surface traps and reduce charge recombination. Here we investigate the

photodynamic of gradient Cd1-xZnxSe1-ySy core–shell QDs in solar cells. Growing the

shell in the gradient way reduces the interfacial defects and also the unwanted AugR

compared with step-like counterparts. In this context, we were interested to study the

electron photoinduced injection from the gradient Cd1-xZnxSe1-ySy CSQDs to n-type

ZnO NPs. We observed the typical exponential injection rate dependence on the shell

thickness (β = 0.51 Å-1). Compared to step-like CSQDs (β = 0.35 Å-1), gradient CSQDs

have stronger electron injection rate dependence on the shell thickness. We attributed

this enhanced dependence to the gradient feature where both the barriers’ width and

height change as the shell thickness increase. Although the shell materials always serve

as barriers against the charge injection, we found that ~ 1.2 nm shell thickness still

offers ~ 80% electron injection efficiency from the CSQDs to ZnO NPs and at the

same time keeps high PL QY (~ 70%) which reflects high surface passivation.

Combining our findings regarding the hole trapping process and the performance of

CSQDs-based solar cells, both in respect to the shell thickness. We found that the hole

traps are almost diminished for ~1.3 nm shell thickness. As a results, solar cell based on

the CSQDs exhibits efficiency 2.5 times higher than neat core CdSe QDs-based device.

Finally, we managed to predict the solar cell device performance based on both electron

and hole injection dynamics. Such way to predict the device performance provides


53

opportunity to optimize the core–shell QD materials for photovoltaics prior to making

real device.

5.2 5.2 5.2 5.2 Future workFuture workFuture workFuture work::::

In the last few years, the limiting factors in QDSCs have been more clear thanks to the

understanding of charge transfer processes. In the following paragraph we will overview

some of these factors in order to draw the possibility of future work for the QDs solar

cell applications.

First of all, based on our findings in this thesis, we revealed that establishing core–

shell structure is indispensable to diminish the hole trapping sites which are the main

restriction to the hole transfer efficiency. Therefore, study the hole dynamics, especially

the injection through the gradient shell will add more important information to

complete the full picture about charge transfer in the CSQDs system.

Secondly, better understandings for the QDs crystal structure and the way it

correlates to the electronic and the optical properties of CSQDs materials are needed.

This open the door to modify and optimize these materials not only in solar cell

applications but also in other applications such as light-emitting-diodes.

Thirdly, as for board interests in the community, engineering the CSQDs structures

to optimize the light harvesting and charge separation is needed.

Last but not least, another important building block in QDSCs: the electrolyte also

need to be noticed. Better understanding of the charge transfer and recombination

dynamics in both liquid and solid electron/hole transfer materials would be meaningful

to boost the device efficiency.

Svensk sammanfattning

54

Svensk sammanfattningSvensk sammanfattningSvensk sammanfattningSvensk sammanfattning

Balansen mellan våra energibehov och den energi som konsumeras är ej längre i

jämvikt. Därför är jakten på alternativa energikällor nödvändig. Hållbara energikällor

erbjuder ett bra alternativ till fossilbränslen. Bland många typer av hållbara energikällor

erbjuder solenergi möjligheten för mer energi än dagens konsumtion. Solceller är de

smarta verktyg som omvandlar ljusets fotoner till användbar elektricitet. Utvecklingen

av solceller har redan genomgått några generationer där två viktiga faktorer styr

marknaden. Solcellernas verkningsgrad samt produktionskostnaderna för att tillverka

dem är hörnstenar inom solcellsmarknaden. Tredje generationens solceller försöker

optimera verkningsgrad mot kostnad via billiga material som fortfarande är tillräckligt

’smarta’ för att skapa laddningsbärare när ljus absorberas och därefter genererar

elektricitet. Samtidigt som elektricitet genereras så är det också ett mål att övervinna

termodynamikens effektivitetsgräns. Nanomaterial har framkommit som byggstenar

för att samla solenergi i tredje generationens solceller. Bland dessa så har kvantprickar

visat sig vara starka kandidater eftersom de har goda egenskaper som hög

extinktionskoefficient, justerbar absorption samt möjligheten att generera och samla

flera excitoner via en enda högenergetisk foton.

Beteendet hos både fotoexciterade elektroner och dito hål bestämmer den generella

verkningsgraden hos en solcell baserad på kvantprickar. Denna avhandling presenterar

en systematisk studie av den ultrasnabba foto-aktiverade elektrondynamiken inom

kvantprickmaterial för solceller. Dessa processer inkluderar laddningsöverföring,

excitonmigration, fällnivåers laddningsbärarinfångning och deras verkan på solcellers

prestanda. De studerade materialen börjar med konventionella enkla CdSe-

kärnkvantprickar och fortsätter med Cd1−xSe1−yZnxSy-gradient-kärn–skal-kvantprickar

(CS). Det sistnämnda används för att förbättra optik- och apparatur-prestandan.

Elektroninjektionen från CdSe till ZnO-nanotrådar observerades först att hända väldigt

snabbt (några ps). Denna snabba elektroninjektionen uppmuntrar oss att studera

möjligheten att injicera multipla elektroner från en kvantprick under höga

excitationsförhållanden. Vi visade att det uppstår en konkurrens mellan

elektoninjektion och Augerrekombination.

Jämfört med elektroner så har fotoinducerade hål större sannolikhet att fångas. Men

sådana typer av fälltillstånd kan ibland vara radiativa med långa livstider, upp till några


55

10-tals mikrosekunder, när de befinner sig i oleinsyratäckta CdSe-kvantprickar. I detta

scenario är hålinjektion i p-typ-kvantpricksolceller mindre effektivt (<10 %) jämfört

med elektroninjektion i motsvarande n-typ-kvantprickar. Det påverkas ytterst av fällor

på ytan som skapats av länkmolekylutbytesprocessen. Hålinjektionen kan då förbättras

genom att passivera fällorna på ytan genom att använda kärn–skalstrukturer. Utöver

elektron- eller hål-injektion kan excitonmigration också ske via FRET (Förster

resonance energy transfer, Förster-resonansenergiöverföring). Vi upptäckte att FRET

mellan kvantprickar kunde möjliggöra användning av ljusabsorption via de indirekt

kopplade kvantprickarna i QD-sensitiserade metalloxidanoder (MO). Inom

välstrukturerade blandningar av kvantprickar med olika storlekar är energiöverföring

även tydligare. Vi kunde experimentellt observera FRET-processen i slumpmässigt

arrangerade kvantprickar av olika storlekar samt i tandemstaplade QD-lager via

tidsupplösta metoder och steady state-spektroskopi. Teoretiska simuleringar där en

dipolfördelningsmodell introducerades för kopplingsberäkningar överensstämmer väl

med experimentella resultat. För att minimera effekten från defekter på ytan och

förbättra fotostabiliteten hos solceller gjorda av kvantprickar så studerade vi kärn–

skalkvantpricksystem där laddningsbärarinfångning i ytfällor kan vara väl passiverat av

skalmaterial som har förbättrade optiska egenskaper och apparaturprestanda. Här

används en halvledare med bredare bandgap som en sköld runt den aktiva kärnan i

gradienttillväxt, känt som Cd1−xSe1−yZnxSy-gradient-kärn–skal-kvantprickar. Sådana

kvantprickar erbjuder högre fotostabilitet, bättre kvantutbyte för fluorescens samt

mindre ytdefekter där materialen möts jämfört med de konventionella stegvisa kärn–

skalkvantprickarna. Först karaktäriserade vi våra CS-kvantprickar via steady state-

spektroskopi tillsammans med HR-TEM-bilder för att fastställa dimensionerna av våra

kvantprickar och utvärdera tjockleken på det skyddande skalet. Sen användes XRD och

EDX för att karaktärisera den kemiska sammansättningen samt kristallstrukturen.

Fotodynamiken hos dessa CS-kvantprickar i fotovoltaiska system studerades också.

Vi trodde först att elektroninjektionen från den aktiva kärnan till MO av n-typ visade

relativt stort exponentiellt beroende av tjockleken på skalet jämfört med stegvisa kärn–

skalkvantprickar. Vi fastställde att det högsta elektroninjektionutbytet (~80 %) kan

uppnås när skalet har en tjocklek upp till 1,3 nm. Ett sådant skal tillåter också hög

ytpassivering vilket ger optimala förhållanden för laddningsuppsamling i solceller. Till

slut integrerade vi vår kunskap om beteendet av elektroner och hål för att förklara

solcellsprestanda enligt kärn–skalstrukturer. Vi bekräftade att hålfällor är den kritiska


56

faktorn för verkningsgraden hos solceller gjorda av kvantprickar. Fällornas påverkan

kan gottgöras väl genom att använda optimala kärn–skalstrukturer.

الملخص العربي

57

الملخص العربىل الشغل ثـ مي خرى للطاقة ال غنى عنه و أ أصبح البحث عن مصادر قد و ةمستمر زيادة ين استهالك العالم للطاقة فإ

بر ت عي يلذا مصادر الطاقة المتجددة أحد أهم البدائل للوقود االحفوريل مثـ ت الشاغل للعديد من الدول والحكومات. الطاقة ل مثـ ت على الطاقة. من بين العديد من مصادر الطاقة المتجددة عتمادها إ يفحجر الزاوية للعديد من الدول

من الطاقة %1فقط نأالى ا العلماء حديث توصل. لقد ةومجانيتاحة من حيث أنها م وذلك األهم المصدر الشمسية وقود.و أكهرباء إلى تحولتما ذا إالستهالك العالم كله يتكفرض األسطح ىالقـادمة إلالشمسية

ل حصد فوتونات من خال يمكنها تحويل الطاقة الشمسية الى كهرباء يالخاليا الشمسية االداة الذكية التعتبر ت . وعلى ذلك ةيكهربائالطاقة ال ة بدورها تنتقـل الى االقطاب المختلقة مولد يالتالطاقة الضوئية وتوليد حامالت للشحنة

ل للخاليا الشمسية األو ل المعادلة بين السعر والكفـاءة. قـام الجيل بهدف ح كثر من جيلأت الخاليا الشمسية بفـلقد مر في أما ،ل هذا الجيل أعلى كفـاءة حتى االن ولكن على حساب التكلفةل للضوء وسج كون كمستقب يعلى إستخدام السيل

. سجلت خاليا الجيل رلالغير متبكون يالسيلمثل ستخدام مواد رخيصة الثمن امن الخاليا الشمسية فقد تم يالجيل الثان فياءة عالية نفس الوقت لها كفـ يلة رخيصة الثمن وفدى إلى البحث عن مواد مستقب أكفـاءة ضعيفة مما يالثان

وكان هذا هو هدف الجيل الثالث من الخاليا الشمسية. ،امتصاص الطاقة الضوئية

الموصالت أشباهبرزها أالجيل الثالث من الخاليا الشمسية من فيت م خد ست إ يهناك العديد من المواد التس تجعلها مناف ةز الكوانتم دوت بخواص مميز ). تتمي نانومتر 10-8بقطر يتراوح من النانومترية (الكوانتم دوت

ر يتغي فيسهل م الحك الت –للضوء جدا يعالمتصاص إالخصائص: معامل هذمجال الخاليا الشمسية. من أهم ه في يقو من حامل كثر أيد القدرة على تول – يالطيف ياصها للضوء على المنحنمنطقة امتص فيحجمها مما يترتب عليه التحكم

عد من اإللكترون و الفجوة ب فجوة) من فوتون واحد ذو طاقة عالية. يعد مسار كال -للشحنة (زوج من إلكترونر أحد ب عت الشحنات ي هدراسة مسار هذفـإن وعليه ،حديد كفـاءة الخلية الشمسيةت يفالفوتون العبا هاما جدا متصاص ا

المواد. ههذأهم الطرق لتحسين كفـاءة الخاليا الشمسية القـائمة على

ههذتحسين خواص رضغ الفوتون ل متصاص ا ب ق من اإللكترون والفجوة ع االطروحة تم دراسة حركية كال ههذ يفنتقـال إن أالمواد. لقد تم التوصل إلى ههذالمواد قدر المستطاع لتحسين كفـاءة الخاليا الشمسية القـائمة على

بضع من ي ف يتم ةالحال ههذ يف) ZnO Nanowiresاإللكترون من الكوانتم دوت إلى مستقبل اإللكترونات (ظل استخدام فوتونات يف ة فـائقة السرعة فتحت الباب لدراسة إمكانية انتقـال إلكترون ثان يالعمل ه. هذالبيكوثانية


58

ما يعرف بإتحاد و ياإللكترون الثاننتقـال االدراسة على أن هناك منافسة بين ههذذات طاقة عالية جدا. أسفرت ).Auger Recombinationأوجر (

كفـاءة الخلية يفمن اإللكترون والفجوة بعد تفعليهما مع الضوء يلعب دورا هاما كما ذكرنا آنفـا أن مسار كال وقيفها من خالل الفجوات إذا ما تم ت ههذعمر راسة مسار الفجوة بعد االثارة. مبدئيا تم دراسة الشمسية لذلك قمنا بد

ههذ يتلك العيوب كمصائد للشحنات ولقد وجدنا أن عمر الفجوات فتعمل عيوب على سطح الكوانتم دوت. ب بعد دراسة ديناميكية إصطياد الفجوات بمصائد السطح (عيو المصائد طويل جدا (العشرات من الميكروثانية).

). توصلنا إلى أن NiOالسطح) قمنا بدراسة دينامكية إنتقـال الفجوات من الكوانتم دوت إلى مستقبل الفجوات (بتقـليل د أثبتنا أنهنتقـالها إلى المستقبل. ولقإمصائد السطح وعملية يفعملية إصطياد الفجوات بين يهناك تنافس قو

المصائد على سطح الكوانتم دوت تزداد عملية انتقـال الفجوات من الكوانتم دوت إلى المستقبل من هعدد هذ. من الطرق المعروفة لتقـليل عدد المصائد على سطح الكوانتم دوت استخدام غطاء من شبة موصل %50إلى 10%

).Core–shell quantum dots) أوسع (band gapذو (

نفصال وهوإلالفجوة قبيل ا –وهو عبارة عن زوج من اإللكترون excitonاألطروحة بعد ذلك حركية التناقش ال الطاقة وجود مستقبل لإللكترونات حيث يتم إنتقـ يالدراسة ف هنتقـال الطاقة أو هجرة الطاقة. تمت هذاعرف بما ي

خيرا إلى تم فصل الشحنات وتنتقـل اإللكترونات أ م إلى جارتها كبيرة الحجم ومن ثم يمن الكوانتم دوت صغيرة الحج. )Size distributionإلى طريقة التحضير ( يعودجدير بالذكر أن اختالف حجم الكوانتم دوت من الالمستقبل.

ام مختلفة من الكوانتم د ألحجتعم مستقبل لإللكترونات وبعمل رص م غياب يفتم إستكمال الدراسة ولكن ثم بعد ذلك دوت.

ات لفترات هو عامل الثبتطبيقـات الخاليا الشمسية يفتحد من إستخدام الكوانتم دوت يمن أهم العوامل الت–Coreبناء غطاء من شبه موصل أخر ( المواد هو كما ذكرنا سابقـا هوأكثر الطرق كفـاءة لزيادة ثبات هذ ،طويلة

shell quantum dots.( بناء الغطاء بطريقتين عموما يتم: ) األولى يكون فيها التحول من القـلبCore إلى ( Interfacialالحد الفـاصل بين القـلب والغطاء ( يف) حاد مما يسفر عن تكوين عيوب داخلية Shellالغطاء (

defects( يدريجتبشكل تعمل كمصائد للشحنات. الثانية يكون فيها التحول من القـلب إلى الغطاء يوالت )Gradient core–shell quantum dots.ن الجزء اآلخر إوعليه فـ ) وهو ما يؤدى إلى تقـليل العيوب الداخلية

تم دراسة من األطروحة قـائم على دراسة حركية كال من اإللكترونات و الفجوات عقب امتصاص الفوتون. مبدئيا ZnO( إلى المستقبل) Gradient core–shell quantum dots(إنتقـال اإللكترون من الكوانتم دوت

Nanoparticles (هذا الغطاء تأثير سماكة ) ودراسةShell thickness ( على إنتقـال االلكترونات. تم التوصلر حماية عالية وف ت نفس الوقت يفو على كفـاءة إنتقـال اإللكترونات ي السماكة التي ال تؤثروه سماكة المثلى إلى ال

من عيوب السطح (مصائد الشحنات).


59

) إلى Gradient core–shell quantum dotsبدراسة إنتقـال الفجوات من الكوانتم دوت (قمنا ايضا فجوات ال النتقـال ا. تم التوصل إلى أن الغطاء على إنتقـال الفجوات سماكة) ودراسة تأثير NiOمستقبل الفجوات (

شمسية. تم التوصل اليا خالالالغطاء على كفـاءة سماكةالغطاء. أخيرا قمنا بدراسة تأثير سماكةيعتمد بصورة كبيرة على يفغطاء). يأعلى من القـلب بدون أ ةمر 2.5تم تسجيل أعلى كفـاءة (نانومتر 1.3حوالى سماكةإلى أنه عند

ات.من اإللكترونات والفجو من خالل دراسة حركية كال سماكةنفس السياق تم حساب الكفـاءة المتوقعة لكل

References

60

ReferencesReferencesReferencesReferences

1. BP statistical review of world energy 2013201320132013, www.bp.com/statisticalreview.

2. America's Climate Choices: Panel on Advancing the Science of Climate Change;

National Advancing the Science of Climate Change; The National Academies Press,

2010201020102010.

3. http://listverse.com/2009200920092009/05/01/top-10-renewable-energy-sources/.

4. Total Primary Energy Consumption. U.S. Energy Information Administration

2010201020102010.

5. Kamat, P. V., Quantum Dot Solar Cells. The Next Big Thing in Photovoltaics. J.

Phys. Chem. Lett. 2013201320132013, 908–918.

6. http://en.wikipedia.org/wiki/File:Solar Spectrum.png.

7. http://www.nrel.gov/ncpv/images/efficiency_chart.jpg.

8. QD Image by using Mercury softwear.

9. Frenkel, J., On the Transformation of Light into Heat in Solids. Phys. Rev. 1931193119311931, , , ,

37, 17–44.

10. Wannier, G. H., The Structure of Electronic Excitation Levels in Insulating

Crystals. Phys. Rev. 1937193719371937, , , , 52, 191–197.

11. Mott, N. F., On the Absorption of Light by Crystals. Proc. R. Soc. A. 1938193819381938, , , , 167,

384–391.

12. Ekimov, A.; Onushchenko, A., Quantum Size Effect in the Optical-Spectra of

Semiconductor Micro-Crystals. Sov. Phys. Semicond. 1982198219821982, , , , 16, 775–778.

13. Dingle, R.; Wiegmann, W. C.; Henry, H., Quantum States of Confined Carriers

in Very Thin AlxGa1−xAs-GaAs-AlxGa1−xAs Heterostructures, Phys. Rev. Lett., 1974197419741974,

33, 827–830.

14. Petroff, P. M.; Gossard, A. C.; Logan, R. A.; Wiegmann, W., Toward Quantum

Well Wires: Fabrication and Optical Properties. Appl. Phys. Lett. 1982198219821982, , , , 41, 635–

638.

15. Efros, A. L.; Efros, A. L., Interband Absorption of Light in a Semiconductor

Sphere. Sov. Phys. Semicond. 1982198219821982, 16, 772–775.

16. Rossetti, R.; Nakahara, S.; Brus, L. E., Quantum Size Effects in the Redox

Potentials, Resonance Raman Spectra, and Electronic Spectra of CdS Crystallites in

Aqueous Solution. J. Chem. Phys. 1983198319831983, , , , 79, 1086–1088.

References

61

17. Weller, H.; Koch, U.; Gutierrez, M.; Henglein, A. Photochemistry of Colloidal

Metal Sulfides. 7. Absorption and Fluorescence of Extremely Small Zinc Sulfide

Particles (The World of the Neglected Dimensions). Ber. Bunsen-Ges. Phys. Chem.

1984198419841984, 88, 649–656.

18. Alivisatos, A. P., Semiconductor Clusters, Nanocrystals, and Quantum Dots.

Science 1996199619961996, 271, 933–937.

19. Michalet, X.; Pinaud, F. F.; Bentolila, L. A.; Tsay, J. M.; Doose, S.; Li, J. J.;

Sundaresan, G.; Wu, A. M.; Gambhir, S. S.; Weiss, S., Quantum Dots for Live Cells,

in Vivo Imaging, and Diagnostics. Science 2005200520052005, 307, 538–544.

20. Bae, W. K.; Char, K.; Hur, H.; Lee, S., Single-Step Synthesis of Quantum Dots

with Chemical Composition Gradients. Chem. Mater. 2008200820082008, 20, 531–539.

21. Alivisatos, P., The Use of Nanocrystals in Biological Detection. Nat. Biotech. 2004200420042004,

22222222, 47–52.

22. Medintz, I. L.; Uyeda, H. T.; Goldman, E. R.; Mattoussi, H., Quantum Dot

Bioconjugates for Imaging, Labelling and Sensing. Nat. Mater. 2005200520052005, 4, 435–446.

23. Pisanic Ii, T. R.; Zhang, Y.; Wang, T. H., Quantum Dots in Diagnostics and

Detection: Principles and Paradigms. Analyst 2014201420142014, 139, 2968–2981.

24. Meulenberg, R. W.; Lee, J. R. I.; Wolcott, A.; Zhang, J. Z.; Terminello, L. J.; van

Buuren, T., Determination of the Exciton Binding Energy in CdSe Quantum Dots.

ACS nano 2009200920092009, 3, 325–330.

25. Nozik, A. J.; Beard, M. C.; Luther, J. M.; Law, M.; Ellingson, R. J.; Johnson, J.

C., Semiconductor Quantum Dots and Quantum Dot Arrays and Applications of

Multiple Exciton Generation to Third-Generation Photovoltaic Solar Cells. Chem.

Rev. 2010201020102010, 110, 6873–6890.

26. Ellingson, R. J.; Beard, M. C.; Johnson, J. C.; Yu, P.; Micic, O. I.; Nozik, A. J.;

Shabaev, A.; Efros, A. L., Highly Efficient Multiple Exciton Generation in Colloidal

PbSe and PbS Quantum Dots. Nano Lett. 2005200520052005, 5, 865–871.

27. Sambur, J. B.; Novet, T.; Parkinson, B. A., Multiple Exciton Collection in a

Sensitized Photovoltaic System. Science 2010201020102010, 330, 63–66.

28. Semonin, O. E.; Luther, J. M.; Choi, S.; Chen, H.-Y.; Gao, J.; Nozik, A. J.; Beard,

M. C., Peak External Photocurrent Quantum Efficiency Exceeding 100% via MEG

in a Quantum Dot Solar Cell. Science 2011201120112011, 334, 1530–1533.

29. Smith, A. M.; Nie, S., Semiconductor Nanocrystals: Structure, Properties, and

Band Gap Engineering. Acc.Chem. Res. 2009200920092009, 43, 190–200.

References

62

30. Pokrant, S.; Whaley, K. B., Tight-Binding Studies of Surface Effects on Electronic

Structure of CdSe Nanocrystals: the Role of Organic Ligands, Surface

Reconstruction, and Inorganic Capping Shells. Eur. Phys. J. D 1999199919991999, 6, 255–267.

31. Underwood, D. F.; Kippeny, T.; Rosenthal, S. J., Ultrafast Carrier Dynamics in

CdSe Nanocrystals Determined by Femtosecond Fluorescence Upconversion

Spectroscopy. J. Phys. Chem. B 2000200020002000, 105, 436–443.

32. Ghosh Chaudhuri, R.; Paria, S., Core/Shell Nanoparticles: Classes, Properties,

Synthesis Mechanisms, Characterization, and Applications. Chem. Rev. 2011201120112011, 112,

2373–2433.

33. Laurent, S.; Forge, D.; Port, M.; Roch, A.; Robic, C.; Vander Elst, L.; Muller, R.

N., Magnetic Iron Oxide Nanoparticles: Synthesis, Stabilization, Vectorization,

Physicochemical Characterizations, and Biological Applications. Chem. Rev. 2008200820082008,

108, 2064–2110.

34. Salgueiriño-Maceira, V.; Correa-Duarte, M. A., Increasing the Complexity of

Magnetic Core/Shell Structured Nanocomposites for Biological Applications. Adv.

Mater. 2007200720072007, 19, 4131–4144.

35. Knopp, D.; Tang, D.; Niessner, R., Review: Bioanalytical Applications of

Biomolecule-Functionalized Nanometer-Sized Doped Silica Particles. Anal. Chim.

Acta 2009200920092009, 647, 14–30.

36. Wang, G.; Harrison, A., Preparation of Iron Particles Coated with Silica. J. Colloid

Interface Sci. 1999199919991999, 217, 203–207.

37. Katherine, B., Quantum Dots Go on Display. Nature 2013201320132013, 493, 283.

38. Sargent, E. H., Colloidal Quantum Dot Solar Cells. Nat. Photon. 2012201220122012, 6, 133–

135.

39. Nozik, A. J., Quantum Dot Solar Cells. Physica E: Low-dimensional Systems and

Nanostructures 2002200220022002, 14, 115–120.

40. McDonald, S. A.; Konstantatos, G.; Zhang, S.; Cyr, P. W.; Klem, E. J. D.; Levina,

L.; Sargent, E. H., Solution-Processed PbS Quantum Dot Infrared Photodetectors

and Photovoltaics. Nature Mater. 2005200520052005, 4, 138–142.

41. Luther, J. M.; Law, M.; Beard, M. C.; Song, Q.; Reese, M. O.; Ellingson, R. J.;

Nozik, A. J., Schottky Solar Cells Based on Colloidal Nanocrystal Films. Nano Lett.

2008200820082008, 8, 3488–3492.

42. Ma, W.; Luther, J. M.; Zheng, H.; Wu, Y.; Alivisatos, A. P., Photovoltaic Devices

Employing Ternary PbSxSe1-x Nanocrystals. Nano Lett. 2009200920092009, 9, 1699–1703.

References

63

43. Debnath, R.; Tang, J.; Barkhouse, D. A.; Wang, X.; Pattantyus-Abraham, A. G.;

Brzozowski, L.; Levina, L.; Sargent, E. H., Ambient-Processed Colloidal Quantum

Dot Solar Cells via Individual Pre-Encapsulation of Nanoparticles. J. Am. Chem. Soc.

2010201020102010, 132, 5952–5953.

44. Ma, W.; Swisher, S. L.; Ewers, T.; Engel, J.; Ferry, V. E.; Atwater, H. A.; Alivisatos,

A. P., Photovoltaic Performance of Ultrasmall PbSe Quantum Dots. ACS nano

2011201120112011, 5, 8140–8147.

45. Tang, J.; Kemp, K. W.; Hoogland, S.; Jeong, K. S.; Liu, H.; Levina, L.; Furukawa,

M.; Wang, X.; Debnath, R.; Cha, D.; Chou, K. W.; Fischer, A.; Amassian, A.;

Asbury, J. B.; Sargent, E. H., Colloidal-Quantum-Dot Photovoltaics Using Atomic-

Ligand Passivation. Nature Mater. 2011201120112011, 10, 765–771.

46. Chuang, C.-H. M.; Brown, P. R.; Bulović, V.; Bawendi, M. G., Improved

Performance and Stability in Quantum Dot Solar Cells through Band Alignment

Engineering. Nature Mater. 2014201420142014, advance online publication

(DOI:10.1038/NMAT3984).

47. Nawrot, T.; Plusquin, M.; Hogervorst, J.; Roels, H. A.; Celis, H.; Thijs, L.;

Vangronsveld, J.; Van Hecke, E.; Staessen, J. A., Environmental Exposure to

Cadmium and Risk of Cancer: a Prospective Population-Based Study. Lancet Oncol.

2006200620062006, 7, 119–126.

48. Yong, K.-T.; Law, W.-C.; Hu, R.; Ye, L.; Liu, L.; Swihart, M. T.; Prasad, P. N.,

Nanotoxicity Assessment of Quantum Dots: from Cellular to Primate Studies. Chem.

Soc. Rev. 2013201320132013, 42, 1236–1250.

49. Bullen, C. R.; Mulvaney, P., Nucleation and Growth Kinetics of CdSe

Nanocrystals in Octadecene. Nano Lett. 2004200420042004, 4, 2303–2307.

50. Greene, L. E.; Yuhas, B. D.; Law, M.; Zitoun, D.; Yang, P., Solution-Grown Zinc

Oxide Nanowires. Inorg. Chem. 2006200620062006, 45, 7535–7543.

51. Fischer, M.; Georges, J., Fluorescence Quantum Yield of Rhodamine 6G in

Ethanol as a Function of Concentration Using thermal Lens Spectrometry. Chem.

Phys. Lett. 1996199619961996, 260, 115–118.

52. Chen, J.; Song, J. L.; Sun, X. W.; Deng, W. Q.; Jiang, C. Y.; Lei, W.; Huang, J.

H.; Liu, R. S., An Oleic Acid-Capped CdSe Quantum-Dot Sensitized Solar Cell.

Appl. Phys. Lett. 2009200920092009, 94, 153115-3.

53. Wheeler, D. A.; Zhang, J. Z., Exciton Dynamics in Semiconductor Nanocrystals.

Adv. Mater. 2013201320132013, 25, 2878–2896.

References

64

54. Žídek, K.; Zheng, K.; Ponseca, C. S.; Messing, M. E.; Wallenberg, L. R.; Chábera,

P.; Abdellah, M.; Sundström, V.; Pullerits, T., Electron Transfer in Quantum-Dot-

Sensitized ZnO Nanowires: Ultrafast Time-Resolved Absorption and Terahertz

Study. J. Am. Chem. Soc. 2012201220122012, 134, 12110–12117.

55. Žídek, K.; Zheng, K.; Abdellah, M.; Lenngren, N.; Chábera, P.; Pullerits, T.,

Ultrafast Dynamics of Multiple Exciton Harvesting in the CdSe–ZnO System:

Electron Injection versus Auger Recombination. Nano Lett. 2012201220122012, 12, 6393–6399.

56. Abdellah, M.; Karki, K. J.; Lenngren, N.; Zheng, K.; Pascher, T.; Yartsev, A.;

Pullerits, T., Ultra Long-Lived Radiative Trap States in CdSe Quantum Dots. J.

Phys. Chem. C 2014201420142014, 118, 21682–21686.

57. Zheng, K.; Žídek, K.; Abdellah, M.; Zhang, W.; Chábera, P.; Lenngren, N.;

Yartsev, A.; Pullerits, T., Ultrafast Charge Transfer from CdSe Quantum Dots to p-

Type NiO: Hole Injection vs Hole Trapping. J. Phys. Chem. C 2014201420142014, 118, 18462–

18471.

58. Zheng, K.; Žídek, K.; Abdellah, M.; Torbjörnsson, M.; Chábera, P.; Shao, S.;

Zhang, F.; Pullerits, T., Fast Monolayer Adsorption and Slow Energy Transfer in

CdSe Quantum Dot Sensitized ZnO Nanowires. J. Phys. Chem.A 2013201320132013, 117, 5919–

5925.

59.... Zheng, K.; Zidek, K.; Abdellah, M.; Zhu, N.; Chabera, P.; Lenngren, N.; Chi, Q.;

Pullerits, T., Directed Energy Transfer in Films of CdSe Quantum Dots: Beyond the

Point Dipole Approximation. J. Am. Chem. Soc. 2014201420142014, 136, 6259–6268.

60. Brus, L. E., Electron-Electron and Electron‐Hole Interactions in Small

Semiconductor Crystallites: The Size Dependence of the Lowest Excited Electronic

State. J. Chem. Phys. 1984198419841984, 80, 4403–4409.

61. Lian, H.; Huizhen, W.; Lingxiao, D.; Hongying, G.; Xin, C.; Ning, D., The Effect

of Annealing and Photoactivation on the Optical Transitions of Band–Band and

Surface Trap States of Colloidal Quantum Dots in PMMA. Nanotechnology 2011201120112011,

22, 125202-6.

62. Luo, X.; Liu, P.; Truong, N. T. N.; Farva, U.; Park, C., Photoluminescence Blue-

Shift of CdSe Nanoparticles Caused by Exchange of Surface Capping Layer. J. Phys.

Chem. C 2011201120112011, 115, 20817–20823.

63. Wang, M.; Abbineni, G.; Clevenger, A.; Mao, C.; Xu, S., Upconversion

Nanoparticles: Synthesis, Surface Modification and Biological Applications.

Nanomedicine: Nanotechnology, Biology and Medicine. 2011201120112011, 7, 710–729.

References

65

64. Bae, W. K.; Kwak, J.; Park, J. W.; Char, K.; Lee, C.; Lee, S., Highly Efficient

Green-Light-Emitting Diodes Based on CdSe@ZnS Quantum Dots with a

Chemical-Composition Gradient. Adv. Mater. 2009200920092009, 21, 1690–1694.

65. Haynes, J. R.; Westphal, W. C., Radiation Resulting from Recombination of Holes

and Electrons in Silicon. Phys. Rev. 1956195619561956, 101, 1676–1678.

66. Masia, F.; Langbein, W.; Moreels, I.; Hens, Z.; Borri, P., Exciton Dephasing in

Lead Sulfide Quantum Dots by X-Point Phonons. Phys. Rev. B 2011201120112011, 83, 201309-

4.

67. Schoenlein, R.; Mittleman, D.; Shiang, J.; Alivisatos, A.; Shank, C., Investigation

of Femtosecond Electronic Dephasing in CdSe Nanocrystals Using Quantum-Beat-

Suppressed Photon Echoes. Phys. Rev. Lett. 1993199319931993, 70, 1014–1017.

68. Chia, C.-H.; Fan, W.-C.; Lin, Y.-C.; Chou, W.-C., Radiative Recombination of

Indirect Exciton in Type-II ZnSeTe/ZnSe Multiple Quantum Wells. J. Luminescence

2011201120112011, 131, 956–959.

69. Dahan, P.; Fleurov, V.; Thurian, P.; Heitz, R.; Hoffmann, A.; Broser, I., Properties

of the Intermediately Bound α-, β- and γ-Excitons in ZnO:Cu. J. Phys. Condens.

Matter 1998199819981998, 10, 2007–2019.

70. Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W., Environmental

Applications of Semiconductor Photocatalysis. Chem. Rev. 1995199519951995, 95, 69–96.

71. Wu, F.; Zhang, J. Z.; Kho, R.; Mehra, R. K., Radiative and Nonradiative Lifetimes

of Band Edge States and Deep Trap States of CdS Nanoparticles Determined by

Time-Correlated Single Photon Counting. Chem. Phys. Lett. 2000200020002000, 330, 237–242.

72. Bakulin, A. A.; Neutzner, S.; Bakker, H. J.; Ottaviani, L.; Barakel, D.; Chen, Z.,

Charge Trapping Dynamics in PbS Colloidal Quantum Dot Photovoltaic Devices.

ACS nano 2013201320132013, 7, 8771–8779.

73. Marcus, R. A., On the Theory of Oxidation‐Reduction Reactions Involving

Electron Transfer. J. Chem. Phys. 1956195619561956, 24, 966–978.

74. Tvrdy, K.; Frantsuzov, P. A.; Kamat, P. V., Photoinduced Electron Transfer from

Semiconductor Quantum Dots to Metal Oxide Nanoparticles. Proc. Natl. Acad. Sci.

2011201120112011, 108, 29–34.

75. Němec, H.; Rochford, J.; Taratula, O.; Galoppini, E.; Kužel, P.; Polívka, T.;

Yartsev, A.; Sundström, V., Influence of the Electron-Cation Interaction on Electron

Mobility in Dye-Sensitized ZnO and TiO2 Nanocrystals: A Study Using Ultrafast

Terahertz Spectroscopy. Phys. Rev. Lett. 2010201020102010, 104, 197401-4.

References

66

76. Stockwell, D.; Yang, Y.; Huang, J.; Anfuso, C.; Huang, Z.; Lian, T., Comparison

of Electron-Transfer Dynamics from Coumarin 343 to TiO2, SnO2, and ZnO

Nanocrystalline Thin Films: Role of Interface-Bound Charge-Separated Pairs. J.

Phys. Chem. C 2010201020102010, 114, 6560–6566.

77. Shockley, W.; Queisser, H. J., Detailed Balance Limit of Efficiency of p‐n Junction

Solar Cells. J. Appl. Phys. 1961196119611961, 32, 510–519.

78. Beard, M. C.; Knutsen, K. P.; Yu, P.; Luther, J. M.; Song, Q.; Metzger, W. K.;

Ellingson, R. J.; Nozik, A. J., Multiple Exciton Generation in Colloidal Silicon

Nanocrystals. Nano Lett. 2007200720072007, 7, 2506–2512.

79. Trinh, M. T.; Polak, L.; Schins, J. M.; Houtepen, A. J.; Vaxenburg, R.; Maikov,

G. I.; Grinbom, G.; Midgett, A. G.; Luther, J. M.; Beard, M. C.; Nozik, A. J.; Bonn,

M.; Lifshitz, E.; Siebbeles, L. D. A., Anomalous Independence of Multiple Exciton

Generation on Different Group IV−VI Quantum Dot Architectures. Nano Lett.

2011201120112011, 11, 1623–1629.

80. Klimov, V. I., Quantization of Multiparticle Auger Rates in Semiconductor

Quantum Dots. Science 2000200020002000, 287, 1011–1013.

81. Jha, P. P.; Guyot-Sionnest, P., Trion Decay in Colloidal Quantum Dots. ACS nano

2009200920092009, 3, 1011–1015.

82. Yang, Y.; Rodriguez-Cordoba, W.; Xiang, X.; Lian, T., Strong Electronic Coupling

and Ultrafast Electron Transfer between PbS Quantum Dots and TiO2

Nanocrystalline Films. Nano Lett. 2012201220122012, 12, 303–309.

83. Wang, X.; Ren, X.; Kahen, K.; Hahn, M. A.; Rajeswaran, M.; Maccagnano-Zacher,

S.; Silcox, J.; Cragg, G. E.; Efros, A. L.; Krauss, T. D., Non-Blinking Semiconductor

Nanocrystals. Nature 2009200920092009, 459, 686–689.

84. Chepic, D. I.; Efros, A. L.; Ekimov, A. I.; Ivanov, M. G.; Kharchenko, V. A.;

Kudriavtsev, I. A.; Yazeva, T. V., Auger Ionization of Semiconductor Quantum

Drops in a Glass Matrix. J. Luminescence 1990199019901990, 47, 113–127.

85. Kambhampati, P., Hot Exciton Relaxation Dynamics in Semiconductor Quantum

Dots: Radiationless Transitions on the Nanoscale. J. Phys. Chem. C 2011201120112011, 115,

22089–22109.

86. Hines, M. A.; Guyot-Sionnest, P., Synthesis and Characterization of Strongly

Luminescing ZnS-Capped CdSe Nanocrystals. J. Phys. Chem. 1996199619961996, 100, 468–471.

87. Kramer, I. J.; Sargent, E. H., Colloidal Quantum Dot Photovoltaics: A Path

Forward. ACS nano 2011201120112011, 5, 8506–8514.

References

67

88. Baker, D. R.; Kamat, P. V., Tuning the Emission of CdSe Quantum Dots by

Controlled Trap Enhancement. Langmuir 2010201020102010, 26, 11272–11276.

89. Rosson, T. E.; Claiborne, S. M.; McBride, J. R.; Stratton, B. S.; Rosenthal, S. J.,

Bright White Light Emission from Ultrasmall Cadmium Selenide Nanocrystals. J.

Am. Chem. Soc. 2012201220122012, 134, 8006–8009.

90. Chen, W.; Wang, Z.; Lin, Z.; Lin, L., Thermoluminescence of ZnS Nanoparticles.

Appl. Phys. Lett. 1997199719971997, 70, 1465–1467.

91. Hoheisel, W.; Colvin, V. L.; Johnson, C. S.; Alivisatos, A. P., Threshold for

Quasicontinuum Absorption and Reduced Luminescence Efficiency in CdSe

Nanocrystals. J. Chem. Phys. 1994199419941994, 101, 8455–8460.

92. Leatherdale, C. A.; Kagan, C. R.; Morgan, N. Y.; Empedocles, S. A.; Kastner, M.

A.; Bawendi, M. G., Photoconductivity in CdSe Quantum Dot Solids. Phys. Rev. B

2000200020002000, 62, 2669–2680.

93. Klimov, V. I., Spectral and Dynamical Properties of Multiexcitons in

Semiconductor Nanocrystals. Annu. Rev. Phys. Chem. 2007200720072007, 58, 635–673.

94. Kaniyankandy, S.; Rawalekar, S.; Verma, S.; Palit, D. K.; Ghosh, H. N., Charge

Carrier Dynamics in Thiol Capped CdTe Quantum Dots. Phys. Chem. Chem. Phys.

2010201020102010, 12, 4210–4216.

95. Peterson, J. J.; Krauss, T. D., Photobrightening and Photodarkening in PbS

Quantum Dots. Phys. Chem. Chem. Phys. 2006200620062006, 8, 3851–3856.

96. Stinaff, E. A.; Scheibner, M.; Bracker, A. S.; Ponomarev, I. V.; Korenev, V. L.;

Ware, M. E.; Doty, M. F.; Reinecke, T. L.; Gammon, D., Optical Signatures of

Coupled Quantum Dots. Science 2006200620062006, 311, 636–639.

97. Carlson, B.; Leschkies, K.; Aydil, E. S.; Zhu, X. Y., Valence Band Alignment at

Cadmium Selenide Quantum Dot and Zinc Oxide (1010) Interfaces. J. Phys. Chem.

C 2008200820082008, 112, 8419–8423.

98. Santra, P. K.; Kamat, P. V., Mn-Doped Quantum Dot Sensitized Solar Cells: A

Strategy to Boost Efficiency Over 5%. J. Am. Chem. Soc. 2012201220122012, 134, 2508–2511.

99. Pernik, D. R.; Tvrdy, K.; Radich, J. G.; Kamat, P. V., Tracking the Adsorption

and Electron Injection Rates of CdSe Quantum Dots on TiO2: Linked versus Direct

Attachment. J. Phys. Chem. C 2011201120112011, 115, 13511–13519.

100. Salter, C. L.; Stevenson, R. M.; Farrer, I.; Nicoll, C. A.; Ritchie, D. A.; Shields,

A. J., An Entangled-Light-Emitting-Diode. Nature 2010201020102010, 465, 594–597.

References

68

101. Kambhampati, P., Multiexcitons in Semiconductor Nanocrystals: A Platform for

Optoelectronics at High Carrier Concentration. J. Phys. Chem. Lett. 2012201220122012, 3, 1182–

1190.

102. Wolcott, A.; Doyeux, V.; Nelson, C. A.; Gearba, R.; Lei, K. W.; Yager, K. G.;

Dolocan, A. D.; Williams, K.; Nguyen, D.; Zhu, X. Y., Anomalously Large

Polarization Effect Responsible for Excitonic Red Shifts in PbSe Quantum Dot

Solids. J. Phys. Chem. Lett. 2011201120112011, 2, 795–800.

103. Kim, M. R.; Xu, Z.; Chen, G.; Ma, D., Semiconductor and Metallic Core–Shell

Nanostructures: Synthesis and Applications in Solar Cells and Catalysis. Chem. Eur.

J. 2014201420142014, 20, 11256–11275.

104. Steckel, J. S.; Snee, P.; Coe-Sullivan, S.; Zimmer, J. P.; Halpert, J. E.; Anikeeva,

P.; Kim, L.-A.; Bulovic, V.; Bawendi, M. G., Color-Saturated Green-Emitting QD-

LEDs. Angew. Chem. Int. Ed. 2006200620062006, 45, 5796–5799.

105. Zimmer, J. P.; Kim, S.-W.; Ohnishi, S.; Tanaka, E.; Frangioni, J. V.; Bawendi,

M. G., Size Series of Small Indium Arsenide−Zinc Selenide Core−Shell Nanocrystals

and Their Application to In Vivo Imaging. J. Am. Chem. Soc. 2006200620062006, 128, 2526–2527.

106. Reiss, P.; Protiere, M.; Li, L., Core/Shell Semiconductor Nanocrystals. Small

2009200920092009, 5, 154–168.

107. Dabbousi, B. O.; Rodriguez-Viejo, J.; Mikulec, F. V.; Heine, J. R.; Mattoussi,

H.; Ober, R.; Jensen, K. F.; Bawendi, M. G., (CdSe)ZnS Core−Shell Quantum

Dots: Synthesis and Characterization of a Size Series of Highly Luminescent

Nanocrystallites. J. Phys. Chem. B 1997199719971997, 101, 9463–9475.

108. Cragg, G. E.; Efros, A. L., Suppression of Auger Processes in Confined Structures.

Nano Lett. 2009200920092009, 10, 313–317.

109. Xie, R.; Kolb, U.; Li, J.; Basché, T.; Mews, A., Synthesis and Characterization of

Highly Luminescent CdSe−Core CdS/Zn0.5Cd0.5S/ZnS Multishell Nanocrystals. J.

Am. Chem. Soc. 2005200520052005, 127, 7480–7488.

110. Zhu, H.; Song, N.; Lian, T., Controlling Charge Separation and Recombination

Rates in CdSe/ZnS Type I Core−Shell Quantum Dots by Shell picknesses. J. Am.

Chem. Soc. 2010201020102010, 132, 15038–15045.

111. Yang, C. C.; Mai, Y. W., Size-Dependent Absorption Properties of CdX (X = S,

Se, Te) Quantum Dots. Chem. Phys. Lett. 2012201220122012, 535, 91–93.

112. Zhu, H.; Lian, T., Wavefunction Engineering in Quantum Confined

Semiconductor Nanoheterostructures for Efficient Charge Separation and Solar

Energy Conversion. Energy Environ. Sci. 2012201220122012, 5, 9406–9418.

References

69

113. Lao, P.; Guo, Y.; Siu, G.; Shen, S., Optical-Phonon Behavior in Zn1-xMnxSe:

Zinc-Blende and Wurtzite Structures. Phys. Rev. B 1993199319931993, 48, 11701–11704.

114. Agrawal, B.; Yadav, P.; Agrawal, S., Ab Initio Calculation of the Electronic,

Structural, and Dynamical Properties of Zn-Based Semiconductors. Phys. Rev. B

1994199419941994, 50, 14881–14887.

115. Dworak, L.; Matylitsky, V. V.; Breus, V. V.; Braun, M.; Basché, T.; Wachtveitl,

J., Ultrafast Charge Separation at the CdSe/CdS Core/Shell Quantum

Dot/Methylviologen Interface: Implications for Nanocrystal Solar Cells. J. Phys.

Chem. C 2011201120112011, 115, 3949–3955.

116. Sharma, S. N.; Pillai, Z. S.; Kamat, P. V., Photoinduced Charge Transfer between

CdSe Quantum Dots and p-Phenylenediamine. J. Phys. Chem. B 2003200320032003, 107, 10088–

10093.

117. Robel, I.; Kuno, M.; Kamat, P. V., Size-Dependent Electron Injection from

Excited CdSe Quantum Dots into TiO2 Nanoparticles. J. Am. Chem. Soc. 2007200720072007,

129, 4136–4137.

118. Phillips, J. C., Stretched Exponential Relaxation in Molecular and Electronic

Glasses. Rep. Prog. Phys. 1996199619961996, 59, 1133–1207.

119. Kamat, P. V.; Christians, J. A.; Radich, J. G., Quantum Dot Solar Cells: Hole

Transfer as a Limiting Factor in Boosting the Photoconversion Efficiency. Langmuir

2014201420142014, 30, 5716–5725.

120. Abdellah, M.; Žídek, K.; Zheng, K.; Chábera, P.; Messing, M. E.; Pullerits, T.,

Balancing Electron Transfer and Surface Passivation in Gradient CdSe/ZnS Core–

Shell Quantum Dots Attached to ZnO. J. Phys. Chem. Lett. 2013201320132013, 4, 1760–1765.

121. Abdellah, M.; Marschan, R.; Žídek, K.; Messing, M. E.; Abdelwahab, A.;

Chábera, P.; Zheng, K.; Pullerits, T., Hole Trapping: The Critical Factor for

Quantum Dot Sensitized Solar Cell Performance. J. Phys. Chem.C 2014201420142014, 118,

25802–25808.

122. Omogo, B.; Aldana, J. F.; Heyes, C. D., Radiative and Nonradiative Lifetime

Engineering of Quantum Dots in Multiple Solvents by Surface Atom Stoichiometry

and Ligands. J. Phys. Chem. C 2013201320132013, 117, 2317–2327.

Documents

Ultrafast Photoinduced Processes in Core and Core–Shell ...lup.lub.lu.se/search/ws/files/5516980/5051929.pdf · Mohamed Mohamed Ahmed Ahmed Ahmed AbdellahAbdellahAbdellah Doctoral