Charge Separation Dynamics between Semiconductor Nanoparticles and Molecular Adsorbates Öffentlichkeit

Abstract

Charge Separation Dynamics between Semiconductor Nanoparticles and Molecular Adsorbates
By Jier Huang

The understanding of the interfacial charge transfer dynamics between semiconductor nanoparticles and molecular adsorbates is essential to their potential applications in solar cells. In this dissertation, we investigated two types of semiconductor nanoparticle - molecular adsorbate systems: (1) quantum dots (QD)-adsorbate complexes and (2) dye sensitized semiconductor films, by using transient absorption spectroscopy and time resolved fluorescence spectroscopy. For the first system, we conducted a series of studies on exciton dissociation dynamics in QDs through charge transfer to the adsorbed molecules. We investigated single exciton dissociation dynamics in CdSe QDs through electron transfer (ET) to a Re-bipyridyl complex, methylene blue (MB⁺) molecules, and Flavin mononucleotide by monitoring the spectral features of both the QDs and molecular adsorbates. It was found that the ET rate depends on the number of adsorbatess per QD as well as the QD particle size. The fastest observed ET rates were on the sub-ps time scale, which is much faster than the exciton-exciton annihilation process, indicating the possibility to dissociate multiple excitons through ET from QDs to molecular adsorbates. Additionally, we have investigated exciton dissociation dynamics in CdSe QDs through hole transfer to phenothiazine molecules (PTZ). It was shown that the hole transfer time was ~ 2.5 ns in 1:1 CdSe-PTZ complexes and reached ~ 300 ps in samples with an average of ~ 6 PTZ per QD. Furthermore, we demonstrated the capability to dissociate multiple excitons in CdSe-MB⁺ complex. It was shown that ~ 3 excitons in CdSe QDs can be dissociated through ET to MB⁺. For the second system, we examined the ET dynamics from Rhodamine (Rh) dyes to different semiconductor films. Electron injection kinetics from RhB to In2O3, SnO2, and ZnO films were compared to examine the effect of the semiconductor nature on the ET dynamics. It was found that ET rate follows the order of In2O3 ≈ SnO2 > ZnO. Additionally, we also explored the impact of dye energetics on ET dynamics by comparing the injection kinetics from RhB, Rh101, and Rh6G to the same semiconductor. The results showed that the ET rate decreases with a decrease in the excited state oxidation potential.

Charge Separation Dynamics between Semiconductor Nanoparticles and Molecular Adsorbates

By
Jier Huang
B.S. Lanzhou University, P.R. China, 2001
M.S. Lanzhou University, P.R. China, 2004
Advisor: Tianquan Lian, Ph.D.
A dissertation submitted to the Faculty of the
James T. Laney School of Graduate Studies of Emory University
in partial fulfillment of the requirements for the degree of
Doctor of Philosophy
in Chemistry
2010

Table of Contents

Table of Contents
Chapter 1. Introduction...1

1.1 Ultrafast Exciton Dissociation in CdSe Quantum Dots through Charge Transfer to Molecular Adsorbates...1

1.1.1. Introduction...1
1.1.2. Single Exciton Dissociation...4
1.1.3. Multiexciton Dissociation...9

1.2 ET Dynamics from Molecular Adsorbates to Semiconductor Nanocrystalline Thin Films...12
1.3 Summary...13
References...13

Chapter 2. Experimental Section...23

2.1 Preparation of QDs and Semiconductor Thin Films...23
2.2 Preparation of QD-adsorbate Complex and Dye Sensitized Semiconductor Thin Films...26
2.3 Spectroscopic Measurement...28
References...30

Chapter 3. Exciton Dissociation Dynamics in CdSe Quantum Dots by Electron Transfer to Re-bipyridyl Complexes...32

3.1. Introduction...32
3.2 Results...34

3.2.1. Exciton Dissociation Pathway in CdSe-ReC0A Assemblies...34

3.2.1.1. Static Absorption Measurement...34
3.2.1.2. Fluorescence Lifetime Measurement...35
3.2.1.3. Transient IR Absorption Measurement...36
3.2.1.4. Transient Visible Absorption Measurement...40

3.2.2. The Effect of the Number of ReC0A per QDs on ET rate...42
3.2.3. The Effect of QD Particle Sizes on ET rate...45

3.3 Discussion...46
3.4 Summary...48
References...49

Chapter 4. Exciton Dissociation Dynamics in CdSe Quantum Dots by Electron Transfer to Flavin...51

4.1 Introduction...51
4.2 Results...53

4.2.1 Static Absorption Measurement...53
4.2.2 Transient Visible Absorption Measurement...54

4.3 Summary...57
References...57

Chapter 5. Exciton Dissociation in CdSe Quantum Dots by Hole Transfer to Phenothiazine...62

5.1 Introduction...62
5.2 Results...65

5.2.1 Static Absorption Measurement...65
5.2.2 Fluorescence Lifetime Measurement...66
5.2.3 Transient Absorption Measurement...67

5.3 Discussion...72
5.4 Summary...76
References...77

Chapter 6. Multiple Exciton Dissociation in CdSe through Electron Transfer to Methylene Blue...81

6.1 Introduction...81
6.2 Results and Discussion...84

6.2.1 Single Exciton Dissociation Dynamics in CdSe(510nm)-MB⁺ Complex...84

6.2.1.1 ET Pathway in CdSe(510nm)-MB+ Complex...84
6.2.1.2 Energy Transfer Efficiency in CdSe(510nm)-MB⁺ Complex...89

6.2.2 Single Exciton Dissociation Dynamics in CdSe(510nm)-MB⁺ Complex...91

6.2.2.1 ET Pathway in CdSe(553nm)-MB⁺ Complex...91
6.2.2.2 Energy Transfer Efficiency in CdSe(553nm)-MB+ Complex...94

6.2.3 Exciton-Exciton Annihilation...96

6.2.2.1 Quantirying Exciton-Exciton Annihilation Rate...96
6.2.2.2 Quantifying the Number of Excitons per QD...101

6.2.4 Multiexciton Dissociation...103

6.2.4.1 Multiexciton Dissociation Dynamics...104
6.2.4.2 Quantifying the Number of Reduced MB⁺...109

6.3 Summary...111
References...112

Chapter 7. Interfacial Electron Transfer Dynamics from Organic Dyes to Semiconductor Nanocrystalline Thin Films...117

7.1 Introduction...117
7.2 Results...120

7.2.1 Effects of Semiconductors on the Injection Rate...120

7.2.1.1 Non-ET Dynamics in RhB Sensitized ZrO2 Films...120
7.2.1.2 ET Dynamics in RhB Sensitized In2O3 Films...123
7.2.1.3 ET Dynamics in RhB Sensitized SnO2 Films...130
7.2.1.4 ET Dynamics in RhB Sensitized ZnO Films...135
7.2.1.5 Comparison of ET from RhB to In2O3, SnO2 and ZnO Films...141

7.2.2 Effects of Dye Energetics on the Injection Rate...143

7.3 Discussion...145

7.3.1 Effects of Semiconductors on the Injection Rate...145
7.3.2 Effects of Dye Energetics on the Injection Rate...146

7.4 Summary...147
References...148

About this Dissertation

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School	Laney Graduate School
Department	Chemistry
Degree	PhD
Submission	Dissertation
Language	English
Research Field	Chemistry, Physical
Stichwort	Molecular adsorbates Solar cells Semiconductor nanoparticles Interfacial electron transfer Transient spectroscopy
Committee Chair / Thesis Advisor	Lian, Tim, Emory University
Committee Members	Heaven, Michael, Emory University Bowman, Joel M, Emory University

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