Molecular Design and Synthesis of D-π-A Structured Organic Sensitizers Based on Thieno[3,2-b][1]benzothiophene as a New Chromophore for Highly Efficient Dye-Sensitized Solar Cells
- 주제(키워드) D-π-A structural sensitizers
- 발행기관 고려대학교 대학원
- 지도교수 김환규
- 발행년도 2015
- 학위수여년월 2015. 8
- 학위구분 박사
- 학과 대학원 소재화학과
- 원문페이지 176 p
- 실제URI http://www.dcollection.net/handler/korea/000000060380
- 본문언어 영어
- 제출원본 000045841658
초록/요약
The development of D-π-A structural sensitizers by molecular engineering is responsible for the significant PCE enhancement in DSSCs. For achieving the higher PCE, the π-bridge unit as the coupling mediator should be preferred planar conformation to facilitate the electron transfer from donor to acceptor. The new thieno[3,2-b][1]benzothiophene (TBT)-based D-π-A sensitizers, coded as SGT sensitizers, have been designed and synthesized for dye-sensitized solar cells (DSSCs). The thieno[3,2-b][1]benzothiophene (TBT), prepared by fusing thiophene unit with the phenyl unit of triphenylamine (TPA) donor, was utilized as the π-bridge for all sensitizers with good planarity. The dihexyloxyphenyl-substituted biphenylamine as the donor (D) group was introduced to retard charge recombination with the electrolyte and to prevent dye aggregation. The combination of a dihexyloxyphenyl-substituted biphenylamine donor and the TBT π-bridge plays multifunctional roles, e.g., the enhanced ability of the π-bridge and donor, slow charge recombination and prevention of dye aggregation in the D?π?A sensitizer. The photovoltaic performance of SGT sensitizer-based DSSCs employing Co(bpy)32+/3+ (bpy=2,2'-bipyridine) redox couple were systematically evaluated. To evaluate the extension effect of π-bridge length, SGT-134 was developed as the reference sensitizer with the introduction of benzothiophene (BT) derivative as π-bridge unit, compared to SGT-130 including TBT π-bridge unit. As a comparison of DSSC performance between SGT-130 and SGT-134, the SGT-130-based DSSC exhibited the high PCE of 9.02 % with Jsc = 14.02 mA cm-2, Voc = 850 mV and FF = 76% compared to that of SGT-134, which demonstrated that the extention effect of π-bridge length caused better results of increasd Jsc and Voc values. As a strategy for the improvement of absorption abilities, the various thiophene derivatives, e.g., those with thiophene (T, SGT-121), bithiophene (BT, SGT-123) and thienothio[3,2-b]thiophene (TT, SGT-125) moieties, were incorporated as π-spacers between the TBT π-bridge and the acceptor unit. The introduction of thiophene π-spacers significantly improved the photovoltaic performance (in particular, in terms of the photocurrent Jsc and open-circuit voltage Voc) compared to SGT-127 without the thiophene unit. The SGT sensitizers were systematically evaluated for DSSCs based on the Co(bpy)32+/3+ (bpy=2,2'-bipyridine) redox couple. As a result, the SGT-123-based DSSC including the BT moiety exhibited a power conversion efficiency of 9.69%, Jsc of 16.16 mA cm?2, Voc of 830 mV and fill factor of 0.72, whose values are much higher than those of SGT-series sensitizers. To compensate the weak donating ability of TBT π-bridge, the benzothiadiazole (BTD) unit as the additional electron-withdrawing unit was introduced into the D-π-A sensitizer, leading to red-shift the intramolecular charge transfer absorption band and reducing the bandgap for enhancing the photocurrent. The BTD-phenyl unit (SGT-130) as an auxiliary acceptor between TBT π-bridge and A unit was designed and synthesized to overcome the weak-donating ability of the TBT π-bridge. For the evaluation of π-bridge ability of TBT unit, the SGT-129 was also synthesized by modification of Y123 sensitizer. Among them, SGT-130 exhibited the highest power conversion efficiency (PCE) of 10.47% with Jsc = 16.77 mA cm-2, Voc = 851 mV and FF = 73.34 %, whose PCE was much higher than that of Y123 (9.5%). The benzothiadiazole (BTD) unit into the D-π-A sensitizer was known as the electron-withdrawing unit. Base on the above result of SGT-130-based DSSC, the SGT-126 and SGT-132 sensitizers were introduced the BTD-thiophene (T) derivative as an auxiliary acceptor for comparison of BTD-phenyl (P) derivative-based SGT-130 and SGT-131 sensitizers. Also, to compare the ability of anchoring group between cyanoacrylic acid and carboxylic acid, the cyanoacrylic acid was introduced as an anchoring group for SGT-126 and SGT-130, and the carboxylic acid was also introduced for SGT-131 and SGT-132. From the point of anchoring group, the λICT of SGT-130 and SGT-126 including cyanoacrylic acid as an anchoring group were significantly red-shifted compared to SGT-131 and SGT-132 with carboxylic acid, indicates that the electron accepting ability of cyanoacrylic acid is much stronger than that of the carboxylic acid. In this SGT series, DSSC based on SGT-132 with BTD-T unit gave a high Jsc of 15.95 mA cm-2, Voc of 832 mV and FF of 0.73, achieving an PCE of 9.82%, with respect to SGT-130 with BTD-P unit, indicating the improvement of the light harvesting ability. To strengthen the donating ability of TBT π-bridge unit, alkyl-substituted thieno[3,2-b]indole (TI) derivatives were developed as the new π-bridge unit. The electron donating ability of TI π-bridge is stronger than the TBT unit as the substitution of the core atom from sulfur (S) to nitrogen (N). As the introduction of hexyl-substituted TI π-bridge instead of TBT unit, DSSCs based on SGT-137 exhibited the highest PCE of 10.73% with Jsc = 17.43 mA cm-2, Voc = 825 mV and FF = 74% using the Co(bpy)32+/3+ electrolyte, indicating the effect of enhanced π-bridge ability. The introduction of alkyl-substituted TI derivative as the π-bridge unit can be achieved for highly efficient organic sensitizers in light of their good light harvesting ability at loger wavelengths.
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Abstract-----------------------------------------------------------------------1
Contents----------------------------------------------------------------------4
List of Schemes--------------------------------------------------------------7
List of Figures---------------------------------------------------------------8
List of Tables---------------------------------------------------------------12
Contents
1. Introduction-------------------------------------------------------------13
2. Background-------------------------------------------------------------15
2.1 Operating Principle of Dye-Sensitized Solar Cells (DSSCs)------------15
2.2 Photovoltaic Key Parameters of DSSCs----------------------------------17
2.2.1 Incident-Photon-to-Current Conversion Efficiency (IPCE)------ 17
2.2.2 Short-Circuit Photocurrent Density (JSC)-----------------------------18
2.2.3 Open-Circuit Photovoltage (VOC)---------------------------------------19
2.2.4 Fill Factor (FF)-------------------------------------------------------------21
2.2.5 Photocurrent-Photovoltage Curves (J-V Curves)-------------------22
2.2.6 Power Conversion Efficiency (PCE)-----------------------------------23
2.2.7 Long-Term Stability-------------------------------------------------------23
2.3 Key Parameters for Enhancement of PCE---------------------------------24
2.3.1 Enhancement of JSC--------------------------------------------------------24
2.3.1-1 Improvement of Aborption--------------------------------------24
2.3.1-2 Optimization of Energy Levels for Efficient Electron Injection-------------------26
2.3.1-3 Reduction of Dye Aggregation for Efficient Electron Injection-------------------26
2.3.1-4 Good Electronic Interaction of Sensitizer for Efficient Electron Injection-------------------27
2.3.2 Enhancement of VOC ------------------------------------------------------28
2.3.2-1 Improvement of Dye Regeneration-------------------------------28
2.3.2-2 Reduction of Charge Recombination-----------------------------28
2.3.2-3 Electrolytes-------------------------------------------------------------29
2.3.2-4 Co-adsorbents---------------------------------------------------------32
2.3.3 Enhancement of FF -------------------------------------------------------33
2.3.3-1 Nanocrystalline Semiconductor for Photoanode---------------33
2.3.3-2 Counter Electrodes---------------------------------------------------34
2.3.3-3 Device Fabrication----------------------------------------------------35
2.4 Key Materials for Highly Efficient DSSCs-------------------------------36
2.4.1 Ruthenium Complex-based Sensitizers--------------------------------37
2.4.2 Metal-Free Organic Sensitizers-----------------------------------------38
2.4.3 Development of π-Bridge Units in D-π-A Organic Sensitizers----40
2.4.3-1 BDT π-Bridge Unit-based Organic Sensitizers-----------------41
2.4.3-2 CPDT π-Bridge Unit-based Organic Sensitizers---------------42
2.4.3-3 DTS π-Bridge Unit-based Organic Sensitizers------------------44
2.4.3-4 DTP π-Bridge Unit-based Organic Sensitizers------------------46
2.4.4 Development of Benzothiadiazole (BTD) Derivatives as an Auxiliary Acceptor---------------48
3. Research Objectives--------------------------------------------------------------------52
4. Experimental----------------------------------------------------------------------------56
4.1 Reagents-------------------------------------------------------------------------56
4.2 Measurements------------------------------------------------------------------56
4.3 DFT Calculation ---------------------------------------------------------------58
4.4 Synthesis ------------------------------------------------------------------------59
4.5 DSSC Fabrication--------------------------------------------------------------83
4.6 Photoelectrochemical measurements of DSSC---------------------------84
5. Results and Discussion-----------------------------------------------------------------85
5.1 Benzothiophene-based D-π-A Organic Sensitizers for Efficient Dye-Sensitized Solar Cells: Effect of Extended π-Bridge Units--------------------------------------------------------------------85
5.2 New Thieno[3,2-b][1]benzothiophene-based Organic Sensitizers Containing π-Extended Thiophene Spacers for Efficient Dye-Sensitized Solar Cells: Effect of π-Extended Thiophene Spacers--------------------------------------------------------------------------------------93
5.3 Thieno[3,2-b][1]benzothiophene Derivative as a New π-Bridge Unit in D-π-A Structural Organic Sensitizers with Over 10.47% Efficiency for Dye-Sensitized Solar Cells
: Introduction Effect of BTD Derivative as an Auxiliary Acceptor----------------------------------106
5.4 Influence of Anchoring Group of Organic Sensitizers Containing Benzothiadiazole-based Auxiliary Acceptors for Efficient Dye-Sensitized Solar Cells: Effect of BTD Auxiliary Acceptors-based Anchoring Group -------------------------------------------------------------------------------------127
5.5 Alkylated Thieno[3,2-b]indole Derivatives as π-Bridge Unit in D-π-A Organic Sensitizers for Dye-Sensitized Solar Cells: Substitution Effect of Core Atom for Efficient π-Bridge Unit----------------139
6. Conclusions-----------------------------------------------------------------------------149
7. References------------------------------------------------------------------------------153
국문요약------------------------------------------------------------------------------------168
