臺灣博碩士論文加值系統

本論文著重於聚合物太陽能電池，其主動層內相分離現象之研究。目前最熱門的主動層材料，是以『聚三己烷塞吩(P3HT)』作為電子施體，而『苯基碳61丁酸甲酯 (PCBM)』則做為電子受體；兩種有機材料均勻混合，再以旋轉塗佈的方式做成主動層薄膜。
配置主動層溶液的過程中，雖然P3HT與PCBM能均勻混合；不過由於兩種材料物理性質與化學性質的差異，會造成塗佈上去的主動層薄膜有相分離的現象；也就是在靠近主動層的表層(空氣界面)，有較多的P3HT聚集，而靠近主動層底層，則有較多的PCBM聚集；就能帶圖來看，這種現象對傳統結構的聚合物太陽能電池，也就是ITO當陽極、金屬當陰極，是不利的；分別聚集在陰極與陽極附近的P3HT與PCBM，對光生的電子與電洞而言就像是能障，如此一來便會造成部分光生載子復合，導致光電流下降，進而影響功率轉換效率。
本論文分兩個研究主題：第一個主題，著重於提升聚合物太陽電池的光電流：蒸鍍金屬電極前，在主動層上面鍍上一層PCBM薄膜，然後再進行後退火使蒸鍍的PCBM分子能滲入主動層中，並與主動層表層的P3HT混合，如此一來靠近陰極處的能障消失了，光電流自然能增加，功率轉換效率也因此而提升。
第二個研究主題，則是利用新製程來製作聚合物太陽電池，看是否能減少相分離效應；由於過去有研究團隊認為相分離的原因，是因為兩種材料間『表面能』的差異所導致；我們想知道相分離是否也跟另一項因素有關：密度；故嘗試新製程來製作太陽電池，以驗證相分離是否也與P3HT與PCBM間的密度差異有關。

The thesis focuses on the research of “Vertical Phase Segregation”, which occurs in the active layer of polymer solar cells.
Typically, for conventional polymer solar cells which have bottom anode and top cathode, the active layer solution is composed of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) as electron donor and acceptor materials, respectively, and is fabricated on the patterned ITO substrate by spin coating method. Because of the difference of physical and chemical properties between P3HT and PCBM, as the blend film is fabricated, the “Phase Segregation” occurs. That is, the free (air) surface of the active layer is donor-enriched whereas the bottom of the active layer is acceptor-enriched.
The phase segregation is unfavorable for the solar cells with conventional structure. The accumulated P3HT and PCBM near the cathode and anode, respectively, are just like energy barriers for photo-induced electrons and holes. The phenomenon may cause the recombination of photo-induced charges, which reduces the photocurrent and further affect the power conversion efficiency.
The first topic of this thesis focuses on enhancing the photocurrent in polymer solar cells. Before thermally depositing metal cathode, PCBM is deposited onto the active layer in advance. After finishing the thermal evaporation process, the post annealing is performed to let the PCBM molecules diffuse into the active layer to mix with the accumulated P3HT on the top active layer. That way, the barrier between the active layer and cathode disappears, which increases the photocurrent as well as the power conversion efficiency.
Recently, A. Kumar et al have proposed that the phase segregation is related to the surface energy difference between P3HT and PCBM. We suspect that the phase segregation may be related to the density difference between the two materials, too. Therefore, in the second topic, we adopt a novel process to fabricate polymer solar cells in order to demonstrate whether the phase segregation is related to the density difference or not.

Chapter 1:
General Introduction-------------------------------------01
1.1 Background of Solar Cells------------------------------------------------------------------01
1.2 The History of Solar Cells-------------------------------------------------------------------02
1.3 The Classification of Solar Cells-----------------------------------------------------------03
1.4 Basic Principles of Solar Cells--------------------------------------------------------------04
1.5 Definition of Basic Parameters of Solar Cells--------------------------------------------04

Chapter 2:
Polymer Solar Cells--------------------------------------------------------------09
2.1 Overview---------------------------------------------------------------------------------------09
2.2 Working Mechanisms of Polymer Solar Cells--------------------------------------------10
2.3 Differences between Inorganic and Organic Semiconductors--------------------------11
2.4 Development of Polymer Solar Cells------------------------------------------------------12
2.5 Formation of the Short Circuit Current----------------------------------------------------13
2.6 Origins of the Open Circuit Voltage--------------------------------------------------------15
2.7 Charge Transfer Complex (CTC) States---------------------------------------------------19

Chapter 3:
Backgrounds and Motivation--------------------------------------------------23
3.1 Introduction to Vertical Phase Segregation------------------------------------------------23
3.2 Prior Arts to Overcome the Phase Segregation-------------------------------------------27
3.3 Experimental Motivation--------------------------------------------------------------------30
Chapter 4:
Experimental Setups-------------------------------------------------------------34
4.1 Substrate and Solution Preparation---------------------------------------------------------34
4.1.1 Substrate Preparation-------------------------------------------------------------------34
4.1.2 “Doped” PEDOT: PSS Solution Preparation----------------------------------------34
4.1.3 Active Layer Solution (P3HT: PCBM Solution) Preparation---------------------35
4.2 Equipments------------------------------------------------------------------------------------37
4.2.1 Vacuum Thermal Evaporator (Thermal Evaporation Chamber) ------------------37
4.2.2 Super Solar Simulator------------------------------------------------------------------39
4.2.3 Ultraviolet and X-ray Photoelectron Spectroscopy---------------------------------40
4.3 Experimental Process------------------------------------------------------------------------44
4.3.1 Standard Device Fabrication-----------------------------------------------------------44
4.3.2 Proposed Device Fabrication----------------------------------------------------------46

Chapter 5:
Results and Discussion----------------------------------------------------------50
5.1 Data Statistics---------------------------------------------------------------------------------50
5.2 Results and Discussion-----------------------------------------------------------------------51
5.2.1 Analysis of Experimental Results-----------------------------------------------------51
5.2.2 UPS Spectra and AFM Images--------------------------------------------------------52
5.3 Comparison of Proposed Devices with Different Structures---------------------------62
5.3.1 Comparison of Several Approaches---------------------------------------------------62
5.3.2 Experimental Processes, Device Performance, and Properties Comparison between Ours and A. Kumar’s Proposed Devices-----------------------------------------63
5.4 Summary---------------------------------------------------------------------------------------66
Chapter 6:
Is Phase Segregation Related to the Density Difference between P3HT and PCBM-------------------------------------------------------------------------68
6.1 Introduction of Phase Segregation---------------------------------------------------------68
6.2 The First Novel Process----------------------------------------------------------------------69
6.2.1 Proposed Device Fabrication----------------------------------------------------------69
6.2.2 Reference Device Fabrication---------------------------------------------------------69
6.2.3 Result and Discussion------------------------------------------------------------------70
6.3 The Second Novel Process------------------------------------------------------------------72
6.3.1 Proposed Device Fabrication----------------------------------------------------------72
6.3.2 UPS and XPS Spectra of the Surface of P3HT: PCBM Layer--------------------72
6.3.3 Result and Discussion------------------------------------------------------------------72
6.4 Summary---------------------------------------------------------------------------------------77

Chapter 7:
Conclusion and Future Work--------------------------------------------------78
7.1 Conclusion-------------------------------------------------------------------------------------78
7.1.1 The First Topic---------------------------------------------------------------------------78
7.1.2 The Second Topic-----------------------------------------------------------------------79
7.2 Future Work-----------------------------------------------------------------------------------80

Ch1 第 8頁
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Ch2 第21頁
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Ch3 第32頁
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Ch4 第48頁
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5.Q. Wei, T. Nishizawa, K. Tajima, and K. Hashimoto, “Self-organized buffer layers in organic solar cells”, Adv. Funct. Mater. 20, 2211 (2008)
6.(Thesis) The Effects of C60 in Organic Light Emitting Diodes, Shu-han Hsu
7.(Thesis) Temperature Dependence of Current-Voltage Characteristics of Organic Devices & Properties of Dipotassium Phthalate (K2Ph) as Electron Injection Layers in Organic Light Emitting Devices, Jung-Hung Chang
8.張正華、李陵嵐、葉楚平、楊平華 編著，馬振基 校訂；有機與塑膠太陽能電池 (五南，2008)
9.(Dissertaeion) Investigation on N-type Doping Effect on Electronic Structures and Interfacial Chemical Reactions in Organic Light-Emitting Devices, Mei-Hsin Cnen
10.H. Kim, W. W. So, and S. J. Moon, “The importance of post-annealing process in the device performance of poly(3-hexylthiophene): Methanofullerene polymer solar cell”, Sol. Energy Mater. Sol. Cells 91, 581 (2007)
11.A. Kumar, G. Li, Z. Hong, and Y. Yang, “High efficiency polymer solar cells with vertically modulated nanoscale morphology”, Nanotechnology 20, 165202 (2009)

Ch5 第67頁
1.Z. Xu, L. M. Chen, G. Yang, C. H. Huang, J. Hou, Y. Wu, G. Li, C. S. Hsu, and Y. Yang, “Vertical phase separation in poly(3-hexylthiophene): fullerene derivative blends and its advantages for inverted structure solar cells”, Adv. Funct. Mater. 19, 1227 (2009)
2.C. W. Liang, W. F. Su, and L. Wang, “Enhancing the photocurrent in poly (3-hexylthiophene)/ [6,6]-phenyl C61 butyric acid methyl ester bulk heterojunction solar cells by using poly (3-hexylthiophene) as a buffer layer”, Appl. Phys. Lett. 95, 133303 (2009)
3.A. Kumar, G. Li, Z. Hong, and Y. Yang, “High efficiency polymer solar cells with vertically modulated nanoscale morphology”, Nanotechnology 20, 165202 (2009)
4.Q. Wei, T. Nishizawa, K. Tajima, and K. Hashimoto, “Self-organized buffer layers in organic solar cells”, Adv. Funct. Mater. 20, 2211 (2008)

Ch6 第77頁
1.Z. Xu, L. M. Chen, G. Yang, C. H. Huang, J. Hou, Y. Wu, G. Li, C. S. Hsu, and Y. Yang, “Vertical phase separation in poly(3-hexylthiophene): fullerene derivative blends and its advantages for inverted structure solar cells”, Adv. Funct. Mater. 19, 1227 (2009)