電子施體/受體共軛有機材料於異質接面太陽能電池元件之應用

Abstract

隨著能源危機的時代漸漸來臨，太陽能電池的發展也逐漸受重視。除了目前廣泛被使用的矽晶圓太陽能電池及無機薄膜太陽能電池技術外，有機太陽能電池因為具有低成本、耐衝擊、高產出及方便攜帶等優點，已成為新世代發展攜帶型電子產品的重點發展技術。本論文共分五部分主要探討由低成本的濕式製程以材料與製程的角度針對太陽能電池元件的效率進行討論。 本文的第一部分針對高分子太陽能電池的電洞傳輸層進行改質，藉由改變單體的化學結構並搭配電化學法聚合多孔性電洞傳輸材料，利用多孔的結構增加主動層與電洞傳輸層之間的接觸面積以達到平衡電性的效果，目前光電轉換效率可達3.57% (AM 1.5G) 。另一方面，也利用溶劑的處理來增加電洞傳輸層的導電性，發現經由多醇類溶劑的處理以後，PEDOT高分子鏈段的構型可由糾纏的結構轉變為較延伸的狀態，如此可幫助電荷的傳導。將此高導電度PEDOT應用於高分子太陽能電池，發現可提升元件之光電流，目前於最適化以後之電池效率於100 mW/cm2 (AM 1.5G)光強度下可達4.30%。 第二部分本研究利用中研院化學所林建村教授實驗室開發之有機小分子材料(TQTFA)搭配RR-P3HT，探討光譜互補式之高分子太陽能電池之行為。發現TQTFA除了光譜與RR-P3HT互補之外具，也擁有雙性電荷傳導的能力，因此可以有效地提升元件之光電流。經由製程最適化以後，發現TQTFA的最佳添加量為0.5%並且效率可達4.50%。 第三部分本研究在不破壞元件的前提之下，利用共具焦顯微儀搭配時間解析之螢光光譜系統來觀察薄膜內部之相之分離以及激子生命週期的分佈，進而重建出薄膜內之三維形貌。探討三維形貌對於元件表現之影響，研究結果顯示利用薄膜慢乾製程可以成功地獲得連續相分之傳導通路，然後在快乾的製程之下，薄膜內部材料混合均勻。搭配元件效率的分析發現在有傳導通路的形貌底下，由於電荷傳導的效果較佳因此有較高的光電流，而均勻混合的形貌卻容易產生電荷的再結合，因而降低的元件的效能。目前在慢乾製程之下所得之元件效率可達3.67%。此分析未來將可提供不同於TEM、SEM、AFM等分析微結構技術，而是針對薄膜之光物理特性進行分析。 本文的第四部分將以製程與材料的觀點討論polyfluorene系列的共聚合共軛高分子於多接面高分子太陽能電池之應用。嘗試利用polyfluorene-based copolymer的窄能隙特性來提高元件的電壓。研究發現，利用PC[70]BM相較於PC[60]BM較不對稱的結構所產生的共振性，可以有效地提升元件對可見光的吸收範圍，進而提升元件的光電轉換效率，在經過製程的最適化以後，目前最高效率可達2.0% 。另一方面，研究結果顯示以polyfluorene為主體之共軛高分子於高溫退火之後的行為非常不同於以polythiophene為主體之材料，由於其熱液晶相的產生使其表面形貌產生規則的排列，雖然規則的排列可以有效地提升材料的電洞遷移率，但在此狀況下也同時產生嚴重的相分離，使得電子與電洞產生劇烈的再結合。實驗結果顯示F8T2的最佳退火條件為70 oC。是針對薄膜之光物理特性進行分析。 本文的第五部分為開發一新式的轉印製程製作有機太陽能電池元件，此製程不僅無污染、低溫、非破壞性並且可以整合於濕式製程之中。目前本研究已成功利用此新式轉印技術製作多接面有機太陽能電池以及反式之雙層有機太陽能電池，效率可分別達到3.20%和2.83%。未來此製程亦可製作疊層元件以及其他有機光電元件。

Over the past two decades, satisfying the world’s growing demand for energy is one of the most significant challenges facing society. Therefore, the development of solar energy is viewed as an ideal technology for power generation because it is clean and renewable. Although the photovoltaic (PV) technology platforms of silicon-based PV and thin-film PV are now undergoing a rapid expansion in production, the next generation PV—organic solar cells —could soon be playing a major role with the advantages of ultralow production costs, rugged and lightweight. The main purpose of this thesis is to fabricate PV cells via an all-solution-process and investigate the influences of materials and fabrication parameters on the device performance. In the first part of this thesis (Chapter 3 and 4), we prepared nanofiber shaped hole collection layer with highly porous structure by changing the structure of EDOT monomer. The highly porous hole collection layer prepared from electrochemical deposition can offer a great deal of interface between the hole collection layer and active layer leading to a more balanced charge mobility. The power efficiency of the device fabricated with porous hole collection layer can achieve 3.57% so far. Furthermore, we also enhance the conductivity of the hole collection layer (PEDOT) by treating the PEDOT with some polyalchols. From the results, it revealed that the conformation of PEDOT can be changed from coiled structure to linear structure after the treatment leading to a higher conductivity. The highly conductive PEDOT was also applied to fabricate PV cells and the power efficiency is about 4.30%. In the second part (Chapter 5), a novel solution-processed small molecule (DFTh-TP) for use in electron donor has been incorporated into the organic solar cells based on P3HT and PC[70]BM. The combination of DFTh-TP with P3HT and PC[70]BM allows not only a broad absorption but also tuning the inter energy level leading to a higher JSC and VOC. The best performing devices exhibited a power conversion efficiency of 4.50 %. The efficiency is increased of almost 15 % compared with the one without incorporating DFTh-TP. In the third part (Chapter 6), we performed a comprehensive analysis of the 2D nanoscale morphology related to the exciton lifetime by combining confocal optical microscopy with a fluorescence module. The results revealed that the film prepared through rapidly grown process leads to an extremely homogeneous blend. The homogeneous phase cannot offer a continuous pathway for charge transport leading to a serious recombination. In the case of slowly grown film, although not all of these pathways may have been ideal, due to the presence of some terminated channels, this system still offered several connected pathways, leading to an interdigitated nanostructure that was responsible for efficient charge transport and the superior value of JSC. This approach provides much fundamental information that is unavailable when using conventional microscopy techniques in the future. In the fourth part (Chapter7~9), we have fabricated organic photovoltaic devices with blends of F8T2 and fullerene as an electron donor and electron acceptor, respectively. A significant improvement of the photovoltaic efficiency was found in device by using PC[70]BM as active material with complementary spectra. Moreover, we also study the effects of nanomorphological chnages on polymer PV devices with blends of F8T2 and PC[60]BM. The morphological changes of blended films were observed upon thermal annealing temperature near and above glass transition temperature (130 oC). Such microstructural transformations resulted in modified charge transport pathways and therefore grately influenced the device performance. The highest PCE of 2.14 % with an VOC of 0.99 V and a JSC of 4.24 mA/cm2 was achieved by device annealing at 70 oC for 20 min. In the final part (Chapter 10), we modified the printing method by increasing the affinity of PDMS for organic solvent via non-destructive solvent treatment. This stamping method eliminates the necessity of any plasma treatment and any possible damages on the PDMS surface and would give full control over the chemical composition and film thickness of each layer. The multilayer polymer structure also demonstrated for photovoltaic applications.