高分子太陽能電池之形態控制與結構設計對於元件效 率之影響探討

Abstract

本論文分成五個研究主題，第一個研究主題：我們利用Bingel-Hirsch reaction合成一系列苯環對位位置帶有不同取代基結構的碳六十衍生物，作為光伏打作用層內部之電子受體，衍生物分別為B-C60、MB-C60、EB-C60、nBB-C60、iPB-C60以及tBB-C60。碳六十衍生物官能基的設計影響著碳六十與聚己基噻吩之相容性，隨著苯環上的烷基碳數增加，碳六十衍生物上的苯環官能基共平面特性開始增加，與碳球間的π-π作用力也跟著增加，亦使得碳六十衍生物容易傾向以規則性堆疊的方式排列，導致MB-C60、EB-C60形成結晶形態的聚集，倘若苯環接上更為立體對稱的烷基結構，將使得碳六十衍生物溶解度增加，並破壞苯環官能基與碳球的π-π吸引力，擴大苯環與碳球間的吸引距離，最終碳六十衍生物彼此堆疊更為鬆散，故能與聚己基噻吩產生更好的相容性與分佈形態，因此iPB-C60、tBB-C60能均勻分散於聚己基噻吩薄膜之中。我們利用UV-vis、PL、TEM、XRD來了解電子施體與電子受體之薄膜於熱退火前後，聚己基噻吩結晶形態以及碳六十衍生物聚集行為之變化。另外針對電子與電洞遷移特性做進一步的分析，結果顯示兩物質的相容特性、異質接觸面積大小以及電子與電洞遷移率之平衡和元件所呈現出的效能有著重要的關係。光伏元件結構為ITO/ PEDOT:PSS/ P3HT:Alkyl-C60/ Ca/ Al。一系列的碳六十衍生物中以tBB-C60作為電子施體的元件有著最大的電流密度Jsc~10.03 mA/cm2，而光電轉換效率提升至3.59%。 第二個研究主題：我們證明出藉由添加少量的bis-PCBM (PCBM合成過程中所產生的副產物)置換，即能夠有效改善高溫下光作用層中PCBM分子間嚴重聚集的行為，並降低在P3HT混合體中PCBM群聚尺寸過度擴大的現象。此外亦可增加兩材料的接觸面積和太陽能元件的短路電流(Jsc)，只要8.3 wt% bis-PCBM的置換添加即可使Jsc與FF提升17%，進一步得到更好的光電轉換效率，提供光作動層混合材料最穩定的形態，而P3HT/PCBM:bisPCBM高分子太陽能元件更可在150℃下加熱15小時，異質接面形態皆無明顯變化，卻仍保有穩定的光電轉換效率。 第三個研究主題目：本實驗我們使用常見的poly(3-hexylthiophene)和PCBM當作施體和受體材料，另外，我們同時採用GISAXS和GIWAXS對剛塗佈完未經熱退火、經熱退火形成BHJ結構的製程(150℃加熱5分鐘)和長時間加熱(150℃加熱360分鐘)，這三種不同製程的P3HT/PCBM混參薄膜樣品進行分析。針對加熱過後BHJ結構中的高分子的結晶性與富勒烯聚集行為，進行前所未有的分析研究。於奈米尺度(~10 nm)、中尺度(10-100 nm)以及微米尺度(~μm)下，做形態的動態發展及結構的統計分析。並對於獨立的相分離區域做不同尺度的分析，也能將各層結構中不同相的構造、成長、變形以及交互作用的相關數據呈現出來。此外，我們亦將添加不同添加量的bis-PCBM於P3HT/PCBM混參薄膜中的案例，進行熱穩定性提升機制的探討。提供GISAXS和GIWAXS系統性的研究和定量調查：(1)了解bis-PCBM如何在P3HT/PCBM塊狀混摻薄膜中有效穩定薄膜的形態。(2)探討電子受體bis-PCBM與PCBM的兩種碳六十衍生物的差異性，進而與高分子施體交互作用，發展成不同的結構和相分離狀態。這樣系統性的研究提供人們深入了解，在高溫加熱下導電高分子/富勒烯形態不穩定的原因。 第四個研究主題：我們成功將常用於液晶高分子的摩擦配向技術，運用於塊狀異質接面有機高分子太陽能電池的製造。藉由不同的摩擦配向方法會使得表面產生不同的表面粗糙度，進而影響高分子的結構、形態和光學性質。因此，這些週期性的溝槽結構就能作為有效的光捕獲裝置，應用於P3HT：PCBM塊狀異質接面有機高分子太陽能電池，來提升其光電轉換效率。這種不用藉由黃光製程就能使PEDOT：PSS產生週期性奈米溝層的技術，我們稱之為摩擦配向技術。利用這些週期性的凹槽結構，來製造入射光於光作用層與電洞傳導層間的散射現象，延長光在內部行走的距離，大幅製造光電高分子吸收光的機會。此外，藉由摩擦配向所建構的PEDOT：PSS週期性溝槽，增大了PEDOT：PSS材料與光作用層之接觸面積，有助於電洞載子的傳遞與蒐集。更重要的是，我們不需要對所使用的PEDOT：PSS做進一步的調整或是加入額外的添加劑。由簡單且快速的摩擦配向方法，在有機高分子太陽能中建構一層具有週期性凹槽結構的PEDOT：PSS，將有助於其太陽能電池效率的提升。由這些具有摩擦配向PEDOT：PSS緩衝層的太陽能電池之相關數據與分析結果，證實此新興的製程技術的確適用於有機高分子太陽能未來的生產製造。 第五個研究主題：此處我們將末端接有-CF3官能基的矽烷偶合劑，藉由奈米轉印技術使其在ITO表面形成自組裝多層膜達到改質和修飾的目的。接續探討研究這些製程對光作用層形態、ITO電極功函數以及OPV效能之影響。在塊狀異質接面太陽能電池中，我們選擇使用P3HT：PCBM系統。藉由表面能和ITO功函數的調整使得光作用層形態產生改變，另外更降低了電洞注入時的能障。將改質修飾完成之ITO基板進行接觸角的量測，紫外光電子能譜(ultraviolet photoemission spectroscopy, UPS)，紫外光吸收光譜以及原子力顯微鏡等觀察。從以上結果得知，修飾過的ITO功函數較靠近P3HT的HOMO，也就是達到了降低電洞注入能障的效果。藉由帶有-CF3官能基的矽烷偶合劑材料進行改質後，其太陽能電池元件之短路電流(Jsc)以及填充因子(F.F.)均有所提升，由此可證明帶有-CF3官能基的材料改善了電洞注入ITO的效能，並且提高了光電轉換效率，等同在元件中嵌入一層類似PEDOT：PSS的緩衝層。此奈米轉印自組裝多層膜技術不僅能夠有效提供陽極功函數的調整，更能適用於可繞曲式高分子太陽能電池大面積的生產。

This thesis consists of five topics concerning polymer photovoltaics. In the first part, in this study, a series of novel soluble fullerene derivatives with different alkyl substitutions, including B-C60、MB-C60、EB-C60、nBB-C60、iPB-C60 and tBB-C60, were synthesized from Bingel-Hirsch reaction and employed as acceptor to fabricate polymer solar cells. The compatibility between these C60 derivatives and poly(3-hexylthiophene) (P3HT) increased with the increase of extent branches number of alkyl substitutions. In these molecules, both MB-C60 and EB-C60 tend to crystallize whereas iPB-C60 and tBB-C60 can homogeneously distribute inside polymer matrix upon the drying of their blend solutions with P3HT. UV-vis, PL, TEM and XRD were used to characterize the blending films after annealing. Besides, the mobility of electrons and holes were measured to analyze the photoelectric properties of the films. The results clearly indicate that both interfacial properties of two phases and mobility of electrons and holes play an important role in the performance of devices. The cells were fabricated with the structure of ITO/ PEDOT:PSS/ P3HT:Alkyl-C60/ Ca/ Al. The device based-on tBB-C60 exhibited the highest current density of 10.03 mA/cm2 and the best energy conversion efficiency of 3.59%. In the second part, we demonstrates that the bis-adduct of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) is an effective inhibitor of the aggregation of PCBM inside the poly(3-hexylthiophene) (P3HT) matrix. Substituting some of the PCBM with bis-PCBM apparently reduces the size of PCBM-rich clusters, enhancing both the short-circuit current density (JSC) and the fill factor (FF), leading to a 17% increment in power conversion efficiency (PCE) for a cell with 8.3 wt% bis-PCBM replacement. More importantly, a tiny amount of bis-PCBM significantly improves the morphological stability of P3HT/PCBM blend against high-temperature aging. All P3HT/PCBM:bis-PCBM devices exhibit extremely stable PCEs, which do not visibly change upon heating at 150 ℃for 15 hours. In the third part, we simultaneously employed grazing incidence small-angle and wide-angle X-ray scattering (GISAXS and GIWAXS) techniques to quantitatively study the structural evolution and kinetic behavior of poly(3-hexylthiophene) (P3HT) crystallization, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) aggregation and amorphous P3HT/PCBM domains from a bulk heterojunction (BHJ) to a thermally unstable structure. The independent phase separation regimes on the nanoscale (~10 nm), mesoscale (~100 nm) and macroscale (~μm) are revealed for the first time. Bis-PCBM molecules as inhibitors incorporated into the P3HT/PCBM blend films were adopted as a case study of a control strategy for improving the thermal stability of P3HT/PCBM solar cell. The detailed information on the formation, growth, transformation and mutual interaction between different phases during the hierarchical structural evolution of P3HT/PCBM:xbis-PCBM (x＝8~100%) blend films are presented herein. This systematic study proposes the mechanisms of thermal instability for a polymer/fullerene-based solar cell. We demonstrate a new fundamental concept that the structural evolution and thermal stability of mesoscale amorphous P3HT/PCBM domains during heating are the origin of controlling thermal instability rather than those of nanoscale thermally stable BHJ structures. It leads to a low-cost and easyfabrication control strategy for effectively tailoring the hierarchical morphology against thermal instability from molecular to macro scales. The optimum treatment achieving high thermal stability, control of mesoscale domains, can be effectively designed. It is independent of the original BHJ nanostructure design of a polymer/fullerene-based solar cell with high performance. It advances the general knowledge on the thermal instability directly arising from the nanoscale structure. In the forth part, Organic Photovoltaic (OPV) cells represent a compelling candidate for renewable energy by solar energy conversion. In recent years, versatile light-trapping measures via structures have been intensively explored to optimize photovoltaic performance. In this work, a unique rubbing technique is demonstrated to create nanoscale grooves on the PEDOT:PSS surface and the grating-like features are 500 nm wide and 10 nm deep. The PEDOT:PSS film with grooved surface is used as buffer layers for OPV cell devices based on a P3HT:PCBM bulk heterojunction. The patterned surface has a profound effect on carrier mobility, light trapping, and hole collection efficiency, leading to an increase in the short circuit density, filling factor, and power conversion efficiency. These results indicate the feasibility of the rubbing method can be applicable to high efficiency OPV cells. In the last part, optimized performances of polymer solar cells has been of magnificent interest in recent years. A variety of approaches have been reported to alter or replace the polymer buffer layers in solar device structures. In this present work, surface modification of indium tin oxide (ITO) coated substrates through the use of self-assembled multilayers by the soft-imprinting method has been applied to adjust the anode work function and device performance in polymer solar cells based on a P3HT:PCBM heterojunction. The efficiency and morphology of the solar device with CF3-terminal group materials as a buffer layer have been measured and investigated. These results demonstrate that the soft-imprinting method is an effective and rapid procedure that enhances the quality of polymer solar cells and indicates potential implications for other organic devices containing an interface between a blended organic active layer and an electrode layer.