添加劑對PC71BM結晶行為與高分子太陽能電池效率之影響

Abstract

本研究使用PTB7-Th與PC71BM混摻合適比例之各式添加劑1,8-二碘辛烷(DIO)、苄醚(BE)、1,8-二溴辛烷(DBO)、1,8-辛二硫醇(ODT)、二苯醚(DPE)、1,4-丁二硫醇(BT)、1-氯萘(CN)，製作反式結構的太陽能電池。並藉由原子力顯微鏡(AFM)、光學顯微鏡(OM)、紫外/可見光吸收光譜儀(UV-Vis)與穿透式X光散射實驗分析PTB7-Th/PC71BM系統的形貌，以探討相異添加劑對PTB7-Th/PC71BM系統的形貌與太陽能電池效率的影響。透過原子力顯微鏡，摻入添加劑的PTB7-Th/PC71BM系統會出現觀察至PTB7-Th富相聚集結構(domain)，進而發現此有序結構可有效提升元件效率。本研究引用比爾-朗伯定律(Beer-Lambert law)建立測量材料溶解度之方法，根據主動層材料與添加劑溶解度差異，推論在旋轉塗佈的過程中，因主體溶劑揮發而析出的高分子PTB7-Th會先形成預排列狀態，導致PC71BM由PTB7-Th層與層間排至PTB7-Th不定形區。然而當主體溶劑近乎揮發時，PTB7-Th則由預排列狀態形成PTB7-Th富相聚集結構，此外PC71BM亦會受到添加劑劑量之影響，隨著添加劑量減少而逐漸析出進行排列，造成PC71BM的延遲排列現象。本研究發現添加劑溶解度將主導PTB7-Th與PC71BM在旋轉塗佈時的排列方式，亦為摻入添加劑的PTB7-Th/PC71BM系統效率變化的主因之一。並且根據溶解度測試結果，建構出摻入添加劑之半結晶性高分子與富勒烯系統的旋轉塗佈形貌模型。 藉由穿透式X光散射實驗，觀察到PC71BM因八碳長烷基鏈添加劑(DIO、ODT與DBO)摻入而形成PC71BM誘導共晶。該誘導共晶具有固定層距的散射訊號，以及使PC71BM自身結晶產生更為規整排列的特性。推論八碳長烷基鏈添加劑填入PC71BM的苯基丁酸甲酯間的空間，將使PC71BM的C70碳球排列變得更為緊密，而達到更穩定的狀態。除此之外，透過示差掃描量熱儀測得各八碳長烷基鏈添加劑所形成誘導共晶之熔點，以及形成最高結晶程度之誘導共晶中所含添加劑莫耳比例。

In this study, PTB7-Th/PC71BM blends with different ratios of additives, including 1,8-diiodooctane(DIO), Benzyl ether(BE), 1,8-dibromooctane(DBO), 1,8-octanedithiol(ODT), Diphenyl ether(DPE), 1,4-butanedithiol(BT), 1-chloronaphthalene(CN) were fabricated into organic solar cells in inverted structure and the effects of the additives were investigated and compared. Atomic Force Microscope(AFM), Optical Microscope(OM), UV/Vis Absorption Microscope(UV-vis), and X-ray Scattering were used to correlate the morphology of PTB7-Th/PC71BM system with the efficiency of solar cells. According to the AFM results, PTB7-Th/PC71BM blends with the additives that can induce ~ 50 nm PTB7-Th-rich phase domains show higher efficiencies. We investigated the solubility of PTB7-Th and PC71BM in the additives following Beer-Lambert law and found that the additives that can improve the efficiency generally show a high solubility for PC71BM while a very low solubility for PTB7-Th. During solvent evaporation, PTB7-Th can thus precipitate first to form the 50 nm domains that facilitate hole transportation, followed by the aggregation of PC71BM that enhances the electron transportation. The aggregation and stacking of PTB7-Th and PC71BM controlled by the additives under the spin coating process is the key to affect the efficiency of solar cells. In addition, from the X-ray scattering and DSC measurements, we found that the additives with 8-carbon alkyl chains can induce an usual layered arrangement of PC71BM crystals. We suggest that such additives can insert into the space between the methyl phenyl butyrate of PC71BM and cause a closer packing of PC71BM. The additive-induced crystals may create new electrical properties for PC71BM.