PTB7-Th/PCBM混摻形態與太陽能電池效能的關係

Abstract

本研究使用不同分子量的PTB7-Th分別混摻PC61BM與PC71BM，來製作反式結構的有機太陽能電池，並利用原子力顯微鏡與掠角X光散射實驗來分析PTB7-Th/PCBM系統的形貌，以探討分子量和添加劑DIO對於PTB7-Th/PCBM系統的形貌與太陽能電池效率的影響。 利用原子力顯微鏡能觀察到PTB7-Th/PCBM系統會出現直徑為15-30 nm長條狀的結構，藉由分析掠角小角X光散射實驗所觀察到的PTB7-Th/PCBM系統隆起訊號，可得知長條狀的結構為PTB7-Th的聚集結構；低分子量的PTB7-Th會比較容易進行排列，故在低分子量系統中長條狀的結構會比較明顯。而添加劑DIO能進行溶劑退火來改善PTB7-Th/PCBM系統的形貌，使太陽能電池效率從5%增加到8.5%。DIO除了促使PTB7-Th長條狀結構的生成，我們還發現添加劑DIO能幫助PC61BM與PC71BM結晶，而PC71BM還會形成平行於基板的結晶結構。 我們利用熱退火實驗來分析PTB7-Th/PCBM系統的熱穩定性，發現PTB7-Th/PCBM系統的熱穩定性非常差。藉由擬合PTB7-Th/PCBM系統的小角X光散射剖線，可以得知PTB7-Th聚集大小並不會受到熱退火影響。而加熱會破壞由添加劑DIO所誘導的PCBM緊密聚集結構，這是加入DIO的PTB7-Th/PCBM系統經過熱退火處理後，其太陽能電池效率會大幅下降的主因之一。

In this study, we blended PTB7-Ths of different molecular weights with PC61BM or PC71BM for fabricating organic solar cells in inverted structure. We used the atomic force microscope (AFM) and grazing incidence X-ray scattering (GIXS) to investigate the effects of molecular weight and 1,8-diiodooctane (DIO) additive on the morphology and the solar cell performance of PTB7-Th/PCBM blends. We find long stripe structures of a diameter 15-30 nm in the AFM images of PTB7-Th/PCBM blends. By analyzing the GIXS data, these long stripes are attributed to the PTB7-Th aggregates. The long stripes are more pronounced in the blends of PTB7-Th of lower molecular weight due to a better packing of shorter chains. The addition of DIO improves the morphology of PTB7-Th/PCBM blends by solvent annealing and therefore the solar cell efficiency is increased from 5% to 8.5%. We found that the DIO additive facilitates the formation of the PTB7-Th stripe structures as well as the crystallization of PC61BM and PC71BM, which in turn contributes to the increase of the solar cell performance. The thermal stability of PTB7-Th/PCBM cells was studied. We found that the performance of the PTB7-Th/PCBM cells sharply decreases after short heating. X-ray scattering profiles reveal that the size of PTB7-Th aggregates is not significantly changed while the scattering intensity of the PCBM close packing induced by DIO is decreased after heating. The destruction of the compact PCBM aggregates upon heating is one of the reason responsible for the poor thermal stability of PTB7-Th/PCBM cells with DIO additive.