Sifting α,ω-di(thiophen-2-yl)alkanes as solvent additives to boost the photovoltaic performance of the PTB7-Th:PC71BM blend

Abstract

With a higher boiling point and much improved solubility for PC₇₁BM, α,ω-di(thiophen-2-yl)alkanes (butane, hexane, octane and decane) were applied as processing solvent additives to fabricate PTB7-Th:PC₇₁BM bulk heterojunction (BHJ) polymer solar cells (PSCs). A large gain in photocurrent was achieved for the devices processed with these α,ω-di(thiophen-2-yl)alkanes as additives. The alkyl chain length of the α,ω-di(thiophen-2-yl)alkanes was found to play an important role in regulating the molecular packing, orientation and crystallinity of crystals within the PTB7-Th:PC₇₁BM blend. Increasing the alkyl chain length from butyl to hexyl and to octyl improved the photovoltaic performance and ordering of film microstructures, whereas the decyl chain based additive led to deteriorated performance and ordering. Among all studied additives, 1,8-di(thiophen-2-yl)octane results in the most optimized PTB7-Th:PC₇₁BM blend with more uniform polymer crystals with better crystallinity and improved stacking of PC₇₁BM, thereby leading to the most enhanced power conversion efficiency of 9.87% when compared with the reference device (8.65%) processed with 1,8-diiodooctane (DIO). In addition, the device processed with 1,8-di(thiophen-2-yl)octane showed improved stability. These results demonstrate the potential of 1,8-di(thiophen-2-yl)octane as a new solvent additive to realize high-performance PSCs.