Effect of fullerene substituent on thermal robustness in polymer:fullerene bulk heterojunction solar cells

1,2,4-trimethylbenzene is a halogen-free solvent which dissolves unmodified and modified fullerenes and conjugated polymers. The unique feature enables the study of how the substituent of fullerene affects thermal robustness in bulk heterojunction solar cells using a conjugated polymer PTB7-Th. While 175 degrees C is the best annealing temperature for a solar cell with unmodified C-70, it deteriorates a device with a substituted C70 (C-70-PCBM). Additionally, annealing at 175 degrees C does not change the surface of PTB7-Th:C-70 film but makes the surface of PTB7-Th:C-70-PCBM film bumpy. The results suggest that the substituent promotes the migration of fullerene in a polymer:fullerene solid composite. (C) 2019 The Japan Society of Applied Physics