Influence of bridging atom on the vertical phase separation of low band gap bulk heterojunction solar cells

Vertical phase separation of the polymer and fullerene molecules in bulk heterojunction organic solar cells influences the exciton dissociation, charge carrier transport and collection. This work compares the vertical phase separation of poly2,1,3‐benzothiadiazole‐4,7‐diyl4,4‐bis(2‐ethylhexyl)‐4H‐cyclopenta 2,1‐b:3,4‐b′]dithiophene‐2,6‐diyl]] (C‐PCPDTBT):6,6]‐phenyl C71 butyric acid methyl ester (PC₇₁BM) and poly2,1,3‐benzothiadiazole‐4,7‐diyl4,4‐bis(2‐ethylhexyl)‐4H‐cyclopenta 2,1‐b:3,4‐b′]dithiophene‐siloe2,6‐diyl]] (Si‐PCPDTBT):PC₇₁BM blend films, using X‐ray photoemission spectroscopy depth profiles. The difference between the two polymers is the bridging atom, which is carbon for C‐PCPDTBT and silicon for Si‐PCPDTBT. Si‐PCPDTBT exhibits enhanced polymer chain packing and crystallinity. We believe this enhanced chain packing provides a driving force during film drying which alters the vertical morphology. The different nature of vertical phase separation plays a role in determining the increased device performance observed for Si‐PCPDTBT:PC₇₁BM solar cells. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)