Structure Formation in Low-Bandgap Polymer:Fullerene Solar Cell Blends in the Course of Solvent Evaporation

The drying process of the bulk heterojunction (BHJ) layer has a strong impact on the solar cell performance for the well-investigated material system P3HT:PC61BM. For higher performing low-bandgap polymers and C-71 fullerene derivatives, no comprehensive studies of the BHJ structure evolution during film drying are available. In this work we investigate the structure formation of the low-bandgap polymer poly{[4,40-bis(2-ethylhexyl)dithieno(3,2-b;20,30-d)silole]-2,6-diyl-alt-(2,1,3-benzothidiazole)-4,7-diyl} (PSBTBT) and [6,6]-phenyl C-71-butyric acid methyl ester (PC71BM) in the transition from wet to solid by in-situ grazing incidence Xray diffraction (GIXD) and laser reflectometry simultaneously. The nucleation and crystallization of PSBTBT differs from the interface-induced crystallization of P3HT and occurs partially in the solution. It is shown that PSBTBT:PC71BM blend nanomorphology and optoelectronic device properties are rather insensitive to the drying process in the investigated temperature range of 40-85 degrees C. This is beneficial for fast drying at elevated temperatures which enables high throughput fabrication of efficient organic photovoltaics.