Morphology evolution of spin-coated films of poly(thiophene-phenylene- thiophene) and [6,6]-Phenyl-C₇₁-butyric acid methyl ester by solvent effect

This paper describes the influence of the solvent on the morphological evolution and performance of polymer solar cells (PSCs) based on blended films of poly(thiophene-phenylene-thiophene) and [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM). The blends are spin-coated with solvents exhibiting various evaporation rates, including o-dichlorobenzene (DCB), chlorobenzene (CB), chloroform (CF), and tetralin. The changing morphologies of these blended films are monitored using atomic force microscopy (AFM) and transmission electron microscopy (TEM). A solvent having a higher boiling point [1,8-octanedithiol (OT)] is also introduced as an additive to further fine-tune the morphology of the bulk heterojunction (BHJ). We demonstrate herein that the morphology of a blend-and, hence, the performance of a BHJ device - can be manipulated by controlling the rate of solvent evaporation during film formation. The resulted fine-scale phase separation leads to enhanced performance of such organic photovoltaic devices. The highest power efficiency for our PSCs (5.8%, AM 1.5G irradiation (100 mW/cm²)) resulted when we use DCB as the solvent with OT as a processing additive.