Improving the Efficiency of Bulk Heterojunction Polymer Solar Cells Via Binary-Solvent Treatment

The effects of different binary-solvent treatments on the performance of polymer solar cells (PSCs) with thieno [3, 4-b] thiophene/benzodithiophene (PTB7):(6, 6)-phenyl-C ₇₁ -butyric acid methyl ester (PC ₇₁ BM) as active layers are systematically investigated. The binary-solvents are obtained by mixing a primary solvent of methanol (MeOH) with a secondary solvent comprising of 2-chlorophenol (2-CP), dimethylsulfoxide (DMSO), or o-xylene. By analyzing the variation of photovoltaic performance and the nanostructure morphology of active layers, we find that both polarity and boiling point of the secondary solvent are playing an important role in controlling the nanostructure morphology of PTB7:PC ₇₁ BM blends. In particular, 2-CP and DMSO with higher polarity and boiling point mixed with MeOH could significantly increase the power conversion efficiency from 6.47% (MeOH-treated device) to 7.81% and 7.19%, respectively. The improved photovoltaic performance of binary-solvent-treated PSCs is mainly due to the enhanced light absorption, optimized blend morphology, and increased charge carrier mobilities, resulting in efficient exciton generation, effective charge separation, and balanced charge transport. Moreover, the impedance spectroscopy results indicate that the binary-solvent treatment can achieve electrically ideal interfacial capacitance and, accordingly, suppress the bimolecular recombination in the blends.