Impact of methanol top-casting or washing on the polymer solar cell performance

The performance of photovoltaic devices comprising of a donor-acceptor copolymer (benzothiadiazole-fluorenediketopyrrolopyrrole-BTD-F-DKPP) and phenyl-C60-butyric acid methyl ester (PCBM) has been investigated. The ascast devices performed with very poor power conversion efficiency. Upon application of an additional top-cast treatment by methanol, considerable increase in the device performance was observed. Indeed, using transient photocurrent we found improved charge extraction of the methanol treated devices. In order to check, whether this effect could be obtained by removing smaller molecular weight fractions from the copolymer, we washed out these components by fractionation with methanol. However, this treatment only resulted in minor improvements and thus cannot be assigned to be the major effect/cause.