As reported earlier, the photovoltaic performance of PCDTBT:PCBM polymer solar cells drastically decreases upon thermal annealing. It was demonstrated in the literature, that thermal annealing leads to increased trap formation and as a consequence disturb solar cell performance, especially via a reduced fill factor. This has been demonstrated by space-charge-limited-current analysis and ellipsometry, as well as, structural changes analyse of PCDTBT upon annealing. However, we decided in addition to investigate morphological changes occurring within PCDTBT:PCBM photoactive blend layers upon thermal annealing, as these must have an impact on charge transport. By application of several characterizations techniques, and especially supported by results of Impedance Spectroscopy and Auger Electron Spectroscopy (AES), indeed the existence of an unfavourable compositional gradient within the photoactive layer could be revealed. This compositional gradient may be in part accounted for harming the transport of electrons and holes in either direction.
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.