The driving force controls charge separation distance in organic donor/acceptor semiconductors

The importance of the Gibbs free energy at the donor/acceptor interface remains a highly debated topic in the study of photocurrent generation in organic semiconductors. Similarly, there is still no consensus as to how charge transfer states formed at the interface may separate into free carriers. Here, we show how these two ideas are connected. We employed photoinduced absorption detected magnetic resonance spectroscopy where the spectra are sensitive to the distances between electrons and holes. We report electron/hole pair distance distributions for a set of driving force-tunable dilute donor/acceptor systems and show that formation of more localized CT states at high driving forces can explain an observed Marcus “inverted regime” in free carrier generation. These results suggest that the driving force controls the distance of electron transfer at the interface, where in the optimal case for free carrier generation, charge transfer skips over the most localized CT states.