Direct observation of the ultrafast electron transfer process in a polymer/fullerene blend

Photoinduced electron transfer in organic molecules is an extensively investigated topic both because of fundamental interest in the photophysics and for applications to artificial photosynthesis. Highly efficient ultrafast electron transfer from photoexcited conjugated polymers to C60 has been reported, the back transfer is heavily hindered, thus providing an intrinsic stabilization mechanism of the photogenerated charges. Although an upper limit for the forward electron transfer time of 1 ps has been reported, its detailed time resolution is still missing and is highly needed to shed light on the photophysics of the charge transfer mechanism. We perform ultrafast experiments on conjugated polymer/C60 blends with sub-10-fs time resolution. We are able to time resolve for the first time the charge transfer process, obtaining a forward electron transfer time constant τct≈45 fs