When energy transfer kills free charge generation

Energy transfer between materials in organic photovoltaics often assists energy transport to the site of free charge generation. Here we present a case where the opposite is true: dilute donor molecules sensitizing a fullerene matrix. We show via a combination of time-resolved microwave conductivity (TRMC), femtosecond transient absorption (fsTA), and photoluminescence excitation (PLE) spectroscopy that fast energy transfer from the donor to the acceptor ultimately results in charge transfer, but not photoconductivity. Instead, the excited states are lost as tightly bound charge-transfer states that do not subsequently dissociate to from free charge in this system. This behavior is caused by an asymmetry in the entropy associated with charge transfer in each direction and is well described by a model in which free charge generation is governed by a combination of entropic gain and competition between multiple Marcus-like charge transfer events to a distribution of distances.