  The formation of bound electron-hole pairs, also called charge-transfer (CT)
states, in organic-based photovoltaic devices is one of the dominant loss
mechanisms hindering performance. While CT state dynamics following electron
transfer from donor to acceptor have been widely studied, there is not much
known about the dynamics of bound CT states produced by hole transfer from the
acceptor to the donor. In this letter, we compare the dynamics of CT states
formed in the different charge-transfer pathways in a range of model systems.
We show that the nature and dynamics of the generated CT states are similar in
the case of electron and hole transfer. However the yield of bound and free
charges is observed to be strongly dependent on the HOMOD-HOMOA and LUMOD-LUMOA
energy differences of the material system. We propose a qualitative model in
which the effects of static disorder and sampling of states during the
relaxation determine the probability of accessing CT states favourable for
charge separation.
