Hot Charge-Transfer States Determine Exciton Dissociation in the DTDCTB/C60 Complex for Organic Solar Cells: A Theoretical Insight

To understand charge-transfer (CT) processes at the donor/acceptor interface of DTDCTB/fullerene solar cells, we have investigated the electronic couplings and the rates for exciton-dissociation and charge-recombination processes based on two representative intermolecular geometries of the DTDCTB/C-60 complex by means of quantum-chemical calculations. Consistent with the experimental measurements of the time scale of over subns or even ns for charge recombination (CR), the calculated CR rates are lower than 10(10) s(-1) and in most cases, below 10(9) s(-1). The calculated rates for exciton dissociation into the CT ground state are mostly lower than 10(10) s(-1), which is, however, in sharp contrast with the ultrafast charge separation (similar to 100 fs) observed experimentally. Interestingly, our calculations point out that excitons are able to dissociate into a higher-energy excited CT state much faster, with the rates being as large as about 10(12) and 10(14) s(-1) in all cases for excitons based on C-60 and DTDCTB, respectively. Thus, exciton dissociation in the DTDCTB/C-60 complex is determined by the hot CT states. As the excess energy of the excited CT state can facilitate the geminate electron and hole to further separate at the donor/acceptor interface, our theoretical results suggest that the high performance of the DTDCTB/fullerene-based solar cell can be mainly attributed to the fact that excitons dissociate via the hot CT states to effectively form mobile charge carriers.