Theoretical prediction of the rate constants for exciton dissociation and charge recombination to a triplet state in pcpdtbt with different fullerene derivatives

The exciton dissociations and charge recombinations to a triplet state in the donor acceptor heterojunction solar cells of [2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta-[2,1-b;3,4-13]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) blended with ten different fullerene derivatives are theoretically investigated by using electronic structure calculations together with a Marcus formula. The detailed discussions of available accuracy in the evaluation of all quantities entering the rate expression (driving force, electronic coupling, and internal and external reorganization energies) are provided. The results reveal that the exciton dissociations in most blends are barrierless reactions because the corresponding values of driving forces and reorganization energies are very close; however, the recombinations from the charge transfer states to the triplet state of PCPDTBT occur in the Marcus normal regime. The predicted rates for both the exciton dissociation and charge recombination are in quite good agreement with experimental measurements. In addition, as the triplet charge transfer states are formed, their recombination rates become two orders larger than those for the singlet ones and have orders similar to the exciton dissociations. It is thus expected that the triplet charge recombinations are dominant channels, whereas the singlet charge recombinations can be safely neglected because of quite small rates compared to exciton dissociation ones.