A comparative theoretical study of exciton-dissociation and charge-recombination processes in oligothiophene/fullerene and oligothiophene/perylenediimide complexes for organic solar cells

The exciton-dissociation and charge-recombination processes in donor-acceptor complexes found in alpha-sexithienyl/C-60 and alpha-sexithienyl/perylenetetracarboxydiimide (PDI) solar cells are investigated by means of quantum-chemical methods. The electronic couplings and exciton-dissociation and charge-recombination rates have been evaluated for various configurations of the complexes. The results suggest that the decay of the lowest charge-transfer state to the ground state in the PDI-based devices: (i) is faster than that in the fullerene-based devices and (ii) in most cases, can compete with the dissociation of the charge-transfer state into mobile charge carriers. This faster charge-recombination process is consistent with the lower performance observed experimentally for the devices using PDI derivatives as the acceptor.