Charge Separation and Recombination of Charge-Transfer Excitons in Donor-Acceptor Polymer Solar Cells

The generation and recombination of photocarriers in a series of donor-acceptor polymers were examined with a focus on the two unique absorption bands that originate from low-energy charge-transfer (CT) excitation and high-energy main-chain (MC) excitation, respectively. The activation energies for the generation of photocurrents were different for CT and MC excitations in pristine polymer films, whereas they were identical to one another in blended films containing a fullerene derivative. The results show that the photocarriers are generated directly by MC excitation, whereas they are generated indirectly by CT excitation through the formation of bound electron-hole pairs, the binding energies of which are correlated with their recombination lifetimes.