Formation of relaxed charge-transfer excitons in donor-acceptor-type polymer solar cells

The formation of relaxed charge-transfer (CT) excitons and its effect on photocarrier generation in a series of donor-acceptor (DA)-type polymers were examined with a focus on the two unique absorption bands that originate from low-energy charge-transfer (CT) and high-energy main-chain (MC) excitations, respectively. The photoluminescence in response to the CT excitation demonstrated Stokes shift, which is a clear evidence of the formation of bound CT excitons associated with a strong lattice relaxation. Results indicate that photocarriers are generated indirectly by CT excitation through the formation of bound electron-hole pairs, the binding energy and lifetime of which correlate with the donor-acceptor CT degree in the DA-type polymers. (C) 2014 The Japan Society of Applied Physics