Influence of Photon Excess Energy on Charge Carrier Dynamics in a Polymer-Fullerene Solar Cell

An important question in research on polymer solar cells is the mechanism of free charge carrier generation. In organic bulk heterojunction solar cells, photoinduced electron transfer between the donor and acceptor molecules leads to population of a charge transfer (CT) state at the donor-acceptor interface. The residual electrostatic binding energy between electron and hole in the CT state may be considerable; experimental studies indicate a binding energy of 0.1-0.2 eV which is much higher than the thermal energy ( k b T = 0.025 eV). [ 1-4 ] Given the low dielectric constant of organic materials ( ε r = 3-4) charges need to be separated over a large distance (10-15 nm) before their electrostatic interaction energy becomes comparable to k b T . Even though the lifetime of the CT state can be as short as only a few nanoseconds, [ 5 , 6 ] the effi ciency of free charge carrier generation can be very high. In optimized polymer solar cells, charge carrier generation occurs with an effi ciency close to unity. [ 7 , 8 ] The mechanism by which the large energy barrier for charge separation from the CT state is overcome within the short lifetime is not known. Several explanations have been proposed, but no consensus on this issue has yet been reached. [ 9 , 10 ] © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.