Identifying the Nature of Charge Recombination in Organic Solar Cells from Charge-Transfer State Electroluminescence

Charge-transfer (CT) state electroluminescence is investigated in several polymer:fullerene bulk heterojunction solar cells. The ideality factor of the electroluminescence reveals that the CT emission in polymer:fullerene solar cells originates from free-carrier bimolecular recombination at the donor-acceptor interface, rather than a charge-trap-mediated process. The fingerprint of the presence of nonradiative trap-assisted recombination, a voltage-dependent CT electroluminescence quantum efficiency, is only observed for the P3HT:PCBM system, which is explained by a reduction of the competing bimolecular recombination rate. These results are in agreement with measurements of the illumination-intensity dependence of the open-circuit voltage.