UC emission excited by 750 nm LED irradiation was clearly observed at 0.94 V, which was the opencircuit condition (OC) of the device, as illustrated in Figure . It was steeply quenched under a smaller applied voltage than the OC. The reason for the quenching was that photogenerated charges were extracted to the electrode under a smaller voltage than the OC. The results demonstrated that the origin of the UC emission was photogenerated charge recombination at the D/A interface. The peak intensity of the UC emission was plotted in the same graph as the J-V curve of the rubrene/ITIC-Cl bilayer photovoltaic device under 1 sun irradiation, as illustrated in Figure . We observed a difference in the voltage dependence: the photocurrent increased continuously even under a negative bias voltage, whereas the UC emission was almost quenched at less than +0.5 V. Charge recombination in OPVs can be classified as geminate recombination (GR) from the charge transfer (CT) state, which is Coulombically bounded at the D/A interface prior to complete free charge separation, and bimolecular recombination (BR) from free charges. GR exhibits strong voltage dependence even in the negative voltage region, whereas BR is the dominant recombination mechanism near the OC. Therefore, steep quenching near the OC strongly suggests that the origin of the UC at the interface was BR from free charges.