Influence of Singlet and Charge-Transfer Excitons on the Open-Circuit Voltage of Rubrene/Fullerene Organic Photovoltaic Device.

We demonstrated that the open-circuit voltage (V) of rubrene/C organic photovoltaic (OPV) devices can be substantially improved by changing the rubrene thickness. A shoulder exhibited in a range of 500-550 nm was observed. This result indicated that the singlet excitons of rubrene were increased when the thickness of the rubrene layer was increased. Capacitance-voltage measurements were conducted for estimating the built-in potential of the devices. The calculated V was higher than that of the experiment, thus indicating that energetic losses occurred in the devices. We reused the reciprocity and revised Marcus theory for determining the charge-transfer (CT) properties of the devices. The CT properties of the CT states at the rubrene/C interface remained similar. The nonradiative energetic losses become smaller when the rubrene layer was increased, thus indicating the bimolecular recombination was increased. The increased recombination thermally activated the electrons in C into rubrene for forming the singlet excitons in rubrene. The reduction in reorganization energy indicated that the electroluminescence of rubrene was enhanced, thereby improving V. These results proved that the two-step thermal activation of C electrons and the improved V of rubrene were caused by the increased singlet excitons of rubrene.