We report the dependence of the characteristics of photovoltaic organic light-emitting diodes (PVOLEDs) on the composition of the light-emission and electron-donating layer (EL-EDL). 5,6,11,12-Tetraphenylnaphthacene (rubrene): dibenzo{[f,f′]-4,4′,7,7′-tetraphenyl}diindeno[1,2,3-cd:1′,2′,3′-lm]perylene (DBP) was used to form the EL-EDL, and C60was used as an electron-accepting layer (EAL) material. A half-gap junction was formed at the EAL/EL-EDL interface. As the rubrene ratio in the EL-EDL increased, the emission spectra became blue-shifted and the light-emission efficiency increased. The highest emission efficiency was achieved with an EL-EDL composed of 95% rubrene and 5% DBP. The short-circuit current decreased as the rubrene content increased up to 50% and then saturated, while the open-circuit voltage was almost unchanged regardless of the rubrene content. The power-conversion efficiency and fill factor increased as the composition of the EL-EDL approached that of pure materials. By controlling the rubrene : DBP ratio, the emission color could be adjusted. The emission efficiency of devices with mixed rubrene/DBP EL-EDLs could be greater than that of either pure rubrene or pure DBP devices. We obtained an overall power-conversion efficiency of 3% and a fill factor greater than 50%.
Progress in Emission Efficiency of Organic Light-Emitting Diodes: Basic Understanding and Its Technical Application