Although organic light emitting diodes are generally well characterized, their mechanism of decay, e.g. formation of black spots, is still not fully understood. Here we present a new technique allowing for deeper insight into the degradation process of an OLED by measuring its photovoltaic properties. The results show the possibility to record maps of crucial photovoltaic values with a lateral resolution of 50 microns. Based on these results, we propose a mechanism for the decay process. Black spots in the device are formed during the fabrication process, and the lifetime is determined by the active materials' chemical stability.
At present, heterojunction polymer solar cells are typically fabricated with an active layer thickness of approximately 80 nm to 100 nm. This active layer thickness has traditionally been chosen based upon convenience and empirical results. However, a detailed mechanistic study of the effects of active layer thickness on the short circuit current and efficiency has never been performed for polymer solar cells. We demonstrate that using the high mobility materials regio regular poly(3-hexylthiophene and [6,6]-phenyl (P3HT) and C61-butyric acid methyl ester (PCBM), that high efficiency solar cells can be fabricated with active layer thickness greater than 100 nm. Devices with an active layer thickness of 200 nm are fabricated with a power efficiency of 4.1% under AM1.5 illumination at and intensity of 80 mW/cm2. In addition, we explain the variation in short circuit current density as a function of thickness using calculations of the distribution of the optical electric field intensity as a function of device thickness.