Light-Soaking Effects on the Open-Circuit Voltage of a-Si:H Solar Cells

We present measurements on the decline of the open-circuit voltage VOC in a-Si:H solar cells during extended illumination (light-soaking) at 295 K. We used a near-infrared laser that was nearly uniformly absorbed in the intrinsic layer of the cell. At the highest photogeneration rate (about 2x10 cm), a noticeable decline (0.01 V) occurred within about 10 minutes; VOC stabilized at 0.04 V below its initial value after about 200 hours. We found that both the kinetics and the magnitudes of VOC are reasonably consistent with the predictions of a calculation combining a bandtail+defect picture for recombination and a hydrogen-collision model for defect generation. The version of the hydrogen-collision model that we used assumes that only bandtail recombination drives the hydrogen collision processes. Within this picture, the crossover between bandtail and defect recombination occurs on the same timescale as the “light-induced annealing” process that accounts for stabilization of the optoelectronic properties for long lightsoaking times.