Determination and analysis of the performance and degradation of a-Si modules using outdoor, simulator and open-circuit-voltage decay (OCVD) measurements

The authors attempted to obtain a reproducible outdoor performance evaluation for a-Si modules and to conduct a systematic study of the degradation effect. With this aim, the modules were exposed to natural sunlight and periodically characterized in a solar simulator by the measurement of the I/V curve and OCVD. These procedures were applied to state-of-the-art a-Si modules from various manufacturers, realized by different technologies, in order to compare their performance and degradation. Performance degradation of a-Si modules results mostly from a reduction of the fill factor. This is caused by an increase of the serial resistance due to corrosion in the intercell zone and a decrease of the lifetime of the minority carriers in the intrinsic layer due to the light-induced degradation effect. The long-term performance is very sensitive to the intrinsic layer thickness. It is shown that it is possible to reduce the degradation significantly by designing a tandem cell structure having the same bandgap and low i-layer thicknesses.