On the effect of varying spectral conditions on amorphous silicon solar cell performance

An opto-electrical modelling platform has been designed to model the effects of illumination spectra on amorphous silicon solar cells of different i-layer thickness and degradation state. The illumination spectra, which were investigated in this work, are solar simulator spectra and solar spectra recorded outdoors at CREST, Loughborough. These spectra are used to probe the effect of spectral variation on a-Si:H solar cell performance and its co-dependence with the state of the device. For the case of indoor evaluation of performance of a-Si:H solar cells, it is shown that the performance of the device remains relative to the illumination source of the solar simulator. Spectra with Average Photon Energy (APE) higher than AM1.5G tend to overestimate the performance parameters (JSC, MPP, VOC) of the device, while spectra with APE lower than AM1.5G tend to underestimate the values of the performance parameters of the device. The maximum power deviation of most class-A solar simulators is less than 1% of the actual STC values, but the performance deviation may arise up to 4% for the case of LED light sources. It is suggested to apply voltage dependant corrections to the J-V characteristics, whenever the spectral mismatch between the illumination spectra and AM1.5G is significant. The effects of outdoor spectral variation on the performance of a-Si:H solar cells has been investigated. The results show that light intensity is primarily responsible for a- Si:H outdoor performance. The APE of the outdoor spectra is also identified a significant factor for the variation of performance. The magnitude of maximum power deviations due to APE changes is in the range of ±3% as compared to power output of the device under the AM1.5G spectrum. The percentage of performance variation to STC differed for a-Si:H solar cells of different i-layer thickness and level of degradation. Specifically devices with thicker i-layer, which have suffered degradation, are prone to performance variations. Finally, the energy yield and the performance ratio of amorphous silicon solar cells were reviewed in respect to outdoor spectral changes. The performance ratio is a useful method for cases where prediction of power output is necessary. However, it is suggested that PV modules should be rated on the basis of their annual energy yield, when possible.