Understanding and mitigating effects of nonuniformities on reliability of thin film photovoltaics

Thin-film photovoltaics (PV) are sensitive to lateral nonuniformities (LN) that manifest themselves in spatial variations of the device local characteristics and in the variability of the measured parameters between nominally identical devices. LN affect all the aspects of device operations and stability and appear as a hidden cost of the otherwise inexpensive technology. They are omnipresent as originating from multiple factors typical of thin-film PV: deposition geometry, wet and heat treatments, dispersion in grain and amorphous phase parameters, and fluctuations in metal-semiconductor barriers. LN are seen in the device mappings, including that of PL, Voc, OBIC, EBIC, thermography, and electroluminescence. Stresses localized on certain vulnerable spots drive the entire device degradation. We present a general summary of physical processes related to LN, including modeling aspects, characteristic length and variability scales, statistics, degradation mechanisms, and superadditive effects between different device components, such as a negative correlation between the resistive and LN related loss, and a positive correlation between LN and device shunting failures under stress. We then review the known practical techniques of mitigating LN effects patented by different groups from 1970s to nowadays and show how nonuniformity treatments play the key role in the existing technologies.