Water Vapor Introduction During Cu(In1-xGax)Se2 Thin-Film Deposition and its Effect on Solar Cell Performance

We have developed a novel technique to improve Cu(In_1-xGa_x)Se₂ (CIGSe) cell performance by means of water vapor introduction during CIGSe deposition. We have examined thus far the effectiveness of water vapor introduction on CIGSe with particular emphasis on a Ga/(In+Ga) ratio x of around 0.5. In this study, the effects of water vapor introduction on thin film properties and solar cell performance for other x-compositions, specifically CuInSe₂ (CISe) and CGSe₂ (CGSe) were studied. Variations in the electrical properties observed in CIGSe (x~0.5), that is, increasing hole density and conductivity with water vapor introduction, were also observed in CISe and CGSe. Water vapor introduction affected solar cell performance as well. The observed improvements in cell performance are thought to be related to the annihilation of donor defects arising from Se vacancies by incorporation of oxygen from the water vapor. In addition, the sodium content in the CIGSe layers was found to increase with increasing oxygen content with water vapor introduction. This result suggests that the mechanism behind the improvement is also related with an enhancement of so-called Na effects