Development of High-Efficiency CuInxGa1-xSe2 Thin-Film Solar Cells by Selenization with Elemental Se Vapor in Vacuum

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Abstract
Achievements in the development of CuInSe2 (CIS) and CuIn x Ga1- x Se2 (CIGS). thin-film absorbers with efficiency over 12% by the two-stage method are reported as follows: 12.3% efficiency with CIS, 13.3% efficiency with CIGS (Ga/III ratio=0.25) and 10.1% efficiency with CIGS (Ga/III ratio=0.40, graded band-gap structure). These results clearly indicate that the simple two-stage method developed in this study has high capability to fabricate high-quality CIS and CIGS thin-film absorbers. The Se content in the precursor layer and the Se beam flux intensity during the selenization stage are key factors that must be controlled precisely in order to achieve a high efficiency of over 15%. It is reported, for the first time, that the spontaneous formation of a graded band-gap structure is possible by changing Se and Ga contents in the precursor layer, which should be applicable to the preparation of a high-quality CIS or CIGS thin-film absorber. The formation chemistry models of CIS and CIGS thin films related to this study are proposed.