Detrimental copper-selenide bulk precipitation in CuIn1-xGaxSe2 thin-film solar cells. A possible reason for the limited performance at large x?

In this paper, we propose a possible explanation of the limited CuIn1-xGaxSe2 (CIGSe) performance at high Ga contents, related to the properties of detrimental copper selenide (Cu delta Se) secondary phases. We study CIGSe layers at different x, by means of X-ray diffraction analyses, Raman and energy dispersive X-ray spectroscopy. Our results reveal that for high Ga contents, Cu delta Se secondary phases either precipitate at the interfaces between grains or remain within the layer as infra- or inter-grain clusters, deteriorating the electronic properties of the absorber layer. On the contrary, for low Ga contents, the copper selenide segregates at the surface of the CIGSe and hence it can be easily removed by KCN surface etching. To understand the Cu delta Se precipitation within the bulk of the film for a high Ga ratio, we also investigate In-free CGSe samples at low and high Cu content. Our observations demonstrate that i) Cu delta Se has a preferential grain orientation to segregate and ii) the Cu-enriched regions within the bulk of the high Ga content films are more a kinetic than a thermodynamic issue.