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

• Detrimental Cu δ Se phases precipitate within the bulk of Ga-rich CuIn 1-x Ga x Se 2. • Cu δ Se bulk precipitation in the Ga-rich CuIn 1-x Ga x Se 2 films is a kinetic issue. • A relaxation stage is necessary to make the Cu δ Se segregate at the surface. • The Cu δ Se has a preferential orientation to segregate. In this paper, we propose a possible explanation of the limited CuIn 1-x Ga x Se 2 (CIGSe) performance at high Ga contents, related to the properties of detrimental copper selenide (Cu δ 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 δ Se secondary phases either precipitate at the interfaces between grains or remain within the layer as intra- 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 δ 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 δ 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. [ABSTRACT FROM AUTHOR]