Structural study and electronic band structure investigations of the solid solution NaxCu1 − xIn5S8 and its impact on the Cu(In,Ga)Se2/In2S3 interface of solar cells

The present work reports investigations on the new In2S3 containing Cu and/or Na compounds, which are expected to be formed at the Cu(In,Ga)Se2/In2S3 interface. The knowledge of these materials properties is very important in order to better understand the operation of the devices based on these junction partners.It has been observed that a solid solution NaxCu1 − xIn5S8 exists from CuIn5S8 (x = 0) to NaIn5S8 (x = 1) with a spinel-like structure. The single crystal structure determination shows that indium, copper and sodium atoms are statistically distributed on the tetrahedral sites.XPS investigations on the CuIn5S8, Na0.5Cu0.5In5S8 and NaIn5S8 compounds combined with the band gap changes reported in a previous work show that these variations are mainly due to valence band maximum shift; it is moved downward when x increases from 0 to 1. These observations are confirmed by the electron structure calculations based on the density functional theory, which additionally demonstrate that the pure sodium compound has direct gap whereas the copper-containing compounds have indirect gaps.