Ghorbani, Elaheh and Albe, Karsten (2018):
Influence of Cu and Na incorporation on the thermodynamic stability and electronic properties of β-In_2S_3.
In: Journal of Materials Chemistry C, 6 (27), Royal Society of Chemistry, pp. 7226-7231, ISSN 2050-7526,
DOI: 10.1039/c8tc01341a,
[Online-Edition: https://doi.org/10.1039/c8tc01341a],
[Article]

Abstract

The aim of this study is to understand the effect of Na and Cu incorporation in In_2S_3, which is representing a Cd-free buffer system for chalcopyrite-type thin film solar cells based on Cu(In,Ga)Se_2 (CIGS). The formation energies and charge states of sodium and copper dopants in In_2S_3 are investigated by means of calculations based on electronic hybrid density functional theory using supercells of 320-atoms. Our results reveal a negative formation enthalpy of sodium in both In-rich and S-rich samples, which indicates the occurrence of side reactions and explains the existence of a chemically modified buffer layer in the presence of a Na-reservoir. Copper, in contrast, can be incorporated in large concentrations in In-rich In_2S_3 under n-type conditions, acting as an acceptor and thus limiting the n-type conductivity. For lower Fermi energies, however, reactions between Cu and the buffer material lead to the formation of Cu-containing secondary phases in the buffer side which is in qualitative agreement with experimental observations of Bär et al. [Bär et al., Appl. Mater. Interfaces, 2016, 8, 2120]. Sulfur rich samples are found to be more heavily doped under n-type conditions and we expect to have Na- and Cu-containing secondary phases formed under metal-poor growth conditions.

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