Ghorbani, Elaheh ; Erhart, Paul ; Albe, Karsten (2019):
Energy level alignment of Cu(In,Ga)(S,Se)2 absorber compounds with In2S3, NaIn5S8, and CuIn5S8 Cd-free buffer materials.
In: Physical Review Materials, 3 (7), pp. 075401-1. American Physical Society, ISSN 2475-9953,
DOI: 10.1103/PhysRevMaterials.3.075401,
[Article]

Abstract

Motivated by environmental reasons, In_2S_3 is a promising candidate for a Cd-free buffer layer in Cu(In,Ga)(S,Se)_2 (CIGSSe)-based thin-film solar cells. For an impactful optimization of the In_2S_3 alternative bufferlayer,however,acomprehensiveknowledgeofitselectronicpropertiesacrosstheabsorber-bufferinterface is of foremost importance. In this respect, finding a favorable band offset between the absorber and the buffer layers can effectively reduce the carrier recombination at the interface and improve open-circuit voltage and fill factor, leading to higher conversion efficiencies. In this study, we investigate the band alignment between the most common CIGSSe-based absorber compounds and In_2S_3. Furthermore, we consider two chemically modified indium sulfide layers, NaIn_5S_8 and CuIn_5S_8, and we discuss how the formation of these secondary phases influences band discontinuity across the interface. Our analysis is based on density functional theory calculations using hybrid functionals. The results suggest that Ga-based absorbers form a destructive clifflike conduction-band offset (CBO) with both pure and chemically modified buffer systems. For In-based absorbers, however, if the absorber layer is Cu-poor at the surface, a modest favorable spikelike CBO arises with NaIn_5S_8 and CuIn_5S_8.

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