The Energy Level Alignment at the Buffer/Cu(In,Ga)Se2 Thin‐Film Solar Cell Interface for CdS and GaOx

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The energy level alignment at the GaOx/CIGSe and CdS/CIGSe interfaces are compared by means of direct and inverse photoemission. For the GaOx/CIGSe we find a (0.04 ± 0.07) eV (i.e., a small spike-like) conduction band offset (CBO) and a (-3.21 ± 0.19) eV (i.e., a large cliff-like) valence band offset (VBO), which suggests a nearly ideal charge-selective contact. The derived GaOx band gap of (4.80 ± 0.25) eV confirms its utility as a highly transparent buffer layer. However, the GaOx (with x derived to be 1.1 ± 0.1) exhibits considerable defect-related occupied states above the valence band maximum. It is proposed that these states may increase charge carrier recombination and decrease open circuit voltage in respective devices; also explaining why solar cells with standard CdS buffer outperform devices with GaOx buffer, despite less ideal electronic interface properties (CBO: [-0.18 ± 0.07] eV, VBO: [-0.98 ± 0.15] eV) and the smaller CdS band gap of (2.35 ± 0.22) eV.

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  • 08/03/2024
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