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Abstract Sputter‐deposited GaOx (i.e., oxygen‐deficient gallium oxide) films are evaluated as a potential replacement for the standard CdS buffer layers in Cu(In,Ga)Se2 (CIGSe) based thin‐film photovoltaics. 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 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) are found, 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 (presumably) 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.

Cu(In,Ga)Se2 (CIGSe) is a promising absorber material for thin‐film photovoltaic devices. A key procedure to achieve high efficiencies is the application of alkali fluoride postdeposition treatments (PDT) of the CIGSe surface. While the effects of the PDT on the directly impacted CIGSe front surface have been subject to extensive studies, less is known about the impact on the deeply buried CIGSe/Mo interface. Exposing the CIGSe absorber backside by stripping it off the Mo back contact allows to use surface‐sensitive photoelectron spectroscopy to study the chemical and electronic structure of this interface in unprecedented detail. CIGSe /Mo stacks prepared using NaF only and combined NaF/RbF (with optimal and excess amount of RbF) PDT are studied. Rb is detected accumulating at the backside of the RbF‐treated CIGSe absorbers in conjunction with a depletion of Na. The exposed CIGSe backsides display a Cu‐deficient surface region, with increased presence of Rb correlating with a decreased Cu content. Rb appears to be incorporated into the Cu‐deficient absorber region as previously detected on the front surface. However, in contrast to the front surface, no distinct, secondary RbInSe‐type phase is found at the back surface.