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Doping an indium sulfide buffer layer with sodium is a promising route to replace the "state-of-the-art" CdS buffer layer in chalcopyrite-based thin-film solar cells, as it achieves efficiencies as high as 17.9% for large-area devices (30 cm × 30 cm). We report on the chemical and electronic structure of the In x S y :Na/CuIn(S,Se) 2 (CISSe) interface for thin-film solar cells by means of photoelectron, soft x-ray emission, and inverse photoemission spectroscopy. For as-deposited In x S y :Na buffer layers, we find a sulfur-poor surface and, in comparison to undoped In x S y and the standard CdS buffer, derive a large electronic surface band gap of 2.60 ± 0.11 eV.The conduction band offset at the buffer/absorber interface is a spike of 0.32 ± 0.10 eV. After annealing at 200°C to simulate the thermal load of subsequent cell manufacturing processes, an additional diffusion of copper and selenium from the absorber towards the buffer layer surface is observed, leading to a distinct electronic surface band gap decrease of the In x S y :Na buffer layer (to 2.11 ± 0.11 eV). We speculate that the diffusion of absorber elements causes a band gap widening at the former absorber surface and that both effects lead to a reduction of the conduction band spike for the buried In x S y :Na/CISSe interface after annealing. KEYWORDS band alignment, electronic structure, photoelectron spectroscopy, thin-film solar cells, x-ray emission spectroscopy

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Highly conductive nominally undoped ZnO (b-ZnO), obtained by means of an additional plasma near the substrate during sputter deposition, represent an attractive alternative for ZnO:Al (AZO) commonly employed in transparent windows of thin film solar cells. b-ZnO layers exhibit more than twice higher charge carrier mobility in comparison to AZO layers of the same resistivity (1•10 Ω cm −3). In consequence, a better transparency in near infrared region and an enhanced short circuit current can be achieved for low band gap thin film solar cells. Replacement of AZO for b-ZnO thus enhances their energy output. In order to allow assessment of suitability of these b-ZnO films for deployment in photovoltaic industry, we examine their stability in various environments, and show pathways to improve it. We demonstrate that the b-ZnO films can exhibit comparable stability to ZnO:Al films in both ambient and heated air over the period of 24 months. However, the examined b-ZnO films degrade faster in accelerated open damp heat (DH) conditions, which we attribute to the lower compactness of columnar microstructure. In order to circumvent this limitation, we introduce a novel multilayered b-ZnO film with an improved environmental stability, as verified by the enhanced optoelectrical performance of DH-treated Cu(InGa)(SSe) 2 solar cells.