Improving performance by Na doping of a buffer layer—chemical and electronic structure of the In x S y :Na/CuIn(S,Se) 2 thin‐film solar cell interface

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.