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_xS_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_xS_y:Na buffer layers, we find a sulfur‐poor surface and, in comparison to undoped In_xS_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_xS_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_xS_y:Na/CISSe interface after annealing.

中文翻译：

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通过Na掺杂缓冲层来提高性能-InxSy：Na / CuIn（S，Se）2薄膜太阳能电池界面的化学和电子结构

用钠掺杂硫化铟缓冲层是一种替代基于黄铜矿的薄膜太阳能电池中“最先进的” CdS缓冲层的有前途的途径，因为它可以实现高达17.9％的大面积效率设备（30厘米×30厘米）。我们通过光电子，软X射线发射和反光发射光谱法报告了薄膜太阳能电池的In _x S _y：Na / CuIn（S，Se）₂（CISSe）接口的化学和电子结构。对于沉积的In _x S _y：Na缓冲层，我们发现了硫含量低的表面，并且与未掺杂的In _x S _y相比和标准CdS缓冲器，得出的电子表面带隙为2.60±0.11 eV。缓冲液/吸收剂界面的导带偏移为0.32±0.10 eV的尖峰。在200°C退火以模拟后续电池制造过程的热负荷后，观察到铜和硒从吸收体向缓冲层表面的额外扩散，导致In _x S _y的电子表面带隙明显减小：Na缓冲层（至2.11±0.11 eV）。我们推测吸收体元素的扩散会导致前一个吸收体表面的带隙变宽，并且这两种作用都会导致退火后掩埋的In _x S _y：Na / CISSe界面的导带峰减小。