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

Abstract

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

Surface and Interface Properties in Thin‐Film Solar Cells: Using Soft X‐rays and Electrons to Unravel the Electronic and Chemical Structure

“…The XPS data of the as‐deposited samples with varying buffer layer thickness with and without the addition of Na points towards an interface with a small degree of Cu diffusion from the absorber into the buffer layer . Our XPS/XES study of In 2 S 3 layers from the Institut des Matériaux Jean Rouxel (IMN, University of Nantes) revealed a similar diffusion of Cu into the buffer …”

“…In the case of the annealed system, a straightforward correction of the interface‐induced band bending is not possible, since the annealing step has to be performed with the full‐thickness buffer layer, making it impossible to access the then hidden interface with the surface‐sensitive electron spectroscopies. Nevertheless, the detailed characterization of the changes in chemical structure allows to qualitatively discuss the possible effect of these changes on the band alignment . Following the above‐discussed diffusion of Cu into the buffer layer, the expected corresponding Cu depletion at the (former) absorber surface will lead to an increase of its bandgap, which is illustrated in Figure b by the gray bars.…”

“…To understand the impact of this additional thermal load, it was simulated in our experiment by annealing the buffer layer samples to 100 °C (for 30 min) and subsequently to 200 °C (for 30 min) under UHV conditions. We find that this strongly enhances the diffusion of Cu into the buffer layer, and, furthermore, that this diffusion is considerably stronger for films without Na addition as compared to the Na‐doped In x S y films . To further analyze the compounds formed during the buffer layer deposition and the subsequent annealing process, samples were investigated using XES.…”

“…The respective resulting bandgaps (±0.11 eV) are listed in the center. Reproduced with permission . Copyright 2018, Wiley.…”

Intermixing at the InxSy/Cu2ZnSn(S,Se)4 Heterojunction and Its Impact on the Chemical and Electronic Interface Structure

“…Please note that, at room temperature, indium sulfide crystalizes in the β-In 2 S 3 phase with a high number of vacancies. They can easily host other atoms 36 , e.g., Na 27 or Cu 7 18,44 and kesterites 45,46 and can be attributed to a stoichiometry variation (often: a copper depletion) at the surface.…”