Soft X-Ray Emission Spectroscopy to Determine the Chemical Structure of ZnTe/CdSe_xTe_1-x Thin Films with Group V Dopants and Post-Deposition Treatments

Cadmium telluride (CdTe) solar cells have become a leading contender in the thin-film photovoltaics industry, with record efficiencies now reaching 22.6% [1]. Further performance improvements are reliant upon a more comprehensive understanding of how various dopants and post-deposition treatments can alter the chemical and electronic properties of CdTe, which subsequently impact key device characteristics such as charge separation, electron transport, and recombination at interfaces. Our research utilizes soft x-ray emission spectroscopy (XES) to investigate the local chemical structure in CdSe_xTe_1-x thin films with group V dopants and after CdCl₂ activation, aiming to clarify their effects on the electronic structure and the overall device performance.

XES measurements were conducted with the SALSA endstation [2] at Beamline 8.0.1 of the ALS probing the valence bands of alternatively doped ZnTe/CdSe_xTe_1-x thin films. The measurements and their comparison with spectra of a wide variety of reference compounds offer information about the occupied valence states and chemical bonding environments of the N, O, P, Cl, As, Cd, and Te atoms involved. The high chemical specificity of XES makes it an ideal tool for identifying these species in complex thin-film systems, particularly in multilayer devices where intermixing and diffusion at interfaces may impact the recombination rates and electronic properties of the device. In our contribution, we will discuss these results and compare them with additional observations from surface-sensitive photoelectron spectroscopy (PES) and density-functional theory (DFT) calculations to demonstrate the effectiveness of such a combined approach probing the chemical and electronic environments of CdTe thin films.

References:
[1] G. Martin et al., Solar Cell Efficiency Tables (Version 64). Progress in Photovoltaics Research and Applications, vol. 32, no. 7, pp. 425–441 (2024).
[2] M. Blum et al., Solid and Liquid Spectroscopic Analysis (SALSA) – a Soft x-Ray Spectroscopy Endstation with a Novel Flow-through Liquid Cell, Rev. Sci. Instrum. 80, 123102 (2009).