Spectroscopic Analysis of Zn_(1−x)Mg_xO (ZMO) Thin Films and Their Integration with CZTSe Solar Cells

This study investigates Zn_(1−x)Mg_xO (ZMO) as a non-toxic n-type layer for Cu₂ZnSnSe₄ (CZTSe) thin-film solar cells, focusing on the impact of Mg concentration on the electronic properties and its influence on band alignment with CZTSe absorbers. ZMO films containing 5%, 10%, 15%, and 20% Mg were deposited at room temperature via radio-frequency magnetron sputtering onto molybdenum-coated soda-lime glass (Mo/SLG) and double-side polished sapphire substrates. The as-deposited films were characterized using X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), reflection electron energy loss spectroscopy (REELS), and optical transmittance (UV-Vis) prior to integration with CZTSe absorbers. The electronic and optical bandgap (Eg) of the ZMO films, estimated from REELS and UV-Vis, respectively, increased from 3.3 to 3.7 eV with higher Mg content, correlating with changes in Zn/Mg composition. Spatial mapping over a 10 cm deposition distance revealed distinct variations in Eg, valence band maximum (VBM), and Zn/Mg ratio, with more pronounced fluctuations at higher Mg concentrations. The Eg was lower in the center of the deposition and increased towards the edges, correlating with changes in the Zn/Mg ratio, likely due to preferential re-sputtering of Zn during the sputtering process.¹ The VBM of the ZMO films did not show a straightforward correlation with Mg concentration. Moreover, XPS and UPS were used to determine valence band offset (VBO) and conduction band offset (CBO) at the interface with CZTSe. Understanding the spatial fluctuations in the electronic and optical properties of ZMO is essential for optimizing band alignment at the CZTSe/ZMO interface.² Spectroscopic data for CZTSe will be thus presented to reveal the interface characteristics. Additionally, results on the performance of CZTSe solar cells using ZMO as the n-type layer will be discussed.

References:
(1) Norrman, K., Norby, P., & Stamate, E. (2022). Preferential zinc sputtering during the growth of aluminum doped zinc oxide thin films by radio frequency magnetron sputtering. Journal of Materials Chemistry C, 10(39), 14444-14452.
(2) Gansukh, M., Li, Z., Rodriguez, M. E., Engberg, S., Martinho, F. M. A., Mariño, S. L., ... & Canulescu, S. (2020). Energy band alignment at the heterointerface between CdS and Ag-alloyed CZTS. Scientific Reports, 10(1), 18388.