CuBi₂O₄: Electronic Structure, Optical Properties, and Photoelectrochemical Performance Limitations of the Photocathode

As CuBi₂O₄ is an emerging p-type semiconductor for applications as a photocathode in photoelectrochemical (PEC) solar fuel production, there is much to be understood about the uniqueness and commonalities the material exhibits in comparison to other, more well-known metal oxide semiconductor systems. We examine p-CuBi₂O₄ thin films grown by reactive co-sputtering with a comprehensive spectroscopic and first principles characterization methodology to describe its fundamental electronic structure and optical properties while addressing intrinsic limitations in the observed PEC performance. The optical properties are evaluated from 180 to 2500 nm with a multi-modal approach using spectroscopic ellipsometry, UV–vis, and photothermal deflection spectroscopy to obtain the complex dielectric function and 5 orders of magnitude of the absorption coefficient. The films are evaluated under PEC conditions appropriate for CO₂ reduction conditions (0.1 M HCO₃^2–) with the inclusion of electron scavenger (S₂O₈^2–) to minimize catalytic limitations. While the theoretical maximum photocurrent density was 4.68 mA cm^–2, the realized photocurrent was 1.18 mA cm^–2 with front-side illumination and an onset potential of about 1.1 V_RHE. The thickness dependence of the photocurrent under back-side illumination exposed a limited electron diffusion length of 45 nm attributed to electron small polaron transport. Connections are established between electronic structure, optical properties, and PEC performance through a combination of X-ray spectroscopies (X-ray absorption spectroscopy, X-ray emission spectroscopy, resonant inelastic X-ray scattering, and X-ray photoelectron spectroscopy) and ab initio modeling. These results not only provide the basis for understanding the observed polaron limitations but also form the basis of a broader connection to other material systems which are governed by polaronic limitations. This study provides a conceptual framework to interconnect observations made through the multiple types of advanced characterization methodologies presented. Ultimately, this work aims to assist the development of CuBi₂O₄ beyond its intrinsic limitations for its application in solar fuel production.