Oxidation Temperature-Dependent Electrochemical Doping of WO3 Deposited via Atomic Layer Deposition

Abstract

Silicon-based photoelectrochemical devices show promise for the performance of light-driven CO₂ reduction but suffer from instability under photoelectrochemical conditions relevant to CO₂ reduction. Coating silicon electrodes with thin layers of metal oxides has shown promise to passivate unstable silicon surfaces, and many different metal oxides can be deposited on silicon using various techniques. In this study, we investigate the fundamental photoelectrochemical performance of WO₃-coated silicon photoelectrodes, which were generated by oxidation of W-metal films deposited via atomic layer deposition on both degenerately doped (nSi⁺) and low-doped (pSi) silicon. Two different oxidation temperatures were investigated (400 and 600 °C), and it was found that the monoclinic phase of WO₃ predominates at both temperatures but that more grain boundaries are present in the 600 °C film. From X-ray photoelectron spectroscopy, the stoichiometry of both films was found to be 1:3 W:O, and low electron energy loss experiments indicate band gaps of 3.0 and 3.1 eV for 400 and 600 °C films, respectively. Cyclic voltammetry experiments showed that the electron transfer kinetics increased after continued redox cycling, particularly for the material produced at 400 °C. X-ray photoelectron spectra suggest that the observed increase in electrode conductivity is due to the formation of oxygen vacancies in the film. Electrochemical impedance spectroscopy indicated that charge transport through the films was impacted by the grain boundaries that formed during oxidation of the film. Photoelectrochemical studies on pSi/WO₃ electrodes were highly variable, only producing a photocurrent and photovoltage with some samples. Our best sample, formed at 400 °C, produced a photovoltage of 180 mV, which is lower than what has previously been reported for WO₃-coated silicon (500 mV). We hypothesize that the variability in photoelectrochemical experiments arose from a roughened WSiO_x interface that is generated during film preparation. WO₃ shows promise as a metal oxide coating for silicon, but our results suggest that formation of a high-quality interface between Si and WO₃ is vital for best performance.