Oxidation Temperature-Dependent\nElectrochemical Doping\nof WO<sub>3</sub> Deposited via Atomic Layer Deposition

Silicon-based\nphotoelectrochemical devices show promise for the\nperformance of light-driven CO<sub>2</sub> reduction but suffer from\ninstability under photoelectrochemical conditions relevant to CO<sub>2</sub> reduction. Coating silicon electrodes with thin layers of\nmetal oxides has shown promise to passivate unstable silicon surfaces,\nand many different metal oxides can be deposited on silicon using\nvarious techniques. In this study, we investigate the fundamental\nphotoelectrochemical performance of WO<sub>3</sub>-coated silicon\nphotoelectrodes, which were generated by oxidation of W-metal films\ndeposited via atomic layer deposition on both degenerately doped (nSi<sup>+</sup>) and low-doped (pSi) silicon. Two different oxidation temperatures\nwere investigated (400 and 600 °C), and it was found that the\nmonoclinic phase of WO<sub>3</sub> predominates at both temperatures\nbut that more grain boundaries are present in the 600 °C film.\nFrom X-ray photoelectron spectroscopy, the stoichiometry of both films\nwas found to be 1:3 W:O, and low electron energy loss experiments\nindicate band gaps of 3.0 and 3.1 eV for 400 and 600 °C films,\nrespectively. Cyclic voltammetry experiments showed that the electron\ntransfer kinetics increased after continued redox cycling, particularly\nfor the material produced at 400 °C. X-ray photoelectron spectra\nsuggest that the observed increase in electrode conductivity is due\nto the formation of oxygen vacancies in the film. Electrochemical\nimpedance spectroscopy indicated that charge transport through the\nfilms was impacted by the grain boundaries that formed during oxidation\nof the film. Photoelectrochemical studies on pSi/WO<sub>3</sub> electrodes\nwere highly variable, only producing a photocurrent and photovoltage\nwith some samples. Our best sample, formed at 400 °C, produced\na photovoltage of 180 mV, which is lower than what has previously\nbeen reported for WO<sub>3</sub>-coated silicon (500 mV). We hypothesize\nthat the variability in photoelectrochemical experiments arose from\na roughened WSiO<sub><i>x</i></sub> interface that is generated\nduring film preparation. WO<sub>3</sub> shows promise as a metal oxide\ncoating for silicon, but our results suggest that formation of a high-quality\ninterface between Si and WO<sub>3</sub> is vital for best performance.