Tailoring the oxidation state of vanadium in V2Ox films by UV-ozone treatment and its impact on V2Ox/c-Si(n) solar cells

The work function of vanadium oxide (V₂O_x) films is strongly influenced by the average oxidation state of vanadium atoms. In this work, UV-Ozone (UVO) treatment was applied to thermally evaporated V₂O_x films to examine its impact on vanadium oxidation states and oxygen vacancies. Film transparency and optical bandgap improved with longer UVO durations, indicating fewer defect states. XPS analysis shows an increase V⁵⁺ content from 76.75% in as deposited states to 84.96% after 20 min UVO and a reduction in oxygen vacancies from 46.46% (as deposited) to 24.42% (after 20 min UVO). UV-PES analysis also showed an increase in work function from 4.65 eV (as-deposited) to 5.12 eV after 20 min of UVO, attributed to a rise in V⁵⁺ content and a reduction in oxygen vacancies after UVO treatment. The strong oxidizing nature of UVO helps filling the vacancies and shifting the stoichiometry towards V₂O₅, thereby enhancing its work function. A higher work function of V₂O_x (\({\phi }_{{V}_{2}{O}_{x}}\)) enables it to function effectively as a hole selective layer (HSL) in c-Si(n) heterojunction solar cells. The increase in V⁵⁺ oxidation state and reduced defect density after UVO treatment enhanced hole selectivity improving the performance of the fabricated V₂O_x/c-Si(n) solar cell (3.8%) compared to the device with as-deposited V₂O_x layer (2.5%). While the device demonstrates a relative efficiency improvement of 1.3%, the absolute efficiency remains low (3.8%), indicating that significant optimization is still required. These findings demonstrate UVO treatment as an innovative post-deposition approach for improving TMO film quality and enhancing the efficiency of TMO based solar cells. This strategy addresses the critical issue of work function sensitivity to air exposure by restoring and stabilizing the work function, enabling the fabrication of high-performance devices. Simulation studies show that UVO treatment more strongly modifies near-surface vanadium oxidation states and hence improvement in work function with relatively less effect at the V₂O_x/c-Si(n) interface. This suggests the necessity of employing thinner V₂O_x films as HSLs in future studies.