Transition metal oxides as hole-selective contacts in silicon heterojunctions solar cells

• Transition metal oxide/n-crystalline silicon solar cells were fabricated.
• V2Ox, MoOx and WOx were obtained after thermal evaporation under vacuum.
• XPS analyses revealed the presence of oxygen vacancies and/or gap states.
• Highest efficiency (open-circuit voltage) was 15.7% (606 mV) for V2Ox/silicon.
• Current-voltage response is limited by diffusion of injected minority carriers.

This work reports on a comparative study comprising three transition metal oxides, MoO3, WO3 and V2O5, acting as front p-type contacts for n-type crystalline silicon heterojunction solar cells. Owing to their high work functions (>5 eV) and wide energy band gaps, these oxides act as transparent hole-selective contacts with semiconductive properties that are determined by oxygen-vacancy defects (MoO3−x), as confirmed by X-ray photoelectron spectroscopy. In the fabricated hybrid structures, 15 nm thick transition metal oxide layers were deposited by vacuum thermal evaporation. Of all three devices, the V2O5/n-silicon heterojunction performed the best with a conversion efficiency of 15.7% and an open-circuit voltage of 606 mV, followed by MoO3 (13.6%) and WO3 (12.5%). These results bring into view a new silicon heterojunction solar cell concept with advantages such as the absence of toxic dopant gases and a simplified low-temperature fabrication process.