Tandem Si Micropillar Array Photocathodes with Conformal Copper Oxide and a Protection Layer by Pulsed Laser Deposition

This work demonstrates the influence of high-quality protection layers on Si-Cu₂O micropillar arrays created by pulsed laser deposition (PLD), with the goal to overcome photodegradation and achieve long-term operation during photoelectrochemical (PEC) water splitting. Sequentially, we assessed planar and micropillar device designs with various design parameters and their influence on PEC hydrogen evolution reaction. On the planar device substrates, a Cu₂O film thickness of 600 nm and a Cu₂O/CuO heterojunction layer with a 5:1 thickness ratio between Cu₂O to CuO were found to be optimal. The planar Si/Cu₂O/CuO heterostructure showed a higher PV performance (J_sc = 20 mA/cm²) as compared to the planar Si/Cu₂O device, but micropillar devices did not show this improvement. Multifunctional overlayers of ZnO (25 nm) and TiO₂ (100 nm) were employed by PLD on Si/Cu₂O planar and micropillar arrays to provide a hole-selective passivation layer that acts against photocorrosion. A micropillar Si/ITO-Au/Cu₂O/ZnO/TiO₂/Pt stack was compared to a planar device. Under optimized conditions, the Si/Cu₂O photocathode with Pt as a HER catalyst displayed a photocurrent of 7.5 mA cm^-2 at 0 V vs RHE and an onset potential of 0.85 V vs RHE, with a stable operation for 75 h.