Engineering Defects and Interfaces of ALD TiO_x Protective Coatings for Highly Efficient III-V Photocathodes

Atomic layer deposited (ALD) protection layers have been crucial for enabling the integration of chemically sensitive III-V semiconductors into photoelectrochemical energy conversion devices. Although high photovoltage systems have been demonstrated, there remain significant open questions regarding the roles of semiconductor/protection layer interfaces and defects within the ALD films on carrier injection, electronic transport, and recombination processes. Here, we aim to elucidate how the optical and electronic properties of TiO_x adlayers influence the PEC performance of III-V photocathodes. To unravel these factors, we report on the effect of ALD TiO_x defect energies and concentrations on interfacial charge transport and photoelectrochemical performance using p-InP/TiO_x/Pt structures as model systems. In particular, we demonstrate highly tunable stoichiometry and defect concentrations within ultrathin, conformal ALD TiO_x using different metalorganic precursors and oxidants. Photothermal deflection spectroscopy reveals defect-induced sub-bandgap optical absorption within TiO_x that can be tuned over three orders of magnitude. These results are corroborated by X-ray photoelectron spectroscopy, which confirms the presence of Ti³⁺ in sub-stoichiometric films and the emergence of electronic states within the bandgap. Consistent with this analysis, in-plane electronic transport measurements reveal conductivities that vary over five orders of magnitude for these films. In addition, <i>in situ</i> spectroscopic ellipsometry during deposition is used to monitor not just the growing TiO_x films but also the oxidation of the underlying InP, thereby allowing us to tailor ALD processes that suppress InPO_x interlayer formation and reduce the out-of-plane charge transport resistance. Photoelectrochemical testing of p-InP/TiO_x/Pt photocathodes under hydrogen evolution reaction conditions confirms the importance of avoiding interface oxidation. In addition, we find once interface oxidation is suppressed, a higher defect concentration within the TiO_x layer leads to the reduction of the HER onset potential. This can be explained by an increased recombination rate in those films, which is evident from surface photovoltage measurements. While high defect concentrations within protection layers significantly reduce photoelectrochemical performance, the interface characteristics rather than film properties play a dominant role for moderate and low defect content TiO_x films. These findings highlight the crucial role of controlling interface properties and defects of protection layers to not only preserve but even enhance the photoelectrochemical energy conversion efficiency of III-V semiconductor electrodes.