Electron dynamics of modified p-InP surfaces

Within the transition towards renewable energies, green hydrogen is expected to play a prominent role due to its high gravimetric energy density and the complete absence of CO₂ emissions . A promising way of producing green hydrogen is photo-assisted electrochemical water splitting, using semiconductor absorbers with band gaps near the optimum for solar energy conversion. The highest solar-to-hydrogen efficiencies to date of up to 19% have been achieved using InP based absorbers protected by a TiO₂ layer through atomic layer deposition (ALD), both for voltage bias-assisted (p-InP) ¹ and unassisted water splitting (AlInP window layer, GaInP top cell) ².<br/><br/>However, understanding of electron dynamics both at the pure p-InP surface as well as at the p-InP / TiO₂ interface remains limited, in part due to the difficulty in experimentally probing surface conduction band states. We chose to study the (2×1/2×2)-reconstructed P-rich, p-type InP (100) surface prepared through metalorganic chemical vapor deposition (MOCVD), which is commonly utilized in record-breaking cells. Measurements using XPS, UPS, LEED, AFM and time-resolved two-photon photoemission spectroscopy (tr-2PPE) were performed, the latter accessing unoccupied states with a time resolution of 30fs.<br/><br/>We for the first time report detailed dynamics in sub-surface and surface conduction band states up to 2 eV above the p-InP conduction band minimum (CBM), tracking electron lifetimes and average decay paths. Moreover, we explore the impact of TiO₂ layers with varying thickness deposited via ALD on the formation of TiO₂ features, unveiling distinct TiO₂ conduction band states in thin layers, deviating from the conventional TiO₂ density of states reported in the literature. Dynamics and lifetimes of electrons photoexcited in the p-InP bulk are again for the first time tracked across the interface and through the observed TiO₂ states.<br/><br/>A surface treatment protocol using water- and heat exposure in ALD is described, which allows for TiO₂ conduction band states with favourable band alignment to be observed at considerably reduced layer thicknesses. Our measurements indicate enhanced interfacial charge transfer as well as a thinner interlayer region. These improvements are linked to changes in P-dimer surface domains of the p-InP surface during water- and heat exposure, leading to more homogenous P-oxide layer formation and TiO₂ nucleation. Such pre-treatment of the p-InP surface holds the potential to substantially enhance interfacial quality, allowing for thinner TiO₂ layers and reduced defect-mediated recombination, opening exciting opportunities for optimizing cell performance.<br/><br/>(1) Yin, X.; Battaglia, C.; Lin, Y.; Chen, K.; Hettick, M.; Zheng, M.; Chen, C.-Y.; Kiriya, D.; Javey, A. 19.2% Efficient InP Heterojunction Solar Cell with Electron-Selective TiO(2) Contact. <i>ACS Photonics</i> <b>2014</b>, <i>1</i> (12), 1245–1250. https://doi.org/10.1021/ph500153c.<br/>(2) Cheng, W.-H.; Richter, M. H.; May, M. M.; Ohlmann, J.; Lackner, D.; Dimroth, F.; Hannappel, T.; Atwater, H. A.; Lewerenz, H.-J. Monolithic Photoelectrochemical Device for Direct Water Splitting with 19% Efficiency. <i>ACS Energy Lett.</i> <b>2018</b>, <i>3</i> (8), 1795–1800. https://doi.org/10.1021/acsenergylett.8b00920.