Atomic Layer Deposition of Defect-Engineered TiO_x and TaO_x Protective Coatings for Photoelectrochemistry

Photoelectrochemical (PEC) energy conversion provides a viable route to chemical fuel production from solar light. Several different PEC design approaches exist, ranging from single photoelectrodes to tandem configurations of photoanodes and -cathodes to buried photovoltaic junctions. However, common to each of these designs is a central materials challenge of achieving simultaneous stability, efficiency, and scalability. Furthermore, the coupling of the semiconducting photo absorber and catalytic components demands sophisticated solid state interface engineering. In this talk, we focus on the stabilization of this semiconductor/electrolyte interface with functional conformal coatings grown by plasma-enhanced atomic layer deposition (PE-ALD). We investigate defect-engineered electron-selective titanium oxide (TiO_x) protection layers for efficient InP photocathodes and identify ALD pathways to optimize interfacial and bulk charge transport, as well as to tune the driving force for photovoltage generation [1]. Furthermore, we use PE-ALD to deposit ultra-thin tantalum oxide (TaO_x) protective coatings showing improved chemical stability compared to titanium oxide in alkaline and acidic environments. In addition to possessing excellent optical transparency, the conductivities of TaO_x layers can be tuned over several orders of magnitude by introducing electronic defects via hydrogen plasma sub-cycles during the ALD process. Spectroscopic methods including X-ray photoelectron spectroscopy (XPS), photothermal deflection spectroscopy (PDS), and spectroscopic ellipsometry (SE) are employed to characterize and understand the introduced defects and correlate the electronic properties to the PEC performance. Overall, the demonstrated route to ALD-based defect-engineering enables precise control over the properties of ultra-thin layers, facilitating not just improved corrosion protection layers but also tunable semiconductor/dielectric interfaces.<br/><br/>[1] Bienek, O. et al. Engineering Defects and Interfaces of Atomic Layer Deposited TiO_x Protective Coatings for Efficient III-V Semiconductor Photocathodes. <i>ACS Photonics</i> (in press). https://doi.org/10.1021/acsphotonics.3c00818