Determining the Conduction Band-Edge Potential of Solar-Cell-Relevant Nb₂O₅ Fabricated by Atomic Layer Deposition

Often key to boosting photovoltages in photoelectrochemical and related solar-energy-conversion devices is the preferential slowing of rates of charge recombination—especially recombination at semiconductor/solution, semiconductor/polymer, or semiconductor/perovskite interfaces. In devices featuring TiO 2 as the semiconducting component, a common approach to slowing recombination is to install an ultrathin metal oxide barrier layer or trap-passivating layer atop the semiconductor, with the needed layer often being formed via atomic layer deposition (ALD). A particularly promising barrier layer material is Nb 2 O 5 . Its conduction-band-edge potential E CB is low enough that charge injection from an adsorbed molecular, polymeric, or solid-state light absorber and into the semiconductor can still occur, but high enough that charge recombination is inhibited. While a few measurements of E CB have been reported for conventionally synthesized, bulk Nb 2 O 5, none have been described for ALD-fabricated versions. Here, we specifically determine the conduction-band-edge energy of ALD-fabricated Nb 2 O 5 relative to that of TiO 2 . We find that, while the value for ALD-Nb 2 O 5 is indeed higher than that for TiO 2, the difference is less than anticipated based on measurements of conventionally synthesized Nb 2 O 5 and is dependent on the thermal history of the material. The implications of the findings for optimization of competing interfacial rate processes, and therefore photovoltages, are briefly discussed.