Photo-electrochemical Properties of Thin-Film InVO4 Photoanodes: the Role of Deep Donor States

The material properties and photoelectrochemical performance of compact thin-film InVO4 photoanodes prepared by spray pyrolysis are investigated. Nearly phase-pure orthorhombic InVO4 can be obtained by a postdeposition anneal treatment in air between 450−550 °C. Optical absorption spectra indicate that InVO4 has an indirect bandgap of ∼3.2 eV with a pronounced sub-bandgap absorption starting at ∼2.5 eV. A dielectric constant of 50 and a flatband potential of −0.04 V vs RHE are determined, which confirms that this material is able to evolve hydrogen. Few shallow donors are present in this material, which is markedly different from what is usually observed for simple binary oxides. The main photocurrent response occurs in the UV (<400 nm) and the incident photon-to-current conversion efficiency is less than 1%. The impedance data show that the poor photo response is due to a high density of deep donors and a concomitantly small depletion layer. The visible light absorption of InVO4 is attributed to the presence of ionized deep donors in the space charge region of the material, which explains why InVO4 powders show a much stronger visible light absorption than thin films. The defect-chemical origin of the deep donor state is discussed and some general considerations for the use of ternary and more complex metal oxides as photoelectrodes are outlined.