Tuning Aerosol Deposition of BiVO4 Films for Effective Sunlight Harvesting

Abstract

Bismuth vanadate (BiVO4) offers high photon efficiencies in solar photo-anodes, due to its suitable semiconductor band gap energies and associated visible light absorption. In well-tuned conditions, such anodes enable green hydrogen generation in photoelectrochemical water splitting cells. Bismuth vanadate films have to ensure high efficiencies in electron/hole pair generation and sufficiently high rates of charge transfer to the conducting substrate and the electrolyte, respectively. Thus, the tuning of coating properties has to aim for high phase purity, good layer integrity as well as optimum diffusion path lengths. In order to explore the potential of aerosol deposition to produce BiVO4 films with high photoelectrochemical activity and to elucidate influences on microstructure and application properties, powder sizes and spraying parameters had to be tailored. By ball milling over durations of up to 20 min, particles sizes in the range from 8.3 down to 0.6 µm were obtained. With respect to spray conditions, the process gas pressure was varied from 1.0 to 2.1 bar corresponding to gas flow rates of 10-40 l/min. The wide range of powder sizes and parameters in aerosol deposition allowed for developing a window of deposition in order to derive the most promising combinations for layer build-up. Optimum parameter sets in application on stainless steel substrates were transferred to FTO-coated glass substrates for backlit cell layouts. The thickness and conductivity of the layers were adjusted to a layer thickness range of 200-500 nm in order to achieve maximum photocurrents. The production of homogeneous, large-scale prototypes demonstrates that aerosol deposition is suitable for processing layers for solar energy harvesting with high photo current densities of up to 3.55 mA/cm2.