Mrs. Hadar Mor, M.Sc. Candidate
Photoelectrochemical (PEC) water splitting produces hydrogen and oxygen using solar energy, providing an elegant route to produce renewable fuel. The photoelectrodes in PEC solar cells should absorb visible light, be stable in aqueous electrolytes, and have appropriate energy band positions for water oxidation and reduction. Iron oxide (α-Fe2O3), also known as hematite, is considered as a promising candidate for solar water splitting. It has a favorable band gap of 2.1 eV, it is stable in alkaline solutions, and it is an abundant and low cost material. Despite these merits, hematite photoelectrodes display lower efficiency than expected, supposedly due to fast recombination that gives rise to short diffusion length of photo-generated charge carriers. One strategy to reduce charge recombination at the interface between the hematite layer and the substrate is by using underlayers that enhance electron extraction while suppressing holes from reaching the substrate.
This work examines the influence of metal oxide underlayers on the photoelectrochemical properties of thin film hematite photoanodes. Different metal oxides were applied as underlayers to hematite films with different thicknesses, using pulsed laser deposition (PLD) on (001) oriented Al2O3 substrates. Among the different undelayers that were scrutinized, Nb-doped TiO2 underlayers were found most effective to improve thin film hematite photoanodes. Unlike previous reports on porous hematite photoanodes, the improved performance did not arise from morphological and structural effects. Detailed analysis of the experimental results, including intensity-modulated and photocurrent action spectra measurements, revealed the effect of the underlayer on the driving forces that govern charge separation within the photoanode.