2, Technische Universiteit Eindhoven, Eindhoven, , Netherlands
Hematite (α-Fe2O3) is widely known as a promising material for photoelectrochemical (PEC) water splitting, due to a suitable band gap energy (1.9-2.2 eV), high chemical stability, non-toxicity and its abundance in nature. However, it suffers from limitations that result in low water splitting efficiency, such as a short charge carrier lifetime (∼10−12 μs) and small minority carrier diffusion length (2−4 nm). This leads to higher charge carrier recombination at the surface as well as at the interface with the transparent conducting oxide (TCO) . Both surface and interface recombination have been shown to be reduced by introducing an underlayer between the hematite thin film and the TCO .
In this work, a ZnO underlayer was deposited on FTO-glass substrate via atomic layer deposition (ALD) with thicknesses between 2 nm and 10 nm. Although ZnO thin films have been widely used as an electron-selective layer in solar cells , this is the first time where it is has been used as an underlayer for improving the PEC activity of hematite thin films. The ALD deposition of ZnO was followed by DC magnetron sputtering of a 20 nm Fe thin film. The films were then annealed at 645°C for 10 min to convert the Fe layer to α-Fe2O3. The PEC performance of the films were studied using cyclic voltammetry and chopped light measurements. It was observed that deposition of a 2 nm thick ZnO underlayer resulted in a 36% increase in photocurrent at 1.23 VRHE compared to a plain hematite thin film.
Electrochemical impedance spectroscopy was used in order to elucidate the mechanism behind this improvement in PEC performance. The Mott-Schottky plots indicated that the charge carrier density (ND) for the hematite film with 2 nm ZnO underlayer was 5.6 times higher than for the plain hematite film. Furthermore, the surface state recombination resistance was 3-4 times lower than for plain hematite films at potentials greater than 1.2 VRHE. This was related to the presence of Zn dopant in the hematite film as confirmed by XPS analysis. This suggests that the ZnO layer acts as a doping agent which allows for a larger band bending effect, thus improving the charge transfer properties at the hematite-electrolyte interface.
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