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Roel Van de Krol1

1, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Berlin, , Germany

Ternary oxides represent a large but relatively little-explored class of semiconductors that are of interest for photoelectrochemical energy conversion applications. One of the best performing candidates thus far is BiVO4, an n-type photoanode with a bandgap of 2.4 eV. One well known ‘trick’ to improve this material is doping. Donor doping with tungsten (W) supposedly improves the photocurrent by increasing the conductivity, but is also found to reduce the magnitude as well as the lifetime of the photoconductivity. Another dopant that was recently found to greatly improve the photocurrent of BiVO4 is hydrogen [1]. We found that in contrast to tungsten, hydrogen actually increases the carrier lifetime and diffusion length in BiVO4 [2]. This is due to the passivation of trap states, the possible origins of which will be discussed. A second ‘trick’ to improve the performance of BiVO4 is by deposition of a cobalt phosphate (CoPi) co-catalyst on the surface. We recently showed that the main role of this ‘co-catalyst’ is not to enhance the charge transfer kinetics, but to passivate surface defects on BiVO4 [3]. To get a better understanding of the chemical nature of these states, we employed ambient pressure resonant photoemission (AP-resPES) and operando XPS methods. These experiments revealed the presence of two separate states in the bandgap of BiVO4 and a redistribution of the phosphate species in the electrolyte under illumination [4]. These initial results are the first steps towards a molecular-level understanding of the BiVO4/electrolyte interface that may eventually help to design efficient solar fuel generators. In the last part of the talk I will discuss recent results on CuBi2O4, a photocathode material with a bandgap of ~1.7 eV. We developed a modified solution chemistry that enables us to spray highly homogeneous films that show photocurrent densities up to 2 mA/cm2 [5]. To enhance the charge separation in these films, we introduced a gradient of copper vacancies across the film thickness. This results in the formation of a homojunction in CuBi2O4 that increases the carrier diffusion length and reduces charge recombination [6]. The resulting films showed AM1.5 photocurrent densities of up to 2.5 mA/cm2 at +0.6 V vs. RHE in the presence of a hole scavenger, which is a new benchmark for this material.

References
[1] J. Cooper et al., Chem. Mater. 28, 5761 (2016)
[2] J.W. Jang et al., Adv. Energy Mater. 1701536 (2017)
[3] C. Zachäus et al., Chem. Sci. 8, 3712 (2017)
[4] M. Favaro et al., J. Phys. Chem. B (in press, DOI:10.1021/acs.jpcb.7b06942)
[5] F. Wang et al., J. Mater. Chem. A 5, 12838 (2017)
[6] F. Wang et al., J. Am. Chem. Soc. 139, 15094 (2017)

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