Interface engineering for the chemical and electronic passivation of group III phosphide semiconductors to be used in highly efficient photoelectrochemical tandem cells for water splitting
Solid State and Surface Chemistry, Material Synthesis
Synthesis and Properties of Functional Materials
Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
Final Report Abstract
The objective of this project was the study of the physical and photoelectrochemical processes occurring at the interfaces of III–phosphide semiconductors (particularly, GaInP2) with water and aqueous electrolyte solutions. At the first stage of the project reproducible procedures for obtaining clean and welldefined GaInP2(100) surfaces were developed. Surprisingly, it was found that the standard technique using argon-ion sputtering and subsequent annealing in UHV cannot be used for surface preparation, since it results in amorphization of the near-surface layer a few nanometers thick and the removal of indium. Therefore, wet etching procedures were developed, which leave the Ga/In ratio intact. The best results were obtained by etching with a commercial 1.25M solution of HCl in 2-propanol under Ar atmosphere. After this treatment the GaInP2(100) surface was free of phosphates containing only a sub-monolayer amount of residual gallium and indium oxides, and it exhibited stable electronic properties: The p-GaInP2(100) surface has a downward band bending of about 0.5 eV, whereas the n-doped surface shows an upward band bending of about 0.4 eV. The work functions of the etched p- GaInP2(100) and n-GaInP2(100) surfaces are in the range of 4.8–5.3 eV and 4.0–4.2 eV, respectively. At the second stage, the interaction of the GaInP2(100) surfaces with water and aqueous electrolyte solutions was investigated at room temperature. Water molecules dissociate on the surface so that the hydrogen atoms are adsorbed on the surface oxygen atoms and the gallium and indium oxides are transformed to hydroxides. While the indium hydroxides remain stable at the semiconductor/water interface, the gallium hydroxides transform to metallic gallium. Interaction with alkaline and acidic aqueous solutions causes the formation of a thin intermediate layer at the GaInP2(100) surface with a thickness of about one monolayer. It consists of different indium and gallium oxides/hydroxides/phosphates, whose exact composition depends on the electrolyte solution. Under cathodic conditions, the hydrogen evolution at the p-GaInP2(100)/solution interface is accompanied by the formation of metallic gallium at the semiconductor surface. Thus, the formation of metallic gallium at the GaInP2(100) surface during hydrogen evolution is the main mechanism of surface degradation in the course of the water splitting process. The results obtained during the third stage show, that the formation of a p-GaInP2/nmetal oxide heterojunction with a sufficiently high built-in potential to drive the water splitting reaction was not possible by the pre-treatments and metal oxide deposition techniques employed so far. Fermi level pinning at the interface of the GaInP2 leads to a reduced onset potential of only 0.7 V, which is more than 1 V lower than the bandgap of the material. This problem was resolved by using a buried pn+-junction, which allowed us to reach photovoltages up to 1.1 V. To achieve even higher photovoltages for the GaInP2/TiO2 system, further optimization of our TiO2 deposition process is necessary. But, alternative promising photoabsorber materials should be considered as well as for example AlGaAs, since it should be easier to produce a clean and well-defined surface for this material. During the final stage the effect of the catalyst on the chemical and charge transfer processes occurring at the semiconductor surface in contact with the aqueous electrolyte solution was investigated using both photoemission spectroscopy and photoelectrochemical methods, such as potentiostatic, galvanostatic or potentiodynamic experiments. As catalyst material Pt nanoparticles were used, since platinum is one of the best catalysts for the hydrogen evolution reaction and can be easily deposited by electrodeposition from aqueous solution. The overall photoelectrochemical performance of the pn+-GaInP2/TiO2/Pt photocathodes showed an overall favorable alignment of the energy levels for an efficient electron transfer from the photoabsorber to the electrolyte solution. The electron transfer energy loss together with the overpotential of the hydrogen evolution reaction was in a range of 0.3–0.6 eV. Further research will have to concentrate on the optimization of the metal oxide interface material e.g. testing other oxides as e.g.NbO2 or Ga2O3 in combination with buried junctions and non-noble electrocatalysts for the HER as well as for the OER.
Publications
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Comparison of wet chemical treatment and Ar-ion sputtering for GaInP2(100) surface preparation. Materials Science in Semiconductor Processing 51, 81 (2016)
Lebedev, M. V.; Kalyuzhnyy, N. A.; Mintairov, S. A.; Calvet, W.; Kaiser, B.; Jaegermann, W.
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Semiconductor/electrolyte interfaces for solar energy conversion: Interface studies by synchrotron induced photoelectron spectroscopy. J. Electron Spectrosc. Relat. Phenom. 221, 116 (2017)
Mayer, T.; Schwanitz, K.; Kaiser, B.; Hajduk, A.; Lebedev, M. V.; Jaegermann, W.
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Synchrotron photoemission spectroscopy study of p-GaInP2(100) electrodes emersed from aqueous HCl solution under cathodic conditions. J. Phys. Chem. C 121, 8889 (2017)
Lebedev, M. V.; Calvet, W.; Kaiser, B.; Jaegermann, W.
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Interaction of liquid water with the p-GaInP2(100) surface covered with submonolayer oxide, Physical Chemistry Chemical Physics 20, 21144 (2018)
Hajduk, A.; Lebedev, M. V.; Kaiser, B.; Jaegermann, W.
