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Interfacial Oxide Formation Limits the Photovoltage of α‐SnWO 4 /NiO x Photoanodes Prepared by Pulsed Laser Deposition
Author(s) -
Schnell Patrick,
Kölbach Moritz,
Schleuning Markus,
Obata Keisuke,
Irani Rowshanak,
Ahmet Ibbi Y.,
Harb Moussab,
Starr David E.,
van de Krol Roel,
Abdi Fatwa F.
Publication year - 2021
Publication title -
advanced energy materials
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 10.08
H-Index - 220
eISSN - 1614-6840
pISSN - 1614-6832
DOI - 10.1002/aenm.202003183
Subject(s) - non blocking i/o , materials science , x ray photoelectron spectroscopy , water splitting , oxide , band gap , band bending , optoelectronics , atomic layer deposition , pulsed laser deposition , surface photovoltage , nanotechnology , analytical chemistry (journal) , thin film , chemical engineering , spectroscopy , photocatalysis , catalysis , biochemistry , chemistry , physics , quantum mechanics , engineering , metallurgy , chromatography
α‐SnWO 4 is a promising metal oxide photoanode material for direct photoelectrochemical water splitting. With a band gap of 1.9 eV, it ideally matches the requirements as a top absorber in a tandem device theoretically capable of achieving solar‐to‐hydrogen (STH) efficiencies above 20%. It suffers from photoelectrochemical instability, but NiO x protection layers have been shown to help overcome this limitation. At the same time, however, such protection layers seem to reduce the photovoltage that can be generated at the solid/electrolyte junction. In this study, an extensive analysis of the α‐SnWO 4 /NiO x interface is performed by synchrotron‐based hard X‐ray photoelectron spectroscopy (HAXPES). NiO x deposition introduces a favorable upwards band bending, but also oxidizes Sn 2+ to Sn 4+ at the interface. By combining the HAXPES data with open circuit potential (OCP) analysis, density functional theory (DFT) calculations, and Monte Carlo‐based photoemission spectra simulation, the presence of a thin oxide layer at the α‐SnWO 4 /NiO x interface is suggested and shown to be responsible for the limited photovoltage. Based on this new‐found understanding, suitable mitigation strategies can be proposed. Overall, this study demonstrates the complex nature of solid‐state interfaces in multi‐layer photoelectrodes, which needs to be unraveled to design efficient heterostructured photoelectrodes for solar water splitting.