Interface engineering of semiconductor electrodes for photoelectrochemical water splitting: Application of surface characterization with photoelectron spectroscopy

Abstract

In this chapter we discuss at first bulk and interface related requirements of efficient photoelectrochemical device structures for water splitting. Maximized conversion efficiencies need photovoltages produced in the photovoltaic component of the device, which are adapted to the electrochemical performance of the electrolyzer components without energetic losses in their coupling across the involved interfaces. The photocurrents must approach quantum efficiencies of one for all absorbed photons above the band gap, which will only be possible for adjusted minority carrier diffusion lengths. We emphasize that to our expectations only multi-junction devices will provide photovoltages high enough for water splitting without any additional bias. Appropriate interface engineering layers must be developed for proper chemical and electronic surface passivation. In addition, highly efficient electrocatalysts, either for the hydrogen or oxygen evolution reaction, must be adjusted in their energetic coupling to the semiconductor band edges and to the redox potentials in the electrolyte with minimized losses in the chemical potentialsIn the second part of this chapter we address our surface science approach to investigate the interface properties of photoelectrodes as relevant for water splitting. This is mainly photoelectron spectroscopy for solid-state contacts and a quasi in situ approach for electrolyte contacts combining transfer techniques from the electrolyte as well as model experiments to simulate the electrolyte by adsorption. Furthermore, possible routes to deposit and modify passivation layers and catalysts under controlled conditions are discussed.In the last part of this chapter we illustrate our experimental results on interface engineering strategies and perspectives of photoelectrochemical water splitting devices concentrating on silicon based single and tandem cells. We present experimental data combining surface science and photoelectrochemical investigations, which are discussed in the framework of the above given conceptual considerations. Based on the obtained results the observed improvements but also the still given limitations can be related to clearly identified nonidealities in surface engineering either related to recombination losses at the semiconductor surface reducing photocurrents or due to not properly aligned energy states leading to potential losses across the interfaces.We anticipate that photoelectrochemical thin film multi-junction devices with properly aligned interfaces will provide a competitive route to solar H2.