Experimental Approach for Determining Semiconductor/liquid Junction Energetics by Operando Ambient-Pressure X-ray Photoelectron Spectroscopy

Operando Ambient Pressure X-ray photoelectron spectroscopy (operando AP-XPS) investigation of semiconductor/liquid junctions provides quantitative understanding of the energy bands in these photoelectrochemical solar cells. Liquid junction photoelectrochemical cells allow a uniform contact between the light-absorbing semiconductor and its contacting electrolyte phase. Standard Ultra High Vacuum (UHV) based X-ray photoelectron spectroscopy (XPS) has been used to analyze the electronic energy band relations in solid-state photovoltaics. We demonstrate how operando AP-XPS may be used to determine these relationships for semiconductor/liquid systems. The use of "tender" X-ray synchrotron radiation produces photoelectrons with enough energy to escape through a thin electrolyte overlayer; these photoelectrons provide information regarding the chemical and electronic nature of the top ~10 nm of the electrode as well as of the electrolyte. The data can be analyzed to determine the energy relationship between the electronic energy bands in the semiconductor electrode and the redox levels in the solution. These relationships are critical to the operation of the photoelectrochemical cell and for understanding such processes as photoelectrode corrosion or passivation. Through the approach described herein, the major conditions for semiconductor-electrolyte contacts including accumulation, depletion, and Fermi-level pinning are observed, and the so-called flat-band energy can be determined.