Voltage-Induced Inversion of Band Bending and Photovoltages at Semiconductor/Liquid Interfaces

Abstract

At semiconductor/liquid interfaces, the surface potential and photovoltages are produced by a combination of band bending and quasi-Fermi-level splitting at the semiconductor surface, which are usually treated in a qualitative fashion. As such, it is important to develop quantitative metrics for the band energies and photovoltaics at these interfaces. Here, we present a spectroscopic method for monitoring the photovoltages produced at semiconductor/liquid junctions. The surface reporter molecule mercaptobenzonitrile (MBN) is functionalized on the photoelectrode surface (p-type silicon) and is measured using in situ surface-enhanced Raman scattering (SERS) spectroscopy with a water immersion lens under electrochemical working conditions. In particular, the vibrational frequency of the C≡N stretch mode (ω CN ) around 2225 cm –1 is sensitive to the local electric field in solution at the electrode/electrolyte interface via the vibrational Stark effect. Over the applied potential range from −0.8 to 0.6 V vs Ag/AgCl, we observe ω CN to increase from 2220 to 2229 cm –1 (at low laser power). As the incident laser power is increased from 83.5 μW to 13.3 mW, we observe additional shifts of Δω CN = ±1 cm –1, corresponding to photovoltages produced at the semiconductor/liquid interface Δ V = ±0.2 V. Based on Mott–Schottky measurements, the flat band potential (FBP) occurs at −0.39 V vs Ag/AgCl. For applied potentials above the FBP, we observe Δω CN > 0 (i.e., blue-shifts ∼1 cm –1 ) corresponding to positive photovoltages, whereas for applied potentials below the flat band potential, we observe Δω CN < 0 (i.e., red-shifts ∼1 cm –1 ) corresponding to negative photovoltages. These spectroscopic observations reveal voltage-induced changes in the band bending at the semiconductor/liquid junction that, thus far, have been difficult to measure.

Topics & Concepts