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

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 ΔV = ±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.