Contactless Optical Probing of Photovoltage in Metal‐Insulator‐Semiconductor Electrodes via MV ^{2 +} /MV ⁺ Redox Transi

ABSTRACT

We investigate the photovoltage of n‐Si/5nm Al ₂ O ₃ /7nm Au electrodes using potential‐dependent second harmonic generation (SHG) in methyl viologen (MV) solutions. For planar 50 nm‐thick Au electrodes immersed in MV‐containing solutions, the SHG intensity exhibits a pronounced suppression near −0.65 V vs. Ag/AgCl. The observed SHG suppression is attributed to the reduction of MV ²⁺ to the radical cation MV ⁺ , which has a 4× higher absorption at 400 nm and 7× higher absorption at 800 nm—leading to decreased SHG intensity. This sharp, voltage‐triggered optical response provides a direct signature of the local surface potential. By correlating the SHG suppression with the applied electrode potential, we demonstrate that the SHG signal can serve as a sensitive, contactless probe of the local surface potential at electrified interfaces. This method is used to probe the photovoltage of n‐Si/5nm Al ₂ O ₃ /7nm Au electrodes. Without UV excitation, the pronounced suppression begins at −1.2 V vs. Ag/AgCl, unlike on planar Au, due to band bending and leakage resistance. With UV excitation, there is a blue shift in the suppression potential due to the photo‐induced voltage. This approach offers a promising strategy for non‐invasive measurement of photovoltage in MIS electrodes without functionalizing the surface with electric field‐sensitive molecules like 4‐mercaptobenzonitrile.