Surface and Electrolyte Steric Factors Modulate Photovoltage at Silicon|Liquid Junctions

High Resolution Image Download MS PowerPoint Slide We demonstrate that molecular steric bulk modulates interfacial capacitance, potential drop, double-layer formation, and photovoltage at silicon–liquid junctions. We show that sub-monolayer and covalent, anthracene-based functionalization of p -type Si(111) with varied alkyl substituents provides molecular-scale control over interfacial electron transfer due to the Si–Anth site serving as the ‘hot-spot’ for electron transfer—without the introduction of material- or polymer-based overlayers. Specifically, the series of surface modifiers anthracene, 9-butyl-anthracene, and 9-dodecyl-anthracene produce varying interfacial dielectric gaps or ‘solvent gaps’ (Sundararaman, R. et al. J. Phys. Chem. Lett. 2017, 8 (21), 5344–5348), characteristics that modulate the potential drop within the interface-molecular layer versus the Helmholtz layer. Mott–Schottky analyses resolved two capacitive elements for sterically insulated anthracene surfaces, which produced two discrete flat-band potentials: one across the silicon–molecule moiety and one across the molecule–solution interface. Additionally, alkyl(anthracene) installment decreased electrolyte access to the interface, thus decreasing the total capacitance and modulating the molecular contribution to the potential drop—all without compromising the improved onset photovoltage achieved with increased steric isolation (dodecyl anthracene, +80 mV). Electrolyte-dependent cyclic voltammetry studies revealed surface-specific diffusion and double-layer responses that scaled with both cation size and molecular sterics which resulted in a large change in onset voltage (–130 mV dodecyl anthracene vs anthracene). Bulky surface substituents amplified resistive behavior and double-layer expansion in aprotic media by between 0.2–0.5 nm, highlighting solvent-dependent micro-environmental control. Overall, the results demonstrate that key performance metrics (photovoltage, V onset ), as well as the interfacial potential drop—with distinct semiconductor, interface-molecular and double-layer contributions—control the interface microenvironment.