Illuminated Semiconducting Electrodes Show Saturation Kinetics under Conditions of Inversion

Semiconductor-electrolyte interfaces are essential in governing electron transfer (ET) in photoelectrochemical cells that drive fuel-forming and fuel-utilizing reactions using sunlight. Electron transfer kinetics at semiconductor-electrolyte interfaces under depletion and in the dark are well understood; however, reductive-fuel forming reactions typically occur at very negative potentials under conditions of inversion and under illumination. Thus, determining the empirical relationship between the rate of ET, light, and potential under conditions of inversion is vital for comparing electron transfer across photoelectrochemical systems. Here, we measure observed electron transfer rate constants, kobs (cm s-1), for ET between low-doped p-type methyl-terminated silicon and three cobalt metallocenes as a function of irradiance. We find that kobs saturates with respect to light and can be described using a simple steady-state approximation to extract maximum light-dependent observed ET rate constants, kobs,max. We observe that kobs saturates at the same irradiance irrespective of the formal reduction potential of the metallocene in solution. Light-dependent cyclic voltammograms and differential capacitance measurements also demonstrate that the observed half-wave potential and total capacitance saturate at the same irradiance as kobs. These observations imply that all three electrochemical parameters are governed by the same underlying factors, which we attribute to the maximum number of electrons that can be stored in the inversion layer of the semiconductor. This work demonstrates that a series of chemical kinetic expressions can be used to describe light-dependent physical parameters in semiconducting photoelectrodes under inversion and aids in understanding the kinetics of electron transfer at semiconductor-electrolyte interfaces of interest for solar-driven fuel-forming reactions.