Real-time measurements of interfacial charge transfer rates at silicon/liquid junctions

Abstract

The rates of interfacial charge transfer from photogenerated carriers in semiconductor electrodes to solution donors and acceptors are fundamentally important in the area of semiconductor electrochemistry. However, to date there have been no real-time measurements of the charge-transfer rates between semiconductors and outer sphere redox couples. Previous steady-state current-voltage experiments have led to the suggestion that such rates might be extremely fast (>10¹¹ s⁻¹), because evidence has been obtained for interfacial charge transfer events competing with thermalization of the photoexcited carriers (i.e. "hot carrier injection processes"). Additionally, luminescence decay measurements on CdS surfaces in contact with strongly adsorbing, inner sphere redox reagents such as 1.0 M KOH/l.O M S²⁻ have indicated high surface recombination velocities (> 10⁵ cm/s) and have yielded photoexcited carrier lifetimes of less than 100 ps. In contrast, simple theoretical estimates of the interfacial charge transfer rate constants for outer sphere redox systems can be used to predict carrier lifetimes between 10⁻⁶ and 10⁻³ s under common experimental conditions. To address these issues, we report herein the first real-time kinetic rate measurements from carriers in a semiconductor to outer sphere redox donors and acceptors in the liquid phase.

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