Theoretical and Experimental Upper Bounds on Interfacial Charge-Transfer Rate Constants between Semiconducting Solids and Outer-Sphere Redox Couples

Abstract

Theoretical expressions for the charge-transfer rate constant at a semiconductor/liquid junction have been modified to include the effects of adiabaticity and the existence of a Helmholtz layer at the solid/liquid interface. These expressions have yielded an estimate of the maximum interfacial charge-transfer rate constant, at optimal exoergicity, for a semiconductor in contact with a random distribution of nonadsorbing, outer-sphere redox species. An experimental upper bound on this interfacial charge-transfer rate constant has been obtained through the determination of key energetic and kinetic properties for stable semiconductor electrodes in contact with outer-sphere redox species. For this purpose, n-Si/CH 3 OH-dimethylferrocenium-dimethylferrocene, n-GaAs/CH 3 CN-ferrocenium-ferrocene, and p-InP/CH 3 CN-cobaltocenium-cobaltocene contacts were investigated using a combination of current density-potential and differential capacitance-potential methods. The upper limits for the interfacial charge-transfer rate constant at these semiconductor/liquid contacts were found to be consistent with the upper limits predicted by theory. The current density-potential behavior of n-InP and p-InP/Fe(CN) 6 3-/4-(aq) junctions was also examined in order to assess the validity of prior kinetic measurements on these interfaces.

Use of near-surface channel conductance and differential capacitance versus potential measurements to correlate inversion layer formation with low effective surface recombination velocities at n-Si/liquid contacts

“…The Si/liquid contacts that showed a low effective S value as probed by photoconductivity decay measurements also formed an inversion layer in contact with the electrolyte solutions. Although the near-surface channel conductance experiments reported herein were performed using ͑100͒-oriented Si surfaces, the flatband potentials of ͑100͒-and ͑111͒-oriented Si surfaces are sufficiently similar 11 that the general conclusions derived from these measurements regarding the electrochemical potentials of the electrolyte solutions required to form an inversion layer are expected to be applicable, with only minor changes, to ͑111͒-oriented Si surfaces.…”

Interfacial behavior of alkyltrichlorosilane monolayers on silicon: Control of flat-band potential and surface state distribution using chain length variation

“…This equation is based on the assumption that the capacitance of the space charge layer is much less than that of the Helmholtz layer and at high frequency (1 kHz), the contribution of Helmholtz capacitance to the measured electrode capacitance is negligible [62]. Thus the capacitance of the semiconductor/electrolyte interface mainly expresses the capacitance of the space charge layer of the semiconductor [63][64][65].…”

Role of inversion layer formation in producing low effective surface recombination velocities at Si/liquid contacts

“…11 Redox couples that have very positive redox potentials are capable of extensive charge transfer from the Si into the electrolyte, thereby establishing an inversion layer at the surface of n-type Si. [12][13][14] Simulations of the surface recombina- tion rate using an extended Shockley-Read-Hall formalism, which incorporates the effects of band bending, have in fact shown that S decreases significantly with increases in positive charge at the surface of n-type Si. 11 Removal of the sample from the electrolyte will generally change the surface potential in a direction that reduces the band bending, so the observed value of S should increase and the variations in the surface trap density, N t , can then be reflected in the measurements of S under these conditions.…”

“…[12][13][14] Near-surface channel conductance measurements in p ϩ -n-p ϩ Si structures exposed to electrolytes have clearly demonstrated the formation of an inversion layer at n-Si/CH 3 OH-5.4 mM Me 2 Fc-2.9 mM Me 2 Fc ϩ contacts but not at CH 3 OH-Me 10 Fc ϩ/0 electrolytes. 12 Mott-Schottky measurements of the n-Si/CH 3 OH-Me 2 Fc ϩ/0 contact have indicated an equilibrium barrier height of Ϸ1.03 V, [14][15][16] which is sufficient to drive the system into carrier inversion. The surface recombination velocity of Si/CH 3 OH contacts after treatment with CH 3 OH-Me 2 Fc ϩ/0 , deduced from current density versus potential data, is Ͻ100 cm s Ϫ1 , whereas higher S values were deduced for these Si samples in contact with redox systems such as CH 3 OH-Me 10 Fc ϩ/0 that cannot produce an inversion layer at the Si surface.…”