Simulation of Charge Transport and Recombination across Functionalized Si(111) Photoelectrodes

Abstract

A method is described for modeling charge transfer and recombination across functionalized Si(111) photoelectrodes using a 1D solid state drift-diffusion device simulation -wxAMPS. To demonstrate the approach, the effect of charge transport mechanisms on the photoelectrochemical performance of hydrogen and methyl-terminated Si(111) electrodes was analyzed. When contacted with electrolyte, prior literature proposed three charge transfer mechanisms that can affect the performance of functionalized surfaces: a shift in interfacial dipole, a change in surface recombination, or Fermi level pinning. By modeling all three processes using wxAMPS, a shift in interfacial dipole of −0.35 ± 0.05 eV between hydrogen and methyl-terminated Si(111) was concluded to be the predominant effect in these electrodes. This verifies prior hypotheses while predicting a more accurate value of the shift in interfacial dipole than previous theoretical treatment and experimental interpretations.

Computational insights into charge transfer across functionalized semiconductor surfaces

“…must match the experimental surface when predicting surface dipoles using DFT. These findings also suggest that the effective band-edge energies of a surface can be engineered to have a specific [33] Photoelectron Spectroscopy [48,49], [86,87] wxaMPS [83] dfT & MBPT [71] −0.40 v (n-Si) −0.49 ev −0.35 ev…”

“…Because an interfacial dipole modifies the effective electron affinity of the surface of the semiconductor, the magnitude of the electron affinity was adjusted until the calculated trend in V oc versus redox formal potential matched the experimental data. The results predicted a dipole shift of −0.35 eV between Si(111)-H and Si(111)-CH 3 [83].…”

“…wxAMPs is available free of charge on the web and has an easy-to-use graphical user interface [81]. wxAMPS has been demonstrated to model a variety of complex devices including dye-sensitized solar cells [82] and functionalized photoelectrodes [83,84].…”

“…from band-to-band and/or via Shockley-Read-Hall recombination, while G op (x) is the optical generation rate from illumination. A more detailed description of the model and the implementation of boundary conditions is described in previous work [83].…”

“…R(x) is the net recombination rate value by tuning the interfacial dipole with the chemical make-up of a mixed monolayer. wxAMPS can also be used to predict the magnitude of an interfacial dipole on a functionalized semiconductor by fitting experimental data [83]. For example, Lewis et al previously measured the V oc for n and p-type Si(111)-H and Si(111)-CH 3 photoelectrodes in contact with a series of redox couples in CH 3 CN-1.0 M LiClO 4 [33].…”