Computational insights into charge transfer across functionalized semiconductor surfaces

Abstract

A 1.23 V electrochemical difference is required as a minimum to drive PEC water-splitting, which can only be provided by a single semiconductor with an excessively large band-gap. An alternative strategy is to use two small band-gap semiconductors placed electrically in series. One semiconductor operates as the photocathode to drive H 2 -evolution while the other operates as the photoanode to drive O 2 -evolution, as shown in Figure 1. In this case, the electrochemical potential difference required to split water is partially provided by each semiconductor. Photoelectrochemical water-splitting is a promising carbon-free fuel production method for producing H 2 and O 2 gas from liquid water. These cells are typically composed of at least one semiconductor photoelectrode which is prone to degradation and/or oxidation. Various surface modifications are known for stabilizing semiconductor photoelectrodes, yet stabilization techniques are often accompanied by a decrease in photoelectrode performance. However, the impact of surface modification on charge transport and its consequence on performance is still lacking, creating a roadblock for further improvements. In this review, we discuss how density functional theory and finite-element device simulations are reliable tools for providing insight into charge transport across modified photoelectrodes.

Computational Approaches to Photoelectrode Design through Molecular Functionalization for Enhanced Photoelectrochemical Water Splitting

“…A detailed review of surface dipole calculations and relating them to barrier‐height shifts can be found in Ref. .…”

“…The surface dipole moves the semiconductor bands up or down depending on its sign. Experimentally, it is known that the band energies at the photoelectrode/electrolyte junction are fixed (dashed line in Scheme ) . Hence, the surface dipole results in a concomitant change in the semiconductor band bending or barrier height …”

“…The Fermi energy of the photoelectrode is equal to the water redox potential (NHE/ E F ) after equilibration. The band energies at the photoelectrode/electrolyte junction are experimentally known to be fixed (dashed line) . A positive μ surf (green) increases (decreases) the barrier height of p‐type (n‐type) photoelectrodes.…”

“…The surface dipole is an Ångström‐scale quantity that can be calculated from the ground‐state charge density obtained from a first‐principles DFT simulation . It can be converted to a change in the surface potential energy and band edges (| μ surf | in Scheme ), which can in turn be approximated as a shift in the photoelectrode's barrier height .…”

“…This creates a bottleneck in further improving PEC device efficiency because of a lack of design rules for candidate functional groups. However, first‐principles density functional theory (DFT), first‐principles molecular dynamics (MD), and device modeling have recently proven to be powerful tools to both support experimental findings and provide predictive insights …”