Enhancing Photovoltage of Silicon Photoanodes by a High Work-Function Coordination Polymer

A metal–insulator–semiconductor (MIS) structure based on an inhomogeneous junction has been recently proven to be highly efficient at photoelectrochemical (PEC) water oxidation. Engineering the surrounding layer of the MIS nanojunction is crucial to maximizing the photovoltage. Specifically, for an n-type photoanode, a high work-function material is required to create a large Schottky barrier that assists the hole transfer to oxidize water while blocking the electron transfer. Normally, the surrounding layer is a native oxidized phase of the metal layer that forms unintentionally. Herein, we demonstrate the use of copper(I) thiocyanate (CuSCN), a transparent p-type coordination polymer semiconductor with a high work function, to specifically surround the n-Si/SiOx/Cu nanojunctions, resulting in an increased effective barrier height from 0.71 to 1.03 eV. This phenomenon, known as the pinch-off effect, is also applied to improve the performance of the planar n-Si/SiOx/Cu electrode via a PEC dissolution method that creates an inhomogeneous surface covered with CuOx. The Cu/CuSCN nanojunction still shows superior characteristics due to the favorable energetics of CuSCN. This work shows a rational route for engineering the surrounding layer to improve the performance of Si-based MIS photoelectrodes, enabled by the facile chemistry of coordination polymers.