Underlayer engineering into the Sn-doped hematite photoanode for facilitating carrier extraction

Abstract

A semiconductor underlayer(s) has been extensively used to improve the performance of photoelectrochemical (PEC) cells. Unfortunately, in many cases, the incorporation of underlayers leads to degraded system performances. A comprehensive study on the functions and manipulations of underlayers is therefore of high significance for achieving high-performance PEC cells. This study indicates that Sn-doped hematite photoanodes decorated with various underlayer materials show substantially distinguished photocurrent responses, leading to qualitatively different PEC cells. With an optimized TiO₂ (ITO, Al₂O₃) underlayer, the photocurrent density at 1.23 V versus RHE can be enhanced from 0.25 to 0.71 (0.59, 0.42) mA cm⁻², while it is decreased to 0.14 mA cm⁻² by using NiO. Our further analysis reveals that the performance differences come mainly from the distinguished bulk and surface carrier recombination effects, i.e., (1) metal doping (i.e., Ti⁴⁺, In³⁺ and Al³⁺) from the underlayers improves the conductivity of hematite film and thus reduces the bulk recombination; (2) the underlayers of TiO₂, ITO and Al₂O₃ can effectively suppress the carrier recombination at the bottom/top surfaces of the hematite layer, while the NiO underlayer leads to a higher surface recombination. Our work provides a basis for selecting an underlayer and a general guideline for the interface engineering for high performance photoelectrodes.