Transparent cuprous oxide photocathode enabling a stacked tandem cell for unbiased water splitting

Abstract

Photoelectrochemical water splitting represents an attractive method of capturing and storingthe immense energy of sunlight in the form of hydrogen, a clean chemical fuel. Given the largeenergetic demand of water electrolysis, and the defined spectrum of photons available from incidentsunlight, a two absorber tandem device is required to achieve high efficiencies. The twoabsorbers should beofdifferentandcomplementary bandgaps, connected in series to achieve thenecessary voltage, and arranged in an optical stack configuration to maximize the utilization ofsunlight.This latterrequirement demands atop device that is responsive to high-energy photonsbut also transparent to lower-energy photons, which pass through to illuminate the bottom absorber.Here, cuprous oxide (Cu2O) is employed as a top absorber component, and the factors influencingthe balance between transparencyandefficiency towardoperationinatandem configurationarestudied. Photocathodes based on Cu2O electrodeposited onto conducting glass substratestreated with thin, discontinuous layers of gold achieve reasonable sub-bandgap transmittancewhile retaining performances comparable to their opaque counterparts. This new high-performancetransparent photo- cathode is demonstrated in tandem with a hybrid perovskite photovoltaic cell,resulting in a full device capable of standalone sunlight-driven water splitting.