Enabling Efficient Charge Separation for Optoelectronic Conversion via an Energy-Dependent Z-Scheme...

Achieving good charge separation while maintaining energetic electronic states in heterostructures is a challenge in designing efficient photocatalyst materials. Using first-principles calculations, we propose a Z-scheme S_n-m-S_p (n-semiconductor-metal-p-semiconductor) heterojunction as a viable avenue for achieving broad-spectrum sunlight absorption and, importantly, energy-dependent charge separation. As a proof-of-concept investigation, we investigated two ternary heterostructures, CdS-Au-PdO and SnO₂-W-Ag₂O, in which the electronic Fermi levels line up by virtue of the presence of an intermediate metal layer. A cascade of work functions in the relative order W_n < W_m < W_p drives electrons flowing from S_n to m and from m to S_p. The inner electric fields established at the S_n-m and m-S_p Schottky junctions selectively guide low-energy photoexcited electrons from S_n (CdS/SnO₂) and low-energy holes from S_p (PdO/Ag₂O) to the interposing Au or W metal, respectively. Importantly, relatively low Schottky barriers enforce charge separation by constraining high-energy photogenerated charges to the individual semiconductor layers. Operating together, these two mechanisms enable the achievement of highly efficient optoelectronic conversion.