Probing Charge Carrier Transport and Recombination Pathways in Monolayer MoS2/WS2 Heterojunction Photoelectrodes

Monolayer heterojunctions such as MoS2/WS2 are attractive for solar energy conversion applications because the interfacial electric field spatially separates charge carriers in less than 100 fs. Photoelectrochemical cells represent an intriguing platform to collect the spatially separated carriers. However, the recombination, transport, and interfacial charge transfer processes that take place following the ultrafast charge separation step have not been investigated. Here we demonstrate novel charge recombination and transport pathways in monolayer MoS2/WS2 photoelectrochemical cells by spatially resolving the net collection of carriers (i.e., the photocurrent) at the single nanosheet level. We discovered an excitation-wavelength-dependent recombination pathway that depends on the heterojunction stacking configuration and the carrier generation profile in the heterostructure. Photocurrent mapping measurements revealed that charge transport occurs parallel to the layers over micrometer-scale distances even though the indium tin oxide electrode and liquid electrolyte provide efficient charge extraction pathways via intimate electron- and hole-selective contacts. Our results reveal how composition heterogeneity influences the performance of bulk heterojunction electrodes made from randomly oriented nanosheets and provide critical insight into the design of efficient heterojunction photoelectrodes for solar energy conversion applications.