  Two-dimensional transition metal dichalcogenides (TMDCs) have recently
attracted attention due to their superlative optical and electronic properties.
In particular, their extraordinary optical absorption and semiconducting band
gap have enabled demonstrations of photovoltaic response from heterostructures
composed of TMDCs and other organic or inorganic materials. However, these
early studies were limited to devices at the micrometer scale and/or failed to
exploit the unique optical absorption properties of single-layer TMDCs. Here we
present an experimental realization of a large-area type-II photovoltaic
heterojunction using single-layer molybdenum disulfide (MoS2) as the primary
absorber, by coupling it to the organic {\pi}-donor polymer PTB7. This
TMDC-polymer heterojunction exhibits photoluminescence intensity that is
tunable as a function of the thickness of the polymer layer, ultimately
enabling complete quenching of the TMDC photoluminescence. The strong optical
absorption in the TMDC-polymer heterojunction produces an internal quantum
efficiency exceeding 40% for an overall cell thickness of less than 20 nm,
resulting in exceptional current density per absorbing thickness in comparison
to other organic and inorganic solar cells. Furthermore, this work provides new
insight into the recombination processes in type-II TMDC-polymer
heterojunctions and thus provides quantitative guidance to ongoing efforts to
realize efficient TMDC-based solar cells.
