Marco Bernardi , Maurizia Palummo , and Jeffrey C. Grossman
Abstract: Graphene and monolayer transition metal dichalcogenides (TMDs) are promising materials for next-generation ultrathin optoelectronic devices. Although visually transparent, graphene is an excellent sunlight absorber achieving 2.3% visible light absorbance in just 3Å thickness. TMD monolayers also hold potential as sunlight absorbers, and may enable ultrathin photovoltaic (PV) devices due to their semiconducting character. In this work, we show that the three TMD monolayers MoS2, MoSe2, and WS2 can absorb up to 5−10% incident sunlight in a thickness of less than 1 nm, thus achieving one order of magnitude higher sunlight absorption than GaAs and Si. We further study PV devices based on just two stacked monolayers: 1) a Schottky barrier solar cell between MoS2 and graphene, and 2) an excitonic solar cell based on a MoS2/WS2 bilayer. We demonstrate that such 1 nm thick active layers can attain power conversion efficiencies of up to ~1%, corresponding to 100−1,000 times higher power densities than the best existing ultrathin solar cells. Our work shows that two-dimensional monolayer materials hold yet untapped potential for solar energy absorption and conversion at the nanoscale.