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 3.3 angstrom thickness. TMD monolayers also hold potential as sunlight absorbers, and may enable ultrathin photovoltaic (PV) devices due to their semi-conducting 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 1 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 similar to 1%, corresponding to approximately 1-3 orders of magnitude 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

Light emission in two-dimensional (2D) transition metal dichalcogenides (TMDs) changes significantly with the number of layers and stacking sequence. While the electronic structure and optical absorption are well understood in 2D-TMDs, much less is known about exciton dynamics and radiative recombination. Here, we show firstprinciples calculations of intrinsic exciton radiative lifetimes at low temperature (4 K) and room temperature (300 K) in TMD monolayers with the chemical formula MX 2 (X = Mo, W, and X = S, Se), as well as in bilayer and bulk MoS 2 and in two MX 2 heterobilayers. Our results elucidate the time scale and microscopic origin of light emission in TMDs. We find radiative lifetimes of a few picoseconds at low temperature and a few nanoseconds at room temperature in the monolayers and slower radiative recombination in bulk and bilayer than in monolayer MoS 2 . The MoS 2 /WS 2 and MoSe 2 /WSe 2 heterobilayers exhibit very long-lived (∼20−30 ns at room temperature) interlayer excitons constituted by electrons localized on the Mo-based and holes on the W-based monolayer. The wide radiative lifetime tunability, together with the ability shown here to predict radiative lifetimes from computations, hold unique potential to manipulate excitons in TMDs and their heterostructures for application in optoelectronics and solar energy conversion. KEYWORDS: Monolayer materials, transition metal dichalcogenides, luminescence, radiative lifetime, excitons, optoelectronics T wo-dimensional (2D) transition metal dichalcogenides (TMDs) are promising materials for ultrathin electronic, optoelectronic, photocatalytic, and photovoltaic devices. 1−8 Out of approximately 40 existing TMDs, 9,10 some have received particular attention due to their semiconducting nature and tunable band gap. In particular, group 6 monolayer TMDs with chemical formula MX 2 (M = Mo, W and X = S, Se) are direct gap semiconductors with relatively intense photoluminescence (PL), while bilayers and thicker multilayers exhibit indirect gap and weaker PL. 1,10−17 The peculiar nature of the excited states has stimulated intense research efforts to investigate exciton dynamics and radiative/nonradiative lifetimes in TMDs. 13,18−24 Recent time-resolved experiments found a range of characteristic times for exciton dynamics in monolayer TMDs, including fast (1−10 ps) recombination attributed to exciton trapping at defects, and slower processes on a 0.1−1 ns time scale interpreted as radiative exciton recombination. 18,20−22 However, the attribution of the observed signals to radiatiave and nonradiative processes can be ambiguous in time-resolved spectroscopies since defects and impurities can modulate the excited state dynamics. In TMDs, the interpretation of time signals and comparison among different experiments is further complicated by the use of micrometer-size flakes where the edges can play a significant role in exciton recombination. This situation has stimulated an ongoing quest for the intrinsic time scale of exciton recombinatio...

Hot carriers (HC) generated by surface plasmon polaritons (SPPs) in noble metals are promising for application in optoelectronics, plasmonics and renewable energy. However, existing models fail to explain key quantitative details of SPP-to-HC conversion experiments. Here we develop a quantum mechanical framework and apply first-principles calculations to study the energy distribution and scattering processes of HCs generated by SPPs in Au and Ag. We find that the relative positions of the s and d bands of noble metals regulate the energy distribution and mean free path of the HCs, and that the electron–phonon interaction controls HC energy loss and transport. Our results prescribe optimal conditions for HC generation and extraction, and invalidate previously employed free-electron-like models. Our work combines density functional theory, GW and electron–phonon calculations to provide microscopic insight into HC generation and ultrafast dynamics in noble metals.