Broadband Absorption Engineering to Enhance Light Absorption in Monolayer MoS₂

Here we take a first step toward tackling the challenge of incomplete optical absorption in monolayers of transition metal dichalcogenides for conversion of photon energy, including solar, into other forms of energy. We present a monolayer MoS₂-based photoelectrode architecture that exploits nanophotonic light management strategies to enhance absorption within the monolayer of MoS₂, while simultaneously integrating an efficient charge carrier separation mechanism facilitated by a MoS₂/NiO_x heterojunction. Specifically, we demonstrate two extremely thin photoelectrode architectures for solar-fuel generation: (i) a planar optical cavity architecture, MoS₂/NiO_x/Al, that improves optical impedance matching and (ii) an architecture employing plasmonic silver nanoparticles (Ag NPs), MoS₂/Ag NPs/NiO_x/Al, that further improves light absorption within the monolayer. We used a combination of numerical simulations, analytical models, and experimental optical characterizations to gain insights into the contributions of optical impedance matching versus plasmonic near-field enhancement effects in our plasmonic photoelectrode structures. By performing three-dimensional electromagnetic simulations, we predict structures that can absorb 37% of the incident light integrated from 400 to 700 nm within a monolayer of MoS₂, a 5.9× enhanced absorption compared to that of MoS₂ on a sapphire (Al₂O₃) substrate. Experimentally, a 3.9× absorption enhancement is observed in the total structure compared to that of MoS₂/Al₂O₃, and photoluminescence measurements suggest this enhancement largely arises from absorption enhancements within the MoS₂ layer alone. The results of these measurements also confirm that our MoS₂/NiO_x/Al structures do indeed facilitate efficient charge separation, as required for a photoelectrode. To rapidly explore the parameter space of plasmonic photoelectrode architectures, we also developed an analytical model based on an effective medium model that is in excellent agreement with results from numerical FDTD simulations.