2, Electrical Engineering, University of Pennsylvania, Philadelphia, Pennsylvania, United States
3, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
The class of layered van der Waals (vdW) materials, materials where their weakly-bonded layered structure provides intrinsically passivated surfaces along the basal plane, allows one to mechanically cleave these materials to produce a single monolayer of atoms. Graphene, cleaved from graphite, was the first member discovered in this layered two-dimensional (2D) family. The group of layered vdW materials now span nearly every material class imaginable, from insulators and semiconductors to superconductors and topological insulators. Furthermore, because of their weak out-of-plane van der Waals interaction, they can be mechanically stacked to form a high-quality heterostructure between the most disparate materials, allowing one to design arbitrary heterostructures with a vast array of unexplored properties and applications.
An interesting member of this 2D family for photovoltaic applications is the semiconducting transition-metal dichalcogenides (TMDCs), compounds of the form MX2, where M is a transition metal and X is a chalcogen (e.g. sulfur, selenium, or tellurium). TMDCs have some of the highest absorption coefficients of photovoltaic materials, and in their monolayer form, can be passivated in a way to reach nearly perfect radiative efficiency. Moreover, they offer a wide range of bandgaps (~1.1 – 2.0 eV), which can be tuned through quantum confinement effects and chemical alloying. Combined with their inherent stability and use of earth-abundant metals, vdW heterostructures of TMDCs are therefore attractive candidates for photovoltaic applications.
A conventional photovoltaic device that has high efficiency must maximize three figures of merit: optical absorption, subsequent carrier collection, and high photovoltage. Through proper nanophotonic engineering and device design, we show that atomically-thin vdW materials can achieve these three figures of merit and therefore act as efficient photovoltaic materials.
First, we experimentally show that in ultrathin (<10 nm) transition metal dichalcogenides (TMDCs), it’s possible to achieve near-unity (>90%) broadband absorption in the visible spectrum. We therefore utilize this optical geometry along with an optimized carrier collection scheme (IQE > 70%) to create a device with record photocurrent density (EQE > 50%) in these materials. Second, we modified the recombination dynamics in these materials through the use of carrier selective contacts, leading to record values of open circuit voltage (Voc > 700 mV). Lastly, we present an outlook for large-area integration and schemes to further enhance the efficiency in these atomically-thin devices.