Interfacing Boron Monophosphide with Molybdenum Disulfide for an Ultrahigh Performance in Thermoelectrics, Two-Dimensional Excitonic Solar Cells, and Nanopiezotronics.

Abstract

A stable ultrathin 2D van der Waals (vdW) heterobilayer, based on the recently synthesized boron monophosphide (BP) and the widely studied molybdenum disulfide (MoS2), has been systematically explored for the conversion of waste heat, solar energy, and nanomechanical energy into electricity. It shows a gigantic figure of merit (ZT) > 12 (4) for p (n)-type doping at 800 K, which is the highest ever reported till date. At room temperature (300 K), ZT reaches 1.1 (0.3) for p (n)-type doping, which is comparable to experimentally measured ZT = 1.1 on the PbTe-PbSnS2 nanocomposite at 300 K, while it outweighs the Cu2Se-CuInSe2 nanocomposite (ZT = 2.6 at 850 K) and the theoretically calculated ZT = 7 at 600 K on silver halides. Lattice thermal conductivity (κl ≈ 49 W m-1 K-1) calculated at room temperature is lesser than those of black phosphorene (78 W m-1 K-1) and arsenene (61 W m-1 K-1). The nearly matched lattice constants in the commensurate lattices of the constituent monolayers help to preserve the direct band gap at the K point in the type II vdW heterobilayer of MoS2/BP, where BP and MoS2 serve as donor and acceptor materials, respectively. An ultrahigh carrier mobility of ∼20 × 103 cm2 V-1 s-1 is found, which exceeds those of previously reported transition metal dichalcogenide-based vdW heterostructures. The exciton binding energy (0.5 eV) is close to those of MoS2 (0.54 eV) and C3N4 (0.33 eV) single layers. The calculated power conversion efficiency (PCE) in the monolayer MoS2/BP heterobilayer exceeds 20%. It surpasses the efficiency in MoS2/p-Si heterojunction solar cells (5.23%) and competes with the theoretically calculated ones, as listed in the manuscript. Furthermore, a high optical absorbance (∼105 cm-1) of visible light and a small conduction band offset (0.13 eV) make MoS2/BP very promising in 2D excitonic solar cells. The out-of-plane piezoelectric strain coefficient, d33 ≈ 3.16 pm/V, is found to be enhanced 4-fold (∼14.3 pm/V) upon applying 7% vertical compressive strain on the heterobilayer, which corresponds to ∼1 kbar of hydrostatic pressure. Such a high out-of-plane piezoelectric coefficient, which can tune top-gating effects in ultrathin 2D nanopiezotronics, is a relatively new finding. As BP has been synthesized recently, experimental realization of the multifunctional, versatile MoS2/BP heterostructure would be highly feasible.