Solar Energy Harvesting in Type II van der Waals Heterostructures of Semiconducting Group III Monochalcogenide Monolayers

Type II van der Waals (vdW) heterobilayers of six group III chalcogenide monolayers (MX; M = Ga, In; X = S, Se, Te) have been systematically investigated and compared with the pristine monolayers via hybrid HSE06 functional based DFT calculations in order to evaluate their utility in photocatalysis and 2D excitonic solar cells. The interfacial binding energies of the type II vdW heterobilayers reported herewith, using DFT-D3, is found to vary from -15 to -25 meV/angstrom(2), which is higher than that in graphite (-12.0 meV/ angstrom(2)). Out of the three stacking orders in the heterobilayers studied in this work, the most energetically favorable one has been considered for photocatalytic and solar cell applications. The various aspects of stabilities of these heterobilayers, such as, energetic, lattice dynamical, and mechanical, have been confirmed. Our investigation shows that InS/InSe and GaTe/InS heterobilayers are semiconductors having a direct band gap of 2.31 and 1.28 eV respectively, although the individual monolayers show indirect band gaps. The power conversion efficiency is found to reach 12.66% and 13.17% in GaTe/InTe and InS/InSe heterostructures, respectively, which are higher than the highest efficiencies reported in organic solar cells (11.7%) and MoS2/p-Si heterojunction solar cells (5.23%). GaTe/InTe shows photocatalytic water splitting abilities over a large range in pH (1-12), while the acidic to neutral medium (pH < 8) is found to be suitable for GaSe/InS. The solar energy absorbance in these heterostructures is observed to be of the order of 10(4) cm(-1). The immense potential of these type II vdW heterobilayers in 2D excitonic solar cells and photocatalysis is also underlined.