Design of new photovoltaic systems based on two-dimensional group-IV monochalcogenides for high performance solar cells

It is of great significance to design high performance photovoltaic systems for application in excitonic solar cells. Here, we propose a new photovoltaic system based on single layer group-IV monochalcogenides MX (M = Ge, Sn; X = S, Se) via density functional theory combined with the Keldysh nonequilibrium Green's function (DFT-NEGF) approach. Our results reveal that the proposed MX monolayers are rich in many attractive characteristics, that are, light effective mass, suitable band gaps (1.3-2.5 eV), and small exciton binding energies, which make MX monolayers promising candidates for fabricating advanced appliances of photovoltaic devices. The photoresponse and the photovoltaic performance of MX monolayers are evaluated by means of quantum transport simulations. Under illumination, the MX based systems exhibit high photoresponsivity (R-ph = 0.16 A W-1) and external quantum efficiency (EQE = 30.3%) in the visible region. In addition, the investigation of the interfacial properties of GeSe monolayers with contact metals (Al, Cu, Ag, and Au) indicates that there are tunneling barriers and Schottky barriers in the contact of GeSe with Ag and Al, while a Schottky barrier is absent in that of GeSe with Au. Interestingly, both tunneling and Schottky barriers vanish in that of GeSe with Cu due to the strong interactions between GeSe and Cu, which means an ohmic contact forms at the GeSe-Cu interface, leading to the possible high performance of the devices. These results can provide an insight into the design of next-generation solar cell devices.