Abstract
Graphene has shown superior properties suitable for photovoltaic devices, including high optical transmittance, high mechanical flexibility, low resistivity, and high carrier mobility. In theory, with the property of semi-metal, graphene is able to form Schottky junction with any semiconductor having moderate carrier density. Monolayer Transition Metal Dichalcogenides (TMDs) are also promising two-dimensional materials in optoelectronic devices. Monolayer TMDs are semiconductors with direct band gap which is beneficial to optoelectronic usage. Combination of monolayer TMDs and bulk materials potentially can overcome the limited absorption problem of 2D materials and difficulties in mass production. This work consists of two studies: Transferring the chemical vapor deposition (CVD) grown TMDs onto p-type Si substrates to produce heterojunction p-n solar cells, and transferring graphene which is also grown by CVD onto n-type GaAs to produce Schottky junction solar cells. In TMDs heterojunction p-n solar cells, various back electrode materials and top electrode structures were tested. The best power conversion efficiencies obtained for the MoS2 and WS2 heterojunction solar cells are 4.23 and 2.08 %, respectively. In grapheme/GaAs Schottky junction solar cells, we used either the GaAs substrate or GaAs thin films with various doping concentrations grown by molecular beam epitaxy. The best power conversion efficiency of grapheme/GaAs Schottky junction solar cells is 2.08 %.