Enabling high efficiencies in MoS2 homojunction solar cells

Van der Waals structures made of layered semiconductor materials, such as transition metal dichalcogenides (TMDCs), have been proposed for the development of ultra-thin solar cells. The performance of these devices was initially limited by low open-circuit voltages (V
OC
), a problem that has been recently overcome with the demonstration of a V
OC
of 1 V in a MoS
2
homojunction. However, the conversion efficiency of that proof-of-concept device was still limited, especially by the quality of the metal/semiconductor contacts. It was found that in this type of solar cells the presence of photoactive Schottky barriers at the contacts reduces the V
OC
and the fill factor (FF). Also, MoS
2
homojunctions suffer from high reflectance at the front surface due to the large refractive index of this material. Here we present two MoS
2
homojunctions, one with metallic contacts deposited onto the MoS2 flakes, and the second with the flakes placed onto pre-patterned contacts. We demonstrate that it is possible to achieve ideal contacts that follow the Schottky-Mott rule in the second case if the metal is deposited with a photolithography process that produces perfectly flat surfaces. We also show that light absorption in MoS2 homojunction solar cells can be significantly enhanced by introducing a hexagonal boron nitride (h-BN) top layer.