Optoelectronics of Multijunction Heterostructures of Transition Metal Dichalcogenides.

Among p-n junction devices with multilayered heterostructures with WSe and MoSe, a device with the MoSe-WSe-MoSe (NPN) structure showed a remarkably high photoresponse, which was 1000 times higher than the MoSe-WSe (NP) structure. The ideality factor of the NPN structure was estimated to be ∼1, lower than that of the NP structure. It is claimed that the NPN structure formed a thinner depletion region than that of the NP structure because of the difference of carrier concentrations of MoSe and WSe. Hence, the built-in electric field was weaker, and the motion of the photocarriers was facilitated. These behaviors were confirmed experimentally from a photocurrent mapping analysis and Kelvin probe force microscopy. The work function depended on the wavelength of the illuminator, and quasi-Fermi level was estimated. The surface photovoltage on the MoSe region was higher than that on WSe because the lower bandgap of MoSe induces more electron-hole pair generation.