Optoelectronics of Multijunction Heterostructures of Transition Metal Dichalcogenides

Among p–n junction devices with multilayered heterostructures with WSe2 and MoSe2, a device with the MoSe2–WSe2–MoSe2 (NPN) structure showed a remarkably high photoresponse, which was 1000 times higher than the MoSe2–WSe2 (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 MoSe2 and WSe2. 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 MoSe2 region was higher than that on WSe2 because the lower bandgap of MoSe2 induces more electron–hole pair generation.