Few-Layer WSe2 Schottky Junction-Based Photovoltaic Devices through Site-Selective Dual Doping

Doping-Free High-Performance Photovoltaic Effect in a WSe2 Lateral p-n Homojunction Formed by Contact Engineering

Two-dimensional transition metal dichalcogenides (TMDs) are promising materials for semiconductor nanodevices owing to their flexibility, transparency, and appropriate band gaps. A variety of optoelectronic and electronic devices based on TMDs p-n diodes have been extensively investigated due to their unique advantages. However, improving their performance is challenging for commercial applications. In this study, we propose a facile and doping-free approach based on the contact engineering of a few-layer tungsten di-selenide to form a lateral p-n homojunction photovoltaic. By combining surface and edge contacts for p-n diode fabrication, the photovoltaic effect is achieved with a high fill factor of ≈0.64, a power conversion efficiency of up to ≈4.5%, and the highest external quantum efficiency with a value of ≈67.6%. The photoresponsivity reaches 283 mA/W, indicating excellent photodiode performance. These results demonstrate that our technique has great potential for application in next-generation optoelectronic devices.

Synthesis of mono- and few-layered n-type WSe2 from solid state inorganic precursors

Tuning the charge transport properties of two-dimensional transition metal dichalcogenides (TMDs) is pivotal to their future device integration in post-silicon technologies. To date, co-doping of TMDs during growth still proves to be challenging, and the synthesis of doped WSe2, an otherwise ambipolar material, has been mainly limited to p-doping. Here, we demonstrate the synthesis of high-quality n-type monolayered WSe2 flakes using a solid-state precursor for Se, zinc selenide. n-Type transport has been reported with prime electron mobilities of up to 10 cm2 V-1 s-1. We also demonstrate the tuneability of doping to p-type transport with hole mobilities of 50 cm2 V-1 s-1 after annealing in air. n-Doping has been attributed to the presence of Zn adatoms on the WSe2 flakes as revealed by X-ray photoelectron spectroscopy (XPS), spatially resolved time of flight secondary ion mass spectroscopy (SIMS) and angular dark-field scanning transmission electron microscopy (AD-STEM) characterization of WSe2 flakes. Monolayer WSe2 flakes exhibit a sharp photoluminescence (PL) peak at room temperature and highly uniform emission across the entire flake area, indicating a high degree of crystallinity of the material. This work provides new insight into the synthesis of TMDs with charge carrier control, to pave the way towards post-silicon electronics.

Gate-Modulated Ultrasensitive Visible and Near-Infrared Photodetection of Oxygen Plasma-Treated WSe2 Lateral pn-Homojunctions

We investigate the development of gate-modulated tungsten diselenide (WSe₂)-based lateral pn-homojunctions for visible and near-infrared photodetector applications via an effective oxygen (O₂) plasma treatment. O₂ plasma acts to induce the p-type WSe₂ for the otherwise n-type WSe₂ by forming a tungsten oxide (WO_x) layer upon O₂ plasma treatment. The WSe₂ lateral pn-homojunctions displayed an enhanced photoresponse and resulted in open-circuit voltage (V_OC) and short-circuit current (I_SC) originating from the pn-junction formed after O₂ plasma treatment. We further notice that the amplitude of the photocurrent can be modulated by different gate biases. The fabricated WSe₂ pn-homojunctions exhibit greater photoresponse with photoresponsivities (ratio of the photocurrent and incident laser power) of 250 and 2000 mA/W, high external quantum efficiency values (%, total number of charge carriers generated for the number of incident photons on photodetectors) of 97 and 420%, and superior detectivity values (magnitude of detector sensitivity) of 7.7 × 10⁹ and 7.2 × 10¹⁰ Jones upon illumination with visible (520 nm) and near-infrared lasers (852 nm), respectively, at low bias (V_g = 0 V and V_d = 1 V) at room temperature, demonstrating very high-performance in the IR region superior to the contending two-dimensional material-based photonic devices. These superior optoelectronic properties are attributed to the junctions induced by O₂ plasma doping, which facilitate the effective carrier generation and separation of photocarriers with applied external drain bias upon strong light absorption.

Modulation of Junction Modes in SnSe2/MoTe2 Broken-Gap van der Waals Heterostructure for Multifunctional Devices

We study the electronic and optoelectronic properties of a broken-gap heterojunction composed of SnSe₂ and MoTe₂ with gate-controlled junction modes. Owing to the interband tunneling current, our device can act as an Esaki diode and a backward diode with a peak-to-valley current ratio approaching 5.7 at room temperature. Furthermore, under an 811 nm laser irradiation the heterostructure exhibits a photodetectivity of up to 7.5 × 10¹² Jones. In addition, to harness the electrostatic gate bias, V_oc can be tuned from negative to positive by switching from the accumulation mode to the depletion mode of the heterojunction. Additionally, a photovoltaic effect with a fill factor exceeding 41% was observed, which highlights the significant potential for optoelectronic applications. This study not only demonstrates high-performance multifunctional optoelectronics based on the SnSe₂/MoTe₂ heterostructure but also provides a comprehensive understanding of broken-band alignment and its applications.

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.

Transfer of transition-metal dichalcogenide circuits onto arbitrary substrates for flexible device applications

Transition-metal dichalcogenide (TMD) materials with two-dimensional layered structures and stable surfaces are well suited for transparent and flexible device applications. In order to completely utilize the advantages of thickness control and fabrication of various heterostructure stacks, we proposed a transfer method of TMD field-effect transistors (FETs) and TMD complementary metal-oxide-semiconductor (CMOS) circuits from a Si/SiO2 substrate to a flexible substrate. We compared the characteristics of transferred MoS2 and WSe2 FETs with those of the corresponding devices transferred after channel passivation with an Al2O3 layer on a flexible substrate. Al2O3 passivation further stabilized the transfer of the entire device with electrodes. A CMOS circuit with MoS2 and WSe2 materials could be successfully transferred to a polyethylene terephthalate substrate after the channel passivation. This implies that TMD circuits can be easily fabricated on polymer substrates, which makes them suitable for use in semiconductor processes, for various applications.