Heterostructures comprising two-dimensional (2D) semiconductors fabricated by individual stacking exhibit interesting characteristics owing to their 2D nature and atomically sharp interface. As an emerging 2D material, black phosphorus (BP) nanosheets have drawn much attention because of their small band gap semiconductor characteristics along with high mobility. Stacking structures composed of p-type BP and n-type transition metal dichalcogenides can produce an atomically sharp interface with van der Waals interaction which leads to p-n diode functionality. In this study, for the first time, we fabricated a heterojunction p-n diode composed of BP and WS. The rectification effects are examined for monolayer, bilayer, trilayer, and multilayer WS flakes in our BP/WS van der Waals heterojunction diodes and also verified by density function theory calculations. We report superior functionalities as compared to other van der Waals heterojunction, such as efficient gate-dependent static rectification of 2.6 × 10, temperature dependence, thickness dependence of rectification, and ideality factor of the device. The temperature dependence of Zener breakdown voltage and avalanche breakdown voltage were analyzed in the same device. Additionally, superior optoelectronic characteristics such as photoresponsivity of 500 mA/W and external quantum efficiency of 103% are achieved in the BP/WS van der Waals p-n diode, which is unprecedented for BP/transition metal dichalcogenides heterostructures. The BP/WS van der Waals p-n diodes have a profound potential to fabricate rectifiers, solar cells, and photovoltaic diodes in 2D semiconductor electronics and optoelectronics.

We present a van der Waals p-MoTe2/n-MoTe2 homojunction p–n diode with low resistance metal contacts.

has been dominated by various singlejunction solar cells with a practical efficiency of up to 22%. To date, photovoltaic devices with high efficiency, long lifetime, compact size, and low cost as a highlighter key still require more attention. Current commercially available solar panels based on mono-crystalline silicon (c-Si) wafers for single-junction solar cells dominate the current PV market. So far, laboratory solar cells have been fabricated with an efficiency of nearly 26.3%. Even though the energy conversion efficiency reaches a maximum value of ≈33.5% for the upper theoretical energy conversion efficiency with a bandgap of 1.15 eV. [2,3] Since Russell Shoemaker Ohl's experiment over 80 years ago, the p-n junction has become an important part of modern electronics and optoelectronics. [4] This device is constructed by connecting two types of dopants, n-type and p-type, together. [5,6] As a result, an intrinsic electric field is present at the interface, which could be employed by electron-hole pair separation created by the absorption of incoming photons. The photovoltaic (PV) effect is the phenomenon of voltage and current generation in materials while they are illuminated. Non-centrosymmetric materials are made up of only a single component. But a photocurrent is an electric current that can also be made when there is no built-in potential It is highly desirable for exploring and discovering new materials and outcome-based approaches to exceed the Shockley-Queisser limit for singlejunction photovoltaic cells. Low-dimensional piezoelectric materials have the potential to generate the optoelectronic phenomenon called the bulk photovoltaic effect, which is not limited by the theoretical limit for solar radiation into electricity conversion. The recent development of 2D materials has demonstrated that by using the bulk photovoltaic effect (BPVE) for crystals lacking inversion symmetry, it is possible to overcome this limit. So far, the exploration of p-n junction designs has been addressed in several review articles. However, the mechanism of BPVE differs from traditional p-n junctionbased photovoltaics in 2D materials. In this focused review, various concepts regarding the shift-current response are explored, both from theoretical and experimental points of view, which are generated in the framework of deformed 2D materials. Finally, prospective approaches for building BPVEbased next-generation solar cells using ultrathin 2D materials are presented. These materials are expected to work better than current methods of turning energy into electricity.

Schottky-barrier diodes have great importance in power management and mobile communication because of their informal device technology, fast response and small capacitance.

2D layered germanium selenide (GeSe) with p-type conductivity is incorporated with asymmetric contact electrode of chromium/Gold (Cr/Au) and Palladium/Gold (Pd/Au) to design a self-biased, high speed and an efficient photodetector. The photoresponse under photovoltaic effect is investigated for the wavelengths of light (i.e. ~220, ~530 and ~850 nm). The device exhibited promising figures of merit required for efficient photodetection, specifically the Schottky barrier diode is highly sensitive to NIR light irradiation at zero voltage with good reproducibility, which is promising for the emergency application of fire detection and night vision. The high responsivity, detectivity, normalized photocurrent to dark current ratio (NPDR), noise equivalent power (NEP) and response time for illumination of light (~850 nm) are calculated to be 280 mA/W, 4.1 × 10 9 Jones, 3 × 10 7 W −1 , 9.1 × 10 −12 WHz −1/2 and 69 ms respectively. The obtained results suggested that p-GeSe is a novel candidate for SBD optoelectronics-based technologies. Two-dimensional (2D) materials have chronically been the most widely studied materials, particularly after the successful scotch tape test to exfoliate graphene by Andre Geim and Kostya Novoselov in 2004 1. 2D materials possess excellent electrical and mechanical properties toward diverse electronic device applications. Graphene, being the prototype of 2D materials 2,3 , has been studied broadly for its exotic electrical, optical, and mechanical properties 3,4. Besides, the group-IV transition metal dichalcogenides (TMDs) having a bandgap of around 1 to 2 eV 5-7 have attracted increasing interest because of their promising electronic and optoelectronic device applications 3,8-21. Graphene possesses extremely high carrier mobility (>10 5 cm 2 V −1 s −1), but the absence of band gap limits its electronic and optoelectronic applications 22. Therefore, TMDs with the properties of graphene-like stature, bandgap tunability, weak van der Waals-like forces and stability have intrigued the interest of the scientific community. TMDs are the family of 2D materials having the chemical composition of MX 2 , where M stands for the transition metal elements (M = Mo, W, Ta, Ge…etc) and X for the chalcogen elements (X = Se, S and Te). Among TMDs, Ge-based materials are preferred for applications due to their abundance on earth and environmentally friendly nature 23. With Se, the p-type Germanium from a narrow bandgap semiconductor material as p-GeSe having exciting application in near-infrared (NIR) photodetectors and electron tunnelling devices. p-GeSe has an indirect bandgap of 1.08 eV in the bulk 24,25 , and a direct bandgap of ~1.7 eV in monolayers 24,26,27. Few layers of p-GeSe can be obtained from bulk by mechanical exfoliation method 28. Among the many applications, p-GeSe shows tremendous capability in the realm of photovoltaics, because of its excellent optical, material and electrical properties. Therefore, it is well known as substitution of phosphorene 29. Moreover, GeSe is considered as an amb...