**3.2 Photovoltaic based on 2d homojunction**

Due to the very low efficiency of the Schottky junction, more research efforts are required to improve photovoltaic processes in the semiconducting p-n junction.


#### **Table 1.**

*Main terms to demonstrate the photovoltaic device.*

**Figure 2.**

*Photovoltaic effect with Pd-Au bias configuration. (a) Optical image of the device. The spacing between the electrodes is 2* μ*m. (b) Current vs. source-drain voltage (at VG = 0) showing strong asymmetry and photoresponse with diode-like behavior for the Pd-Au bias configuration indicated in (a). (c) Effect of sourcedrain bias on a device with one Au and one Pd electrode. Reprinted with permission from [58]. Copyright (2013) springer nature.*

Using a splitting gate on monolayer WSe2, Pospischil *et al*. [61], Baugher *et al.* [62], and Ross *et al.* [63] effectuated experimentally p-n junctions. This demonstration of WSe2 monolayer flake has been achieved with mechanical exfoliation after that shifted onto a pair of split gates slipover with previously formed gate dielectric materials (SiN, HfO2). The charge density and the conduction type (into the monolayer thick channel) can be modulated by electrostatic control after dissimilar voltages have been applied on the two local gates and the automatically thin p-n junction was formed. Due to this remarkable rectification in the diode behavior occur, finally able to photovoltaic generation. Taking the gap within two gates as a photoactive area the power conversion of 0.5% was demonstrated by Pospischil *et al.* with Voc of 0.64 V and illumination of 140 mW/cm<sup>2</sup> (halogen source). The remarkable results are in the picture with very high efficiency of photovoltaic energy conversion, assuming the monolayer WSe2 (95% transparency), which opens a pipeline of single-layer TMDs for semi-transparent solar cells. Memaran *et al.* [64] successfully demonstrated the composition of electrostatically generated MoSe2 multilayer p-n junction to achieve high photovoltaic performance.

In addition to modifying the photovoltaic parameters, the 2d black phosphorous (BP) has attracted more attention of researchers due to its remarkable optical and electrical properties, keeping in mind its unique bandgap (≈0.3–2.0 eV), in-plane anisotropy and high carrier mobility i.e. 1000 cm2 /Vs, hence BP shows the possibility for broadband optoelectronic applications [65–67]. Choi *et al.* [68] develop a technique to form a lateral homogeneous 2d MoS2 p-n junction by partially stacking 2d h-BN as a mask to p-dope MoS2. The fabricated lateral MoS2 p-n junction with asymmetric electrodes of Pd and Cr/Au displayed a highly efficient photo response such as maximum external quantum efficiency of ∼7000%, specific directivity of ∼5 x1010 Jones, and light switching ratio of ∼103 . **Figure 3** shows the fabrication of the MoS2 p-n diode. **Figure 3b** shows the optical microscopy image of MoS2 p-n diode with hetero electrodes.

**271**

**Figure 4.**

*Chemical Society.*

**Figure 3.**

*Two-Dimensional Materials for Advanced Solar Cells DOI: http://dx.doi.org/10.5772/intechopen.94114*

*Copyright (2014) American Chemical Society.*

**3.3 Photovoltaic based on 2d heterojunction**

The p-n heterojunctions work as a basic backbone of various optoelectronic devices and applications due to various theoretical and experimental restrictions, there is the need for manufacturing designed heterostructures. Duan *et al.* [69] demonstrate the modulated MoS2-MoSe2 and WS2-WSe2 lateral heterostructures by thermal chemical vapor deposition (CVD) technique. The well-known n-type (WS2) and p-type (WSe2) builds natured heterojunction p-n diode with current rectification. When such heterojunction p-n diodes are characterized. The

*(a) Schematic diagram of the photodiode based on the WSe2/WS2 heterojunction. (b) Optical image of the photodiode device. (c) Photocurrent and responsivity as a function of the incident power. d) the photocurrent image is taken from the device in (b). Reprinted with permission from [71]. Copyright (2017) American* 

*Fabrication of the MoS2 p-n diode. (a) Cross-sectional diagram and (b) optical microscopy image of MoS2 p-n diode with hetero electrodes. The scale bar indicates 10* μ*m. (c) Three-dimensional schematic and circuit diagrams of the fabricated MoS2 p-n diodes under light illumination. Reprinted with permission from [68].* 

*Two-Dimensional Materials for Advanced Solar Cells DOI: http://dx.doi.org/10.5772/intechopen.94114*

#### **Figure 3.**

*Solar Cells - Theory, Materials and Recent Advances*

Using a splitting gate on monolayer WSe2, Pospischil *et al*. [61], Baugher *et al.* [62], and Ross *et al.* [63] effectuated experimentally p-n junctions. This demonstration of WSe2 monolayer flake has been achieved with mechanical exfoliation after that shifted onto a pair of split gates slipover with previously formed gate dielectric materials (SiN, HfO2). The charge density and the conduction type (into the monolayer thick channel) can be modulated by electrostatic control after dissimilar voltages have been applied on the two local gates and the automatically thin p-n junction was formed. Due to this remarkable rectification in the diode behavior occur, finally able to photovoltaic generation. Taking the gap within two gates as a photoactive area the power conversion of 0.5% was demonstrated by Pospischil

*Photovoltaic effect with Pd-Au bias configuration. (a) Optical image of the device. The spacing between the electrodes is 2* μ*m. (b) Current vs. source-drain voltage (at VG = 0) showing strong asymmetry and photoresponse with diode-like behavior for the Pd-Au bias configuration indicated in (a). (c) Effect of sourcedrain bias on a device with one Au and one Pd electrode. Reprinted with permission from [58]. Copyright* 

remarkable results are in the picture with very high efficiency of photovoltaic energy conversion, assuming the monolayer WSe2 (95% transparency), which opens a pipeline of single-layer TMDs for semi-transparent solar cells. Memaran *et al.* [64] successfully demonstrated the composition of electrostatically generated

In addition to modifying the photovoltaic parameters, the 2d black phosphorous (BP) has attracted more attention of researchers due to its remarkable optical and electrical properties, keeping in mind its unique bandgap (≈0.3–2.0 eV), in-plane

ity for broadband optoelectronic applications [65–67]. Choi *et al.* [68] develop a technique to form a lateral homogeneous 2d MoS2 p-n junction by partially stacking 2d h-BN as a mask to p-dope MoS2. The fabricated lateral MoS2 p-n junction with asymmetric electrodes of Pd and Cr/Au displayed a highly efficient photo response such as maximum external quantum efficiency of ∼7000%, specific directivity of

of the MoS2 p-n diode. **Figure 3b** shows the optical microscopy image of MoS2 p-n

MoSe2 multilayer p-n junction to achieve high photovoltaic performance.

(halogen source). The

/Vs, hence BP shows the possibil-

. **Figure 3** shows the fabrication

*et al.* with Voc of 0.64 V and illumination of 140 mW/cm<sup>2</sup>

anisotropy and high carrier mobility i.e. 1000 cm2

∼5 x1010 Jones, and light switching ratio of ∼103

diode with hetero electrodes.

**270**

**Figure 2.**

*(2013) springer nature.*

*Fabrication of the MoS2 p-n diode. (a) Cross-sectional diagram and (b) optical microscopy image of MoS2 p-n diode with hetero electrodes. The scale bar indicates 10* μ*m. (c) Three-dimensional schematic and circuit diagrams of the fabricated MoS2 p-n diodes under light illumination. Reprinted with permission from [68]. Copyright (2014) American Chemical Society.*

#### **Figure 4.**

*(a) Schematic diagram of the photodiode based on the WSe2/WS2 heterojunction. (b) Optical image of the photodiode device. (c) Photocurrent and responsivity as a function of the incident power. d) the photocurrent image is taken from the device in (b). Reprinted with permission from [71]. Copyright (2017) American Chemical Society.*

#### **3.3 Photovoltaic based on 2d heterojunction**

The p-n heterojunctions work as a basic backbone of various optoelectronic devices and applications due to various theoretical and experimental restrictions, there is the need for manufacturing designed heterostructures. Duan *et al.* [69] demonstrate the modulated MoS2-MoSe2 and WS2-WSe2 lateral heterostructures by thermal chemical vapor deposition (CVD) technique. The well-known n-type (WS2) and p-type (WSe2) builds natured heterojunction p-n diode with current rectification. When such heterojunction p-n diodes are characterized. The

photovoltaic effect with a Voc of ≈ 0.47 V and Isc of ≈1.2 nA was established with laser illumination conditions (514 nm and 30nW). The internal quantum efficiency (IQE) and external quantum efficiency (EQE) were found to be 43% and 9.9% respectively The active regime was selected as lightly doped WS2 and WS2-WSe2 interface region through photocurrent mapping, implies with the fact "A large fraction of the depletion layer is localized to the lightly doped WS2 of the diode" [69]. Atomically sharp in-plane WSe2/MoS2 interface automatically comes into the picture with a high-magnification scanning transmission electron microscope (STEM), these force the photovoltaic effect of the intrinsic single layer p-n heterojunction was indorsed [70]. Li *et al.* [71] successfully fabricated a composition graded doped lateralWSe2/WS2 heterostructure by ambient pressure CVD in a single heat-cycle. The optoelectronic device (**Figure 4a-b**) based on the lateral WSe2/WS2 heterostructure shows improved photodetection performance in terms of a reasonable responsivity and a large photoactive area. The photocurrent and photo-responsivity are also depicted in **Figure 4c**. The photocurrent appears to increase nonlinearly, whereas the photoresponsivity decreases as the light power increases, with the highest obtained photoresponsivity of 6.5 A.W−1. The reduction of the photoresponsivity at higher light powers may be ascribed to the reduction of unoccupied states in the conduction bands of WSe2 and WS2.
