2.1.

Molybdenum Disulfide

Among 2D materials, the TMD family has received the most attention.⁸ TMDs are hexagonal in nature and absorb visible- to infrared-ranged light.⁹ They are promising semiconducting materials with extremely thin bodies that enhance carrier confinement and electrostatic gate control as a result of programmable bandgaps and the absence of surface dangling bonds.⁸ This study focuses on molybdenum disulfide (${\mathrm{MoS}}_2$), a well-known and thoroughly investigated TMD material with Mo atoms sandwiched between two layers of S atoms that has environmental stability and natural availability (Fig. 1).^8,10

Fig. 1

${\mathrm{MOS}}_2$ structure.⁸

Moreover, it appears in nature as an n-type material,¹¹ with visible to near-infrared light absorption,⁹ a large work function of 5.1 eV,¹² and mechanical stability and transparency for flexible devices.⁸

The most attractive feature of ${\mathrm{MOS}}_2$ is the variable energy gap from 1.2 to 1.8 eV as the material moves from the bulk indirect bandgap to the direct bandgap monolayer due to the quantum confinement effect.

Furthermore, ${\mathrm{MoS}}_2$ has optical properties that are thickness dependent.^13–15 The number of layers in ${\mathrm{MoS}}_2$ influences its electron affinity. The monolayer ${\mathrm{MoS}}_2$ had an electron affinity of 4.0 eV,¹⁶ three layers of ${\mathrm{MoS}}_2$ had an affinity of 4.0 eV,¹⁷ and multilayers (28 nm) had an affinity of 4.1 eV.¹⁸ Thus the ${\mathrm{MoS}}_2$, with a monolayer thickness of 0.65 nm, has more than 80% of its transparency, making it ideal for wearable applications.¹⁹ The ${\mathrm{MoS}}_2$’s physical properties are given in Table 1.

Table 1

Physical properties of the MoS2.

As mentioned, the contact Schottky barrier height, contact resistivity, and carrier mobility are all impacted by thickness.⁸ Depending on the application, the ${\mathrm{MoS}}_2$ layer’s thickness needs to be carefully adjusted for maximum performance. ${\mathrm{MoS}}_2$ nanofiber bundles were made by the hydrothermal technique in 2007 by Nagaraju et al.,²⁴ using ammonium molybdate in an acidic medium and maintained at 180°C for several hours. The sample’s PXRD pattern may be easily identified as hexagonal 2H-${\mathrm{MoS}}_2$. According to the ${\mathrm{MoS}}_2$ FTIR spectra, the band at $480{\mathrm{cm}}^{-1}$ is called (Mo-S). ${\mathrm{MoS}}_2$ nanofibers are found in bundles that are 20 to 25 m long and 120 to 300 nm in diameter.

According to Li et al.,²⁵ advanced materials for photoelectrochemical water splitting are essential for the field of renewable energy. This study demonstrated selective solvothermal synthesis of ${\mathrm{MoS}}_2$ NPs on sheets of reduced graphene oxide (rGO) floating in solution. The resulting ${\mathrm{MoS}}_2/\mathrm{rGO}$ hybrid material, in contrast to the enormous aggregated ${\mathrm{MoS}}_2$ particles grown freely in solution without GO, had nanoscopic few-layer ${\mathrm{MoS}}_2$ structures with a lot of exposed edges piled on graphene. The ${\mathrm{MoS}}_2/\mathrm{rGO}$ hybrid demonstrated greater electrocatalytic activity in the hydrogen evolution process when compared with conventional ${\mathrm{MoS}}_2$ catalysts [hydrogen evolution reaction (HER)].

The study by Helmly et al.²⁶ investigated the production and characterization of molybdenum (IV) chalcogenide NPs. The process involves the interaction of molybdenum hexacarbonyl with sulfur or selenium. The NPs absorb light in the UV area of the spectrum, whereas bulk molybdenum (IV) chalcogenides absorb light in the near-IR. When stimulated in the UV range, the particles also emit a blue fluorescence.

Also Li et al.²⁷ used temporally structured femtosecond laser ablation of bulk ${\mathrm{MoS}}_2$ targets in water to create monolayer ${\mathrm{MoS}}_2$ QDs. The findings of the investigation reveal that very pure, homogeneous, and monolayer ${\mathrm{MoS}}_2$ QDs may be produced effectively.

A combination of multilayer photoexfoliation of monolayer ${\mathrm{MoS}}_2$ and water photoionization enhanced light absorption is the underlying process. Because of the many active edge sites, high specific surface area, and superior electrical conductivity, the as-prepared ${\mathrm{MoS}}_2\mathrm{QDs}$ have remarkable electrocatalytic activity for hydrogen evolution processes. The second important material for postsilicon electronics that are used in optoelectronics applications is ${\mathrm{MoS}}_2$.²⁷

${\mathrm{MoS}}_2$ NPs were generated using the laser ablation technique ($\mathrm{Nd}:\mathrm{YAG}=1064\mathrm{nm}$) in the polymer ethylene glycol solution as a function of laser fluence, according to Moniri and Hantehzadeh.²⁸ The resulting NPs are hexagonal crystals with diameters ranging from 4 to 160 nm, according to the findings. Furthermore, according to the laser fluence, the majority of them had a maximum wavelength that ranged from 212.5 to 216.5 nm. As the laser flux increases, the size of these particles will increase from 10 to 18 nm.

2.2.

Graphene

Graphene, as shown in Fig. 2, is the basic structure of all forms of graphite that is now used in most modern fields in the last decade. As a result of its unusual physical and chemical features, it has piqued the interest of researchers all over the world, proving its theoretical significance.³⁰ The physical characteristics of graphene are provided in Table 2.

Fig. 2

Structure of graphene.²⁹

Table 2

Physical properties of graphene.31

3.1.

Mechanical Exfoliation Technique

The mechanical exfoliation technique is a method that involves extending layered materials by outside forces (such as chemical or physical forces) to lessen interlayer noncovalent interactions connections, allowing them to gradually separate into a single structure of very few layers.³³ The first synthesis method for making graphene was the method of mechanical exfoliation, known as the adhesive tape method; it has since been successfully used for multilayer 2D materials. The few-layer ${\mathrm{MoS}}_2$ nanosheets were generated by Li et al.³⁴ utilizing an exfoliation technique, as shown in Fig. 3(a). A second sticky side of the tape was applied to the scotch tape with crystal after the first one was pushed onto the crystal surface for about 5 s. The scotch tape is then carefully removed at the tiniest possible angles. The two tapes are then torn, and crystals of the same size are glued to each. When this procedure is repeated, the bulk ${\mathrm{MoS}}_2$ flakes become thinner and thinner until they are reduced to one or a few layers. The ${\mathrm{MoS}}_2$ layers on tape were next successfully adhered to a spotless ${\mathrm{Al}}_2{\mathrm{O}}_3$ ceramic substrate. Before removing the scotch tape, the adhesion state was maintained for 6 h. After that, acetone was poured over the substrate to eliminate the scotch tape’s adhesive residue. It is a simple, inexpensive, and straightforward technology for creating high-quality 2D materials, but it has limitations due to the size of the created material being $<100\mathrm{m}$ and the inability to change the layer thickness. As a result, it is unsuitable for materials research and device demonstration at the industrial level in pioneering work.

Fig. 3

(a) Diagram of the mechanically exfoliated ${\mathrm{MoS}}_2$ nanosheets,³⁰ (b) a schematic of the liquid exfoliation process’s mechanism, and (c) diagram of the CVD method used to create ${\mathrm{MoSe}}_2$ crystals on molten glass.³⁵

3.2.

Liquid Exfoliation Technique

The liquid exfoliation method is an inexpensive method. Hernandez³⁶ used sonication to exfoliate graphite in an organic solution containing N-methyl pyrrolidone. The graphene dispersion was obtained at a concentration of $0.01\mathrm{mg}/\mathrm{ml}$, and a monolayer graphene yield was 1 wt.%. Various liquid exfoliation procedures, such as high shear mixing,³⁵ microoxidization,³⁷ and jet cavitation,³⁸ have been documented. As illustrated in Fig. 3(b), sonication-assisted liquid exfoliation is widely used in synthesis. Nanosheets are created simply by dissolving the beginning bulk material in a solvent to disperse the two-dimensional components in the solution, and ultrasound or centrifugation is used.

3.3.

Chemical Vapor Depositions Technique

Using active chemical reactions of precursors in a designed environment, CVD is used to create 2D materials. Numerous operational factors, such as substrates, the position of substrates, carrier gas, pressure, temperature, and flow rate, require in-depth knowledge and engineering to produce crystals of high quality.^32,39 Chen et al.⁴⁰ used CVD to rapidly create high-quality millimeter-sized monolayer ${\mathrm{MoSe}}_2$ crystals on molten glass. As shown in Fig. 3(c), glass substrates were placed inside a quartz tube that was installed inside an oven. To contain the molten glass, a piece of ${\mathrm{SiO}}_2/\mathrm{Si}$ was used with Mo sheets, which are placed the above the ${\mathrm{MoO}}_3$ source.

3.4.

Pulse Laser Ablation Technique

The laser ablation technique is one of the most effective physical methods for producing NPs. The principle of this technique is summarized by absorbing the laser beam falling on the surface of a solid target, which leads to the expulsion of its components and the formation of nanostructures. When the solid target is removed in a vacuum or gas, nanoclusters are deposited on a substrate placed at a distance from the solid target (usually a few centimeters). When the ablation is carried out in a liquid environment (e.g., water, ethanol, and acetone), the NPs are released into the liquid medium and form a colloidal nanosolution. The properties of the resulting nanostructures can be controlled by different laser parameters (wavelength, pulse duration, laser energy, and number of pulses) as well as by the properties of the medium liquid.⁴¹

3.5.

Pulsed-Laser Deposition Technique

When material is ablated from a solid target in a gaseous environment, it can be recovered as a powder or as a deposit on a substrate. Although some uses, such as fuel cell applications of carbon nanotubes and nanohorns,⁴² necessitate the manufacture of nanostructured powders, the majority of applications necessitate the deposition of thin nanostructured films.⁴³ The traditional PLD technology was found to be well suited for these jobs as it does not need any synthetic cluster size selection. The ablation of material is commonly conducted in PLD nanofabrication investigations using a nanosecond excimer laser (KrF and ArF) operating at UV wavelengths (248 and 193 nm, respectively) or the second harmonic of a Nd:YAG laser, as illustrated in Fig. 3 (532 nm). Because these wavelengths are lightly absorbed by plasma, they have little impact on the ablation process. At an incidence angle of roughly 45 deg, the radiation is focused on the surface of a revolving target (e.g., Si). The radiation intensity is typically between 108 and 109 W/cm. The plume of the laser-induced plasma extends perpendicular to the target surface. The substrates are placed at a distance (usually a few centimeters) from the target on a rotatable substrate holder. To increase the adherence of the deposited substance, the substrate is either held at room temperature or heated. The deposition is done in the presence of residual inert (He and Ar) or reactive (O and N) gases, at a low pressure (about 0.01 to 20 Torr), prior to filling with gases (Fig. 4).

Fig. 4

Schematic of the PLD experiment for Si-based nanostructured film deposition.

5.1.

Sensitivity ($R$)

The ratio of photocurrent ($I_{\mathrm{ph}}$) via the detector to input light power $P_{\text{optical}}$, where $I_{\mathrm{ph}}=I_{\text{illuminated}}-I_{\text{dark}}$ is

5.2.

External Quantum Effectiveness

The proportion of charge carriers captured by the photodetector to incident light is the external quantum effectiveness (EQE), which is influenced by the material coefficient and the thickness of the absorbing material:

5.3.

On/Off Ratio

The proportion of photocurrent to dark current is the on/off ratio. The device should have a high on/off ratio because this indicates an improved photocurrent and reduced dark current, both of which increase device noise. The on/off ratio is calculated as

5.4.

Detection Specificity ($D*$)

The photodetector’s responsivity and dark current determine how feeble a light the gadget can detect. Dark current should be reduced to the greatest extent possible because it makes it difficult to distinguish very weak optical signals and adds to the device’s noise; it is expressed as follows, where $\mathrm{\Delta }f$ is the electrical bandwidth, $n$ is the noise current, and $A$ is the effective area of the detector:

5.5.

Response Time

The rise and fall times of the optical signal response range from 10% to 90% and 90% to 10% of the maximum photocurrent, respectively.

5.6.

Power Equivalent to Noise

The NEP is the incident power required to achieve a signal-to-noise ratio of 1 at 1 Hz bandwidth, as determined by the noise spectral density.

6.1.

Graphene-Based Photodetectors

The photoconductor and photovoltaic effects lie at the heart of graphene PDs. The external bias voltage is not required for graphene PDs because the high conductivity of graphene may create a huge dark current. A metal–graphene Schottky junction can be used to add built-in electric fields. Xia et al.⁵⁰ was the first to announce graphene PD with a bandwidth of 40 GHz and a photoresponsivity of $0.5\mathrm{mA}/\mathrm{W}$. Most PDs have a low response⁵¹ or high dark current due to the graphene channel bias⁵² or fabrication method.⁵³

In addition, Huang et al.⁵⁴ discovered that the graphene/HfO/Si photodetector has a significantly greater interfacial gating effect than the graphene/SiO/Si photodetector. The graphene/HfO/Si photodetector has a photo responsivity of 45.8 A/W.

By modifying the rGO film with hydrophilic polymers, Shih et al.⁵⁵ demonstrated a feasible method for designing surface homogeneity and improving photodetector performance. The on/off ratio of the PVA/rGO photodetector is 3.5 times higher than that of the rGO photodetector, and the detective improves by 53% even when the photodetector is operated at a low bias of 0.3 V.

6.2.

MoS₂-Based Photodetectors

Graphene PDs when compared with 2D ${\mathrm{MoS}}_2$-based PDs rely primarily on photovoltaic effects, resulting in higher responsivity and smaller dark currents. ${\mathrm{MoS}}_2$ is one of the most thoroughly investigated TMDs because of its high carrier mobility of up to $200{\mathrm{cm}}^2/\mathrm{V}\mathrm{s}$, high absorption efficiency, and electrical operation (on/off) ratio of more than ${10}^8$. As a result of quantum confinement, the bulk ${\mathrm{MoS}}_2$ band gap can be converted from indirect to direct 1.2 to 1.8 eV by reducing it to a single layer.

Yin et al.⁵⁶ demonstrated a single-layer phototransistor prepared by the mechanical exfoliation method with a responsivity to light of $7.5\mathrm{mA}/\mathrm{W}$ at a wavelength of 750 nm and a response time of about 50 ms. Following that, researchers observed that charge trapping in ${\mathrm{MoS}}_2$, surface modification, preparation techniques, layer number, and the electrode contact all play a key role in ${\mathrm{MoS}}_2$ PD performance.^57–60 Meanwhile, Selamneni et al.⁶¹ established flexible broadband for all ${\mathrm{MoS}}_2$ PD based on ${\mathrm{MoS}}_2$ QDs and ${\mathrm{MoS}}_2$ heterojunctions recently, with a responsivity ($R$) value of $12.8\mathrm{mA}/\mathrm{W}$ and specific detectivity ($D*$) value of $7.2\times {10}^9\text{Jones}$. Furthermore, Han et al.⁶² created a ${\mathrm{MoS}}_2$ PD with a responsivity ($R$) of $1.4\times {10}^5\mathrm{A}/\mathrm{W}$ and specific detectivity ($D*$) of $9\times {10}^{15}\text{Jones}$, both of which were many magnitudes greater than Si and Ge PDs.

Also Kumar et al.⁶³ constructed a highly effective UV photodetector based on ${\mathrm{MoS}}_2$ layers using the PLD technology, with a $D*$ of $1.8\times {10}^{14}\text{Jones}$.

Recently, the photoelectric performance of the ${\mathrm{MoS}}_2/\mathrm{PSS}$ photodetector was increased under three layers of 365, 460, and 660 nm.⁶⁴ The photocurrent rose thrice under the 365-nm laser, the noise equivalent power decreased to $1.77\times {10}^{-14}\mathrm{W}/{\mathrm{Hz}}^{1/2}$, and the specific detectivity ($D*$) increased by $1.2\times {10}^{10}\text{Jones}$.

Also Li et al.⁶⁵ described a waveguide-integrated ${\mathrm{MoS}}_2$ photodetector functioning for hot electron-assisted photodetection to enable the telecom band. The low bias voltage of 0.3 V resulted in a photo responsiveness of $15.7\mathrm{mA}{\mathrm{W}}^{-1}$ at a wavelength of 1550 nm, which was also fairly uniform throughout the entire telecom band. Table 3 displays a comparison of ${\mathrm{MoS}}_2$ production methods.⁶⁶

Table 3

Comparison of MoS2 synthesis techniques.

6.3.

Photodetectors Based on Polymer–Graphene/MoS₂

${\mathrm{MoS}}_2$ is considered to be a possible 2D material for optoelectronic applications due to its exceptional electrical and optical properties. The response range of ${\mathrm{MoS}}_2$-based photodetectors can be increased using a semimetal material with a zero bandgap, such as graphene. Table 4 illustrates the various strategies for building a polymer/${\mathrm{MoS}}_2/\text{graphene}$-based photodetector.

Table 4

Different methods for fabricating polymer/MoS2/graphene-based photodetector.

6.4.

Solar Cell Based on MoS₂/Graphene

Many photovoltaic applications use transition metal dichalcogenides (TMDCs), particularly ${\mathrm{MoS}}_2$ with a 2H crystal structure.⁷⁴ The bandgap of TMDCs ranges from 1.0 to 2.0 eV, which corresponds to the Sun’s spectrum and with solar cell based on semiconductors, such as Si (1.1 eV), GaAs (1.4 eV), and CdTe (1.5 eV).⁷⁵ It has been reported that 2D TMDCs with a thickness of 1 nm absorb 5% to 10% of incoming light per unit volume and have the absorption efficiency of the Sun that is greater than commonly used light absorbers such as Si and GaAs. The quantity of light absorbed by the TMDC single layer is equivalent to the amount absorbed by the 50-nm-thick Si layer, resulting in greater photocurrent values.⁷⁶

In dye-sensitive solar cells, the antpolar plays an important role in the reduction process that occurs by transferring electrons through an external circuit. The electrocatalytic action of the counter electrode substantially influences the photo electrochemical process in dye-sensitized solar cells (DSSCs).⁷⁷ In DSSCs, ${\mathrm{MoS}}_2$ is employed as a counter electrode instead of the conventionally used high-priced Pt electrodes.^78–80

Lei et al.⁸¹ employed ${\mathrm{MoS}}_2$ as a counter electrode in three different structural: ${\mathrm{MoS}}_2$ NPs, multilayered ${\mathrm{MoS}}_2$, and few-layered ${\mathrm{MoS}}_2$. The photon conversion efficiency of various ${\mathrm{MoS}}_2$ morphologies is (5.41%) for ${\mathrm{MoS}}_2$ NPs, (2.92%) for $\mathrm{ML}\text{-}{\mathrm{MoS}}_2$, and (1.74%) for $\mathrm{FL}\text{-}{\mathrm{MoS}}_2$.

Also Lin et al.⁸² obtained a power conversion efficiency (PCE) of 6.07% as a counter electrode, and ${\mathrm{MoS}}_2$ was implanted with graphene flakes. ${\mathrm{MoS}}_2$ allows for a quick reduction of triiodide ions, and graphene flakes, which are extremely conductive, allow for a quick charge transfer. In 2012, the same team employed a ${\mathrm{MoS}}_2/\mathrm{rGO}$ nanocomposite as the opposite electrode and achieved a PCE of 6.04%.⁸³

Furthermore, ${\mathrm{MoS}}_2$ was identified as an HTL rather than PEDOT:PSS (poly(4,8-bis[(2-ethylhexyl)oxy)[1,2-b:4,5-b]dithiophene-2,6-diyl-3-fluoro-2-[(2-ethoxyhexyl)-carbonyl][3,4-b]dithiophene-2,6-diyl-3-fluoro-2-[(2-ethoxyhexyl)-carbonyl]thiophenediyl) PTB7:PC71BM solar cells and (3-alkylthiophenes).⁸⁴ Organic solar cells based on P3HT:PC71BM ([6,6]-phenyl C71 butyric acid methyl ester) demonstrated a PCE of 8.11% and 4.02%, respectively. The PCE varies from 6.97% to 8.11% in PTB7:PC71BM-based solar cells when the layers of ${\mathrm{MoS}}_2$ are changed, demonstrating the reliance of the PCE on the number of layers of ${\mathrm{MoS}}_2$. The solar cell based on monolayer ${\mathrm{MoS}}_2$ achieved the VOC = 0.69 V, the fill factor FF = 64%, the JSC = $15.78{\mathrm{mA}/\mathrm{cm}}^2$, and the PCE = 6.97%.

Li et al.⁸⁵ demonstrated that ${\mathrm{MoS}}_2$ may be employed as an anode buffer layer in organic solar cells to avoid current leakage between ITO and PC61BM; the result was a PCE = 3.9%. Solar cells made of ITO without a buffer layer have a PCE = 2.14%. A nanocomposite of $\mathrm{PEDOT}:\mathrm{PSS}/{\mathrm{MoS}}_2$ may be employed as a hole extraction layer in solar cells based on ${\mathrm{P}}_3\mathrm{HT}:\mathrm{PCBM}$ ([6,6]-phenyl-C61-butyric acid methyl ester) with a PCE = 3.74%.⁸⁶

Yang et al.⁸⁷ employed ${\mathrm{MoS}}_2$ as a hole layer in an organic solar cell. The organic solar cell ${\mathrm{PTB}}_7:{\mathrm{PC}}_{71}\mathrm{BM}$ was employed as an active layer; PFN poly[(9,9-bis(3′-(N-N-dimethylamino)-propyl)-2,7-fluorene)-alt-2,7-(9,9dioctylfluorene)-alt-2,7-(9,9dioctylfluorene)-alt-2,7-9, in comparison with PEDOT:PSS, ${\mathrm{MoS}}_2$ nanosheets and oxygen-containing ${\mathrm{MoS}}_2$ nanosheets demonstrated superior optical transparency in range the wavelength 400 to 900 nm, with a PCE of 4.99%. This is due to the possibility of using graphene as layers for extracting electrons and holes in organic solar cells.

${\mathrm{MoS}}_2$ was used as both an electron extraction layer and a hole withdrawal layer.⁸⁸ P-type doped ${\mathrm{MoS}}_2$ was used for the hole withdrawal layer, and n-type doped ${\mathrm{MoS}}_2$ was used for the electron extraction layer. The photon conversion efficiency increased from 2.8% to 3.4% in organic solar cells using p-type doped ${\mathrm{MoS}}_2$ as a hole extraction layer.

${\mathrm{MoS}}_2$ was used as an interfacial layer, and silver nanowires were used in conjunction with ${\mathrm{MoS}}_2$ to create a transparent electrode.⁸⁹ At 550 nm, Ag $\mathrm{NWs}\text{-}{\mathrm{MoS}}_2$ exhibited 93.1% transparency as a transparent electrode and had a reduced sheet resistance 9.8/sq. When compared with the Ag NW electrode alone, ${\mathrm{MoS}}_2$ in the Ag $\mathrm{NWs}\text{-}{\mathrm{MoS}}_2$ composite electrode enhanced environmental dependability and offered the best oxidation and moisture resistance. A PBDTTT-C-T:A PC70BM-based organic solar cell with a cathode made of Ag $\mathrm{NWs}\text{-}{\mathrm{MoS}}_2$ nanosheets/n-${\mathrm{MoS}}_2$ nanosheets and an anode made of $\mathrm{p}\text{-}{\mathrm{MoS}}_2$ nanosheets/Ag for hole extraction produced a PCE of 8.72%. As seen in Fig. 5(a), the general cell structure is $\text{glass}/\mathrm{Ag}\text{-}{\mathrm{MoS}}_2/\mathrm{n}\text{-}{\mathrm{MoS}}_2/\mathrm{PBDTTT}\text{-}\mathrm{C}\text{-}\mathrm{T}:{\mathrm{PC}}_{70}\mathrm{BM}/\mathrm{p}\text{-}{\mathrm{MoS}}_2/\mathrm{Ag}$.

Fig. 5

Tapping-mode AFM images of electrodes. (a) Ag NWs bottom electrode film height image. (b) Ag $\mathrm{NWs}\text{-}{\mathrm{MoS}}_2$ bottom electrode film height image.

Qin et al.⁹⁰ and Yang et al.⁹¹ employed ${\mathrm{MoS}}_2$ in conjunction with the ${\mathrm{MoO}}_3$ layer and ${\mathrm{MoS}}_2$ layer with Au NPs as hole extraction layers in solar cells. Also Chuang, et al.⁹² used ${\mathrm{MoS}}_2$ nanosheets with Au NP as an electron extraction layer, yielding a PCE = 4.91%. Jian et al.⁹³ investigated the role of the ${\mathrm{MoS}}_2$ layer in graphene/silicon organic solar cells. ${\mathrm{MoS}}_2$ was used as the interlayer in this case and provided a 4.4% PCE.

The same group⁹³ investigated the influence of annealing temperature on the PCE of the ${\mathrm{MoS}}_2$ prototype. Tsuboi et al.⁹⁴ employed ${\mathrm{MoS}}_2$ as an electron blocking and passivation layer between graphene and silicon of heterojunction solar cells based on $\text{graphene}/{\mathrm{MoS}}_2/\mathrm{n}\text{-}\mathrm{Si}$ in the same year. When comparing the solar cell that contains the ${\mathrm{MoS}}_2$ layer, the PCE of 1.35% is higher than the PCE of the solar cells without the ${\mathrm{MoS}}_2$ layer, with a PCE of 0.07%. The use of three-layer graphene increased the efficiency of PCE to 8.0%.

Rehman et al.⁹⁵ used the ${\mathrm{Al}}_2{\mathrm{O}}_3$ layer on the ${\mathrm{MoS}}_2$ surface to improve the performance of the solar cells based on ${\mathrm{MoS}}_2/\text{silicon}$. The PCE = 5.6% was obtained; this is higher than the conversion efficiency of the solar cell based on ${\mathrm{MoS}}_2/\text{silicon}$ without adding the ${\mathrm{Al}}_2{\mathrm{O}}_3$ layer, which was PCE = 2.21%. Also Saraswat et al.⁹⁶ made a solar cell based on TMDCs/silicon and gave highly efficient results and excellent photovoltaic performance. Meanwhile, Krishnamoorthy and Prakasam⁹⁷ reported on the simple hydrothermal synthesis of diverse compositions of ${\mathrm{MoS}}_2/\text{graphene}$ nanocomposites and their subsequent application in DSSCs. The $\mathrm{PCE}=8.92\%$ was for ${\mathrm{MoS}}_2/\text{graphene}$ and was higher than the conversion efficiency of ${\mathrm{MoS}}_2$ without graphene ($\mathrm{PCE}=3.36\%$) because graphene increased the charge collection faster and sensitized the dye better.

Maa et al.⁹⁸ described a unique approach to synthesizing layers of ${\mathrm{MoS}}_2$ on diverse substrates such as n-Si that act as photon absorbing layers. This significantly improved the photovoltaic conversion efficiency of graphene-based Schottky junction solar cells. In heterojunction solar cells based on 2D materials, a comparatively high conversion efficiency of 12% was successfully obtained.

Flexible solar cells were manufactured from ${\mathrm{MoS}}_2$, which works as a hole transfer layer with a transparent conductive electrode.⁹⁹ The PCE of these solar cells depends on the number of ${\mathrm{MoS}}_2$ layers as well as on the concentration of doping, with the value of the PCE being the highest when the number of layers is ($L_n=2$) and the concentration of doping is ($n_D=20\mathrm{mM}$). When the active layer with quantum dots was added to the solar cell installation, the efficiency of the conversion power was increased to 4.23%, and this efficiency was maintained after conducting 1000 cycles of tests, which indicates the mechanical stability of this solar cell.