Two-Dimensional Materials for Advanced Solar Cells

Inorganic crystalline silicon solar cells account for more than 90% of the market despite a recent surge in research efforts to develop new architectures and materials such as organics and perovskites. The reason why most commercial solar cells are using crystalline silicon as the absorber layer include long-term stability, the abundance of silicone, relatively low manufacturing costs, ability for doping by other elements, and native oxide passivation layer. However, the indirect band gap nature of crystalline silicon makes it a poor light emitter, limiting its solar conversion efficiency. For instance, compared to the extraordinary high light absorption coefficient of perovskites, silicon requires 1000 times more material to absorb the same amount of sunlight. In order to reduce the cost per watt and improve watt per gram utilization of future generations of solar cells, reducing the active absorber thickness is a key design requirement. This is where novel two-dimensional (2d) materials like graphene, MoS2 come into play because they could lead to thinner, lightweight and flexible solar cells. In this chapter, we aim to follow up on the most important and novel developments that have been recently reported on solar cells. Section-2 is devoted to the properties, synthesis techniques of different 2d materials like graphene, TMDs, and perovskites. In the next section-3, various types of photovoltaic cells, 2d Schottky, 2d homojunction, and 2d heterojunction have been described. Systematic development to enhance the PCE with recent techniques has been discussed in section-4. Also, 2d Ruddlesden-Popper perovskite explained briefly. New developments in the field of the solar cell via upconversion and downconversion processes are illustrated and described in section-5. The next section is dedicated to the recent developments and challenges in the fabrication of 2d photovoltaic cells, additionally with various applications. Finally, we will also address future directions yet to be explored for enhancing the performance of solar cells.


Introduction
Because of excessive utilization and consumption, the conventional fuel sources started depleting rapidly. In this direction, there is an urgent need for reconstruction of energy infrastructure, which is based on environmentally sustainable energy technologies such as wind, water, and solar. The worldwide research attracted towards solar energy, converting light energy into electrical energy. Solar photovoltaic is a pollution-free, efficient, renewable, reliable, rich, and continual source of energy. The photovoltaic solar cell, well-known technique, provides the solution of energy source crises in the 21st century. The main mechanism for the conversion of light to electricity: photovoltaic effect, photoconductivity, and photovoltaic effect (bulk). There is the requirement of a p-n junction in which electron and holes (photo-induced) in p-type and n-type materials partitioned transport a gathered to an electrode for production of photocurrent. In 1839, Edmond Becquerel first of all showed the demonstration of photovoltaic effect [1][2]. In the absence of p-n junction the conductivity of the semiconductor sample rises (by the illumination), it happens when the number of free electrons is increased, this is famed as photoconductivity. The electricity generated through the photovoltaic solar cells is not so cost-efficient in comparison to the grid power which we are using today [3]. At the large scale the solar energy conversion which should be low cost, there is a need for such type semiconducting materials that will make the production processes easily measurable and economically feasible [4]. In this direction two-dimensional (2d) material is referred to as impediment in one dimension between the size range 0-100 nanometers (nm), while the rest of the two dimensions are of micrometer range [5]. Furthermore, the configuration of atom and bond strength in 2d is identic and much stronger than that of bulk materials [6]. Also, ultrathin 2d nanomaterials have uncommon properties from their alternative nanostructured materials, such as three-dimensional (3d) nanocubes, one-dimensional (1d) nanotubes, and zero-dimensional (0d) quantum dots. First, the ultra-thickness of 2d nanomaterials provides high charge carrier, high charge mobility both at low and 300 kelvin (K) temperature, and high thermal conductivity [7][8][9]. Second, quantum confinement of 2d nanomaterials especially single layer or atomic thick layer, displays a number of properties, such as conductivity, tunable bandgap, surface activity, and magnetic anisotropy [10][11]. Third, the quantum Hall Effect (QHE) is shown by defect-free 2d materials, even at 300 K. The defect-free 2d materials have the electrons with a concentric (scatter-less) motion that allows the high charge carrier [12][13]. Fourth, the large ultrahigh surface area, keeping atomic-sized thickness, shows them ultrahigh specific surface area [14][15]. Therefore, photovoltaic solar cell manufactured by two-dimensional materials is a well-versed method in between of scientific community.
In the present chapter, we aim to follow up on the most important and novel developments that have been recently reported on solar cells. Section-2 is devoted to the properties, synthesis techniques of different 2d materials like graphene, transition metal dichalcogenides (TMDs), and perovskites. In the next section-3, various types of photovoltaic cells, 2d Schottky, 2d homojunction, and 2d heterojunction have been described. Systematic development to enhance the power conversion efficiency (PCE) with recent techniques has been discussed in section-4. Also, 2d Ruddlesden-Popper perovskite explained briefly. New developments in the field of the solar cell via upconversion and downconversion processes are illustrated and described in section-5. The next section is dedicated to the recent developments and challenges in the fabrication of 2d photovoltaic cells, additionally with various applications. Finally, we will also address future directions yet to be explored for enhancing the performance of solar cells.

Graphene
The dimension is the key factor to classify carbon allotropes/nanostructures into four groups, 0d (quantum dots, fullerenes), 1d (nanohorns, nanoribbons, carbon nanotubes), 2d (graphene) and 3d (diamond, graphite) structures [16][17]. A new area of research started with the groundbreaking discovery of graphene in 2004 by Novoselov and his co-authors in his famous publication "Electric field effect in atomically thin carbon films" and awarded jointly Nobel prize for it [18]. Graphene is a single layer structure with sp 2 hybridization in which carbon atoms are arranged in a hexagonal honeycomb lattice. It is a semimetal with zero-bandgap, large specific surface area (2630 m 2 g −1 ), high Young's modulus (1.1 TPa), and high thermal conductivity (3 × 10 3 W m −1 K −1 at 300 K) [6,[19][20][21][22]. Graphene also provides the optical and electrical properties as excellent transparency (97.7% in the visible spectrum) and electrical conductivity (≈10 4 Ω −1 cm −1 ) [23][24]. These exotic properties of graphene make it special in several optoelectronic applications. In solar cells, instead of indium doped tin oxide (ITO) and fluorine-doped tin oxide (FTO), graphene attracted attention due to flexibility, chemical stability, and high transmittance [20,[25][26]. These excellent dimensional, structural, optical, and electrical properties depict the graphene as a suitable aspirant for photovoltaic cells.
One of the well-known methods to synthesis the graphene is thermal chemical vapor deposition. In the thermal chemical vapor deposition (CVD), copper substrate placed into the quartz tube and then precursor gases (in the specific ratio) are allowed to flow at very high temperatures in the furnace [27]. After some time, single layer, bilayer, or multilayer deposition of graphene revels, this depends upon the internal conditions of experiments like temperature, pressure, reaction time, and gas flow rate [28]. The more advancement in the synthesis of graphene on Ni was achieved by Somani et al. [29]. In this, the camphor (C 10 H 16 O) has been taken as the precursor. Moreover, the large-scale monolayer graphene was produced by Obraztsov and co-others via a CVD method [30]. Another attempt has been performed to manufacture graphene on Cu foil (industrial base) via thermal CVD of methane with 1000°C temperature by Lia and co-workers in 2009 [31].

Transition metal dichalcogenides
Although graphene has various excellent properties, due to zero-bandgap, work-function, and toxic nature, the research on new atomically thin 2d materials gained attention. These necessities have been fulfilled by TMDs. These2d materials attracted more attention as they have grown on a flexible surface and can be bears the stress and deformation [32][33][34]. Generally, TMDs are formulized as MX 2 where M expresses the transition metal from group IV-VIII, (M = Ti, Zr, Hf, V, Nb, Cr Ta, Mo, W, etc.) and X is a chalcogen atom (X = S, Se, Te) [35][36]. TMDs have opened the new pipeline of research as having tunable bandgap (1-2 eV) and explore an excellent picture of electrical, optical, and mechanical properties [37][38][39]. Various combinations of TMDs such as MoS 2 , CrS 2 , WS 2 , TiS 2 , MoSe 2 , CrSe 2 , WSe 2 , TiSe 2 etc. found in metallic, semiconductor and insulator phase [40]. TMDs are a collection of big crystal family, found in different phases such as 1 T, 2H, and 3R., having two-third materials with layered structure [41]. In particular, MoS 2 shows mechanically 30% more strength than steel and can be ruptured after warping 1%. It generates the most distensible and strongest semiconducting materials [36,42]. Counter electrodes manufactured by platinum (Pt) were replaced by MoS 2 in photovoltaic devices [43].

Perovskites
Perovskites are a mixture of organic-inorganic materials, which offer high absorption coefficients, direct bandgap, high charge carrier mobility, and long charge carrier diffusion length [49][50]. This is why the research groups attracted more and more attention by 2d perovskites for a long time. There are three types of halide perovskite (2d) (i) organic-inorganic mixed halide perovskite, (ii) 2d Ruddlesden-Popper perovskites, (iii) inorganic halide perovskite [51]. The typically Perovskite structure is given by ABX 3 , where A indicates monovalent cation such as methyl rubidium (Rb), ammonium, and formamidinium; B represents heavy materials like tin (Sn) and lead (Pb); and X shows a halogen anion (i. e. chlorine, bromine, iodine). A unique type of properties provides highly defected bulk structures, indicate chemical compound through which the device operation power has been smoothed. The performance of 2d perovskite solar cells can be improved by obtaining a very high output voltage (under the circumstance of open circuit V oc ). The photovoltaic solar cells should be free from all recombination losses and this can be achieved by suppressing losses up to unity while quantum yield must be highest. [52][53].
The synthesis of 2d organic-inorganic mixed halide perovskite fabricated in two steps: (i) formation of lead halide (nano-platelets) on muscovite mica using van der Waals epitaxy in vapor transport CVD system, (ii) Ag as-solid heterophase reaction (using methylammonium halide molecules) used to obtain perovskite from platelets. However, the structure fabricated via this method is a 3d perovskite but using the universal scotch tape-based mechanical exfoliation method 2d perovskites is obtained [54][55][56][57]. Figure 1 shows schematic illustration of exotic properties of 2d materials useful for solar cell devices.

Photovoltaic based on 2d Schottky junction
During the photovoltaic processes (under illumination), electron-hole pairs are formed. These pairs are also termed as photogenerated carriers and they can be equal and more energetic (with incident photons) by the bandgap of the semiconductor. The conjunction of electron-hole pairs accorded on the electrodes and they are isolated through the junction internal field (electric) [58]. When the difference between the Fermi level of semiconductor and metal work function is generated, a Schottky junction enters in the pictures and photocurrent starts to develop. Net photocurrent has been maintained in asymmetric Schottky barriers (metal having different work function), whereas symmetric metal contact structure produces no net photocurrent. The important characteristics terms associated with the photovoltaic device illustrated in Table 1. Fontana et al. [58] synthesized a MoS 2 based (50 nm thick) phototransistor with palladium (Pd) and gold (Au) for drain contact and source, respectively. When two different materials are used for the source and drain contacts, such as hole-doping Pd and electron-doping Au, the Schottky junctions formed at the MoS 2 contacts generates a photovoltaic effect. Figure 2a displays the optical image of the device. Figure 2b shows the current vs. voltage curve at zero gate voltage, corresponding to the branch of the hysteresis with higher current, where the Fermi energy is shifted into the MoS 2 conduction band. Shin et al. [59] reported the graphene/porous silicon Schottky-junction solar cells by employing graphene transparent conductive electrodes doped with silver nanowires. The Ag nanowires-doped graphene/PSi solar cells show a maximum PCE of 4.03%. Yi and his co-workers developed Schottky junction photovoltaic cells based on multilayer Mo 1-x W x Se 2 with x = 0, 0.5, and 1 [60]. To generate built-in potentials, Pd and Al were used as the source and drain electrodes in a lateral structure, while Pd and graphene were used as the bottom and top electrodes in a vertical structure.

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.

Short-circuit current (I sc )
It is defined as the current flowing through the device (under illumination) and at zero external bias having contact shorted.

Power conversion efficiency (PCE)
It is defined as the ratio of electrical power generated to the incident light power.

External quantum efficiency (EQE)
The ratio defines by the amount of charge carriers moving through the device (under short-circuit current) to the all number of colliding photons on it.

Internal quantum efficiency (IQE)
Shows the ratio of the amount of charge carriers moving through the device (under short-circuit current) to all numbers of absorbed photons.

Open-circuit voltage (V oc )
The voltage produced by the device having no current flow (under illumination) Fill factor (FF) It is describing the ratio of maximum electric power generated to the product of its open-circuit voltages and its short-circuit current. Table 1.
Main terms to demonstrate the photovoltaic device.
Using a splitting gate on monolayer WSe 2, Pospischil et al. [61], Baugher et al. [62], and Ross et al. [63] effectuated experimentally p-n junctions. This demonstration of WSe 2 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, HfO 2 ). 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 V oc of 0.64 V and illumination of 140 mW/cm 2 (halogen source). The remarkable results are in the picture with very high efficiency of photovoltaic energy conversion, assuming the monolayer WSe 2 (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 MoSe 2 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 cm 2 /Vs, hence BP shows the possibility for broadband optoelectronic applications [65][66][67]. Choi et al. [68] develop a technique to form a lateral homogeneous 2d MoS 2 p-n junction by partially stacking 2d h-BN as a mask to p-dope MoS 2 . The fabricated lateral MoS 2 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 x10 10 Jones, and light switching ratio of ∼10 3 . Figure 3 shows the fabrication of the MoS 2 p-n diode. Figure 3b shows the optical microscopy image of MoS 2 p-n diode with hetero electrodes.

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 MoS 2 -MoSe 2 and WS 2 -WS e 2 lateral heterostructures by thermal chemical vapor deposition (CVD) technique. The well-known n-type (WS 2 ) and p-type (WSe 2 ) builds natured heterojunction p-n diode with current rectification. When such heterojunction p-n diodes are characterized. The  photovoltaic effect with a V oc of ≈ 0.47 V and I sc 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 WS 2 and WS 2 -WS e 2 interface region through photocurrent mapping, implies with the fact "A large fraction of the depletion layer is localized to the lightly doped WS 2 of the diode" [69]. Atomically sharp in-plane WSe 2 /MoS 2 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 lateralWSe 2 /WS 2 heterostructure by ambient pressure CVD in a single heat-cycle. The optoelectronic device (Figure 4a-b) based on the lateral WSe 2 /WS 2 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 WSe 2 and WS 2 .

Perovskite 2d materials for photovoltaic cells 4.1 Transport layers in regular (n-i-p) photovoltaic
The layer-by-layer deposition technique is used to manufacture photovoltaic solar cell devices (PSCs). In these types of constructions, the order of charge selective layers in the manner can prosecute subdivide the devise configuration in two ways, regular PSCs (n-i-p) and inverted PSCs (p-i-n). The PSCs have two parts, (a) metal contact, (b) transparent conductive glass (TCO), while a slice of the observer has been arranged between hole transporting layer (HTL) and electron transporting layer (ETL). When the perovskite absorbs the light, an exciton i. e. the carriers are partitioned and moved towards the adequate layer, HTL, and ETL. Hence the charge carriers are shifted to the different electrodes. Moreover, ETL and HTL are performing two main roles, control the perovskite crystal growth, and extract and move the charge carriers. It is well versed that the hysteresis phenomenon is chiefly linked with the characteristics and interface of the charge selective layers to the perovskite [72][73]. Some of the remarkable features of ideal ETL and HTL materials are high transparency, high charge mobility, inherent stability, low-cost manufacturing, and appropriate energy alignment.
Yin et al. [78] reported the PCE of 17.37% by taking TiS 2 nanosheets (8-9 layers) suspended in isopropanol alcohol (IPA) in the perovskite solar cells. Uniquely, encapsulated perovskite solar cells with TiS 2 ETL demonstrate the highest stable under the conditions (UV irradiation 10 mW cm −2 and in ambient atmosphere for 50 h), with the result of 90% PCE. A double of SnO 2 and 2d TiS 2 synchronously, used for the PCE of 21.73% performed by Huang et al., as every potential ETLs [79].
Applying the self-assembly stacking deposition method, the PCE of SnS 2 ETL based device, up to 20.12% has reported by Zhao et al. [80]. The Ti 3 C 2 integration into a SnO 2 ETL for low-temperature planer perovskite solar cells by Yang and his co-workers by varying the loading of MXene from 0 to 2.5 wt% [81]. The device formed has the value of PCEs modified from the value of 17.23% to 18.34% at 1 wt% (increasing the loading of MXene). In the addition, the device fabricated through a SnO 2 -T i 3 C 2 is highly stable (with, RH-20%), and shows the PCE up to 80% of their initial performance after 700 h (identical with SnO 2 -only devices) [81].
To increase the charge transfer efficiency, the perovskite crystal size, and lower the defect density, Guo et al. [82] explained Ti 3 C 2 T X MXene as an external material for the perovskite precursor solution. The results reveal that the incorporated device experimentally verified a PCE of 17.41% with short-circuit current (J SC ) of 22.26 mA cm −1 which is comparably high with the pristine perovskite device having the PCE of 15.54% and J SC of 20.67 mA cm −1 .
Wang and his research group [83] experimentally verified that whenever a tiny amount of black phosphorus added to the MAPbI 3 starting solution (precursor), the photostability and efficiency of perovskite solar cells have been critically enhanced. The few-layer black phosphorus is proved to obtain ample perovskite to the size greater than 500 nm with the comparison bare MAPbI 3 film size (<400 nm). Taking the complex structure FTO/c-TiO 2 /SnO 2 /perovskite/Spiro-OMeTAD/Ag and MAPbI 3 /BP for PSCs, the unique PCE of 20.65% was achieved, having less hysteresis and high reproducibility. Under the continuous white light-emitting diode (illumination of 100 mW cm −2 within the N 2 glove box) the MAPbI 3 /BP-based PSCs show an excellent PCE limit of 94% (1000 h) [83][84].
The spiro-OMeTAD HTL and perovskite on active buffer layer (liquid phase exfoliated few-layer MoS 2 nanosheets) instead by Cappaso and co-workers and tried to solve the problem [85]. The above arrangement completes two necessary conditions i.e. prominent layer to ease the injection process and hole collection and performing like a barrier so that metal electrode migration can be curbed. The N-methyl-2-pyrrolidone solvent is very famous for the experimentalist to efficient MoS 2 [86][87]. On the other hand, some study proves this solvent not suitable for perovskite, to make it perovskite favorable solvent (IPA), a solvent exchange process has to be required. Najafet al. [88] developed "graphene interface engineering" strategy based on van der Waals MoS 2 QD/graphene hybrids that enable MAPbI 3 -based PSCs to achieve a PCE up to 20.12% (average PCE of 18.8%).The van der Waals hybridization of MoS 2 quantum dots (QDs) with functionalized reduced graphene oxide (f-RGO), obtained by chemical silanization induced linkage between RGO and (3-mercaptopropyl)trimethoxysilane. This results in homogenize the deposition of the hole transport layer (HTL) or active buffer layer (ABL) onto the perovskite film since the two-dimensional nature of RGO effectively plugs the pinholes of theMoS 2 QD films.

Transport layers in inverted (p-i-n) photovoltaic
The organic solar cell is the key to fabricate the p-i-n PSC structures [89]. Huang et al. [90] successfully showed that with coverage optimization, a planar p-i-n ++ device with a PCE of over 11% was achieved. This also suggests that the ETL may not be necessary for an efficient device as long as the perovskite coverage is approaching 100%. Figure 6a-b presents the device architecture of the perovskite solar cells with (a) and without (b) a TiO 2 ETL. Figure 6c shows the current density-voltage curves of two typical CH 3 NH 3 PbI 3-x Cl x -based solar cells grown on FTO substrates with and without UVO treatment under simulated AM1.5G solar irradiation (100 mW/cm 2 ). Jeng and co-workers reported that perovskites have the ability to transport the holes [91]. To achieve a PCE of 15.1%, Hu et al. proposed a surface-modification technique in which the ITO surface/optimize energy level by using the cesium salt solution [92].
Liu and his co-workers reported layer free PSC with an efficiency of 13.5% to obtain this, perovskite layer directly put together with the ITO surface by using a sequential layer deposition method. The above arrangement proves that to enhance device efficiency ETL is not required always [93]. Ke et al. [94] manufactured ETL free PSC with efficiency 14.14% and V oc 1.4 V deposited directly on FTO substrate (via a one-step solution process) in which no hole blocking layers  are required. He further described that TiO 2 (electron-transporting material) is not a perfect interfacial material. It is also described Liu et al. [93], Chen et al. [95], and Prochowicz et al. [96] that the efficiency of compact layer-free devices can be higher when various film-forming techniques are to be used.
Various quantities of black phosphorus quantum dots (BPQDs) mixed with MAPbI 3 precursor solution to form p-i-n inverted devices [97]. These BPQDs based perovskite films revealed less non-reactive defects, larger grain size, and higher crystallinity, with a comparison of no BPQDs perovskite films. Further, it is clear from some experimental facts that BPQDs also work as heterogeneous nucleation sites. This leads to the growth of perovskite crystals with homogeneity. The PCE of 20% was obtained for additive-assisted perovskite film. Adding the BPQDs on the lower surface of MAPbI 3 -the enhanced crystalline of MAPbI 3 -BPQDs film has been achieved.

2d Ruddlesden-popper perovskite
The hybrid organic-inorganic halide perovskite (OIHPs) are very unsuitable in the industrial fabrication of solar device as they lose the stability on heating, moisture, and light. These major issues have been resolved by S. N. Ruddlesden and P. Popper to fabricate the perfect candidates known as Ruddlesden-Popper perovskite (RPPS). This is the mixture of 2D/3D materials and can be used in LEDs as it has an intense photoluminescence feature [98]. The 2D Ruddlesden-Popper (2DRP) perovskites are the topic of great interest and research because highly stable PSCs can be fabricated by them [99]. The general form of 2DRP is (A ' ) 2 (A) n-1 B n X 3n + 1 , where A ' indicates R-NH 3 or H 3 N-R (R an aromatic ligand or large aliphatic alkyl chain) and works as an insulating layer to partitioned the various inorganic layers. A describes small cation  [90]. Copyright (2016) American Chemical Society. such as CH 3 NH 3 + and Cs + . B represents Pb 2+ and Sn 2+ , divalent metal cation, while X describes the halides. Various values of small n provide us strict 2D structure (n = 1), quasi-2D structure (n = 2-5), conventional 3D structure (n = ∞), and represents the number of metal halide, monolayer sheets [100]. These 2DRP excellently perform thermal stability, humidity stability, and structure stability [101][102][103][104][105][106]. Giorgi et al. [98] showed a lateral and top view of the nanosheets Ruddlesden-Popper organicinorganic halide perovskites (NS-RPPs) optimized structure in Figure 7a-b. Also, lateral and top view of the quantum-well Ruddlesden-Popper organic-inorganic halide perovskites (QW-RPPs) structures in Figure 7c-d. The solution base synthesis, colloidal base method, liquid, and vapor-based epitaxy, exfoliation method, and single crystal growth are the well-known growing technique to fabricate 2DRP perovskites [107]. Niu et al. [108] prepared mono and few layers (C 6 H 9 C 2 H 4 NH 3 ) 2 PbI 4 flakes via the method of micromechanical exfoliation.

Downconversion of perovskite photovoltaic cell
In the previous investigations, various techniques and methods have been adopted to improve the efficiency of solar cells, higher by the Shockley and Queisser limit (32%). The phenomenon of splitting, low energy photons by high energy photons (single) is known as downconversion (quantum cutting). An ample work has been done on downconversion for photovoltaic devices. This was performed  with lanthanide ions because of its excellent optical properties. Various experiments also proved the use of nanomaterials as down converters. If we chose the lanthanide ions and design of the solar cell, there will be a maximum benefit to using the down-conversion materials. The host materials should possess properties like low scattering, absorption strength, thermal and chemical strength, high transmittance, photo-stability, and excitation energy [109][110]. The necessary conditions to pick the lanthanide ion are good electrical and chemical stability and high emission lifetime. Downconversion Tb 3+ -Y b 3+ has also been demonstrated in GdAl 3 (BO 3 ) 4 [111], GdBO 3 [112], Y 2 O 3 [113], CaF 2 nanocrystals [114] and lanthanum borogermanate glass [115]. Tsai et al. [116] explored the graphene quantum dots (GQDs) as a down conversion in then-type Si heterojunction (SHJ) solar cells.
The photographic image and the cross-sectional schematic of the SHJ solar cell are shown in Figure 8a-b, respectively. Figure 8c-d shows the low-magnification and high-magnification top-view SEM images of the micro pyramids of SHJ solar cells. The device with 0.3 wt % of GQDs shows the highest short-circuit current (J SC ) and fill factor (FF) of 37.47 mA/cm 2 and 72.51%, respectively, which leads to the highest PCE of 16.55% (Figure 8f). The external quantum efficiency (EQE) of the devices with 0.3 wt % and without GQDs and the EQE enhancement are shown in Figure 8g. The efficiency enhancement is due to the photon down conversion phenomenon of GQDs to make more photons absorbed in the depletion region for effective carrier separation, leading to the enhanced photovoltaic effect. Various down conversion materials were described and synthesized to enhance UV stability and UV photon harvesting [117][118].

Upconversion of perovskite photovoltaic cell
The conversion (nonlinear optical process) in which minimum two low energy photons, present in the near-infrared region into high energy photon with the visible region known as upconversion [119][120]. The upconversion materials contain large bandgap, seem to be most favorable for solar cell applications. Various uses of upconversion materials are optical data storage, medical therapy, display technology, light-harvesting, temperature sensors, and solid-state lighting. Trupke et al. [121] theoretically investigated that if a perfect upconvertor is used with conventional single-junction bifacial solar cells (bandgap 2 eV), we can obtain PCE of 47.6% (non-concentrated sunlight) and 63.2% concentrated sunlight. Lanthanide based upconverters and organic upconverters are very common to improve the efficiency of photovoltaic devices. The elements lanthanum to lutetium are used as the upconverters. In addition, enhancement in the photocurrent has been achieved by the use of two commercial upconverters on both sides of Si solar cells (Pan et al.) [122].
The nano precursor upconversion materials Er 3+/ Yb 3+ co-doped with TiO 2 and LaF 3 have been explored by Shan et al. [123]. Shang et al. [124] also explored the various techniques to enhance efficiency via upconversion. The utilization of light beyond the visible region is not possible by PSCs (CH 3 NH 3 PBI 3 ), due to their intrinsic bandgap. The upconversion is a specific way, to harvest this regime and convert it in the visible regime, so that the PSCs IR response can be increased. Chen and co-workers reported that the efficiency will be increased if LiYF 4 : Yb 3+ , Er 3+ single-crystal attached in the front part of PSCs [125]. Taking nano prisms NaYF4: Yb 3+ , Er 3+ , which is hydrothermally formed to the TiO 2 layer in PSCs, the efficiency enhancement has been demonstrated by Roh et al. [126]. In another study, Wang et al. introduced efficiency increment via hydrothermally grown 3% Er 3+ and 6% Yb 3+ co-doped TiO 2 nanorod in PSCs [127]. In 2016, He et al. integrated NaYF 4 :Yb 3+ , Er 3+ nanoparticles as mesoporous electrode with PSCs (CH 3 NH3PbI 3 based), this leads a short circuit current density 0.74 mAcm −2 excite with 980 nm laser with 28 W cm −2 [128]. The sample (semiconductor plasmon-sensitized nanocomposites i.e. mCu 2-x S@SiO 2 @ Er 2 O 3 ), which changes the broadband infrared light to visible light by the localized surface plasmon resonance (LSPR) of Cu 2-x S integrated with TiO 2 paste [129]. This arrangement is used in PSCs as an electron extraction layer with an enhanced response of 800-1000 nm. With the dependency of the power density of the 980 nm extraction layer (>0.85 mAcm −2 ), the short circuit current density was noticed to rise 45 Wcm −2 .

Recent developments on 2d perovskite photovoltaic cells
The 2d layered perovskites are more advanced and useful than their 3d structures. The unique electroluminescent property of 2d perovskites makes them more  superior to their 3d counterparts. 2d perovskites structures have large exciton binding energy than 3d structures [130][131]. Due to this fact, 2d perovskite has a high photoluminescence quantum yield (PLQY) with enhanced radiative recombination. The appearance of cascaded energy structures in 2d perovskites films (mixed n layer thickness) leads to a fast and efficient energy transfer from lower-n quantum wells to higher-n quantum wells. These results in the decreased exciton quenching effect: occurring the enhanced van der Walls interactions in organic molecules and hydrophobic organic ligands, 2d perovskites show enhanced and ambient and thermal stability in the comparison with 3d perovskites. In 2d perovskites, the electrical and optical properties can be tuned more and advanced applications like circular-polarized emission and broadband emission due to their excellent chemical tenability [132][133][134][135].
Yo u et al. [136] developed a novel annealing approach formethylammonium lead iodide (MAPbI 3 ) and (CsPbI 3 ) 0.05 (FAPbI 3 ) 0.95 (MAPbBr 3 ) 0.05 mixed perovskite Reprinted with permission from [140]. Copyright (2020) American Chemical Society. films by fast laser beam scanning. Under optimum conditions, high-quality perovskite films with good crystallinity, preferred orientation, and low density of defects device offers PCE nearly about 20%. Wang et al. [137] showed an efficient strategy to tune the band structure and electron mobility of the ETL by adding NH 4 Cl to the sol-gel-derived ZnO precursor. Low temperature (160°C) fabricated CsPbIBr 2 solar cells recorded high efficiency of 10.16%. Li et al. [138] fabricated the MAPbI 3 perovskite solar cells and showed the device performance is strongly influenced by the TiO 2 electron transport layer. Oz et al. [139] studied the effect of lead(II)propionate additive on the stabilization of CsPbI 2 Brall-inorganic perovskite, and the use of a novel dopant-freepolymer hole transport material (synthesized by us) for photovoltaic performance assessment of CsPbI 2 Br solar cells.
Fu and his co-workers report a dual-protection strategy via incorporating monomer trimethylolpropane triacrylate (TMTA) intoCsPbI 2 Br perovskite bulk and capping the surface with 2-thiophenemethylammonium iodide (Th − NI) [140]. The fabricated devices show a greatly improved efficiency from 12.17 to 15.58% with an opening circuit voltage (V oc ) of 1.286 V. Figure 9a presents a schematic illustration of the device and the dual-protection CsPbI 2 Br film. The UPS measurements are conducted to reveal the electronic structure changes the calculated results are drawn in Figure 9b. The photocurrent density-voltage (J-V) curves of the optimal devices under AM 1.5 illumination are presented in Figure 9c. The ref. device shows the best efficiency of 12.17% with a V oc of 1.151 V, and TMTA doped film (BT) devices show an improved efficiency of 13.88% after incorporating 1 mg/mL of TMTA. In Figure 9e, the Th − NI modified BT film (BTSTh) device exhibits the improved output efficiency with 14.93% for 1000 s, while the ref. shows the attenuated efficiency and remains 10.51% under the same operation, which indicates the better operational stability for the BTSTh devices.
Li et al. [141] reported vertically aligned 2D/3D Pb − Sn perovskites with enhanced charge extraction and suppressed phase segregation for efficient printable solar cells. Wang et al. [142] successfully fabricated high-quality CsPbBr 3 films via additive engineering with NH 4 SCN. The incorporation of NH 4 + and pseudohalide ion SCN − into the precursor solution, a smooth and dense CsPbBr 3 film with good crystallinity and low trap state density can be obtained.

Neoteric challenges in development of 2d photovoltaic cells
It is crucial that the entire energy of any absorbed photon is harvested for next-generation photovoltaics. The unique features of 2d layered materials such as high crystalline quality, transparency, atomic thickness, make them special to use in photovoltaic solar cells. Thus, it is important to synthesized and demonstrate 2d flexible photovoltaic devices on industrial scale. While some different issues and problems will be faced by the experimentalist regarding an efficiency performance. The specific requirements in the fabrication of photovoltaic devices are the large-area synthesis, highly controllable, low cost, atomically thin, and recyclable fabrication of materials and their devices. The above features are specified not only for all potential electronic devices but also for optoelectronic devices. Increased number of layers of 2d materials, the conductivity of the material improves at the cost of reduced transparency. The application of 2dmaterials like TMDs and perovskites in photovoltaic devices has also been investigated over the last few years. The advantages of using such materials for solar cells have been explored based on the high absorption coefficient of these materials in the visible to the near-infrared part of the solar spectrum. So there is a need for more investigation of the heterostructure based on these materials, which can synergize the performance of the device. Although the efficiency of perovskite solar cells has been boosted to over 25%, further improving the efficiency towards their theoretical Shockley-Queisser efficiency limit of more than 30% and improving the stability towards commercial application deserve more intensive research. The micromechanical cleavage method provides us structural study and device performance but this cannot be used as industry level as low production rate and low yield. This lack noted by the scientific community and remarkable advancement has been achieved by fabricating 2d materials at the industry level within the past few years. Moreover, TMDs, hBN, graphene, and perovskite are critically fabricated with different methods like physical vapor transport, CVD, layer by layer conversion; etc. The samples obtained in this way have properties like controllable thickness, electronic properties scalable sizes, and high crystal quality. Also, a liquid-based wet chemical method provides two unique requirements high control power and low-cost manufacturing of atomically thin 2D nanomaterials. Moreover, it is necessary to achieve excellent crystal quality and film uniformity before using the LBWC technique in large scale manufacturing.

Conclusion
In this chapter, we have enlightened specific properties and synthesis techniques of graphene, TMDs, and perovskite. We have described recent progress made with graphene, graphene-based 2D materials for solar photovoltaics. In addition, 2d Schottky junction, homojunction, and heterojunction are explained briefly. The unique account of the charge carrier transport layer as ETL and HTL has been done fairly. Moreover, regular n-i-p and inverted p-i-n structure are the key features. Furthermore, 2DRP perovskite, upconversion/downconversion of perovskite cells are also discussed briefly. In the last of the section very recent developments on 2d perovskite photovoltaic cells have been critically explained with the facts. Various challenges in the fabrication and development of today's devices are pointed out. Therefore, the outlook towards 2d materials should be optimistic and needs more attention in the near future.
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