**2. Photovoltaic materials**

#### **2.1 Graphene**

The dimension is the key factor to classify carbon allotropes/nanostructures into four groups, 0d (quantum dots, fullerenes), 1d (nanohorns, nanoribbons,

**267**

devices [43].

2009 [31].

**2.2 Transition metal dichalcogenides**

prepare the desired size of TMDs [44–48].

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

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

which carbon atoms are arranged in a hexagonal honeycomb lattice. It is a semi-

[6, 19–22]. Graphene also provides the optical and electrical properties as excellent transparency (97.7% in the visible spectrum) and electrical conductivity (≈104 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

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 (C10H16O) 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

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 [32–34]. Generally, TMDs are formulized as MX2where 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 [37–39]. Various combinations of TMDs such as MoS2, CrS2, WS2, TiS2, MoSe2, CrSe2, WSe2, TiSe2 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, MoS2 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 MoS2 in photovoltaic

Typically, the synthesis approaches like exfoliation, hydrothermal, CVD, molecular beam epitaxy (MBE), and atomic layer deposition (ALD) are used to

hybridization in

W m−1 K−1 at 300 K)

g−1), high Young's

prize for it [18]. Graphene is a single layer structure with sp2

metal with zero-bandgap, large specific surface area (2630 m<sup>2</sup>

modulus (1.1 TPa), and high thermal conductivity (3 × 103

graphene as a suitable aspirant for photovoltaic cells.

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

*Solar Cells - Theory, Materials and Recent Advances*

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 [7–9]. 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

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

The dimension is the key factor to classify carbon allotropes/nanostructures into four groups, 0d (quantum dots, fullerenes), 1d (nanohorns, nanoribbons,

**266**

community.

enhancing the performance of solar cells.

**2. Photovoltaic materials**

**2.1 Graphene**

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 sp2 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<sup>2</sup> g−1), high Young's modulus (1.1 TPa), and high thermal conductivity (3 × 103 W m−1 K−1 at 300 K) [6, 19–22]. Graphene also provides the optical and electrical properties as excellent transparency (97.7% in the visible spectrum) and electrical conductivity (≈104 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 (C10H16O) 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].
