**2.3 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 ABX3, 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 Voc). 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 [54–57]. **Figure 1** shows schematic illustration of exotic properties of 2d materials useful for solar cell devices.

**269**

**Table 1.**

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

**3. Photovoltaic in domain of 2d materials**

**3.1 Photovoltaic based on 2d Schottky junction**

**3.2 Photovoltaic based on 2d homojunction**

**Term Description**

*Main terms to demonstrate the photovoltaic device.*

Short-circuit current

Power conversion efficiency (PCE)

External quantum efficiency (EQE)

Internal quantum efficiency (IQE)

Open-circuit voltage

(Isc)

(Voc)

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 MoS2 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 MoS2 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 MoS2 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 Mo1-xWxSe2 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.

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.

at zero external bias having contact shorted.

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.

It is defined as the current flowing through the device (under illumination) and

It is defined as the ratio of electrical power generated to the incident light power.

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.

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

The voltage produced by the device having no current flow (under illumination)

*Solar Cells - Theory, Materials and Recent Advances*

**268**

**2.3 Perovskites**

**Figure 1.**

highest. [52–53].

materials useful for solar cell devices.

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 ABX3, 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 Voc). 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

*Schematic illustration of exotic properties of 2d materials used for solar cell devices.*

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 [54–57]. **Figure 1** shows schematic illustration of exotic properties of 2d
