**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.

2,2,7,7-tetrakis (N,Npdimethoxyphenylamino)-9,9-spirobifluorene (Spiro-OMeTAD) and TiO2 are the wall known materials that can be used as hole transport material (HTM) and electron transport material (ETM) respectively in the formation of n-i-p PSCs. On the other hand poly(3,4 ethylenedioxythiophene)–polystyrene sulfonate (PEDOT:PSS) and fullerene derivatives (e.g., 6,6-phenyl-C61-butyric acid methyl ester (PCBM)) has been taken as the HTM and ETM to manufactured the p-i-n PSCs [74–76]. Singh *et al.* [77] described the direct synthesis of MoS2 (transparent, thin, and uniform) films on FTO- coated glass by the use of microwave irradiation and utilize this ETL in perovskite solar cells. TiO2-coated FTO, SnO2, MoS2, and FTO-only substrates were prepared and their photovoltaic performances were checked and they have maximum PCEs with 17.15%, 15.80%, 13.14%, and 6.11% respectively [77]. When the article examination of the lifetime of charge

**273**

has to be required.

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

the PCE of 15.54% and JSC of 20.67 mA cm−1.

show an excellent PCE limit of 94% (1000 h) [83–84].

extraction.

carriers and charge recombination dynamics of ETL/Perovskite seems to be MoS2 as shortest charge carrier lifetime as with other ETLs, which shows the charge

suspended in isopropanol alcohol (IPA) in the perovskite solar cells. Uniquely, encapsulated perovskite solar cells with TiS2 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 SnO2 and 2d TiS2 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 SnS2 ETL based device, up to 20.12% has reported by Zhao *et al.* [80]. The Ti3C2 integration into a SnO2 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 SnO2-Ti3C2is highly stable (with, RH-20%), and shows the PCE up to 80% of their

initial performance after 700 h (identical with SnO2-only devices) [81].

To increase the charge transfer efficiency, the perovskite crystal size, and lower the defect density, Guo *et al.* [82] explained Ti3C2TX 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 (JSC) of 22.26 mA cm−1 which is comparably high with the pristine perovskite device having

Wang and his research group [83] experimentally verified that whenever a tiny amount of black phosphorus added to the MAPbI3 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 MAPbI3 film size (<400 nm). Taking the complex structure FTO/c-TiO2/SnO2/perovskite/Spiro-OMeTAD/Ag and MAPbI3/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 N2 glove box) the MAPbI3/BP-based PSCs

The spiro-OMeTAD HTL and perovskite on active buffer layer (liquid phase exfoliated few-layer MoS2 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 MoS2 [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

Najaf*et al.* [88] developed "graphene interface engineering" strategy based on van der Waals MoS2 QD/graphene hybrids that enable MAPbI3-based PSCs to achieve a PCE up to 20.12% (average PCE of 18.8%).The van der Waals hybridization of MoS2 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 theMoS2 QD films. **Figure 5a** represents the sketch of mesoscopic MAPbI3-based PSC exploiting MoS2 QDs:f-RGO hybrids as both HTL and ABL. The normalized

Yin *et al.* [78] reported the PCE of 17.37% by taking TiS2 nanosheets (8–9 layers)

*Solar Cells - Theory, Materials and Recent Advances*

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.

low-cost manufacturing, and appropriate energy alignment.

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,

2,2,7,7-tetrakis (N,Npdimethoxyphenylamino)-9,9-spirobifluorene (Spiro-OMeTAD) and TiO2 are the wall known materials that can be used as hole transport material (HTM) and electron transport material (ETM) respectively in the formation of n-i-p PSCs. On the other hand poly(3,4 ethylenedioxythiophene)–polystyrene sulfonate (PEDOT:PSS) and fullerene derivatives (e.g., 6,6-phenyl-C61-butyric acid methyl ester (PCBM)) has been taken as the HTM and ETM to manufactured the p-i-n PSCs [74–76]. Singh *et al.* [77] described the direct synthesis of MoS2 (transparent, thin, and uniform) films on FTO- coated glass by the use of microwave irradiation and utilize this ETL in perovskite solar cells. TiO2-coated FTO, SnO2, MoS2, and FTO-only substrates were prepared and their photovoltaic performances were checked and they have maximum PCEs with 17.15%, 15.80%, 13.14%, and 6.11% respectively [77]. When the article examination of the lifetime of charge

**4. Perovskite 2d materials for photovoltaic cells**

**4.1 Transport layers in regular (n-i-p) photovoltaic**

**272**

carriers and charge recombination dynamics of ETL/Perovskite seems to be MoS2 as shortest charge carrier lifetime as with other ETLs, which shows the charge extraction.

Yin *et al.* [78] reported the PCE of 17.37% by taking TiS2 nanosheets (8–9 layers) suspended in isopropanol alcohol (IPA) in the perovskite solar cells. Uniquely, encapsulated perovskite solar cells with TiS2 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 SnO2 and 2d TiS2 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 SnS2 ETL based device, up to 20.12% has reported by Zhao *et al.* [80]. The Ti3C2 integration into a SnO2 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 SnO2-Ti3C2is highly stable (with, RH-20%), and shows the PCE up to 80% of their initial performance after 700 h (identical with SnO2-only devices) [81].

To increase the charge transfer efficiency, the perovskite crystal size, and lower the defect density, Guo *et al.* [82] explained Ti3C2TX 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 (JSC) of 22.26 mA cm−1 which is comparably high with the pristine perovskite device having the PCE of 15.54% and JSC 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 MAPbI3 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 MAPbI3 film size (<400 nm). Taking the complex structure FTO/c-TiO2/SnO2/perovskite/Spiro-OMeTAD/Ag and MAPbI3/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 N2 glove box) the MAPbI3/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 MoS2 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 MoS2 [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.

Najaf*et al.* [88] developed "graphene interface engineering" strategy based on van der Waals MoS2 QD/graphene hybrids that enable MAPbI3-based PSCs to achieve a PCE up to 20.12% (average PCE of 18.8%).The van der Waals hybridization of MoS2 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 theMoS2 QD films. **Figure 5a** represents the sketch of mesoscopic MAPbI3-based PSC exploiting MoS2 QDs:f-RGO hybrids as both HTL and ABL. The normalized

**Figure 5.**

*(a) Sketch of mesoscopic MAPbI3-based PSC exploiting MoS2 QDs:f-RGO hybrids as both HTL and ABL. (b) Scheme of the energy band edge positions of the materials used in the different components of the assembled mesoscopic MAPbI3-based PSC. (c) Normalized PCE trends vs. time extracted by I* − *V characteristics under 1 sun illumination. Reprinted with permission from [88]. Copyright (2018) American Chemical Society.*

PCE trends vs. time extracted by I − V characteristics under 1 SUN illumination, periodically acquired during the shelf life test (ISOS-D-1), shows in **Figure 5c**.

#### **4.2 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 TiO2 ETL. **Figure 6c** shows the current density-voltage curves of two typical CH3NH3PbI3-xClx-based solar cells grown on FTO substrates with and without UVO treatment under simulated AM1.5G solar irradiation (100 mW/cm<sup>2</sup> ). 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 Voc 1.4 V deposited directly on FTO substrate (via a one-step solution process) in which no hole blocking layers

**275**

**Figure 6.**

*American Chemical Society.*

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

are required. He further described that TiO2 (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

*Device architecture of the perovskite solar cells (a) with TiO2 ETL and (b) without TiO2 ETL (c) J-V curves of CH3NH3PbI3-xClx-based solar cells with and without UVO treatment (d) histograms of their corresponding PCE values measured for 36 separate devices (c). Reprinted with permission from [90]. Copyright (2016)* 

Various quantities of black phosphorus quantum dots (BPQDs) mixed with MAPbI3 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 MAPbI3-the enhanced crystalline of MAPbI3-BPQDs film has been achieved.

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

R-NH3 or H3N-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

)2 (A)n-1Bn X3n + 1, where A'

indicates

higher when various film-forming techniques are to be used.

**4.3 2d Ruddlesden-popper perovskite**

by them [99]. The general form of 2DRP is (A'

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

**Figure 6.**

*Solar Cells - Theory, Materials and Recent Advances*

PCE trends vs. time extracted by I − V characteristics under 1 SUN illumination, periodically acquired during the shelf life test (ISOS-D-1), shows in **Figure 5c**.

*(a) Sketch of mesoscopic MAPbI3-based PSC exploiting MoS2 QDs:f-RGO hybrids as both HTL and ABL. (b) Scheme of the energy band edge positions of the materials used in the different components of the assembled mesoscopic MAPbI3-based PSC. (c) Normalized PCE trends vs. time extracted by I* − *V characteristics under 1 sun illumination. Reprinted with permission from [88]. Copyright (2018) American Chemical Society.*

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 TiO2 ETL. **Figure 6c** shows the current density-voltage curves of two typical CH3NH3PbI3-xClx-based solar cells grown on FTO substrates with and without UVO treatment under simulated AM1.5G solar

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

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 Voc 1.4 V deposited directly on FTO substrate (via a one-step solution process) in which no hole blocking layers

). Jeng and co-workers reported that perovskites have the

**4.2 Transport layers in inverted (p-i-n) photovoltaic**

**274**

**Figure 5.**

irradiation (100 mW/cm<sup>2</sup>

using the cesium salt solution [92].

*Device architecture of the perovskite solar cells (a) with TiO2 ETL and (b) without TiO2 ETL (c) J-V curves of CH3NH3PbI3-xClx-based solar cells with and without UVO treatment (d) histograms of their corresponding PCE values measured for 36 separate devices (c). Reprinted with permission from [90]. Copyright (2016) American Chemical Society.*

are required. He further described that TiO2 (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 MAPbI3 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 MAPbI3-the enhanced crystalline of MAPbI3-BPQDs film has been achieved.

## **4.3 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-1Bn X3n + 1, where A' indicates R-NH3 or H3N-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

**Figure 7.**

*(a) Lateral view and (b) top view of the n = 2 sheet (NS-RPP) optimized structure. Same (c, d) for the bulk QW (QW-RPP). [gray: Pb; purple: I; Brown: C; light blue: N; white: H atoms]. Reprinted with permission from [98]. Copyright (2018) American Chemical Society.*

such as CH3NH3 + and Cs+ . B represents Pb2+ and Sn2+, 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 [101–106]. Giorgi *et al.* [98] showed a lateral and top view of the nanosheets Ruddlesden–Popper organic– inorganic 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 (C6H9C2H4NH3)2 PbI4 flakes via the method of micromechanical exfoliation.
