*2.1.1. Origin of perovskite*

Although these materials already possessed useful physical properties, organic‐like mobil‐ ity, nonlinear optical properties, enhanced exciton binding energies, electroluminescence, magnetic properties, and conductivity, they have emerged as DSSCs only in 2009 [63–68]. The performance of DSSCs is assessed by three major parameters: short‐circuit photocurrent (*J* SC), open‐circuit voltage (*V*OC), and fill factor (FF), which are further used to calculate the efficiency (PCE). *V*OC is proportional to the HOMO‐LUMO energy gap and *J* SC reflects the mobility, efficient light‐harvesting, and carrier generation. These values of different device structures are presented in **Table 3**.

The first perovskite‐sensitized TiO2 solar cell gave the efficiency of 3.8 and 3.1%, respectively [13]. Later on the titania's surface and CH3 NH3 PbI3 ‐based iodide liquid electrolyte solar cell have increased the efficiency to 6.5% [25]. In 2012, the liquid electrolyte was replaced with a solid electrolyte and a PCE of 9.7% was achieved [69]. A sequential deposition method for the formation of the perovskite pigment within the porous metal oxide film was developed with a PCE of 15% in 2013 (short‐circuit current density *J* SC = 21.5 mA/cm2 , open‐circuit voltage *V*OC = 1.02 V, and fill factor FF = 0.71) [27]. An efficiency of 20% at low temperature was achieved in a processed solar cell, through the end of 2013 [70, 71]. Further, it is reported that the achieved efficiency has above 30% in 2016.

#### *2.1.2. Photoanodes*

Mesoporous metal oxide films act as a working electrode for perovskite cells. The charge extraction rates are relatively faster for the perovskite solar cells than the conventional DSSCs [39]. Again the mesoporous TiO2 was replaced by Al2 O3 with similar mesomorphology and it was seen that the PCE unexpectedly reached to 10.9% giving hopes for the future increase in efficiency. Furthermore, the DSSC efficiency has improved to 15.9% [27], yet there is the difficulty in pore filling because of the labyrinthine maze structure [72], which was alterna‐ tively substituted by a vertically aligned nanowire (NW) and nanotube (NT) structure. These nanotubes and nanowires can be used in pore filling due to their open porous structures. Moreover, they are reported to be better in electron transportation and recombination behav‐ ior and hole conductors presenting faster recombination than nanoparticulate films in liquid‐ based DSSCs [73–75].

As the absorption properties of perovskite are excellent, a possible decrease in the total sur‐ face area of the NWs/NTs compared to the nanoparticles does not stimulate the significant reduction of photocurrent. Later it was concluded that perovskite semiconductors in their simple architecture can exhibit sufficiently good ambipolar charge transport and the principal roles of photovoltaic operation, including charge generation, light absorption, and transport


**Table 3.** comprehensive summary of the performance of perovskite solar cells, including the perovskite materials, photoanodes, hole‐transport materials (HTMs), *J* SC (mA/cm), *V*OC (v), FF and PCE (%)

of both electrons and holes. Now the challenge is to determine whether mesostructure is essential or the thin‐film p‐i‐n can lead to a better performance [76].

#### *2.1.3. Perovskite thin films*

become unstable and a perovskite structure will not form, although these factors provide a guidelines for the formation of halide perovskite, yet they are not sufficient to predict the

Although these materials already possessed useful physical properties, organic‐like mobil‐ ity, nonlinear optical properties, enhanced exciton binding energies, electroluminescence, magnetic properties, and conductivity, they have emerged as DSSCs only in 2009 [63–68]. The performance of DSSCs is assessed by three major parameters: short‐circuit photocurrent

SC), open‐circuit voltage (*V*OC), and fill factor (FF), which are further used to calculate the

mobility, efficient light‐harvesting, and carrier generation. These values of different device

NH3 PbI3

have increased the efficiency to 6.5% [25]. In 2012, the liquid electrolyte was replaced with a solid electrolyte and a PCE of 9.7% was achieved [69]. A sequential deposition method for the formation of the perovskite pigment within the porous metal oxide film was developed with

*V*OC = 1.02 V, and fill factor FF = 0.71) [27]. An efficiency of 20% at low temperature was achieved in a processed solar cell, through the end of 2013 [70, 71]. Further, it is reported that

Mesoporous metal oxide films act as a working electrode for perovskite cells. The charge extraction rates are relatively faster for the perovskite solar cells than the conventional DSSCs

it was seen that the PCE unexpectedly reached to 10.9% giving hopes for the future increase in efficiency. Furthermore, the DSSC efficiency has improved to 15.9% [27], yet there is the difficulty in pore filling because of the labyrinthine maze structure [72], which was alterna‐ tively substituted by a vertically aligned nanowire (NW) and nanotube (NT) structure. These nanotubes and nanowires can be used in pore filling due to their open porous structures. Moreover, they are reported to be better in electron transportation and recombination behav‐ ior and hole conductors presenting faster recombination than nanoparticulate films in liquid‐

As the absorption properties of perovskite are excellent, a possible decrease in the total sur‐ face area of the NWs/NTs compared to the nanoparticles does not stimulate the significant reduction of photocurrent. Later it was concluded that perovskite semiconductors in their simple architecture can exhibit sufficiently good ambipolar charge transport and the principal roles of photovoltaic operation, including charge generation, light absorption, and transport

O3

was replaced by Al2

solar cell gave the efficiency of 3.8 and 3.1%, respectively

SC = 21.5 mA/cm2

‐based iodide liquid electrolyte solar cell

with similar mesomorphology and

SC reflects the

, open‐circuit voltage

efficiency (PCE). *V*OC is proportional to the HOMO‐LUMO energy gap and *J*

structural formations within the perovskite family [62].

**2.1. Experimental scenario**

structures are presented in **Table 3**.

The first perovskite‐sensitized TiO2

[39]. Again the mesoporous TiO2

*2.1.2. Photoanodes*

based DSSCs [73–75].

[13]. Later on the titania's surface and CH3

a PCE of 15% in 2013 (short‐circuit current density *J*

the achieved efficiency has above 30% in 2016.

*2.1.1. Origin of perovskite*

250 Nanostructured Solar Cells

(*J*

While using the methylammonium lead halide (CH3 NH3 PbX3 , X‐halogen) and its mixed‐ halide crystals, corresponding to the 3D perovskite structures as light harvesters in solar cells, it is observed that substituting the I with Cl/Br ions, bandgap tuning of MAPbX3 is achieved, which occurred due to the strong dependence of electronic energies on the effective exci‐ ton mass [76]. The entire visible region was controlled by tuning the bandgap. Apart from that, the addition of Cl/Br into an iodide‐based structure shows a drastic improvement in the charge transport, relative stability, and separation kinetics within the perovskite layer [77]. It was also observed that the bandgap is reduced (1.48–2.23 eV), leading to high short‐circuit currents of >23 mA/cm2 and a PCE of up to 14.2%, when the cation size of perovskite materials is increased [42].

There are a few solution‐based techniques that has been used for the fabrication of thin films, where a mixture of two precursors is used to form final absorber, but due to the lack of suit‐ able solvents and high‐reaction rate of the perovskite component, the process results in thin film with pinhole formation and incomplete surface coverage, which deteriorates the film quality and thus effect the device performance.

The two‐step deposition technique that was used previously to prepare the films of organic‐ inorganic systems has incompatible solubility characteristics where the organic component is difficult to evaporate. Devices based on the planar CH3 NH3 PbI3 thin film via the modified two‐step deposition technique have also achieved the efficiency of 12.1% [78].

Another technique that was developed was dual‐source vapor‐deposited organometallic trihalide perovskite solar cells based on a p‐i‐n thin‐film architecture with high efficiency. However, the deposition with the vacuum evaporation method will make it cost effective [26].

## *2.1.4. Hole‐transporting materials (HTMs)*

The conductivity of perovskite is high, which requires a thick layer of HTM to avoid pin‐ holes. Spiro‐OMeTAD, due to being less conductive, offers high resistance because of thick capping layers. A wide variety of polymer hole conductors are also used, which is shown in **Figure 4**. Protic ionic liquids (PILs) are used as effective p dopants in hybrid solar cells [78] based on triarylamine hole‐transporting materials. Further, the efficiency is improved by replacing the lithium salts, p‐dopants for spiro‐OMeTAD with PILs [79]. While using other HTMs as P3HT and DEH HTM, the efficiency of spiro‐OMeTAD is much slower than P3HT and DEH HTM, respectively. However, a recent synthesis based on the pyrene‐based derivative Py‐C also exhibited an overall PCE of 12.7% [76]. As the hole conductors, spiro‐ OMeTAD and P3HT are costly, so an inexpensive, stable, solution‐processable inorganic CuI as the hole conductor has been demonstrated [80]. A solution‐processed p‐type direct bandgap semiconductor CsSnI3 with a perovskite structure can also be used for hole con‐ duction replacing a liquid electrolyte [34]. Overall we can say that perovskite materials play both the role of light harvesters and hole conductors. Recently, a hole‐conductor‐free mesoscopic CH3 NH3 PbI3 perovskite/TiO2 heterojunction solar cell has reported with a PCE of 5.5% [30], yet the photovoltaic performance was inferior to that of HTM. The tuning of bandgap of perovskite materials plays an important role in photophysical properties. The energy bandgaps of different hybrid materials and the hole‐transporting materials are given in **Figures 5** and **6**.

Perovskite as Light Harvester: Prospects, Efficiency, Pitfalls and Roadmap http://dx.doi.org/10.5772/65052 253

**Figure 4.** Structural representation of hole‐transporting materials (HTMs).

it is observed that substituting the I with Cl/Br ions, bandgap tuning of MAPbX3

currents of >23 mA/cm2

quality and thus effect the device performance.

*2.1.4. Hole‐transporting materials (HTMs)*

bandgap semiconductor CsSnI3

NH3 PbI3

mesoscopic CH3

given in **Figures 5** and **6**.

is difficult to evaporate. Devices based on the planar CH3

is increased [42].

252 Nanostructured Solar Cells

which occurred due to the strong dependence of electronic energies on the effective exci‐ ton mass [76]. The entire visible region was controlled by tuning the bandgap. Apart from that, the addition of Cl/Br into an iodide‐based structure shows a drastic improvement in the charge transport, relative stability, and separation kinetics within the perovskite layer [77]. It was also observed that the bandgap is reduced (1.48–2.23 eV), leading to high short‐circuit

There are a few solution‐based techniques that has been used for the fabrication of thin films, where a mixture of two precursors is used to form final absorber, but due to the lack of suit‐ able solvents and high‐reaction rate of the perovskite component, the process results in thin film with pinhole formation and incomplete surface coverage, which deteriorates the film

The two‐step deposition technique that was used previously to prepare the films of organic‐ inorganic systems has incompatible solubility characteristics where the organic component

Another technique that was developed was dual‐source vapor‐deposited organometallic trihalide perovskite solar cells based on a p‐i‐n thin‐film architecture with high efficiency. However, the deposition with the vacuum evaporation method will make it cost effective [26].

The conductivity of perovskite is high, which requires a thick layer of HTM to avoid pin‐ holes. Spiro‐OMeTAD, due to being less conductive, offers high resistance because of thick capping layers. A wide variety of polymer hole conductors are also used, which is shown in **Figure 4**. Protic ionic liquids (PILs) are used as effective p dopants in hybrid solar cells [78] based on triarylamine hole‐transporting materials. Further, the efficiency is improved by replacing the lithium salts, p‐dopants for spiro‐OMeTAD with PILs [79]. While using other HTMs as P3HT and DEH HTM, the efficiency of spiro‐OMeTAD is much slower than P3HT and DEH HTM, respectively. However, a recent synthesis based on the pyrene‐based derivative Py‐C also exhibited an overall PCE of 12.7% [76]. As the hole conductors, spiro‐ OMeTAD and P3HT are costly, so an inexpensive, stable, solution‐processable inorganic CuI as the hole conductor has been demonstrated [80]. A solution‐processed p‐type direct

duction replacing a liquid electrolyte [34]. Overall we can say that perovskite materials play both the role of light harvesters and hole conductors. Recently, a hole‐conductor‐free

of 5.5% [30], yet the photovoltaic performance was inferior to that of HTM. The tuning of bandgap of perovskite materials plays an important role in photophysical properties. The energy bandgaps of different hybrid materials and the hole‐transporting materials are

perovskite/TiO2

two‐step deposition technique have also achieved the efficiency of 12.1% [78].

and a PCE of up to 14.2%, when the cation size of perovskite materials

NH3 PbI3

with a perovskite structure can also be used for hole con‐

heterojunction solar cell has reported with a PCE

is achieved,

thin film via the modified

**Figure 5.** Energy bandgap diagram of hybrid perovskite materials.

**Figure 6.** Energy level diagram of hole transporting materials (HTMs).

#### *2.1.5. Measurement of charge‐carrier mobility, lifetime and diffusion lengths*

Regarding the exciton or the electron and hole diffusion length, it was observed that 100‐nm long range diffusion length was obtained in solution‐processed CH3 NH3 PbI3 by applying femtosecond transient optical spectroscopy to bilayers that interface this perovskite with either selective‐electron or selective‐hole extraction materials [38]. The higher efficiency of these materials is only due to the comparable optical absorption length and charge‐car‐ rier diffusion lengths. Photoluminescence quenching measurements were performed to extract the electron‐hole diffusion lengths in triiodide (CH3 NH3 PbI3 ) and mixed‐halide (CH3 NH3 PbI3−*<sup>x</sup>* Cl*<sup>x</sup>* ) perovskite thin films [39]. In mixed‐halide perovskite cells, the larger dif‐ fusion length is due to the much longer recombination time, requires both low recombination rates and high charge‐carrier mobility; however, the mechanism causing the extended diffu‐ sion length is still unclear. Few other things that remain unclear is the relative fraction of free and bound charge pairs at room temperature, the nature of the excited state, and the role of the two species [81, 82].

#### **2.2. Theoretical scenario**

There are reports that prove that Density functional theory (DFT) calculations have already carried out before the first perovskite solar cell was reported experimentally [4, 13]. Various theoretical methods were adopted using exchange‐correlation functionals such as Local density approximation (LDA) [83], Generalized gradient approximation (GGA) [51], hybrid functional methods (HSE), quasiparticle GW methods, spin‐orbit‐coupling (SOC), and van der Waals interactions. LDA underestimates and GGA overestimates the lattice parameters. It is observed that when dispersion interactions are included, the calculated results match well with the experimental results. It is found that adding dispersion corrections increases the binding and corrects the GGA errors.

However, the defects does not affect much as they do not create a detrimental deep level within the bandgap [84, 85] that could be carrier traps and recombination centers for electron‐hole in solar cells. Ringwood [86] has included that the contribution of charges depends on the differences in electronegativity. Since lead is considered as a provider of the charge and size, it holds the perovskite crystals all together.

#### *2.2.1. Ambipolar activities*

*2.1.5. Measurement of charge‐carrier mobility, lifetime and diffusion lengths*

**Figure 6.** Energy level diagram of hole transporting materials (HTMs).

long range diffusion length was obtained in solution‐processed CH3

extract the electron‐hole diffusion lengths in triiodide (CH3

(CH3 NH3

PbI3−*<sup>x</sup>* Cl*<sup>x</sup>*

254 Nanostructured Solar Cells

the two species [81, 82].

**2.2. Theoretical scenario**

Regarding the exciton or the electron and hole diffusion length, it was observed that 100‐nm

femtosecond transient optical spectroscopy to bilayers that interface this perovskite with either selective‐electron or selective‐hole extraction materials [38]. The higher efficiency of these materials is only due to the comparable optical absorption length and charge‐car‐ rier diffusion lengths. Photoluminescence quenching measurements were performed to

fusion length is due to the much longer recombination time, requires both low recombination rates and high charge‐carrier mobility; however, the mechanism causing the extended diffu‐ sion length is still unclear. Few other things that remain unclear is the relative fraction of free and bound charge pairs at room temperature, the nature of the excited state, and the role of

There are reports that prove that Density functional theory (DFT) calculations have already carried out before the first perovskite solar cell was reported experimentally [4, 13]. Various theoretical methods were adopted using exchange‐correlation functionals such as Local density approximation (LDA) [83], Generalized gradient approximation (GGA) [51], hybrid functional methods (HSE), quasiparticle GW methods, spin‐orbit‐coupling (SOC), and van der Waals interactions. LDA underestimates and GGA overestimates the lattice parameters.

NH3 PbI3

NH3 PbI3

) perovskite thin films [39]. In mixed‐halide perovskite cells, the larger dif‐

by applying

) and mixed‐halide

The ambipolar activities of these materials can be defined by taking effective mass into con‐ sideration which is defined by formula:

$$\hat{\theta}\,m^\* = \hbar^2 \left[\frac{\delta^2 \mathcal{E}\{k\}}{\delta^2 \mathcal{E}\{k\}}\right]^{-1} \tag{3}$$

where *ε*(*k*) is the energy dispersion relation functions, described by the band structures. If the band is more dispersive (flat), near the band edges, the effective mass is lighter (heavier). In perovskite materials, the lone‐pair Pb s electrons play a vital role. The electronic structure of CH3 NH3 PbI3 is inverted. The conduction band matrix is derived from Pb p orbitals, and the valence band matrix is a mixture of Pb s and I p (s‐p semiconductor) orbitals. A cation Pb p orbital has a much higher energy level than anion p orbitals, although the CBM is derived from Pb p orbitals, Therefore, the lower conduction band of CH3 NH3 PbI3 is more dispersive than the upper valence band, similarly the upper valence band of CH3 NH3 PbI3 is dispersive due to the strong s‐p coupling around the Valence band maximum (VBM). Due to the balance between the hole effective mass and the electron effective mass, CH3 NH3 PbI3 leads to higher ambipolar activities. It might be possible that many‐body effect plays a role for small carrier effective mass, as the effective mass calculated by the GW + SOC method [87] is even lower. The effective hole and electron masses are given in **Table 4**.


**Table 4.** Calculated effective masses (electron and holes) and bandgap (eV) for different materials. Experimental values are in parenthesis

#### *2.2.2. Optical absorption spectra*

The optical absorption spectra of perovskite materials are determined by the energy band‐ gaps and partial density of states (pdos). The pdos graph for different materials is depicted in **Figure 7**. The energy bandgap measures the probability of each photoelectric transition and the density of states measures the total number of possible photoelectric transitions. Thus, we can easily conclude that the optical absorption coefficient of a material is closely related to its electronic structure. However, the effect of optical absorption spectra is not considered in the Shockley‐Queisser limit [42]. The theoretical maximum efficiency depends on the thickness of the absorber layer. Recently, a method has been developed by Yu et al. [88], in which they calculated the maximum efficiency based on the absorber thickness by taking absorption coef‐ ficient and absorber layer thickness both into consideration. So theoretical calculations were carried out on this basis and it was found that halide perovskites (CH3 NH3 PbI3 and CsPbI3 ) exhibit much higher conversion efficiencies for any given thickness. These materials are also capable of achieving high efficiencies with very thin absorber layers. On the basis of experi‐ mental calculations, it is proved that CH3 NH3 PbI3 perovskite has the capability of achieving a high fill factor. Improved interfaces and contact layers also improve the performance of a solar cell, while Pb chalcogenides exhibit abnormal bandgap changes with lattice constant and strain [89].

**Figure 7.** (a) The periodic structural model of Σ5 (310) GB for CH3 NH3 PbI3 . (b) Comparison of DOS of bulk CH3 NH3 PbI3 calculated from unit cell. (c–f) pdos of selected atoms highlighted in the above structure. Adapted with permission from reference [137].
