*3.2.1. Optical constants*

A lot of research has been conducted on tuning the bandgap of perovskite, but a more detail understanding of these materials awaits further research. The major problem that occurs in perovskite materials is the difficulty of producing continuous films of sufficient smoothness [121] to avoid measurement artifacts from spectroscopic measurements of transmittance, reflectance, and ellipsometry. The absorption coefficients determined from the absorption of CH3 NH3 PbI3 films on quartz [122] and glass [123] yield values of ~104 cm−1 near the band edge without providing any corrections for the surface's inhomogeneity, so for accurate measure‐ ments is important to calculate the absorption coefficients based on the optical constants of CH3 NH3 PbI3 [124]. It is observed that the absorption spectrum for CH3 NH3 PbI3 differs, when deposited within a mesoscopic template and planar substrates, which might be due to the changes in the crystallite morphology that affects the optical transitions [125, 126].

#### *3.2.2. Excitons*

The electron concentration was measured to be ~1017–1018 cm−3, and it was proposed that the iodide vacancies are responsible for the n‐type conductivity [107]. The electron mobility for

/V/s from

/V/s) [110] CIGS,

/V/s) [101]. Film

PbI3−*<sup>x</sup>* Cl*<sup>x</sup>*

prepared by a solid‐state reaction in a vac‐

/V/s. It was observed that the electron mobility of poly‐

is 1.5–1.9 μm [40]. The carrier diffusion length is compa‐

films is larger than the thin‐film mobility of polymers [107, 108] and

/V/s) [109] comparable to CdTe (10 cm2

/V/s) [111, 112], and polycrystalline Si (40 cm2

/V/s [24], while solution processed

n‐type films deposited from stoichiometric precursors was determined to be 3.9 cm2

NH3 SnI3

morphology plays an important role as the dark and light conductivities of CH3NH3

The techniques used to measure the electrical parameters are given in subsections.

deposited on a planar scaffold on mesostructured aluminum oxide are quite different [113]. To further increase the photovoltaic performance and radiative lifetime, solvent annealing

This technique is used to identify the frequency dependence of capacitance, to measure charge diffusion lengths and lifetimes and to investigate carrier trapping and recombina‐ tion. The carrier diffusion length was derived and has been estimated to be about ~1 μm for

Another method to obtain the electrical parameters is EBIC from which the calculated carrier

rable or longer than that of other polycrystalline semiconductors with direct bandgaps used

It is very important to understand the optical response of the materials, as optical properties are the most important feature of perovskite materials and they provide insights into the electronic and chemical structures. The ability to tune the optoelectronic properties with ease presents a major attraction among researchers. Few important parameters that are used to

A lot of research has been conducted on tuning the bandgap of perovskite, but a more detail understanding of these materials awaits further research. The major problem that occurs in perovskite materials is the difficulty of producing continuous films of sufficient smoothness [121] to avoid measurement artifacts from spectroscopic measurements of transmittance,

the Hall measurements, although CH3

material measured mobility of 200 cm2

NH3 PbI3

colloidal quantum dots (10−3–1 cm2

*3.1.2. Extrinsic electrical properties:*

*3.1.2.1. Impedance spectroscopy (IS) [115, 116]*

*3.1.2.2. Electron beam‐induced current (EBIC) [117]*

define these properties are discussed herein:

NH3

PbI3−*<sup>x</sup>* Cl*<sup>x</sup>*

(CZTS) (10–102

crystalline CH3

260 Nanostructured Solar Cells

ZnSnS4

Cu2

CH3 NH3

PbI3−*<sup>x</sup>* Cl*<sup>x</sup>* [83].

diffusion length forCH3

**3.2. Optical properties**

*3.2.1. Optical constants*

in solar cells [76, 77, 118–120]

uum‐sealed tube showed an electron mobility of 2320 cm2

cm<sup>2</sup>

has been applied to increase the grain size of the films to ~1 μm [114].

Exitons play an important role in perovskites. The studies indicate, however, that there is not significant population of excitons in photovoltaics made from CH3 NH3 PbI3 , whose exci‐ ton‐binding energy has been reported between 20 and 50 meV, comparable to the thermal energy at room temperature [127, 128]. These values have been obtained by fitting temper‐ ature‐dependent absorption spectra using the measured [88] reduced mass of the exciton. Excitonic radius from the binding energy and an appropriate dielectric constant study is still a subject of debate [129]. The excitonic transition significantly enhances the absorption of hybrid perovskites near the band edge [130, 131].

#### *3.2.3. Photoluminescence*

The photoluminescence (PL) efficiency depends on the pump fluence. The trapping of pho‐ togenerated charges competes effectively with direct radiative recombination of electrons while holes reduce luminescence at low excitation energies. The PL efficiency ofCH3 NH3 PbI3 is ~17–30%. The PL efficiency falls at higher pumping and high charge carrier densities. The PL lifetime measurements reported shorter lifetime (between 3 and 18 ns) at low pump fluen‐ cies [127, 132–134]. These longer lifetimes have been found in a semiconductor in doped and undoped GaAs films. This might be due to the photon recycling and the PL lifetime depen‐ dency on surface recombination than radiative recombination. So we can conclude that photon cycling plays a major role in their excited state dynamics, when nonradiative decay pathways are suppressed. The absorption spectra and photoluminescence for perovskite materials are shown in **Figure 9**.

#### *3.2.4. Vibrational spectroscopy*

IR spectroscopy also plays an important role in determining the chemical composition. If we look into the chemical structure of CH3 NH3 PbI3 , CH3 NH3 PbBr3 , and CH3 NH3 PbCl3 , the first one is tetragonal, while the other two are cubic. Raman‐active modes are precluded in the symmetry of the lattice for cubic structures [135], though a weak broadband at 66 cm−1 is observed in CH3 NH3 PbCl3 . For CH3 NH3 PbI3 , the resonant Raman spectrum (DFT calcu‐ lations) has been observed with nodes below 100 cm−1 (approximately) related to the inor‐ ganic octahedron. The higher energy modes indicate the disorder of CH3 NH3 + cations. A lot of work in this field is still required to investigate how the modes shift occurs with the struc‐ tural changes. Raman nodes can provide better tool in understanding the in homogeneity of perovskite films with submicron spatial resolution.

**Figure 9.** (a) Absorption spectra, (b) photoluminescence spectra of FAPbI*<sup>x</sup>* Br3−*<sup>x</sup>* (varying I:Br ratio), (c) XRD spectra of the phase transition Br‐rich cubic phase to the I‐rich tetragonal phase. Adapted with permission from reference [37].
