**3. Properties**

*2.2.5. Point defects*

258 Nanostructured Solar Cells

lency of the system [104].

*2.2.6. Structural disorder*

The p‐ or n‐type absorbers were made from materials with intrinsic defects, or using inten‐ tional doping intrinsic defects that create deep energy levels in the absorber usually act as Shockley‐Read‐Hall nonradiative recombination centers and carrier traps, reducing the carrier lifetime and thus *V*oc. A good solar cell absorber must exhibit proper doping and defect prop‐ erties. There are many types of defects as a donor and acceptor which lies in the semiconduc‐ tors. The formation energy of a defect depends on the chemical potential and environmental factors such as precursors, partial pressure, and temperature. So we can conclude that these experimental conditions play a vital role to determine the formation energies of all the pos‐ sible defects and further impact the polar conductivity in these materials. Defect formation energies determine the polar conductivity of a semiconductor, whereas defect transition lev‐

Besides point defects, Kim et al. [102] used DFT‐GGA to calculate the DOS and partial

(equal numbers of positive and negative vacancies) and (b) Frenkel defects (equal numbers of vacancies and interstitials of the same ion). The tunable polar conductivity and shallow defect properties may help to explain why high‐performance perovskite solar cells, with extremely long carrier lifetimes [40, 103] can be produced by a diverse range of growth approaches and a wide variety of solar cell architectures. These point defects would suggest new methods for perovskite solar cell architecture. It was observed that deep point defect levels could exist through large atomic relaxations, which is attributed to the strong cova‐

disordered with a local perovskite structure extending over a range of only 1.4 nm, which is

The mesoporous scaffold confined need the perovskite within the pores and reshaped the structures of perovskites. On the other hand, the low‐temperature growth process inevita‐ bly leads to polycrystalline perovskites with grain boundaries (GBs). Experimentally, it is very difficult to investigate the structural and electronic properties directly, as it requires a high resolution transmission spectroscopy (HRTEM). So, we have to rely on the theoretical calculations that can give direct insights into the electrical properties of structural disorders and topological defects in hybrid perovskites. Recent combined theoretical and experimental studies [106] have demonstrated that Cl segregated into the GB part of polycrystalline CdTe

> NH3 PbI3

. It was observed that the DOS of the supercells with GBs are very similar to those of single‐crystal phases. None of these GBs introduce defect states within the bandgap region.

NH3 PbI3

> NH3 PbI3

, the GB structures were constructed based on

: (a) Schottky defects

(70%) is highly

els determine the electrical effect of any particular defect [101].

charge densities of two types of neutral defects in β phase CH3

In a recent investigation, Choi et al. [105] found that most of CH3

solar cells effectively taming the detrimental effects at GBs.

The GW band structure diagram is given in **Figure 8**.

about 2 lattice constants of the α phase [106].

Due to the structural complicity of CH3

CsPbI3

#### **3.1. Electrical properties**

Hybrid perovskites exhibit unprecedented carrier transport properties that enable their stel‐ lar performance in photovoltaics. So more attention is needed to develop understanding the material properties and ways to improve these properties in all key directions for research. The electrical properties of perovskite materials are seen in the ambipolar carrier transport behavior and long carrier lifetime. These electrical properties are further investigated on the basis of corresponding device structure.

#### *3.1.1. Intrinsic electrical properties*

The electrical characteristics of the materials are determined by the carrier type, concentra‐ tion, and mobility, which is dependent on the method of preparation. It is necessary to use smooth and uniform films to perform measurements. The carrier type is determined by Hall measurements of the conductivity's response to an applied magnetic field, thin‐film tran‐ sistor's response to a gating electric field, and thermoelectric measurements of the Seebeck coefficient. For example, CH3 NH3 PbI3 indicated n‐type conductivity, a carrier concentration of ~109 cm−3, and an electron mobility of 66 cm2 /V/s [24]. Carrier concentration can also be adjusted by tuning the stoichiometry of the precursors during solution‐phase synthesis and even switch the carrier type to the p‐type when excess CH3 NH3 I is used in two‐step synthesis. 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 n‐type films deposited from stoichiometric precursors was determined to be 3.9 cm2 /V/s from the Hall measurements, although CH3 NH3 SnI3 prepared by a solid‐state reaction in a vac‐ uum‐sealed tube showed an electron mobility of 2320 cm2 /V/s [24], while solution processed material measured mobility of 200 cm2 /V/s. It was observed that the electron mobility of poly‐ crystalline CH3 NH3 PbI3 films is larger than the thin‐film mobility of polymers [107, 108] and colloidal quantum dots (10−3–1 cm2 /V/s) [109] comparable to CdTe (10 cm2 /V/s) [110] CIGS, Cu2 ZnSnS4 (CZTS) (10–102 cm<sup>2</sup> /V/s) [111, 112], and polycrystalline Si (40 cm2 /V/s) [101]. Film morphology plays an important role as the dark and light conductivities of CH3NH3 PbI3−*<sup>x</sup>* Cl*<sup>x</sup>* deposited on a planar scaffold on mesostructured aluminum oxide are quite different [113]. To further increase the photovoltaic performance and radiative lifetime, solvent annealing has been applied to increase the grain size of the films to ~1 μm [114].
