*2.2.3. Ferroelectricity*

*2.2.2. Optical absorption spectra*

256 Nanostructured Solar Cells

mental calculations, it is proved that CH3

**Figure 7.** (a) The periodic structural model of Σ5 (310) GB for CH3

and strain [89].

CH3 NH3 PbI3

permission from reference [137].

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

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‐

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

> NH3 PbI3

calculated from unit cell. (c–f) pdos of selected atoms highlighted in the above structure. Adapted with

. (b) Comparison of DOS of bulk

NH3 PbI3 NH3 PbI3

perovskite has the capability of achieving

and CsPbI3

)

carried out on this basis and it was found that halide perovskites (CH3

One more theoretical aspect is the dipole moment of the noncentrosymmetric organic cation in perovskite materials. It was shown from electric dipole calculations of the organic cation that hybrid perovskites exhibit spontaneous electric polarization, which might be due to the two reasons: the alignment of the dipole moments of organic cations and the intrinsic lat‐ tice distortion breaking the crystal centrosymmetry. On the basis of this concept, it was pro‐ posed in the studies that the presence of ferroelectric domains will result in internal junctions might support electron‐hole separation and transportation. However, the calculated value of CH3 NH3 PbI3 bulk polarization is 38 mC/cm2 , which is comparable to the value of ferroelec‐ tric oxide perovskites such as KNbO3 (30 mC/cm2 ) [90]. Frost and coworkers [91] suggested that it may be possible that the boundaries of ferroelectric domains may form "ferroelectric highways" that facilitate the transportation of electrons and holes. Furthermore, it was pro‐ posed that the favorable highways are energetically chosen in such a way that the holes and electrons avoid any collision with the opposite charges. It is directly seen in the recent experi‐ ment by direct observation of ferroelectric domains in the β phase of CH3 NH3 PbI3 . Another important factor is that *V*OC can be larger than the bandgap, and charge separation and carrier lifetime can be enhanced due to the internal electric field [92].

#### *2.2.4. Interface and surface*

The surface and interface between the absorber, carrier transport layers, and electrode contact layers are also important for efficient carrier transportation. However, the two‐step method, vacuum deposition and vapor‐assisted solution processing methods [85], have improved the quality much better by the one‐step method. The vacuum deposition method is used in small molecule‐based devices, which makes the use of insoluble materials more stable than their soluble analogues. There are at least three aspects worth consideration.

#### *2.2.4.1. Band alignment*

The bandgaps and band alignments of perovskites can also be tuned by the chemical manage‐ ment of compositional elements, including organic cations [93, 94], Pb [95–97], and halogen elements [98, 99]. This is another way to optimize band alignment at interfaces.

#### *2.2.4.2. Interface structure and passivation*

The unusual hysteresis of the *I*–*V* curve of perovskite solar cells, which would reduce the working cell efficiency, was suspected to be related to the interface properties [99, 100].

#### *2.2.4.3. Surface*

Abate et al. [79] reported the existence of trap states at the perovskite surface, which gener‐ ated charge accumulation and consequent recombination losses in working solar cells. They identified under coordinated iodine ions as responsible and used supramolecular halogen bond complexation for passivation.
