**4.1. Hysteresis**

Perovskite solar cells exhibit an anomalous hysteresis in the current‐voltage and resistiv‐ ity‐temperature dependence curves [136]. Though it was predicted that the hysteresis on resistivity verses temperature curves is associated with the structural phase transition while the reason for current‐voltage curves are still unknown. In an extensive [E‐CE6] studies car‐ ried out by Prof. Erik Christian Garnett et al. [136], several explanations have been proposed as ion migration, filling of interface, or surface trap states, accumulation of charges at grain boundaries and ferroelectricity, yet no convinced conclusion has been drawn. In structural perception, the cubic phases of the chloride and bromide perovskites do not allow a polar ferroelectric distortion. Various hypotheses have been suggested and it was further predicted that hysteresis should depend on the magnitude of the dipole moment of the organic cationic species and the connecting halide cage. Though the origin of this phenomenon is not yet understood properly, a number of possible causes have been proposed in which the noted causes are ferroelectricity or the presence of mobile ionic species [136]. The illustration for the hysteresis in the electrical transport in hybrid perovskites is given in **Figure 10**.

Here, it is necessary to mention that reporting results from single *J*–*V* sweeps, even in the absence of hysteresis, or choosing scan rates to report the highest efficiencies, will lead to Perovskite as Light Harvester: Prospects, Efficiency, Pitfalls and Roadmap http://dx.doi.org/10.5772/65052 263

**Figure 10.** Hysteresis representation in hybrid perovskites. (a) *I*‐*V* graph of CH3 NH3 PbI3 (single crystal) at room temperature, (b) schematic *I*‐*V* curve, (c) proposed phenomena for its origin. Adapted with permission from reference [104].

misleading results. As it might be possible that the certified efficiencies for perovskite solar cells are deemed "not stabilized" though they were measured with negligible hysteresis.

#### **4.2. Thermal and operational stability**

There are so many reports that claim that perovskite solar cells have been shown to be stable for many hundreds of hours without any encapsulation. However, the solar cells were stored in the dark and only measured occasionally. So we can conclude that the sealing from envi‐ ronmental ageing is necessary because of operation at elevated temperature and humidity. Stability has become a bigger problem for tin (II) perovskites due to the decrease in stability of the oxidation state of tin (II) compare to lead (II).

#### **4.3. Toxicity**

**Figure 9.** (a) Absorption spectra, (b) photoluminescence spectra of FAPbI*<sup>x</sup>*

**4. Pitfalls**

**4.1. Hysteresis**

262 Nanostructured Solar Cells

Br3−*<sup>x</sup>*

phase transition Br‐rich cubic phase to the I‐rich tetragonal phase. Adapted with permission from reference [37].

Perovskite solar cells exhibit an anomalous hysteresis in the current‐voltage and resistiv‐ ity‐temperature dependence curves [136]. Though it was predicted that the hysteresis on resistivity verses temperature curves is associated with the structural phase transition while the reason for current‐voltage curves are still unknown. In an extensive [E‐CE6] studies car‐ ried out by Prof. Erik Christian Garnett et al. [136], several explanations have been proposed as ion migration, filling of interface, or surface trap states, accumulation of charges at grain boundaries and ferroelectricity, yet no convinced conclusion has been drawn. In structural perception, the cubic phases of the chloride and bromide perovskites do not allow a polar ferroelectric distortion. Various hypotheses have been suggested and it was further predicted that hysteresis should depend on the magnitude of the dipole moment of the organic cationic species and the connecting halide cage. Though the origin of this phenomenon is not yet understood properly, a number of possible causes have been proposed in which the noted causes are ferroelectricity or the presence of mobile ionic species [136]. The illustration for the

hysteresis in the electrical transport in hybrid perovskites is given in **Figure 10**.

Here, it is necessary to mention that reporting results from single *J*–*V* sweeps, even in the absence of hysteresis, or choosing scan rates to report the highest efficiencies, will lead to

(varying I:Br ratio), (c) XRD spectra of the

Due to the toxic nature of lead, concerns have been raised on the possible environment and legal‐ ization problems from perovskite solar cells based on water soluble lead compounds. So efforts have been made to replace lead with other metal ions without degrading the photophysical proper‐ ties with quantum mechanical calculations. As lead halogen perovskites are water soluble, the most pessimistic view is the consequences of damaged solar cells and panels with potential exposure to water followed by dissolution and distribution of lead ions into buildings, soil, air, and water.

**Figure 11.** Pictorial representation of replacement of lead by strontium in perovskite solar cells [138].

Lead is known to damage the nervous system and cause brain disorders. In this direction, a theoretical study carried out by De Angelis and group [137] has replaced Pb by Sn (**Figure 11**) with effective development of the GW method with spin‐orbit coupling to accurately model the properties of CH3 NH3 SnI3 and then compared it to the CH3 NH3 PbI3 . They predicted that MASnI3 is a better electron transporter than MAPbI3 by the SR‐DFT method. Another study carried out by Jesper Jacobsson and group [138] has provided deep physical insights into the photophysical nature of a metal‐halogen perovskite by removing lead with strontium, which is relatively nontoxic and inexpensive. CCSD calculations and DFT study were performed on the two basic structures of CH3 NH3 SrI3 and CH3 NH3 PbI3 to extract and compare the electronic structures and the optical properties. This is based on the fact that the ionic radii of Sr2+ and Pb2+ are almost identical, so the exchange could be made as it will not affect the crystal struc‐ ture. CH3 NH3 SrI3 gives a bandgap of 1.6 eV, which is fairly close to the experimental value reported to be around 1.55 eV [5, 42]. The second effect that was caused by shifting Sr for Pb is that the shape of the pdos graphs for both the halogen and the organic ion is shifted and slightly distorted. The lower electronegativity of Sr compared to Pb shifts the electronic cloud closer to the iodine atoms in the lattice, which perturb the local dipole moment as well as the bonding angles between the iodine octahedra and consequently their columbic interaction with the methylammonium dipoles. The charge distribution is similar to the two structures, with higher charge density around lead compared to strontium due to the higher atomic number of lead.
