## 3 Defect Passivation through Compositional Engineering of Perovskites
In contrast to these studies, other work shows that the relation between optoelectronic quality and the orientation and size of crystalline domains must be interpreted with caution. [PERSON] et al. (2019)used electron back-scattered diffraction (EBSD), which has higher spatial resolution than the conventional XRD and scanning electron microscopy (SEM) methods, to obtain crystallographic parameters [31]. They correlated the results with local photoluminescence properties. The PL and carrier lifetimes did not show any dependence on grain size or orientation. They argued that the contrasting findings with previous studies can be explained by the following: First, morphological clusters observed in SEM and real crystallographic grains are often misinterpreted, clusters seen in SEM can be wrongly assigned to grains. Secondly, the chemical additives that create enlarged grains and preferential orientation can also passivate defects or grains. Also [PERSON] et al. (2019) showed that zinc does not necessarily influence the crystal grain size or orientation but can aid homogeneous incorporation of elements [32]. Lead-tin based perovskite cells were fabricated employing different concentrations of ZnI\({}_{2}\) precursor and from the chemical spatial distribution mapping of the elements (by using STEM/EDX = Scanning transmission electron microscopy / Energy-dispersive X-ray spectroscopy), as shown in Figure 10, it was observed that Zn doping leads to more homogeneously distributed Sn through the films (Figure 10b). The majority of the Zn localizes along the bottom of the film (Figure 10c) and, therefore, they proposed that high doping concentration can form an insulating layer. This also explained the current and voltage enhancement for the 1% and 2% ZnI\({}_{2}\) doping devices and the decreasing device performances at 10% doping. The XRD results revealed a small increase in the lattice parameter so that it is expected that Zn is incorporated in small amount but that the structure is not altered nor its grain sizes. They speculated on the role of Zn as an active dopant, as suggested by [PERSON] et al. [33], and as additive assisting and seeding the growth of higher-quality grains that are chemically more homogeneous. When perovskite material is tolerant to the inclusion of extrinsic dopant, additional passivating effects are observed. Here, we discuss several dopants that are able to incor  
Figure 10: Distribution of elements of FA (Pb\({}_{0.5}\)Sn\({}_{0.5}\))I\({}_{3}\) with and without 5% ZnI\({}_{2}\) added measured by STEM/EDX. (**a**) Pb. (**b**) Sn. (**c**) Zn. (**d**) Change in lattice parameter with doping content. Reproduced with permission from _ACS Energy Lett._**2019**, 4, 2301–2307. Reference [32]. X-site or at interstitials and the subsequent influence on the lattice. The tolerance for the inclusion of foreign atoms by the perovskite lattice depends on the ionic radius. Small amounts of extrinsic doping at the A, B, X sites in the perovskite crystal can lead to increased disorder if the radius is either too large or too small. These lattice distortions affect the phase stability of the perovskite as empirically described by the Goldschmidt tolerance factor:  
\[t=\frac{r_{A}+r_{X}}{\sqrt{2}(r_{B}+r_{X})}\]  
where \(r\) are the ionic radii and \(0.8<t<1\) to maintain the 3D perovskite lattice, with a perfect cubic base at ideal 1. For MAPbI3 the reported factor ranges between 0.92-0.95. The incorporation of Zn at the B-site has been frequently reported and can also be expected from the calculated tolerance factor of 1.05 for MAZnI3[34]. [PERSON] et al. argued that the incorporation of Zn(II) in MAPbI3\({}_{\text{3-x}}\)Cl\({}_{\text{x}}\) crystals caused the lattice to shrink, they observed contraction of \(BX_{6}\) octahedron, which is beneficial to fix the A-site cation and relax the lattice strain [35]. This also promoted the formation of ordered and uniform perovskite crystal with tetragonal structure.