## 3 Doping of Perovskites with Zinc(II)  
Following up on the [PERSON] report [9] (2016), much more effort was put into research on the partial replacement of lead with zinc. [PERSON] et al. reported that the perovskite lattice is tolerant to non-VI group elements, such as: Co\({}^{2+}\), Cu\({}^{2+}\), Sn\({}^{2+}\) and Zn\({}^{2+}\) (from zinc-acetate). Among these elements, Zn stands out due to certain characteristics that make it particularly interesting. At first, Zn is abundantly available in the Earth's crust, non-toxic and therefore a welcome partial replacement for lead. Zn(II) is a medium-sized transition metal and its atomic radius (74 pm) is considerably smaller than the one of lead(II) (119 pm [22]). Furthermore, zinc has a suitable valence state and is reported to be resistant against reduction and oxidation processes [16]. The theoretical Goldsmith tolerance factor (\(t\)) for a generic ABX\({}_{3}\) perovskite is calculated as shown in Equation (1). \[t=\frac{r_{A}+r_{X}}{\sqrt{2}(r_{B}+r_{X})} \tag{1}\]  
Figure 2: The proposed perovskite thin film growth under the influence of Al\({}^{3+}\). C, N, H, O, I, Pb and Al are depicted in black, blue, pink, red, yellow, grey and cyan respectively. Figure adapted with permission from _Energy Environ. Sci._**2016**, 9, 2892–2901, Reference [22]. Here, \(r_{A}\), \(r_{B}\), and \(r_{X}\) are the (average) ionic radii for the cation (\(A\)), metal (\(B\)), and halide (\(X\)) sites, respectively. If the tolerance factor is in the range 0.813-1.107, perovskites can form. The Goldsmith factor expresses how close the geometry is to an ideal undistorted perovskite packing, with a perfect cubic base as the ideal 1. For MAPbI\({}_{3}\) and FAPbI\({}_{3}\) these reported factors range between 0.92-0.95 and 1.03-1.04, respectively. The tolerance factor of CH\({}_{3}\)NH\({}_{3}\)ZnI\({}_{3}\) is calculated to be 1.05. In this calculation the atomic radii for methyl ammonium (\(R_{A}\)) and iodine (\(R_{X}\)) are 0.18 nm and \(R_{X}\) = 0.22 nm, respectively [12]. Due to the tolerance factor of CH\({}_{3}\)NH\({}_{3}\)ZnI\({}_{3}\) being very close to 1, it is expected that Zn\({}^{2+}\) has the ability to form CH\({}_{3}\)NH\({}_{3}\)Pb\({}_{1-x}\)ZnI\({}_{x}\)I\({}_{3-y}\)Cl\({}_{y}\) and this can be considered a first step in the assumption that Zn\({}^{2+}\) (in contrast to Al\({}^{3+}\)) actually incorporates in the perovskite lattice and that Zn can have profound effects on the electronic and structural properties of the resulting material [13]. Values of \(t\) between 0.94 and 0.98 have been correlated with optimal device efficiencies. The Goldsmith tolerance factor (\(t\)) is a simple first limited approach to correlate structure to function. Crystal size, grain boundaries effects and dark quenching states however, are of more practical importance for device performance of perovskite materials. It is important to note that for some ions various radii are available depending on coordination. [PERSON] et al. reported [16] FA/MA-based lead perovskite materials with enhanced stabilization due to doping with 2.5% ZnI\({}_{2}\) and the addition of 3-chloropropyl ammonium chloride (CPACI). The material, FA\({}_{0.85}\)MA\({}_{0.15}\)Pb\({}_{0.975}\)Zn\({}_{0.025}\)I\({}_{2.85}\)Br\({}_{0.15}\)-[CPACI] was spin-coated and annealed and resulted in a compact dense homogenous pinhole-free layer. (Additives are denoted in square brackets in this review). During the annealing step CPACI was gradually evaporated from the structure gradually changing the FA/MA: CPACI ratio. During this gradual change, the lattice organization is induced and leads to the stacking of preformed crystalline units yielding an organized and protected final product. Goldsmith tolerance factor for the perovskite containing 2.5 wt% Zn(II) is 0.981 compared to 0.978 for the full Pb-perovskite. The process is schematically depicted in Figure 3. They used X-ray diffraction (XRD) to confirm that that Zn\({}^{2+}\) is incorporated in the lattice on a Pb\({}^{2+}\) position.