## 6 Discussion  
In the quest for a perovskite device structure and composition that features low production costs, low toxicity and great stability, compositional tailoring is a great avenue to pursue. In this context, many dopants have been tried and it was recently discovered that the introduction of a few percent of zinc has profound effects on device morphology, efficiency and stability. The bandgap however, remains virtually unchanged. Selecting suitable dopants is not a random process and the mechanisms behind the changing properties are (partly) known. The anionic composition influences the bandgap of the material, because the combination of metal cation and anion influences these properties, the organic cation is mainly responsible for the structural stability of the perovskite material. [PERSON] et al. [9] were the first to dope perovskites with non-group IV-metals and found out that perovskite was tolerant towards Co, Cu, Zn and Sn. Soon it was discovered that Zn had effects on morphology and stability and many groups started experimenting with Zn doping. Zn was immediately popular due to its non-toxic nature, its resistance towards oxidation and reduction reactions and its abundant presence in the Earth's crust. Probably the most promising advantage is the improved morphology, efficiency and stability of the Zn-doped perovskite material. The exact mechanism behind these improved properties were only partially reported until recently. Unlike the report, Reference [22], on hetero-valent doping with Al\({}^{3+}\), where Al\({}^{3+}\) concentrates at grain boundary edges aiding the formation of bigger grains and fewer pinholes, Zn is actually replacing Pb homogeneously over the thin film. Elemental mapping confirms that Zn does not accumulate on grain edges but is found to be evenly dispersed over the whole film. All reports indicate that Zn is incorporated in the lattice on a Pb position and XRD measurements confirm that lattice contraction takes place due to the presence of smaller Zn atoms on Pb positions in the crystal structure. Another \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Material** & **x (Zn)** & **PCE (\%) Undoped** & **PCE (\%) Zn-Doped** & **d/ud** & **Reference** \\ \hline MAPPb\({}_{1+\text{Zn}}\)N\({}_{3}\)-[OAc\({}^{-}\)] & 0.016 & 7.27 & 9.69 & 1.33 & [9]1  \\ MAPPb\({}_{1+\text{Zn}}\)N\({}_{3}\) & 0.03 & 17.13 & 18.35 & 1.07 & [14] \\ MAPPb\({}_{1+\text{Zn}}\)N\({}_{3}\) & 0.05 & 17.11 & 18.25 & 1.07 & [15] \\ MAPPb\({}_{1-\text{Zn}}\)N\({}_{3}\) & 0.01 & 17.94 & 20.06 & 1.12 & [23] \\ \hline FA\({}_{0.9\text{ZA}}\)M\({}_{0.05}\)Pb\({}_{3}\)-[PEAI, Cl\({}^{-}\)] & - & 23.32 & - & - & [25]2  \\ FA\({}_{0.9\text{S}}\)MA\({}_{0.05}\)Pb\({}_{2.85}\)Pb\({}_{0.15}\)-[PEAI, Cl\({}^{-}\)] & - & 22.10 & - & - & [26]3  \\ FA\({}_{0.8\text{ZA}}\)N\({}_{3}\)-[PEAI] & - & 17.91 & - & - & [28]5  \\ MAPPb\({}_{3}\) & - & 3.81 & - & - & [28]6  \\ FASnI\({}_{3}\)-[PEAI] & - & 9.00 & - & - & [29] \\ MASnIBr\({}_{2}\) & - & 5.75 & - & - & [8] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Reported power conversion efficiencies (PCE) for materials without zinc (undoped, ud) and Zn-doped (d) materials, together with their enhancement factors (d/ud). Compositions are estimated from their References. Additives are indicated in square brackets. A selection of certified world record efficiencies is given in the last seven entries. consistent observation throughout literature is the increased grain size and smoother pinhole-free perovskite surface after the addition of Zn(II). This is attributed to the unit cell contraction, which has effects on the crystallization thermodynamics resulting results in a lower defect density within the film. It is hard to speculate about effects of the presence of zinc on actual device production, but we do not anticipate interference with metal electrode deposition and sealing procedures, zinc being relatively inert and applied in industry in multiple areas. Exemption of RoHS (Regulations of Hazardous Substances) is certainly needed for Pb containing materials.