## 4 Influence of Zinc(II) on the Crystallization Mechanism  
[PERSON] et al. published [13] a thorough reasoning on the positive effects that Zn content has on the properties of perovskite solar cells (MA(Pb:Zn)I\({}_{3}\)-[Cl']). They hypothesized that Zn\({}^{2+}\) forms a perovskite structure with MA\({}^{+}\) and I\({}^{-}\). Additionally, they report that Zn has a higher interaction with MA. This was confirmed by using attenuated total refraction Fourier Transform Infra-Red spectroscopy (ATF-FTIR) to study the stretching frequency of symmetrical NH\({}_{3}\)\({}^{+}\) in a pristine sample of CH\({}_{3}\)NH\({}_{3}\)I and when incorporated in a perovskite structure. As shown, stretching vibration of symmetrical NH\({}_{3}\)\({}^{+}\) in a CH\({}_{3}\)NH\({}_{3}\)I sample appears to be at 3086 cm\({}^{-1}\) while the stretching vibration shifts to 3122 cm\({}^{-1}\) in pure and doped perovskite due to its interaction with Pb or Zn. However, it is now obvious that the stretching intensity increases when CH\({}_{3}\)NH\({}_{3}\)I is in the direct presence of Zn. This indicates that there is a stronger interaction between CH\({}_{3}\)NH\({}_{3}\)I and Zn. From this it can be stated that Zn\({}^{2+}\) increased the crystallinity of the hh0-plane of the unit cell in which the Pb is replaced by Zn. This contracts the distance between adjacent lattice planes (\(d\)-space), which could cause the formation of bigger grains. A more exact mechanism is described as follows: the precursors that were used to prepare the device were PbCl\({}_{2}\) and CH\({}_{3}\)NH\({}_{3}\)I. Zinc was introduced in the form of ZnCl\({}_{2}\). After annealing the CI' is released from the perovskite system by forming CH\({}_{3}\)NH\({}_{3}\)Cl gas and the resulting perovskite structure is CH\({}_{3}\)NH\({}_{3}\)Pb\({}_{1-x}\)Zn\({}_{x}\)I\({}_{3-y}\)Cl\({}_{y}\). Due to the replacement of a small quantity of Pb\({}^{2+}\) by Zn\({}^{2+}\)  
Figure 6: XRD spectrum of MA(Pb:Zn)I\({}_{3}\)-based thin films after incorporation of Zn\({}^{2+}\) in concentrations of 0, 5, 10 and 15% (**a**). A shift of the peak representing the 110 plane is clearly visible in (**b**). Figure reprinted with permission from _Mater. Today Energy_**2017**, 5, 205–213, Reference [15]. the crystallization process has been changed. As shown in Figure 7d, it was hypothesized that the stronger interaction of Zn\({}^{2+}\) with the anions and the strong coordination with the organic group makes the perovskite system to grow into bigger grains. This is in line with the observation that the over-doped perovskite systems result in grains that are so big that areas with pinholes arise in between and the general coverage is affected. Note that we indicate additives in square brackets after the composition, e.g., [Cl\({}^{-}\)]. To fully confirm that Zn is indeed incorporated in the lattice X-ray diffraction measurements were carried out at different concentrations of Zn\({}^{2+}\): upon increase of the Zn\({}^{2+}\) content the peak representing the 110 plane is slightly shifted to a higher degree, which indicates that the \(d\)-space of cells contract when they are doped with Zn (as shown in Figure 6b, reported in Reference [15]). The contraction of the \(d\)-space can be explained the aforementioned higher chemical bond with MA\({}^{+}\) and I and Zn's smaller ionic radius [11; 13; 15]. Zinc chloride and lead chloride precursors were used in Reference [13]. A last conformation for the researchers that Zn was evenly dispersed over the lattice was obtained through elemental mapping using energy dispersive X-ray spectroscopy (EDS). The obtained image is displayed in Figure 8 and reveals that Zn\({}^{2+}\) ions are uniformly distributed in the seeds of the perovskite film and not aggregated at the grain boundaries. Figure 7: Binding energies of Pb 4f (**a**) and I 3d (**b**) which diminish upon increase of Zn\({}^{2+}\) content. (**c**): Stretching vibration of the NH\({}_{3}\)\({}^{+}\) group in pristine CH\({}_{3}\)NH\({}_{3}\)I and in perovskite.