## 3 Defect Passivation through Compositional Engineering of Perovskites  
Compositional engineering [27; 28; 29; 30; 31; 32; 33] has become an attractive approach to optimize perovskite structure because of the advantage of directly tailoring other intrinsic material properties, which enables higher-quality perovskite with enhanced photovoltaic performances. Reference [34] contains a condensed historic representation of compositional changes related to record perovskite devices (see Table 2 of reference [34]). We can distinguish between two types of additives: Inorganic (metal cations and inorganic acids) and organic additives (small molecules, polymers, and fullerenes). Most additives can control the crystallization of perovskite in some way and, therefore, enhance stability [28]. However, organic molecules generally only passivate defect states at the surface and not do not alter intrinsic material properties, while the bulk trap density might be reduced by the incorporation of inorganic molecules into the lattice. In literature a variety of dopant- effects that enhance perovskite performance are reported [28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41], which can be divided into influences on the crystallization and influence on the intrinsic lattice properties. Most effects described in literature are still not well-explained making rational compositional tailoring challenging. Here, current literature on Zn(II) and several other inorganic dopants are reviewed with the aim to obtain a more mechanistic picture on how dopants affect different properties and control the defect density. A considerable amount of literature has been published on the use of inorganic dopants that improve photovoltaic performance of perovskite materials and many attribute this to the improved crystallization of doped perovskite [29; 33; 34; 35; 41]. While the enhanced crystallization mechanisms do not seem to be fully understood, we review here several studies on dopants that effectively improve crystallization, leading to larger crystal domains and reduced defect density, and identify mechanisms behind this dopant effect. [PERSON] et al. discovered that Al\({}^{3+}\), at low concentrations (\(<\)0.3%), facilitates crystallization of MAPbI3, and specifically enhanced the growth of the (\(hh0\)) crystal plane [29]. This was ascribed to reduced micro-strain in the lattice during the crystallization process, resulting in better oriented crystalline domains and consequently a reduced density of defects. Crystallography and mass spectrometry revealed that Al\({}^{3+}\) was not significantly incorporated in the lattice, which is expected from the ionic radius (53 pm and 199 pm for Al and Pb, respectively). Crystal defects that potentially occur in the form of terraces are reduced via a selective growth of these terraces induced by Al\({}^{3+}\). Unlike Al\({}^{3+}\), Zn\({}^{2+}\)-ions have been proven to be incorporated at the lattice B-site, replacing lead ions. [PERSON] et al. argued that this incorporation leads to better crystallization of MAPbI\({}_{3}\) perovskite [30]. Using X-ray diffraction (XRD) they showed successful doping of the perovskite and with attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), they demonstrated that the interaction of the Zn\({}^{2+}\) and MA ions are stronger in comparison to the Pb-MA interaction. Upon 0.1% Zn\({}^{2+}\) doping of MAPbI\({}_{3}\), photovoltaic performances were increased the most. This was attributed to higher chemical interaction and coordination between Zn\({}^{2+}\) and anions, and the organic group, which makes the doped system tend to grow into larger grains. They also observed an _over-doping_ effect for high Zn\({}^{2+}\) concentration, which was believed to result in the accumulation of bigger grain seeds on the previous small grain seeds affecting the general coverage and therefore reducing the photovoltaic performances.