## 1 Introduction: Perovskite Solar Cells on the Rise  
The search for novel photovoltaic materials has intensified over the past few decades as the demand for cleaner energy sources grows. A huge surge in research focusing on third generation photovoltaic devices has brought forth dramatic improvements in device performances as power conversion efficiencies start approaching the Shockley-Queisser limit [1]. In particular, perovskites have received a tremendous amount of attention, since these materials provide a cheap route to highly efficient photovoltaic materials. The rapid increase in performance of lead-based metal-halide perovskite solar cells over the past years has resulted in solar cells with efficiencies of 24.2% [2]. In conjunction with reports that perovskite devices are stable over longer periods of time [3,4] this demonstrates that it is not unimaginable that perovskites will be suitable for commercialization in the foreseeable future. In this quest it is imperative to compositionally tailor the devices by engineering the ionic composition of the ABX\({}_{3}\) lattice [5]. The bandgap of a device can be tuned by using mixed halides [6]. Similarly, the thermal stability and photo-stability of lead-based perovskites can be improved by (partially) replacing the conventional organic cation, methylammonium (MA), by formamidinium (FA) [5]. Despite the extensive research efforts focusing on these perovskites, there have been surprisingly few reports discussing the effects of the partial substitution of the Pb\({}^{2+}\) ions at the B-site of the perovskite crystal lattice. It remains elusive how partial substitution of the lead ions in the B-site with a particular alternative metal species influences the material properties and photovoltaic performance. It has been demonstrated that the introduction of certain hetero-valentmetals in perovskite films influences both the optoelectronic and crystallographic properties of the resulting film. The introduction of In\({}^{3+}\) increases crystallinity and morphology of the film [7]. Through the homo-valent substitution of Pb\({}^{2+}\) with Sn\({}^{2+}\) the first example successfully replacing lead on the B-site of the crystal lattice (with tin) was observed, resulting in a narrower bandgap [8]. However, a big drawback is the ability of Sn\({}^{2+}\) to oxidize to Sn\({}^{4+}\) in situ making Sn less suitable as doping material. In 2017, a whole new set of homo-valent metals were explored as dopants in perovskite lattices. It was soon discovered that lead could partially be replaced by Co\({}^{2+}\), Zn\({}^{2+}\), Mg\({}^{2+}\), Mn\({}^{2+}\), Ni\({}^{2+}\) due to their similar atomic radii and divalent nature [9], also in perovskite nanocrystals [10]. Not long after this, several extensive studies focused on doping with Zn\({}^{2+}\) commenced [11; 12; 13; 14; 15; 16]. Zinc is particularly interesting due to its non-toxic nature, its inability to oxidize or reduce easily, and its abundance on Earth. The result of doping with several percentages of Zn(II) were smoother and denser films with fewer pinholes. Zn\({}^{2+}\) features a stronger interaction towards the MA\({}^{+}\) cation and I\({}^{-}\) anion. The smaller ionic radius of Zn(II) compared to Pb(II) causes a unit cell contraction of the hh0-plane and this results in enhanced nucleation and crystallization during fabrication, resulting in bigger grains and fewer pinholes. This is beneficial for efficiency because non-radiative losses are generally attributed to Grain Boundaries (GBs), so by reducing the number of GBs (due to the enlargement of perovskite domains) losses are also reduced. However, this also induces increased stabilization of the resulting film towards degradation, which commences at grain boundaries and at pinholes. In this review, we compile recent results on the use of zinc-iodide and zinc-chloride for doping or alloying of perovskite materials for solar cells.