## 1 Introduction: Recombination Processes in Perovskites  
The impressive photovoltaic performance of perovskites has made them one of the most promising high-efficiency photovoltaic materials. Perovskite-based solar cells have achieved power conversion efficiencies of 25.2% [1]. The high performance of these materials can be attributed to the remarkable optoelectronic properties of perovskites: high absorption coefficient, long carrier diffusion lengths and low Urbach energy [2]. Moreover, perovskites offer simple band gap tunability by changing composition and low-cost production. Despite the rapid progress of perovskite solar cells, they are still far from their full potential [2]. The high absorption coefficient combined with device optimizations (at the electron and hole conducting layers) has led to increased light management and carrier collection and, therefore, minimized current loss. The voltage loss is also relatively small, but still below the theoretical limit. Similar to other thin-film semiconductors (GaAs, CIGS, CdTe), perovskite materials contain intrinsic defects, including point defects within the perovskite crystals and impurities at grain boundaries and surfaces [3, 4]. As result of these defects, trap states for both electrons and holes can induce non-radiative recombination. This pathway of recombination for the photogenerated carriers is undesirable and lowers the voltage and, consequently, decreases the power conversion. To progress further toward the theoretical limit of power conversion efficiency, a better understanding of the defect states and knowledge of how to manipulate them is essential. Here, we consider the role of defects in perovskites, their chemical nature, and the defect chemistry. Next, compositional engineering as passivation method for defects within perovskite crystals is described, and reports on different mechanisms that are proposed to reduce the defect density and boost the voltage and PCE (power conversion efficiency) are analyzed. This review covers work published up to September 2019. A perovskite solar cell is a type of solar cell that includes a perovskite material as light-absorbing layer. Perovskites possess the crystal structure ABX\({}_{3}\), where A and B are cations jointly bound to X, an anion (Figure 1). One of the most investigated perovskites for solar applications is the methylammonium (MA) lead halide perovskite with the chemical structure CH3 NH2 PbXnY3-n (where X and Y are usually I, Br or Cl anions) [5]. Perovskite materials are considered to be direct band gap semiconductors, and their band gap is primarily determined by the chemical composition and structure. In a solar cell, the photovoltaic properties (open-circuit voltage (\(V_{OC}\)), short-circuit current (\(J_{SC}\)), \(FF\) is fill factor) depend on the band gap and the charge transport. The carriers that contribute to charge transport are electrons in the conduction band (CB) and holes in the valence band (VB). The valence band is separated from the conduction band by forbidden energy levels [6]. The difference between the conduction band minimum (CBM) and valence band maximum (VBM) is the band gap energy. The Fermi energy (Fermi level) is the electrochemical potential of the electrons, representing the average energy of electrons in the material. When incident light excites electrons from the VB into the CB this causes the splitting of the so-called quasi-Fermi levels of electrons and holes in a working solar cell. These levels describe the population of the electrons (in the CB) and holes (in the VB) separately, when their populations are displaced from equilibrium. The output voltage of the cell is determined by the difference of the quasi hole and electron Fermi levels [6]. The open-circuit voltage (\(V_{OC}\)) is the maximum voltage generated by a solar cell, which occurs when no current flows through the external circuit (the forward bias voltage).