## 4 Discussion and Conclusions  
With this work, the current understanding of intrinsic defects and their passivation in lead halide perovskites has been reviewed. Intrinsic defects in the perovskite bulk are at the origin of reduced VOC by creating non-radiative recombination centers. Recent research has shown that the most important defects are iodine interstitials, which create effective hole and electron traps. The redox chemistry of the electron traps explains the apparent defect tolerance of lead halide semiconductors, deactivating the traps over longer times. However, other research highlights the formation of vacancies, which might create shallow traps but are reported to be mobile, affecting the stability and photovoltaic performances. We show that the addition of computational methods is essential to predict the stability and density of point defects. Combined computational and experimental techniques create strong evidence of the physical and chemical properties of perovskite point defects [26]. However, more complications in the description of defects rise from the effects of external stimuli, resulting in a trap density that is never fixed and always dynamic. The external stimuli (like oxygen and water) that may create and destroy electronic traps are outside the scope of this review but are important to take into account in future research. In Section 3, the current understanding of how compositional engineering can control the density of defects is analyzed. Literature describes that compositional engineering of lead halide perovskite can implicate the two following factors: (1) crystallization; (2) lattice properties. The interplay of these dopant effects influences the optoelectronic properties so that for the incorporation of dopants, such as Zn, Sn, Cd, Cs, and halides, carrier lifetimes and photoluminescence are improved, hinting reduced non-radiative recombination because of reduced defect density. First, the crystallization mechanisms can be altered upon the addition of dopants in the precursor solution creating better-orientated domains or larger crystal sizes. Proposed mechanisms included reduction of strain, selective crystal growth, and stronger chemical coordination. Interestingly, a recent report that investigated the optoelectronic properties of MAPbI3 with different grain sizes and orientations did not report any correlation between enlarged grains and improved optoelectronic properties [31]. Problems with the interpretation of SEM images or other simultaneous passivating effects can lead to wrongly correlating improved crystallinity (orientation and size) with improved PL and carrier dynamics. [PERSON] et al. [32] showed that the effect of Zn can also be characterized as more homogeneous element incorporation for a tin-lead based perovskite. Next, dopants that are effectively incorporated at lattice positions give rise to other effects: (a) lattice strain relaxation; (b) chemical bonding enhancement; (c) band-gap tuning. The lattice strain relaxation was well explained by [PERSON] et al. [33] showing that formation energies for defects, such as vacancies, increase when the intrinsic strain in the lattice is reduced by incorporation of smaller elements at the B and X site. Chemical bond enhancement immobilizes ions reducing vacancies and \(J-V\) hysteresis. The distinction between the two mechanisms is still missing in literature. Furthermore, band gap tuning with dopants can also occur as a consequence of the changes in lattice structure or as a result of electronic contribution to orbitals that govern the band gap. At the A-site, small dopants, such  
Figure 14: Band gap of MAPbI3. The orbital contributions of Pb and I. Reproduced with permission from _Chem. Mater._**2016**, _28_, 4259–4266. Reference [44]. as Cs, can have competing effects; lattice contractions can increase the orbital overlap and decrease the band gap, whereas octahedral tilting increases the band gap by destabilizing the overlap.