## 3 Defect Passivation through Compositional Engineering of Perovskites
They ascribed the formation of more stable perovskite to the increased electron-donating ability at the B site, creating stronger B-site-X-site interactions. Chemical bond enhancement can also be achieved through doping with more electronegative halides. [PERSON] et al. fabricated a triple-cation perovskite (Cs0.05 FA0.54 MA0.41)Pb(10.98 Br0.02)3 and used several halides for doping, NaX (X = I, Br, Cl or F), in a two-step solution process [36]. All alkali halides improved the quality of the perovskite, with the fluoride-containing material (0.1%) giving the best performance. Time-resolved PL showed increased carrier lifetimes implying that non-radiative recombination is suppressed. The increased electronegativity of the fluoride anion was argued to result in a stronger ionic bond with Pb and in formation of a hydrogen bond with FA ions, leading to stabilizing of local structure and inhibiting mobilization of ions (I and MA/FA). Hydrogen bonding between the NH of the organic ion and the F ion was confirmed by \({}^{1}\)H-nuclear magnetic resonance (NMR). The F ions did mainly occupy the surface and grain boundaries. In contrast to the other ions, devices with F- ions exhibit negligible _J-V_ hysteresis, indicating that the chemical bond enhancement with F immobilizes the counter ions. The stabilization reported by [PERSON] et al. resulting from lattice shrinking can be explained by the strain in the pristine perovskite lattice. Lattice strain in MAPbI3 was first observed by [PERSON] et al. in high-temperature annealed thin films MAPbI3 perovskite [37]. Using XRD and temperature-dependent conductivity measurements, they correlated ion migration to increased lattice strain. [PERSON] et al. speculated that lattice strain is at the origin of vacancy formation [33]. DFT calculations showed that the formation energy of lead iodine vacancies was increased upon replacing FA and I with the smaller Cs/MA and Br. They argued that if the size of the A cation does not match the lead halide octahedral cage, strain will be present in the material. When local strain acts on the lattice, strain can be released through the formation of point defects or by incorporation of small ions. Especially in FAPbI3, where size mismatch is largest, this strain is likely to be reduced by point defect formation since the calculated formation energies lie close to 0 eV. The strain relaxation through the formation of vacancies has also been observed in oxide perovskites [38]. [PERSON] et al. successfully incorporated Cl- into the lattice of triple-cation perovskite films (CsMAFAPbI2\({}_{\text{5-55}}\)Br\({}_{\text{0.45}}\)) [33]. They observed increased long term stability and reduced superoxide generation, which indicates that Cl reduces the density of defects. Using DFT calculations, they found \(\sim\)0.3 eV increase in formation energies for iodine vacancies. Computations also showed that the Pb-X bond length decreased, as did Pb-X-Pb distortion angles. Therefore, they proposed lattice strain relaxation as reason for reduced iodine vacancies. The downside of the substitution with small halogen anions is the widening of the band gap [39]. Although incorporation might reduce non-radiative recombination, it will lower the device efficiency because of a sub-optimal band gap. Furthermore, the incorporation of Cl ions inside other perovskite lattices, such as the MAPbI3, has been unsuccessful as the energetically more favorable location is outside the lattice [40]. In order to relax strain further at the B-site, [PERSON] et al. incorporated Cd because it is isovalent to Pb but has a smaller ionic radius. DFT calculation showed that Cd incorporation relaxes lattice strain and suppresses the formation of I vacancies by \(\geq\)0.5 eV. The Cd incorporation also increased the PL intensity of films tenfold. Similar effects have been observed [41] upon doping with Zn(II) in combination with Cl\({}^{-}\), the latter originating from the alkyl-ammonium additive (see Figure 11). Lattice distortions can also implicate the perovskite band gap (Figure 12).