## 3 Defect Passivation through Compositional Engineering of Perovskites
[PERSON] et al. successfully incorporated Cs in Pb and Sn-based (FAPb/SnI\({}_{3}\)) perovskite and observed that the for tin-based films introduction of Cs at the A-site decreased the band gap; whereas in lead perovskite films, introduction of Cs was observed to increase the band gap [42]. They used grazing incidence X-ray diffraction (GIXD) measurements and revealed the incorporation of Cs has different effects in these two materials (Figure 13). The tin iodide perovskite maintains its cubic crystal structure if FA is partly substituted by Cs and the only observed change is the isotropic contraction of the lattice. However, \"in the lead iodide perovskite, additional X-ray diffraction peaks are observed, indicating that the structure experiences tilting of the BX\({}_{6}\) octahedra in addition to lattice contraction. Electronic structure calculations have shown that the VBM of perovskite material is an antibonding hybrid state of the metal s and halide p orbitals, and thus is sensitive to B-X orbital overlap (more than the CBM)\" [42]. This explains the different band gap response observed in the tin iodide vs. lead iodide perovskites. In the former, lattice contraction increases orbital overlap, raising the antibonding VBM and the optical band gap is decreased. \"In the latter, octahedral tilting decreases orbital overlap, moving the VBM to deeper energies and causing the band gap to increase. Band gap energies were directly measured with ultraviolet and X-ray photoelectron spectroscopy (XPS) and the results support this model\" [43]. Thus, for Cs doping at the A-site competing effects of octahedral tilting and lattice contraction were observed. When the X and B sites are doped, the electronic contributions of the dopant give rise to more complicated band gap interference. Figure 11: Characterization of MA/FA film with and without Cl\({}^{-}\) and Zn(II) dopants. (**a**) Relative yield of emission by time-integrated photoluminescence (PL) spectra. (**b**) Time-dependent PL traces with different dopants, stretched exponential fitting with function. Reproduced with permission from _ACS Appl. Mater. Interfaces_**2019**, _11_, 17555–17562. Reference [41]. Figure 14 depicts the orbital contributions of the PbI framework to the band gap of MAPbI3. \"The top of the valence band consists of an Pb 6 s--I 5p * anti-bonding hybrid state of the metal s and halide p orbitals. The conduction band minimum is a hybrid of metal 6p and halide 5p orbitals with less anti-bonding and more non-bonding character\" [44]. For dopants at the X-site in particular, the tuning of halogens has been frequently reported. Increasing the electron binding energies, for example from I to Br (from 5p to 4p, respectively), the band gap can be tuned from 1.5-2.3 eV by varying x in MAPb(Br\({}_{x}\) I\({}_{(1-x)}\))\({}_{3}\)[45]. However, the band gap does not change linearly indicating increased defect densities upon Br introduction. B-site dopants are reported to infer with the conduction band. The replacement of lead with, for example, tin in MAPbI\({}_{3}\) widens the band gap [46]. However, tin has also led to instability because of oxidation to the IV oxidation state. A recent literature review of Zn doped MAPbI\({}_{3}\) perovskite shows that generally the 3% doping leads to the most enhanced emission intensity and lifetimes, creating PCEs between 18%-20% [34]. The band gap for low concentrations like this remains virtually unchanged [47], with valence bands shifting from \(-\)5.45 to \(-\)5.50 eV upon going from 0 to 10% Zn(II) doping. Conduction bands shifting from \(-\)3.85 to \(-\)3.91 eV in the same doping range. Figure 12: Perovskite lattice and energy levels of undistorted cubic phase (center), with lattice contraction (left) and with octahedral tilting (right). Reproduced with permission from _J. Am. Chem. Soc._**2017**, _139_, 11117–11124. Reference [42]. Figure 13: Optical band gap as function of cesium content for perovskites (FA\({}_{1-x}\)Cs\({}_{x}\))MI\({}_{3}\), with M = Pb or M = Sn. Reproduced with permission from _J. Am. Chem.