## 2 The Mechanisms of Additives
### Coordinate Covalent Bond
The result was a \(V_{oc}\) of 1.12 V, a \(J_{sc}\) of 22.99 mA cm\({}^{-2}\), an \(FF\) of 77.51% and a PCE of 19.96%. Figure 11: (**a**) Control, (**b**) KI and (**c**) KI\({}_{3}\) SEM images of the addition of a perovskite film. (**d**) Normalized XRD peaks of the plane of the same perovskite film (220). (**e**) Time-resolved PL in the absence and absence of KI additives, KI\({}_{3}\) additives and mixtures of KI and KI\({}_{3}\) additives. (**f**) J-V curve of the best performing device for reverse scan and forward scan. Reproduced with permission from Reference [45]. Copyright 2019 Elsevier B.V. Various additives engineering reports are based on Pb-based perovskites. However, in recent years, Sn-based perovskites, which are non-toxic and have a suitable band gap, have also become promising candidates for the preparation of perovskite solar cells. However, the preparation of highly stable and highly efficient tin-based PSCs is challenging, because Sn\({}^{2+}\) in perovskites is readily oxidized to Sn\({}^{4+}\) upon exposure to air. [PERSON] et al. [47] reported the production of air-stable FASnI\({}_{3}\) solar cells by adding hydroxy benzene sulfonic acid or a salt as an antioxidant to the perovskite precursor solution and excess SnCl\({}_{2}\). Figure 13a shows the molecular structure of three representative additives used in their work, including phenomenally acid (PSA), 2-aminophenol-4-sulfonic acid (APSA) and hydroquinone sulfonate potassium salt (KHQSA). The interaction between the sulfonate group and the Sn\({}^{2+}\) ion causes the perovskite particles and the SnCl\({}_{2}\) additive composite layer to be encapsulated in situ, thereby greatly enhancing the oxidative stability of the perovskite film. Compared with the former two, KHQSA containing two hydroxyl (-OH) groups has higher antioxidant activity. Figure 13b appears in a scanning transmission electron microscope (STEM) image showing the distribution of KHQSA on the surface of the perovskite particles. Figure 13c shows a XRD pattern of a FASnI\({}_{3}\) film with different additives. The additive did not cause a significant change in the pattern, indicating that the additive molecules were not incorporated into the perovskite crystal lattice. As shown in Figure 13d, PCE was observed to have a similar dose dependency for PSA, APSA and KHQSA, which can be attributed to a certain amount of additive to prevent phase separation of SnCl\({}_{2}\) and oxidation of perovskite. Excessive additives can significantly reduce photocurrent, because the conductivity of the complex located at the grain boundary and surface of the perovskite is poor. After optimization, the champion PCEs of PSA, APSA and KHQSA was 5.58%, 5.16% and 6.76%, respectively. Figure 13e shows the J-V curves for the best performing raw and KHQSA modified PSCs. The hysteresis of KHQSA-PSC is negligible compared to the apparent JV hysteresis shown in the original PSC, which may be linked to the lower trap density of the perovskite film. Figure 12: (**a**) Schematic representation of the PbPyA2 molecule at grain boundaries (GBs) after film formation and strongly interacting with the perovskite component to passivate GBs. (**b**) XRD patterns of various dopant concentrations and b) SEM images. (**c**) A cross-sectional view of the PSC device. (**d**) J-V curve of the device of the undoped and doped film. (**e**) Repeatability analysis of the device power conversion efficiency (PCE). Reproduced with permission from Reference [46]. Copyright 2019 Wiley-VCH. The synergy between different additives has also been explored. [PERSON] et al. [48] reported that the perovskite solar cell of (FA\({}_{0.65}\)MA\({}_{0.2}\)Cs\({}_{0.15}\))Pb(I\({}_{0.8}\)Br\({}_{0.2}\))\({}_{3}\) was highly efficient by the two complementary additives Phenethylammonium Iodide(PEAI) and Pb (SCN)\({}_{2}\). The coupling of PEA\({}^{+}\) and Thiocyanate ion (SCN\({}^{-}\)) provides a synergistic effect, reduces the formation of excess PbI\({}_{2}\), enhances crystallinity and enhances the quality of the perovskite film finally.