## 2 The Mechanisms of Additives
### Forming an Intermediate or Reacting  
While the alkali halide salt additives offer monovalent cations and anions, bivalent or trivalent metal ions have not yet been widely explored as additives. [PERSON] et al. [49] used the CuBr\({}_{2}\) additive in the perovskite precursor solution to produce devices with a highly efficient. By replacing a part of PbI\({}_{2}\) to CuBr\({}_{2}\), they fabricated high-efficiency PSCs (ITO/PEDOT: PSS/CH\({}_{3}\)NH\({}_{3}\)I(PbI\({}_{2}\))\({}_{1\text{-}X}\)(CuBr\({}_{2}\))X/PCBM/LiF/Al), and there is no significant _J-V_ hysteresis. The PbI\({}_{2}\)-DMSO\({}_{2}\) intermediate cannot be flowed due to the melting and resolving process conduct at the same time during the heat treatment, while the CuBr\({}_{2}\)-DMSO\({}_{2}\) intermediate is flowable [50]. Thus, the CH\({}_{3}\)NH\({}_{3}\)I(PbI\({}_{2}\))\({}_{1\text{-}X}\)(CuBr\({}_{2}\))\({}_{X}\) perovskite can form larger grains more reproducibly than the MAPbI\({}_{3}\) film (Figure 15a-j). Furthermore, the weak bond between DMSO and MAI-PbI2 may accelerate the crystallization of MAPbI\({}_{3}\)[51]. Figure 15k shows the JV curves of CH\({}_{3}\)NH\({}_{3}\)I(PbI\({}_{2}\))\({}_{1\text{-}X}\)(CuBr\({}_{2}\))\({}_{X}\) perovskite solar cells with various CuBr\({}_{2}\) concentration (x = 0, 0.025, 0.050, 0.075 and 0.100). The control device had a _V\({}_{oc}\)_ of 0.945 V, _J\({}_{sc}\)_ of 17.96 mA/cm\({}^{2}\), _FF_ of 0.77, and PCE of 13.18%. For devices doped with CuBr\({}_{2}\), when x = 0.05, _V\({}_{oc}\)_ = 0.961 V, _J\({}_{sc}\)_ = 21.51 mA/cm\({}^{2}\), _FF_ = 0.82, and PCE = 17.09%. [PERSON] et al. reported the same result, confirming again that Br plays the role as well as iodine in the organometallic halide perovskite [52]. In addition, [PERSON] et al. [53] introduced monovalent cations Na\({}^{+}\) and Ag\({}^{+}\) with similar radii of Pb\({}^{2+}\) Studies have shown that those additives can minimize the boundaries between the adjacent crystallites and increase charge transport properties and optimize the device performance. [PERSON] et al. [54] reported on the important role of H\({}_{3}\)PO\({}_{2}\) in controlling the morphology of perovskite films, which has been largely ignored. H\({}_{3}\)PO\({}_{2}\) has been used as a stabilizer in HI, and therefore, introduces MAH\({}_{2}\)PO\({}_{2}\) impurities into the synthesized MAI (unpurified MAI). Pb(H\({}_{2}\)PO\({}_{2}\))\({}_{2}\) mesophase is formed and slows the crystallization processing. Both the MAH\({}_{2}\)PO\({}_{2}\) impurity and the Pb(H\({}_{2}\)PO\({}_{2}\))\({}_{2}\) mesophase all impede the rapid reaction of PbI\({}_{2}\) and MAI, resulting in a highly uniform and smooth perovskite film with larger grain size. The authors used non-purified MAI and purified MAI precursor solutions to fabricate the perovskite film by a two-step processing. The perovskite film is formed on  
Figure 14: SEM image (**a**) and XRD pattern (**b**) of a perovskite film. The symbol \({}^{*}\) indicates a peak associated with PbI\({}_{2}\). (**c**) J-V curve. The illustration shows the device stack: ITO/ poly(triarylamine) (PTAA)/Perovskite/C\({}_{60}\)/ bathocuproine(BCP)/Ag. (**d**) Schematic representation of passivation defects in perovskites from PEAI’s PEA + and Pb (SCN)\({}_{2}\) SCN. Reproduced with permission from Reference [48]. Copyright 2019 Cell Press. the ITO/PEDOT:PSS substrate, non-purified MAI with a mirror-like surface, as shown in Figure 16a. The grain size of perovskite film made with purified MAI is not uniform and has many white spots, as shown in Figure 16b. Therefore, MAH\({}_{2}\)PO\({}_{2}\) will lead to differences morphology of the film. It is worth noting that the conversion from Pb(H\({}_{2}\)PO\({}_{2}\))\({}_{2}\) to perovskite by thermal annealing is much slower than the direct reaction of PbI\({}_{2}\) and MAI. Therefore, the mesophases of Pb(H\({}_{2}\)PO\({}_{2}\))\({}_{2}\) and MAH\({}_{2}\)PO\({}_{2}\) would coexist. The similar phenomenon has also been explored in existence of HPbI\({}_{3}\)[55].