## 2 The Mechanisms of Additives
### Forming an Intermediate or Reacting
This may be due to the delayed conversion of the adduct of the perovskite caused by the lesser influence of the MeO additive, resulting in grain growth. However, higher amounts of MeO did not result in a further increase in grain size. The author attributes this phenomenon to the decomposition of MAI (Figure 18c). Since non-stoichiometric conditions result in a decrease in MAI content, grain growth of perovskites is limited and access PbI\({}_{2}\) precipitates in the grain boundaries. As shown in the XRD pattern of Figure 18d, all films exhibited similar diffraction patterns, which mean that MeO does not affect lattice parameters and crystal structure by substituting the position of the MA cation. The J-V curve of the PSC prepared by various doping amounts of MeO is illustrated in Figure 18e. When a small amount of MeO (MeO5 and MeO10) was added, all equipment parameters were enhanced, especially for MeO10 devices with 16.86% PCE (\(V_{OC}\) = 1.10 V, \(J_{SC}\) = 21.59 mA cm\({}^{-2}\) and \(FF\) = 0.71). The method of reducing the crystallization rate by forming an intermediate phase continues being explored. [PERSON] et al. [61] found that ammonium besylate (ABS) can form a mesophase with the precursor material to reduce the crystallization rate. As shown in Figure 19a, the steric hindrance of ABS and DMSO hindered the conversion of layered PbI\({}_{2}\) to tetragonal phase perovskites. Thereafter, the chlorobenzene accelerated the nucleation and crystallization of the perovskite by rapidly extracting the DMSO solvent, and then formed the MAI-PbI\({}_{2}\)-ABS phase. During the annealing process, the mesophase MAI-PbI\({}_{2}\)-ABS was gradually converted into crystalline MAPbI\({}_{3}\), and finally, large-grain and uniform film formed. The decrease in trap density was qualitatively demonstrated by PL (Figure 19b) and time-resolved photoluminescence (TRPL) (Figure 19c) maps, indicating that the surface defects of the film were small. As shown in Figure 19d, the perovskite solar cell has the structure of FTO/compact titanium dioxide (cTiO\({}_{2}\))/meso-titanium dioxide (mTiO\({}_{2}\))/MAPbI\({}_{3}\) (with or without ABS)/spiral--OMeTAD/Au. Figure 19e shows photovoltaic performance of various PSCs. In the ABS-based device, the crystallinity is good, the series resistance is small, and the FF is significantly increased from 75.68% to 78.62%. As a result, the overall PCE has increased dramatically from 17.29% to an impressive 19.64%. Figure 18: (**a**) Chemical structure of the MeO additive. (**b**) SEM images of perovskite films prepared using MeO, MeO10 and MeO20. (**c**) Schematic representation of the formation of a MAPbI\({}_{3}\) film using different amounts of MeO additive during the annealing process. (**d**) XRD patterns of films prepared using different amounts of MeO additives. (**e**) J-V curve of PSC using five different amounts of MeO additive. Reproduced with permission from Reference [60]. Copyright 2019 Wiley-VCH. The principle of chemical interaction between Lewis acid and Lewis base is also applied to the performance optimization of PSC. [PERSON] et al. [62] used the principle that melamine can interact with Lewis acids such as PbI\({}_{2}\), using melaninium dodide (M) as a solution for the perovskite precursor solution. Figure 20a shows the chemical structure of melamine based on 1,3,5-triazine. Since the triamine contains three primary amines, it can interact with a Lewis acid such as PbI\({}_{2}\). The bifunctionality of Lewis bases and hydrogen bond acceptors is expected to interact with Lewis acids and organic cations in organic-inorganic halide perovskites to minimize unreacted PbI\({}_{2}\). In Figure 20b, the steady state PL intensity increases after the addition of M, indicating that the additive M enhances the radiation recombination without changing the optical band gap. In Figure 20c, the PL is exponentially attenuated by two processes of a fast decay component associated with non-radiative recombination and a slow decay component associated with radiation recombination.