## 2 The Mechanisms of Additives
### Coordinate Covalent Bond  
The coordinate covalent bond, also known as a dative bond or coordinate bond, is pervasive. In all metal aquo-complexes, the bonding between water and the metal cation is described as a coordinate covalent bond. Metal-ligand interactions in most organometallic compounds and most coordination compounds are described similarly. [PERSON] et al. [36] used amine-based polymer poly[9,9-bis (30-(N,_N_-dimethylaminol-propyl)-2,7 -fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN-P1) as a surfactant to product PSCs take excellent performance and stability. The authors used a cationic derivative (PFN-P2) on the cathode side, while the neutral form PFN-P1 served as an additive for the perovskite layer (Figure 8b-d). The ability of cationic or halogenated PFN derivatives to improve the electron selectivity at the cathode interface has been demonstrated [37; 38] The PFN-P1 is spin coated onto PEDOT:PSS firstly, and PbI\({}_{2}\) solution is spin-coated on top, subsequently. Since PFN-P1 is soluble in \(N\),_N_-Dimethylformamide(DMF), it can dissolve in the PbI\({}_{2}\) solution immediately. The coordinate covalent bond is formed between the nitrogen lone electron pair in PFN-P1 and Pb\({}^{2+}\), and makes PFN-P1 act as a surfactant. From Figure 8e-h, it can be seen that the grain size of perovskite on PEDOT:PSS varies from 100 to 480 nm, while the  
Figure 7: (**a**) Chemical structure of ethylamine chloride. (**b**) -OH and -NH\({}_{3}\) in EA-HCl interact with the halogen anion in MAPbI\({}_{3-\times}\). (**c**) J-V curves for different levels of EA-HCl perovskite solar cells. SEM image of perovskite film: (**d**) without 1 wt.% EA-HCl and (**e**) with 1 wt.% EA-HCl. (**f**) X-ray diffraction (XRD)of perovskite films having different contents of EA-HCl additives [35]. Reproduced with permission from Reference [35]. Copyright 2019 Elsevier B.V.  
PFN-P1-doped film has an almost uniform grain size over the entire surface. The PFN-P1 layer flattens the surface and suppresses irregular crystallization of the PbI\({}_{2}\) layer, thereby promoting more uniform crystallization. Figure 8i depicts the J-V characteristics of both devices. The device without PFN-P1 had a PCE of 12.0%, while the device with PFN-P1 had a superior PCE of 13.2%. The efficiency improvement of PFN-P1 based devices is mainly due to the higher \(V_{\alpha}\) and shunt resistance (\(R_{sl}\)), while \(J_{sc}\) and the \(FF\) of devices remained mostly unchanged. The increase in \(V_{\alpha}\) is attributable to the reduction of charge carrier recombination at the interface, and the high \(R_{sl}\) means that charge recombination is suppressed effectively. Moreover, owing to the passivation effect of the separation and accumulation of PFN-P1 molecules at the grain boundary [39], the stability of the device also greatly improved. Methyl formate (MAF) had been employed to produce higher quality crystalline perovskite films, resulting from HCOO-interacts with Pb\({}^{2+}\) and then slows the crystal growth of perovskite. [PERSON] et al. proposed an ionic liquid (IL)-driven crystallization mechanism as a new strategy for the preparation of high-efficiency perovskite solar cells. If a small amount of ionic liquid (IL) is added to the precursor solution, it remains in the perovskite structure after the annealing process [40]. Figure 9a shows the chemical structure of MAF and MAI. They have the same cation (methylammonium) and a different counter anion formate (HCOO-) or iodide (I-). The mechanism is depicted in Figure 9b: (1) HCOO- and Pb\({}^{2+}\) are coordinate in solution and in the early stages of crystal growth; (2) when annealing to 100 \({}^{\circ}\)C, HCOO- and Pb\({}^{2+}\) is replaced by Pb\({}^{2+}\) and I- gradually; (3) the HCOO- and Pb\({}^{2+}\) coordination  
Figure 8: (**a**) A perovskite doped with poly [9,9-bis (30-(\(N\),\(N\)-dimethylaminol-propyl)]-2,7-fluorene)-alt -2,7-(9,9-dioctyllfluorene)] (PFN-P1) formed by a spin coating method. (**b**) Schematic diagram of the device structure.