## 2 The Mechanisms of Additives
### Coordinate Covalent Bond
(**c**,**d**) are the chemical structures of PFN-P2 and PFN-P1, respectively. (**e**) PbI\({}_{2}\) membrane without PFN-P1, (**f**) PbI\({}_{2}\) membrane without PFN-P1, (**g**) perovskite membrane without PFN-P1 and (**h**) perovskite without a PFN-P1 Plane SEM image of the film. (**i**) J–V curve. Reproduced with permission from Reference [36]. Copyright 2016 American Chemical Society. is replaced by Pb\({}^{2+}\) and I- completely, and the MAF is located on the surface of the perovskite crystal. Therefore, crystal growth is slowed down with the presence of MAF, while slower crystal growth results in the larger grain size. Figure 9b are SEM images of a perovskite structure with MAF and without MAF having average grain sizes of 325 and 170 nm, respectively. Large grains can pass through the entire thickness of the perovskite structure to form a local single crystal device. Charge transfer through a single crystal is much faster than that in polycrystalline semiconductor. [41; 42] Figure 9d shows a _J-V_ curve with and without MAF. MAF-doped PSCs demonstrated an excellent _Jsc_ = 23 mA/cm\({}^{2}\), which corresponds well with the above hypothesis. Metal ions play important roles in improving the quality of perovskite films as precursor solution additives. [PERSON] et al. incorporated a series of alkali metal halide (NaCl, KCl, LiCl) into the PbI\({}_{2}\) precursor solution to adjust the morphology of the PbI\({}_{2}\) film [43]. These halogenated additives can chelate with Pb\({}^{2+}\) during film formation, facilitating the growth of the crystals. A significant improvement in perovskite film crystallinity has been obtained by adopting the two-step processing to adjust the morphology of PbI\({}_{2}\) film. The large crystal size be formed with the presence of the alkali metal halide additives results in greatly improved PCE and device stability of the planar heterojunction PSCs. Figure 10a-d show the SEM images of spin-coated PbI\({}_{2}\) films with and without additives; Figure 10e-h show the perovskite films with and without additives. In the two-step processing, the quality of the perovskite film depends on the quality of the precursor PbI\({}_{2}\) film greatly. The optimized film formation process can not only improve device efficiency, but also significantly reduce photocurrent hysteresis (Figure 10j). As shown in Figure 10i, the highest PCE (15.08%) was achieved with a device doped with 0.75% KCl, _Jsc_ = 19.42 mA/cm\({}^{2}\), _Vox_ = 1.04 V, _FF_ = 0.74. Devices fabricated using 1% NaCl as an additive also exhibited improved device performance: PCE = 12.77%. The additive of LiCl reduces the performance of the device at all concentrations. This is due to the high sensitivity for oxygen and moisture of Li\({}^{+}\). In summary, devices fabricated using KCl as an additive provide the best performance. These alkali metal halide additives act as electrically active impurities can passivate the grain boundary and interface states, thereby creating and disassociating the effective charge in the perovskite film [44]. Figure 9: (**a**) Molecular structure of methyl ammonium iodide and formic acid. (**b**) Schematic representation of the growth of perovskite crystals controlled by formate anions. (**c**) A top view SEM image of the perovskite film. (**d**) J-V curve of the device. Reproduced with permission from Reference [40]. Copyright 2016 Wiley-VCH. Potassium iodide (KI) and potassium triiodide complex (KI\({}_{3}\)) are useful inorganic additives for the perovskite solution. [PERSON] et al. prepared a KI\({}_{3}\) additive solution by dissolving equimolar amounts of KI and I\({}_{2}\) in dimethyl sulfoxide (DMSO) [45]. They fabricated the PSCs using the KI\({}_{3}\) additive with the method of gas-quenching.