## 2 The Mechanisms of Additives
### Coordinate Covalent Bond
Due to the halide exchange and defect passivation caused by I\({}_{2}\) provided by the KI\({}_{3}\) complex, the surface morphology of the deposited perovskite film was changed significantly shown in the SEM of Figure 11a-c. As shown in Figure 11d, the (220) peak of the KI sample shifted towards lower angle compared to the control sample, indicating that iodine occupied the interstitial position resulting in lower angle shift. The KI\({}_{3}\) sample showed a larger shift than the KI sample, indicating an even larger lattice constant resultant in the film. Due to the reduced non-radiative recombination, samples with a mixture of KI\({}_{3}\) additives or KI and KI\({}_{3}\) additives exhibited slower decay and longer life than KI and control samples. (Figure 11e) The study also found that KI and KI\({}_{3}\) additives can significantly enhance device stability. This is due to the hydrolysis of DMF that occurs during the aging process, which consumes MA\({}^{+}\) during the formation of dimethyl formate, so the amount of MA\({}^{+}\) used after aging is reduced. However, when the KI\({}_{3}\) complex is present in the precursor, MA\({}^{+}\) does not decrease due to I3-bonding with the MA\({}^{+}\) cation, and it is possible to form a MAI\({}_{3}\) complex that stabilizes the precursor solution. Finally, the best performing device with KI:KI\({}_{3}\) = 7:3 additive achieved 21.3% PCE in reverse scan, 21.0% in forward scan, showing negligible current density--voltage (\(JV\)) The hysteresis of the characteristic is shown in Figure 11f. Figure 10: A top SEM image of (**a–d**) PbI\({}_{2}\) film and (**e–g**) perovskite film prepared as (**a,e**) without any additives and containing (**b,f**) KCl, (**c,g**) NaCl and (**d,h**) LiCl as an additive to the salt (scale bar: 2 \(\mu\)m). J–V characteristic curve: (i) photo and dark current. (j) Effect of scanning direction on the performance of perovskite solar cells prepared with KCl as an additive [43]. Reproduced with permission from Reference [43]. Copyright 2016 Royal Society of Chemistry. Recently, many studies have passed through additive engineering to passivate grain boundaries (GBs) to produce high quality perovskite film. [PERSON] et al. [46] prepared a high performance PSC incorporating a lead salt of pyridine-2-carboxylate (PbPyA\({}_{2}\)). Pyridine and carboxyl groups on PbPyA\({}_{2}\) not only control crystallization, but also passivate GBs to form high quality perovskite film with larger grains and fewer defects. As shown in Figure 12a, during the film formation process, the carboxyl group of PbPyA\({}_{2}\) has a strong interaction with Pb\({}^{2+}\) and MA\({}^{+}\) ions, thereby controlling the crystallization of the perovskite to suppress the formation of morphological defects. After crystallization, PbPyA\({}_{2}\) molecules are mainly distributed around GBs, not only by organic acid ions interacting with GBs, but also excess Pb\({}^{2+}\) ions near GBs can also cover volatile components, thereby reducing deep defects, inhibiting ion migration, and inhibiting the components to volatilize and inhibit external damage. In order to test the effect of PbPyA\({}_{2}\) on the crystal structure of perovskite film, the authors conducted X-ray diffraction(XRD) tests on films with different doping concentrations, and the results of those tests are shown in Figure 12b. At different concentrations, the diffraction pattern of the tetragonal crystal showed the same, while the doped film showed a peak of PbPyA\({}_{2}\) at 9.18, indicating that PbPyA\({}_{2}\) was not embedded in the perovskite crystal lattice and was only distributed around GB. A device having the structure shown in Figure 12c was then fabricated, and the J-V curve is indicated in Figure 12d. Clearly, after the introduction of PbPyA\({}_{2}\), the device exhibited lower hysteresis and increased \(V_{oc}\), indicating that the additive PbPyA\({}_{2}\) minimized trap density and inhibited ion migration.