## 2 The Mechanisms of Additives
### Hydrogen Bonds
As a result, the stability of the PVP-doped device further increases. Figure 1: (**a**) Device architecture diagram. (**b**) Scanning electron microscopy (SEM) cross-section image of the pristine device. (**c**) SEM cross-section image of the treatment with 1 wt.% poly (ethylene glycol) (PEG), and close-up of the image of the red box in (**d**,**c**). (**e**) Current density-voltage (J-V) curve for devices of varying amounts of PEG additive. Reproduced with permission from Reference [23]. Copyright 2015 American Chemical Society. In addition, the polymer can be used as an additive to stabilize the perovskite crystals to prevent moisture degradation. Recently, [PERSON] et al. [30] selected non-electroactive polymer polyethylene oxide (PEO) as an additive to make flexible devices and optimized the concentration of PEO. PEO additive acts as a crystallization inhibitor by physically interfering with crystallization and improving processability, thereby producing a large-area defect-free perovskite film. The oxygen atom in the PEO molecule can form a hydrogen bond with the methylammonium cation (MA\({}^{+}\)) in the perovskite component, hindering the formation of hydrate under hydration conditions. As shown in Figure 4a, in a dry environment, equipment with PEO additives exhibited similar performance to equipment without the added polymer. However, it is apparent from Figure 4b that the difference between the equipment with and without the PEO additive becomes superficial on wet days. To investigate the origin of these differences, we analyzed these films using SEM. As expected, there was no significant difference in the grain size of the film deposited at low humidity, and the difference in grain size of high humidity was more pronounced. This is because PEO is capable of forming stable coordination bonds with positive ions and generating a stable five-membered ring structure. The hydrophobic ethylene group on the surface of the film also prevents the penetration of water molecules, again reducing the negative effects of humidity. Figure 2: Top view SEM image of perovskite film (**a**) without PEG and (**b**) with PEG. (scale bar, 1 mm). (**c**) Cross-sectional SEM images of perovskite solar cells without PEG and (**d**) PEG scaffolds. (scale bar, 3 mm). (**e**) Schematic diagram of the formation of a hydrogen bond between a PEG molecule and MAPbI\({}_{3}\). (**f**) J-V curves for devices with different PEG concentrations. (**g**) Schematic: (1) water uptake on perovskite; (2) perovskite hydrolyzed by water to PbI\({}_{2}\) and MAIH\({}_{2}\)O; (3) after evaporation of the water, the PEG-inhibited methylammonium iodide (MAI) reacts with the nearby PbI\({}_{2}\) to form perovskite. Reproduced with permission from Reference [24]. Copyright 2016 Nature Publishing Group. It is interesting that bifunctional molecules have also been used as templates to direct nucleation or crystal growth. [PERSON] et al. [31] reported that (5-AVA)\({}_{\rm x}\)(MA)\({}_{\rm 1-x}\)PbI\({}_{3}\) perovskite can be obtained by using 5-ammoniumvaleric acid (5-AVA) additive. The doped perovskite film has a better morphology on the TiO\({}_{2}\) surface and a lower defect concentration, resulting in improved conversion efficiency. Before that, [PERSON] et al. reported the effect of 4-ammonium-butyric acid (4-ABA) additive on the crystal structure of perovskite; that is, the ABA molecules form linear hydrogen bond chains [32]. 5-AVA affects the crystal growth of (5-AVA)\({}_{\rm x}\)(MA)\({}_{\rm 1-x}\)PbI\({}_{3}\) by the hydrogen bond between COOH groups and NH\({}_{3}\)\({}^{+}\) groups. The COOH groups of 5-AVA anchors monolayers of amino acids to the surface of mesoporous TiO\({}_{2}\) and ZrO\({}_{2}\) films by cooperative binding with exposed Ti or Zr ions. In this state, the terminal -NH\({}_{3}\)\({}^{+}\) group of 5-AVA faces the perovskite and thus serves as a nucleation site.