## 2 The Mechanisms of Additives  
### Hydrogen Bonds  
A hydrogen bond is a relatively weak bond that hydrogen atoms interaction with the electronegative atoms nitrogen, oxygen or fluorine. Hydrogen bond is usually weaker than ionic, covalent and metallic bonds, but plays an important role in formation of perovskite film. In a typical perovskite precursor solution, hydrogen bond formed between the hydrogen atom and the other atoms (e.g., N, O atoms), which will affect the growth of perovskite crystals and the quality of the films in finally. [PERSON] et al. [23] first discovered that the addition of 1 wt.% poly (ethylene glycol) (PEG) to a CH\({}_{3}\)NH\({}_{3}\)Pb\({}_{3-x}\)Cl\({}_{x}\) perovskite precursor solution can solve the problem of poor coverage of the perovskite film on TiO\({}_{2}\) as shown in Figure 1. The higher coverage of the perovskite layer reduces the possibility of direct contact between the TiO\({}_{2}\) and the 2,2\({}^{\prime}\),7,7\({}^{\prime}\)-Tetrakis[\(N\),\(N\)-di(4-methoxyphenyl)amino]-9,9\({}^{\prime}\)-spirobifluorene (spiro-OMeTAD) layer, which leads to a higher open-circuit voltage (\(V_{\infty}\)) in a device. However, when the PEG content exceeds 1 wt.%, the performance of a device would decrease in all aspects. This is because once the amount of PEG additive is overloaded, the charge cannot efficiently be transferred to the TiO\({}_{2}\) nanoparticles or spiro-OMeTAD layer and will accumulate or recombine in the perovskite layer. [PERSON] et al. [24] reported a PEG polymer scaffold PSCs, and subsequently, the PCE exceeded 16%. The long-chain insulating polymer dissolved in the precursor solution will be assembled into a three-dimensional (3D) scaffold when the precursor solution is spin-coated on the substrate. The formed scaffold can guarantee a uniform thickness and homogeneous morphology of the perovskite films. Therefore, the performance of the solar cell is greatly improved. As we known, PEG is a hygroscopic polymer containing O atoms. There is a strong interaction with perovskite by hydrogen bonds with H atoms in CH\({}_{3}\)NH\({}_{3}\)\({}^{+}\). Figure 2a-d show the Scanning electron microscopy (SEM) images of perovskite films without polymer scaffolds and with polymer scaffolds. This is consistent with the report from [PERSON]. They speculated that PEG molecules are anchored on the surface of the perovskite crystallites by a hydrogen bond, as shown in Figure 2e. Another interesting finding is that PEG has a strong hygroscopicity and a binding effect with methylammonium iodide (MAI), which can protect perovskite from being decomposed. The enhancement of device stability is mainly due to the forces of hydrogen bond between MAI and PEG. The MAI molecule is anchored by a nearby PEG molecule. After annealing, MAPbI\({}_{3}\) phase will be generated by the reacts of PbI\({}_{2}\) and MAI. The instantaneous decomposition regeneration mechanism in Figure 2g explains the rapid self-healing process in perovskite films with PEG scaffolds. They further tried PEG and [6,6]-phenyl-C\({}_{61}\)-methylbutyrate (PC\({}_{61}\)BM) dual additives in the perovskite solution [25]. Among them, PEG still plays a role in ameliorating the film morphology and device stability. Similarly, polyvinylpyrrolidone (PVP) can also serve as an additive for enhancing the stability of the PSCs by forming hydrogen bonds. [PERSON] et al. [26] proposed that doping PVP in the perovskite precursor solution can produce a perovskite film with a smoother surface morphology and minimum crystallite size in ambient air. The production of translucent perovskite films also requires smaller grain sizes [27]. Translucent perovskite film (50-100 nm) is a key device for window applications in zero-energy building technology [28,29]. The mechanism of PVP as an additive to control the crystal size and enhance device stability is shown in Figure 3a-e. The hydrogen bonds formed between O atoms in PVP and H atoms of CH\({}_{3}\)NH\({}_{3}\)\({}^{+}\) in the perovskite can protect the minimum perovskite crystals.