## 2 The Mechanisms of Additives
### Hydrogen Bonds
The optimized device showed the PCE of 11.6% with short-circuit current density (\(J_{\rm sc}\)), \(V_{\rm sc}\) and fill factor (_FF_)of 21.1 mA cm\({}^{-2}\), 843 mV and 0.65, respectively (Figure 5b). For 5-AVA-doped PSCs, the slope of the _JV_ curve at the open circuit voltage is steeper than for MAPbI\({}_{3}\), indicating that the resistance of the doped device is reduced. In brief, the better surface contact had been ensured between perovskite and TiO\({}_{2}\) by the hydrogen bonds between the COOH groups and -NH\({}_{3}\)\({}^{+}\) groups (Figure 5c,d). Recently, [PERSON] et al. [33] also explored the role of 5-AVA as a perovskite precursor solution additive. They found that unpackaged devices with AVA-MAPbI\({}_{3}\) showed a 40-fold longer life than devices with MAPbI\({}_{3}\) in ambient air. This is because AVA-MAPbI\({}_{3}\) also has the effect of inactivating surface defect sites. Figure 3: (**a**) Preparation of a precursor solution with or without polyvinylpyrrolidone (PVP). (**b**) PVP-free perovskite prepared at 100 \({}^{\circ}\)C, and (**c**) quadrilateral PV formed at \(<\) 52 \({}^{\circ}\)C. Cubic to quadrilateral transition = CTT. (**d**) Perovskite synthesized in the presence of 3 wt.% PVP at 100 \({}^{\circ}\)C. (**e**) Schematic representation of the translucent solar cell of this study. (**f**)-V characteristic curve of the device. (**g**) Stability testing of equipment. Under humidity control, the air temperature is 22–25 \({}^{\circ}\)C, and the relative humidity is 24–30% [26]. Reproduced with permission from Reference [26]. Copyright 2016 Wiley-VCH. One additive that has the effect of cross-linking adjacent perovskite crystals is PbAc\({}_{2}\). [PERSON] et al. [34] found that PbAc\({}_{2}\) can act as a cross-linking agent to stabilize the perovskite mesophase by forming strong hydrogen bonds. In addition, PbAc\({}_{2}\) can produce PbI\({}_{2}\) in the perovskite film, which will passivate defects at the grain boundaries. Although PbAc\({}_{2}\) has been used as a lead source for the manufacture of pinhole-free and ultra-smooth perovskite films, it has never previously been reported as an additive for enhancing the crystallization of perovskite films. The efficiency of adding only 3% of PbAc\({}_{2}\) is relatively improved by about 10%, and the device stability is also greatly improved. As shown in Figure 6b, the stable output PCE of the two devices with and without PbAc\({}_{2}\) was 18.50% and 16.13%, respectively. This is because PbAc\({}_{2}\) is introduced into the precursor solution to form a strong hydrogen bond. As shown in Figure 6f, hydrogen bonding can be formed by the interaction between MA\({}^{+}\) and O in PbAc\({}_{2}\). This hydrogen bonding causes PbAc\({}_{2}\) to act as a crosslinking agent in the mesophase to enhance its stability. As shown in Figure 6g, the absorption peak having a wave number of 1580 cm\({}^{-1}\) belongs to the N-H bond of the MA \({}^{+}\) group. For perovskite films containing 3% PbAc\({}_{2}\), the N-H bond was red shifted to 1571 cm\({}^{-1}\), demonstrating that hydrogen bonding affects adjacent N-H bonds by reducing its vibration frequency. Figure 4: J-V curve with and without polyethylene oxide (PEO) in (**a**) low humidity and (**b**) high humidity environments. (**c–f**) SEM images of perovskite films on Indium Tin Oxide (ITO)/Poly (3,4-ethylenedioxythiophene)/poly (styrenesulfonate) (PEDOT:PSS) substrates with and without PEO at low humidity and high humidity [30]. Reproduced with permission from Reference [30]. Copyright 2019 Wiley-VCH. Figure 5: **(a)** Schematic showing a cross section of a perovskite solar cell. **(b)** J-V curve of the device at 100 mW cm\({}^{-2}\) intensity. SEM images of perovskite solar cell cross sections based on MAPbI\({}_{3}\) **(c)** and based on (5-AVA)\({}_{\text{x}}\)(MA)\({}_{1-\text{x}}\)PbI\({}_{3}\) **(d)**. Reproduced with permission from Reference [31]. Copyright 2014 the American Association for the Advancement of Science. Figure 6: **(a)** Schematic diagram of the inverted device.