## 2 The Mechanisms of Additives
### Forming an Intermediate or Reacting
As shown in Figure 16, the performance of the device with 2 wt.% MAH\({}_{2}\)PO\({}_{2}\) and the device performance of the non-purified MAI are better than those of the device after purification. In summary, the reaction of MAH\({}_{2}\)PO\({}_{2}\) and PbI\({}_{2}\) is reversible; thus, Pb(H\({}_{2}\)PO\({}_{2}\))\({}_{2}\) can be slowly converted to perovskite in the presence of excess MAI during thermal annealing. Based on this finding, the authors controlled the crystallization rate of perovskite by adjusting the proportion of MAH\({}_{2}\)PO\({}_{2}\) in the MAI precursor to obtain the optimal film morphology. Phosphorous acid (HPA) is also a stabilizer that has been used in PSCs to optimizing film morphology. [PERSON] et al. [56] showed that the strong reducing agent HPA can act as an additive of a perovskite precursor solution, which is usually added to the HI solution as a stabilizer to prevent the oxidation [57]. By reducing the non-radiative recombination center, the performance of the PSCs is greatly improved. It is well known that a semiconductor device with high performance usually requires high purity and low defect materials. However, due to the obvious non-uniformity of the polycrystalline material and little knowledge of the defect components [58], there is still a good prospect  
Figure 16: Top view SEM images of films made from: (**a**) unpurified MAI, (**b**) purified MAI and (**c**) purified MAI + 2 wt.% MAH\({}_{2}\)PO\({}_{2}\). J–V curve: (**d**) using non-purified MAI, (**e**) using purified MAI, and (**f**) using purified MAI+2 wt% MAH\({}_{2}\)PO\({}_{2}\). [54] Reproduced with permission from Reference [54]. Copyright 2016 Elsevier B.V.  
Figure 15: (**a–j**) top view SEM image of MAI(PbI\({}_{2}\))1-X(CuBr\({}_{2}\))X film with different composition ratios (x = 0, 0.025, 0.050, 0.075, and 0.100). (**k**) J-V curve. Reproduced with permission from Reference [49]. Copyright 2016 Elsevier B.V.  
for further improvement [59]. MAI has strongly hygroscopicity and is not stable in air and light, especially after long-term storage. The SEM image of perovskite film is shown in Figure 17a,b; both samples show complete coverage without pinholes on the substrate. The perovskite grain size is significantly increased by added HPA, increasing from an average of 168 to 769 nm. As shown in Figure 17c, the photoluminescence (PL) spectra of the perovskite films was found that the HPA-modified perovskite films showed a PL quantum efficiency (PLQE) improvement of \(11.6\pm 0.7\%\), compared with \(0.84\pm 0.15\%\) in the control group. It is worth noting that because the HPA additive can reduce the oxidized I\({}_{2}\) (Figure 17d) and reduced the density of metal lead in the perovskite crystals result in a PCE improvement from 13.2% to 16.2%. Recently, [PERSON] et al. [60] proposed a dual additive method in which the morphology of the perovskite film was improved by the simultaneous addition of dimethyl sulfoxide (DMSO) and methoxy ammonium salt (MeO) (Figure 18a). They show that an appropriate amount of MeO additive can help the precursor form a stable intermediate PbI\({}_{2}\)-DMSO adduct during film formation and expand the perovskite grains by preventing the kinetics of adduct conversion to perovskite. As shown in the film plane of SEM of Figure 18b, when the amount of the MeO additive was increased from 0 to 5 and 10%, the grain size was observed to increase from approximately 150 nm to 380 nm and  
Figure 17: Top view SEM images of the perovskite film: (**a**) undoped phosphorous acid (HPA), (**b**) doped with HPA. The scale in the image is 20 \(\upmu\)m and 1 \(\upmu\)m in the insets. (**c**) photoluminescence (PL) spectrum of a perovskite film on glass. (**d**) Ultraviolet-visible (UV-vis) absorption spectrum of MAI or I\({}_{2}\) dissolved in _N,N_-Dimethylformamide(DMF) and absorption quenching by addition of HPA-containing MAI solution. Reproduced with permission from Reference [56]. Copyright 2015 Nature Publishing Group. 120 nm, respectively.