Organic-inorganic hybrid perovskite materials have attracted substantial attention because of their excellent physical properties [1] [2] [3] [4] [5] , which enable them to be employed in solar cells and other application, such as LED. These materials show remarkable optical absorptions across a wide range of the solar spectrum, and a sharp optical band edge, which suggests low levels of disorder [6] [7] . They also exhibit long charge-carrier diffusion lengths (>1 mm) relative to the absorption depth of incident light (~100 nm), [8] [9] meaning that almost all photoexcited species in the perovskite are able to reach the interfaces from where the charges are then transported through suitable hole- and electron-transporting layers to the electrodes. Based on these excellent properties, the power conversion efficiencies of organic-inorganic hybrid perovskite solar cells have increased from around 4% to a certified 22% in the last three years [1] [2] [3] . Now, most of the significant improvements in PCE have been a direct result of improvements in the formation of perovskite films, [10] [11] which led to a better film uniformity and crystalline quality, thus suggesting that the thin film features are of the upmost importance for achieving high performance.

As we know, organic-inorganic hybrid perovskites are a family of materials that share a crystal structure with calcium titanate, that is, ABX₃. These material are crystallized from organic halide and metal halide salts to form crystals in the ABX₃ structure, where A is the organic cation, such as methylammonium (MA = CH 3 NH 3 + ), B is the metal cation, such as lead (B = Pb²⁺) and X is the halide anion (X = I, Br, Cl or mixtures). As for this, there is a general schematic diagram of perovskite synthesis (Figure 1). And a lot of processing techniques of perovskite layers have been reported to obtain high-quality perovskite films so far, such as one-/two-step solution process, vacuum deposition, and solvent vapor/additive assisted crystal growth [12] - [18] . Although these methods give some help to understand the growth mechanisms of perovskite films, however the growth mechanisms of perovskite films are not completely clear. The two-step sequential deposition method in all method for film formation is more helpful for studying the growth mechanisms of perovskite crystallites.

We have found that the characteristics of PbI₂ layer play a significant role in the final perovskite films formation. In this paper, we research the influence of PbI₂ film characteristics on the final perovskite films and its mechanisms based on the two-step sequential deposition method.

Many methods to fabricate organic-inorganic perovskites have emerged, each resulting in varying degrees of surface coverage [19] , and crystal and film quality [20] . Nevertheless, the two-step sequential deposition method can achieve

additional control over the morphology by sequentially depositing the two precursors relative to one-step deposition [21] . Meanwhile the two-step sequential deposition method is more flexible to design the procession of film formation. Here, organic-inorganic perovskite films were fabricated by two-step sequential deposition methods, which is helpful to investigate the effect of PbI₂ layer characteristics on the perovskite films formation.

The crystal structures of the CH₃NH₃PbI₃ films were characterized by X-ray diffraction (XRD, Philips PANalytical X’Pert Pro) with a copper X-ray source, and the surface morphologies were observed by scanning electron microscope (SEM, Hitachi SU8010) and atomic force microscopy (AFM) (Seiko SPA-400SPM UNIT). All samples were tested in air and at room temperature.

To investigated the characteristics of PbI₂ layer prepared by different processes. We prepared different PbI₂ layers by spin coating and thermal evaporation, respectively.

As our previous work described [22] , SEM and XRD measurements of PbI₂ layers fabricated by spin coating with different solution concentrations were taken. In Figures 4(f)-(j), an obvious signature peak at 12.65˚ is observed in all PbI₂ layers, which indicate doubtless PbI₂ material. Figures 4(a)-(e) show the top-view SEM images of PbI₂ films with the different solution concentration. It is observed that the PbI₂ layer has no clear morphology and fuzzy domain boundaries with low PbI₂ solution concentration (Figure 4(a)). With the increase of PbI₂ solution concentration, the morphology of PbI₂ layer becomes

gradually clear (Figures 4(a)-(e)) with pancake-shape crystalline grain, the grain size gradually increases as shown in Figures 4(a)-(e). Meanwhile, the surface roughness also gradually increases as extracted from its AFM measurements (see Table 1), which is consistent with SEM measurements (Figures 4(f)-(j)). We found the PbI₂ grains prepared by spin coating have fuzzy domain boundaries.

Then, the characteristics of PbI₂ layer fabricated by thermal evaporation with different amount of PbI₂ were researched via the SEM images and XRD measurements in Figure 5. However, we found the PbI₂ layers by thermal evaporation appeared a completely different morphology from that by spin coating. As shown in Figure 5, all the PbI₂ layers obtained by thermal evaporation have clear surface morphology and appear rice-shaped crystalline grain. With the increase of the amount of PbI₂, the surface roughness and the grain size of PbI₂ layers gradually increases. On the other hand, the sharp signature peak at 12.65˚ in the XRD measurements implies well crystalline degree of all PbI₂ layers obtained via thermal evaporation (Table 2).

The SEM images and XRD measurements of twos representative PbI₂ layers prepared by spin coating and thermal evaporation are compared as shown in Figure 6. As can be seen, there has an obvious difference between the morphology of the two types of PbI₂ layers. The later has a clear crystal shape and a loose structure comparing with that of the former. However the former have fuzzy domain boundaries and fewer grain boundaries than the later. From the XRD measurements in Figure 6(c), the sharpness of XRD peak of the PbI₂ layers