The Influence of PbI2 on Characteristic of Organic-Inorganic Hybrid Perovskite Thin Films

Organic-inorganic hybrid perovskite materials have attracted significant research efforts because of their outstanding properties. Meanwhile the crystallization of organic-inorganic hybrid perovskite materials can significantly influence the films quality. Here, we research the influence of the characteristics of PbI2 thin film on final perovskite films and the mechanisms of film formation based on the two-step sequential deposition method. We found that the characteristics of PbI2 thin film, such as the grain size, the grain shape, the surface roughness and the film densification, have significant effects on the final perovskite films due to different film crystallization process. According to the analysis on the characteristics of the perovskite films obtained from different PbI2 precursor, we suggested that the formation of perovskite film begins from the PbI2 crystals expanding when they are converted to MAPbI3 perovskite by migration of MA+ cations from the grain boundaries.


Introduction
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 Modeling and Numerical Simulation of Material Science [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, ABX 3 .These material are crystallized from organic halide and metal halide salts to form crystals in the ABX 3 structure, where A is the organic cation, such as methylammonium (MA = CH NH + ), B is the metal cation, such as lead (B = Pb 2+ ) 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 PbI 2 layer play a significant role in the final perovskite films formation.In this paper, we research the influence of PbI 2 film characteristics on the final perovskite films and its mechanisms based on the two-step sequential deposition method.

Experimental Section
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 PbI 2 layer characteristics on the perovskite films formation.
2.1.Organic-Inorganic Perovskite Thin Films Deposition was preheated at 110˚C on a hot plate, followed by spin coating on the substrates at 4000 rpm for 40 s, then the PbI 2 film was put back on the hot plate for 15 min of drying.To obtain the perovskite thin film, substrates with PbI 2 film were then put into a vacuum coating machine, then MAI was deposited by thermal evaporation for 30 min (Figure 2).After cooling down to room temperature, these perovskite thin films were annealed for 18 min at 100˚C.Then the films were rinsed with 2-propanol, and dried under a flow of clean air.

PbI2 Layer Fabricated by Thermal Evaporation
This method to convert PbI 2 to a perovskite is aside from conversions in solution, where the substrate is exposed to the PbI 2 vapor.Then substrates with PbI 2 film were then put into a vacuum coating machine, then MAI was deposited by thermal evaporation for 30 min, sequentially (Figure 3).After cooling down to

Analysis of Characteristics of the Resulted Layers
The crystal structures of the CH 3 NH 3 PbI 3 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.

Results and Discussions
To investigated the characteristics of PbI 2 layer prepared by different processes.
We prepared different PbI 2 layers by spin coating and thermal evaporation, respectively.

The Characteristics of PbI2 Layer Prepared by Different Processes
As our previous work described [22], SEM and XRD measurements of PbI 2 layers fabricated by spin coating with different solution concentrations were taken.
The SEM images and XRD measurements of twos representative PbI 2 layers prepared by spin coating and thermal evaporation are compared as shown in     fabricated by two methods has a significant difference, where the peaks corresponding to PbI 2 obtained by thermal evaporation process is very sharp than that by spin-coating process, illustrating the higher crystalline degree of the PbI 2 obtained by thermal evaporation process.

The Influence of PbI2 Characteristics on Final Organic-Inorganic Hybrid Perovskite Films
To investigate the influence of characteristics of PbI 2 film on final perovskite films, we fabricated perovskite films with different PbI 2 precursor.Figure 7 shows Figure 7. SEM top-view images and X-ray diffraction pattern of MAPbI 3 films: PbI 2 solution concentration varied from (100 mg/mL) (a,f), 250 (mg/mL) (b,g), 285 (mg/mL) (c,h), 345 (mg/mL) (d,i), 500 (mg/mL) (e,j).Modeling and Numerical Simulation of Material Science concentration is too high, finally leading to a rough and irregular surface of perovskite film when PbI 2 layer converts to perovskite films (see Figure 7(e)).
Therefore, we suggest that the characteristics of PbI 2 layers fabricated by spin coating have a main influence on the roughness of perovskite film rather than the grain size.has less grain boundary than that prepared by thermal evaporation, the final perovskite films obtained by thermal evaporation (Figure 8) have smaller grain size than that obtained by spin coating (Figure 7).As we know, the perovskite formation process is the combination of an organic component, such as methylammonium iodide (MAI), with an inorganic component, such as PbI 2 or PbCl 2 , to form the perovskite (MAPbI 3 or MAPbI 3-x Cl x , respectively).We suggested that the formation of perovskite in two-step method is the PbI 2 as nucleus reacts with MAI, in the meantime, the PbI

Conclusions
In summary, we have found that the PbI 2 thin film has different characteristics fabricated by different process based on the two-step sequential deposition method.The PbI 2 thin film fabricated by thermal evaporation usually has a clear crystal shape and a loose structure comparing with that by spin coating.However, the latter has fuzzy domain boundaries and fewer grain boundaries than the former.The final perovskite films are very dependent on the characteristics of PbI 2 precursor.We suggest that the characteristics of PbI 2 layers fabricated by spin coating mainly influence on the roughness of perovskite film rather than the grain size, while, the characteristics of PbI 2 film have an effect on both the grain size and the film surface roughness of the final perovskite films.So, when perovskite applied to TFTs as active layer, in order to make carrier mobility maximized, we need a bigger grain size and higher coverage.Thus, the research of influence of PbI 2 on characteristic of perovskite thin films is important and meaningful.
We suggested the formation of perovskite is that the PbI 2 crystals as the nucleus start to grow into perovskite grains, where they reacted with MAI to convert to MAPbI 3 by migration of MA+ cations from the grain boundaries.The high-quality perovskite films can be achieved by adjusting the PbI 2 crystallization.

Figure 1 .
Figure 1.Schematic illustration of synthesis of perovskite.

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Modeling and Numerical Simulation of Material Science

Figure 2 .
Figure 2. Schematic illustration of the procedure of perovskite thin film prepared via modified vapor-assisted solution process.

Figure 3 .
Figure 3. Schematic illustration of the procedure of perovskite thin film prepared via sequential thermal evaporation process.

Figure 6 .
Figure 6.As can be seen, there has an obvious difference between the morphology of the two types of PbI 2 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 PbI 2 layers
. Peng et al.DOI: 10.4236/mnsms.2017.74004Modeling and Numerical Simulation of Material Science the SEM images of perovskite films made from the PbI 2 precursor fabricated by spin coating with different solution concentrations above discussion, with the increase of PbI 2 solution concentration, the film surface roughness gradually increases, which benefit PbI 2 to contact MAI and convert completely to perovskite.Then the final perovskite films were obtained without PbI 2 residue, as illustrated in Figure7(i), Figure7(j), where the PbI 2 solution concentration is higher than 345 mg/ml.As to the PbI 2 solution concentration below 250 mg/ml, the perovskite films did not have PbI 2 residue, which were mainly due to lower PbI 2 solution concentration (see Figure7(f), Figure 7(g)).Nevertheless, the grain sizes of perovskite films obtained with different PbI 2 solution concentration are almost same (Figures 7(a)-(e)).Additionally, the surface of PbI 2 layer would become rough (see Figure 4(e) andTable 1) when the PbI 2 solution

Figure 8 Figure 8 .
Figure8shows the scanning electron microscope (SEM) images of perovskite films made from the PbI 2 precursor by thermal evaporation with different amount of PbI 2 .Comparing with the PbI 2 film fabricated by spin coating, the PbI 2 films fabricated by thermal evaporation have loose morphologies as shown above, which make it easier for the MAI to diffuse into PbI 2 films and makes complete reaction between the MAI and PbI 2 .Therefore, PbI 2 residue in the final films is fewer than that made from the spin coating PbI 2 precursor as shown in Figure8(e).However when the amount of PbI 2 reaches to 150 mg, an obvious signature peak at 12.65˚ appears, which suggests that the amount of PbI 2 was too much to react completely and there is some PbI 2 residue in the final perovskite films.Different with the case of spin coating PbI 2 precursor, the characteristics of PbI 2 film have an effect on both the grain size and the film surface roughness final perovskite films.Base on the above analysis, we found the final perovskite films are very dependent on the characteristics of PbI 2 precursor with respect to the morphology, the grain size and crystalline degree.Because the PbI 2 prepared by spin coating 2 crystals start to expand to convert to MAPbI 3 crystals by migration of MA+ cations from the grain boundaries.Then the PbI 2 deposited via thermal evaporation has high grain density as the nucleus which grows into perovskite grains, resulting in high perovskite grain density and smaller grain size.As to the PbI 2 deposited via spin coating, grains have fuzzy domain boundaries and fewer grain boundaries, therefore, the final perovskite film has lower grain density and larger grain size when PbI 2 crystals are converted to MAPbI 3 perovskite by migration of MA+ cations from the grain boundaries.The cross-sectional SEM images of MAPbI 3 perovskite made from different PbI 2 precursor are shown in Figure 9: It can be observed that the crystalline structure of MAPbI 3 perovskite thin film prepared by spin coating fused together (Figure 9(a)), nevertheless, that by thermal evaporation has clear grain boundaries and crystal piled up together (Figure 9(b)), which is consistent with the above results and analysis.

2.1.1. PbI2 Layer Fabricated by Spin Coatings Here
, the inorganic framework film was formed by depositing PbI 2 solution on the substrates.PbI 2 solution was prepared in DMF.The prepared PbI 2 solution