Methylammonium Metal Iodide Perovskite Film Preparation from Gaseous and Liquid Sources
Authors: Matsuda Megumi , Murakami Takanori , Matsumura Nanzuki , Kawase Motoaki 
Published: December 2020

Introduction
Perovskite solar cells have attracted great expectation as next-generation solar cells since they were invented in 2009[1] due to their high power conversion efficiency (PCE), which is 25.2 % in 2020[2], and low cost. Now methylammonium lead iodide (MAPI, CH3NH3PbI3) is mainly used for a light-absorbing layer, but the toxicity of lead is the problem for commercial use. Recently, methylammonium bismuth iodide (MABI, (CH3NH3)3Bi2I9) is attracting attention for a new perovskite material because of its low toxicity, and high durability[3,4,5].
For fabricating perovskite thin film, solution process is widely used due to its simplicity. However, it is impossible to use for the industrial scale. Therefore, development of a chemical vapor deposition (CVD) process is required for large-area fabrication of thin films. In this study, a novel CVD process to synthesize MAPI thin films from gaseous and liquid reactants is proposed. Thin films were also synthesized by solution process. The properties of MAPI and MABI were compared in terms of the component metals and the processes.
Experimental
Solution process— MAPI and MABI were synthesized respectively by dropping the precursor solution containing stoichiometric mixture of methylammonium iodide (MAI) and MI (M = Pb, Bi) on FTO substrates (10 ×10 ×1.1 mm3) set on the spin coater. The precursor solution was synthesized in reaction of 40 wt% MAI and MI (M = Pb, Bi) in -butyrolactone or N,N-dimethylformamide. After spin-coated, the solution film was annealed on the hot plate at 413 K.
CVD process— As shown in Fig. 1, the reactor consisting of 3 parts, the iodide generation part, the mixing part, and the film deposition part, was used for CVD. In the iodide generation part, molten Pb was placed in a tube heated at 873 K, and 0.3 % HI in He was supplied for generating PbI2 vapor in Reaction (1).
Pb + 2HI → PbI2 + H2 (1)
PbI2 and HI were mixed with 10 % methylammonium (MA) in He in the mixing part and supplied for the film deposition part where MAPI film was synthesized on FTO substrates (8 ×10 ×1.1 mm3) coated with TiO2 at 413 K in Reaction (2).
PbI2 + HI + MA → MAPI (2)
The total pressure was approximately 2 kPa. HI and MA were supplied at a stoichiometric ratio of 3:1. Respective gases were supplied to inlets 1, 2, and 3 at flow rates of v
1 : v
2 : v
3 = 1 : 1 : 2.
Obtained samples were analyzed by an X-ray diffractometer (Rigaku, Ultima), a UV-vis spectrophotometer (Shimadzu, UV-3100PC), and an SEM (Keyence, VE-7800).
Results and Discussion
Fig. 2 shows the film XRD patterns synthesized by the CVD and solution process. Both patterns show good agreement with MAPI reference pattern. MAPI thin film was successfully obtained by the proposed CVD process. The film synthesized by CVD has higher orientation and crystallinity. Stronger PbI2 peak is observed in the film synthesized by solution process. MAI, due to the lower molecular weight, may evaporate from the surface of solution while annealed.
Fig. 3 shows the MAPI and MABI XRD patterns synthesized by the solution process. Full width at half maximum of main peak of MABI is greater than that of MAPI. Judged from the SEM images, the grain size of MABI is about 20 though MAPI is around 5 . The relatively less domain boundary in MABI thin film is expected to improve the power conversion efficiency.
Conclusions
A novel CVD process from gaseous and liquid sources was developed and MAPI thin film was successfully obtained by the proposed process. More highly oriented and crystallized film were obtained by CVD process than solution process. MABI has higher crystallinity and its grain size was greater than what MAPI has.
References
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Figure 1