**2. Synthesis of methylammonium trihalogenoplumbates (II)**

There are various fabrication processes for the methylammonium trihalogenoplumbates (II) (CH3NH3PbI3) compound with the perovskite structures. Two typical synthesis methods for the CH3NH3PbI3 (MAPbI3) were reported [1]. MAPbI3 could be synthesised from an equimolar mixture of CH3NH3I and PbI2 using the reported method [8]. CH3NH3I was synthesised at first by reacting a concentrated aqueous solution of hydroiodic acid with methylamine, and the cleaned precipitant was mixed with PbI2 in gamma-butyrolactone to obtain the MAPbI3 product. Crystalline MAPbI3 was obtained by drop-casting the solutions on glass substrates, and annealed at 100 °C. Polycrystalline MAPbI3 could be also prepared by precipitation from a hydroiodic acid solution [22]. Lead(II) acetate was dissolved in a concentrated aqueous HI and heated. An HI solution with CH3NH2 was added to the solution, and black precipitates were formed upon cooling from 100 °C.

A typical fabrication process of the TiO2/CH3NH3PbI3 photovoltaic devices is also described here [28]. The details of the fabrication process is described in the reported paper [2] except for the mesoporous TiO2 layer [16]. The photovoltaic cells were fabricated by the following process. F-Doped tin oxide (FTO) substrates were cleaned using an ultrasonic bath with acetone and methanol and dried under nitrogen gas. The 0.30M TiOx precursor solution was prepared from titanium diisopropoxide bis(acetyl acetonate) (0.11 mL) with 1-butanol (1 mL), and the TiOx precursor solution was spin-coated on the FTO substrate at 3000 rpm for 30 s and annealed 125 °C for 5 min. This process was performed two times, and the FTO substrate was sintered at 500 °C for 30min to form the compact TiO2 layer. After that, mesoporous TiO2 paste was coated on the substrate by a spin-coating method at 5000 rpm for 30 s. For the mesoporous TiO2 layer, the TiO2 paste was prepared with TiO2 powder (Aerosil, P-25) with poly(ethylene glycol) in ultrapure water. The solution was mixed with acetylacetone and triton X-100 for 30min. The cells were annealed at 120 °C for 5min and at 500 °C for 30min. For the preparation of pigment with a perovskite structure, a solution of CH3NH3I and PbI2 with a mole ratio of 1:1 in γ-butyrolactone (0.5 mL) was mixed at 60 °C. The solution of CH3NH3I and PbI2 was then introduced into the TiO2 mesopores by spin-coating method and annealed at 100 °C for 15min. Then, the hole transport layer (HTL) was prepared by spin coating. As the HTLs, a solution of spiro-OMeTAD (36.1 mg) in chlorobenzene (0.5 mL) was mixed with a solution of lithium bis(trifluoromethylsulfonyl) imide (Li-TFSI) in acetonitrile (0.5 mL) for 12 h. The former solution with 4-tert-butylpyridine (14.4 μL) was mixed with the Li-TFSI solution (8.8 μL) for 30min at 70 °C. Finally, gold (Au) metal contacts were evaporated as top electrodes. Layered structures of the photovoltaic cells were denoted as FTO/TiO2/CH3NH3PbI3/HTL/Au.
