*2.1. Powder Preparation*

Methylamine iodide (MAI) was synthesized from 24 mL methylamine (MA) solution and 10 mL hydroiodic acid (HI) solution [15]. MA solution and HI solution were firstly mixed in a smaller beaker that was placed in a larger beaker at 0 ◦C. The mixed solution was continuously stirred in an ice bath for 120 min until evenly mixed to form a slightly yellowish solution. Then, the resulting mixture was evaporated in a vacuum oven at 90 ◦C for 7 h until a white precipitate was obtained. After naturally cooling down to room temperature, the obtained precipitate was washed 3 to 4 times with ether in a centrifuge. Eventually, the precipitate was dried at 60 ◦C for 24 h to obtain white crystalline powder MAI.

Preparation of lead methyl iodide triiodide (MAPbI3). An equal molar amount of MAI powder and PbI2 powder were ground in an agate mortar for 20 min until a visually black powder was obtained.
