*2.1.1 Solution temperature-lowering (STL) method*

According to the lower solubility of MAPbX3 in HX (X = Cl, Br, and I) solution as the temperature decreases, Tao's group introduced the STL method to synthesize a MAPbI3 bulk single crystal (**Figure 2a**) [47]. After the reaction between methylamine (CH3NH2) and hydro-iodic acid (HI) in a cold atmosphere, the obtained white microcrystal MAI was reacted with Pb(CH3COOH)2∙3H2O in aqueous HI, and the solution was then cooled to 40°C. A 10 mm × 10 mm × 8 mm black MAPbI3 single crystal was grown in about one month (**Figure 2b**). Lin's group discovered a more efficient way, and they synthesized the single crystals with a size of 5 mm in just around 10 days [48]. Lin et al. selected high-quality seeds and dropped them back into fresh solution and obtained single crystals sized up to 1 cm (**Figure 2c**). Furthermore, MAPbBr3 − xClx and MAPbI3 − xBrx mixed-halide perovskite crystals were studied using such method [49]. Hydro-bromic acid with hydrochloric acid or hydro-iodic acid were mixed in different molar ratios into methylamine and lead (II) acetate solution to fabricate single-halide and mixed-halide perovskite crystals (**Figure 2d**). The time-consuming factor is the biggest drawback of this method, which has indirectly led to the domination of other crystallization methods.
