MAPbBr3 and MAPbCl3 crystals were first ground with a mortar & pestle to form rough powders. The resulting powders were dried in a vacuum oven at ~50 °C for ~24 to ~48 hours. This was followed by two subsequent pulverization steps with a Retsch PM200 Planetary ball mill with 50 mL zirconia jars and milling media. The ball milling parameters for each step were as follows: (i) 20:1 ball to powder mass ratios, (ii) ~60% jar filling ratios, and (iii) 400 rpm for 3 hours (reverse rotational direction every 10 minutes). All ball milling parameters remained the same between the first and second steps except the diameter of the milling media-15 mm zirconia balls in the initial step and 2 mm zirconia balls in the final step. Finally, milled powders were filtered through a 125 m then a 25 m mesh with the aid of mechanical shaker and were stored under vacuum.