Fig. 5

Optical images and photoluminescence (PL) spectra of top-crust peeling-off test. a Schematics of top-crust peeling-off test setup. A glass substrate (40 mm × 50 mm) with 100 µL BA₂MA₃Pb₄I₁₃ precursor solution (2 M in N,N-dimethylformamide (DMF)) on top was placed on a hot plate at 70 °C. The yellow solution begun to turn dark in the edge, and this dark area gradually spread into the center as indicated by the red arrows, forming a top-crust. b A Kapton tape was used to partially peel off the top-crust, and the revealed yellow precursor solution underneath that has not yet crystallized started to turn red under heating. After the solution had dried, the glass substrate was removed from the hot plate. The area inside the white frame represented the crystallized front surface revealed after the top-crust was removed by Kapton tape. c PL spectrum of different areas in b