Fig. 2

(a) Precursor solution, (b) Before annealing MA₃Bi₂I₉ thin film deposited on FTO substrate, (c) After annealing MA₃Bi₂I₉ thin film deposited on FTO substrate, (d) Before annealing MA₃Bi₂I₉ film on TiO_2, (e) After annealing MA₃Bi₂I₉ film on TiO₂.