Photo- and Thermal-Induced Ion Migration and Phase Separation in Mn-Doped Two-Dimensional PEA₂PbX₄ Perovskite

Ion migration and phase separation in perovskite materialshavenegatively affected the solid-state lighting and display. Studyingphoto- and thermal-induced degradation is considered as a promisingapproach to understanding the luminescence mechanism and promotingpractical applications. Herein, the Mn-doped two-dimensional PEA(2)PbX(4) (X = Cl, Br, I) microcrystals with changinghalogen composition were synthesized by an acid-assisted post-processingstrategy. Then, photo- and thermal-induced degradation was studiedby using steady-state and time-resolved photoluminescence (PL) spectroscopy.The band edge exciton PL peak of Mn-doped 2D PEA(2)PbX(4) microcrystals was adjusted from 397 to 500 nm. The reducedMn PL lifetime (1.37 to 0.21 ms) was monitored under ion exchangefrom Cl to Br to I. The degradation mechanism could be divided intotwo cases: (i) The halide ion migration in Mn-doped 2D perovskiteunder continuous illumination was revealed, suggesting that the migrationof Cl ions was more accessible than that of Br and I. (ii) The PLredshift and lifetime reduction were observed after annealing at 420K, which means that thermally induced aggregation of Mn ions resultedin the formation of Mn2+-Mn2+ dimers.In addition, the experimental results indicated that the induced B-sitephase separation at high temperature annealing made the mixed perovskitephase of Pb and Mn ultimately transform into pure PEA(2)PbBr(4) and PEA(2)MnBr(4).