halide vacancies/ions is an important factor in the excitonic and charge carrier properties of perovskites. [4g,h] For example, the hole trap-mediated nonradiative charge recombination in CH 3 NH 3 PbI 3 is correlated with the oxidation state of the iodine vacancy. [4h] While an I V − vacancy does not create a hole trap state, a neutral I V vacancy forms a hole trap near the conduction band and accelerates nonradiative charge recombination. Similarly, an I V + vacancy accelerates nonradiative charge recombination by forming shallow and deep hole traps. Like I vacancies, Cl or Br vacancies with different oxidation states form shallow and deep traps and influence the nonradiative exciton/charge carrier recombination rate. A chemical additive or photoactivation fills these anion vacancies. In the former case, halide ions from halogen gases, [8] halogen compounds AX [9] (A = MA/FA/Cs/Na/K and X = Cl/Br/I), organic pseudohalides, [10] or ligands [11] fill the vacancies. In the latter case, the so-called light soaking effect, molecules like oxygen improve the PL of LHPs by photoinduced defect passivation. [6c,d,12] For example, a superoxide generated by self-sensitization occupies the halide vacancy and improves the optical properties of LHPs. [6c,f ] Also, an oxide layer (e.g., PbO) on the LHP surface increases the sample stability against humidity and decreases the trap-assisted nonradiative recombination rate. [6e] Defect passivation during photoexcitation involves lattice expansion and compositional redistribution through ion migration. [12a,b,h] Crystal shapes and optical properties are equally important for the electro-optical applications of LHPs. [3a,13] Information about the LHP crystal shape-dependent halide vacancy filling by a halide precursor or the light can help optimize the properties and applications of LHPs. Nevertheless, a correlation among the structure, shape, size, defects, and degradation rate of perovskites remains concealed.We report the surface-to-volume (s-t-v) ratio-dependent PL stability, halide vacancy filling, and carrier recombination rates in methylammonium lead bromide (MAPbBr 3 ) single crystals with the cubic, plate, or rod shape. The Br − vacancies are filled by soaking the crystals in a MABr solution or a picosecond laser beam. We discuss the s-t-v ratio-dependent kinetics of Br − vacancy filling from the viewpoint of redistribution in the radiative and nonradiative recombination rates.
Results and DiscussionTo understand the shape-dependent halide vacancy filling, we synthesized MAPbBr 3 microrods, microplates, and microcubes.Halide perovskites show high photoluminescence quantum yields and tunable bandgap. While perovskites' optical properties significantly degrade due to the ionic and electronic defects, a correlation among their structure, size, defects, and degradation rate remains concealed. The authors report the crystal shape-and halide vacancy-dependent stability of methylammonium lead bromide single crystals. The vacancies are filled in the cubic-, plate-, and r...