Improving the Bulk Emission Properties of CH₃NH₃PbBr₃ by Modifying the Halide-Related Defect Structure

Abstract

The peculiar defect chemistry of hybrid organic–inorganic lead halide perovskites is believed to be partially responsible for the outstanding performance of this solution-processed material in optoelectronic devices. While most effort has been put on the management of halide defects (the ones presenting the highest mobility) for CH3NH3PbI3, its bromide counterpart has not been so widely studied. Although the former is the material of choice for photovoltaics, the latter is present in most light-emitting applications. Here, we report how the exposure of CH3NH3PbBr3 single crystals to a bromine atmosphere strongly affects its emission properties. Such improvement takes place in the absence of apparent signs of degradation and remains for tens of hours. We propose an explanation based on the defect structure for this material where bromine-related defects can act as deep or shallow traps. These results are of relevance for a material expected to be present in a new generation of solution-processed light-emitting devices.

Spatially Resolved Analysis of Defect Annihilation and Recovery Dynamics in Metal Halide Perovskite Single Crystals

“…As the enhancement in emission in the illuminated region is not accompanied by any darkening of the surroundings it seems that under this level of external illumination migration of vacancies, acting as deep traps quenching PL in CH 3 NH 3 PbBr 3 , [49] to nearby interstitial ions is induced causing the annihilation of defects which leads to a PL enhancement. [50] This picture matches the one recently proposed by Mosconi and co-workers for CH 3 NH 3 PbI 3 [32] involving the annihilation of optically excited Frenkel pairs formed by iodide vacancies/interstitials present in the material. The driving force for the migration of negatively charged interstitial halide defects is also provided by the formation of negatively charged superoxide layers at the surface of the material as a consequence of the light-induced formation of superoxide species as mentioned above.…”

“…In what follows an interpretation of the above changes in the emission properties is carried out in terms of the spatial redistribution of halide ions which have been reported for different metal–halide perovskites under external visible light illumination. , The initial activation dynamics taking place during the first tens of seconds evidence how under the presence of external irradiation halide redistribution takes place. Because the enhancement in emission in the illuminated region is not accompanied by any darkening of the surroundings, it seems that under this level of external illumination migration of vacancies, acting as deep traps quenching PL in CH 3 NH 3 PbBr 3 , to nearby interstitial ions is induced causing the annihilation of defects which leads to a PL enhancement . This picture matches the one recently proposed by Mosconi and co-workers for CH 3 NH 3 PbI 3 involving the annihilation of optically excited Frenkel pairs formed by iodide vacancies/interstitials present in the material, albeit a similar study for the bromide counterpart taking into account the corresponding migration energy barriers has not been carried out.…”

“…Because the enhancement in emission in the illuminated region is not accompanied by any darkening of the surroundings, it seems that under this level of external illumination migration of vacancies, acting as deep traps quenching PL in CH 3 NH 3 PbBr 3 , 28 to nearby interstitial ions is induced causing the annihilation of defects which leads to a PL enhancement. 29 This picture matches the one recently proposed by Mosconi and co-workers for CH 3 NH 3 PbI 3 20 involving the annihilation of optically excited Frenkel pairs formed by iodide vacancies/interstitials present in the material, albeit a similar study for the bromide counterpart taking into account the corresponding migration energy barriers has not been carried out. According to the measured dynamics, the activation rates (see Figure 3b) depend on the pump intensity as the spatial variation of k a follows that of the illuminated region.…”

Shape‐Dependent Kinetics of Halide Vacancy Filling in Organolead Halide Perovskites

“…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.…”

Effects of alkylamine chain length on perovskite nanocrystals after washing and perovskite light-emitting diodes

“…2(b)] which is a similar emission energy to that of bulk MAPbBr 3 . [24][25][26] Except for ethylamine and propylamine, the PeNCs show high PLQYs (83%-90%) with narrow FWHM close to 20 nm (Table I), which is comparable to the optical properties from the PeNCs using OAm. 7,8) Although ethylamine and propylamine respectively have relatively low PLQY values of 67% and 83%, respectively (Table I) compared to the longer chain ligands, they precipitate stable colloidal PeNCs.…”