Ultraviolet Light-induced Degradation Of Luminescence In Mn-Doped CsPbCl₃ Nanocrystals And Preparation Of White-Light-Emitting Diodes

Improving the photostability of highly luminescent Mn²⁺doped all-inorganic halide perovskite nanocrystals(NCs)is challenging because their excellent optical performances are determined by the stable structure and low defect/trap states.The optical properties of Mn²⁺doped CsPbCl₃(Mn²⁺:CsPbCl₃)NCs with various Mn²⁺doping concentrations dispersed in n-hexane solvent and after film formation under ultraviolet(UV)illumination were studied to unravel their photodegradation by using photoluminescence(PL)spectroscopy at room temperature.The PL intensities of band edge excitons and Mn²⁺ions in the Mn²⁺:CsPbCl₃NCs films significantly decreased due to formation of nonradiative defects/traps in NCs with increasing illumination times.It was surprisingly found that the single-and multi-exponential decay times(1.81-1.15 ms)of Mn²⁺emissions ranging 595 to 640 nm observed in the doped NCs films with low and high Mn/Pb ratios were almost not varied with increasing UV illumination times.However,the blueshift of Mn²⁺emission was observed in high Mn²⁺doping concentration NCs under illumination,which might result from the diffusion of Mn²⁺ions to the surface of doped NCs.Further the surface passivation of Mn²⁺:CsPbCl₃NCs by using a Cs₄PbCl₆shell was found to effectively suppress the photodegradation of Mn²⁺:CsPbCl₃NCs and enhance the PL stability of Mn²⁺doped Mn²⁺:CsPbCl₃Cl_3-xBr_x/Cs₄PbCl₆NCs and green-emitting Cs Pb Br₃/Cs₄Pb Br₆core/shell NCs were prepared.A high-efficiency warm white LED with a color rendering index exceeding 80 and a luminous efficiency exceeding 70 lm/W was prepared.The main research contents are as follows:1.Mn²⁺doped CsPbCl₃NCs with different molar ratios were prepared at 190°C with a one-pot method.The samples with different concentrations were dissolved in N-hexane,and their first exciton absorption peaks were measured after the optical density(OD)value is around 0.05.A 365 nm UV lamp was used to illuminate the sample.Steady-state and time-resolved fluorescence spectra were recorded at different illumination times.The mechanism of light degradation was analyzed with the increase of illumination time.It was found that the band edge emission peak had a red shift of 8 nm.It was considered that the sample in the solution was irradiated with ultraviolet light,causing the ligand to fall off and the sample to aggregate.The TEM images confirmed our speculation after the light illumination.2.After understanding the photodegradation mechanism of samples in the solvent,in order to obtain stable samples under strong UV light,Mn²⁺doped CsPbCl₃NCs with Mn/Pb of 2/1 and different NCs sizes were synthesized.Samples with different NCs sizes were dissolved in N-hexane,and when the first exciton absorption peak was measured at OD value of 2,the NCs in N-hexane was spin-coated on a SiO₂substrate by a spin coater.Ultraviolet light was irradiated to the samples with different NCs sizes,and it was found that only the thin film samples with an average NCs size of 4.36 nm and band edge emission at 390 nm moved to 395 nm after fluorescence quenching,which should be due to the aggregation of NCs.It was found that the sample with band edge emission at 403 nm and average NCs size at 8.82 nm was relatively stable.After that,the photodegradation of NCs with Mn/Pb of 1/1,2/1,3/1and 5/1,band edge emission peaks at 403 nm,and Mn²⁺emission peaks at 595,600,610 and 640 nm,respectively,was analyzed.In the mechanism study,the local environment around the Mn²⁺ions in the doped NCs with a low Mn/Pb ratio was relatively stable,because the fluorescence lifetime did not change with the increase of the illumination time.It was observed that in high concentration Mn²⁺doped NCs,the blue shift of the Mn²⁺emission peak indicated that Mn²⁺ions diffuse out of the doped NCs.3.The light stability of Mn²⁺:CsPbCl₃NCs was further improved by synthesizing Mn²⁺:CsPbCl₃/Cs₄PbCl₆core/shell NCs.After that,Mn²⁺:CsPbCl_3-xBr_x/Cs₄PbCl₆core/shell NCs samples were prepared.By adjusting the Br^-concentration,the band edge emission peak position was adjusted to 450 nm.Band-edge emission was formed,and the Mn²⁺emission peaks were at 450 and 600 nm,respectively.A warm white LED was prepared using Ga N chip with an excitation wavelength of 397 nm and Cs Pb Br₃/Cs₄Pb Br₆core/shell NCs samples.At the same time,the stability of white LEDs prepared with different Mn²⁺doping concentrations of Mn²⁺:CsPbCl_3-xBr_xand Mn²⁺:CsPbCl_3-xBr_x/Cs₄PbCl₆NCs were compared.