Research On Defect Passivation For Quasi-2D Perovskite Light-emitting Diodes

In recent years,the perovskite light-emitting materials have attracted extensive attention,due to the excellent properties such as high efficiency,tunable bandgap,solution processing,low cost,etc.The external quntum efficiencies（EQE）of the red and green perovskite light-emitting diodes（Pe LED）have arealdy exceeded 20%,showing a great potential for the practical application on display and lighting.The formed multiple quantum wells of the quasi-two-dimensional（Q-2D）perovskite films can spontaneously transfer the excited states from the wide bandgap phases（the small n phases;n is the number of inorganic layers,corresponding to the well width.）to the narrow bandgap phases（the large n phases）through charge or energy funneling,effectively increasing the excited state density for efficient light emission.And,the quntum or dielectric confinement effect of the 2D phases can significantly enhance the stability of excited state or exciton,effectively suppressing the fluorescence quenching by defects.Nonetheless,the defect-related nonradiative recombination of the excited states is commonly the main factor,limiting the further increases of light emission efficiency.Therefore,the main work of this dissertation focuses on the defect passivation and energy transfer related-suppression of nonradative recombination to increase the light emission efficiency.Firstly,the ligand of phenethylammonium bromide is used to suppress the formation of the 3D-type defects such as pin holes or cracks.Secondly,using ionic liquid as passivator,the defect-sites of the perovsite layers are sufficiently passivated by the sponteonlaly formed targated distribution of passivator,which can significantly increase the efficiencies of the perovskite films and the related light-emitting diodes.And then,DMSO（Dimethyl sulfoxide）vapor is used to modulate the crystallization of the 2D perovskite phases.The2D-phase distribution and content are simultaneously improved,which,combing with the DMSO vapor induced defect passivation effect,can effectively suppress the energy transfer losses or nonradiative recombination of the excited states and significantly increase the efficiency of the Pe LEDs.Finally,the defect passivation effect is thoroughly investigated through DMSO vapor induced recrystallization for the 2D perovskite phases.The results are listed as follows:1.Phenethylammonium bromide addition for high luminance and efficiency perovskite light-emitting diodes.The nonradiative losses of excited states in the perovskite films are effectively suppressed by the addition of PEABr,through morphology improvement-induced defect healing,confinement effect,or defect passivation of PEABr.And,the Pe LEDs show a maximum luminance of 61800 cd m^-2and current efficiency of 41.8 cd A^-1 with an emission wavelength of 533 nm.2.Targeted distribution of passivator for Quasi-2D perovskite light-emitting diodes with high efficiency.Comparison of the properties of the 1-butyl-3-methylimidazolium tetrafluoroborate（BMIMBF₄）-,1-butyl-3-methylimidazolium bromide（BMIMBr）-,and formamidinium tetrafluoroborate（FABF₄）-modified perovskite films and the corresponding Pe LEDs unambiguously indicates that the BMIM⁺ions are contributed to the sufficient defect passivation.The PI bond in N-C=N of the BMIM⁺ions has strong passivation interaction to the Pb-related defects,as deduced from the largely red shifted IR-absorption peak of C=N stretching vibration.Meanwhile,the BMIM⁺ions are spontaneously targeted to distribute on the crystal surface and on the film top-surface,resulting in that the Pb-related defects are sufficiently passivated,as measured by the time-of-flight secondary ion mass（TOF-SIMS）depth profiling and the nano-Fourier transfer infrared（nano-FTIR）spectroscopic imaging.Specifically,the signals of passivation interaction nearly fully cover the polycrystalline perovskite layers,intuitively indicating that the Pb-related defects are ubiquitously targeted to be sufficiently passivated by the BMIM⁺ions.Consequently,the BMIMBF₄-modified Pe LEDs show a maximum EQE of 22.9%and a current efficiency of 98 cd A^-1 with an emission wavelength of 529 nm,which stands as one of the best efficiencies for green-emitting Pe LEDs,as well as significantly improved operational stability.3.Suppressed energy transfer loss of Dion-Jacobson perovskite enabled by DMSO vapor treatment for efficient sky-blue light-emitting diodes.DMSO vapor treatment can effectively rearrange the phase distribution with a completely suppressed n=1 phase,moderately decreased n=2 phase,and a significantly formed n=3 phase and effectively increased the light emission phase.As a result of the rearranged phase distribution with matched energy landscape and phase content,the energy transfer gap and the undesired light emission from the small-n phases are effectively eliminated,and in addition the electron-phonon coupling and defect-related nonradiative recombination is also significantly suppressed,which all jointly contribute to the efficient energy transfer and light emission.Consequently,the high efficiency sky-blue DJ Pe LEDs show the record EQEs of 13.7%and 15.5%with the emission peaks at 489 and 494 nm,respectively.4.DMSO vapor treatment can induce the recrystallization of the 2D perovskite phases and further effectively suppress the formation of defect.Furthermore,it can avoid the negative influence of passivator addition on the charge transfer.The luminance and EQE of the Pe LEDs are significantly increased to 3207 cd m^-2 and 18.5%,respectively,with the emission wavelengnth of 504 nm.