| Lead halide perovskite nanocrystals,as a new direct bandgap semiconductor material,are promising for next-generation high-definition display,especially in light of their tunable bandgap,wide color gamut,high color purity,high photoluminescence quantum efficiency,advantageous optoelectronic properties,and solution processability.Within the past few years,the external quantum efficiency of perovskite nanocrystalbased light-emitting diodes has progressed rapidly,reaching the standard for commercial applications.However,the low operational stability of these perovskite nanocrystal-based lightemitting diodes remains a crucial issue for their practical applications.Recent research indicates that due to the ionic surface of perovskite nanocrystals and the low activation energy of their halide ion migration,the defects at their surface and grain boundaries can induce the migration of halide ions under an electric field,which will lead to charge accumulation,interface deterioration,light-emitting layer decomposition,and electrode corrosion during device operation,resulting in the performance degradation of the perovskite nanocrystal-based light-emitting diodes.Therefore,for achieving highly stable perovskite nanocrystal-based light-emitting diodes,resurfacing perovskite nanocrystals via surface engineering to reduce the concentration of surface defects for mitigating halide migration has demonstrated to be a vital strategy.Herein,we take the realization of efficient and stable lead halide perovskite nanocrystal-based light-emitting diode devices as the research goal,focusing on the influence of the surface state of perovskite nanocrystals on ion migration during device operation.We adopted a series of resurfacing strategies to passivate the defects on the surface of perovskite nanocrystals and their grain boundaries to alleviate the halide ion migration in light-emitting diode devices.We utilized the optimized perovskite nanocrystal film as the emissive layer to fabricate a high-performance lightemitting diode device.This thesis mainly includes the following three aspects:1.We proposed the surface passivation strategy with an ion layer,prepared highquality potassium bromide surface passivation mixed-halide perovskite nanocrystals,and realized high brightness pure-red emission perovskite nanocrystals-based lightemitting diode with improved spectral stability.Our synthesized potassium bromide surface passivated CsPbI3-xBrx nanocrystals showed pure-red photoluminescence emission and a photoluminescence quantum yield exceeding 90%.Using synchrotron radiation photoelectron spectroscopy analysis,we proved that most potassium ions exist on the surface of CsPbI3-xBrx nanocrystals and form strong binding with surface bromine ions,which can provide a surface passivation effect,improve the photoluminescence stability of nanocrystal thin films and inhibit the halide separation in mixed-halide perovskite nanocrystal thin films.Simultaneously,compared with the pristine perovskite nanocrystal-based light-emitting diodes,the maximum external quantum efficiency of the pure-red perovskite light-emitting diodes based on potassium bromide passivated CsPbI3-xBrx nanocrystals has increased from 1.89%to 3.55%,and exhibited better electroluminescence stability.This work also indicates that increasing the barrier of halide ion migration through the surface passivation layer can help to inhibit the phase separation,thus alleviating the spectral shift in mixed-halide perovskite nanocrystal-based light-emitting diodes.The proposed strategy will open a new avenue for fabricating efficient,stable,and tunable pure-color perovskite nanocrystal-based light-emitting diodes.2.We developed a weak-confined strategy to prepare perovskite nanocrystals with high internal crystallinity and combined them with the surface ligand passivation strategy to achieve green perovskite nanocrystals-based light-emitting diodes with high color purity and high efficiency.The results indicated that the weak-confined CsPbBr3 nanocrystals have better crystallinity,fewer surface insulating ligands,and fewer surface defects.Moreover,with the increase of particle size,the sensitivity of emission line width to the particle size distribution can be remarkably reduced,therefore,the weak-confined CsPbBr3 nanocrystals can achieve ultrahigh color purity.In addition,the surface modification of weak-confined CsPbBr3 nanocrystals by the complex ligand of calcium bromide and tributyl phosphine oxide further reduced the halide vacancy at the surface and brings better photoelectric performance for CsPbBr3 nanocrystals.The CsPbBr3 nanocrystals obtained by weak-confined strategy and surface modification method show ultra-narrow emission line width,high colloidal dispersion,and high luminescence efficiency.Utilizing these weak-confined CsPbBr3 nanocrystals as emitters,we manifested that the detrimental effects caused by the quantum confinement effect can be avoided in the device,thereby realizing the highest color purity in green perovskite nanocrystal-based light-emitting diodes,with a narrow full width at halfmaximum of 16.4 nm and a high corrected maximum external quantum efficiency of 17.85%.More importantly,the large-size effect can efficiently decrease the total number of CsPbBr3 nanocrystals within a given volume in the emissive layer,which substantially decreases the effective density of the state,resulting in that the devices can be operated at much lower voltage.Therefore,the operating half-lifetime of largesize CsPbBr3 nanocrystals-based light-emitting diodes has been significantly enhanced.Our work provides a new avenue for improving the performance of perovskite nanocrystal-based light-emitting diodes based on unconventional large-size effects.3.We developed a new resurfacing strategy for CsPbBr3 nanocrystals in a nonpolar solution system via pseudo-halogen surface reconstruction,which improved the migration barrier of halide ions,resulting in the achievement of perovskite nanocrystalbased light-emitting diodes with high brightness and high stability.This method avoids the serious problem of halide ion migration in perovskite nanocrystal-based lightemitting diode devices caused by traditional rich halide ion passivation.The residual halide ions from stoichiometrically-excess sources would accumulate at the grain boundary of the perovskite nanocrystal films,which are more likely to result in ion migration-induced degradation of perovskite nanocrystal-based light-emitting diodes.Replacing bromide ions with pseudo-halide ions to resurface the CsPbBr3 nanocrystals,the migration barrier of bromine ions increased.As a result,the thiocyanate passivated CsPbBr3 nanocrystal film exhibited higher stability under electric field conditions than the traditional bromine-rich passivated CsPbBr3 nanocrystal film.The green-emitting light-emitting diode prepared by thiocyanate passivated CsPbBr3 nanocrystal film displayed a narrow emission full width at half maximum of less than 17 nm,a maximum luminance of 48000 cd m-2 and a high external quantum efficiency of 17.3%.More importantly,at the initial brightness of 1000 cd m-2,the operating half-lifetime of thiocyanate passivated CsPbBr3 nanocrystal-based light-emitting diode is nearly four times longer than that of traditional bromine-rich passivated CsPbBr3 nanocrystals.This work provides a new method for improving the stability of perovskite nanocrystalbased light-emitting diodes by using pseudo-halide ions resurface strategy. |
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