Research On Perovskites Defect Regulation For Efficient And Stable Light-emitting Diodes

In recent years,halide perovskite has shown its unique advantages in the field of light-emitting diodes（LEDs）,due to its excellent optoelectronic properties including low cost,high luminous efficiency,tunable luminescence spectra,and et al.The development of perovskite LEDs（Pe LEDs）has made great breakthrough and the LEDs have achieved good performance in just a few years.Pe LEDs were hopefully to be used in new generation of high-definition display and change the backward situation of China’s display industry.But there is still a long way to go before the Pe LEDs can be applied in industrialization,and both high efficient and good working stability are the focus of current research in Pe LEDs.As the emitting layer in the sandwich structure of LED device,perovskite film must have high luminous efficiency and strong stability to satisfy the Pe LEDs development needs of efficiency and stability.However,the structure defects in polycrystalline films and the intrinsic instability of perovskite can cause significant problems in radiative recombination for efficient luminance.The erosion of perovskite by water/oxygen in air will destroy its crystal structure,resulting in more non-radiative recombination defect states,and some kinds of defects can become the ion-migration channels in perovskites,futher accelerating the aging of perovskites.And these multiple destabilizing factors are related to each other and jointly affect the performance of perovskite films and LED devices.Consequently,how to effectively regulate the crystal defects to improve the luminance efficiency and stability of perovskites is the diffilculty to be overcame for the development of LEDs,and the investigation of coordinated ligands and preparation technology are the major breakthrough to improve the quality and stability of perovskites.In this dissertation,the halide perovskite film is taken as the main research,and the controllable adjustment of perovskite phase and effective passivation of defects through ligand regulation and process optimization can effectively improve the luminance efficiency and stability of perovskite films.The mechanism of defect regulation and the dynamic mechanism of carrier radiation recombination,as well as the relationship between ligand engineering and optoelectronic properties are studied in depth,so as to improve the efficiency of carrier radiation recombination in perovskite layer and the efficient and stable LEDs are constructed.The main researches are as follows:（1）Construction of stable and efficient red LEDs by（α-δ）-CsPbI₃ phase engineering.The CsPbI₃-based red Pe LEDs suffer low external quantum efficiency（EQE）and poor device stability due to the spontaneous phase transition from cubic CsPbI₃（α-CsPbI₃）to nonradiative orthorhombic phase（δ-CsPbI₃）under ambient conditions.2-（naphthalene-1-yl）ethanamine（NEA）is incorporated in the CsPbI₃ perovskite system,and a lower formation energy ofα-CsPbI₃ is observed.Meanwhile,compact coverage of hydrophobic NEA on theα-CsPbI₃surface can also significantly boost the stability and optoelectronic properties ofα-CsPbI₃.The successful achievement of stableα-CsPbI₃ promises the effective radiative recombination center for red band emission.As a result,a high EQE of 8.65%is successfully achieved for the characteristic red emission at≈682 nm representing the highest value among Cs-based red Pe LEDs up to now.More importantly,the corresponding Pe LEDs exhibit outstanding stability with EQE retaining 90%after 3 months of storage.（2）Novel lewis base cyclam self-passivation of perovskites for efficient LEDs.High density of defects would produce in solution processing inevitably as perovskite film formed,leading to poor exciton recombination efficiency in LEDs with low EQE.1,4,8,11-tetraazacyclotetradecane（cyclam）is introduced in perovskite precursor solution and it spontaneously passivates defect states of Cs Pb Br₃-based perovskite by coaction between amine and uncoordinated lead ions during spin-coating without anti-solvent process.Photophysical studies unraveled that cyclam can significantly reduce the defect states,which is facilitated charge transportation and recombination.The proposed ligand passivation boosts the EQE from1.25%to 16.24%.Further,defects passivation is also conductive to reduce LED degradation paths and improve device stability as the extrapolated lifetime(T₅₀)of LEDs with an initial brightness of 100 cd/m² gain a 140-fold enhancement from 0.9 h to 127 h.（3）Crosslinking-passivator blocking ion-migration for improving stability of LEDs.Perovskite layers in LEDs suffer from much strong electric field due to the thin thickness and large operating voltage,and hence serious ion migration,that impeds charge injection,transport,and recombination,and results in sharp decay of LEDs performance.The continuous network polymer based on methylane-bia-acrylamide（MBA）monomer with alkeny group could coordinate with lead and passivate the bromide vacancy defects,which effectively reduce the instability headstreams from defects.Meanwhile,the crosslinked network anchor to grain boundaries would suppress the ion-migration,evidenced by the increased activation energy（E_a）and weakened electric hysteresis.Consequently,an excellent performance with a peak EQE of16.8%and maximum luminance of 45306 cd/m² are achieved.The Pe LEDs exhibit a 100-fold enhancement in operational stability with a half lifetime T₅₀of 208 h at an initial brightness of100 cd/m² compared to the control device of 2.1 h,which is extraordinary for high-performance green perovskite LEDs.EQE of Pe LEDs can keep above 14%as it is stored in glove box for more than 3 months.（4）Defects regulation by heating-assistance vacuum deposition for multi-color LEDs construction.The vacuum deposition process possesses advantages and competitiveness in the industrialized production.However,the performance of light emitting diodes（LEDs）based on vacuum-deposited is incredibly low.A heating-assisted vacuum deposition（HAVD）method was proposed to construct inorganic perovskite LEDs（Pe LEDs）.The roughness and crystallinity of perovskite film were improved by regulating the heating treatment of substrates.And the perovskite film exhibited largely rise in luminescence,with decreased defect density.Consequently,with the optimized temperature,the green Pe LEDs exhibited 100-fold improvement of external quantum efficiency（EQE）with the luminance of up to 11941 cd/m².At the same time,both red and blue Pe LEDs also exhibited more than 100-fold enhancement in EQE by HAVD method.All red,green and blue light exhibit excellent high color purity in the display applications.