Defect Suppression Of Bromide-Based Perovskite Quantum Dots And Its Application In Light Emitting Diodes

Bromide-based perovskite quantum dots（MA/FA/CsPb Br₃ QDs）have become one of the most promising materials for light-emitting diodes（LEDs）because of their low cost,easy film formation,high color purity,high photoluminescence quantum yield and easy spectral regulation.The three primary colors（blue,green and red）can synthesize the any color gamut,which plays an extremely important role in the fields of backlight display.Bromide-based perovskite quantum dots（PQDs）can emit blue and green light under ultraviolet irradiation.However,it is still difficult to synthesize bromide-based PQDs with high luminous efficiency at present.There are a large number of defects in PQDs,seriously deteriorating the optical properties of PQDs,especially blue-emission bromide-based PQDs.In addition,the researchers always focus on the optimization of materials for improving the performance of perovskite light emitting diodes（PeLEDs）,but the device process has not received enough attention.Therefore,the defect suppression strategy of PQDs and the process optimization of PeLEDs are the key factors for realizing high-performance PeLEDs.In this paper,we proposed the liquid nitrogen passivation strategy to obtain high luminous efficiency deep-blue-emission MAPb Br₃ QDs.The double-ligand collaborative passivation strategy is adopted to realize high-quality green-emission CsPb Br₃ QDs.Also,this paper explores a new spin-coating process regarding colloidal PQD_S for enhancing the performance of FAPb Br₃ PeLEDs.The main research contents are as follows:1.In this paper,liquid nitrogen passivation（LNP）strategy is proposed for passivating Br vacancy defect(V_Br)on the surface of deep-blue MAPb Br₃ QDs.The low temperature environment created by liquid nitrogen can quickly slow down the crystal growth rate,significantly reduce the size of MAPb Br₃ QDs and realize the emission light transformation from green to deep-blue.Also,LNP strategy can promote NH₄Br to firmly anchor the surface Br^-and efficiently passivate V_Br.The prepared deep-blue-emission MAPb Br₃ QDs showed high fluorescence quantum yield of 97.64%.It can also maintain 91%of the initial photoluminenscence intensity under 120 min ultraviolet irradiation,showing excellent irradiation stability.Based on the optimized MAPb Br₃ QDs,we successfully developed deep-blue PeLEDs with wavelength of 455 nm.Therefore,this LNP strategy may open a novel way to obtain deep-blue PQD_S with high-quality conversion luminescence,and effectively promotes the commercial development of deep-blue PeLEDs.2.A dual-ligand collabrotive passivation strategy（DCP）is designed for repairing CsPb Br₃ QDs defects.The inorganic ligand K⁺is closely combined with Br^-to inhibit the regeneration of V_Br defects,and the organic ligand DDAB⁺stabilizes the crystal structure of PQDs.DCP strategy can effectively inhibit the non-radiative defects and halogen ion migration on the surface of QDs.This strategy can also stabilize the crystal structure of QDs,improving the radiation recombination efficiency of CsPb Br₃ QDs.Under the effect of the DCP strategy,the fluorescence quantum yield is significantly increased from 56.87%to93.25%.Accordingly,after optimization,the maximum brightness of CsPb Br₃ PeLEDs is increased from 27089 cd/m² to 52568 cd/m²,and the external quantum efficiency（EQE）is increased from 1.92%to 3.45%as well.Therefore,the DCP strategy can well inhibit the defect state of CsPb Br₃ QDs and significantly improve the performance of green CsPb Br₃ PeLEDs.3.This work explores a new process regarding the spin coating of PQDs to improve the performance of FAPb Br₃PeLEDs.We put forward that the performance of PeLEDs made by static spin-coating process is better than those of the PeLEDs made by traditional dynamic spin coating.Firstly,the PLQY of dynamic spin-coated FAPb Br₃ film is only 29.61%,while the PLQY of static FAPb Br₃ film is as high as 75.60%.Secondly,compared with the dynamic spin-coated film,the valence band top of the static spin-coated film is closer to the HOMO level of the hole transport layer,reducing the hole injection barrier and thus improving the electron-hole recombination rate.Finally,in comparision with the performance of the device made by dynamic spin coating,the maximum brightness（20468.48 cd/m²）of devices made by static spin coating is increased 10 times.12.76%EQE_max of the PeLEDs by static spin coating is three times that of PeLEDs by dynamic spin coating.Therefore,we found that the new static spin coating process is more advantageous than the traditional dynamic spin coating process,promoting the development high-performance FAPb Br₃PeLEDs.