Study On Efficiency And Stability For Perovskite QLED

Since the first perovskite quantum dot light-emitting diodes（Pe-QLEDs）shown bright electroluminescence,the surface passivation,doping and component engineering of perovskite quantum dots（Pe-QDs）have been studied extensively.These strategies greatly suppressed the generation of defects in QDs,promoting the efficient exciton radiation recombination in Pe-QLEDs and ultimately achieving the external quantum efficiency（EQE）close to the theoretical upper limit.However,the dynamic bonding of surface ligands for perovskite QDs（Pe-QDs）due to the ionic nature of the perovskite crystal itself and large surface-to-volume ratios of QDs lead surface sites of perovskite QDs being extremely fragile during electroluminescence（EL）.In addition,perovskite also produces serious ion migration under electric field.These factors make the performance of QLEDs rapidly decay during operation.Compared with the well development of efficiency,the stability of perovskite QLEDs faces greater challenges,and even lags behind the development of film-based perovskite LEDs.By understanding the degradation mechanism of Pe-QDs in the EL process,efficient and stable perovskite QLEDs can be prepared,which is of great significance for promoting their commercial applications.This dissertation studied the instability of Pe-QDs caused by the passivation of commonly used halogen-rich ligands,and clarified the performance degradation mechanism of surface bromine-rich QDs in QLED.On this basis,4-dodecylbenzenesulfonic acid（DBSA）was introduced to modulate bromine defects on the surface and interior of QDs,which eliminated the instability of bromine-rich in QDs.In addition,through the interface passivation and structural design of the device,the regenerated interface defects in the QD film were reduced and the carrier injection in the QLEDs was balanced.These strategies promoted the exciton radiation recombination in the QLEDs and significantly improve the efficiency and stability of the device.The main research findings were summarized as follows:（1）Mechanism of rapid performance degradation based on surface bromine-rich quantum dots in QLED.Halogen atoms can perform in-situ passivation of perovskite QD surfaces,greatly reducing surface defects,and thus halogen-rich ligands are widely used in the preparation of efficient Pe-QDs.However,the effect of such ligands on the stability of QDs has been controversial.In this chapter,we found that Pe-QDs passivated by Br^--rich ligand exhibited high efficiency but terrible stability in QLED,and explored the rapid failure mechanism of QDs with Br-rich surface in QLED.Through the characterization of carrier dynamics and elemental analysis by XPS,it was further demonstrated that the Br^--rich surface was more prone to trap hole carriers,resulting in electrochemical reactions that lead to rapid passivation failure.This study elucidated the reason for the performance degradation of Br^--rich ligand-passivated Pe-QDs in QLED,and provided guidance for further screening of efficient and stable ligand-passivation strategies.（2）Tuning the surface/internal bromine defects of QDs enhances the efficiency and stability of perovskite QLED.In view of the instability caused by Br^--rich ligand passivation,4-dodecylphenylcyclic acid（DBSA）was proposed to be used as alternative to passivate the surface of QDs.It was found that introducing an appropriate amount of DBSA into the precursor could not only provide great surface passivation for QDs,but also suppress the generation of bromine defects(bromine interstitial and bromine vacancy defects:Br_i and V_Br)inside the QDs.Among them,Br_i is a shallow-level defect that restricts the movement of carriers in QDs by trapping carrier and reduces the rate of carrier radiation recombination,while V_Br produces severe nonradiative quenching.By modulating surface/internal bromine defects with DBSA,the QDs exhibited faster radiative recombination rates,higher photoluminescence quantum yields（PLQYs）,and larger ion migration activation energy.These advantages effectively reduced the surface degradation,carrier accumulation and ion migration of the QLED during operation,resulting in a maximum EQE of 20.4%and a nearly 14-fold increase in operating lifetime compared to the control device.（3）Interface optimization of quantum dot film enhances the efficiency and stability of perovskite QLED.Aiming at the interface defects generated during the preparation of perovskite QD films,a bilateral interface passivation strategy was proposed to passivate the interface defects and suppress defect regeneration.Passivating the top and bottom interfaces of QD films by organic molecules could effectively reduce the influence of interfacial defects on carrier injection,transport and recombination,thereby drastically improving the efficiency and stability of perovskite QLEDs.The necessity of bilateral interfacial passivation and the universality of this strategy were further verified by comparative experiments with different molecules.Finally,the passivated device achieved a maximum EQE of 18.7%,a current efficiency of 75 cd/A,and an operating lifetime of 15.8 hours.These findings highlighted the importance of interfacial passivation for efficient and stable QLEDs.（4）Device structure design enhances the efficiency and stability of perovskite QLED.To address the unbalanced charge injection caused by the low mobility of the electron transport layer（ETL）and the thin QDs layer in perovskite QLEDs,a structure with bilayered ETLs was designed to enhance electron injection and modulate the carrier balance in the device.This structure controlled the exciton recombination region by adjusting the relative thicknesses of low-mobility 1,3,5-tris（1-phenyl-1H-benzimidazol-2-yl）benzene（TPBi）or Bis[2-（diphenylphosphino）phenyl]ether oxide（DPEPO）and high-mobility 2,4,6-tris[3-（diphenylphosphinyl）phenyl]-1,3,5-triazine（PO-T2T）,suppressing luminescence quenching and QDs degradation caused by charge accumulation.Ultimately,the balanced carrier injection enabled the perovskite QLED to exhibit a sub-bandgap turn-on voltage of 2.0 V with a maximum EQE of 21.63%.In addition,the operational lifetime of QLED had been increased by nearly 20 times than that of the control device,reaching 180.1 hours.These results demonstrated that rational device engineering had the great potential to improve the performance of perovskite QLEDs toward practical applications in lightings and displays.