Hole Transporting Layer Modulation And Study Of Stability Based On Sky-Blue Quasi-2D Perovskite Light Emitting-Diodes

In the past few years,significant progress has been made in perovskite light-emitting diodes(Pe LED)field.Over 20%external quantum efficiencies have been all achieved in near-infrared,red and green light-emitting devices.However,the maximum efficiency of blue Pe LEDs was only 15.6%to date,which still lags behind the values of green and red Pe LEDs counterparts.Recently,substantial optimization efforts have been devoted to develop high performance blue Pe LEDs,such as dimensional and compositional engineering of perovskites through the strategies by introducing small functional molecular ligands or modulating Cl/Br ratio in precursor.However,few studies focus on the issues occurred at interfaces,which also play a crucial role in improving device efficiency.In a traditional p-i-n Pe LED structure,electron injection is more efficient than hole injection,which results in imbalance of charge carrier injection and hinder the final device efficiency.Therefore,this thesis is dedicated to improve the device performance through the modulation of hole transporting layer(HTL).In chapter II,we firstly compared the performance between PEDOT:PSS and Ni O_x/PVK based HTLs,which shows that Ni O_x/PVK with a cascade-type energy alignment is more suitable for blue Pe LED.Since electron mobility of TPBi is much higher than the hole mobility of PVK in the device structure(ITO/Ni O_x/PVK/Perovskite/TPBi/Li F/Al),resulting in charge imbalance,we then introduced a p-type dopant named CN6-CP with a deep LUMO level,which efficiently doped PVK and increased its hole mobility.However,such deep-LUMO materials like CN6-CP can directly quench excitons,causing poor performance of Pe LEDs.In chapter III,we further studied the Ni O_x/PVK structure and found that the commonly used bi-layer HTL structure suffered from a processing issue that the solvent of perovskite precursor solution could wash away PVK during spin-coating procedure.Through introducing2PACz SAM to form Ni Ox/SAM/PVK HTL(hole transporting layer),we successfully solved this issue via the following aspects:the identical carbazole groups in 2PACz and PVK allow them strongly pack with each other via Van der Waals force,improving adhesion and solvent resistance of the PVK layer;the smooth PVK film could efficiently block the direct contact between Ni O_xand perovskite,suppressing the emission quenching;2PACz could also passivate the surface traps of Ni O_x,which suppress the non-radiative recombination.Finally we successfully demonstrated the high-efficiency blue Pe LED with a EQE over 14.5%,and luminance of 10392 cd/cm~2.In chapter IV,we gave a preliminary study of the stability of quasi-2D perovskite under the condition of light,heat and electric field.We observed a inconsistent stability between single ligand quasi-2D perovskite and double ligand quasi-2D perovskite.Firstly,double ligand quasi-2D perovskite exhibits better stability than single ligand quasi-2D perovskite,which can attribute to the dispersity of quantum well thickness in the perovskite film.And we found that the joule heat generated by electrical current accounts for the major inducement of device degradation.Meanwhile the electric field applied on the device can suppress the drift of emission spectrum and the formation of heterogeneous phase.This research can contribute to the further development of stability research.