Inkjet-Printed Quasi-Two-Dimensional Perovskite Light-Emitting Diodes

Metal halide perovskite materials have attracted much attention because of excellent photoelectronic properties such as high photoluminescence efficiency,narrow emission spectrum and effective wavelength tunability.The external quantum efficiency（EQE）of red and green perovskite light-emitting diodes（PeLEDs）has both exceeded 25%,presenting an attractive prospect in the field of full-color displays.However,perovskite films are mainly prepared by spin-coating at present,which is difficult to meet the display requirements of colorization,large-area fabrication and patterning.As an intelligent additive manufacturing technology,inkjet printing is very promising in realizing flexible,large-area,low-cost display devices.Whereas,the problems of the printed perovskite films,including uneven thickness,rough morphology,wide phase distribution and low-resolution patterning,limit the application of perovskite in the display field.Aimed at the above problems,in this dissertation,research on high-quality pixel films is carried out in terms of material component design,device structure optimization,drying method development as well as new printing process research and development,to provide theoretical and practical guidance to promote the high-efficiency and low-cost perovskite display applications via inkjet printing.（1）The drying mode of the perovskite ink droplet and the effects of organic ligands on the morphology,phase distribution as well as photoluminescence quantum efficiency（PLQY）of perovskite pixel films are mainly studied.In this dissertation,a vacuum rapid drying process is proposed to prepare perovskite pixel films with uniform thickness,flat morphology and high PLQY.The study showed that high vacuum enables rapid evaporation of solvents.On the one hand,the perovskite ink oversaturates quickly and promotes the formation of many crystal nuclei,refining the grains and reducing the surface roughness of the film.On the other hand,the span of capillary outflow is significantly shortened to inhibit the“coffee-ring”effect.From the perspective of ligand selection,it was found that ammonium bromide（PBABr）has small intermolecular interaction energy and low self-aggregation degree,benefitting the preparation of blue light perovskite films with narrow phase distribution and high energy transfer efficiency,which is attributed to the fact that PBABr is not conducive to the formation of n=1 phases and restricts the growth of perovskite phases in the direction of high n-values,thereby inhibiting phase separation.The red,green and blue monochrome devices prepared by inkjet printing exhibited maximum EQE(EQE_max)of 3.5%,3.4%and 1.0%,respectively.Combined with the liquid phase co-drying method,the 120 PPI full-color perovskite electroluminescence device was realized for the first time,which laid the groundwork for the subsequent development of full-color perovskite display.（2）The problems of poor wettability of perovskite ink on the hole transport layer（HTL）and the poor pixel film quality on the HTL were studied.Conventional O₂Plasma treatment for surface energy enhancement damages the surface of the HTL and results in low-quality perovskite pixel films.In this dissertation,PFN-Br is introduced as an interfacial layer to modify HTL for the first time.As a result,a high-efficiency perovskite pixel film with dense-packed grains and a flat-smooth surface was prepared,which contributed to the widely improved performance of red,green and blue perovskite devices.Not only does the polar quaternary ammonium ion in PFN-Br effectively increase the surface energy of HTL,but it can also generate electrostatic interaction with the[PbBr₆]^4-in the ink,inducing nucleation and fixing perovskite grains.Besides,Diphenylphosphonic anhydride（DPA）can reduce the low-n phase and eliminate the unbonded Pb²⁺in the quasi-two-dimensional perovskite films as an additive.Finally,a 120 PPI efficient green PeLED was obtained,whose EQE_maxand maximum luminescence were 10.1%and 12882 cd/m²,respectively.（3）The effects of metal ions on the crystallization path of quasi-two-dimensional perovskite were systematically studied.SrBr₂can effectively dissociate into Sr²⁺in DMSO and be preferentially adsorbed to[PbBr₆]^4-surroundings through strong electrostatic interaction,which increases the formation energy of perovskite,resulting in slow and uniform growth of quasi-two-dimensional perovskite.Finally,a quasi-two-dimensional perovskite film without low n-value phases but with narrow phase distribution and low defect density was obtained,bringing the EQE_maxof green PeLED up to 13.9%.Benefitting from the compatibility of inkjet printing technology,large area（10 cm²）PeLED and flexible PeLED with EQE_maxof 9.3%and 8.6%respectively were obtained.Therefore,employing metal ions to control the phase distribution strategy may provide a new way to the preparation of high-efficiency PeLED via inkjet printing technology.（4）A new inkjet printing solvent etching（JP-SE）technology is proposed to produce high-resolution LEDs,which can be applied to PeLED,QLED and OLED as well.An insulating PS layer was coated on the surface of a spin-coated luminescent layer,where electrohydrodynamic printing technology was employed to print the 2-heptanone for the dissolution of PS and the formation of homogeneous micropores benefiting from the“coffee-ring”effect.On the one hand,this IJP-SE technology takes full advantage of the luminescent films with high PL efficiency and smooth morphology prepared by spin coating as the pixel luminescent layer;on the other hand,unetched PS film subtly prevents large-area luminescence and large current leakage.As a result,high-resolution PeLEDs,QLEDs,and OLED with pixel sizes of 2 and 10μm were achieved,showing decent peak EQEs of 17.5%（10μm）and 13.3%（2μm）for PeLEDs,18.5%（10μm）and 10.7%（2μm）for QLEDs,and2.1%（10μm）for OLED.The results show that JP-SE technology provides a new strategy for the development of high-performance and high-resolution LEDs.