Large-area And Efficient Perovskite Light-emitting Diodes Via Blade-coating

Metal halide perovskites(MHPs)have been intensively studied in recent years and considered as the ideal materials for next-general semiconductors such as solar cells,photodetectors,and light-emitting diodes(LEDs),due to their unique optoelectronic characteristics including low trap density,high absorption coefficients,tunable bandgap,long carrier diffusion lengths.Benefitting from substantial progress in ligand engineering,energy transfer enhancement,trap passivation,light extraction improvement,and carrier injection balance management,the external quantum efficiency(EQE)of the perovskite light-emitting diodes(PeLEDs)with near-infrared,red,and green electroluminescence(EL)emissions has exceeded 20%,and the EQE of blue PeLEDs has exceeded 10%,which shows an important step toward the flat-panel lighting and full-color display.However,these high-performance PeLEDs are fabricated by a spin-coating method and the effective areas are about a few square millimeters,limiting their application in display and flat-panel lighting.Therefore,the fabrication of large-area and efficient PeLEDs using mass-production techniques such as spray-coating,inkjet printing,vacuum deposition,slot-die coating,and blade-coating is an urgent problem to be solved.Considering the low-cost,flexible operation,and high material utilization of blade-coating,we seek to fabricate the large-area and efficient PeLEDs via bladecoating.Nevertheless,perovskite films made by blade-coating are usually very rough and not continuous due to the coffee ring effect and fast crystallization of perovskites.The EQE of PeLEDs made by blade-coating reached only 1.1%with the device area of 0.04 cm2,much lower than the devices fabricated by spin-coating.This thesis aims to optimize the parameters of blade-coating and regulate the crystallization process to fabricate uniform perovskite films with small grains,served as emissive layers of largearea and efficient PeLEDs.The main research contents of this thesis are as follows:1.We choose the methylammonium lead iodide(CH3NH3PbI3,MAPbI3)as the research object and firstly optimize the experimental parameters including bladecoating speed,blade-coating temperature and N2 knife pressure to fabricate uniform perovskite films.We find that the crystallization process of MAPbI3 films can be effectively modified by changing the precursor concentration and incorporation of excess 4-fluorophenylmethylammonium iodide(FPMAI).The diluted precursor with excess FPMAI results in much denser nucleation centers,a slower/eliminated gelation process,and a faster phase transformation process.As a result,the roughness of perovskite films decreases dramatically from 394.7 nm to 0.8 nm,and the grain sizes are only about 10 nm.2.To manifest the robustness of our blade-coating process,large-area perovskite films(6 cm×9 cm)are prepared using blade-coating and spin-coating,respectively.Through experimental comparison,the blade-coated films show better uniformity in terms of thickness,roughness,and optoelectronic properties than spin-coated films.After incorporating the uniform film as an emissive layer,the EQE of PeLEDs reaches 16.1%with an area of 0.04 cm2 and 12.7%with an area of 1 cm2.The red PeLEDs with an ultra-large area of 28 cm2 and uniform emission are also demonstrated.Furthermore,the cost of the emissive layer reaches only 0.02 cents per cm2,demonstrating its huge potential in real applications of flat panel lighting and display.3.Based on the above research,we adjust the CsPb(Br0.84Cl0.16)3 precursor solubility by partially replacing dimethyl sulfoxide(DMSO)with dimethylformamide(DMF)to obtain a more volatile supersaturated solution.In contrast to unsaturated precursors,the nucleation starts from the solution phase instead of the gas-solution interface when blade coating this supersaturated solution,resulting in a high density of nucleus and uniform perovskite films with small grains.Benefiting from the better film morphology,the trap density decreases,and trap-assisted nonradiative recombination is suppressed.Meanwhile,bimolecular radiative recombination becomes more efficient as the crystal size decreases.By this strategy,we realize the sky-blue PeLEDs with a peak EQE of 10.3%for the device area of 0.04 cm2.Moreover,the large-area(28 cm2)sky-blue PeLEDs with uniform emission are also demonstrated.4.The most commonly used hole transport layers(HTLs)in PeLEDs are semiconducting polymers,with corresponding hole mobilities in the range of 10-4-10-6 cm2 V-1 s-1.The low hole mobilities induce the carrier accumulation at the HTLs/perovskite interface under a high injection current density,resulting in severe nonradiative recombination,efficiency roll-off,and decrease in luminance intensity.To solve this problem,we successfully deposit MAPbI3 on top of methylammonium lead chloride(MAPbCl3)using orthogonal solvent to form heterojunction PeLEDs.Due to the high hole mobility of MAPbCl3 around 42 cm2 V-1 s-1,the heterojunction PeLEDs exhibit a peak external quantum efficiency of 10.7%and a high radiance of 157 W sr-1 m-2 at a low applied voltage of 3.9 V.Furthermore,the large-area(28 cm2)heterojunction PeLEDs made by blade coating show a uniform and high radiance of around 180 W sr-1 m-2.Our work provides a novel strategy to fabricate large-area and high-radiance PeLEDs to meet the requirements of commercial applications.