Interface Modulation Of Perovskite Light Emitting Diodes And Their Applications In Flexible Devices

The metal halide perovskite material has the advantages of high carrier mobility,narrow luminescence peak,tunable band gap,solution processability,and strong luminescence.It is expected to be an ideal emission layer material of the next generation display technology.However,although the efficiency of perovskite light-emitting diodes(LEDs)has been improved in recent years and approaching that of organic light-emitting diodes(OLEDs),there are still some issues that need to be solved for potential commercial application,such as poor stability of perovskite materials and low repeatability.This thesis is mainly focus on optimizing the perovskite materials and device interface of perovskite LEDs,developing a novel flexible transparent conductive substrate,and fabricating perovskite LEDs on the flexible substrate.The main research contents are followed:1.Developing a method to passivate defect states on the surface of the all-inorganic perovskite emission layer.Compared with conventional organic-inorganic hybrid perovskite materials,all-inorganic perovskite materials have improved thermal stability.However,all-inorganic perovskites still have many problems,such as poor crystallinity,high density of defect states,and sensitivity to humidity,limiting its further applications.The existence of defect states causes the injected carriers to be confined in the defect trap,free excitons rapidly decay,causing serious non-radiative recombination,and ultimately reducing the device's efficiency.In order to reduce the defect density of the perovskite thin film and increase the film coverage,we fabricated an atomic layer deposition(ALD)processed alumina layer on the surface of all-inorganic perovskite to passivate the defects on perovskite surface and increase the film coverage.The performance of the all-inorganic perovskite LED device based on CsPbBr3 is greatly improved.2.Building a method to assure the quality of organic-inorganic LED film on HTL.At present,a lot of research works are concentrated on organic-inorganic hybrid perovskite LEDs.However,in the process of preparing organic-inorganic hybrid perovskite LEDs by the solution-process method,there are some problems caused by the solvent of next deposited layer:single-layer or multi-layer thin films that have been deposited on the device substrate may be redissolved by the solvent of the next layer,resulting in changes in the surface morphology of the film and the formation of pinholes.This has a bad effect on the film formation of perovskite and has great restrictions on device performance.By depositing an ultra-thin alumina layer between the hole transport layer(HTL)and the perovskite emission layer(EML)using an ALD method,the above problems are solved.The efficiency and repeatability of devices are improved.In addition,the surface tension of the substrate becomes larger after the addition of the alumina layer,making the perovskite precursor solution easier to infiltrate on the HTL,improving the surface morphology and crystallization of the perovskite film,resulting in high-quality perovskite film with less defects.In addition,the alumina layer can reduce the hole transport rate in the perovskite LED and form a balanced electron and hole injection to reduce non-radiative recombination caused by the accumulation of holes.We have conducted optical simulations for LED devices with alumina layer.The simulation results show that the alumina layer has an appropriate optical refractive index,which improves the outcoupling efficiency of the light emission.Based on the above improvements,we have obtained a high-efficiency green perovskite LED with an external quantum efficiency(EQE)of 17.0%.3.Fabricating organic-inorganic perovskite light-emitting device based on a novel flexible transparent conductive substrate.With the rapid development and popularization of smart phones and tablet computers,people's requirements for display devices are also constantly increasing,which promotes the rapid development of a variety of flexible display devices.However,the bending resistance of traditional indium tin oxide(ITO)transparent conductive substrate is poor,so it is important to develop flexible transparent conductive electrode materials with low square resistance and good bending resistance.The metal microgrid based on micro-nano imprinting technology is a kind of flexible transparent conductive electrode,which has the advantages of low square resistance and high light transmittance.However,the flexibility of the existing silver-based embedded metal mesh still needs to be improved,and the resistance greatly increases after multiple bends.In order to improve the flexibility of the silver-based embedded metal mesh,we made the silver-indium gallium conductive paste by mixing the liquid metal indium gallium alloy and nano silver particles uniformly.By filling the silver-indium gallium conductive paste in the mesh on the polyethylene terephthalate(PET),we fabricate a flexible transparent conductive electrode with good bending resistance.This flexible transparent electrode has a square resistance of 1.87 ?/? and a transmittance of about 90%,which is comparable to that of ITO.Because the liquid metal plays a role of flexible welding between the silver nanoparticles,the increase of resistance caused by the cracking of the silver nanoparticles during the bending process is avoided,and the stability of the resistance of the electrode after multiple bendings is improved.In order to prepare high-efficiency perovskite LEDs,we used ALD to deposit a thin alumina interface layer,which effectively suppressed the leakage current,improved the uniformity of the current distribution in the device,and fabricated flexible perovskite LED with current efficiency(CE)of 0.5%.