Highly Efficiency Light-Emitting Diode Based On Perovskites

Exploring high efficiency,low power cost,low cost semiconductor light emitting technology is an effective way to alleviate energy crisis.Perovskite materials have received great attention for their excellent optoelectronic properties,which are expected to be applied in the next-generation light emitting technologies.Since the first report of perovskite electroluminescent devices in 2014,the research in the perovskite light-emitting field has developed rapidly.The highest EQE reported so far has exceeded20%,but the synergistic improvement of device efficiency and stability are an unresolved issue.At present,the main reason for the poor stability of the perovskite LEDs is that the perovskite material is unstable,which is susceptible to temperature,humidity,oxygen,electric field and so on.The regulation of the perovskite material dimensional can effectively enhance the stability.Therefore,the research on the perovskite ligand engineering is an effective way to achieve synergistic improvement of the device efficiency and stability.This thesis focuses on the study of ligand engineering of perovskite materials.By studying the role of different ligands in perovskite materials,high-fluorescence quantum efficiency and high-stability perovskite films are designed to prepare high performance perovskite LED.The main work are stated as following:（1）Highly stable quasi-2D inorganic CsPbBr3 perovskite LEDsCompared with organic-inorganic hybrid perovskite materials,all inorganic perovskites show better stability.However,the problem is how to prepare high-performance inorganic perovskite LEDs.In this thesis,we firstly added phenethylamine（PEA）molecules to prepare low dimensional PEA₂Cs_n-1PbnBr_3n+1 perovskite.We then inserted inorganic crystalline Cs₄PbBr₆ to separate PEA₂Cs_n-1Pb_nBr_3n+1 perovskite.LEDs based on this composite material bring one order of magnitude improvement of maximum external quantum efficiency（EQE）and luminescence intensity compared with the control device based on entirely low dimensional PEA₂Cs_n-1Pb_nBr_3n+1.Meanwhile,the performance degradation under constant current is much reduced as the insertion of the inorganic crystalline walls.The steady device operation time is four times longer than the control device based on PEA₂Cs_n-1Pb_n Br_3n+1 only structure.（2）Highly stable organic-inorganic hybrid perovskite LEDs based on Dion-Jacobson structureIn the perovskite solar cell research field,the Dion-Jacobson structure perovskite shows better device stability,but it has not been reported in the light-emitting field.In this thesis,we performed density functional theory（DFT）simulation to calculate the decomposition energy of quasi-2D structural perovskite and observed that DJ structure has two times higher decomposition energy than typical quasi-2D Ruddlesden-Popper structure.Then a Dion-Jacobson structural perovskite based LED using bidentate organic molecules 1,4-Bis（aminomethyl）benzene（BAB）as bridging molecules was fabricated.We tracked the stability of the device at the point of the highest external quantum efficiency（EQE）and observed T₅₀ lifetime up to 100 hours,which is two orders of magnitude longer than that based on the quasi-2D Ruddlesden-Popper（RP）structure.Furthermore,no obvious change of the wavelength of the film was found after100 hours of continuous operation.We characterized the perovskite structure after continuous operation using transient absorption measurement:no change of the DJ structure was found,in contrast to the obvious transformation of RP structure.In addition,an energy level landscape engineered quasi-2D structure was constructed by manipulating the composition of the film,bringing an external quantum efficiency of over 5.2%.This work raises the expectancy for developing highly stable and efficient perovskite LEDs.（3）Low-temperature,amorphous SnOx ETL for high performance perovskite optoelectronic devicesThe transporting layer material is the basis for the research of perovskite optoelectronic devices,the efficiency and stability of the device are closely related to the transporting layer materials.In this thesis,a novel intermediate synthesis strategy was developed for the preparation of amorphous SnOx electron transport layer.By optimizing the solvent and reaction conditions,we obtained an amorphous electron transporting layer with perfect morphology and excellent electrical properties.Based on the novel amorphous SnOx electron transport layer,we fabricated perovskite solar cell and LEDs.For the solar cell,due to the good carrier transport properties of amorphous SnOx,we obtained maximum PCE of 18.7%and 20.4%with MAPbI₃ and FA_0.85MA_0.15PbI₃ as the active layer,respectively.More important,the device can still maintain 85%of its initial efficiency after 4800 hours of continuous illumination by fluorescent lamps.For the LED device,we use layered PEA₂MA_n-1Pb_n I_3n+1 perovskite as the light-emitting layer to obtain a maximum EQE of 3.05%.The preparation of optoelectronic devices makes us more convinced that the amorphous SnOx prepared by the new method has great research prospects,which provides a new idea for low temperature preparation of electron transporting layers.（4）Highly efficiency perovskite QLEDs based on surface ligand engineeringThe surface defects and internal lattice defects of perovskite quantum dots pose great obstacles for us to prepare high-performance LEDs.In addition,the poor air stability brings many inconveniences for our applications.Through the study of surface ligands and passivation treatments,it is found that the photoelectric properties and stability of perovskite quantum dots can be greatly improved by ligand exchange.In this thesis,we first studied the effect of bidentate ligand IDA on the surface of perovskite quantum dots,and then prepared a perovskite QLED based on the ligand-exchanged quantum dots,IDA ligand-treated QLED shown more efficient exciton radiation recombination,and the efficiency of the device has increased by more than200%.Based on this work,we further studied the effect of defects of perovskite quantum dots on the device performance.By introducing a halogen-containing ligand OAMCl to post-treat quantum dots,combined with other suitable device structure design strategy,we obtained high-efficiency perovskite QLED with red light device exceeding 9%and blue light device exceeding 2.2%.This work aims to clarify that the quantum dot surface passivation scheme is an effective way to improve the performance of perovskite light-emitting devices.