Modulating Phase Distribution Of Quasi-Two-Dimensional Perovskites For Blue Light-Emitting Diodes

Metal halide perovskite has the attractive characteristics of tunable bandgap,narrow full width at half maxima（FWHM）,high color purity,high carrier mobility and low-cost solution processability,which is a promising semiconductor luminescent material.In recent years,perovskite light-emitting diodes（PeLEDs）have developed very rapidly.The external quantum efficiency（EQE）of red and green PeLEDs has exceeded 20%,however,the development of blue PeLEDs is lagging behind.On the one hand,the bandgap of blue perovskite materials is wide（≥2.5 e V）,and more defects lead to serious nonradiative recombination,resulting in low photoluminescence quantum yield（PLQY）.On the other hand,compared with red and green PeLEDs,the injection barrier of charge at the interface of charge transport layer and emissive layer of blue PeLEDs is larger,resulting in charge accumulation and deterioration of PeLEDs.Quasi-two-dimentional（Q2D）perovskite has a natural quantum well structure,which induces dielectric confinement and quantum confinement effects,enabling it with high exciton binding energy and easily bandgap tunability,so that it has good stability and high PLQY.Therefore,Q2D perovskite is considered to be the best way to achieve efficient and stable blue PeLEDs.However,the existence of mixed phase in Q2D perovskite films leads to energy transfer between perovskites with different n values,which is prone to spectral red-shift.To solve this problem,this thesis starts from two aspects of ligand regulation and additive engineering,and modulates the phase distribution in Q2D perovskite films in order to achieve high-performance blue PeLEDs.The main research contents are as follows:（1）The effect of 2-phenylethylammonium bromide（PEABr）ligand on the properties of Q2D blue perovskite films and devices.First,Q2D-PeLEDs was successfully prepared based on PEA₂MAPb₂Br₇（n=2）,and the influence of hole and electron transport layers on the device performance was systematically studied.By matching the hole and electron transport layers with appropriate HOMO and LUMO levels,respectively,PeLEDs with a maximum luminance of 5362.5cd/m² and a maximum current efficiency of 23.8 cd/A was achieved.The effect of PEABr on the optical and electrical properties of Q2D perovskite films was further studied.It was found that the PL of Q2D perovskite films could be regulated from 507 to 488 nm by using a single PEABr ligand,but at the same time,the carrier transport capacity of Q2D perovskite films decreased sharply.Therefore,the blue PeLEDs performed a poor device performance with the luminance of 19.1 cd/m²and the efficiency of 0.31 cd/A.It was difficult to achieve high-performance blue PeLEDs by using single PEABr ligand.（2）Effect of amphiphilic L-arginine ligand on the phase modulation of Q2D blue perovskite films and application in blue PeLEDs.Firstly,due to the strong interaction between guanidinium group on L-arginine and[Pb Br₆]^4-octahedral layers,the results of PL,UV-vis absorption spectrum and X-ray diffraction（XRD）confirmed that the introduction of L-arginine can promote the formation of low-dimensional perovskite nanosheets.Secondly,it was observed by scanning electron microscopy（SEM）and atomic force microscopy（AFM）that the addition of L-arginine significantly enhanced the formation of low-dimensional perovskite nanosheets in Q2D perovskite films.Thirdly,X-ray photoelectron spectroscopy（XPS）and Fourier infrared spectroscopy（FTIR）results showed that the carboxyl group in L-arginine could passivate the uncoordinated Pb²⁺ions,and the lifetime of time-resolved PL（TRPL）was significantly prolonged.Finally,a blue PeLEDs was successfully constructed by Q2D perovskite films modified by L-arginine,with an electroluminescence peak of 492 nm,a luminance of 2152 cd/m²,an EQE of5.4%,and T₅₀ lifetime of 13.81 min.（3）Effect of imidazolium based ionic liquids on Q2D blue perovskite films and blue PeLEDs.Firstly,the effect of 1-benzyl-3-methylimidazolium chloride（BZMIMCl）,1-butyl-3-methylimidazolium chloride（BMIMCl）and 1-carboxymethyl-3-methylimidazolium chloride（HOOCMIMCl）on the optical properties of PEA₂MAPb₂Br₇（n=2）perovskite films was studied.It was found that the PL of Q2D perovskite films can be blue-shifted by～14 nm and the XRD peaks shifted to higher angle after the addition of the ionic liquids.It was confirmed that more chloride ions entered in the perovskite lattice,resulting in the lattice shrinkage and bandgap widening.The phase of n=1 was found to decrease significantly in UV-vis absorption spectrum.TRPL test results showed that the ionic liquids had defect passivation effect.Finally,PeLEDs were fabricated based on three perovskite films modified with chlorimidazole ionic liquids.The EL peaks of BZMIMCl,BMIMCl and HOOCMIMCl modified PeLEDs were all located at 492 nm,their maximum luminances were 548.1,592.7,2116.9 cd/m²,maximum current efficiencies were 9.3,1.6,12.0 cd/A and 10.3,1.8,8.7 min for T₅₀,respectively.The results of XPS and FTIR showed that the carboxyl groups in HOOCMIMCl had remarkable passivation effect,and the corresponding PeLEDs had the best performance.