Defect Passivation For Mixed-halide Perovskites And Their Application In Blue Light-emitting Diodes

Because of the advantages of adjustable band gap,high photoluminescence quantum yield（PLQY）,high carrier mobility,simple preparation process and low cost,metal halide perovskite materials have developed rapidly in the field of light-emitting diodes（LEDs）.At present,the maximum external quantum efficiency（EQEs）of green and red perovskite LEDs（PeLEDs）has already exceeded 20%,while the EQEs of blue PeLEDs,one of the three primary colors,is lower.Using bromine/chlorine mixed-halide perovskite is an effective method to regulate the emission color of perovskites,which plays a crucial role in the realization of blue PeLEDs.However,for the mixed-halide perovskites,halogen ion migration occurs under the electric field,resulting in halogen phase segregation and spectral instability,which seriously deteriorates the efficiency and stability of blue PeLEDs.Therefore,it is very important to suppress the formation of defects in mixed-halide perovskite films.This thesis carries out the following research from the perspective of defect passivation:（1）Polyvinylidene chloride（PVDC）improved the stability of blue perovskite films.Firstly,the effect of Cl/Br ratio on the emission color and Photoluminescence（PL）spectra in mixed-halide perovskites was studied.Blue perovskite films with emission peak at 480 nm were prepared based on PEA_0.5MAPb Br_2.14Cl_0.86.Secondly,the effects of the concentration of perovskite precursor solution and annealing temperature on the PL properties of perovskite films were systematically studied.Furthermore,the hydrophobic chlorine-containing polymer PVDC was introduced,which significantly improved the stability of blue perovskite films.Finally,blue PeLEDs were fabricated based on PEA_0.5MAPb Br_2.14Cl_0.86 films modified by PVDC.However,due to the intrinsic insulation property of PVDC,the maximum brightness of the blue device was 532.8 cd/m² and the current density was only 0.57 cd/A.（2）Trichloroacetic acid（TCA）additive improved the spectral stability and luminous efficiency of blue PeLEDs.Here,a new additive TCA is proposed,which has the ability of ion exchange with bromine ions in the perovskite precursor solution.Meanwhile,the carboxyl groups can passivate the uncoordinated Pb²⁺defects to improve the stability.The results showed that:1)With the increase of TCA content,PL spectra blue-shifted from 483 nm to 477 nm,and X-ray diffraction（XRD）peaks shifted to higher angles,indicating that chlorine content increased after the addition of TCA,resulting in the lattice shrinkage and emission peak blue-shift.2)The PLQY of the perovskite films with TCA increased from 2.5%to 8.1%,the carrier lifetime increased from 8.25 ns to 11.64 ns,and the binding energy of Pb core-level XPS spectra moved towards to higher values,which proved that TCA could passivate the film by binding with uncoordinated Pb²⁺.Thus,the TCA modified perovskite film stability was greatly improved.3)The TEM and AFM morphological characterizations confirmed that the addition of TCA promoted the crystallization of perovskites,leading to more two-dimensional nanosheets,which could improve the device performance.4)By introducing TCA into PeLEDs based on PEA_0.5MAPb Br_2.28Cl_0.72 and PEA_0.71Cs_1.5Pb Br_2.29Cl_0.71 light-emitting layers,it was found that the device performance was significantly improved,especially in the PEA_0.71Cs_1.5Pb Br_2.29Cl_0.71 PeLEDs.The brightness increased from 798.4 to 3440.4 cd/m²,current efficiency increased from 1.59 to 8.00 cd/A,EQE increased from 1.24%to 6.06%,device lifetime increased by 4.3 times,and the EL spectrum stability was improved under various driving bias.（3）Triphenylphosphine（TPP）additives regulated emission color and improved the stability of blue PeLEDs.As cationic surfactants,TPP materials can homogenize the halogen distribution in perovskite precursor solutions.Meanwhile,chloride ions can exchange with bromine ions,which can simultaneously inhibit the phase separation of mixed-halide perovskites and provide additional chloride ions to regulation emission color.In this work,the effects of（methoxymethyl）triphenylphosphoruschloride（TPPOCl）and（4-chlorobenzyl）triphenylphosphonium chloride（TPPCl）on the properties of blue PeLEDs were systematic studied.The results showed that:1)After adding additives of TPPOCl and TPPCl,PL spectra showed blue shift,and XRD diffraction peaks moved to higher angle,indicating that free chloride ions in the additives had ion exchange with bromine ions in perovskites,resulting in lattice shrinkage.2)The film stability of PEA_0.71Cs_1.5Pb Br_2.29Cl_0.71 films under the conditions of the room atmosphere and ultraviolet（UV）irradiation was significantly improved after adding additive,and the phase separation was effectively suppressed.3)In terms of device performance,after adding TPPOCl,the brightness of blue PeLEDs increased from 849.8 to 3063.6 cd/m²,EQE increased from 1.35%to3.81%,the red-shift of EL spectrum under various bias decreased from 12 nm to 0 nm,and device lifetime increased by about 3.7 times.When TPPCl was added,the brightness of blue PeLEDs increased from 779.7 to 3331.7 cd/m²,EQE increased from 0.79%to 4.81%,the red-shift of EL spectrum under various bias decreases from 8 nm to 4 nm,and the device lifetime increased by about3.8 times.In short,the TPP additive is a kind of facile and efficient additives to improve the stability of mixed-halide perovskites and regulate the emission color,which has a great application prospect in blue PeLEDs.