Highly Efficient And Stable Organic-Inorganic Hybrid Perovskite Quantum Dots And Their Light-Emitting Devices Based On Ligand Passivation

Over the past decade,a remarkable progress has been achieved for perovskite solar cells,which power conversion efficiencies have been increased rapidly from 3.8%to 24.2%.It should be noted that their unique optical properties make them exhibiting not only excellent performance in the field of photovoltaics,but also great potentials in the field of displays and lighting.Especially,perovskite quantum dots?PQD?have much improved optical properties,such as high photoluminescence quantum yield?PLQY?,high color purity,obvious quantum confinement effect and large exciton binding energy.Therefore,PQD have attracted intense interest in the field of luminescence.Nevertheless,surface defects are considered as one of major problems to hinder the applications of PQD.Because of surface defects,the photoluminescence quantum yield of PQD significantly decreased due to non-radiative recombination.Furthermore,surface defects also induce aggregation and poor stability.As a result,the performance of devices with PQD as an electroluminescent layer is low.It has potential to improve their performance from several aspects such as device fabrication process and structure optimization.On the basis of above problems,this thesis has improved the performance of MAPbBr₃ quantum dot light-emitting devices according to the synthesis,ligand passivation,device structures and electron transport layer optimization.?1?Synthesis method is improved to increase quantum dots PLQY:A room temperature ligand-assisted reprecipitation method is used to synthesize MAPbBr₃ quantum dots.Large particles were removed after centrifugation,and their supernatant was used for subsequent characterization and device fabrication.By optimizing the ratio of CH₃NH₃Br:PbBr₂ and concentration in precursor solution,the surface defects of quantum dots were reduced with a longer photoluminescence lifetime,and a green organic-inorganic hybrid perovskite quantum dots solution with high PLQY was obtained.Based on these,the perovskite quantum dot light-emitting diodes?PQD-LED?with device structure:ITO/PEDOT:PSS/PVK/MAPbBr₃ QDs/TPBi/LiF/Al were prepared.Their maximum luminance and current efficiency were 845 cd m^-2 and 2.21 cd A^-1,respectively.?2?Trioctylphosphine oxide?TOPO?ligands were introduced to enhance quantum dot stability and luminous efficiency:In order to improve the optical properties and stability of MAPbBr₃ quantum dots,TOPO was employed to passivate MAPbBr₃ quantum dots.Various concentrations of TOPO solution ware added into toluene which is as an anti-solvent to synthesize the MAPbBr₃ quantum dots.The experimental results showed that the optimized volume of TOPO was 100?L.The photoluminescence lifetime of TOPO passivated MAPbBr₃ quantum dots was7.56 ns,which is 65.8%higher than the unpassivated quantum dots.Furthermore,the stability of MAPbBr₃ quantum dot solution was much improved by the passivation of TOPO.The MAPbBr₃quantum dots can be stable at room temperature exceeding 20 days in the condition of adding 1 ml TOPO.Based on these,PQD-LED were prepared with the same device structure as the previous part.By optimizing concentration of TOPO,the stability and luminescence properties of the device were significantly improved.The device half-lifetime was increased from the 44 s to 7 min.Their maximum luminance,current efficiency and external quantum efficiency were 1635 cd m^-2,5.51 cd A^-1,and 1.64%respectively,which showed significant improvement factors of 93.5%,149.3%and168.9%respectively.?3?Electron transport layer blending is employed to improve device performance:For purpose of further improving the performance of PQD-LED,B3PYMPM:TPBi is introduced to replace the pure TPBi in the first part of the device structure as an electron transport layer.The greenPQD-LEDwithdevicestructureITO/PEDOT:PSS/PVK/MAPbBr₃QDs/B3PYMPM:TPBi/LiF/Al were prepared.The blending of the electron transport layer effectively reduced the electron injection barrier in the device and promoted the recombination of electron-hole in the light-emitting layer.The characterization results of single-carrier devices showed that B3PYMPM:TPBi blending is more advantageous for balancing electrons and holes in a device than the single-material electron transport layer device.The maximum luminance,current efficiency and external quantum efficiency of the optimized PQD-LED were 2363 cd m^-2,6.24 cd A^-1 and 1.74%,respectively,which are 44.5%,13.2%and 6.1%higher than the pure TPBi device.In summary,by optimizing synthesis method,we obtain high-performance organic-inorganic hybrid perovskite quantum dots for green electroluminescent devices.The stability and photoluminescence lifetimes of PQD are much improved by TOPO passivation,and the passivation mechanism is systematically analyzed.Furthermore,the photoelectric properties of PQD-LED are further enhanced by blending of B3PYMPM:TPBi in electron transport layers.This work provides an effective solution for preparing high quality colloidal organic-inorganic hybrid perovskite quantum dots and high-performance electroluminescent devices.