Regulation Of Microstructure And Optoelectric Performance Of Low-dimensional Perovskite Light-emitting Devices

Compared with organic light-emitting diodes(OLEDs)and inorganic quantum dot light-emitting diodes(QLEDs),perovskite light-emitting diodes(PeLEDs)have the advantages of low-cost production,high color purity,easy adjustment of the emission spectrum and high carrier mobility.Three-dimensional perovskites usually have a long exciton diffusion length with weak exciton binding energy,enabling them ideal semiconductor for preparing solar cells but not suitable for preparing highly efficient PeLEDs.Low-dimensional perovskites have been proposed to confine exciton diffusion in 3D spaces and improve the exciton binding energy.Low-dimensional perovskites are realized through the regulation of surface ligands to synthesize nanocrystals with a size of about 10 nm,or through the addition of large cationic additives into precursor to promote the formation of two-dimensional(2D)/three-dimensional(3D)mixed perovskites.For nanocrystals,the impact of surface ligands is significant.For 2D/3D films,the regulation of nucleation and crystallization of precursors is critical.In this thesis,all-inorganic Cs Pb X₃nanocrystals were synthesized,whose surface defects and emission spectrum were adjusted by their surface ligands.2D/3D mixed perovskite thin films were prepared,and the relationship between precursor composition and perovskite crystallization was studied.High performance green and blue PeLEDs were prepared,and the effects of microstructure on device performance were discussed.The main research contents are as follows:(1)The short-chain ligands propionic acid(PA)and butylamine(BLA)were used to synthesize Cs Pb Br₃ nanocrystals at room temperature and ambient environment using green solvents isopropanol and n-hexane.By adding different content of cesium salt,the performance of PeLEDs can be tuned.Device study shows that with 300?l of lead precursor fixed,the introduction of 25?L of cesium salt can produce a maximum brightness of 1243 Cd/m²,but the film has poor density and serious leakage current.By introducing long-chain oleic acid(OA)and oleylamine(OAm)ligands,Cs Pb Br₃nanocrystals can be well dispersed in non-polar solvents,and the spin-coated films become uniform and dense.By optimizing the number of purification times,and the thicknesses of the transport and active layers,a maximum brightness of 5033 Cd/m² can be achieved,together with a current efficiency of 18.6Cd/A,and an external quantum efficiency(EQE)of 5.4%.(2)Ligand chain length on the surface of CsPbBr₃NCs was adjusted via ligand exchange,and the effects of ligand chain length on the performance of NC-based PeLEDs were investigated.Long-chain ligands OA and OAm,and short-chain ligands decanoic acid(CA)and octylamine(OLA),were selected to make ligand pairs,namely OA/OAm,OA/OLA,CA/OAm,and CA/OLA ligand pairs.Studies show that after four cycles of purification,the PA/BLA ligand pair employed for the initial synthesis of Cs Pb Br₃NCs can be completely replaced by long-chain ligand pairs.CA/OLA can not disperse the nanocrystals because the chain lengths are too short,resulting in aggregation of NCs.Compared with long-chain OA/OAm ligand pair,short-chain ligand pairs exhibit better charge transport but inferior passivation effect.At the same time,OA/OAm ligand pair endows better thermal stability.(3)2D/3D perovskite films are prepared by solution casting of a perovskite DMSO precursor consisting of Cs Br and Pb Br₂ with the extra addition of an organic amine molecule benzamidine hydrochloride(BMCl).Compared with 3D perovskite films,2D/3D mixed films have improved photoluminescence quantum yields(PLQY)and PeLEDs performance,but Cs₄Pb Br₆ phase was formed.By further introducing appropriate amounts of MABr and Pb Br₂,BM₂(Cs_1-xMA_xPb Br₃)_n-1Pb Br₄ crystals were formed in as-cast films without any additional thermal annealing,without the formation of the Cs₄Pb Br₆ phase.The EQE of PeLEDs can be increased from 1.2%to5.2%.This result suggests that 2D/3D mixed perovskite can be achieved at room temperature via solution casting by dedicated engineering of the precursor solution.(4)Halogen exchange in CsPbBr_xCl_3-xnanocrystals was rationalized to tune the spectrum and performance of Cs Pb Br_xCl_3-xPeLEDs.Results show that bromine in Cs Pb Br_xCl_3-xis substituted by chlorine and the spectrum blue-shifts by introducing tetrabutylammonium p-toluenesulfonate(TBSA)during the NC purification process,whereas chlorine in Cs Pb Br_xCl_3-x is exchanged by bromine and the spectrum red-shifts by introducing sodium dodecylbenzenesulfonate(SDSA)as the ligand during purification.The origin for halogen exchange can be attributed to the synergistic effects of anion and cation of benzenesulfonates.The PLQY of Cs Pb Br_xCl_3-xNCs increases from 7%to 81%due to the effective passivating effect of the strong ionic sulfonate moiety,and the blue PeLEDs prepared by this method show a promising external quantum efficiency of 2.6%.