Blue-emitting Perovskite Nanocrystals And Theri Electroluminance Light-emitting Diodes

CsPbX3(X=I,Br,Cl)nanocrystal perovskite light-emitting diodes(PeLED)have been emerged as one of the most competitive candidates for wide-gamut and high-performance displays.PeLED have several advantages such as tunable emission peaks wavelength,high carrier mobility,high colour purity,narrow full-width at half-maximum(FWHM),wide color gamut(?140%),high photoluminance quantum yield(PLQY),and facile cost-effective solution processable fabrication etc.However,blue PeLED devices,which emits one of the three primary colors,are limited by low brightness,low external quantum efficiency(EQE),poor spectral stability,and short operating lifetime.In addition,the unbalanced carrier transfer in PeLED also reduces device performance.In view of those problems,this study firstly discusses some optimization strategies including quantum confinement effect,doping engineering and surface ligand engineering to suppress phase separation and reduce surface defects,aiming at improving both optical performance and stability of CsPbX3 NC-based blue-emitting PeLED.Moreover,the device structure is then rationally designed and further optimised,which is supposed to improve the carrier recombination efficiency,balance the charge injection,and supress Auger recombination.As a result,high-efficient and stable pure-blue(-470 nm)PeLED can be obtained,while their optical and operating performance are dramatically improved.A breif summary of main research results are below:Blue light emission can be obtained by small-size CsPbBr3 nanocrystals(NCs)because of the strong quantum confinement effect.This blue light-emitting option is beneficial for avoiding the phase separation issue that mixed halides CsPbX3 NCs normally has.However,a large number of surface defects are normally generated in small-size CsPbX3 NCs which have a high surface-to-volume ratio,resulting in unsatifised optical performance and poor stability.Considering this research gap,this paper developed a novel quick-quenching synthesis method to prepare a core-shell nanocrystal.Specifically,small-sized CsPbBr3 with wide bandgap were previously precipitated in solution.Then NCs far from the heating center,whose surface lacks active growth dynamics,would probably in situ forms amorphous CsPbBrx on the surface of CsPbBr3 NCs.Finally,a core-shell nanocrystal(CsPbBr3@A-CsPbBrx)was successfully given.The amorphous shell A-CsPbBrx can effectively passivate the surface defects of the core,thereby greatly reducing the surface defect density of the nanocrystals,assisting excitons with formation in NC cores,and enhancing the radiative recombination process.The experiment highlighs that the PLQY of CsPbBr3@A-CsPbBrx NCs is increased from 54%to 84%,and the tightly-coated A-CsPbBrx shell can resist oxygen and water diffusing into the CsPbBr3 core,which could weaken the optical corrosion of the core by high-energy excitation light,thereby greatly improving the environmental stability and optical stability of the CsPbBr3@A-CsPbBrx NCs.Besides,the corresponding photoluminescence LED exhibits bright and stable pure-blue emission at-465 nm emission(EM)peak.Noting that the amorphous shell layer could undergo a crystallization process at high temperature,which results in thermal stability problems of such NCs.The spectrum of these CsPbBr3@A-CsPbBrx NCs exhibit tendency of red-shift.Although the core-shell CsPbBr3@A-CsPbBrx NCs mentioned above show excellent blue-emitting performance,the amorphous shells have poor thermal stability,which is prone to transfer to CsPb2Br5 and lose optical property.Considering that the size of Cs+ ions in CsPbX3 is small,the tolerance factor of perovskite structure is small too,resulting in slight lattice distortion.In addition,the ionic compound nature of CsPbX3 is one of the main reasons for poor stability of PeLED.In order to prepare blue-emitting CsPbX3 NCs with nice thermal and structural stability,and overcome problems encountered in the previous chapter,ion doping strategy has been put forward.Herein,some of the larger Pb2+ ions(119 pm)in CsPbX3 lattice is replaced by smaller Cu2+ions(73 pm),which cause contraction of the lattice and bond length,and increasing ionic binding energy.Synchrotron radiation extended X-ray absorption fine structure(EXAFS)exhibited that the lattice contraction leads to a stronger interaction between Pb and X orbitals,eliminate the distortion of the octahedra,and supress the formation of halide vacancies.On the other hand,introduction of Cu2+ into the CsPbX3 lattice could improve the order of the local coordination environment of Pb and enhance the short-range order of the lattice.Therefore,a remarkably high PLQY of 80%was achieved for those Cu2+ doped CsPbBr3 NCs.In addition,the shorter bond length improves the formation energy of perovskite lattices,resulting in significant improvement of the thermal stability of Cu2+ doped CsPbBr3 NCs.Experimentally,after being annealed at 250? under ambient conditions,doped NC film still maintains a PLQY of 40%(75%of the initial value(53%)remains),while undoped ones give a PLQY diminished to nearly 0%.This method opens up new possibilities for the synthesis of high-performance blue-emitting perovskite nanocrystals with excellent thermal stability.Nanocrystals used for preparation of PeLED usually need to be purified for several times,which causes unstable ligand shedding at the surface of nanocrystals,thereby deteriorating the stability and even resulting in phase transformation.Here,a novel acid-etching-assisted-ligand engineering was developedd to solve these problems.This work focused on preparing unltra-small-sized(?4 nm)CsPbBr3 NCs with ultralow vacancy defect density to achieve pure-blue light-emitting and excellent environmental stability.Initially,the acid hydrogen bromide(HBr)was utilized to etch imperfect octahedrons,thereby removing surface defects and excessive carboxylate ligand from NCs and achieving accurate coordination of Pb2+ and Br-ions.In this process,didodecylamine(DDDAM)and phenethylamine(PEA)were successively introduced to bond the residual uncoordinated sites and facilitate in-situ exchange with the original long-chain organic ligands at the NCs surface,achieving high-quality and stable NCs with ultralow vacancy defects.Consequently,a near-unity photoluminescence quantum yield(PLQY)of 97%and remarkable stability even in ultraviolet lighting and ambient conditions were obatined.PeLED with the planar structure of ITO/PEDOT:PSS/PVK/CsPbBr3//ZnO/Ag were fabricated based on these treated CsPbBr3 NCs.Except for metal electrode,all functional layers were prepared by spin-coating process.The PeLED combined with the treated NCs were fabricated by our all-solution process,exhibiting pure-blue emission at 470 nm,a EQE of 4.7%,a maximum luminance of 3850 cd m-2(record performance value for pure-blue PeLED),and long-term half-lifetime(T50 above 12 h)under operational condition at the initial luminance of 102 cd m-2,which is more than one order of magnitude higher than that of the state-of-the-art blue PeLED.Till now,the optical performance and operation stability of blue PeLED have been improved,but there is a large room for commercilization.The obvious efficiency roll-off,as one of disadvantages for PeLED,limiting their pratical application in industry.The electron mobility of electron transport layer(ETL)is usually one order higher than that the hole mobility of hole transport layer(HTL)in PeLED,which results in unbalanced charge-carrier tranfer and injection levels.This unbalance phonamena leads to the accumulation of charges and further causes exciton quenching.In a word,the non-uniform spatial distribution of carriers could fasten the Auger recombination process in LEDs,resulting in a low performance od PeLED.Here,a novel structure of emitting layer of PeLED is designed,that an ultrathin interlayer of ZnCl2 was constructed between two NCs layers to form a sandwich panel(SWP)structure.ZnCl2 acted as a barrier layer in the middle of SWP film to reduce the electron mobility,thereby balancing the transport of electrons and holes of PeLED.The interlayer modulated the charge recombination zone away from the previous interface of NCs film and HTL,which is helpful to reduce the interfacial exciton quenching and Auger recombination caused by the accumulation of interfacial carriers.As a result,the PeLED exhibited stabilized high-purity blue emission at 469 nm wavelength with EQE of 5.0%,and the EQE presented low roll-off meanwhile.Besides,the maximum luminance was improved from 7,000 cd m-2 to 10,410 cd m-2,which is one of the highest brightness records reported as far.Strikingly,the PeLED gave a half-lifetime(T50)at 59 h under continuous operation condition,representing the most stable blue PeLED reported at present.