Fabrication And Characterization For Electroluminescent Perovskite Light-Emitting Diodes With High Performance

Perovskite materials have been applied widely in fabricating perovskite light-emitting diodes(PeLEDs)owing to excellent optical property including easy to process,high color purity and big carriers mobility etc..Since 2014,the first PeLED with only external quantum efficiency(EQE)of 0.1%processed at room temperature was reported.Investigation on PeLEDs have surfed fierce competition in scientific community.To improve electroluminance(EL)performance and stability,various methods and technologies have been developed.Until now,EQE of PeLEDs has been over 20%.Meanwhile,the stability of PeLEDs has been also enhanced greatly.However,there is still a large room to meet the requirements of commercialization due to many problems exsisting to be solved.Such as,the ELperformance and stability of green PeLEDs are not enough good.In addition,the spectral stability and EQE of blue PeLEDs are disappointing.Finally,the poison of Pbis unfavorable for health and environment.To enhance EL performance and stability of green PeLEDs,in this work,investigation were conducted by improving perovskite films quality and balancing hole/electron injection.The novel PeLEDs structures are designed and fabricated,and PeLEDs perfromance are detailedly characterizated.The related mechanisms are clarified combining with theoretical knowledge.The main contents are as follows:3D perovskite materials severely limit EL performance of PeLEDs due to with small exciton binding energy,perovskite films with extremely rough and low coverage.In order to improve all inorganic perovskite CsPbBr₃films quality,a novel technology of multilayer coatings was adopted to fabricate emitting layer consisted of spincoating TmPyPB and CsPbBr₃solution in turn.By multilayer coatings,the quality of perovskite films was improved.In detail,the crystal size was reduced and defect densities were also decreased.Meanwhile,due to the doping effect of3,3'-[5'-[3-(3-Pyridinyl)phenyl][1,1':3',1''-terphenyl]-3,3''-diyl]bispyridine(TmPyPB),the electron transport ability of PeLEDs was also evidently enhanced.Most importantly,the multilayer structure make exciton generating interfaces increase and exciton forming region expand,which decrease the number of exciton quenching.Finally,the green PeLED with high purity was realized.The maximum current efficiency(CE)is 9.16 cd/A and corresponding the maximum EQE is 2.37%.In addition,8-Hydroxyquinoline aluminum(Alq₃)as an electron transport layer(ETL)was used to produce organic light-emitting diodes(OLEDs)due to low cost,easy to process in the past.However,it has never been utilized to fabricate PeLEDs because Alq₃with low electron mobility(8.05×10^-6cm²/Vs)will cause unbalance between electron and hole injection in PeLEDs.Moreover,the energy gap(2.70 eV)of Alq₃is only bigger of 0.40 eV than that(2.30 eV)of CsPbBr₃,which is not big enough to confine exciton in CsPbBr₃layer competely.Meanwhile,the valance band(VB,-5.85eV)is deeper than the highest occupied molecular orbital(HOMO,-5.70 eV)of Alq₃,which will lead to a number of hole leakage from CsPbBr₃layer to Alq₃layer and recombine with electron to form exciton in the Alq₃layer.As a result,the color purity will be decreased.To solve problems and take the advantages of both the low cost of Alq₃and the efficient exciton and hole blocking capabilities of the materials with a high hole mobility,a large Eg and a deep HOMO level,a composite ETLis formed by inserting a hole and exciton blocking blayer(HEBL)layer of TmPyPB between CsPbBr₃and Alq₃layer.The large enough barrier of the Eg and the high enough hole injection barrier between the CsPbBr₃and TmPyPB can play efficient roles in avoiding the exciton diffusing and the hole leaking from CsPbBr₃into the neighboring ETL,respectively.Finally,the efficient PeLEDs with high color purity were realized.As we known,PEDOT:PSS is often used to be hole transport layer(HTL)in PeLEDs.However,in fact,PEDOT:PSS with many disadvantages will limit the EL performance of PeLEDs as follows:Firstly,PEDOT:PSS is a strong acid material that result in degradating property of perovskite films.Secondly,PEDOT:PSS with low the lowest occupied molecular orbital(LUMO),narrow Eg will lead to leakage electron in CsPbBr₃/PEDOT:PSS interface and quench.Thirdly,a large hole injection barrier between PEDOT:PSS and CsPbBr₃exists,which limits hole injection efficiently.For this,4,4'-cyclohexylidenebis[N,N-bis(p-tolyl)aniline](TAPC)with high hole mobility(10^-2cm²/Vs),large Eg(3.50 eV)and high LUMO and low HOMO is inserted between PEDOT:PSS and CsPbBr₃to be HTL and exciton blocking layer(EBL).This EBL palys a role in reducing hole injection barrier and enhancing hole transport ability.Importantly,the excitons quenching caused by PEDOT:PSS with strong acid can be avoided in PEDOT:PSS/CsPbBr₃interface.By this method,the efficient PeLED was achieved.Compared to 3D perovskite,quasi-two dimensional(Quasi-2D)perovskite have more merits including large exciton binding energy,films with high coverage and high photoluminance quantum yield(PLQY)etc.Therefore,2-phenylethanamine bromide(PEABr)with different ratios was blended into CsPbBr₃solution to adjust composition and covert the 3D perovskite into quasi-2D perovskite.It was found perovskite films with 40%PEABr exhibit fully coverage and decreased crystal size.And the PeLEDs based on 40%PEABr also show higher ELperformance,comparing to the CsPbBr₃PeLEDs.Unfortunately,coverting 3D perovskite into quasi-2D perovskite will generate more defects owing to reduce crystal size and decrease conductivity of film by inserting insulation cation.As a result,the turn on voltage of PeLEDs will increase,which has an adverse effect on realizing quasi-2D PeLEDs with high performance.To further improve EL performance of quasi-2D PeLEDs base on PEA₂Cs_n-1Pb_nBr_3n+1(40%PEABr),the bilayer structure of poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(4,4?-(N-(4-butylphenyl)(TFB)/Polyvinylcarb-azole(PVK)is adopted as a HTL to enhance hole transport and balance the hole/electron injection.Finally,the quasi-2D PeLED with high CE of 35 cd/A was achieved successfully.