Design,Synthesis And Application Of Tilted-spiro Based Functional Materials In Organic Light-emitting Diodes

For the fast development of OLED,the continuous appearance of novel material systems has been one of the main driving forces;this will also be the main power for solving the future issues that OLED is facing,such as the low efficiency and poor stability in the deep-blue/deep-red/near-infrared color regions.At the same time,in terms of the device structure,the recently emerging ternary system,i.e.the traditional host-TADF sensitizer-traditional fluorescence guest system,also shows great potential in enhancing device stability and efficiency.Understanding the mechanism of this ternary system,investigating the role of each element in this system,and building a system of materialmechanism-efficiency are also of great significance to the further development of OLED.In this dissertation,we focus on two aspects:(Ⅰ)new material systems and(Ⅱ)understanding the material-mechanism-efficiency of the ternary system by applying the tilted spiro TADF materials.We aim to promote the development of new material systems in blue and deep-red regions and an in-depth discussion of device mechanisms.In terms of material design,we focus on the D-σ-A structure TADF material system.By adoptingσ as a linker of donor and acceptor,we designed and synthesized a series of TADF materials with spatially tilted spiro structures.When these TADF materials act as host or guest materials,they have achieved excellent efficiency and stability.On the other hand,we systematically studied the relationship between the tilted spiro geometry and the fluorescence quantum yield,stability,luminescence,and device efficiency.With theoretical guidance,we applied the new tilted spiro structure to the design of blue TADF materials and realized the breakthrough of efficiency in the blue region.In addition to blue emission,we also focused on the working mechanism of the ternary system,using the tilted spiro TADF material as the sensitizer,and systematically studied its difference with 4CzIPN.The role of the doping concentration,radiation transition rate constant,emission spectrum,and device performance of sensitizer,has provided a guiding role for further understanding the working mechanism of the ternary system and improving the device performance.At the same time,we adopted the D-A-A design strategy to design and synthesize near-infrared fluorescence material with high fluorescence quantum yield(70%)and emission wavelength>750 nm and combined it in the ternary system.1.In this contribution,a newly-reported spiro backbone,OSTF(oxygen-based spiro-triphenylamine-fluorene triphenylamine),is utilized to construct TADF type hosts for red phosphor.For the OSTF skeleton,the rigid geometry and strong electron-donating characteristic originating from lone pair electron of oxygen lead to and reduced singlet energy(S1)as well as ΔEST.Actually,the reduced S1 is useful for low driving voltage thus high power efficiency,meanwhile the small ΔEST from effective frontier orbitals(FMO)separation is beneficial for balanced carrier transfer.More importantly,tilted spiro geometry of the OSTF skeleton can enhance intra-and inter-molecular interactions,which enbales effective resonance energy transfer(RET).By utilizing OSTF as the skeleton at the same time appending two acceptors(cyano and bis-(2,4,6trimethylphenyl)borane)on the para-position of fluorene,TADF type hosts(OSTFPCN and OSTFPB)with small ΔEST,balanced carrier transporting ability and effective RET ability are obtained.When utilizing them as hosts,record-high power efficiency(PE)of 63.6 lm/W and EQE of 3 1.2%are achieved for Ir(MDQ)2(acac)based device.Even when the doping concentration is as low as 0.5 wt%,the PE can still reach 58 lm/W.Furthermore,in merits of ultralow concentration(0.5 wt%),efficiency roll-off of the OSTFPCN-based device is quite flat with EQE keeping almost constant in the luminance range of 100 to 1000 cd/m2 and EQE dropped by only 1.5%when the luminance reaches to as high as 5000 cd/m2.These high PE and EQE even under low concentration not only create records for Ir(MDQ)2(acac)based PhOLEDs,but also provide a new platform to promote the commercialization process of red PhOLEDs by reducing their fabrication cost.2.In this charpter,we aimed to greatly improve the efficiency of D-σ-A type emitters by rational molecular modification.Two new molecules,OSTFCN and OSTFB,with tilted spiro core are proposed.For these two compounds,the rigidity and strong donor capacity of oxygen-bridged tri-phenylamine part greatly facilitate the spread of the HOMO distribution thus the enhancement of rate constant of fluorescence.Furthermore,the energies of the lowest triplet state of charge transfer property(3CT)of these tilted molecules were greatly reduced,which have a big effect on minimizing ΔESTt;this situation is quite different from the dominant lowest localized triplet state(3LE)in previous D-σ-A systems.Noticeably,X-ray data reveal that the imperfect vertical configuration makes the donor part and acceptor part approach to each other which is believed to be useful for enhancing the device performance.As expected,good device performance with current efficiency(CE)/power efficiency(PE)/EQE of 68.8 cd/A1/52.1 lm/W1/20.4%with peak emission around 530 nm for OSTFCN is successfully achieved.This is believed to be the first sp3 carbon-based D-σ-A type TADF emitter exceeding 20%EQE,Additionally,a long lifetime of 18,000 h at practical brightness of 100 cd/m2 is successfully achieved as well.3.In this work,we designed and synthesized a novel sky-blue spiro-type TADF emitter QAFCN,in which the donor and acceptor were not perpendicular to each other although they were connected through a spiro carbon.This was because the rigid donor moiety,5,9-dσihydroquinolino[3,2,1-de]acridine(QA),had a deformed conformation that could bend the donor towards the acceptor moiety and thus shorten the D/A distance to facilitate the TSCT.As compared to the classic spiro TADF emitter ACRFLCN,the QAFCN had blue-shifted emission,obviously higher PLQY and faster rate of RISC.Consequently,the QAFCN-based device achieved an EQE of 17.9%with an emission peak at 488 nm,largely exceeding the ACRFLCN-based device’s performance(11.5%),which could provide new understandings in promoting the efficiency of D-σ-A type TADF materials in TSCT mechanism.4.In this work,a new spiro skeleton spiro[quinolino[3,2,1-kl]phenoxazine-9,9’thioxanthene](SQPT)is proposed and developed via coupling sulfur-bridged fluorene(SBF)(acceptor)with quasi-planar 10-(2-bromophenyl)-10Hphenoxazine(BPX)moiety.Density functional theory(DFT)indicates the SBF acceptor show much high 3LE(2.96 eV)than the traditional fluorene moiety(2.55 eV).The quasi-planar geometry of BPX with good rigidity and delocalized characteristics can suppress the vibration-induced energy loss.Concerted research efforts have proven that the tilted spiro structure using a quasi-planar donor could enhance the through-space interaction and thus enhance the oscillator strength.By appending two different kinds of acceptors(cyano(SQPT-CN)and fluorodimesitylborane(SQPT-FB))on the para-positions of SBF acceptor,two blue TADF emitters are developed.OLEDs with SQPT new skeleton show EQE of over 15%and 12%for SQPT-CN and SQPT-FB,with corresponding a CIE of(0.14,0.15)and(0.14,0.20).The desirable CIE together with the high EQE suggests our design strategy in blueshifting the emission at the same time maintains the performance is valid.5.In this work,we study the role of TADF sensitizer in TAF-OLEDs by choosing two TADF emitters(4CzIPN and OSTFB)with similar photoluminescence spectra for two red emitters(POZ-BT-CN and OTPA-BT-CN).For the new red emitters,both of them show electroluminescence(EL)peak at 750 nm and good photoluminescence quantum yield(PLQY),which is suitable for research of red TAF-OLEDs.For the TADF sensitizers,commercially available 4CzIPN and novel spiro-based OSTFB are used.In demonstrating which parameters of TADF sensitizer takes a more important position in increasing the overall device performance and resonance energy transfer(RET)process,the overlap of TADF sensitizer and red emitter and the radiative constant(kr)are compared.Furthermore,different types of devices are also fabricated to prove the phenomenon.As a result,the device based on OSTFB,which shows a similar emission spectrum but larger kr exhibits more favorable energy transfer and higher device performances.While the 4CzIPN based devices show obvious emission from TADF sensitizer when changing its doping concentration from 10%to 50%.Consequently,the device performances based on OSTFB exhibit higher performance of 12.4%/10.1%with EL peak at 613/626 nm,while 4CzIPN based devices show inferior performances of 6.3%/5.9%with EL peak at 610/623 nm.During my PhD,I also made achievements in perovskite LEDs.From September 2018,I was supported by the China Scholarship Council and studied in Edward H.Sargent group of University of Toronto in Canada for two years.During that period,my research was perovskite materials and emitting devices.By combining my experience in OLED with perovskite materials,I have achieved a series of distinctive and important results:Ⅰ.Perovskite quantum dots are developing rapidly as a new candidate for solutionprocessed devices;however,the current research mainly focuses on red and green quantum dot materials with little attention on blue emisison.In addition,the long-chain organic ligands used in the synthesis often make the perovskite have low mobility and conductivity.Therefore,it is important to develop a ligand exchange strategy that can improve both the conductivity and mobility.In this work,we found that the stability of perovskite quantum dots can be significantly improved by adopting a bipolar shell structure,that is,cations as the inner shell combined with quantum dots,and polar solvents dissolving anions as the outer shell to further protect the quantum dots.The carrier mobility of the quantum dots prepared by this method exceeds 0.01 cm2 V-1s-1,enabling 12.3%external quantum efficiency in blue and 22%in green light-emitting diodes.(This work is published in Nature Nanotechnology,2020,15,668).Ⅱ.Blue perovskite materials can be obtained by halogen doping,however mixed halides are prone to phase separation during the operation of light-emitting diodes,which leads to spectral red shift and poor stability of the device.Therefore,it is important to develop blue perovskite materials with single halide.In this work,we tune the emission spectrum of the pure bromide perovskite by adding amino acid derivatives to the perovskite precursor solution.Detail experiments and theoretical calculations proved that the amino acid derivative can control the growth of perovskite quantum wells,thus controlling the size of the perovskite quantum well and the strength of the quantum confinement effect.As a result,light-emitting diodes based on this material achieved ideal CIE color coordinates(0.12,0.14)and an external quantum efficiency of over 6%.(This work is published in Nature Communications,2020,11,3674).