Synthesis And Optoelectronic Properties Of Side-Chain-Type Delayed Fluorescent Polymer

Organic light-emitting diodes(OLEDs)have attracted great attention because of their promising applications.In OLEDs,luminescent materials are of vital importantance.The luminous materials used in the early OLEDs are fluorescent materials.Due to the exciton ratio of singlet and triplet excitons in OLED is 1:3,the theoretical internal quantum efficiency(IQE)of OLED based on fluorescent materials can only reach 25%,which greatly limits the application of fluorescent devices.Due to the spin orbital coupling of heavy atoms in heavy metal complex of phosphorescent material,both singlet and triplet excitons can be used to achieve 100%IQE.However,the commonly used heavy metals are Ir?Pt and other precious metals,and heavy metal complex of phosphorescent material in the blue emissionhas yet to be bottleneck.Pure organic thermally activated delayed fluorescence(TADF)materials own a small gap(?EST)between the lowest singlet and triplet states,so that the triplet excitons can be converted to the singlet excitons by the reverse intersystem crossing(RISC),which can simultaneously use both single and triplet excitons to achieve 100%IQE.For TADF materials,the large constant of RISC(kRISC)and the high photoluminescence quantum yield(PLQY)are necessary for high performance OLED.At present,comparing to a large number of phosphorescent materials,TADF materials are still relatively scarce,especially for TADF polymers.In this thesis,a series of TADF polymers with high TADF ratios and PLQYs were synthesized,and their properties were studied.Finally,a series of high-performance OLEDs based on these luminescent materials were achieved.In chapter 1,a concise introduction of OLEDs,the basic principles of organic electroluminescence,device structures and device parameters are given.Then the various functional materials in OLED are introduced,in particular,the progress of TADF materials are reviewed.In chapter 2,we designed and synthesized a single polymer with TADF characteristics by grafting TADF emitter,10-(4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl)-10H-phenoxazine,to the side chain of the polymer backbone of polycarbazole.Employing these copolymers as the emitting layer achieved an efficient bluish-green polymer LED with a maximum external quantum efficiency of 4.3%,corresponding to a high exciton utilization efficiency(EUE)of 63.7%.In chapter 3,by using non-conjugated norbornene with high Si and T1 as the main chain,four polymers,named PBD-5%?PBD-20%with different percentage of blue TADF small molecule units were synthesized.Those polymers have good thermodynamic stability and film-forming properties.Further transient spectra show that those polymers owning obvious TADF properties inherited from the small molecules.Those polymers also have high PLQYs and large kRISCS.Finally,the OLEDs based on PBD-15%as emitting material achieved a maximum EQE of 1.95%,which was the highest efficiency of OLEDs based on blue TADF polymers.In chapter 4,we reported a series of bluish-green thermally activated delayed fluorescence(TADF)polymers by grafting the TADF emitter to the side chain of the polycarbazole backbone.This new strategy ensures the independence of the side-chain TADF emitters,and consequently the resulting polymers inherit the characteristics of the TADF small molecules.These pristine polymers show intensive delayed fluorescence with very efficient reverse intersystem crossing(RISC)rate of 8.6 × 105 s-1 and high PLQYs of up to 74%,which are superior to previously reported TADF polymers(PLQYs less than 50%).By employing these efficient TADF polymers as the emitters in the solution-processed light-emitting diodes,a maximal external quantum efficiency of 16.1%was achieved in a bluish-green device with cascade energy transfer via a small-molecule TADF assistant dopant.To our best knowledge,this is the highest efficiency for the TADF polymer-based devices.For the first time,we sensitized the TADF polymers by mixing a TADF small molecule into the emissive layer and thus boosted the internal and external quantum efficiencies.In chapter 5,we synthesized four white polymers,named W-0.05%?W-1%,by using blue polyfluorene as the main chain and orange TADF small molecules as the side chain.Those polymers have very good thermodynamic stability.The transient PL spectra show that the four white polymers own obvious TADF properties.Finally,the OLED based on W-0.5%as the emitting material achieved a maximum EQE of 2.0%with the color coordinates of(0.26,0.33),which was the first reported TADF white polymer.In chapter 6,we synthesized two solution-processed blue TADF emitters by multi-carbazole encapsulation for an emissive core of DMAC-DPS.These compounds possess good solution processability,excellent thermal stability,obvious TADF feature and relatively high PLQYs.By employing these TADF emitters as the non-doped emissive layers in solution-processed OLEDs,a greenish-blue double-layer device achieves a peak EQE of 12.2%with the CIE coordinates of(0.22,0.44),which is among the highest efficiency to date for non-doped solution-processed fluorescent OLEDs.In chapter 7,three fluorescent emitters with sky-blue,yellow and green emission,respectively,are obtained by introducing various encapsulation groups into multi-positions of a blue thermally activated delayed fluorescence emissive core.Such simple modification of encapsulation groups functionalizes these emitters to exhibit tunable emissive color,different transient emissive behavior,high thermal and morphological stability,and good film-forming ability.The singlet-triplet energy splitting of diphenylamine-encapsulated fluorescent emitter is significantly smaller than those of phenyl-and triphenylamine-encapsulated fluorescent emitters,which makes diphenylamine-encapsulated fluorescent emitter exhibit obvious thermally activated delayed fluorescence feature with a short delayed fluorescence lifetime of 0.45 ?s.Consequently,a solution-processed yellow organic light-emitting diode based on DDA-DP accomplishes a maximum external quantum efficiency of 8.1%,a maximum luminance of 37300 cd m-2,and also exhibits slow external quantum efficiency roll-offs of 1.2%at the luminance of 1000 cd m-2 and 18.5%at 10000 cd m-2,which represents the lowest attenuation of external quantum efficiency in thermally activated delayed fluorescence-based organic light-emitting diodes.In chapter 8,two TADF small molecules,3DMACIPN and 4DMACIPN,which are based on isophthalonitrile and 9,10-dihydro-9,9-dimethylacridine as electron acceptors and donors,respectively,were synthesized by simple nucleophilic substitution reaction.Due to the small ?EST,the two molecules own very high TADF ratios(86%and 79%).Finally,the OLED based on 4DMACIPN as emitting materials achieved maximum EQE of 10.0%.