Investigation Of Chemical Stability And Degradation Mechanism Of Blue-Emitting Materials For Organic Light-emitting Diodes

With the realization of highefficiencies,limited operational stability becomes instead the keyimpediment to wide commercialization of OLEDs,in particularfor high-energy blue-emitting devices.It was reported that chemical stability of organic light-emitting materials toward excitons and charge carriers is intrinsically associatedwith the operational stability and economics of state-of-the-artorganic light-emitting devices.To reveal the reasons for the short-lifetime of blue OLEDsand enable the development of novel materials with enhanced stabilities,this dissertation mainly investigated the chemical stability and degradation mechanismof the typical organic blue-emitting materials toward excitons and charge carriers.Accordingly,the design strategy of blue-emitting materials with high stability by comparing the energy of the excited states with the bond dissociation energies(BDEs)of the correspondingsingle bonds was proposed.The main conclusions of this dissertation are as follows:1.By comprehensive experiments and theoretical calculations,the intrinsic chemical stability toward excitons and charge carriers of organic materials,which contain typical electron-accepting moieties of sulfonyl,phosphine-oxide,and carbonyl group have been investigated.The materials with adiphenylsulfonyl or phosphine-oxide moiety suffered a fatal chemical instabilityoriginating from the cleavage of C-S or C-P single bond whether underUV irradiation or in electrical-stressed devices.The materialwith a dibenzothiophene-S,S-dioxide moiety exhibited significantly improved chemical stability because of effective shieldingof the weak C-S single bond in a ring.In contrast,the commercially used carbonyl-containing compound demonstrated thehighest chemical stability with negligible degradation under the same condition.Comparing theenergy of the excited states with the BDEs of the correspondingsingle bonds by quantum chemical calculations,it was found that the bond strength of the weak chemical bonds of the molecules would determine theintrinsic chemical stability of the organic materials in their excited and charged states,which might be a plausible origin of thelimited stability of high-energy blue-emitting materials and devices.Besides,it has been found that comparing the energy of the excited states with the BDEs of the correspondingsingle bonds bythe quantum chemical calculation is a prediction tool for evaluating the intrinsic chemical stabilities of organic materials.2.The relationship between the chemical stabilitytowards excitons of the materialsand the lifetime of corresponding OLEDs have been investigated based on two blue-emitting thermal activated delayed fluorescence materials with sulfonyland carbonyl group respectively.The X-ray photoelectron spectroscopyandX-ray absorption near-edge structurewere carried out to analysis the chemical stability of the materials under UV irradiation andin electrical stressed OLEDs.It was found that the chemical stability of material with carbonyl groupis much better than the material with sulfonylgroup.The material with sulfonyl group suffered a fatal chemical instabilityoriginating from the cleavage of C-S single bond.Furthermore,the electrically stressed OLEDs and the materials under UV irradiation showed similar degradation behavior which indicated that the chemical stability of the material towards excitonsis the main factors affecting the lifetime of the OLEDs.3.The chemical stability towards excitons and charged states of the materials containing typical donor and acceptor moieties has beencomprehensive investigated via quantum chemical calculations.Accordingly,the BDEs of single bond in thesetypical donor and acceptor moieties and suitable application were obtained,such as the carbazole,cyano,10H-phenoxaborin,tetraphenylsilane andN–phenylbenzimidazole are appropriate to be used in the design of blue emitting materialsowing to the strong bond(BDE >3.60 eV)in these moieties.While the BDEs of the single bonds in phenoxazineand fluorenemoieties are less than 2.70 eV,which arenot appropriate to be used in the design of blue emitting materials.In addition,for the twistedmolecularstructure,although it can enhance the singlet and triplet energy of the molecule,the bond strength of the corresponding bond would decline about 3~5 kcal/mol.