Theoretical Investigation Of Carbazole And Annulated Fluorene Derivatives As Bipolar Host Materials For Organic Light Emitting Diodes

Phosphorescent organic light-emitting diodes(PhOLEDs)have attracted tremendous attention due to their harvesting of both electrogenerated singlet and triplet excitons for emission and their great potential in full-color flat-panel displays and solid-state lighting.In general,phosphorescence-based OLEDs adopt guest–host systems in which the phosphors as guests are normally doped into host materials to reduce concentration quenching and triplet–triplet(T–T)annihilation etc.Choosing an efficient bipolar host material for blue emitters remains one of the critical issues compared with green and red phosphorescence OLEDs.A series of host materials have been designed by linking different electron acceptors.To reveal their electronic structures,optoelectronic properties and structure-property relationships of the host materials,an investigation was studied through density functional theory(DFT)and time-dependent density functional theory(TDDFT)calculations.In addition,we built a host–guest model to investigate the charge transfer and energy transfer between host and guest materials.Detailed research contents are divided into five parts as follows:First,a brief history and the current development status of OLEDs were introduced.Then,the device structures and its working principle were addressed.Furthermore,we present the research significance of our work.Second,we presented the theoretical methods in detail which applied in our study in detail and light principle.Third,we have designed a series of host molecules for blue/green electrophosphorescence by combining the electron donor carbazolyl unit and the electron acceptor aryl phosphine oxide unit into a single molecular unit by density functional.We have systematically investigated the influence of the phenyl spacers,phenyl substitution and the type of bridge(diphenylether-bridge or diphenylamine-bridge)on an array of electronic properties of the designed ambipolar hosts.Different linkage strategies of the o-substituted phenyl groups can suppress the extension of the triplet state exciton for highly efficient ambipolar host materials.The decrease of the phenyl spacer ?-?* conjugation can lead to an increase of the triplet energy.The corresponding materials based on diphenylamine-bridges show relatively poor performance,compared with diphenyletherbridge host materials.We have characterized the electronic structures,predicted the host performances,and considered the possible emission mechanism in the studied hosts and reference guest systems.Forth,a series of novel host materials with different electron acceptor unit were designed using triphenylamine annulated fluorene as the experimental molecule.We have systematically investigated the influence of the proportion of electron-rich groups(D)and electron-deficient groups(A).No matter what proportion for D-A type,the diphenylborane based ambipolar host materials show a low charge-injection barrier,a small ?EST and better charge-transport performance.Fifth,we continued the study of spiro-annulated triphenylamine derivatives.We modified the molecule structures by replacing triphenylamine unit with phenylcabarzole.A detailed theoretical study on the electronic structures and properties as host molecules for a series of STF /CzSTF series by introducing different electron-deficient units.It has been proved that a spiro-annulated structure can be modified in constructing high ET bipolar host material with prominent performance.The results clearly show that the host molecules in which the donor moiety is substituted with triphenylamine group(STF series)exhibit smaller ?EST than similar hosts in which the donor unit is substituted with N-phenylcarbazole group(CzSTF series).A smaller ?EST is beneficial to the formation of triplet excitons and can reduce the driving voltage generally.Among all of the hosts,as SOSTF presented a high ET,a low charge-injection barrier,better charge-transport performance,and a small ?EST,it can be found to be good host material for blue phosphorescent organic light-emitting diodes.