Theoretical Study Of Phosphine Oxide/Carbazole And BN Replacement Derivatives As Hosts For Blue Phosphorescent OLEDs

Organic light emitting diodes have received considerable attention during the past decades as potential candidates for the ultrathin, large-sized flat-panel displays in the future, owing to the rapid response rate, low driving voltage, compatibility with digital image and inexpensive technology cost. To avoid the triplet-triplet annihilation and concentration quenching, phosphorescent emitters are usually dispersed into the host materials, namely doped into the devices. Based on this design, efficient energy transfer can be achieved between the host and guest materials. Up to now, great progress has been obtained in the development of high-efficiency green and red-emitting OLEDs.However, the development of blue-emitting devices moves slowly, which can be ascribed to the lack of efficient host materials for blue emitting. Therefore, the improvement in the relevant performance of host materials suitable for blue guest materials is significantly important. With an attempt to prevent the adverse energy transfer from guest to host and guarantee the triplet excitons are confined into the guest,the triplet state of host should lie at higher energy levels than that of the guest.Unfortunately, the increase in the triplet state energy is usually accompanied by the widened band gap, which may lead to bigger carrier injection barrier and higher device voltage. In this study, we made theoretical study on a series of phosphine oxide/carbazole polymer as well as tetrahphenylethene and polycyclic aromatic hydrocarbons by BN replacement derivatives as hosts for blue phosphorescent OLEDs by employing quantum chemical calculation aim at investigation on the influence of the electronic structure on the triplet state energy and the injection and transportation of carrier. Moreover, we also established the host-guest model to study the energy transfer between them. We hope that our work can provide useful guidance for the design and characterization of host materials. This article mainly includes the following three respects.First, density functional theory calculations were performed to investigate theelectronic structures of bipolar host materials comprising a backbone of linked acceptor moieties where each acceptor was also linked to a pendant donor moiety. The acceptor was triphenylphosphine oxide with two of its phenyls substituted with fluorine atoms or nitrile groups(CN). The donor was carbazole(CZ) substituted, or not, with t-butyl groups. The HOMO and LUMO energy levels of these host molecules were mainly influenced by their respective hole- and electron-transport units. The t-butyl substituents on the CZ moieties had an adverse effect on the triplet energies(ET) of the host molecules, especially for molecules where the phenyls of the backbone chain were substituted with CN groups. While introducing CN substituents onto the backbone chain decreased the energy difference between the lowest singlet and triplet excited states(ΔEST), it also caused the energy gap between the HOMO and LUMO to narrow. Among the host molecules investigated, that in which one of the phenyls in the acceptor moiety was linked to the donor while the other two phenyls in the acceptor were substituted with CN substituents exhibited the highest ET, balanced charge transport, a low charge-injection barrier, and a small ΔEST, and is therefore a promising candidate host material for use in bluephosphorescent devices.Second, we designed and investigated a series of host materials based on tetrahphenylethene derivatives. Density functional theory was employed to study on the effect of replacement of B-N bond with C=C bond and the nature of substituent as well as substitution position on the electronic structure and performance of the host materials.As revealed, incorporation of BN can improve the energy of the triplet state as two times. Moreover, the host materials bearing BN were adopted to establish the host-guest model with the corresponding guest materials, to elucidate the energy transfer between the host and guest. The complexes with electron-withdrawing TPO substituted at the para-position of the N atom have not only higher triplet state energy, but also effective energy transfer with the blue-emitting guest FIrpic. Furthermore, there is also a good match between the emission spectra of host and the absorption spectra of guest. All the discussed results indicate that these materials are suitable as blue-emitting host materials.Third, based on the above work, density functional theory was also employed to study the effect of substitution of B-N bond and the substitution position on electronic structure and performance of the polycyclic aromatic hydrocarbon derivatives as potential bipolar host materials for blue-emitting. In the previous discussion, although the complex with the B-N substituted has higher triplet state energy, the carriertransportation performance has not been improved, which may lead to the unbalance between hole and electron transportation. Herein, the replacement of tetrahphenylethene with polycyclic aromatic hydrocarbons not only improves the triplet state energy but also perfects the hole and electron transportation, thus the bipolar host materials for blue-emitting are obtained, as well as efficient Singlet-Singlet F?rster energytransfer and Triplet-Triplet Dexter energy transfer with blue-emitting guest FIrpic.