Abstract
In the field of organic light-emitting diodes (OLED) development, the scientific community's attention is currently focused on third-generation OLEDs, which based on luminophores exhibiting the thermally activated delayed fluorescence (TADF). In TADF luminophores, the singlet and triplet levels S1 and T1 are close enough in energy that efficient thermally activated reverse intersystem crossing from the triplet state to the singlet state (T1->S1) occurs, which is where the (delayed) fluorescence (S1->S0) occurs. Therefore, both triplet and singlet excitons contribute to the EL, so the internal EL quantum yield of third-generation OLEDs can reach 100%. In 2016 a new type of so-called multiple resonant TADF luminophores was proposed. These compounds are polyaromatic hydrocarbons with heteroatoms (nitrogen and boron), where the spatial separation of the frontier molecular orbitals (HOMO and LUMO), necessary for low energy differences between the S1 and T1 levels, occurs at the donor (nitrogen) and acceptor (boron) atoms. Such luminophores exhibit a narrow luminescence band (about 20–30 nm) due to their rigid molecular structure, which, in addition, minimizes nonradiative relaxation (by suppressing various types of vibrations), which ensures high photoluminescence efficiency of about 90%.i The most promising representative of such compounds is considered to be a molecule called DABNA (5,9-Diphenyl-5,9-diAza-13b-BoraNaphtho[3,2,1-de]Anthracene, DABNA-1), as well as its derivatives. They are promising candidates for the development of TADF materials with a pure deep blue color. In the present work, we have fabricated and investigated the OLED samples based on the new phenoxazine-substituted DABNA derivative, DABNA-DAP. The device structure and its main characteristics are shown in Figure 1. Its electroluminescence (EL) provides a pure deep blue light with CIE coordinates x=0.141, y=0.074. The EL spectrum of the DABNA-DAP-based OLED (Figure 1(c), blue solid line) has a maximum at 444 nm with a FWHM of 31 nm and slightly blue-shifted relatively to the EL spectra of the reference DABNA-1-based OLED sample (Figure 1(c), red dashed line). However, in the present work, we obtain a rather low EL EQE (about 0.6%, see Figure 1(b)), which can be associated with some issues during OLED samples fabrication and characterization. Approaches for EQE enhancement are discussed.