Elaboration of the appropriate host materials proved to be not less important for the fabrication of a highly efficient OLED than the design of emitters. In the present work, we show how by simple variation of molecular structure both blue emitters exhibiting delayed fluorescence and ambipolar high triplet energy hosts can be obtained. The compounds with a para-junction revealed higher thermal stability (T up to 480 °C), lower ionization potentials (5.51-5.60 eV), exclusively hole transport, and higher photoluminescence quantum efficiencies (0.90-0.97). Meta-linkage leads to ambipolar charge transport and higher triplet energies (2.82 eV). Introduction of the accepting nitrile groups in the para-position induces intensive delayed fluorescence via a triplet-triplet annihilation up-conversion mechanism. By utilization of the para-substituted derivative as an emitter and the meta-substituted isomer as the host, a deep-blue OLED with the external quantum efficiency of 14.1% was fabricated.

We fabricated a yellow organic light-emitting diode (OLED) based on the star-shaped donor compound tri(9-hexylcarbazol-3-yl)amine, which provides formation of the interface exciplexes with the iridium(III) bis[4,6-difluorophenyl]-pyridinato-N,C2']picolinate (FIrpic). The exciplex emission is characterized by a broad band and provides a condition to realize the highly effective white OLED. It consists of a combination of the blue phosphorescent emission from the FIrpic complex and a broad efficient delayed fluorescence induced by thermal activation with additional direct phosphorescence from the triplet exciplex formed at the interface. The fabricated exciplex-type device exhibits a high brightness of 38 000 cd/m(2) and a high external quantum efficiency.

A new interface engineering method is demonstrated for the preparation of an efficient white organic light-emitting diode (WOLED) by embedding an ultrathin layer of the novel ambipolar red emissive compound 4,4-difluoro-2,6-di(4-hexylthiopen-2-yl)-1,3,5,7,8-pentamethyl-4-bora-3a,4a-diaza-s-indacene (bThBODIPY) in the exciplex formation region. The compound shows a hole and electron mobility of 3.3 × 10 and 2 × 10 cm V s, respectively, at electric fields higher than 5.3 × 10 V cm. The resulting WOLED exhibited a maximum luminance of 6579 cd m with CIE 1931 color coordinates (0.39; 0.35). The bThBODIPY dye is also demonstrated to be an effective laser dye for a cholesteric liquid crystal (ChLC) laser. New construction of the ChLC laser, by which a flat capillary with an optically isotropic dye solution is sandwiched between two dye-free ChLC cells, provides photonic lasing at a wavelength well matched with that of a dye-doped planar ChLC cell.

The starburst carbazole derivative and phosphorescent bis-cyclometalated iridium(III) complex (IC2) were used for the preparation of multilayered “warm-white” organic light-emitting diodes (OLEDs), the emission spectra of which are modulated by the thickness of the phosphorescent layer. It was shown that the electroluminescence spectra of the fabricated devices are more extended into the visible region compared with the photoluminescence spectra of both component materials. The observed extension of the electroluminescence spectra can be assigned to the phosphorescent emission of the low-energy exciplex formed at the interface of the emissive layers. The quantum-chemical calculations performed by the DFT and (TD) DFT methods support the formation of the low-energy triplet exciplex at the interface of the IC2 layer and the neighboring layer of the starshaped carbazole-based compound, (4,4′,4″-tris[3-methylphenyl­(phenyl)amino] triphenylamine, tri(9-hexylcarbazol-3-yl)amine (THCA). Indeed, the triplet excited state of such bimolecular complex corresponds to intermolecular charge transfer between IC2 and THCA. The experimentally observed electrophosphorescence of these exciplexes is induced by strong spin–orbit coupling in the THCA:IC2 complexes due to the Ir(III) heavy atom effect. With dependence on the iridium(III)-complex film thickness (5–9 nm), the CIE coordinates changed from (0.41, 0.41) to (0.52, 0.47), corresponding to the warm white and orange color. The brightness of the fabricated OLEDs at the 15 V bias was in the range from 500 to 6000 cd/m2.

Diphenilamino-substituted carbazoles were used as guest compounds for the preparation of highly efficient blue organic light-emitting diodes based on the phenomenon of delayed fluorescence. It was shown that the spectra of the delayed fluorescence of host−guest systems are identical to those of the prompt fluorescence and in general coincide with the photoluminescence spectra of the guest films. The congruence of the prompt and delayed fluorescence spectra is explained by the effective intermolecular triplet−singlet (T → S) energy transfer from the excited T states of the host to the S states of the guest molecules. High external electroluminescence efficiency of the fabricated electroluminescent devices, reaching 17%, is comparable to that achieved in phosphorescence-based organic light-emitting diodes.