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.
A new triaryl molecule based on a benzene–benzothiadiazole–benzene core has been applied in a WOLED device. This very simple molecule emits from a combination of emissive states (exciton/electromer/exciplex/electroplex) to give white light with CIE coordinates of (0.38, 0.45) and a colour temperature of 4500 K
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.
Recent efficiency
records of organic photovoltaics (OPV) highlight stability as a limiting
weakness. Incorporation of stabilizers is a desirable approach for
inhibiting degradationit is inexpensive and readily up-scalable.
However, to date, such additives have had limited success. We show
that β-carotene (BC), an inexpensive and green, naturally occurring
antioxidant, dramatically improves OPV stability. When compared to
nonstabilized reference devices, the accumulated power generation
of PTB7:[70]PCBM devices in the presence of BC increases by an
impressive factor of 6, due to stabilization of both the burn-in and
the lifetime, and by a factor of 21 for P3HT:[60]PCBM devices, owing
to a longer lifetime. Using electron spin resonance and time-resolved
near-IR emission spectroscopies, we probed radical and singlet oxygen
concentrations. We demonstrate that singlet oxygen sensitized by [70]PCBM
causes the “burn-in” of PTB7:[70]PCBM devices and that
BC effectively mitigates it. Our results provide an effective solution
to the problem that currently limits widespread use of OPV.
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