Thermally Activated Delayed Fluorescence (Green) in Undoped Film and Exciplex Emission (Blue) in Acridone–Carbazole Derivatives for OLEDs (original) (raw)
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Efficient thermally activated delayed fluo-rescence (TADF) has been characterized for a carbazole/ sulfone derivative in both solutions and doped films. A pure blue organic light emitting diode (OLED) based on this compound demonstrates a very high external quantum efficiency (EQE) of nearly 10% at low current density. Because TADF only occurs in a bipolar system where donor and acceptor centered 3 ππ* states are close to or higher than the triplet intramolecular charge transfer (3 CT) state, control of the π-conjugation length of both donor and acceptor is considered to be as important as breaking the π-conjugation between them in blue TADF material design.
In this work we introduce a new acceptor for use in thermally activated delayed fluorescence (TADF) emitters, pyridylbenzimidazole, which when coupled with phenoxazine, allows efficient TADF occurs. N-functionalization of the benzimidazole using methyl, phenyl and tert-butyl groups permits colour tuning and adjustment of the optoelectronic properties and suppression of aggregation-caused quenching (ACQ) with minimal impact on the TADF mechanism. Good photoluminescence quantum yields, FPL, (49% - 68%) in mCP-doped films are reported for the emitters. The functionalized derivatives support a higher doping of 7 wt% before a fall-off in FPL is observed, in contrast with the parent compound, which undergoes ACQ at doping concentrations greater than 1 wt%. We find complex conformational dynamics, reflected in the time-resolved decay profile. DEST is modulated by N-substituents of the benzimidazole and ranges of between 0.22 eV and 0.32 eV in doped films. Density Functional Theory (DFT) ca...
Advanced Optical Materials, 2021
Organic light‐emitting materials exhibiting thermally activated delayed fluorescence (TADF) show great promise for improving display applications. Recently, intermolecular effects between emitting molecules have been given more attention, revealing strong solid‐state solvation or aggregation induced changes of sample performance. Implications of this on device performance are not yet fully covered. In this work, a thorough investigation of a novel TADF emitter, methyl 2,3,4,5,6‐penta(carbazol‐9‐yl)benzoate (5CzCO2Me), is provided. Steady‐state emission spectra reveal a luminescence redshift with increasing emitter concentration in a small molecule host. In all investigated concentrations, the emission profile remains the same; thus, the redshift is attributed to the solid‐state solvation effect. The highest photoluminescence quantum yield (PLQY) is achieved in the 20 wt% sample, reaching 66%. The best organic light‐emitting diode (OLED) in terms of current–voltage–luminance and exte...
Nature communications, 2018
The lack of structural information impeded the access of efficient luminescence for the exciplex type thermally activated delayed fluorescence (TADF). We report here the pump-probe Step-Scan Fourier transform infrared spectra of exciplex composed of a carbazole-based electron donor (CN-Cz2) and 1,3,5-triazine-based electron acceptor (PO-T2T) codeposited as the solid film that gives intermolecular charge transfer (CT), TADF, and record-high exciplex type cyan organic light emitting diodes (external quantum efficiency: 16%). The transient infrared spectral assignment to the CT state is unambiguous due to its distinction from the local excited state of either the donor or the acceptor chromophore. Importantly, a broad absorption band centered at ~2060 cm was observed and assigned to a polaron-pair absorption. Time-resolved kinetics lead us to conclude that CT excited states relax to a ground-state intermediate with a time constant of ~3 µs, followed by a structural relaxation to the or...
Beilstein Journal of Organic Chemistry, 2018
The design of highly emissive and stable blue emitters for organic light emitting diodes (OLEDs) is still a challenge, justifying the intense research activity of the scientific community in this field. Recently, a great deal of interest has been devoted to the elaboration of emitters exhibiting a thermally activated delayed fluorescence (TADF). By a specific molecular design consisting into a minimal overlap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) due to a spatial separation of the electron-donating and the electron-releasing parts, luminescent materials exhibiting small S1–T1 energy splitting could be obtained, enabling to thermally upconvert the electrons from the triplet to the singlet excited states by reverse intersystem crossing (RISC). By harvesting both singlet and triplet excitons for light emission, OLEDs competing and sometimes overcoming the performance of phosphorescence-based OLEDs could be fabricated, j...
ACS Applied Materials & Interfaces, 2019
We present a study of two isomeric thermally activated delayed fluorescence (TADF) emitters 9,9'-(sulfonylbis(pyrimidine-5,2-diyl))bis(3,6-di-tert-butyl-9H-carbazole) (pDTCz-DPmS) and 9,9'-(sulfonylbis(pyrazine-5,2-diyl))bis(3,6-di-tert-butyl-9H-carbazole) (pDTCz-DPzS). The use of pyrimidine and pyrazine as bridging units between the electron donor and acceptor moieties is found to be advantageous compared to the phenyl-(pDTCz-DPS) and pyridine-based analogues (pDTCz-3DPyS and pDTCz-2DPyS). Conformational modulation of the donor groups as a function of the bridge results in high photoluminescence quantum yields (FPL > 68%) and small energy gaps between singlet and triplet excited states (ΔEST < 160 meV). OLEDs using pDTCz-DPmS and pDTCz-DPzS as emitters exhibit blue and green electroluminescence, respectively, with higher maximum external quantum efficiencies (EQEmax of 14% and 18%, respectively) and reduced efficiency roll-off as compared to the reference devices using pDTCz-DPS, pDTCz-3DPyS, and pDTCz-2DPyS as the emitters. Our results provide a more complete understanding on the impact of the bridge structure in D-AD TADF systems on the optoelectronic properties of the emitter, and how the balance between color purity and EQE in the devices can be controlled, advancing the design strategies for TADF emitters.
We report a series of pentacarbazolyl-benzonitrile derivatives such as 2,4,6-tri(9H-carbazol-9-yl)-3,5-bis(3,6-di-(pyridin-3-yl)-9H-carbazol-9-yl)benzonitrile (mPyBN), 3,5-bis-(3,6-bis(4-(trifluoromethyl)phenyl)-9H-carbazol-9-yl)-2,4,6-tri-(9H-carbazol-9-yl)benzonitrile (pCF3BN), 2,4,6-tri(9H-carbazol-9-yl)-3-(3,6-di(pyridin-3-yl)-9H-carbazol-9-yl)-5-(3,6-diphenyl-9H-carbazol-9-yl)benzonitrile (PyPhBN), 3-(3,6-bis(4-(trifluoromethyl)phenyl)-9H-carbazol-9-yl)-2,4,6-tri(9H-carbazol-9-yl)-5-(3,6-di(pyridin-3-yl)-9H-carbazol-9-yl)benzonitrile (PyCF3BN), and 3-(3,6-bis(4-(trifluoromethyl)phenyl)-9H-carbazol-9-yl)-2,6-di(9H-carbazol-9-yl)-5-(3,6-di(pyridin-3-yl)-9H-carbazol-9-yl)-4-(9H-pyrido[3,4-b]indol-9-yl)benzonitrile (CbPyCF3BN) in which some of the carbazoles are substituted with modified 3,5-diphenyl carbazoles, exhibiting thermally activated delayed fluorescence (TADF) properties. These emitters comprised two, three, and four different types of donors, capable of bluish-green emission of around 480 nm with relatively high photoluminescence quantum yields over 90% in solution. Emitters, namely, PyPhBN, PyCF3BN, and CbPyCF3BN, composed of three and four different types of donors endowed a rather short delayed lifetime (τ d) of 4.25, 5.01, and 3.65 μs in their film state, respectively. Bluish-green organic light-emitting diodes based on PyPhBN, PyCF3BN, and CbPyCF3BN exhibit a high external quantum efficiency of 20.6, 19.5, and 19.6%, respectively, with unsurpassed efficiency roll-off behavior. These results indicate that the TADF properties of multidonor type molecules can be manipulated by controlling the types and number of electron donor units.
Purely organic thermally activated delayed fluorescence (TADF) emitting materials for organic light-emitting diodes (OLEDs) enable a facile method to modulate the emission color through judicious choice of donor and acceptor units. Amongst purely organic TADF emitters, the development of TADF molecules that emit at longer wavelengths and produce high-efficiency devices that show low efficiency roll-off remains a challenge. We report a modular synthesis route that delivers three structurally related fluorinated dibenzo[a,c]-phenazine-based TADF molecules, each bearing two donor moieties with different electron-donating strengths, namely 3,6-bis(3,6-di-tert-butyl-9H-carbazol-9-yl)-10-fluorodibenzo[a,c]phenazine (2DTCz-BP-F), 3,6-bis(9,9-dimethylacridin-10(9H)-yl)-10-fluorodibenzo[a,c]phenazine (2DMAC-BP-F) and 10,10'-(10-fluorodibenzo[a,c]phenazine-3,6-diyl)bis(10H-phenoxazine) (2PXZ-BP-F). They exhibit donor strength-controlled color-tuning over a wide color range from green to d...
Journal of Chemical Sciences, 2019
Acridone (acceptor) and naphthylamine (donor) based Donor-Acceptor-Donor (D-AD) compound (1) was synthesised, characterised and its thermally-activated delayed fluorescence (TADF) properties were studied in detail. Compound 1 is fluorescent and emits in the green region (550 nm). The energy gap between the ground and the lowest excited singlet (S 1) state is estimated to be 2.55 eV. The energy gap between the CT singlet and triplet states (DE ST) was found to be *0.3 eV. Small DE S1-T1 is one of the important criteria for TADF to take place in a molecule and thus detailed photophysics has been studied. Transient lifetime measurements showed an increase in the fluorescence lifetime (s) on purging with N 2 , as compared with that in air-saturated solution, indicating the involvement of the triplet state in emission. Emission at 550 nm was also observed with a delay of 100 ls which corresponded to the delayed fluorescence in 1. The lifetime of TADF was found to be 176 ls. Applications of TADF materials in organic lightemitting devices (OLEDs) has gotten attention as TADF materials utilise the triplet excitons which helps in increasing internal quantum efficiency of device. Air-saturated based on 1 were fabricated and their intensity was found to be nearly as high as 17,000 Cd/m 2 at 25 mA/cm 2 which was comparable to many of the known TADF emitters.
Chemical science, 2017
A novel bipolar hosting material, 11-(3-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-12,12-dimethyl-11,12-dihydroindeno[2,1-a]carbazole (DPDDC), was designed, synthesized, and characterized for green phosphorescent organic light-emitting diodes (PhOLEDs). The DPDDC exhibits excellent hole and electron transport properties, superior thermal stability, a high glass-transition temperature and a small singlet-triplet energy gap for efficient reverse intersystem crossing from triplet to singlet, reducing the triplet density of the host for PhOLEDs. The electrophosphorescence properties of the devices using DPDDC as the host and three green phosphorescent iridium(iii) complexes, bis(2-(4-tolyl)pyridinato-N,C2')iridium(iii) acetylacetonate, bis(2-phenylpyridine)iridium(iii) acetylacetonate, and bis(4-methyl-2,5-diphenylpyridine)iridium(iii) acetylacetonate [(mdppy)2Iracac] as the emitter were investigated. The green PhOLED with 5 wt% (mdppy)2Iracac presents an excellent performance, includ...