Blue thermally activated delayed fluorescence emitters for high efficiency OLEDs (2017 - 2020)

 

Project leader Dalius Gudeika

Agreement No 1.1.1.2/16/I/001

Research application No 1.1.1.2/VIAA/1/16/177

Project proposal deals with the development of low-cost, stable blue emitters based on sulfones, pyridines and benzonitriles for applications in high-efficiency organic light emitting diodes (OLEDs) based on Thermally Activated Delayed Fluorescence (TADF). The proposed emitters will contain rigid core, rendering them as potential TADF emitters, by grafting suitable donors on the emitter framework, thus enhancing triplet-singlet back conversion. These materials will be photophysically characterized in the forms of solutions and films using absorption, emission spectroscopies to quantify emission quantum yields, emission life times. OLEDs will be fabricated to evaluate the electroluminescence parameters of the materials. Having new stable TADF emitters, it is planned in the frame of the project to fabricate stable blue OLEDs with CIE chromaticity coordinates as close as possible to blue colour standard of 0.14, 0.08. For those OLEDs, it’s expected obtaining of the maximum external quantum efficiency higher (EQE) than 15%. The maximum brightness will be reached higher than 5000 cd/m2 that will unclose the practical applications of those OLEDs. Another output parameters of OLED such as the turn on voltages, the roll-off, the current (ηc) and power (ηp) efficiencies will be comparable to those of the known prototypes.


Project progress

May 21, 2019

From 1st February 2019 till 30th April 2019 highly efficient thermally activated delayed fluorescence (TADF) emitters based on 3- and 4-substituted diphenylsulfones and dibenzothiophene dioxide as the acceptors and di-tert-butyldimethyldihydroacridine as the donor moiety were synthesized. The glass forming derivatives showed glass transition temperatures in the range from 92 to 131 oC. They showed high thermal stabilities with the temperatures of onsets of thermal degradation ranging from 367 to 416 oC. Cyclic voltammetry measurements revealed close values of the ionization potentials (5.18 - 5.23 eV) and electron affinities (3.46 - 3.73 eV). The derivatives showed bipolar charge transport. 1 At electric field of 5×105 Vcm-1, electron mobilities of 3×10-5 cm2V-1s-1 and hole mobilities of 1.3×10-4 cm2V-1s-were recorded for the layer of bis(4-(2,7-di-tert-butyl-9,9-dimethyl-9,10-dihydroacridine-10-yl)phenyl)sulfone. With this TADF emitter, an organic light emitting device exhibited the high efficiency performances with a maximum current efficiency, power efficiency, and external quantum efficiency of 61.1 cd·A-1, 64.0 lm·W-1, and 24.1%, respectively.

The previously prepared work thesis based on 3-sulfone TADF materials was reported in the Journal of Luminescence (2019, 206, 250).

We will continue our work, hopefully acquiring advanced thermally activated delayed fluorescence materials for organic light emitting diodes with improved performances.

Three lectures were presented for secondary school students in the general field of TADF OLEDs and OLED materials, which are excellent platform to disseminate the advancing technology developed within the project. The students will serve as science ambassadors to the wider communities, raising the profile and prestige of publically-funded scientific research among the general public.

A session of communication activity has been done in the Lithuanian Academy of Science 12th March, 2019 to reach the general public in order to raise positive awareness of this project. This activity was used as an efficient pathway to bridge basic science (low cost, environmentally benign and efficient blue TADF emitters) and emerging technologies (new generation lightning technologies) to a non-specialized public, to contribute to a better understanding and reach social awareness and responsibility.


Project progress

February 19, 2019

From 1st November 2018 till 31th January 2019 synthesis, purification and chemical characterization of 3- and 4-sulfone based materials were be carried out. All the compounds were found to be capabale to form glasses with glass transition temperatures ranging from 82° to 91°C. They exhibited high thermal stabilities, with 5% weight loss temperatures exceeding 385 °C. The photophysical properties of the compounds were performed using techniques of absorption, fluorescence and phosphorescence in solution and solid states. Strong solvatochromism arising from the intramolecular charge transfer in the excited state was evidenced by bathochromic shifts of emission maxima with increasing solvent polarity. The compounds containing acridine and phenoxazine moieties showed relatively high photoluminescence quantum yield (up to 35%) in the non-doped solid state, long delayed fluorescence lifetime (in µs range) and small singlet-triplet energy splitting (ΔEST) that is attributed to thermally activated delayed fluorescence. Studies of electrochemical and charge transporting properties. Solid-state ionization potentials estimated by photoelectron emission spectrometry ranged from 5.6 to 5.9 eV. The compounds exhibited well-balanced hole and electron mobilities exceeding 10-5 cm2/V·s at an electric field higher than 2.5·105 V/cm.  These compounds were tested as emissive species for the fabrication of OLEDs. The sky-blue and green devices showed maximum brightness of 3200 and 12300 cd/m2.

Results based on 4-sulfone derivatives have been presented in the 9th International Conference on Molecular Electronics (ElecMol9) which was held in Paris at the Sorbonne University, France from 17th till 20th December, 2018.


Project progress

August 16, 2018

Based on the theoretical calculations of excited states and semiconducting properties, a new 1,8-naphthalimide derivative having an electron-donating 1-pyrenyl group at the C-4 position was designed and synthesized. This derivative exhibited an excellent thermal stability and bipolar charge carrier transport ability. It was found to be capable of forming glass with the glass transition temperature of 57 °C. The ionization potential of the compound established by the electron photoemission technique in air was found to be 5.93 eV. Electron and hole mobilities of ca. 10−4 cm2V−1s−1 were recorded at an electric field of 3.5 × 105 V cm−1, which are well matched with theoretical data. It was successfully utilized as a host in red phosphorescent organic light-emitting diodes showing an efficient energy transfer from the host to the phosphorescent emitter. The derivative may be a single material electroplex-forming host for PhOLEDs. The best fabricated red emitting device demonstrated maximum current, power, and external quantum efficiencies of 10.8 cd A−1, 7 lm W−1, and 13.6%, respectively. The best device exhibited a high brightness of 15300 cd m−2. Low efficiency roll-offs of the red devices, which are attributed to a deactivation by triplet-polaron quenching processes, were observed and modelled.

The prepared work thesis was reported in the international journal.


Project progress

May 18, 2018

Diphenylsulfones substituted by acridine, carbazole, phenothiazine and phenoxazine moieties were synthesized and characterized by thermal analysis, UV-, steady-state and time-resolved luminescent spectrometries, cyclic voltametry. Quantum chemical calculations on the molecular level were performed to interpret photophysical properties of the derivatives. Structural parameters, electronic properties, HOMO-LUMO gaps, molecular orbital densities, ionization potentials, reorganization energies were determined. The lowest excitation energies and the wavelengths of absorption maxima were also estimated using the time-dependent density functional theory. All the compounds were found to be capabale to form glasses with glass transition temperatures ranging from 82 to 91 oC. They exhibited high thermal stabilities, with 5% weight loss temperatures exceeding 385 oC. Strong solvatochromism arising from the intramolecular charge transfer in the excited state was evidenced by bathochromic shifts of emission maxima with increasing solvent polarity. The compounds containing acridine and phenoxazine moieties showed relatively high photoluminescence quantum yield (up to 35%) in the non-doped solid state, long delayed fluorescence lifetime (in µs range) and small singlet-triplet energy splitting (ΔEST) that is attributed to thermally activated delayed fluorescence. These compounds were tested as emissive species for the fabrication of OLEDs. The sky-blue and green devices showed maximum brightness of 3200 and 12300 cd/m2 and maximum external quantum efficiency of 6.3 and 6.9%, respectively.

The prepared work thesis was submitted to the journal.


Project progress

February 19, 2018

Phenanthroimidazole-based vinyl monomers were obtained by simple three-step procedures, and their thermal, photophysical and electrochemical properties were investigated. The solutions of the compounds exhibit emission peaks in the range from 388 to 398 nm. In the solid state, the emission of these molecules shows red shift which is coherent with the similar red-shifts of the corresponding absorption spectra. The prepared work thesis will be presented at 11-th international conference "Electronic processes in organic and inorganic materials'', 2018, May 21-25, Ivano-Frankivsk, Ukraine. The conference article will be published in the journal of “Molecular Crystals and Liquid Crystals”.

By systematic investigations of molecules with slightly different geometric arrangements between donor and acceptor moieties, the time-consuming development of suitable synthesis routes will be restricted to the most promising candidates.