Organic light-emitting diodes based on heavy metal free emissive materials (2021-2023)

Project leader Anna Pidluzhna

Agreement No 1.1.1.2/16/I/001

Research application No. 1.1.1.2/VIAA/4/20/592

This project focuses on the design and fabrication of light-emitting diodes (OLED) based on organic heavy metal free fluorescent materials. The OLEDs with new materials can become more efficient, be of a simpler device structure, lower fabrication costs and reduced environmental impact.

The main goal of the project is to develop new approaches to design and fabrications of organic heavy metal free emissive materials for OLED by targeting the following objectives with corresponding activities:

• Synthesize and characterise new promising compounds for OLED

• Test materials in device structures to meet industry requirements

To show the project’s goal has been achieved, organic fluorescent materials will be developed and tested in device structures that meet or exceed the 15% of external quantum efficiency.

The project is implemented at the Institute of Solid State Physics of the University of Latvia from 09.02.2021. until 30.06.2023. The total cost of the project is 106 673.02 EUR.


Project progress

31.10.2022

The optical and physical properties of five families (Phenanthroimidazole-based derivatives; Diethynyl Fluorenes; Derivatives of quinazoline and triphenylamine; Derivatives of naphthyl-phenylamine and benzophenone; Derivatives of carbazol and oxadiazole) of synthesised compounds were investigated, the absorption and emission bands for them were determined. The Ionisation potentials and Electron affinity for all synthesised substances were measured and calculated by means of photoelectron emission yield spectroscopy and photoconductivity method respectively.

The basic principles of the OLED fabrication process were analysed, tested and modified for be more suitable for synthesised substances. The main sets of OLED stacks were proposed for investigation.

Results of investigation for the Diethynyl Fluorene family were presented and discussed during joint conference Functional Materials and Nanotechnologies FM&NT – NIBS 2022 (Riga, 3-6 July, 2022).

3 project proposals were written and send for 3 different calls.


30.06.2022.

This period was finished with synthesis all planed families of promising for emission and application in OLEDs substances:

  1. Phenanthroimidazole-based derivatives;
  2. Diethynyl Fluorenes;
  3. Derivatives of quinazoline and triphenylamine
  4. Derivatives of naphthyl-phenylamine and benzophenone
  5. Derivatives of carbazol and oxadiazole

The optical and physical properties of three families of synthesised compounds were investigated completely, the absorption and emission bands for them were determined. The Ionisation potentials and Electron affinity for all synthesised substances were measured and calculated by means of photoelectron emission yield spectroscopy and photoconductivity method respectively.

Results of investigation were presented and discussed at the PROJECT SYNERGY MEETING - EMITTERS, LED, OLED (Riga, Institute of Solid State, April 13, 2022)


19.04.2022

The third class of phenanthroimidazole core-based substances was synthesised via selected by many authors following root. The targeted compounds were prepared by Ullman cross-coupling reaction in presence of CuI, 18-crown-6 (0.2 mmol), and K2CO3 (20.0 mmol) in 1,3-dimethyltetrahydropyrimidin-2(1H)-one (DMPU) at 170 ℃ for 48 h under nitrogen atmosphere. As obtained products were purified by column chromatography on silica gel using dichloromethane as the eluent.

The optical and physical properties of synthesised compounds of phenanthroimidazole family are under investigation, the absorption bands of RV-35_RV-38 compounds lie in violet and ultraviolet region and main emission maxima lie in 400-450nm region. During the standard procedures for photophysical characterisation the appearance of shoulders in a red region in emission spectra was noticed therefore the new fresh samples were fabricated and measured at once. The shoulder disappeared but the instability in these new fresh emission spectra was observed also. Because of this the new thin film samples were fabricated and encapsulated with epoxy resin (Ossilla) and cover glass. The new emission and absorption spectra became stable and shoulder in a red region disappeared as well and these results were taken for further analysis.

The basic principles of the OLED fabrication process have been started to be examined and it was determined that from the point of view of OLED architecture, type of substrate, emitter materials and fabrication procedures from-top-to-bottom and inverted architectures for OLEDs are most suitable for fabrication in ISSP facilities. After thorough data analysis the choice was made in favour of rigid glass substrate because it is in use as a substrate material in OLED lighting panel production and is most appropriate for thermal vacuum deposition technique. As results of data analysis show there are a lot of different applicable materials within hole injection, transport or blocking, electron transport or blocking, emissive materials and host matrixes. Therefore only substances with good electron and hole transporting properties which recommended themselves well will be applied for OLED fabrication.

In spite of the pandemic restrictions the meaning experimental work of the project is not influenced.


4.10.2021

During the previous standard procedures for photophysical characterisation, the appearance of shoulders in a red region in emission spectra for synthesised compounds was noticed therefore the bench of additional experiments were executed with the purpose to understand the nature of the observed phenomenon and to work out the mechanism for preventing the substance from interaction with ambient atmosphere. The new emission and absorption spectra became stable and results were taken for further analysis.

All steps of the project plan are performed according to the schedule and a one-month visit to KTU is under realisation at the moment.


7.06.2021

The second class of fluorene core-based substances was synthesised via selected by many research the following root of fluorene modification by diiodination reaction, then displacement reaction with hexyl bromide, cross-coupling using Pd/Cu catalyst, and finally deprotonation.

The optical and physical properties of synthesised compounds of diethynyl fluorene family are under investigation, the absorption bands of these compounds lie in the violet and ultraviolet region and the main emission maxima lie in 400nm region, which makes them promising as deep blue emitters.


18.04.2021.

During reporting period the Post-Doctorant started to work at the institute as the researcher, got the trainings from the proper work in the Clean Rooms, passed all tests and got the access to Clean Room services, got the trainings from the photoemission yield spectroscopy (PYS) method, photoconductivity measurements, got the access to work with Fluorescence Spectrometer FL1000 for steady state, fluorescence lifetime and phosphorescence lifetime measurements and to Dektak 150 for thin film thickness measurements (step 1.1).

According to the project plan a screening of compounds most promising for synthesis and application in OLEDs is being done and within this stage the derivative of quinazoline and triphenylamine was synthesised successfully and optical and photoelectrical properties of this compound were studied (steps 1.1, 1.2) thoroughly. This compound is fluorescently active and demonstrates emission maximum at 578 nm, 14% of photoluminescence quantum yield for the film of 1 056,5 Å thickness. The Ionisation energy determined from the PYS is 5,35±0.03 eV and the adiabatic gap obtained from the photoconductivity measurements is 2.25±0.03 eV.

All steps of the project plan are performed according to the schedule except of one visit to KTU planed for the third month because of the restrictions to the travelling under strict lockdowns in Latvia and in Lithuania. In spite of the pandemic restrictions the meaning experimental work of the project is not influenced.