Organic light-emitting diodes based on heavy metal free emissive materials

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

30.06.2023

During reporting period the final report for the project was prepared and we are pleased to provide an overview of its final stage. This report highlights the key accomplishments, findings, and outcomes achieved during this phase of the project.

Key Accomplishments:

• Successfully completed the synthesis and characterization of novel compounds from the phenantroimidazole, diethynylfluorene quinazoline and triphenylamine families.
• Conducted extensive photophysical and photoelectrical characterization of the OLED devices
• Achieved significant improvements in photoemission quantum yield for phenantroimidazole derivatives in solid films which is twice more than for solutions.
• Explored and optimized various device architectures to enhance efficiency and stability.

Findings and Results:

• Obtained comprehensive understanding of the optical and electrical properties of the developed materials
• Identified the key factors influencing the device performance, including the unique properties of emissive materials employed in the devices such as their fluorescence, phosphorescence, thermally activated delayed fluorescence (TADF), or exciplex properties. Based on these properties, careful consideration is given to choosing suitable combinations of materials and designing the stack architecture. This approach ensures optimal energy transfer, charge balance, and exciton management within the device, leading to improved efficiency and overall performance.
• Demonstrated the successful integration of the developed materials into functional OLED devices
• Evaluated the performance of the devices through various measurements, such as current-voltage characteristics, luminance, and efficiency

Outcomes and Impact:

• Contributed to the advancement of OLED technology and organic electronics;
• Generated valuable knowledge and insights through the investigation of properties of heavy-metal free organic materials for OLEDs
• Submitted research findings in reputable scientific journals and presented them at 3 international conferences

Future Directions:

Based on the outcomes of this project, there are several promising directions for further research and development:

• Exploration of alternative materials and device architectures to improve efficiency and stability;
• Investigation of scalable fabrication methods for large-area OLED production;
• Evaluation of the long-term reliability and environmental impact of the developed OLED devices.

We would like to express our gratitude to all project team members, collaborators, and funding agency for their support and contributions throughout the duration of the project. The achievements and knowledge gained through this endeavour will undoubtedly contribute to the advancement of Heavy-metal free organic materials for organic electronics.

Project progress

30.05.2023

The main focus of this project period still stayed on the technology of OLEDs fabrication and characterisation of prepared OLEDs.

The process of OLED preparation and measurement was successfully developed and applied for OLEDs fabrication. It involved several steps to ensure the optimal performance of the devices. Glass substrates coated with Indium Tin Oxide (ITO) were procured from Präzisions Glas & Optik GmbH. Prior to usage, the ITO substrates underwent a meticulous cleaning procedure, encompassing multiple stages. Initially, they were subjected to sonication in CHCl₃, followed by sonication in acetone. Subsequently, the substrates were rinsed with deionized (DI) water and further treated with sonication in a 3 vol% solution of Hellmanex II. A thorough rinsing with DI water was performed, and the substrates were then sonicated in a mixture of DI water and isopropyl alcohol. After drying with a stream of N₂, the samples were carefully transferred from the glovebox to the vacuum chamber, inside a sealed container, to facilitate the thermal evaporation of OLED layers.

Using this well-established method, all devices with synthesised substances were fabricated. The electrical characteristics of the OLEDs were assessed by measuring the current-voltage profiles using a Keithley 2700 multimeter. The luminosity characteristics, on the other hand, were evaluated utilizing a Konica Minolta Luminance and Color Meter CS-150, which also helped to determine chromaticity characteristics.

2 reports were made.

1 conference thesis where published: The 23rd International Conference on the Science and Applications of Nanotubes and Low-Dimensional Materials NT`23 was held in Arcachon near Bordeaux, France from 04.06.2023 till 09.06.2023. A poster presentation titled "Promising heavy-metal free emissive materials" was included in the program.

28.02.2023

The main focus of this project period stayed on the technology of OLEDs fabrication. The development of OLED technology depends heavily on its reliability. Thus, the questions of the degradation mechanism of OLEDs were considered during this period. Many factors have been suggested as causes of OLED degradation. The most common causes of operational instability include chemical reactions that occur within the OLED device, such as the formation of non-emissive dark states and exciton-polaron quenching. These reactions can be initiated by various factors, such as the presence of impurities or exposure to moisture and oxygen. Furthermore, electrical degradation can also occur due to the buildup of trapped charges or electric fields within the device. OLEDs can also suffer from storage instability, which is typically caused by chemical reactions that occur within the OLED layers due to exposure to moisture, oxygen, and other contaminants. To address these issues, the encapsulation technique was applied to protect OLED devices from environmental factors.

The presence of a significant energy barrier at the anode interface can generate substantial joule heat, leading to the aggregation of molecules in localized areas. To address this issue, oxygen plasma treatments have been employed to improve the contact between the anode and indium tin oxide (ITO). This approach has proven to be effective in enhancing hole-injection, resulting in improved OLED performance. Specifically, it has led to a reduction in voltage, increased efficiency, and enhanced reliability. Another approach to overcome the barrier at anode involves the use of a layer that injects holes over the ITO anode. This approach is also effective in improving the performance of OLEDs.

The functioning of OLEDs involves the creation of excited states, which are essential for their operation. However, the irreversible chemical reactions of these excited states can result in the removal of emissive species from the device. Therefore, the choice and purification of materials are crucial factors that affect the rate of emitter degradation, and the operational stability of OLED.

The another reason for device degradation is the injection of holes. The mobile ion species, such as metals, from the electrodes or other contaminants introduced during fabrication, could be responsible for this. Although these impurities could explain the initial rapid decay of luminance, it is possible that it originates from contamination introduced during device construction, such as water/oxygen adsorbed on the device's surface or out-gassed products of the epoxy cure. Once the initial contamination is eliminated by reactions at the contact, the subsequent degradation slows down.

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

Results of investigation were presented and discussed during 7th Advances in Functional Materials (AFM) Conference (The Kyushu University School of Medicine Centennial Hall, Kyushu, Japan  from January 09-12, 2023).

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.