Title: Investigation of the Crystalline Emitting Layers Exhibiting Emission Facilitated by Triplet States for White Organic Light Emitting Diodes
Research proposal No: 1.1.1.9/LZP/1/24/140
Duration: 01.04.2025.-31.03.2028.
Project Leader: Oleksandr Bezvikonnyi
Total budget: 184 140 EUR
European Regional Development Fund (ERDF) funding: 156 519 EUR
ISSP UL budget: 9207 EUR
Project description:
The biggest obstacle to the dominance of the technology of white organic light emitting diodes (WOLED) in the market of lighting and displays is low efficiency and stability. The project aims to develop efficient crystalline OLEDs. These devices will utilize emitters exhibiting crystallinity in emitting layers and triplet-facilitated emission. The phenomena like thermally activated delayed fluorescence (TADF), room temperature phosphorescence (RTP), etc. are prioritized as non-toxic, environmentally friendly approaches. State-of-the-art efficiency and stability will be pursued by using newly designed and synthesized crystalline organic semiconducting (COS) compounds with highly oriented dipoles, high drift mobility of charge carriers, and enhanced morphological features. Molecular packing of COS facilitated by crystallization in emitting layers will be used for the enhancement of the efficiency of WOLEDs with target values of 30% of external quantum efficiency (EQE) and color rendering index (CRI) of 90. The target values will be achieved by optimization of the device structure incorporating different emitting layers systems, transport layers, thickness of the layers to improve the charge balance, out-coupling and the quality of the white light.
This project focuses on the development of efficient white organic light emitting diodes (WOLEDs) utilizing emitting organic semiconductors exhibiting crystallinity in emitting layers of the devices and utilizing triplet excitons in electroluminescence. The main goals of the project include design, synthesis and characterization of the crystalline organic emitters exhibiting triplet-facilitated phenomena like thermally activated delayed fluorescence (TADF), room temperature phosphorescence (RTP), etc. The target values of the OLEDs based on the emitters are external quantum efficiency (EQE) of 30% and white light with the color rendering index (CRI) of 90.
The project is implemented at the Institute of Solid State Physics of the University of Latvia from 01.04.2025. until 31.03.2028. The total cost of the project is 184,140.00 EUR.
PROJECT PROGRESS
Time period: 01.09.2025. - 31.01.2026.
During the period of time, the postdoctoral student, in cooperation with the partner institutions and their collaborators, achieved the following results according to the plan of the implementation of the project.
The series of cyanopyridine-based compounds were found to be characterized by high thermal stability, with the 5% weight loss temperature ranging from 456 to 529 °C. Photophysical characterization of the compounds revealed that the emissive properties, the charge transfer (CT) characteristics, and the position of the emission spectral peak are determined by the strength of the electron donor moiety attached to the structure of the compounds. The emission of one of the samples of the compounds exhibited an interplay of local excited state emission and CT. The effective electronic excitation energy transfer from host to guest was achieved for the developed host-guest systems, utilizing the investigated compounds as dopants in films. The study of the samples at different temperatures confirmed the thermal activation of TADF and excluded other phenomena as a possible origin of the detected long-living emission. The strategy for the development of the host-sensitizer-guest system utilizing the investigated compounds and TADF COS for the efficient white light was developed.
A few more series of the compounds were investigated in a similar fashion. The compounds that were found to be COS exhibited the superior characteristics of the long-living triplet-facilitated emission in comparison to the non-crystalline counterparts in each series of the compounds. One of the investigated compounds demonstrated the signs of interplay of RTP and TADF in several samples, including efficient guest-host systems.
The data of thermal, electrochemical, photophysical, charge transport characteristics, and the energy levels of frontier molecular orbitals derived from the photoelectron emission spectroscopy make the investigated compounds suitable for utilization in efficient OLEDs.
The presentation of the preliminary results of the investigation as a part of the implementation of the postdoctoral project was done at the 15th International Conference on Optical Probes of Organic and Hybrid Semiconductors (OP2025) held in Fukuoka, Japan. The poster title was “Derivatives of cyanopyridine exhibiting thermally activated delayed fluorescence for efficient organic light emitting diodes”. During participation in the conference, the postdoctoral researcher was involved in networking, scientific discussion, and dissemination of the obtained results.
The postdoctoral researcher visited the scientific group of Dr. Glib Baryshnikov of the Laboratory of Organic Electronics of Linköping University in Norrköping, Sweden, from 01.12.2025 to 06.12.2026. During the visit, the postdoctoral researcher attended the facilities of the partner institution and had discussions about the theoretical investigations of the investigated cyanopyridine-based compounds in the scope of the project implementation. Additionally, the postdoctoral researcher had an oral presentation at the seminar titled “Triplet-facilitated emission and its practical application" on 04.12.2025.
Time period: 01.04.2025. – 31.08.2025.
During the period of time, the postodoctoral student started to work as a visitor researcher at the Institute of Solid State Physics of the University of Latvia, familiarized himself with the internal regulations and safety procedures and gained access to work in the facilities.
The measures were undertaken by the postdoctorant and partner institutions of the project for the development of the donor-acceptor organic semiconductors exhibiting phenomena of thermally activated delayed fluorescence (TADF) with the possibility of occurrence of other triplet-facilitated phenomena like room temperature phosphorescence (RTP). In accordance with the project plan and in collaboration with the project partner institution Lviv Polytechnic National University and their cooperators, several series of promising compounds were synthesized in the scope of utilization of their crystallinity properties and triplet-facilitated emission.
One of the series is cyanopyridine-based derivatives with different electron donor moieties attached, like phenoxazine, phenothiazine, dimethylacridine and tert-butylcarbazole that are selected for achieving thermal stability, high charge transport and quantum efficiency of the compounds. The donor-acceptor structure must ensure the minimization of the overlap of the highest occupied and lowest unoccupied molecular orbitals, HOMO and LUMO, respectively, stimulating TADF. The scientific group of the partner institution Linköping University works on the theoretical calculations using the density functional theory, which were at the basis of the design strategy and will be continued to explore the distribution of the excited states and their correlation with emissive and crystallinity properties.
Another series is diaminobenzophenone derivatives with four fluorene arms aiming at high hole drift mobility, TADF property due to donor-acceptor interactions (Figure 2a). Differential scanning calorimetry (DSC) scans revealed that one of the compounds of the series exhibits a crystallinity peak making it a promising candidate as the crystalline organic semiconductor for the project implementation.
Several other design strategies are considered during the development of the compounds based on literature review and theoretical calculations. Their synthesis is ongoing.