Melnykov S. Using the effect of delayed fluorescence to increase the efficiency of organic light-emitting diodes

The thesis is devoted to increasing the efficiency of the OLED structure based on thermally activated delayed fluorescence, the search for new and promising organic semiconductors for the exciplex-forming TADF structures in OLEDs and organic lasers. In the first chapter, the mechanism of formation of thermally activated delayed fluorescence (TADF) in exciplex-forming systems is analyzed. The relevance and practical value of organic TADF materials from the point of view of their application in OLED structures have been confirmed. New approaches to the use of organic semiconductors as an active amplification element in distributed-feedback lasers are considered. In the second chapter, the newly synthesized organic semiconductor 4-ethyl-3,5-bis[40-(N,N-diphenylamine)biphenyl-4-yl]-4H-1,2,4-triazole (TPA-TZ) is investigated. By means of thermogravimetric analysis of TPA-TZ, the melting point and glass formation temperature were determined to be 173 ºC and 68 ºC, respectively. The value of the crystallization temperature was not recorded, which negatively affects the transition of the material from the melt phase to the solid state. The values of ionization potential (IP) and electron affinity (EA) were estimated from the onset of the first oxidation and reduction potentials after formation of 5.49 and 3.14 eV, respectively. The photoluminescence and phosphorescence spectra estimated the energy of the first singlet S1 = 3.12 eV and triplet T1 = 2.63 eV excited states, which are in good agreement with the results of theoretical TD DFT calculations. It was established that the intermolecular exciplex based on the newly synthesized donor compound TPA-TZ with the acceptor component based on PO-T2T is characterized by photoluminescence decay times with a long-term (τDF=5.2 μs) component, which confirms the presence of the TADF effect. Based on the organic semiconductor TPA-TZ, an OLED structure with blue electroluminescence was formed with CIE1931 color coordinates dependent on the applied voltage: (0.20, 0.17) at 8 V and (0.20, 0.23) at 14 V. The maximum external quantum efficiency of the device was 3, 5%, and the maximum brightness is 8400 cd/m2 at 13 V. A multilayer light-emitting heterostructure with color coordinates (0.31, 0.48) was also formed on the basis of the TPA-TZ:PO-T2T intermolecular exciplex-forming system. The OLED device demonstrated a brightness of 3995 cd/m2 and an external quantum efficiency of 7% at 10 V. The third chapter presents the results of the research of three newly synthesized organic carbazole-containing semiconductors 2tCzPy, 3tCzPy and 4tCzPy. In these compounds, the energy position of the first triplet (T1) energy level is close to 3 eV, while the position of the singlet energy level S1 varies from 3.3 to 3.17 eV. As a result of singlet-triplet splitting (ΔEST) of the compounds decreased from 0.25 eV for 2tCzPy to 0.14 eV for 4tCzPy. On the basis of these materials formed full-color OLED structures with coordinates (0.31, 0.35), (0.32, 0.34) and (0.38, 0.34) close to natural white light with a maximum brightness of 10,000 cd/m2 and external quantum efficiency from 5% to 7%. In the fourth chapter, new active amplification element in distributed-feedback lasers based on newly synthesized organic semiconductors are investigated and proposed. The influence of the fluorescent impurity concentration on the generation parameters was investigated. The generation of laser emission was obtained using the newly synthesized material DG-21 at a wavelength of 545 nm at a pump energy of 686 nJ by a pulsed laser with an emission wavelength of 447 nm and pulse duration from 40 to 90 ns.