Vygintas Jankus

Triplet Harvesting with 100% Efficiency by Way of Thermally Activated Delayed Fluorescence in Charge Transfer OLED Emitters

Organic light-emitting diodes (OLEDs) have their performance limited by the number of emissive singlet states created upon charge recombination (25%). Recently, a novel strategy has been proposed, based on thermally activated up-conversion of triplet to singlet states, yielding delayed fluorescence (TADF), which greatly enhances electroluminescence. The energy barrier for this reverse intersystem crossing mechanism is proportional to the exchange energy (ΔEST ) between the singlet and triplet states; therefore, materials with intramolecular charge transfer (ICT) states, where it is known that the exchange energy is small, are perfect candidates. However, here it is shown that triplet states can be harvested with 100% efficiency via TADF, even in materials with ΔEST of more than 20 kT (where k is the Boltzmann constant and T is the temperature) at room temperature. The key role played by lone pair electrons in achieving this high efficiency in a series of ICT molecules is elucidated. The results show the complex photophysics of efficient TADF materials and give clear guidelines for designing new emitters.

Colour tuning from green to red by substituent effects in phosphorescent tris-cyclometalated iridium(III) complexes of carbazole-based ligands: synthetic, photophysical, computational and high efficiency OLED studies

Two series of fac-tris-cyclometalated iridium(III) complexes, series 1 from the 2-(carbazol-3′-yl)-pyridine ligands, and series 2 from the isomeric 2-(carbazol-2′-yl)-pyridine ligands, have been characterised. The photoluminescence and electroluminescence from series 2 complexes are red shifted compared to series 1 complexes, due to the increased electron donating ability of the carbazole unit in series 2. The attachment of trifluoromethyl and methoxy substituents to the pyridyl ring in these complexes results in colour tuning of phosphorescence energy maxima over the range 494–637 nm (green to red). These complexes possess predominantly 3MLCT (metal-to-ligand-charge transfer) excited states. DFT/TD-DFT computations correctly predict the phosphorescence emission maxima and show that the HOMOs in these complexes contain mixed iridium and carbazolyl character. The carbazolyl ligand contributions to the excited states increase in series 2 compared to series 1. Complexes of series 1 exhibit high phosphorescence quantum yields whereas complexes of series 2 show lower quantum yields. Solution processed organic light emitting devices (OLEDs) with series 1 complexes using the high triplet poly(9-vinylcarbazole) (PVK) as the host polymer exhibit very high performances of up to 40 cd A−1 and external quantum efficiency of 12%. For series 2 the highest current efficiency is 10.3 cd A−1 and external quantum efficiency of 5.6%.

Bipolar Molecules with High Triplet Energies: Synthesis, Photophysical, and Structural Properties

This article sheds new light on the interplay of electronic and conformational effects in luminescent bipolar molecules. A series of carbazole/1,3,4-oxadiazole hybrid molecules is described in which the optoelectronic properties are systematically varied by substituent effects which tune the intramolecular torsion angles. The synthesis, photophysical properties, cyclic voltammetric data, X-ray crystal structures, and DFT calculations are presented. Excited state intramolecular charge transfer (ICT) is observed from the donor carbazole/2,7-dimethoxycarbazole to the acceptor phenyl/diphenyloxadiazole moieties. Introducing more bulky substituents onto the diphenyloxadiazole fragment systematically increases the singlet and triplet energy levels (E(S) and E(T)) and blue shifts the absorption and emission bands. The triplet excited state is located mostly on the oxadiazole unit. The introduction of 2,7-dimethoxy substituents onto the carbazole moiety lowers the value of E(S), although E(T) is unaffected, which means that the singlet-triplet gap is reduced (for 7bE(S) - E(T) = 0.61 eV). A strategy has been established for achieving unusually high triplet levels for bipolar molecules (E(T) = 2.64-2.78 eV at 14 K) while at the same time limiting the increase in the singlet energy.

Bimetallic Cyclometalated Iridium(III) Diastereomers with Non‐Innocent Bridging Ligands for High‐Efficiency Phosphorescent OLEDs

Two phosphorescent dinuclear iridium(III) diastereomers (ΛΔ/ΔΛ) and (ΛΛ/ΔΔ) are readily separated by making use of their different solubilities in hot hexane. The bridging diarylhydrazide ligand plays an important role in the electrochemistry and photophysics of the complexes. Organic light-emitting devices (OLEDs) that use these complexes as the green-emissive dopants in solution-processable single-active-layer architectures feature electroluminescence efficiencies that are remarkably high for dinuclear metal complexes, achieving maximum values of 37 cd A(-1), 14 lm W(-1), and 11% external quantum efficiency.

Solution-processable ambipolar host oligomers with high triplet energies for phosphorescent green emitters

Four carbazole–diaryloxadiazole oligomers 14–17 based on 9,9′-dioctyl-[3,3′]bicarbazolyl and 2,5-diaryl-[1,3,4]-oxadiazole monomer units have been synthesised by Suzuki cross-coupling reactions. The molecular weights of the oligomers were estimated using gel permeation chromatography: Mw 3130–4266 Da; polydispersity indices Mw/Mn 1.41–1.76 (i.e. 3–4 repeat units). These oligomers are, therefore, more like low molar mass polymers than monodisperse molecular materials. Thermal gravimetric analysis (TGA) indicated their good thermal stability with decomposition temperatures Td5% 381–440 °C. The photophysical properties have been investigated in detail in solution and as thin films. The data are consistent with the existence of an intramolecular charge transfer state (ICT) in all of these oligomers, which is enhanced by the more electron withdrawing pyridyl-oxadiazole substituent in 15. The triplet energy is sufficiently high (ET = 2.57 eV for λmax of the phosphorescence) for oligomers 14–17 to host a soluble green phosphorescent iridium guest emitter, as demonstrated in electroluminescence studies which showed emission exclusively from the guest complex. Single-emitting-layer organic light-emitting diodes (OLEDs) were constructed with the architecture glass/ITO/PEDOT:PSS/oligomer host + Ir guest (10% w/w)/Ba/Al. Devices with 16 as host demonstrated the best results reaching maximum current efficiency, ηc 3.93 cd A−1, external quantum efficiency (EQE) ηext 1.1% and good stability. The solution processability of the oligomers, their ambipolar structure, and the simplicity of the device architecture are attractive for further development.

Carbazole based polymers as hosts for blue iridium emitters: synthesis, photophysics and high efficiency PLEDs

This article reports the synthesis of new carbazole based polymers and their application as hosts in sky-blue polymer light emitting devices (PLEDs) with a solution-processed emitting layer doped with a cyclometalated Ir(III) complex. We systematically investigate their effect on the PLED performance. A current efficiency of 19.7 cd A−1 and a brightness of 1850 cd m−2 were achieved with these polymers. The roll-off in electrophosphorescent quantum efficiency in PLEDs was shown to arise mainly from triplet–triplet annihilation between dopants in the hosts with tert-butyl groups. It has been shown that in the devices with hosts without tert-butyl groups the efficiency roll-off is additionally affected by electric field quenching. In these carbazole based polymers, triplet dimers are formed and tert-butyl groups do not limit the intermolecular interactions to prevent triplet dimer formation, nevertheless tert-butyl groups reduce charge transport.