Search

We thank the EPSRC for financial support (grants EP/P010482/1 and EP/R035164/1). We thank the EPSRC UK National Mass Spectrometry Facility at Swansea University for analytical services. G.C. acknowledges funding by the European Commission through a Marie Skłodowska Individual Fellowship (No. 799302). C.K. acknowledges support from the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2017R1A6A3A03012331). Three novel donor-acceptor molecules comprising the underexplored pyridazine (Pydz) acceptor moiety have been synthesized and their structural, electrochemical and photophysical properties thoroughly characterized. Combining Pydz with two phenoxazine donor units linked via a phenyl bridge in a meta configuration ( dPXZMePydz ) leads to high reverse intersystem crossing rate krISC = 3.9 ∙ 106 s-1 and fast thermally activated delayed fluorescence (TADF) with

The development of deep blue OLED materials is a challenging problem – both for phosphorescent and TADF materials. We show that Ir(III) complexes with N-heterocyclic carbene ligands provide a route to deep blue OLED materials with CIE co-ordinates (0.15, 0.05) and external quantum efficiency above 12% at 100 Cd/m2.

A series of four novel deep-blue to sky-blue thermally activated delayed fluorescence (TADF) emitters (2CzdOXDMe, 2CzdOXD4MeOPh, 2CzdOXDPh, and 2CzdOXD4CF3Ph) have been synthesized and characterized. These oxadiazole-based emitters demonstrated bluer emission compared with the reference emitter 2CzPN thanks to the weaker acceptor strength of the oxadiazole moieties. The oxadiazole compounds doped in hosts (mCP and PPT) emitted from 435 to 474 nm with photoluminescence quantum yields ranging from 14–55%. The emitters possess singlet–triplet excited-state energy gaps (ΔEST) between 0.25 and 0.46 eV resulting in delayed components ranging from 4.8 to 25.8 ms. The OLED device with 2CzdOXD4CF3Ph shows a maximum external quantum efficiency of 11.2% with a sky-blue emission at CIE of (0.17, 0.25), while the device with 2CzdOXD4MeOPh shows a maximum external quantum efficiency of 6.6% with a deep-blue emission at CIE of (0.15, 0.11). E.Z.-C. thanks the University of St Andrews for support. We are grateful to the EPSRC for financial support (grants EP/P010482/1, EP/J01771X, and EP/J00916). I.D.W.S. is a Royal Society Wolfson Research Merit Award Holder. We thank the EPSRC UK National Mass Spectrometry Facility at Swansea University for analytical services. C.M. acknowledges funding by the European Commission through a Marie Skłodowska Curie Individual Fellowship (No. 703387). G.C. acknowledges funding by the European Commission through a Marie Skłodowska Individual Fellowship (No. 799302).

The authors are grateful to the University of St Andrews and EPSRC financial support (grants EP/P010482/1 and EP/M02105X/1). A.K.P thanks the Leverhulme Trust Early Career Fellowship (ECF-2017-326). I.D.W.S. acknowledges a Royal Society Wolfson Research Merit award. High‐efficiency pure blue phosphorescent organic light‐emitting diodes (OLEDs) remain one of the grand challenges, principally because the emissive complexes employed either do not possess sufficiently high photoluminescence quantum yields or exhibit unsatisfactory Commission International de l'Éclairage (CIE) coordinates. Here two deep‐blue‐emitting homoleptic iridium(III) complexes are reported and OLEDs are demonstrated with CIE coordinates of (0.15, 0.05) and maximum external quantum efficiency of 13.4%, which decreases slightly to 12.5% at 100 cd m−2. They represent examples of the most efficient OLEDs surpassing the CIEy requirement of the National Television System Committee (NTSC) and the European Broadcasting Union (EBU). Emitter orientation contributes to the excellent device performance. Postprint

In the past few decades Organic Light-Emitting Diodes (OLEDs) have matured to become a widespread display technology. Despite their commercial success in TVs, smartphones and tablet screens, several key issues still remain, especially if OLEDs are to become a viable technology for lighting. Metalorganic emitters based on Ir or Pt are widely used in OLED technology, due to their facile colour tunability, short phosphorescence lifetime and efficient intersystem crossing (ISC), which enables emission from both singlet and triplet states to be harnessed, leading to 100% internal quantum efficiency (IQE). However, these emitters are based on rare and toxic noble metals. In addition, efficient and stable blue and especially deep-blue phosphorescent emitters, which are essential for displays and lighting, are still to be demonstrated in order to replace currently used less efficient fluorescent blue OLEDs. Recently, purely organic emitters exhibiting thermally activated delayed fluorescence (TADF) mechanism were also shown to be capable of 100% IQE. Since efficient ISC in these compounds is based on the small singlet and triplet gap rather than heavy atom effect, TADF compounds represent a new class of potentially inexpensive emitters for next generation OLED displays and lighting. In this work, a series of four deep blue-to-green emitting TADF compounds have been synthesized and characterised. The compounds are based on the same scaffold as the dicarbazoyldicyanobenzene (2CzPN) reported by Adachi and co-workers, but with the replacement of the cyano groups by less electron-withdrawing oxadiazole moieties. The weaker acceptor strength of oxadiazole compared to cyano groups translates to more blue-shifted emission compared to 2CzPN. Additionally, higher spatial separation of HOMO and LUMO levels between acceptor and donor units, respectively, leads to a smaller singlet-triplet gap. This allows us to demonstrate efficient deep blue TADF OLEDs with CIE coordinates (0.16, 0.13), as compared to (0.16, 0.30) for 2CzPN devices. A thorough photophysical study of model compound 2CzPN and oxadiazole acceptor based derivatives will be presented both for solution and thin films. The blue oxadiazole compounds show up 75% quantum yield in solid state. The electrochemical and photophysical properties, as well as crystal structures are compared to the theoretical quantum chemical calculations of the studied compounds. The efficient emission properties coupled to the small singlet-triplet gap in these molecules allows us to demonstrate efficient electroluminescence (up to 10 % EQE) from the vacuum deposited OLEDs.

New pyrenyl mono- and disubstituted fluorene and carbazole derivatives were synthesized and investigated as blue-emitting materials. Most of the synthesized compounds were capable of forming glasses with glass transition temperatures up to 105 °C. The mono- and disubstituted compounds exhibited efficient emission in a diluted form with the peak wavelengths of 416 and 422 nm and quantum yields of 0.72 and 0.82, respectively. Photophysical properties of the compounds in dilute solutions and solid state were investigated experimentally and rationalized by density functional theory calculations. The calculations revealed a low energy barrier for intramolecular twisting in the ground and excited states. The electronic spectra of the compounds were dominated by an allowed “pyrene-like” S0 → S2 transition prevailing in nearly perpendicularly twisted molecular structures and competitive S0 → S1 transition becoming increasingly allowed for more planar conformers. The disubstituted compounds demonstrated a 3-fold e...

Light-induced transient grating and photoluminescence measurements were employed for carrier recombination studies in variously Mg doped GaN layers. Carrier lifetime and ambipolar diffusion coefficient were found to decrease with doping from 210 to 20 ps and from 2.0 to 0.9 cm2/s,respectively, which proved the degradation of electrical quality of the layers. A threshold of stimulated emission was found to depend non-monotonously on doping and had the lowest value of 0.19 mJ/cm2 in the most doped layer. This dependence was explained in terms of degeneracy of the hole system.

Recently, bilayer resist processing combined with development in hydrofluoroether (HFE) solvents has been shown to enable single color structuring of vacuum-deposited state-of-the-art organic light-emitting diodes (OLED). In this work, we focus on further steps required to achieve multicolor structuring of p-i-n OLEDs using a bilayer resist approach. We show that the green phosphorescent OLED stack is undamaged after lift-off in HFEs, which is a necessary step in order to achieve RGB pixel array structured by means of photolithography. Furthermore, we investigate the influence of both, double resist processing on red OLEDs and exposure of the devices to ambient conditions, on the basis of the electrical, optical and lifetime parameters of the devices. Additionally, water vapor transmission rates of single and bilayer system are evaluated with thin Ca film conductance test. We conclude that diffusion of propylene glycol methyl ether acetate (PGMEA) through the fluoropolymer film is the main mechanism behind OLED degradation observed after bilayer processing.

Novel donor-acceptor compounds consisting of singly bonded fluorene (Fl), benzothiadiazole (BT), and carbazole (Cz) functional units in the same molecule were investigated. Analysis of the optical spectra and fluorescence transients of the compounds revealed the domination of intramolecular charge transfer (ICT) states with high fluorescence quantum yield (72%-85%). A similar Cz-Fl-Cz compound exhibiting 100% fluorescence quantum yield and no ICT character was also studied as a reference to reveal the impact of electron-accepting BT groups. Thorough examination of the optical properties of the compounds in different media, i.e., dilute solution and polymer matrix, indicated their twisted conformations due to steric hindrance in the ground state and flattened geometry in the excited state for both reference and ICT compounds. Remarkable fluorescence efficiency losses (amounting to 70%) observed upon casting the molecular solutions into neat films were determined to originate from the low-fluorescent twisted conformers and migration-facilitated exciton quenching. The majority of emission efficiency losses (over 70%) were caused by the twisted conformers, whereas only less than 30% by exciton-migration-induced nonradiative deactivation.