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.

Simple trilayer, deep blue, fluorescent exciplex organic light‐emitting diodes (OLEDs) are reported. These OLEDs emit from an exciplex state formed between the highest occupied molecular orbital (HOMO) of N,N′‐bis(1‐naphthyl)N,N′‐diphenyl‐1,1′‐biphenyl‐4,4′‐diamine (NPB) and lowest unoccupied molecular orbital (LUMO) of 1,3,5‐tri(1‐phenyl‐1H‐benzo[d]imidazol‐2‐yl)phenyl (TPBi) and the NPB singlet manifold, yielding 2.7% external quantum efficiency at 450 nm. It is shown that the majority of the delayed emission in electroluminescence arises from P‐type triplet fusion at NPB sites not E‐type reverse intersystem crossing because of the presence of the NPB triplet state acting as a deep trap.

Article type: Full PaperHighly efficient TADF OLEDs; how the emitter-host interaction controls both the excited state species and electrical properties of the devices to achieve near 100% triplet harvesting and high efficiency.Vygintas Jankus, * (OLEDs). Molecules that have a charge transfer (CT) excited state can potentially achieve this through the mechanism of thermally activated delayed fluorescence 2 (TADF). Here, it is shown that a D-A charge transfer molecule in the solid state, can emit not only via an intramolecular charge transfer (ICT) excited state, but also from exciplex states, formed between the molecule and the host material. OLEDs based on one of our previously studied D-A-D molecules in a host TAPC achieves >14% external electroluminescence yield and shows nearly 100% efficient triplet harvesting. In these devices it is unambiguously established that the triplet states are harvested via TADF, but more interestingly these results are found to be independent of whether the emitter is the ICT state or the D-A-D/host exciplex. IntroductionArtificial lighting is an essential part of our lives, which consumes 19% of the planet's electricity usage, and cheaper ways of creating energy, and more efficient devices that use less electricity, are needed to reduce energy costs, and greatly cut CO 2 production. Ultraefficient lighting will play an important role in achieving this. In particular, white organic light emitting diodes (OLEDs) could become an integral part of the new lighting technologies; however, alternatives to Ir based phosphors, and especially much improved deep blue OLED emitters, are needed in order to give high quality, efficient white OLEDs that are not reliant on scarce rare-earth metals.Two types of excited states are created when charge recombines in an OLED -singlet and triplet excitons, but only the singlets directly give light, which fundamentally limits external OLED efficiency to 5%. [1] Thus the efficiency can be increased fourfold if the non-emissive triplets can be utilised. Currently, phosphorescent heavy metal complexes are used to 'harvest' the triplet states and generate light. [2][3][4] Unfortunately, pushing the metal-to-ligand charge transfer excited state of these complexes into the blue opens a non-radiative pathway via the 3 metal d-orbitals, which limits efficiency [5] , and makes the complexes thermally and photochemically unstable. [6] The production of singlets via triplet-triplet annihilation (TTA)i.e. triplet fusion (TF), has also been demonstrated in OLEDs, [7,8] but it has been shown that triplet annihilation can contribute to both an increase and a loss in yield in OLEDs, [9] and the maximum theoretical external quantum efficiency (EQE) 12.5%, when accounting for emission arising from TF, has not been reached. However, deep blue emission is essential to achieve the required colour rendering and efficiency in white OLEDs for lighting applications, therefore, other processes that can convert triplets to singlets must be found.An alternative way to convert t...

A new family of anthracene core, highly fluorescent emitters is synthesized which include diphenylamine hole transport end groups. Using a very simple one or two layer organic light emitting diode (OLED) structure, devices without outcoupling achieve an external quantum efficiency of 6% and photonic efficiencies of 20 cd/A. The theoretical maximum efficiency of such devices should not exceed 3.55%. Detailed photophysical characterization shows that for these anthracene based emitters 2T1≤Tn and so in this special case, triplet fusion can achieve a singlet production yield of 0.5. Indeed, delayed electroluminescence measurements show that triplet fusion contributes 59% of all singlets produced in these devices. This demonstrates that when triplet fusion becomes very efficient, fluorescent OLEDs even with very simple structures can approach an internal singlet production yield close to the theoretical absolute maximum of 62.5% and rival phosphorescent‐based OLEDs with the added advantage of much improved stability.

Understanding the delayed fl uorescence mechanism behind the creation of emissive singlets from the non-emissive triplets in exciplexes is vital for the fabrication of highly effi cient blue fl uorescent emitters, and subsequent white light applications. In this article we report the spectroscopic investigation of the exciplex formed between 4,4′,4′′-tris[3-methylphenyl(phenyl)amino] triphenylamine (m-MTDATA) and 2-(biphenyl-4-yl)-5-(4-tert-butylphenyl)-1,3,4oxadiazole (PBD) in a 50:50 blended fi lm. The mechanism behind extra singlet production in the blend is of E-type nature, that is, "thermally activated" delayed fl uorescence. The exciplex singlet-triplet energy splitting is estimated to be around 5 meV, smaller than previously estimated at ≈50 meV. The absence of a well defi ned separation between prompt emission and emission components with very long lifetimes, >100 ns, is indicative of such a small exchange energy, and arises through multiple cycling between the resonant singlet and triplet manifolds before eventually being emitted from a singlet state. An observed redshift of the exciplex emission with time and increasing temperature is attributed to different exciplex species being formed between the m-MTDATA and PBD molecules.