Highly 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

Abstract

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...

Engineering the singlet–triplet energy splitting in a TADF molecule

“…This mechanism uses thermal energy to up-convert the lower energy triplet excitons (dark states) into emissive singlet states of higher energy, thereby surpassing the imposed 25% internal quantum efficiency. [3][4][5][6][7][8] The TADF mechanism relies on thermal energy to raise the triplet state to a vibronic sub level that is isoenergetic with the emissive singlet states by enabling reverse intersystem crossing (RISC), an energy conserving process. Therefore, the energy splitting between the singlet and triplet states (∆EST) plays a key role in the TADF emitters.…”

Using Guest–Host Interactions To Optimize the Efficiency of TADF OLEDs

“…3,4 Emission via the TADF mechanism uses thermal energy to vibronically couple the localised ( 3 LE) and charge transfer ( 3 CT) triplet states to mediate spin orbit coupling (SOC) to up-convert lower energy triplet excitons, which are in general not emissive at room temperature in organic molecules, into emissive singlet states of higher energy, thereby surpassing the imposed 25% internal quantum efficiency. [4][5][6][7][8][9] One way to improve the TADF emission contribution is to minimize the energy splitting (∆E ST ) between singlet and triplet states. Recent experimental studies [10][11][12] along with initial theoretical work [13][14][15] identify that the SOC mechanism in these TADF systems is a complex second order process requiring vibronic coupling between 3 CT and 3 LE to mediate the spin flip back to the 1 CT state, the 3 LE state mediates the SOC between the singlet and triplet CT states 15 .…”

Kinetics of thermal-assisted delayed fluorescence in blue organic emitters with large singlet–triplet energy gap

“…This avoids the quenching of triplet states due to TTA. However, often host-guest interactions give origin to the formation of emissive exciplex states showing extended fluorescence lifetimes, and more complex photophysics [13,14].Significant progress in recent years has been achieved on the design of materials that emit in the green region of the visible spectrum; however, TADF blue emitters are still scarce, and also red emitters with strong TADF contribution are not abundant. TADF is also finding application in other areas: for example, in bio-imaging applications due to the extended fluorescence lifetime and large Stokes-shift of these materials, which allows the tissue auto-fluorescence to be filtered and avoids the use of fluorescent probes based on heavy metals [15].…”

“…This avoids the quenching of triplet states due to TTA. However, often host-guest interactions give origin to the formation of emissive exciplex states showing extended fluorescence lifetimes, and more complex photophysics [13,14].…”