Abstract
AbstractOrganic light emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) utilize molecular systems with a small energy splitting between singlet and triplet states. This can either be realized in intramolecular charge transfer states of molecules with near‐orthogonal donor and acceptor moieties or in intermolecular exciplex states formed between a suitable combination of individual donor and acceptor materials. Here, 4,4′‐(9H,9′H‐[3,3′‐bicarbazole]‐9,9′‐diyl)bis(3‐(trifluoromethyl) benzonitrile) (pCNBCzoCF3) is investigated, which shows intramolecular TADF but can also form exciplex states in combination with 4,4′,4′′‐tris[phenyl(m‐tolyl)amino]triphenylamine (m‐MTDATA). Orange emitting exciplex‐based OLEDs additionally generate a sky‐blue emission from the intramolecular emitter with an intensity that can be voltage‐controlled. Electroluminescence detected magnetic resonance (ELDMR) is applied to study the thermally activated spin‐dependent triplet to singlet up‐conversion in operating devices. Thereby, intermediate excited states involved in OLED operation can be investigated and the corresponding activation energy for both, intra‐ and intermolecular based TADF can be derived. Furthermore, a lower estimate is given for the extent of the triplet wavefunction to be ≥ 1.2 nm. Photoluminescence detected magnetic resonance (PLDMR) reveals the population of molecular triplets in optically excited thin films. Overall, the findings allow to draw a comprehensive picture of the spin‐dependent emission from intra‐ and intermolecular TADF OLEDs.
Highlights
Organic light emitting diodes (OLEDs) represent a promising alternative to conventional LEDs for display applications and room lighting
We investigate the thermally activated delayed fluorescence (TADF) characteristics of the building blocks of warm white OLEDs based on the intramolecular charge transfer (CT) emission from pristine pCNBCzoCF3 and devices based on emission from exciplex states formed between pCNBCzoCF3 and m-MTDATA
The inherently spin-sensitive techniques of electroluminescence and photoluminescence detected magnetic resonance (ELDMR, Photoluminescence detected magnetic resonance (PLDMR)) were employed to study efficient OLEDs based on intra- and intermolecular TADF effects
Summary
Organic light emitting diodes (OLEDs) represent a promising alternative to conventional LEDs for display applications and room lighting. A device combining both characteristics showed warm white emission color with high efficiency of 17.0% EQE (Kaminskiene2018) It is still an important point of discussion how the spin degree of freedom influences the first order forbidden RISC rate in operational TADF OLEDs. Since pCNBCzoCF3 shows both intra- and intermolecular CT states, it is an interesting model system to employ inherently spin sensitive methods that just recently were applied for the first time to donor:acceptor based intermolecular TADF OLEDs (Vaeth2017, Bunzmann2020), but not yet to intramolecular emitters or their combination. Holes overcome the energetic barrier at the m-MTDATA:pCNBCzoCF3 interface and form excitons with electrons in the pCNBCzoCF3 layer, giving rise to the observed shoulder in the EL spectrum This effect, does not need to be considered a deficiency since the use of an intramolecular TADF emitter as one of the constituents for an exciplex emitter can be beneficial for device efficiency (Liu2016). With a combination of the sky-blue emission of pristine pCNBCzoCF3 and the orange emission of the exciplex, a warm white OLED can be realized (Kaminskiene2018, Sych2020). a
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