Spin‐ and Voltage‐Dependent Emission from Intra‐ and Intermolecular TADF OLEDs

Abstract

Organic 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.

Conversions from Normal to Abnormal Current‐Dependent ISC and from Abnormal to Normal Current‐Dependent RISC Processes in Exciplex‐Based OLEDs

“…[3][4][5][6] Moreover, it is difficult to separate ISC and RISC processes from other processes because they generally happen at the same time and influence each other. [7][8][9][10] These complex microscopic processes lead to ambiguous understandings of physical mechanisms related to ISC and RISC processes.…”

Spontaneous exciton dissociation enables spin state interconversion in delayed fluorescence organic semiconductors

“…28), where no evidence for intermolecular charge transfer is observed for timescales up to 2 ns, also supported by modelling (vide infra). As a result, the HFI-ISC channel we observe in the other DF emitters is largely unavailable to 4CzIPN and the ISC processes must proceed via spin-orbit interactions, as generally considered in the literature 6,27 .…”

“…From the full width at half maximum (FWHM) of the negative signal (1.5 mT), when the magnetic field of the applied microwave radiation (B 1 ) is 0.12 mT, we estimate an upper bound of 0.75 mT (21 MHz) for the zero-field splitting D-parameter, which is related to the average inter-spin distance 26 . D ≤ 0.75 mT corresponds to a lower bound of ≥1.5 nm for the inter-CT state electron-hole radius (r e-h ) in neat BF2 (full calculation details in the SI) 27,28 , far larger than expected for an intramolecular exciton 26 . We also observe the presence of this same negative signal in BF2 diluted at 10 wt% in CBP (Supplementary Fig.…”

“…from the crystal value of 0.35 to 0.7 nm (Supplementary Figs. [25][26][27][28][29][30][31][32][33][34]. In Fig.…”

Tetracoordinate Boron‐Based Multifunctional Chiral Thermally Activated Delayed Fluorescence Emitters

“…[5][6][7] Compared with vacuum deposition technology, solution processes, such as spin coating and inkjet printing, are more attractive for mass production. [8][9][10][11][12] Therefore, solution-processable emitters with high efficiency are indispensable for solution-processed OLEDs. However, most reported emitters suffer from concentration quenching, exciton annihilation, and inefficient host-guest energy transfer.…”