98 research outputs found
Less Is More: Dilution Enhances Optical and Electrical Performance of a TADF Exciplex
A surprising yet highly practical approach to improve the performance of a TADF exciplex blend is reported. Using the TSBPA donor and PO-T2T acceptor to form an exciplex, we are able to blue shift the emission, increase PLQY from 58 to 80%, and increase the device EQE from 14.8 to 19.2% by simply diluting the exciplex with an inert high triplet energy host materialâhere either UGH-3 or DPEPO. These effects are explained in terms of an increasing donorâacceptor distance and associated charge separation, while different behaviors observed in the different hosts are attributed to different energy barriers to electron transfer through the host. We expect that the observed performance-enhancing effects of dilution will be general to different exciplex blends and host materials and offer a new way to optimize the electrical properties of exciplex emission layers with narrow blue emission
Spiro donorâacceptor TADF emitters: naked TADF free from inhomogeneity caused by donor acceptor bridge bond disorder. Fast rISC and invariant photophysics in solid state hosts
We have studied the thermally activated delayed fluorescence (TADF) properties of the spiro-bridged donorâacceptor molecule, 10-phenyl-10H,10âČH-spiro[acridine-9,9-anthracen]-10âČ-one, (ACRSA) in guestâhost films and used it as a probe to explore the details of host effects on the TADF mechanism in typical OLED host materials. Linked by the rigid spiro CâC bond rather than a flexible CâN bond, we observe no inhomogeneous effects arising from distributions of donorâacceptor bridge dihedral angles. ACRSA displays no time dependent âapparentâ red shift of the prompt or delayed charge transfer (CT) emission. Moreover, using a range of different hosts, we show that the ground state dipole moment (dielectric value) of the host has very little effect on the ACRSA CT energy, i.e. there is no so-called âsolid state solvatochromismâ. This leads to weak stabilisation of the CT state in all hosts, but has a very small singlet triplet gap and very fast and efficient monoexponential rISC rates in films (reaching nearly 107 sâ1 in zeonex host). We observe no power law decaying DF tail because there is no dispersion of the dihedral angle between donor and acceptor units. The previously much lower reported rISC rates in ACRSA are instead reattributed to intermolecular excimer states. The intermolecular species give rise to additional slow TADF contributions and broaden the overall CT emission band at 10% ACRSA loading and in neat films. Harnessing the rapid and homogenous rISC displayed by isolated ACRSA molecules may unlock higher efficiencies and â crucially â extended operational lifetimes in future TADF OLEDs
Key requirements for ultraefficient sensitization in hyperfluorescence organic light-emitting diodes
Blue organic light-emitting diode (OLED) technology requires further advancements, and hyperfluorescent (HF) OLEDs have emerged as a promising solution to address stability and colour-purity concerns. A key factor influencing the performance of HF-OLEDs is Förster resonance energy transfer (FRET). Here we investigate the FRET mechanism in blue HF-OLEDs using contrasting thermally activated delayed fluorescence (TADF) sensitizers. We demonstrate that the molecular structure of the sensitizer profoundly impacts the FRET efficiency, exemplified by the spiro-linked TADF molecule ACRSA, which suppresses the dihedral-angle inhomogeneity and any lower-energy conformers that exhibit minimal FRET to the terminal emitter. Consequently, the FRET efficiency can be optimized to nearly 100%. Further, we demonstrate how the properties of a near-ideal sensitizer diverge from ideal TADF emitters. As a result, blue HF-OLEDs utilizing a greenish sensitizer exhibit a remarkable tripling of external quantum efficiency (~30%) compared with non-HF devices. This new understanding opens avenues for sensitizer design, indicating that green sensitizers can efficiently pump blue terminal emitters, thereby reducing device exciton energies and improving blue OLED stability
The Critical Role of nÏ* States in the Photophysics and Thermally Activated Delayed Fluorescence of Spiro Acridine-Anthracenone
The molecular photophysics and thermally activated delayed fluorescence (TADF) in spiro compounds are distinct because of the rigid orthogonal CâC bridging bond between donor and acceptor. The photophysics is found to be highly complex, with unprecedented multiple anti-Kasha emissions from three different singlet states, two of which are one-photon forbidden. The TADF mechanism is critically controlled by local acceptor nÏ* states; the singlet nÏ* state undergoes rapid intersystem crossing populating an energetically close acceptor ÏÏ* triplet state. The acceptor triplet nÏ* state couples nonadiabatically to a CT triplet state mediating reverse intersystem crossing. When the nÏ* and CT states are energetically close, TADF is greatly enhanced with rISC rate reaching 107 sâ1. We observe neither DF from the singlet nÏ* state nor electron transfer (ET) to form the 1CT because there is no ET driving force; however, ET from the higher-energy donor singlet ÏÏ* state readily occurs along with donor emission
Canny Algorithm: A New Estimator for Primordial Non-Gaussianities
We utilize the Canny edge detection algorithm as an estimator for primordial
non-Gaussianities. In preliminary tests on simulated sky patches with a window
size of 57 degrees and multipole moments up to 1024, we find a
distinction between maps with local non-Gaussianity (or
) and Gaussian maps. We present evidence that high resolution CMB
studies will strongly enhance the sensitivity of the Canny algorithm to
non-Gaussianity, making it a promising technique to estimate primordial
non-Gaussianity.Comment: 4 pages, 4 figures; v2. 5pp, as submitted to PRD; v3. 5pp, minor
clarifications and added discussion of negative fNL value
Laplace Transform Fitting as a Tool To Uncover Distributions of Reverse Intersystem Crossing Rates in TADF Systems
Donorâacceptor (DâA) thermally activated delayed fluorescence (TADF) molecules are exquisitely sensitive to DâA dihedral angle. Although commonly simplified to an average value, these DâA angles nonetheless exist as distributions across the individual molecules embedded in films. The presence of these angle distributions translates to distributions in the rates of reverse intersystem crossing (krISC), observed as time dependent spectral shifts and multiexponential components in the emission decay, which are difficult to directly quantify. Here we apply inverse Laplace transform fitting of delayed fluorescence to directly reveal these distributions. Rather than a single average value, the crucial krISC rate is instead extracted as a density of rates. The modes and widths of these distributions vary with temperature, host environment, and intrinsic DâA torsional rigidity of different TADF molecules. This method gives new insights and deeper understanding of TADF hostâguest interactions, as well as verifies future design strategies that target DâA bond rigidity
Are the Rates of Dexter Transfer in TADF Hyperfluorescence Systems Optically Accessible?
Seemingly not, but for unexpected reasons. Combining the triplet harvesting properties of TADF materials with the fast emission rates and colour purity of fluorescent emitters is attractive for developing high performance OLEDs. In this âhyperfluorescenceâ approach, triplet excitons are converted to singlets on the TADF material and transferred to the fluorescent material by long range Förster energy transfer. The primary loss mechanism is assumed to be Dexter energy transfer from the TADF triplet to the non-emissive triplet of the fluorescent emitter. Here we use optical spectroscopy to investigate energy transfer in representative emissive layers. Despite observing kinetics that at first appear consistent with Dexter quenching of the TADF triplet state, transient absorption, photoluminescence quantum yields, and comparison to phosphor-sensitised âhyperphosphorescentâ systems reveal that this is not the case. While Dexter quenching by the fluorescent emitter is likely still a key loss mechanism in devices, we demonstrate that â despite initial appearances - it is inoperative under optical excitation. These results reveal a deep limitation of optical spectroscopy in characterizing hyperfluorescent systems
Benzo[1,2-b:4,5-b']dithiophene as a weak donor component for push-pull materials displaying thermally activated delayed fluorescence or room temperature phosphorescence
In the search for high-performance donor-acceptor type organic compounds displaying thermally activated delayed fluorescence (TADF), triisopropylsilyl-protected benzo[1,2-b:4,5-b']dithiophene (BDT-TIPS) is presented as a novel donor component in combination with two known acceptors: dimethyl-9H-thioxanthenedioxide (TXO2) and dibenzo[a,c]phenazinedicarbonitrile (CNQxP). For a broader comparison, the same acceptors are also combined with the well-studied 9,9-dimethyl-9,10-dihydroacridine (DMAC) donor. Optimized BDT-TIPS-containing structures show calculated dihedral angles of around 50° and well-separated highest occupied and lowest unoccupied molecular orbitals, although varying singlet-triplet energy gaps are observed experimentally. By changing the acceptor moiety and the resulting ordering of excited states, room temperature phosphorescence (RTP) attributed to localized BDT-TIPS emission is observed for TXO2-BDT-TIPS, whereas CNQxP-BDT-TIPS affords a combination of TADF and triplet-triplet annihilation (TTA) delayed emission. In contrast, strong and pure TADF is well-known for TXO2-DMAC, whereas CNQxP-DMAC shows a mixture of TADF and TTA at very long timescales. Overall, BDT-TIPS represents an alternative low-strength donor component for push-pull type TADF emitters that is also able to induce RTP properties
Intramolecular Hydrogen Bonding in Thermally Activated Delayed Fluorescence Emitters: Is There Evidence Beyond Reasonable Doubt?
Intramolecular hydrogen bonding between donor and acceptor segments in thermally activated delayed fluorescence (TADF) materials is now frequently employed toâpurportedlyârigidify the structure and improve the emission performance of these materials. However, direct evidence for these intramolecular interactions is often lacking or ambiguous, leading to assertions that are largely speculative. Here we investigate a series of TADF-active materials incorporating pyridine, which bestows the potential ability to form intramolecular H-bonding interactions. Despite possible indications of H-bonding from an X-ray analysis, an array of other experimental investigations proved largely inconclusive. Instead, after examining computational potential energy surfaces of the donorâacceptor torsion angle we conclude that the pyridine group primarily alleviates steric congestion in our case, rather than enabling an H-bond interaction as elsewhere assumed. We suggest that many previously reported âH-bondingâ TADF materials featuring similar chemical motifs may instead operate similarly and that investigation of potential energy surfaces should become a key feature of future studies
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