Themed collection Materials for thermally activated delayed fluorescence and/or triplet fusion upconversion

Guest editors Malika Jeffries-EL, Nobuhiro Yanai and Eli Zysman-Colman introduce this themed issue on thermally activated delayed fluorescence, and triplet–triplet annihilation-based photon upconversion, materials.

Photon upconversion (UC) stands for the conversion of low to high energy photons, a promising approach to improve solar cells. While high efficiencies can be obtained in liquid UC, will a solid UC device be able to reach such levels?

The photophysical insights of thermally activated delayed fluorescence emitters for OLEDs obtained by time-resolved EPR spectroscopy were summarized.

This review presents design strategies to obtain novel materials, for generating unique sets of photophysical properties with structure-functionality that can influence their triplet-to-singlet upconversion, leading to efficient OLEDs.

Intermolecular TADF including bulk and interface exciplexes, can be used as emitters to achieve high efficiency, and also as hosts to extend the device lifetime by reducing the triplet exciton concentration and improving charge balance.

The experimental requirements for reproducible, all-optical and minimally invasive temperature sensing, based on the temperature dependence of the triplet–triplet annihilation photon upconversion (TTA–UC) in a soft matter environment are reviewed.

Recent progress of efficient OLEDs based on sulphur-containing heterocyclic compounds, thiophene, phenylsulfone, thiazole and phenothiazine, as appealing building blocks for the new generation of luminogens, is systematically summarized.

We provide a review of the progress in materials and applications of TTA-UC: biological, environmental/energy, OLED, and other applications. Moreover, an excellent demonstration of TTA-UC based technologies are presented in each chapter.

In this review, we illustrate how the field of luminescent copper(I) compounds has developed with a focus on ionic copper(I) complexes and those exhibiting thermally-activated delayed fluorescence (TADF).

Herein is described a blue-emitting gold(III)–TADF complex serving as a sensitizer for SP-OLEDs, in which an MR-TADF emitter is employed as the emissive dopant. The SP-OLEDs attained EQE_max of up to 16.6%, high color purity and CIE_x,y of (0.14, 0.18).

The highest efficiency of heavy metal-free visible-to-UV photon upconversion is achieved by employing a ketocoumarin sensitizer with strong visible absorption, weak UV absorption, and high intersystem crossing efficiency.

Solid mixture of PbS quantum dot and a directly attached emitter penetrating the ligand layer enables photon upconversion from near infrared.

This research reports the next generation of solid-state triplet–triplet annihilation upconversion (TTA-UC) host material (polycaprolactone, PCL) for highly efficient, processable, and biocompatible solid-state TTA-UC.

This work not only gives a new functional group for the construction of deep-red pure organic efficient excimer materials, but also further verifies that the “hot exciton” theory can also be effective in excimer-based OLEDs.

Biphenyl at its best: a (triisopropylsilyl)ethynyl group in para position converts biphenyl into a UV annihilator that is successfully employed for blue-to-UV upconversion with unprecedented output photon energies.

A family of thermally activated delayed fluorescence (TADF) materials functionalized with carboxylate esters is developed.

The structural stability of CsPbI₃ has been studied using reactive molecular dynamics simulation under mechanical-load and temperature induced stress.

We compare the effect of donor strength on the optoelectronic properties of thermally activated delayed fluorescence poly(dendrimer)s and their dendrimer analogues.

Structural diversity of different BTPs in exciplexes together with TCTA is investigated, to improve their performance in TADF OLEDs.

Engineering of donor–acceptor light-harvesting systems based on geometrical features: applications in energy transfer & energy/photon upconversion.

A new iptycene unequivocally validates homoconjugation as a viable strategy to simultaneously enhance TADF reverse intersystem crossing and radiative decay.

Simplified thermal evaporation of rubrene in an inert atmosphere is demonstrated to alter the morphology of photon upconverting (UC) films, and subsequently ramp their NIR-to-vis UC quantum yield up to a record value of 1.2% (out of maximum 50%).

4DTPIPN has a strong tendency for dimer formation, even in dilute film, and possesses a higher colour purity than 4CzIPN but this is traded-off against the photoluminescence quantum yield. (Credit to LeStudio/Shutterstock for the James Bond themed background.)

A phosphorescent OLED with an external quantum efficiency of 22.31% and an extremely low efficiency roll-off of 0.67% at 10 000 cd m⁻² was achieved by reducing the carrier mobility of the donors in an exciplex co-host system.

An Osmium sensitizer and anthracene annihilator are incorporated into a metal ion linked multilayer photoanode that harnesses NIR light in an integrated triplet–triplet annihilation upconversion solar cell.

It is shown here that positional isomerism of perylene substitution affects high energy triplet states differently. This in turn influences the quantum efficiency of triplet–triplet annihilation photon upconversion.

A series of twelve two-coordinate coinage metal, Cu, Ag and Au, complexes with carbene-metal-amide structures were prepared and used to study the correlation of the overlap between the hole and electron NTOs with the electronic and photophysical properties of these TADF emitters.

In addition to reverse intersystem crossing, triplet-to-singlet exciton transfers plays an important role in the triplet harvesting mechanism in hyperfluorescent organic light-emitting diodes.

Impacts of intramolecular non-covalent interactions of TADF emitters on their electroluminescent performances were investigated using donor-acceptor compounds containing moiety 1,4-bis(trifluoromethyl)benzene.

Understanding the role of non-radiative decay rates in TADF OLEDs by comparing PLQY and ELQY simulated values. Quantification of the non-radiative decay rates in TADF compounds with a sophisticated fitting procedure.

High efficiency and reduced efficiency roll-off were realized for a cationic iridium complex with deep-red emission based on an indeno-anthraquinone host endowing thermally activated delayed fluorescence.

We have successfully developed blue-emitting thermally activated delayed fluorescence (TADF) compounds derived from 2CzPN. They display novel electrochemiluminescence properties, these revealing correlations with measured ΔE_ST values.

Regioisomeric and substituent effects in a twisted D–A–D molecular scaffold on the photophysical properties has been revealed. The studied compounds display distinct difference in TADF, dual TADF&RTP, and dual RTP, depending on the host used.

A new microfluidic approach was used to screen the oxygen tolerability of photon upconverting nanocarriers and to quantify their performance at oxygen levels ranging from severely hypoxic tumours to healthy peripheral tissue.

A TADF emitter exhibiting high OLED device performance of 25% and narrow emission (FWHM = 58 nm) in the deep blue region (λ_em = 458 nm, CIE = 0.14, 0.13) due to balanced charge transfer interactions and locked molecular geometry.

We present the first example of a MR-TADF extended helicene.

Four new perhalophenylgold(I)–diphosphino complexes have been described as TADF emitters. The position and the electronegativity of the halogen atoms in the aromatic ring allow tuning of the photophysical properties.

A hetero-bichromophore thermally activated delayed fluorescence (TADF) emitter named BOQAO was designed and synthesized, which consists of two multi-resonance TADF (MR-TADF) cores, tBuBO and tBuQAO.

TTA and TADF mechanisms with moderate rate constants on the order of 10⁻¹ to 10¹ s⁻¹ have been incorporated into dopant–matrix systems to significantly enhance the organic afterglow efficiency under ambient conditions.

Metathesis of the halide co-ligands X from chloro to iodo improves the TADF properties in a diplatinum(II) complex of the form (N^C^N–N^C^N)Pt₂X₂, leading to a ∼3-fold increase of radiative decay rate constant.

Novel thermally-activated delayed fluorescence (TADF) materials 2Cz2SB and 2Mi2SB, possessing N-heterocycle and arylsulfonyl groups, were synthesized and their photophysical properties in solution and the solid state were characterized.

In an efficient hyperfluorescent ternary blend, most of the singlet excitons from the TADF sensitizers will efficiently transfer to the emitters while the triplet excitons will first undergo a reverse intersystem crossing transition.

Designing luminescent organic materials exhibiting narrowband emission is crucial for achieving high resolution and energy efficient organic light emitting diodes (OLEDs), but remains a significant challenge.

Different from spiro-blocking, frontier molecular orbital engineering has been proposed to develop AT-spiro-DMACF, which shows interesting aggregation-induced delayed fluorescence (AIDF) for non-doped OLEDs.

Here a new electron donor is developed to endow a near-IR TADF molecule with good solubility for solution processing and AIE behavior. This is the first AIE TADF material with all PL > 700 nm.

Building on an effective model Hamiltonian for DMAC–TRZ, a non-adiabatic approach is proposed to estimate (reverse) inter-system crossing rates and to address the role of the polarizability of the matrix in the properties of TADF emitters.

Three phenoxazine-π-bridge-benzothiazole dyes are investigated by a joint experimental-computational approach. Depending on the linker, TADF or RTP (also in solution) is observed.

A blue TADF host with and without steric hindrances was utilized to manipulate Fӧrster and Dexter interactions with a red phosphorescent iridium complex, realizing the state-of-the-art PhOLEDs with a sufficiently high EQE of 22.2%.

Two sky-blue TADF emitters bearing the tris(triazolo)triazine acceptor moiety were designed and synthesized. The solution-processed OLEDs based on these two emitters achieved a maximum EQE of 10.01% at 474 nm.

A pair of chiral aromatic-imide-based TADF emitters were conveniently constructed for the fabrication of highly efficient CP-OLEDs.

An asymmetric carbazole-donor motif in a D–A–D type naphthyridine-based TADF emitter enables CT/LE state mixing, thus enhancing the RISC rate, shortening the delayed fluorescence lifetime and reducing the OLED efficiency roll-off.

Assistant dopants with reduced Dexter energy transfer rates were designed by replacing the donor moiety of 2,3,5,6-tetra(9H-carbazol-9-yl)terephthalonitrile (4CzTPN) with 5H-benzo[4,5]thieno[3,2-c]carbazole (BTCz).

Solution-processed hyperfluorescent red organic light-emitting diodes with external quantum efficiencies of 15.3% was achieved by using cibalackrot as the fluorescent emitter, where attaching tert-butyl groups to the assistant host significantly contributes to device performance.

Ligand-protected Au₂Cu₆ cluster exhibits not only fluorescence but also phosphorescence at room temperature and serves as a triplet sensitizer in triplet fusion upconversion.

Fluorinated dibenzo[a,c]-phenazine-based green to red thermally activated delayed fluorescent OLED emitters are accessible through judicious choice of donor.

An unusual methyl group effect for designing TADF emitters was revealed, and the resulting blue OLEDs achieved a maximum EQE of nearly 20%.

DMAC-DPS showing AIE behavior increased TTA and STA, whereas Ph-OBNA showing ACQ behavior showed relatively small TTA and STA values due to the generation of excimers.

Two bipolar molecules CzT2.1 and CzT2.2 are examined as electron acceptors to form exciplexes with electron donors 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) and 4,4′,4′′-tris(carbazol-9-yl)-triphenylamine (TCTA), respectively.

Electronic communication uncovered between donor groups in a pair of D–A–D TADF emitters lowers their triplet energies, and shows that they cannot be described purely as combinations of the equivalent D–A systems.

Suitable intermolecular hydrogen bonding interactions enable head-against-tailed 3D supramolecular frameworks and high-performance orange-red non-doped OLEDs were thus realized.

Blue-efficient thermally-activated delayed fluorescence emitters are widely desired in organic light-emitting diodes due to their advantages in both improving display resolution and providing better pixels.

Conversion of near-infrared photons to visible light in rubrene based systems is made 20x more efficient when mixed with 0.5% wt DBP. This is not because singlet fission in rubrene is supressed, but because of reduced triplet losses.

Colloidal CsPbBr₃ nanocrystals effectively sensitize surface-attached molecular triplets which can further translate their energy to non-conventional light-emitting materials such as phosphorescent and radical molecules.