Thermally Activated Delayed Fluorescence (TADF) in Transition Metal Complexes

Realizing External Quantum Efficiency over 25% with Low Efficiency Roll-Off in Polymer-Based Light-Emitting Diodes Synergistically Utilizing Intramolecular Sensitization and Bipolar Thermally Activated Delayed Fluorescence Monomer

Novel secondary acceptor based molecular design for superb lifetime in thermally activated delayed fluorescent organic light-emitting diodes through high bond energy and fast up-conversion

Rational molecular design of deep blue thermally activated delayed fluorescent emitters for high efficiency fluorescent and hyperfluorescent devices

This work demonstrates successful large area inkjet printing of a thermally activated delayed fluorescence (TADF) material as the emitting layer of organic light-emitting diodes (OLEDs). TADF materials enable efficient light emission without relying on heavy metals such as platinum or iridium. However, low-cost manufacturing of large-scale TADF OLEDs has been restricted due to their incompatibility with solution processing techniques. In this study, we develop ink formulation for a TADF material and show successful ink jet printing of intricate patterns over a large area (6400 mm2) without the use of any lithography. The stable ink is successfully achieved using a non-chlorinated binary solvent mixture for a solution processable TADF material, 3‐(9,9‐dimethylacridin‐10(9H)‐yl)‐9H‐xanthen‐9‐one dispersed in 4,4’-bis-(N-carbazolyl)-1,1’-biphenyl host. Using this ink, large area ink jet printed OLEDs with performance comparable to the control spin coated OLEDs are successfully achieved. In this work, we also show the impact of ink viscosity, density, and surface tension on the droplet formation and film quality as well as its potential for large-area roll-to-roll printing on a flexible substrate. The results represent a major step towards the use of TADF materials for large-area OLEDs without employing any lithography.

Fabricating solution-processible host material for thermally activated delayed fluorescence (TADF) emitter remains a formidable challenge for organic light-emitting diodes (OLEDs). In this work, two new host materials, namely 3CzAcPy and 9CzAcPy, are found to exhibit high triplet energy levels, high thermal stability, and excellent film morphology from a solution process. An in-depth analysis on the photophysical data and device performance reveals the isomeric effect of the host materials has a significant impact not only on the host properties, but also on the host-dopant interactions and thus the performance of the resulting solution-processed TADF OLEDs. Impressively, the new hosts are proven to be suitable for both donor-acceptor type and multi-resonance TADF emitters, achieving state-of-the-art device performance. By using the new host 9CzAcPy, solution-processed OLED based on a donor-acceptor TADF emitter of DPAC-PCN, a maximum external quantum efficiency (EQE) of 29.5% is achieved, and solution-processed narrowband OLED based on a multiple-resonance TADF emitter of BN-CP1 acquires a maximum EQE of 26.6%. These efficiencies represent the highest values among the solution-processed TADF OLEDs. This study highlights the significance of host-dopant interactions in modulating the electroluminescence performance of TADF emitters, and provides an effective design principle for solution-processible host materials.

Macrocyclic compounds have garnered considerable attention in the realms of supramolecular chemistry and optoelectronic materials owing to their distinctive geometries, exceptional stability, and potential chiral properties. As third‐generation organic light‐emitting diodes (OLEDs) emitters, thermally activated delayed fluorescence (TADF) materials present themselves as promising contenders due to their ability to achieve complete exciton utilization through the harnessing of triplet excitons. The incorporation of macrocyclic structures with TADF characteristics holds the potential to yield TADF emitters endowed with unique attributes, thereby synergistically enhancing the performance and longevity of TADF materials for diverse device applications, ultimately propelling their commercialization. Within this comprehensive review, a thorough exploration of the molecular structures, synthesis methodologies, structure‐property relationships, and applications of the reported macrocyclic TADF systems in OLEDs are delved. Additionally, the prevailing challenges and prospective research directions are highlighted that demand attention in the realm of macrocyclic TADF materials. It is hoped that this review will provide researchers engaged in optoelectronic materials research with a lucid comprehension and inspiration concerning the intricacies of these macrocyclic TADF systems.

Thermally activated delayed fluorescence (TADF) is a desirable emission mechanism that can translate into highly efficient organic light‐emitting diodes. Conventionally, the TADF kinetics have been extracted by analysing the transient photoluminescence (PL) of the emitters in combination with measurements of their PL quantum yield. However, measuring transient PL can be challenging, as the emission decay of a TADF emitter occurs over a large range of intensities and timescales. The huge dynamic range can make it difficult to match data from early and late processes in the decay together. Here, we introduce a novel measurement that enables rate constants to be extracted from the turn‐on of the PL of TADF emitters. In our approach, the prompt and delayed PL can be measured in a single measurement where the PL intensity changes only over one order of magnitude, thereby overcoming the problem of a huge dynamic range. This approach enables a much faster measurement of the excited‐state dynamics than conventional transient PL methods, whilst using less expensive equipment. Our method also allows for the reliable estimation of prompt and delayed emission contributions. It could enable high‐throughput screening of the photophysics of TADF materials and so accelerate the identification of highly promising TADF emitters.

Recently, the external quantum efficiency and lifetime of organic light-emitting diodes (OLEDs) have been dramatically upgraded due to development of organic materials and device structure. In particular, an intramolecular or intermolecular complex based on thermally activated delayed fluorescent (TADF) materials has greatly contributed to improving OLED device performance. Although high external quantum efficiency has been the main objective of the development of TADF materials as hosts and emitters, recent interest has been directed towards the lifetime of TADF-material-based OLEDs. For the past several years, remarkable advances in the lifetime of phosphorescent and TADF OLEDs have been made using TADF materials as hosts or emitters in the emitting layer. Therefore, since TADF materials are useful as both hosts and emitters for a long lifetime, this work discusses the recent progress made in developing TADF materials for long-lifetime OLEDs.

Robust thermally activated delayed fluorescence (TADF) materials have shown promising applications as emitters and sensitizers for next‐generation organic light‐emitting diodes (OLEDs). However, most TADF materials have to be dispersed in host matrices at precisely low concentrations to prevent serious emission quenching and exciton annihilation, which is unfavorable in the OLED industry. Herein, four tailored TADF luminogens built with spiro‐fused xanthone acceptors and acridine‐based spiro donors are developed. Thanks to the rigid and bulky donor and acceptors, these new TADF luminogens hold fast reverse intersystem crossing process and emit strong cyan to green light with high quantum yields of up to 91% and 99% in neat and doped films, respectively. They have balanced bipolar charge transport ability and prefer horizontal dipole orientation. The non‐doped OLEDs using them as emitters show an excellent external quantum efficiency (EQE) of 30.6%, with ultra‐small roll‐offs. They enjoy concentration insensitivity in doped OLEDs, and the EQEs stay stable (29.1%‒36.8%) in various concentrations (10‒90 wt%) with very small roll‐offs. Finally, they can serve as efficient sensitizers for multi‐resonance emitters in narrow‐spectrum OLEDs, providing impressive EQEs of 41.0% with suppressed roll‐offs. This work reveals an applicable molecular design strategy of TADF materials for high‐performance OLEDs.

Deep-blue thermally activated delayed fluorescence (TADF) molecules present promising potential in organic light-emitting diodes (OLEDs), especially for display applications. Here, an efficient molecular engineering approach to modifying the donor or acceptor features of the D-π-A-configured TADF molecules for deep-blue emission is reported. By introducing oxygen and sulfone as a bridge unit onto the macrocyclic donor, two emitters, c-ON-MeTRZ and c-NS-MeTRZ, are synthesized and characterized, respectively. The reduced donor strength of c-ON-MeTRZ and c-NS-MeTRZ as compared to that of the model molecule c-NN-MeTRZ leads to blue-shifted emissions with high photoluminescence quantum yields (PLQYs) and retains TADF characters, while the new emitter c-NN-MePym with the most blue-shifted emission only exhibits a pure fluorescent nature because of the electron-accepting feature of pyrimidine that is insufficient for inducing the TADF property. In the presence of macrocyclic donors, these new emitters show high horizontal dipole ratios (Θ// = 85-89%), which are beneficial for improving the light out-coupling efficiency. Deep-blue TADF OLEDs incorporating c-ON-MeTRZ as an emitter doped in the mCPCN host achieves a high maximum external quantum efficiency (EQEmax) of 30.2% together with 1931 Commission Internationale de I'Eclairage (CIE) coordinates of (0.14, 0.13), while the counter device employing c-NS-MeTRZ as a dopant gives EQEmax of 15.4% and CIE coordinates of (0.14, 0.09). The EQEmax of c-ON-MeTRZ- and c-NS-MeTRZ-based devices can be significantly improved to 34.4 and 29.3%, respectively, with a polar host DPEPO, which stabilizes the charge transfer (CT) S1 state to give lower ΔEST for improving the reverse intersystem crossing process. The efficient TADF character, high PLQYs, and high anisotropic emission dipole ratios work together to render the superior electroluminescence (EL) efficiencies. Based on the detailed characterizations of physical properties, theoretical analyses, and comprehensive study on the corresponding devices, a clear structure-property-performance relationship has been successfully established to verify the effective molecular design strategy of modulating the macrocyclic donor characters for efficient deep-blue TADF emitters.

Circularly polarized luminescence (CPL) molecules, especially those with thermally activated delayed fluorescence (TADF) properties, have attracted considerable attention due to their great potential for chiroptical organic light emitting diode (OLED) devices. Here we developed a new pair of TADF emitters with CPL based on boron complexes using chiral donor (cD) binaphthalene, acceptor (A) biphenyl boron β-diketonate, and donor (D) biphenylamine in a cD-A-D architecture. With this design, both efficient intramolecular charge transfer (ICT) and chiral ICT for high-performance CPL were established, leading to high dissymmetry factors (|glum|) up to 2.2 × 10-3 in solution and significantly red-shifted emission around 600 nm for red TADF with a quantum yield over 15% in doped films. More impressively, with these chiral TADF emitters, solution-processed red circularly polarized OLEDs (CP-OLEDs) exhibit external quantum efficiencies (EQEs) up to 2.0% and efficient circularly polarized electroluminescence with dissymmetry factors of 2.6 × 10-3, which are among the best performances of the reported solution-processed orange-red and red TADF CP-OLEDs. These results illustrate the great success of the cD-A-D strategy in designing high-performance CPL TADF emitters with axially chiral boron complexes, providing important clues to understand efficient chiral transfer for large |glum| values and high device performance of CP-OLEDs.

Organic light-emitting diode (OLED) technology is steadily gaining more and more popularity, and, as of 2019, is the most commercially successful application of organic electronics. Compared to light-emitting diodes (LEDs), OLEDs possess several advantages such as colour tunability, quick switching times, higher brightness, better contrast, larger fields of view, the possibility to use lighter substrates instead of commonly used glass, and, potentially, cheaper production costs. Improving the performance of OLED devices could open new opportunities for the development of the technology and might be crucial for it to strengthen its position in the market. Upon electrical excitation singlets and triplets are formed in 1:3 ratio, and one of the challenges is to design materials that can efficiently utilise triplet excited states to generate electroluminescence. In purely organic fluorescent compounds normally only singlets are emissive at room temperature, so in fluorescent devices only 25% of the excited states are used. Phosphorescent OLEDs are known for their high efficiency compared to their fluorescent predecessors, however the structure of these materials is limited to heavy metal-containing organic compounds. Thus, creation of purely organic light emitting materials is of a great interest for the field. One of the solutions is to use thermally activated delayed fluorescence (TADF) emitters where dark triplets can be converted into singlet excited states and thus 100% internal quantum efficiency could be achieved. The design and synthesis of new TADF emitters is the main scope of this thesis. Chapter 1 gives a brief overview of the history of lighting technology with an emphasis on solid-state lighting. The milestones in the development of organic semiconductors are also described. The main concepts of organic electronics and the working principles of OLED devices are briefly approached. Furthermore, the existing ways of enhancing the efficiency of luminescent organic materials in light-emitting devices is discussed. A substantial part of the chapter is dedicated to the description of the main principles used for the design of materials exhibiting thermally-activated delayed fluorescence (TADF), recent advances in their development and future perspectives. Chapter 2 describes the photophysical properties of new helicene-based TADF emitters, in particular their behaviour in the solid state. This includes the presentation of time resolved measurements of helical compounds in crystals and films. The performance of the materials in OLEDs was also investigated. In Chapter 3 the synthesis and physical properties of novel-star-shaped donor-acceptor structures based on a truxene-benzothiadiazole-truxene core are discussed. The application of these materials in fluorescent OLEDs and as colour-converters in hybrid LEDs is described. Chapter 4 demonstrates a new way of preparing materials exhibiting delayed fluorescence (DF) from non-DF starting materials. These compounds exhibit either TADF or triplet-triplet annihilation (TTA) depending on the relative position of donor and acceptor moieties in the molecule. They also show aggregation-induced emission and mechanochromic properties. The photophysical behaviour of these compounds was investigated mainly in the solid state. The experimental procedures for chapters 2–4 are presented in Chapter 5.

An efficient thermally activated delayed fluorophore featuring an oxygen-locked azaryl ketone acceptor

Thermally activated delayed fluorescent (TADF) materials are those that have a small energy gap between the lowest singlet and triplet excited states, enabling singlet excitons through thermal upconversion of triplet excitons even at room temperature. Thus, they can achieve external electroluminescence (EL) quantum efficiency (EQE) equal to that of phosphorescence based organic light-emitting diodes (OLEDs) [1]. Considering fluorescence and phosphorescence based OLEDs as the first and second generations, respectively, TADF materials can be positioned as the third generation realizing highly efficient OLEDs. Recently, the best EQE of 31.2% in green TADF based OLEDs was demonstrated [2]. Further, EQE can be greatly improved by transition dipoles horizontally oriented to a substrate surface; such a preferential dipole orientation is achievable by controlling molecular orientation.

Until now, thermally activated delayed fluorescence (TADF) materials based on bridged boron-based acceptors have been primarily developed as dopants. However, in this study, we synthesized and characterized multifunctional deep-blue TADF materials─t-OBO-DMAC and t-OBO-DPAC─using bridged boron-based acceptors in combination with dimethylacridine or diphenylacridine as donors. These materials serve as both dopants and hosts. Theoretical calculations and experimentally measured photophysical properties of t-OBO-DMAC reveal a smaller singlet-triplet energy difference, higher photoluminescence quantum yield, and more efficient reverse intersystem crossing compared to t-OBO-DPAC. When evaluated as TADF emitters, t-OBO-DMAC and t-OBO-DPAC exhibited maximum external quantum efficiency (EQE) of 14.4 and 7.3% with deep-blue color coordinates of (0.14, 0.11) and (0.15, 0.07), respectively. Both materials were further assessed as hosts in various configurations, including host-only, TADF, phosphorescent, and phosphor-sensitized fluorescence (PSF)-emitting systems. Notably, t-OBO-DMAC demonstrated a high maximum EQE of 13.9% with deep-blue color coordinates of (0.15, 0.07) in a nondoped host-only device. Remarkably, both materials achieved EQEs exceeding 20% in the PSF devices. Our study marks a critical advancement in the field that breaks the conventional boundaries of the dopant and host and demonstrates unprecedented multifunctionalities for advanced organic light-emitting diodes.