Role of Linker Functionality in Polymers Exhibiting Main-Chain Thermally Activated Delayed Fluorescence

The Golden Age of Thermally Activated Delayed Fluorescence Materials: Design and Exploitation

Since the seminal report by Adachi and co-workers in 2012, there has been a veritable explosion of interest in the design of thermally activated delayed fluorescence (TADF) compounds, particularly as emitters for organic light-emitting diodes (OLEDs). With rapid advancements and innovation in materials design, the efficiencies of TADF OLEDs for each of the primary color points as well as for white devices now rival those of state-of-the-art phosphorescent emitters. Beyond electroluminescent devices, TADF compounds have also found increasing utility and applications in numerous related fields, from photocatalysis, to sensing, to imaging and beyond. Following from our previous review in 2017 ( Adv. Mater. 2017, 1605444), we here comprehensively document subsequent advances made in TADF materials design and their uses from 2017-2022. Correlations highlighted between structure and properties as well as detailed comparisons and analyses should assist future TADF materials development. The necessarily broadened breadth and scope of this review attests to the bustling activity in this field. We note that the rapidly expanding and accelerating research activity in TADF material development is indicative of a field that has reached adolescence, with an exciting maturity still yet to come.

Achieving Record External Quantum Efficiency of 11.5 % in Solution-Processable Deep-Blue Organic Light-Emitting Diodes Utilizing Hot Exciton Mechanism

High performance solution-processable deep-blue emitters with a Commission International de l'Eclairage (CIE) coordinate of CIEy≤0.08 are highly desired in ultrahigh-definition display. Although, deep-blue materials with hybridized local and charge-transfer (HLCT) excited-state feature are promising candidates, their rigidity and planar molecular structures limit their application in solution-processing technique. Herein, four novel deep-blue solution-processable HLCT emitters were first proposed by attaching rigid imide aliphatic rings as functional units onto the HLCT emitting core. The functional units not only improve solubility, enhance thermal properties and morphological stability of the emitting core, but also promote photoluminescence efficiency, balance charge carrier transport, and inhibit aggregation-caused quenching effect due to the weak electron-withdrawing property as well as steric hindrance. The corresponding solution-processable organic light-emitting diodes (OLEDs) substantiate an unprecedented maximum external quantum efficiency (EQEmax) of 11.5 % with an emission peak at 456 nm and excellent colour purity (full width at half maximum=56 nm and CIEy=0.09). These efficiencies represent the state-of-the-art device performance among the solution-processable blue OLEDs based on the "hot exciton" mechanism. This simple strategy opens up a new avenue for designing highly efficient solution-processable deep-blue organic luminescent materials.

Acceptor Copolymerized Axially Chiral Conjugated Polymers with TADF Properties for Efficient Circularly Polarized Electroluminescence

Chiral conjugated polymer has promoted the development of the efficient circularly polarized electroluminescence (CPEL) device, nevertheless, it remains a challenge to develop chiral polymers with high electroluminescence performance. Herein, by the acceptor copolymerization of axially chiral biphenyl emitting skeleton and benzophenone, a pair of axially chiral conjugated polymers namely R-PAC and S-PAC are synthesized. The target polymers exhibit obvious thermally activated delayed fluorescence (TADF) activities with high photoluminescence quantum yields of 81%. Moreover, the chiral polymers display significant circularly polarized luminescence features, with luminescence dissymmetry factor (|glum|) of nearly 3 × 10-3. By using the chiral polymers as emitters, the corresponding circularly polarized organic light-emitting diodes (CP-OLEDs) exhibit efficient CPEL signals with electroluminescence dissymmetry factor |gEL| of 3.4 × 10-3 and high maximum external quantum efficiency (EQEmax) of 17.8%. Notably, considering both EQEmax and |gEL| comprehensively, the device performance of R-PAC and S-PAC is the best among all the reported CP-OLEDs with chiral conjugated polymers as emitters. This work provides a facile approach to constructing chiral conjugated TADF polymers and discloses the potential of axially chiral conjugated luminescent skeletons in architecting high-performance CP-OLEDs.

Two Birds with One Stone: Polymerized Thermally Activated Delayed Fluorescence Small Molecules

Thermally activated delayed fluorescence (TADF) polymers show a great potential in low-cost, large-area and flexible full-color flat-panel displays. One of the most promising design rules is based on TADF+Linker, where a small molecular TADF unit is bonded to each other by a simple linker. Unlike the expensive vacuum deposition for small molecules, these polymerized TADF small molecules (Poly-TADF-SMs) are capable of cost-effective solution processing. Meanwhile, the good luminescent property of small molecular TADF emitters can be well inherited by Poly-TADF-SMs so as to bridge the efficiency gap between small molecules and polymers. Herein, we will highlight the recent progress of Poly-TADF-SMs, together with emphasis on their molecular design, photophysical and electroluminescence properties.

Unveiling the In Situ and Solvent Polymerization Engineering for Highly Efficient and Flexible Thermally Activated Delayed Fluorescence Organic Light-Emitting Diodes

Thermally activated delayed fluorescence (TADF) polymer has great potential for the construction of flexible solution-processed organic light-emitting diodes (OLEDs). However, the relationship between polymerization engineering and device functions has rarely been reported. Here, two novel TADF polymers, P-Ph4CzCN and P-Ph5CzCN, with a small energy gap between the first excited singlet and triplet states (ΔE_ST; <0.16 eV) were newly developed by both solvent and in situ polymerization of a styrene component. Detailed device performance testing indicates that both polymerization strategies ensure that the TADF polymer achieves comparable high efficiencies in commonly rigid devices, and the maximum external quantum efficiencies (EQE_max) were 11.9%, 14.1%, and 16.2% for blue, green, and white OLEDs, respectively. Although in situ polymerization provides a simplified device fabrication process, which avoids the complicated synthesis and purification of the polymer, the inevitable high-temperature annealing makes it fail in a plastic substrate device. In contrast, P-Ph5CzCN achieved by solvent polymerization enables the successful fabrication of a flexible device on a poly(ethylene terephthalate) (PET) substrate, which was the first reported flexible OLED based on a TADF polymer. This work provides a strong guideline for the simple fabrication of TADF polymer devices and the application of TADF polymer materials in OLED flexible panels and flexible lighting.