High-Performance, Solution-Processable Thermally Activated Delayed Fluorescent Organic Light-Emitting Diodes Realized via the Adjustment of the Composition of the Organoboron Acceptor Monomer in Copolymer Host Materials

Charge Transport Regulation in Solution-Processed OLEDs by Indenocarbazole-Triazine Bipolar Host Copolymers

This work reports the synthesis of a series of bipolar host polymers, specifically PPPIC-co-PPDPT (9:1), PPPIC-co-PPDPT (8:2), and PPPIC-co-PPDPT (7:3), which incorporate a hole-transporting unit, 7,7-dimethyl-5-phenyl-2-(4-vinylphenyl)-5,7-dihydroindeno[2,1-b]carbazole (PPIC), and an electron-transporting unit, 2,4-diphenyl-6-(4-vinylphenoxy)-1,3,5-triazine (PDPT). These polymers are meticulously designed to achieve efficient bipolar charge transport while exhibiting high triplet energy levels (T1), robust thermal stability, and smooth surface morphology. The tunability of the recombination zone position was achieved by adjusting the relative ratio of the hole and electron transport units in the emissive layer, leading to optimized charge balance and enhanced device performance. The maximum current efficiency of 40.71 cd/A and an EQE of 11.19% were achieved through a simple device structure. Overall, this study highlights the potential of these bipolar host materials for advancing the OLED technology by enabling superior charge transport and reducing efficiency roll-off.

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.

Organoboron-based multiple-resonance emitters: synthesis, structure-property correlations, and prospects

Boron-based multiple-resonance (MR) emitters exhibit the advantages of narrowband emission, high absolute photoluminescence quantum yield, thermally activated delayed fluorescence (TADF), and sufficient stability during the operation of organic light-emitting diodes (OLEDs). Thus, such MR emitters have been widely applied as blue emitters in triplet-triplet-annihilation-driven fluorescent devices used in smartphones and televisions. Moreover, they hold great promise as TADF or terminal emitters in TADF-assisted fluorescence or phosphor-sensitised fluorescent OLEDs. Herein we comprehensively review organoboron-based MR emitters based on their synthetic strategies, clarify structure-photophysical property correlations, and provide design guidelines and future development prospects.

Relating Structure to Properties in Non-Conjugated Pendant Electroactive Polymers

Non-conjugated pendant electroactive polymers (NCPEPs) are an emerging class of polymers that offer the potential of combining the desirable optoelectronic properties of conjugated polymers with the superior synthetic methodologies and stability of traditional non-conjugated polymers. Despite an increasing number of studies focused on NCPEPs, particularly on understanding fundamental structure-property relationships, no attempts have been made to provide an overview on established relationships to date. This review showcases selected reports on NCPEP homopolymers and copolymers that demonstrate how optical, electronic, and physical properties of the polymers are affected by tuning of key structural variables such as the chemical structure of the polymer backbone, molecular weight, tacticity, spacer length, the nature of the pendant group, and in the case of copolymers the ratios between different comonomers and between individual polymer blocks. Correlation of structural features with improved π-stacking and enhanced charge carrier mobility serve as the primary figures of merit in evaluating impact on NCPEP properties. While this review is not intended to serve as a comprehensive summary of all reports on tuning of structural parameters in NCPEPs, it highlights relevant established structure-property relationships that can serve as a guideline for more targeted design of novel NCPEPs in the future.

Advances in Solution-Processed OLEDs and their Prospects for Use in Displays

This review outlines problems and progress in development of solution-processed organic light-emitting diodes (SOLEDs) in industry and academia. Solution processing has several advantages such as low consumption of materials, low-cost processing, and large-area manufacturing. However, use of a solution process entails complications, such as the need for solvent resistivity and solution-processable materials, and yields SOLEDs that have limited luminous efficiency, severe roll-off characteristics, and short lifetime compared to OLEDs fabricated using thermal evaporation. These demerits impede production of practical SOLED displays. This review outlines the industrial demands for commercial SOLEDs and the current status of SOLED development in industries and academia, and presents research guidelines for the development of SOLEDs that have high efficiency, long lifetime, and good processability to achieve commercialization.

FRET mechanism to enhance the quantum yield of the PCz/gC3N4 nanocomposite, an emissive material for OLED applications

Conjugated polymers such as polycarbazole (PCz) have captivated more attention than other carbazole-based derivatives due to their superior electrical and optical properties. Accordingly, we synthesized PCz/gC3N4 nanocomposites via the in situ polymerization method using FeCl3 as the oxidative reagent. The synthesized nanocomposites were subjected to characterization techniques to examine their optical and electrical parameters and decide whether the materials were suitable as emissive materials. X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) were carried out to ascertain the crystalline or amorphous nature, surface interactions, and functional groups present in them. The surface microstructural and topographical investigations were conducted using field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (HRTEM) techniques. Optical parameters, such as refractive index ∼2.06, optical absorbance, optical band energy ∼2.77 eV, and the photoluminescence emission range, were studied using UV-Visible and photoluminescence spectrometry. The theoretical relative emission quantum yield of ∼67.9% and 87.7% energy transfer from the donor to the acceptor ion via the Förster energy transfer mechanism are illustrated by the PL data. The Förster energy transfer mechanism has been elaborated. The carrier mobility ∼32.03 m2 V-1 S-1, sheet resistance ∼1.6977 × 102 Ω m, carrier density ∼11.96 × 1014 cm-3 and conductivity ∼5.90 × 10-3 S cm-1 were computed using Hall effect measurements. The dielectric constant, dielectric loss, and IV characteristic curve were estimated by the LCR and Four-probe IV measurement methods. The high PL emission intensity, CIE coordinates in the blue emission region, and the CCT value indicate that it is a suitable emissive layer material for OLED applications.

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.

Boosting external quantum efficiency to 38.6% of sky-blue delayed fluorescence molecules by optimizing horizontal dipole orientation

To achieve high electroluminescence efficiency, great efforts are devoted to enhancing photoluminescence quantum yield (Φ_PL) and exciton utilization of luminescent molecule, while another important factor, light out-coupling efficiency (η_out), receives less attention in molecule design. Here, we focus on horizontal dipole orientation engineering of the molecule to increase η_out and external quantum efficiency (η_ext). A series of tailor-made luminescent molecules consisting of an electron-accepting carbonyl core plus double electron-donating groups of spiro[acridine-9,9′-fluorene] and carbazole derivatives [e.g., 1,3,5-tri(carbazol-9-yl)benzene] are developed and systematically investigated. These molecules hold distinguished merits of strong sky-blue delayed fluorescence with excellent Φ_PL values, large horizontal dipole ratios, and balanced bipolar carrier transport, which furnish record-high η_ext values of up to 26.1 and 38.6% in nondoped and doped sky-blue organic light-emitting diodes (OLEDs), respectively. Moreover, the state-of-the-art nondoped hybrid white OLED and all-fluorescence single-emitting layer white OLED are also realized, demonstrating great potentials in OLED industry of these molecules.

Simple-Structured Blue Thermally Activated Delayed Fluorescence Emitter for Solution-Processed Organic Light-Emitting Diodes with External Quantum Efficiency of over 20

Solution-processed organic light-emitting diodes (OLEDs) are much preferred for the manufacture of low-temperature, low-cost, large-area, and flexible lighting and displaying devices. However, these devices with high external quantum efficiency are still limited, especially for blue ones. In addition, the molecular configurations of emitters are usually complicated, indicative of high costs. In this study, two simple-structured thermally activated delayed fluorescent emitters M1 and its polymer P1 were synthesized with acridine as a donor and benzophenone as an acceptor. Solution-processed OLEDs were prepared based on M1 and P1 as doped light-emitting layer, and M1-based doped device could achieve maximum external quantum efficiency of up to 20.6% with blue-light emission.

New hole transport styrene polymers bearing highly π-extended conjugated side-chain moieties for high-performance solution-processable thermally activated delayed fluorescence OLEDs

New polystyrene-based polymers with high π-extended hole transport pendants were synthesized to obtain a low turn-on voltage and high efficiency in solution-processed green TADF-OLEDs.