Efficient and Stable Deep-Blue Fluorescent Organic Light-Emitting Diodes Employing a Sensitizer with Fast Triplet Upconversion.

Multiple donor-acceptor-type carbazole-benzonitrile derivatives that exhibit thermally activated delayed fluorescence (TADF) are the state of the art in efficiency and stability in sky-blue organic light-emitting diodes. However, such a motif still suffers from low reverse intersystem crossing rates (k ) with emission peaks <470 nm. Here, a weak acceptor of cyanophenyl is adopted to replace the stronger cyano one to construct blue emitters with multiple donors and acceptors.

Understanding and Manipulating the Interplay of Wide-Energy-Gap Host and TADF Sensitizer in High-Performance Fluorescence OLEDs.

Comprising an emitting layer (EML) constituting a wide-energy-gap host, a thermally activated delayed fluorescence (TADF) sensitizer and a conventional fluorescent dopant, TADF-sensitizing-fluorescence organic light-emitting diodes (TSF-OLEDs) highly depend on component interplay to maximize their performance, which, however, is still under-researched. Taking the host type (TADF or non-TADF) and the recombination position (on the host or on the TADF sensitizer) into consideration, the interplay of host and TADF sensitizer is comprehensively studied and manipulated. A wide-energy-gap host with TADF and recombination of charges on it are both required to maximize device performances by triggering multiple sensitizing processes to eliminate exciton losses.

Exciplex System with Increased Donor-Acceptor Distance as the Sensitizing Host for Conventional Fluorescent OLEDs with High Efficiency and Extremely Low Roll-Off.

Exciplex systems with efficient thermally activated delayed fluorescence as the sensitizing hosts for fluorescent organic light-emitting diodes (OLEDs) have been flourished recently, while the device performances are still lagging behind. Here, a donor molecule sterically encapsulated with tert-butyl units is designed and synthesized to increase the donor-acceptor separation in an exciplex system, leading to reduced singlet-triplet energy gap (Δ Es) and improved reverse intersystem crossing (RISC) efficiency. OLEDs utilizing exciplexes with increased donor-acceptor distance ( r) as the hosts for conventional fluorescent dopants exhibit a maximum external quantum efficiency (EQE) as high as 16.

Thermally Activated Delayed Fluorescent Materials Combining Intra- and Intermolecular Charge Transfers.

A novel thermally activated delayed fluorescent (TADF) compound, 9-(3-((4,6-diphenyl-1,3,5-triazin-2-yl)oxy)phenyl)-3,6-diphenyl-9 H-carbazole (PhCz- o-Trz), with a donor-σ-acceptor (D-σ-A) motif is developed. A flexible small space σ-junction is adopted to partly suppress the intramolecular charge transfer (intra-CT) while inversely enhancing the intermolecular charge transfer (inter-CT) between D/A moieties, realizing the coexistence of both intra-CT and inter-CT in an amorphous aggregate. The coexistence of dual CTs increases the complexity of the singlet and triplet state mixing, enhancing the triplet-to-singlet spin-flip transition and thereby the TADF emission.

Unveiling the Role of Langevin and Trap-Assisted Recombination in Long Lifespan OLEDs Employing Thermally Activated Delayed Fluorophores.

Recent research studies on noble-metal-free thermally activated delayed fluorescent (TADF) materials have boosted the efficiencies of organic light-emitting diodes (OLEDs) to unity. However, the short lifespan still hinders their further practical application. Carrier recombination pathways have been reported to have a significant influence on the efficiencies of TADF devices, though their effects on device lifetimes remain rarely studied.

High-Performance Fluorescent Organic Light-Emitting Diodes Utilizing an Asymmetric Anthracene Derivative as an Electron-Transporting Material.

Fluorescent organic light-emitting diodes with thermally activated delayed fluorescent sensitizers (TSF-OLEDs) have aroused wide attention, the power efficiencies of which, however, are limited by the mutual exclusion of high electron-transport mobility and large triplet energy of electron-transporting materials (ETMs). Here, an asymmetric anthracene derivative with electronic properties manipulated by different side groups is developed as an ETM to promote TSF-OLED performances. Multiple intermolecular interactions are observed, leading to a kind of "cable-like packing" in the crystal and favoring the simultaneous realization of high electron-transporting mobility and good exciton-confinement ability, albeit the low triplet energy of the ETM.

Versatile Indolocarbazole-Isomer Derivatives as Highly Emissive Emitters and Ideal Hosts for Thermally Activated Delayed Fluorescent OLEDs with Alleviated Efficiency Roll-Off.

Maintaining high efficiency at high brightness levels is an exigent challenge for real-world applications of thermally activated delayed fluorescent organic light-emitting diodes (TADF-OLEDs). Here, versatile indolocarbazole-isomer derivatives are developed as highly emissive emitters and ideal hosts for TADF-OLEDs to alleviate efficiency roll-off. It is observed that photophysical and electronic properties of these compounds can be well modulated by varying the indolocarbazole isomers.

Blocking Energy-Loss Pathways for Ideal Fluorescent Organic Light-Emitting Diodes with Thermally Activated Delayed Fluorescent Sensitizers.

Organic light-emitting diodes (OLEDs) based on thermally activated delayed fluorescence-sensitized fluorescence (TSF) offer the possibility of attaining an ultimate high efficiency with low roll-off utilizing noble-metal free, easy-to-synthesize, pure organic fluorescent emitters. However, the performances of TSF-OLEDs are still unsatisfactory. Here, TSF-OLEDs with breakthrough efficiencies even at high brightnesses by suppressing the competitive deactivation processes, including direct charge recombination on conventional fluorescent dopants (CFDs) and Dexter energy transfer from the host to the CFDs, are demonstrated.

Multifunctional Materials for High-Performance Double-Layer Organic Light-Emitting Diodes: Comparison of Isomers with and without Thermally Activated Delayed Fluorescence.

Organic light-emitting diodes (OLEDs) with simple structures are attracting a lot of attention nowadays, though their performances are always inferior to those of the more complicated structures as multifunctional materials are rare. Here, we have designed and synthesized multifunctional isomers by combining electron-donating carbazole (Cz) and triphenylamine (TPA) units with electron-accepting triazine (Trz), namely, N-[4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl]-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-[1,1'-biphenyl]-4-amine (CzTPA-p-Trz) and N-[3-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl]-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-[1,1'-biphenyl]-4-amine (CzTPA-m-Trz). The use of multiple electron-donating groups gives them suitable highest occupied molecular orbitals for hole injection and high mobilities for hole transport.

Highly Efficient Full-Color Thermally Activated Delayed Fluorescent Organic Light-Emitting Diodes: Extremely Low Efficiency Roll-Off Utilizing a Host with Small Singlet-Triplet Splitting.

Numerous efforts have been devoted to boost the efficiency of thermally activated delayed fluorescence (TADF) devices; however, strategies to suppress the device efficiency roll-off are still in urgent need. Here, a general and effective approach to suppress the efficiency roll-off of TADF devices is proposed, that is, utilizing TADF materials as the hosts for TADF emitters. Bearing small singlet-triplet splitting (ΔE) with donor and acceptor units, TADF materials as the hosts possess the potential to achieve matched frontier energy levels with the adjacent transporting layers, facilitating balanced charge injection as well as bipolar charge transport mobilities beneficial to the balanced charges transportation.