A Multifunctional Bipolar Luminogen with Delayed Fluorescence for High‐Performance Monochromatic and Color‐Stable Warm‐White OLEDs

Abstract

(1 of 11)Increasing exciton utilization and reducing exciton annihilation are crucial to achieve high performance of organic light-emitting diodes (OLEDs), which greatly depend on molecular engineering of emitters and hosts. A novel luminogen (SBF-BP-DMAC) is synthesized and characterized. Its crystal and electronic structures, thermal stability, electrochemical behavior, carrier transport, photoluminescence, and electroluminescence are investigated. SBF-BP-DMAC exhibits enhanced photoluminescence and promotes delayed fluorescence in solid state and bipolar carrier transport ability, and thus holds multifunctionality of emitter and host for OLEDs. Using SBF-BP-DMAC as an emitter, the nondoped OLEDs exhibit maximum electroluminescence (EL) efficiencies of 67.2 cd A −1 , 65.9 lm W −1 , and 20.1%, and the doped OLEDs provide maximum EL efficiencies of 79.1 cd A −1 , 70.7 lm W −1 , and 24.5%. A representative orange phosphor, Ir(tptpy) 2 acac, is doped into SBF-BP-DMAC for OLED fabrication, giving rise to superior EL efficiencies of 88.0 cd A −1 , 108.0 lm W −1 , and 26.8% for orange phosphorescent OLEDs, and forwardviewing EL efficiencies of 69.3 cd A −1 , 45.8 lm W −1 , and 21.0% for two-color hybrid warm-white OLEDs. All of these OLEDs can retain high EL efficiencies at high luminance, with very small efficiency roll-offs. The outstanding EL performance demonstrates the great potentials of SBF-BP-DMAC in practical display and lighting devices.luminous efficiencies and efficient exciton utilization approaching 100%. [2] However, commercial phosphors depend on rare metal elements such as iridium and platinum, and thus are usually expensive. What is more, high doping concentrations (5-20 wt%) of phosphors were recently reported to optimize the device performance, leading to a high manufacturing cost. [3] As promising alternatives, Adachi and co-workers developed purely organic luminescent materials with thermally activated delayed fluorescence (TADF), which can fully utilize the electrogenerated excitons in OLEDs and thus afford excellent external quantum efficiencies (ƞ ext ) of >20% via reverse intersystem crossing (RISC) process based on small singlet-triplet energy gaps (ΔE ST ≤ 0.3 eV) of the molecules. [4] However, on account of the long triplet lifetimes, most phosphors and TADF emitters suffer from negative nonradiative processes in OLEDs, such as aggregation and concentration caused quenching, [5] triplet-triplet annihilation (TTA), [6] singlet-triplet annihilation (STA), [7] and so on, [8] which greatly limits their practical applications. To address the issue, robust luminogens that can alleviate emission quenching and exciton annihilation are extremely desired. According to the previous works, reducing the intermolecular interactions (e.g., π-π interactions) has been evidenced to be an effective strategy to develop efficient luminescent materials with high photoluminescence quantum yields (Φ F s) in neat films and prominent delayed fluorescence. [9] By this way, the emitters are usually insensitive ...

A π–D and π–A Exciplex‐Forming Host for High‐Efficiency and Long‐Lifetime Single‐Emissive‐Layer Fluorescent White Organic Light‐Emitting Diodes

“…The efficiency of FET ( Φ FET ) can be calculated as following equations [ 24 ] where τ h is the decay time of the host in the absence of guest. The distance between the host and guest ( R hg ) can be calculated by using where β represents the guest fraction in the doping film, ρ stands for the density of the doping film, of which the value is usually taken 1 g cm −3 .…”

Twisted Biphenyl‐Diimide Derivatives with Aggregation‐Induced Emission and Thermally Activated Delayed Fluorescence for High Performance OLEDs

“…As illustrated in Figure a, the two compounds showed good electrochemical stability with excellent reversibility of reduction/oxidation processes, favoring charge injection and transport in OLEDs. [ 53 ] The HOMO energy levels of BPI‐PhPXZ and BPI‐PhDMAC were calculated as −4.86 and −5.07 eV, respectively, according to the onset oxidation potentials ( E ox ) and the formula of HOMO = –( E ox + 4.4) eV. The LUMO energy levels were −2.91 and −2.89 eV for BPI‐PhPXZ and BPI‐PhDMAC by using the formula of LUMO = –( E red + 4.4) eV, and E gs were 1.95 and 2.18 eV (calculated from the E g = HOMO − LUMO), respectively.…”

Dendritic Interfacial Exciplex Hosts for Solution‐Processed TADF‐OLEDs with Power Efficiency Approaching 100 lm W−1

“…[ 22 ] In addition, owing to the TADF feature of exciplex, triplet excitons can be converted to singlet ones through reverse intersystem crossing (RISC) process and then transferred to dopant through Förster resonance energy transfer (FRET), leading to alleviated triplet‐triplet and triplet‐polaron annihilation, and improved efficiency of the resultant devices. [ 23–25 ] Generally, exciplex could be formed in bulk phase by mixing donor and acceptor together (denoted as bulk exciplex) or at the interface of donor and acceptor layers (named interfacial exciplex). [ 26 ] Although a series of vacuum‐deposited exciplex hosts have been developed and achieved very perfect device performance, [ 8,20–22 ] exciplex‐type hosts used for vacuum‐deposited OLEDs might not suitable for solution‐processed OLEDs, because of the very limited solubility and strong intermolecular interaction, [ 27 ] which may induce phase separation or serious intermolecular π‐π stacking.…”