Strategy for the Realization of Highly Efficient Solution-Processed All-Fluorescence White OLEDs-Encapsulated Thermally Activated Delayed Fluorescent Yellow Emitters

Synthesis and Luminescent Properties of 1,4,5-Triphenylimidazole‒Phenothiazine Fluorophores

Two blue donor-acceptor fluorophores with 1,4,5-triphenylimidazole as the electron-transporting unit and phenothiazine as the hole-transporting unit were synthesized by grafting 1,4,5-triphenylimidazole moieties onto 3- and 3,7-position of the phenothiazine core and characterized by spectroscopic methods. Their thermal stability, photophysical, electrochemical and electroluminescence properties were systematically investigated. These compounds exhibit good thermal stability and show blue emission in dichloromethane solution and thin solid films. The solution-processed doped devices were fabricated by using these fluorophores as the emitting dopant in 1,3-bis(N-carbazolyl)benzene host, in which the device fabricated from the fluorophore containing two 1,4,5-triphenylimidazole moieties exhibited blue emission with a luminance of 648 cd/m² and external quantum efficiency of 1.48%.

Circularly Polarized White Organic Light-Emitting Diodes Based on Spiro-Type Thermally Activated Delayed Fluorescence Materials

In this study, we report the first circularly polarized white organic light-emitting diodes (CP-WOLEDs) based on all thermally activated delayed fluorescence (TADF) materials. Two pairs of spiro-type TADF enantiomers, (R/S)-SPOCN (5,5'-((2,2',3,3'-tetrahydro-1,1'-spirobi[indene]-7,7'-diyl)bis(oxy))bis(4-(10H-phenoxazin-10-yl)phthalonitrile)) and (R/S)-OSFSO (2'-(trifluoromethyl)-spiro[quinolino[3,2,1-kl]phenoxazine-9,9'-thioxanthene]-10',10'-dioxide), serve as emitters with complementary emission. The CP-OLEDs exhibit warm white emission with a CIE coordinate of (0.35, 0.46). Besides, decent device performances are observed with an external quantum efficiency of up to 21.6 % at maximum and 11.8 % at 1000 cd m^-2 . Obvious circularly polarized electroluminescence signals are detected with a dissymmetry factor |g_EL | of around 3.0×10^-3 . This is the first report of CP-WOLEDs that can harvest both singlet and triplet excitons, which provides a feasible strategy for the development of CP-WOLEDs with remarkable device performances.

Solution-processed white OLEDs with power efficiency over 90 lm W-1 by triplet exciton management with a high triplet energy level interfacial exciplex host and a high reverse intersystem crossing rate blue TADF emitter

Solution-processed white organic light-emitting diodes (WOLEDs) have shown much lower device efficiency than their vacuum-deposited counterparts, due to the lack of triplet exciton management in a single-emissive-layer device structure, which will induce triplet-triplet annihilation (TTA) and triplet-polaron annihilation (TPA). Here, two kinds of solution-processed WOLEDs, including thermally activated delayed fluorescence (TADF)/phosphorescence hybrid WOLEDs and all-TADF WOLEDs, with high power efficiency are developed by using a high triplet energy level (T1) interfacial exciplex as a host and a high reverse intersystem crossing (RISC) rate TADF emitter as a blue dopant for triplet exciton management. The interfacial exciplex host with high T1 can ensure that triplet excitons transfer from the host to the blue emitter, and the blue TADF emitter with high RISC rate (1.15 × 107 s-1) can rapidly upconvert triplet excitons to singlet ones to avoid TTA and TPA. The solution-processed TADF/phosphorescence hybrid and all-TADF WOLEDs exhibit maximum external quantum efficiencies of 31.1% and 27.3%, together with maximum power efficiencies of 93.5 and 70.4 lm W-1, respectively, which are the record efficiencies for solution-processed WOLEDs, and quite comparable to those of most vacuum-deposited counterparts.

Recent Advances of Interface Exciplex in Organic Light-Emitting Diodes

The interface exciplex system is a promising technology for reaching organic light-emitting diodes (OLEDs) with low turn-on voltages, high efficiencies and long lifetimes due to its unique virtue of barrier-free charge transport, well-confined recombination region, and thermally activated delayed fluorescence characteristics. In this review, we firstly illustrate the mechanism frameworks and superiorities of the interface exciplex system. We then summarize the primary applications of interface exciplex systems fabricated by doping and doping-free technologies. The operation mechanisms of these OLEDs are emphasized briefly. In addition, various novel strategies for further improving the performances of interface exciplex-based devices are demonstrated. We believe this review will give a promising perspective and attract researchers to further develop this technology in the future.

π-Stacked Donor-Acceptor Dendrimers for Highly Efficient White Electroluminescence

π-Stacked dendrimers consisting of cofacially aligned donors and acceptors are developed by introducing three dendritic teracridan donors with orthogonal configuration and three triazine acceptors in periphery of hexaphenylbenzene skeleton. The dendritic structure and orthogonal configuration of teracridan not only make their outer acridan segments approaching to acceptor in close distance, but also fix donor and acceptor in face-to-face alignment, leading to through-space charge transfer emission with thermally activated delayed fluorescence (TADF) effect. By regulating charge transfer strength via substituent effect of acceptor, emission color of the dendrimers can be tuned from blue to yellow/red region. Solution-processed two-color white organic light-emitting diodes (OLEDs) based on blue and yellow π-stacked donor-acceptor dendrimers exhibit the maximum external quantum efficiency of 20.6 % and maximum power efficiency of 58.9 lm W^-1 , representing the state-of-the-art efficiency for all-TADF white OLEDs by solution process.

Efficient white light-emitting polymers from dual thermally activated delayed fluorescence chromophores for non-doped solution processed white electroluminescent devices

Conjugated TADF copolymers comprised of two TADF molecules linked with carbazole exhibited stable pure white emission from non-doped OLEDs with CIE coordinates (0.32, 0.35), a maximum luminance efficiency of 9.13 cd A⁻¹, and a maximum EQE of 4.17%.

Solution-Processed White Circularly Polarized Organic Light-Emitting Diodes Based on Chiral Binaphthyl Emitters

By combining the blue and orange CPL or functionalized bis-benzoxanethones emitters (S-/R-BN-tCz and S-/R-BN-PXZ), warm white CP-OLEDs were fabricated using solution-processed single emitting layer strategy. The successful realization of white CP-EL benefited from the same stable binaphthyl chirality and similar rigid structure of the two emissive CPL emitters. The devices exhibited the low turn-on voltage of ≈4.3 V, maximum luminance of ≈10200 cd m^-2 and maximum current efficiency of ≈2.0 cd A^-1 . Most significantly, the devices with CIE coordinates of (0.32, 0.45) displayed intense CP-EL signals in the spectral range of 450 to 650 nm, and showed stable g_EL values of ≈10^-3 as the luminance increased from 100 to 6000 cd m^-2 . To the best of our knowledge, this work provides for the first time a simple and feasible strategy to fabricate solution-processed white CP-OLEDs based on the co-doping of the CPL emitters.

Highly Efficient and Color-Stable Thermally Activated Delayed Fluorescence White Light-Emitting Diodes Featured with Single-Doped Single Emissive Layers

Despite their merits of environmental friendliness, low cost, and large-scale production, thermally activated delayed fluorescence (TADF) based white organic light-emitting diodes (WOLEDs) for daily lighting applications still face the formidable challenges of structural simplification and controllable exciton allocation. Here, the state-of-the-art full-TADF WOLEDs with features of the single-doped single emissive layers (EMLs) and ultrasimple trilayer structure are demonstrated. The EMLs are binary systems as yellow TADF emitter (4CzTPNBu) doped blue TADF matrix (ptBCzPO₂ TPTZ) with the large steric hindrance and mismatched frontier molecular orbital energy levels to effectively restrain excessive blue-to-yellow triplet exciton transfer and host-dopant interaction induced triplet quenching. Simultaneously, Förster resonance energy transfer is utilized to optimize exciton allocation for the balance of blue and yellow emissions, giving rise to the photoluminescence quantum yield beyond 90%. Consequently, these single-doped EMLs endow their cool white, pure white, and warm white diodes with the high-quality and ultrastable white light and the 100% exciton utilization efficiencies through the extremely simple structures, making them competent for the diverse daily lighting applications.

Highly Efficient Solution-Processed Thermally Activated Delayed Fluorescence Bluish-Green and Hybrid White Organic Light-Emitting Diodes Using Novel Bipolar Host Materials

Two pyridine-containing bipolar host materials with high triplet energy, 9,10-dihydro-9,9-dimethyl-10-(3-(6-(3-(9,9-dimethylacridin-10(9H)-yl)phenyl)pyridin-2-yl)phenyl acridin (DDMACPy) and N-(3-(6-(3-(diphenyl amino)phenyl)pyridin-2-yl)phenyl)-N-phenylbenzenamine (DTPAPy), are synthesized from the modification of the commonly adapted host material 2,6-bis(3-(9H-carbazol-9-yl)phenyl)pyridine (DCzPPy). The highest occupied molecular orbital levels of DDMACPy (5.50 eV) and DTPAPy (5.60 eV) are found to be shallower than that of DCzPPy (5.90 eV) that leads to the improvement in hole injection from the hole transport layer PEDOT:PSS (WF = 5.10 eV). These host materials are used in the emitting layer of bluish-green organic light-emitting diode (OLED) with the thermally activated delayed fluorescence (TADF) emitter, 9,9-dimethyl-9,10-dihydroacridine-2,4,6-triphenyl-1,3,5-triazine, as the guest. The DDMACPy-based device shows the highest performance among them, with the maximum external quantum efficiency (EQE_max), current efficiency (CE_max), and power efficiency (PE_max) of 21.0%, 53.1 cd A^-1, and 44.0 lm W^-1 at CIE (0.17, 0.42), respectively. By further doping with the red-emitting phosphor iridium(III) bis(2-phenylquinoline)(2,2,6,6-tetramethylheptane-3,5-ionate) [Ir(dpm)PQ₂] and yellow-emitting phosphor iridium(III) bis(4-(4-t-butylphenyl)thieno[3,2-c]pyridinato-N,C20)acetylacetonate (PO-01-TB) emitters into the bluish-green emitting layer, a TADF-phosphor hybrid white OLED (T-P WOLED) is obtained with excellent EQE_max, CE_max, and PE_max of 17.4%, 48.7 cd A^-1, and 44.5 lm W^-1 at CIE (0.35, 0.44), respectively. Moreover, both the bluish-green and WOLED show a low efficiency roll-off with external quantum efficiencies at the brightness of 1000 cd m^-2 (EQE₁₀₀₀) of 18.7 and 16.2%, respectively, which are the highest performance records among the solution-processed TADF bluish-green and T-P WOLEDs.

Optimizing energy transfer for highly efficient single-emissive-layer white thermally activated delayed fluorescence organic light-emitting diodes

Thermally activated delayed fluorescence (TADF) white organic light-emitting diodes (WOLEDs) with simplified structures have great potential for daily lighting applications. However, the complicated energy and charge transfer processes between TADF emitters impede the development of single-layer white TADF systems. Here we demonstrate high-efficiency WOLEDs with single-emissive layers composed of blue and yellow TADF emitters with appropriate steric hindrances and energy gaps, which optimize the energy transfer from blue to yellow dopants for rational exciton allocation. As a consequence, the single-emissive-layer WOLEDs achieve the maximum external quantum efficiency beyond 20% and small roll-offs, which are among the best results of full-TADF WOLEDs.

White light emitting diode based on purely organic fluorescent to modern thermally activated delayed fluorescence (TADF) and perovskite materials

White organic/polymer light emitting diode (WOLED/WPLED) processed from solution has attracted significant research interest in recent years due to their low device production cost, device flexibility, easy fabrication over large area including roll to roll and ability to print in various designs and shapes providing enormous design possibilities. Although WOLEDs fabricated using solution process lack their thermally evaporated counterparts in terms of device efficiency, remarkable progress has been made in this regard in recent years by utilizing new materials and device structures. In the present review, we have summarized and extrapolated an excellent association of old and modern concept of cost-effective materials and device structure for realization of white light. In particular, this article demonstrated and focused on design, and development of novel synthesis strategy, mechanistic insights and device engineering for solution process low cost WOLEDs device. Herein, an overview of the prevailing routes towards white light emitting devices (WLEDs) and corresponding materials used, including polymer based WLED, small molecules emitters based thermally activated delayed fluorescence (TADF), perovskite light-emitting diodes (PeLEDs) and hybrid materials based LEDs, color down-converting coatings with corresponding best efficiencies ever realized. We presume that this exhaustive review on WLEDs will offer a broad overview of the latest developments on white SSL and stonework the approach en route for innovations in the immediate future.