Non-doped Electroluminescent Devices Research Articles

Efficient deep blue fluorescent emitters based on bipolar phenanthroimidazole-carbazole hybrid for non-doped electroluminescent device with small CIEy

In this work, four bipolar deep-blue emitting materials of 2-(4-(9H-carbazol-9-yl)phenyl)-1-(4-methylphenyl)-1H-phenanthro[9,10-d]imidazole (MePPIM-Cz), 2-(4-(9H-carbazol-9-yl)phenyl)-1-(4-chlorophenyl)-1H-phenanthro [9,10-d]imidazole (ClPPIM-Cz), 2-(4-(9H-carbazol-9-yl)phenyl)-1-(4-tert-butyl phenyl)-1H-phenanthro[9,10-d]imidazole (BuPPIM-Cz) and 2-(4-(9H-carbazol-9-yl) phenyl)-1-(4-(trifluoromethyl)phenyl)-1H-phenanthro[9,10-d]imidazole (TFMPPIM-Cz) based on phenanthroimidazole and carbazole group have been designed and synthesized by introducing different electron active substitutions into the N1 position of phenanthroimidazole. The thermal, photophysical, electrochemical and computational properties of four compounds were investigated. All compounds exhibited the deep blue emission and good thermal stability with glass transition temperature (Tg) in range of 126°C–137 °C. Single carrier devices were also fabricated and demonstrated that the four compounds have bipolar transporting properties. The non-doped organic light emitting diodes (OLEDs) based on MePPIM-Cz, ClPPIM-Cz, BuPPIM-Cz and TFMPPIM-Cz showed deep blue emission with the Commission Internationale de L'Eclairage (CIE) coordinates of (0.16, 0.07), (0.16, 0.06), (0.16, 0.05) and (0.16, 0.10) and the maximum external quantum efficiencies (EQEmax) of 2.67%, 2.24%, 1.81% and 1.09%, respectively. The CIE coordinates of MePPIM-Cz, ClPPIM-Cz and BuPPIM-Cz is extremely close to the National Television System Committee (NTSC) standard blue CIE of (0.14, 0.08), which is very significant for high color purity full-color display.

The effect of electron donor and acceptor conformations on thermally activated delayed fluorescence

A novel organic molecule, 2-(9,9-dimethyl-9 H -acridin-10-yl)-anthraquinone (DMAC-AQ), is designed and synthesized by introducing a dual-conformation electron donor 9,9-dimethyl-9,10-dihydroacridine to a typical electron acceptor anthraquinone. DMAC-AQ exhibits decent thermally activated delayed fluorescence (TADF) property. However, using DMAC-AQ as an emitter, the doped and non-doped electroluminescence devices show maximum external quantum efficiency values of 5.63% and 0.32%, respectively, which are much lower than expected. Crystallographic analysis reveals that DMAC-AQ has two nearly orthogonal conformers (C1 and C2), in which planar and non-planar DMAC moieties are both linked to the AQ group with high twisting angles. Interestingly, the non-planar conformations of donor (D) and acceptor (A) units contribute to the generation of luminescent enantiomers in C2. C1 exhibits obvious TADF while C2 shows no TADF. The results suggest that nonplanar conformation of the donor in the emitter may inhibit TADF and significantly reduce device efficiency. The present work may provide new insight into understanding the effect of D and A conformations in TADF emitters. The non-planar conformations of electron donor and acceptor groups in thermally activated delayed fluorescence (TADF) emitters may inhibit TADF and significantly reduce device efficiency. • A TADF emitter with dual conformations was designed and synthesized. • The effect of donor and acceptor conformations on TADF was investigated. • Nonplanar conformation of the donor in the emitter may inhibit TADF.

Highly Efficient Near-Infrared Thermally Activated Delayed Fluorescent Emitters in Non-Doped Electroluminescent Devices.

Constructing organic near-infrared (NIR) luminescent materials to confront the formidable barrier of "energy gap law" remains challenging. Herein, two NIR thermally activated delayed fluorescence (TADF) molecules named T-β-IQD and TIQD were developed by connecting N,N-diphenylnaphthalen-2-amine and triphenylamine with a novel electron withdrawing unit 6-(4-(tert-butyl)phenyl)-6H-indolo[2,3-b]quinoxaline-2,3-dicarbonitrile. It is confirmed NIR-TADF emitters concurrent with aggregation-induced emission effect, J-aggregate with intra- and intermolecular CN⋅⋅⋅H-C and C-H⋅⋅⋅π interactions, and large center-to-center distance in solid states can boost the emissive efficiencies both in thin films and non-doped organic light-emitting diodes (OLEDs). Consequently, the T-β-IQD-based non-doped NIR-OLED achieved the maximum external quantum efficiency (EQEmax ) of 9.44 % with emission peak at 711 nm, which is one of the highest efficiencies reported to date for non-doped NIR-OLEDs.

A solution-processable hybridized local and charge-transfer (HLCT) phenanthroimidazole as a deep-blue emitter for efficient solution-processed non-doped electroluminescence device

Most highly efficient hybridized local and charge-transfer (HLCT)-based organic light‐emitting diodes (OLEDs) are multi-layer devices fabricated by thermal vacuum evaporation techniques, which are inappropriate for the simplified device fabrication process. However, there are only a couple of reported examples of solution-processed HLCT OLEDs, particularly deep-blue OLEDs. Herein, we design and synthesize a solution-processable HLCT emitter, CPPI, in which 1,2-diphenyl-phenanthroimidazole (PI) as an acceptor core is substituted by three 3,6-di-tert-butyl-N-phenyl-carbazole (CP) moieties as a donor and hole-transporting/solubilizing unit. The HLCT and photophysical properties are thoroughly examined by theoretical and experimental methods. CPPI exhibits HLCT characteristics with intense deep-blue color emission, high thermal and electrochemical stability, and decent hole-transporting ability. The molecule is successfully fabricated as a solution-processed non-doped emitter in organic light-emitting diodes (OLED). The solution-processed OLED retains efficient and stable deep-blue color emission (CIE coordinates of (0.157, 0.089)) with a narrow FWHM of 66 nm, an EQEmax of 3.39% and a high exciton utilization efficiency of 42.4%. Crucially, this result signifies an advance in developing a solution-processable HLCT molecule as an emitter for solution-processed non-doped OLEDs.

Exceptionally efficient deep blue anthracene-based luminogens: design, synthesis, photophysical, and electroluminescent mechanisms

Achieving high-efficiency deep blue emitter with CIEy < 0.06 (CIE, Commission Internationale de L’Eclairage) and external quantum efficiency (EQE) >10% has been a long-standing challenge for traditional fluorescent materials in organic light-emitting diodes (OLEDs). Here, we report the rational design and synthesis of two new deep blue luminogens: 4-(10-(4′-(9H-carbazol-9-yl)-2,5-dimethyl-[1,1′-biphenyl]-4-yl)anthracen-9-yl)benzonitrile (2M-ph-pCzAnBzt) and 4-(10-(4-(9H-carbazol-9-yl)-2,5-dimethylphenyl)anthracen-9-yl)benzonitrile (2M-pCzAnBzt). In particular, 2M-ph-pCzAnBzt produces saturated deep blue emissions in a non-doped electroluminescent device with an exceptionally high EQE of 10.44% and CIEx,y (0.151, 0.057). The unprecedented electroluminescent efficiency is attributed to the combined effects of higher-order reversed intersystem crossing and triplet-triplet up-conversion, which are supported by analysis of theoretical calculation, triplet sensitization experiments, as well as nanosecond transient absorption spectroscopy. This research offers a new approach to resolve the shortage of high efficiency deep blue fluorescent emitters.

Cyanophenyl spiro[acridine-9,9′-fluorene]s as simple structured hybridized local and charge-transfer-based ultra-deep blue emitters for highly efficient non-doped electroluminescent devices (CIEy ≤ 0.05)

Cyanophenyl substituted spiro[acridine-9,9′-fluorene]s featured with HLCT excited states yielded highly efficient non-doped OLEDs with emissions in high-definition television standard blue color, CIEy ≤ 0.05, and CEmax/EQEmax of 6.54 cd A−1/4.63%.

Purely organic and saturated red emitters for a non-doped electroluminescent device with an EQE of 6.3% and low efficiency roll-off

Using a 2,1,3-naphthodiazole derivative as the fluorescent emitter, a non-doped red OLED has been obtained with a maximum LE of 5.3 cd A−1, a maximum EQE value of 6.3%, good CIE coordinates of (0.64, 0.35) and low efficiency roll-off feature.

Highly fluorescent solid-state benzothiadiazole derivatives as saturated red emitters for efficient solution-processed non-doped electroluminescent devices

Benzothiadiazole derivatives exhibit strong solid-state red fluorescence emission with bipolar properties.

A highly efficient near infrared organic solid fluorophore based on naphthothiadiazole derivatives with aggregation-induced emission enhancement for a non-doped electroluminescent device.

Two regioisomers of naphthothiadiazole derivatives with aggregation-induced emission enhancement exhibited a strong solid-state fluorescence emission in the range of 666-760 nm. A non-doped EL device emitted brilliant near infrared emission peaked at 754 nm with a high maximum radiance of 22 050 mW Sr-1 m-2, an EQE as high as 1.48%, and relatively low efficiency roll-off.

Efficient deep blue emitter based on carbazole-pyrene hybrid for non-doped electroluminescent device

A new deep blue fluorescent material based on carbazole-pyrene hybrid, namely PyEtCz, was synthesized by palladium catalyzed C–C Suzuki coupling reaction, and structurally characterized by NMR, mass spectra and elemental analysis. The thermal stability, photophysical and optoelectronic properties were fully investigated. The material shows good thermal stability and can form an amorphous glass state. Its CH2Cl2 solution exhibits strong deep-blue emission with high fluorescent quantum yield of 75%. PyEtCz also possesses high HOMO level of −5.48 eV that would facilitate efficient hole injection into the emitting layer. The non-doped OLED using PyEtCz as emitter through vacuum evaporation was fabricated for purpose of investigating its electroluminescence properties. The device exhibited stable blue electroluminescence with CIE coordinate of (0.15, 0.10), a good performance with maximum luminance of 6573 cd m−2 and maximum luminance efficiency of 3.24 cd A−1 (3.35%) was achieved.

Remarkable mechanofluorochromism and low efficiency-off electroluminescence from a fully aromatic D-A cruciform emitter

Two-dimensional aryl center-crossed cruciforms generally exhibit the strongly twisted conformation and the spatial separation of frontier molecular orbitals, which can render the emitters destructible packing structures and high solid-state fluorescence efficiency. In the current work, we design and synthesize a benzene-centered cruciform emitter (DOT) with two adjacent phenyloxadiazoles and two adjacent triphenylamines starting from commercially available chemicals. Single crystal analysis indicates that DOT molecules loosely pack together in a twisted conformation and weak inter-molecular interactions. Upon mechanical grinding, the crystalline DOT is changed into an amorphous state, accompanying with a remarkable (56 nm) mechanofluorochromism (MFC). In view of the crossed donor and acceptor structure and the strong solid-state fluorescence, the non-doped electroluminescence (EL) devices with the structures ITO/HATCN/NPB or TAPC/DOT/TPBi/LiF/Al are fabricated. The devices efficiently emit 500 nm green EL with the low turn on voltage of 2.7 V and the low efficiency roll-off. Thus, we have provided a feasible synthetic strategy for various isomers and analogues of fully aromatic benzene-centered D‒A cruciforms and demonstrated a new class of candidates for MFC and EL materials.

Rational molecular design of bipolar phenanthroimidazole derivatives to realize highly efficient non-doped deep blue electroluminescence with CIEy ˂ 0.06 and EQE approaching 6%

We report a series of novel phenanthroimidazole (PI) based bipolar deep blue-emitting compounds. The structure-property relationship is systematically studied. A star-shape steric group tetraphenylbenzene (TPB) is substituted to the C2 position of PI, which contributes to high-efficiency deep blue photoluminescence (PL) in solid state; while electron-withdrawing or donating groups are attached to the N1 position to realized tunable charge transfer character for high exciton utilization. Among all the emitters, the non-doped electroluminescent devices based on TPBPPI-PY and TPBPPI-PBI display superior EQE of 5.70% and 5.94% with color coordinates of (0.16, 0.049) and (0.16, 0.059), respectively. These performances are among the best deep blue OLEDs with CIEy < 0.06.

Through-space charge transfer in luminophore based on phenyl-linked carbazole- and phthalimide moieties utilized in cyan-emitting OLEDs

Two new carbazole-containing aromatic imides were synthesized by three-step synthetic pathway with the moderate yields up to 76%. The compounds were investigated theoretically. TD-DFT computational studies revealed low singlet-triplet energy differences. The compounds were found to have relatively high thermal stability with 5% mass loss temperatures in the range of 280–310 °C. Phthalimide-based compound exhibited aggregation-induced emission enhancement and thermally activated delayed fluorescence with photoluminescence quantum efficiency of 20% in the solid state. Structure-properties relationship of this compound was investigated and it was found that charge-transfer through space mechanism is responsible for the emission. Series of green-emitting doped and non-doped electroluminescent devices were fabricated based on the carbazole-phthalimide derivative were fabricated to reveal best-performing mCP-doped device demonstrating maximum external quantum efficiency of 2.4% with current efficiency of 6.6 cd/A and power efficiency of 4.0 lm/W with maximum brightness of 8300 cd/m2.

Efficient Near‐Infrared (NIR) Organic Light‐Emitting Diodes Based on Donor–Acceptor Architecture: An Improved Emissive State from Mixing to Hybridization

AbstractHerein, a high‐efficiency near‐infrared (NIR) material PXZ‐3‐NZP is designed and sythesized using the concept of hybridized local and charge‐transfer (CT) state (HLCT), which is composed of donor (D) and acceptor (A) moieties as well as 10‐substituted isomeride PXZ‐10‐NZP for the purpose of comparison and deep understanding on the essential difference of their excited state properties. As a result, the nondoped electroluminescent (EL) device of PXZ‐3‐NZP exhibits an excellent NIR emission (λmax = 738 nm) with a maximum external quantum efficiency (ηEQE) of 0.82% and a Commission International de L'clairage coordinate of (0.70, 0.29), which is record‐setting among NIR fluorescent organic light emitting diodes with similar EL chromaticity. Also, a high ηEQE of 2.03% is achieved in a doped device with a deep red emission at λmax = 676 nm. As a comparison, PXZ‐10‐NZP shows far inferior performance to those of PXZ‐3‐NZP in both nondoped and doped devices, due to the instinct CT character of its S1 excited state. In terms of excited state, HLCT state is exactly superior to mixed state in high‐efficiency luminescence, which is a particularly useful strategy to design narrow‐bandgap light‐emitting materials beyond energy gap law with donor–acceptor architectures.

Blue AIEgens bearing triphenylethylene peripheral: adjustable intramolecular conjugation and good device performance

The new approaches to construct deep blue aggregation-induced emission (AIE) materials have been explored, which control the conjugation by two different strategies, to make a great step for the commercialization of organic light-emitting diodes. In order to shorten the intramolecular conjugation length, triphenylethylene (tPE) was utilized to construct blue AIEgens as peripheral groups, instead of tetraphenylethylene (TPE), the famous AIE star molecule, to yield three blue AIEgens of 3,4-BtPE-PI, 4,4-BtPE-PI and 4,4-BtPE-PPI. Nondoped electroluminescence devices are fabricated by using these three AIEgens as the emitting material layer, the best performance of 3.8cd/A as the maximum current efficiency achieved at the commission internationale de l’Eclairage coordinates of (0.17, 0.18).

Synthesis, aggregation-induced emission, and electroluminescence properties of a novel emitter comprising tetraphenylethene and carbazole moieties

In this paper, a new tetraphenylethene-based molecule with two carbazole-containing moieties, 1,2-bis(4-(9H-carbazol-9-yl)phenyl)-1,2-diphenylethylene (BCPD), has been successfully prepared by the McMurry coupling reaction of (4-(9H-carbazol-9-yl)phenyl)(phenyl)methanone (1). The structure of BCPD was fully characterized by elemental analysis, mass spectrometry, NMR spectroscopy and infrared spectroscopy. Its optical properties in both solution and solid-state were investigated systematically. The results show that BCPD has almost no emission in solution but is highly emissive in the aggregated form, demonstrating a typical aggregation-induced emission phenomenon. An extensive investigation of its thermal properties verifies that BCPD enjoys excellent thermal and morphological stabilities with Td of 335°C and Tg of 105°C. Eventually, non-doped electroluminescence device by utilizing BCPD as emitting layer was fabricated. The device exhibits sky blue light with the maximum luminance and the maximum luminescent efficiency reaching 8624cdm−2 and 4.243cdA−1, respectively.

Excellent deep-blue emitting materials based on anthracene derivatives for non-doped organic light-emitting diodes

Two deep-blue emitting materials 2-tert-butyl-9,10-bis(3,5-diphenylphenyl)anthracene (An-1) and 2-tert-butyl-9,10-bis(3,5-diphenylbiphenyl-4′-yl)anthracene (An-2) were successfully synthesized by the Pd-catalyzed Suzuki coupling reaction. Both of these compounds have high thermal stabilities and show strong deep-blue emission as solid-state film as well as in n-hexane solution. Two non-doped electroluminescent devices employing An-1 and An-2 as emitting layers were fabricated by vacuum vapor deposition. These devices exhibited highly efficient and stable deep-blue emission with high color purity. The CIE coordinate and maximum EQE of An-1 based device are 4.2% and (0.16, 0.06), respectively. Device based on An-2 achieved a maximum EQE of 4.0% and a CIE coordinate of (0.16, 0.10).

Aggregation-induced emission (AIE) active iridium complexes toward highly efficient single-layer non-doped electroluminescent devices

For the first time, AIE-active Ir(iii) complexes are applied successfully in single-layer non-doped phosphorescent devices, and a high current efficiency of 25.7 cd A−1 and 7.6% external quantum efficiency are achieved.

Self-host thermally activated delayed fluorescent dendrimers with flexible chains: an effective strategy for non-doped electroluminescent devices based on solution processing

Due to the encapsulation of the emissive core, the concentration quenching effect of the TADF material can be effectively restrained.

Fluoranthene derivatives as blue fluorescent materials for non-doped organic light-emitting diodes

In this study, we report synthesis of symmetrically and non-symmetrically functionalized fluoranthene-based blue fluorescent molecular materials for non-doped electroluminescent devices.