Deep-blue Organic Light-emitting Diodes Research Articles

Oligocarbazole Modified Hybridized Local and Charge Transfer Emitters for Efficient Solution-Processed Deep Blue Organic Light-Emitting Diodes

Herein, two new blue solution-processable hybridized local and charge transfer (HLCT) fluorophores (pCTPI and mCTPI) were designed and synthesized by incorporating oligocarbazole fragment into the HLCT luminogenic triphenylamine-phenanthroimidazole (TPI) molecule. Their HLCT and photoluminescence properties were experimentally and theoretically probed by the solvation effect and density functional theory calculations. These materials exhibited an intense blue emission (440 nm) with a good solution-processed film-forming quality and decent hole mobility (3.98 – 4.55 × 10-6 cm2 V-1s-1), resulting in a successful utilization as non-doped emissive layers in simple solution-processed organic light-emitting diodes (OLEDs). The devices showed deep blue emission (430 nm) with excellent electroluminescence (EL) performance and narrow EL emission as compared to the TPI-based device. Notably, pCTPI-based solution-processed OLED achieved a luminance of 4,938 cd m-2 and an external quantum efficiency of 6.35%.

Nitrogen-Embedding Strategy for Short-Range Charge Transfer Excited States and Efficient Narrowband Deep-Blue Organic Light Emitting Diodes.

The development of deep-blue organic light-emitting diodes (OLEDs) featuring high efficiency and narrowband emission is of great importance for ultrahigh-definition displays with wide color gamut. Herein, based on the nitrogen-embedding strategy for modifying the short range charge transfer excited state energies of multi-resonance (MR) thermally activated delayed fluorescence (TADF) emitters, we introduce one or two nitrogen atoms into the central benzene ring of a versatile boron-embedded 1,3-bis(carbazol-9-yl)benzene skeleton. This approach resulted in the stabilization of the highest occupied molecular orbital energy levels and the formation of intramolecular hydrogen bonds, and thus systematic hypsochromic shifts and narrowing spectra. In toluene solution, two heterocyclic-based MR-TADF molecules, Py-BN and Pm-BN, exhibit deep-blue emissions with high photoluminescence quantum yields of 93% and 94%, and narrow full width at half maximum of 14 and 13 nm, respectively. A deep-blue hyperfluorescent OLED based on Py-BN exhibited a maximum external quantum efficiency of 27.7% and desired color purity with Commission Internationale de L'Eclairage (CIE) coordinates of (0.150, 0.052). These results demonstrate the significant potential for the development of deep blue narrowband MR-TADF emitters.

Realizing Efficient Deep Blue Light‐Emitting Diodes and Single Component White Light‐Emitting Diodes with Low Efficiency Roll‐Offs from Anthracene‐Based Organic Emitters

AbstractPurely organic emitters that can efficiently utilize triplet excitons are highly desired for organic light‐emitting diodes (OLEDs). Hot exciton channel has emerged as a competitive strategy for the efficient harvest of triplet exciton to improve the OLEDs performance and suppress efficiency roll‐off in devices. Here, two deep blue emitters, OPACN and SPACN are designed and synthesized, employing diphenylanthracene (PAnP) unit as central core, sp3 hybridized O and S atom as conjugation blocker, and electron‐withdrawing cyano group as intramolecular charge transfer (ICT) introducer. Especially, OPACN presents bluer emission with electroluminescence (EL) peak at 436 nm, CIE coordinates of (0.16, 0.10), and a maximum external quantum efficiency (EQE) of 8.7%, while SPACN shows an EL peak at 444 nm, CIE coordinates of (0.17, 0.12) and a maximum EQE of 9.0%. Furthermore, the single‐component white organic light‐emitting diode (WOLED) based on OPACN exhibits dual emission from monomer and excimer with EQE of 5.4%. And even the luminance increases to 1000–10 000 cd m−2, the EQE still remains as high as 5.2% and 4.4%, respectively, showing very low efficiency roll‐offs. These results demonstrate an alternative approach for designing high‐performance deep blue OLEDs and WOLEDs based on single component.

Realizing highly efficient deep-blue organic light-emitting diodes towards Rec.2020 chromaticity by restricting the vibration of the molecular framework.

Deep-blue organic light-emitting diodes (OLEDs) with narrow emission spectra and high efficiency, meeting the Rec.2020 standard, hold significant promise in the realm of 4K/8K ultrahigh-definition displays. However, the development of light-emitting materials exhibiting both narrowband emission and high efficiency, particularly in the realm of deep-blue thermally activated delayed fluorescence (TADF), confronts substantial challenges. Herein, a novel deep-blue TADF emitter, named BOC-PSi, was designed by integrating a rigid B-heterotriangulene acceptor (A) with a rigid phenazasiline donor (D). The replacement of a sp3 carbon atom with a sp3 silicon atom in the D moiety helps to restrict the low-frequency bending vibration throughout the entire D-A molecular backbone, while concurrently accelerating the multi-channel reverse intersystem crossing (RISC) processes. Notably, OLEDs using the BOC-PSi emitter exhibit exceptional performance, with a high maximum external quantum efficiency (EQEmax) approaching 20%, and a superior color purity closely aligning with the Rec.2020 blue standard.

Correction: Orthogonal anthracene and pyrene derivatives for efficient pure deep-blue organic light-emitting diodes

Correction for ‘Orthogonal anthracene and pyrene derivatives for efficient pure deep-blue organic light-emitting diodes’ by He Jiang et al., J. Mater. Chem. C, 2023, 11, 6438–6443, https://doi.org/10.1039/D3TC00594A.

Efficient and low roll-off deep-blue organic light-emitting diodes with anthracene-based compounds as hosts

We synthesize four anthracene-based bipolar host materials. The external quantum efficiency and the max luminance of the deep blue OLEDs reaches 8.10% and 7973 cd m−2, and the CIE is (0.1298, 0.1712) with a low efficiency roll-off of only 2.9%.

Windmill-Type Molecules for Efficient Deep-Blue Organic Light-Emitting Diodes via Hybridized Local and Charge-Transfer Excited-State

Twisted bipolar molecules with hybridized local and charge transfer (HLCT) excited state have been demonstrated to be an efficient strategy for achieving high-performance deep-blue electroluminescence (EL) by harvesting both singlet...

An Ideal Molecular Construction Strategy for Ultra-Narrow-Band Deep-Blue Emitters: Balancing Bathochromic-Shift Emission, Spectral Narrowing, and Aggregation Suppression.

Narrowband emissive multiple resonance (MR) emitters promise high efficiency and stability in deep-blue organic light-emitting diodes (OLEDs). However, the construction of ideal ultra-narrow-band deep-blue MR emitters still faces formidable challenges, especially in balancing bathochromic-shift emission, spectral narrowing, and aggregation suppression. Here, DICz is chosen, which possesses the smallest full-width-at-half-maximum (FWHM) in MR structures, as the core and solved the above issue by tuning its peripheral substitution sites. The 1-substituted molecule Cz-DICz is able to show a bright deep-blue emission with a peak at 457nm, an extremely small FWHM of 14nm, and a CIE coordinate of (0.14, 0.08) in solution. The corresponding OLEDs exhibit high maximum external quantum efficiencies of 22.1%-25.6% and identical small FWHMs of 18nm over the practical mass-production concentration range (1-4 wt.%). To the best of the knowledge, 14 and 18nm are currently the smallest FWHM values for deep-blue MR emitters with similar emission maxima under photoluminescence and electroluminescence conditions, respectively. These discoveries will help drive the development of high-performance narrowband deep-blue emitters and bring about a revolution in OLED industry.

Hydrogen bond boosts EQEs to 30+% for acridone-carbazole based deep-blue TADF emitters in simple-structure OLEDs

A group of D-A-D type thermally activated delayed fluorescence (TADF) emitters were developed with 10-(pyridin-2-yl)acridin-9(10H)-one as acceptor and substituted carbazoles as donors. The design was focused on incorporating pyridyl to the 10-site nitrogen atom of acridin-9(10H)-one ring, which improved the reverse intersystem crossing rate constant kRISC from 103 s−1 to 105 s−1. More importantly, the linkage of pyridyl uniquely from its 2-position to acridin-9(10H)-one ring induced intramolecular hydrogen bond, which suppressed nonradiative transition rates by three to four orders of magnitude and universally increased the photoluminescence quantum yields from 54 % to nearly 100 %. In combination with the intrinsic high horizontal orientation ratios over 90 % of acridin-9(10H)-one based molecules, such pyridyl decoration boosted the external quantum efficiencies of simple-structure deep blue organic light-emitting diodes to 31–33 % with CIEx of 0.15 and CIEy of 0.09 ∼ 0.12, being double of that for hydrogen-bond-free parent emitter and representing the state-of-the-art efficiencies for the deep blue twisted intramolecular charge transfer type TADF materials with CIEy ≤ 0.10 so far. These TADF emitters are comparable with the prevailing multiresonance blue emitters in terms of distinctive efficiencies, but advantageous in much facile syntheses.

A multifunctional emitter with synergistical adjustment of rigidity and flexibility for high-performance data-recording and organic light-emitting devices with “hot exciton” channel

An intramolecular dual-locking design is proposed for organic luminescent materials, aiming at synergistically adjusting the molecular rigidity and flexibility for applications in data storage and light-emitting devices. According to this concept, a multifunctional donor–acceptor type emitter, LPCz-TRZ, is developed, featuring a “soft” diphenylmethane lock and a “hard” single-bond lock in its donor moiety. The highly locked structure results in appreciable rigidity that endows LPCz-TRZ with high luminescence efficiency, narrow emission, promoted charge conductivity and triplet harvesting “hot exciton” channels, allowing high-performance deep-blue organic light-emitting diodes (OLEDs) with an external quantum efficiency as high as 7.88%, which is among the best deep-blue “hot exciton” OLEDs. Notably, the structure of LPCz-TRZ is semi-flexible, leading to unique mechanochromic luminescence (MCL) behavior. Compared to the conventional MCL emitters, the orchestration of rigidity and flexibility in LPCz-TRZ affords high-contrast, fatigue-resistance MCL, as evidenced by a simple, rewritable data-recording demonstration with negligible fatigue after up to 15 wiring-erasing cycles.

Stable Spiro‐Linked Terfluorenes with Triphenylamine Pendants as Deep Blue Emitters for Solution‐Processed Organic Light‐Emitting Diodes

AbstractHerein, we reported new spiro‐linked terfluorenes (TF1 and TF2) as solution‐processible hole‐transporting‐free deep‐blue fluorescent emitters for simple structured organic light‐emitting diodes (OLEDs). They were designed and synthesized by incorporating triphenylamine pendants into the C9 positions of the spiro‐linked terfluorene, resulting in a criss‐cross configuration of the two terfluorene backbones and a highly bulky molecular geometry from the pendant groups. Consequently, TF1 and TF2 exhibited good quality thin films by spin‐coating, stable deep blue emissions with no unwanted long wavelength emission after prolonged thermal annealing and UV exposure, shallow highest occupied molecular orbital energy levels (−5.36–−5.37 eV), decent hole mobilities (1.09–1.23×10−7 cm2 V−1 s−1), and high thermal stability, and effectively employed as non‐doped emissive layers in the OLEDs. Particularly, TF2‐based solution‐processed OLED displayed a bright deep blue emission with superior EL performance (turn‐on voltage (Von) of 3.4 V, external quantum efficiency (EQEmax) of 3.82 %, and luminance efficiency (LEmax) of 5.22 cd A−1). This outstanding EL performance of the non‐doped solution‐processed deep blue OLED could be attributed to the effective molecular design, and this work presented a new approach for constructing effective blue emitters based on oligofluorenes.

Extremely stable deep-blue organic light-emitting diodes employing diindolophenazine-based fluorophore with narrow-band emission and a shallow LUMO level

Extremely stable deep-blue organic light-emitting diodes employing diindolophenazine-based fluorophore with narrow-band emission and a shallow LUMO level

Highly efficient non-doped deep-blue organic light-emitting diodes based on pyreneimidazole derivative with external quantum efficiency > 11% and negligible efficiency roll-off

Highly efficient non-doped deep-blue organic light-emitting diodes based on pyreneimidazole derivative with external quantum efficiency > 11% and negligible efficiency roll-off

Expanded multiple-resonance structure for highly efficient narrowband deep-blue organic light-emitting diodes

Excellent color purity and high external quantum efficiency (EQE) are major requirements in the development of deep-blue organic light-emitting diodes (OLEDs). To achieve this, multiple-resonance (MR)–thermally activated delayed fluorescence (TADF) emitters have been considered as promising options. Herein, we suggest a novel expanded MR design strategy to fabricate deep-blue MR–TADF emitters derived from a fused indolo[3,2,1-jk]carbazole framework. The expanded MR structure managed a triplet excited state for the accelerated spin–vibronic coupling-assisted reverse intersystem crossing and increased the emission dipole orientation while maintaining the high efficiency and deep-blue emission color. The rigid and planar structure of the MR core yielded a small full-width at half-maximum (FWHM; less than 16 nm), high photoluminescence quantum yield (over 97%), and high horizontal emitting dipole orientation (over 90%), and facilitated a second-order spin–vibronic coupling-assisted triplet-to-singlet spin crossover. The fabricated MR–TADF OLEDs recorded a high EQE of 24.3% and FWHM of 21 nm at a CIEy of 0.044, thereby satisfying the BT.2020 blue standard. Additionally, further optimized device architecture provided an EQE of 26.8%.

The Novel Organic Emitters for High-Performance Narrow-Band Deep Blue OLEDs.

Narrow-band deep-blue organic light-emitting diodes (OLEDs) have played a key role in the field of high-quality full-color displays. However, because of the considerable challenges of inherent band gaps, unbalanced carrier injection and the lack of molecular structures, narrow-band deep-blue emitters develop slowly compared with red- and green-emitting materials. Encouragingly, with the continuous efforts of scientists in recent years, great progress has been made in the molecule design and material synthesis of highly efficient narrow-band deep-blue emitters. The typical deep-blue emitters which exhibit narrow emission with a full width at half maximum of < 50nm are summarized in this article. They are divided into the three categories: fluorescence, phosphorescence and thermally activated delayed fluorescence. The methods of molecular design for realizing narrow-band deep-blue emission are described in detail and future research directions are also discussed in this article.

Efficient blue and deep blue fluorescent OLEDs based on anthracene with upper-level intersystem crossing

Two new emitters based on anthracene core, 6-(10-phenylanthracen-9-yl)nicotinonitrile (CNPyAnPh) and 4-(10-(pyridin-2-yl)anthracen-9-yl)benzonitrile (CNPhAnPy) are designed and synthesized for realizing blue and deep blue organic light-emitting diodes (OLEDs). Both CNPyAnPh and CNPhAnPy show nearly orthogonal structures and the suppressed π-conjugation is favorable for blue and even deep blue emission. According to the theoretical calculations, small energy differences between T2 and S1 are observed in these two isomers, endowing the “hot exciton” channel. Furthermore, the S1 of CNPyAnPh possesses both locally excited (LE) and charge transfer (CT) components. By employing CNPyAnPh and CNPhAnPy as the emissive guests in the devices, their maximum external quantum efficiencies (EQEmaxs) are 4.15% and 3.71%, respectively. Thanks to the “hot exciton” and hybrid local and charge transfer (HLCT) properties, the exciton utilization efficiencies (EUEs) of the two molecules are higher than 25%, namely, 42% for CNPyAnPh and 34% for CNPhAnPy. The Commission International de l’Eclairage (CIE) coordinate of CNPhAnPy is (0.15, 0.08), approaching the National Television System Committee (NTSC) standard index for pure blue emission.

Color Stable Deep Blue Multi-Resonance Organic Emitters with Narrow Emission and High Efficiency.

The development of highly efficient and deep blue emitters satisfying the color specification of the commercial products has been a challenging hurdle in the organic light-emitting diodes (OLEDs). Here, deep blue OLEDs with a narrow emission spectrum with good color stability and spin-vibronic coupling assisted thermally activated delayed fluorescence are reported using a novel multi-resonance (MR) emitter built on a pure organic-based molecular platform of fused indolo[3,2,1-jk]carbazole structure. Two emitters derived from 2,5,11,14-tetrakis(1,1-dimethylethyl)indolo[3,2,1-jk]indolo[1',2',3':1,7]indolo[3,2-b]carbazole (tBisICz) core are synthesized as the MR type thermally activated delayed fluorescence emitters realizing a very narrow emission spectrum with a full-width-at-half-maximum (FWHM) of 16nm with suppressed broadening at high doping concentration. The tBisICz core is substituted with a diphenylamine or 9-phenylcarbazole blocking group to manage the intermolecular interaction for high efficiency and narrow emission. The deep blue OLEDs achieve high external quantum efficiency (EQE) of 24.9%, small FWHM of 19nm, and deep blue color coordinate of (0.16, 0.04) with good color stability with increase in doping concentration. To the authors' knowledge, the EQE in this work is one of the highest values reported for the deep blue OLEDs that achieve the BT.2020standard.

Judicious Heteroatom Doping Produces High Performance Deep Blue/Near UV Multiresonant Thermally Activated Delayed Fluorescence OLEDs.

We present two multi-resonant thermally activated delayed fluorescence (MR-TADF) emitters and show how further borylation of a deep blue MR-TADF emitter, DIDOBNA-N, both blue shifts and narrows the emission producing a new near-UV MR-TADF emitter, MesB-DIDOBNA-N. DIDOBNA-N emits bright blue light (lPL = 444nm, FWHM = 64nm, FPL = 81%, td = 23 ms, 1.5 wt% in TSPO1). The deep blue organic light-emitting diode (OLED) based on this twisted MR-TADF compound shows a very high maximum external quantum efficiency (EQEmax) of 15.3% for a device with CIEy of 0.073. The fused planar MR-TADF emitter, MesB-DIDOBNA-N shows efficient and narrowband near UV emission (lPL = 402nm, FWHM = 19nm, FPL = 74.7%, td = 133 ms, 1.5 wt% in TSPO1). The best OLED with MesB-DIDOBNA-N, doped in a co-host, shows the highest efficiency reported for a near UV OLED at 16.2%. With a CIEy coordinate of 0.049, this device also shows the bluest EL reported for a MR-TADF OLED to date. This article is protected by copyright. All rights reserved.

Triptycene‐Fused Sterically Shielded Multi‐Resonance TADF Emitter Enables High‐Efficiency Deep Blue OLEDs with Reduced Dexter Energy Transfer

AbstractDesigning multi‐resonance (MR) emitters that can simultaneously achieve narrowband emission and suppressed intermolecular interactions is challenging for realizing high color purity and stable blue organic light‐emitting diodes (OLEDs). Herein, a sterically shielded yet extremely rigid emitter based on a triptycene‐fused B,N core (Tp‐DABNA) is proposed to address the issue. Tp‐DABNA exhibits intense deep blue emissions with a narrow full width at half maximum (FWHM) and a high horizontal transition dipole ratio, superior to the well‐known bulky emitter, t‐DABNA. The rigid MR skeleton of Tp‐DABNA suppresses structural relaxation in the excited state, with reduced contributions from the medium‐ and high‐frequency vibrational modes to spectral broadening. The hyperfluorescence (HF) film composed of a sensitizer and Tp‐DABNA shows reduced Dexter energy transfer compared to those of t‐DABNA and DABNA‐1. Notably, deep blue TADF‐OLEDs with the Tp‐DABNA emitter display higher external quantum efficiencies (EQEmax=24.8 %) and narrower FWHMs (≤26 nm) than t‐DABNA‐based OLEDs (EQEmax=19.8 %). The HF‐OLEDs based on the Tp‐DABNA emitter further demonstrate improved performance with an EQEmax of 28.7 % and mitigated efficiency roll‐offs.

Efficient solution-processed OLEDs featuring deep-blue triplet-triplet annihilation emitter based on pyrene derivative

Achieving high efficiency deep-blue organic light-emitting diodes (OLEDs) with a CIEy < 0.08 is still terribly urgent yet challenge. Herein, we designed and synthesized a novel blue triplet-triplet annihilation (TTA) emitter with a fan-shape torsional molecular structure, namely TbuPyB, which is based on benzene as core and three tert-butylpyrene as arm. The photophysical, electrochemical and electroluminescent performances of TbuPyB were systematically investigated. Intense deep-blue/blue emissions peaked at 413 and 472 nm are observed in the 7 wt% doped 1,3-bis(N-carbazolyl)benzene (mCP) thin film and neat film, accompanied by absolute photoluminescence (PL) quantum yield (PLQY) of 66.3% and 74.8%, respectively. Contributed by the TTA process, the solution-processed non-doped device exhibits blue emission peak at 470 nm with an impressive EQE of 10.65% and a CIE coordinates of (0.15, 0.20) at the voltage of 7.8 V. In contrary, at the 7 wt% dopant concentration, an outstandingly 57 nm violet-shifted deep-blue emission at 413 nm with an EQE of 6.87% and a CIE coordinates of (0.15, 0.06) was achieved in the doped device. These results schedule a novel avenue for designing highly efficient deep-blue emitter in the solution processable OLEDs.