Phosphorescent Light-emitting Diodes Research Articles

Achieving pure room temperature phosphorescence (RTP) in phenoselenazine-based organic emitters through synergism among heavy atom effect, enhanced n → π* transitions and magnified electron coupling by the A-D-A molecular configuration.

The weak spin-orbit coupling (SOC) in metal-free organic molecules poses a challenge in achieving phosphorescence emission. To attain pure phosphorescence in RTP organic emitters, a promising molecular design concept has been proposed. This involves incorporating n → π* transitions and leveraging the heavy atomic effect within the spin-orbit charge transfer-induced intersystem crossing (SOCT-ISC) mechanism of bipolar molecules. Following this design concept, two bipolar metal-free organic molecules (PhSeB and PhSeDB) with donor-acceptor (D-A) and acceptor-donor-acceptor (A-D-A) configurations have been synthesized. When the molecular configuration changes from D-A to A-D-A, PhSeDB exhibits stronger electron coupling and n → π* transitions, which can further enhance the spin-orbit coupling (SOC) together with the heave atom effect from the selenium atom. By the advanced synergism among enhanced n → π* transitions, heavy atom effect and magnified electron coupling to efficiently promote phosphorescence emission, PhSeDB can achieve pure RTP emission in both the solution and doped solid film. Thanks to the higher spin-orbit coupling matrix elements (SOCMEs) for T1 ↔ S0, PhSeDB attains the highest phosphorescence quantum yield (ca. 0.78) among all the RTP organic emitters reported. Consequently, the purely organic phosphorescent light-emitting diodes (POPLEDs) based on PhSeDB achieve the highest external quantum efficiencies of 18.2% and luminance of 3000 cd m-2. These encouraging results underscore the significant potential of this innovative molecular design concept for highly efficient POPLEDs.

Low-Temperature Cross-Linkable Hole Transport Materials for Solution-Processed Quantum Dot and Organic Light-Emitting Diodes with High Efficiency and Color Purity.

Cross-linkable hole transport materials (HTMs) are ideal for improving the performance of solution-processed quantum dot light-emitting diodes (QLEDs) and phosphorescent light-emitting diodes (OLEDs). However, previously developed cross-linkable HTMs possessed poor hole transport properties, high cross-linking temperatures, and long curing times. To achieve efficient cross-linkable HTMs with high mobility, low cross-linking temperature, and short curing time, we designed and synthesized a series of low-temperature cross-linkable HTMs comprising dibenzofuran (DBF) and 4-divinyltriphenylamine (TPA) segments for highly efficient solution-processed QLEDs and OLEDs. The introduction of divinyl-functionalized TPA in various positions of the DBF core remarkably affected their chemical, physical, and electrochemical properties. In particular, cross-linked 4-(dibenzo[b,d]furan-3-yl)-N,N-bis(4-vinylphenyl)aniline (3-CDTPA) exhibited a deep highest occupied molecular orbital energy level (5.50 eV), high hole mobility (2.44 × 10-4 cm2 V-1 s-1), low cross-linking temperature (150 °C), and short curing time (30 min). Furthermore, a green QLED with 3-CDTPA as the hole transport layer (HTL) exhibited a notable maximum external quantum efficiency (EQEmax) of 18.59% with a remarkable maximum current efficiency (CEmax) of 78.48 cd A-1. In addition, solution-processed green OLEDs with 3-CDTPA showed excellent device performance with an EQEmax of 15.61%, a CEmax of 52.51 cd A-1, and outstanding CIE(x, y) color coordinates of (0.29, 0.61). This is one of the highest reported EQEs and CEs with high color purity for green solution-processed QLEDs and OLEDs using a divinyl-functionalized cross-linked HTM as the HTL. We believe that this study provides a new strategy for designing and synthesizing practical cross-linakable HTMs with enhanced performance for highly efficient solution-processed QLEDs and OLEDs.

Highly thermal stable and efficient carbazole/pyridine/dibenzothiophene based bipolar host material for red phosphorescent light-emitting diodes

In this work, we designed and synthesized two bipolar host materials, DBTPC1 and DBTPC2 through different linkage ways between carbazole, pyridine and dibenzothiophene. The photophysical properties and thermal properties of these hosts were systematically investigated. Their rigid molecular structures result in high glass transition temperatures, contributing to excellent thermal stability of molecules. Employing DBTPC1 and DBTPC2 as hosts, red phosphorescent organic light-emitting devices (PHOLEDs) with Ir(piq)2acac as an emitter display maximum external quantum efficiencies (EQEs) of 18.8 and 13.2%, respectively. The experimental results reveal that DBTPC1 shows better performance than commercial host 4,4’-bis(carbazol-9-yl)biphenyl (CBP), thus, would be a promising host material for practical application in red PHOLEDs and for the future exploration in multi-color PHOLEDs.

High performance of solution-processed green phosphorescent light emitting-diodes based on a new Pt(II)-complex

New Pt(II)-complex named as AT8809 was used as dopant to prepare green phosphorescent light emitting diodes (PhOLEDs) by solution -processed method. According to the research results, the device with the structure of ITO/PEDOT: PSS/PVK/CBP: AT8809/TPBi/LiF/Al achieved the highest performance, such as 10561 cd/m2 of the luminance, 41.7 cd/A of the current efficiency with the peak around 518 nm. The spectra of devices with different dopant ratio or thickness of ETL red shifted, even there was a second peak, which should be caused by the interaction between excitons and the aggregation luminescence of AT8809. The PL and TRPL were tested to discuss the reason of wavelength red shift by changing the doping ratio of AT8809. In PL spectra, the peak red shifts gradually as the doping ratio increasing, even in pure AT8809 film, the PL shows a red peak at 660 nm rather than green peak at 520 nm, which corresponds to the aggregation luminescence and traditional luminescence of AT8809 respectively. The film with a high doping ratio shows a fast PL decay, which means that the increasing concentration of dopant leads to a severe intermolecular reaction.

High-performance red phosphorescent light-emitting diodes built on bipolar host materials obtained from dibenzo[F,H]quinoxaline and dibenzothiophene

DTBDQ, a bipolar host material with an acceptor unit dibenzo[F,H]quinoxaline and a donor unit dibenzothiophene connected by two interstitial benzenes, was employed. DTBDQ exhibits deep LUMO and HOMO energy levels, along with a triplet energy of 2.75 eV, owing to rational molecular design. Red phosphorescent organic light-emitting diodes (PhOLEDs) devices with DTBDQ here as the host and Ir(piq)2acac as the guest layer have an external quantum efficiency(EQE) of 19.79% with a limited roll-off and an EQE of 18.08% at 1000 cd/m2, with CIE coordinates of (0.68,0.32), which is quite comparable to the National Television Systems Committee (NTSC) CIE coordinates of (0.67,0.33). The device performance is significantly better than that of devices using hole-type host CBP, electron-type host TPBI, and electron and hole-type hosts together, indicating that the performance of bipolar host materials transferring both electrons and holes is superior to that of simply two hosts as a double light-emitting layer separately, which is beneficial to carrier transport balancing and avoiding carrier buildup. DTBDQ demonstrates that it is an excellent bipolar host candidate.

How molecular aggregation affects internal quantum efficiency in organic phosphorescent light-emitting diodes

We investigate whether typical host–guest phosphorescent organic light-emitting diodes are at an intrinsic efficiency limit due to the long-range nature of the bimolecular loss mechanism in these devices. Using kinetic Monte Carlo simulations and developing models that can mimic realistic and ideal distributions of emitter molecules, we discuss how the interplay among exciton relaxation, transport, and annihilation is affected by the prevalent molecular aggregation in state-of-the-art emission layers. Regarding the internal quantum efficiency, we find a potential increase of ∼15% (and a minimum relative increase of 20%) for devices with suppressed aggregate formation. We also discuss conditions under which the system reaches its inherent limitation of efficiency.

Transparent organic upconversion device targeting high- grade infrared visual image

Infrared-to-visible transparent upconverter offers an appealing prospect of demonstrating in situ invisible information without interfering with the intrinsic vision of human eyes. However, previous studies required high operational voltage for visible light emission failing to meet the prerequisite of practical application. Here, we report an organic upconversion device composed of a single photoactive material ClAlPc for sensing infrared light and subsequent visualization by BCzPh:CN-T2T exciplex co-hosted phosphorescent light-emitting diode. Incident NIR signal (λmax at 780 nm) is upconverted into visible light emission (λmax at 523 nm) under an onset bias voltage of merely 1.7 V by the virtue of barrier-less bipolar exciplex design. Particularly, the device exhibits linear dynamic luminance toward NIR power density down to 34 μW/cm2 and real-time response to frequency-modulated NIR signal up to 10 kHz. By comprising a transparent multilayered electrode for see-through upconverter, a high-quality infrared visual image with a maximum resolution of 1588 dpi is demonstrated.

Evoking non-bonding S-π interaction by aryl phosphine sulfide for selectively enhanced electronic property of organic semiconductors

Selective tuning of optoelectronic properties of organic semiconductors is one of the most significant challenges in organoelectronics. Herein, on the basis of aryl phosphine sulfide (APS), we show that the electronic performance can be selectively enhanced by evoking the non-bonding S-π interactions between the sulfur atom of APS and π-conjugated carbazole moiety, on account of the large scope 3p lone-pair electrons of S for effective electronic communications. It is indicated that the material properties and device performances are strongly dependent on the beingness of the successfully established intramolecular S-π interaction. Specifically, APS-based DPSSi exhibits excellent solubility and film-forming ability and enhanced carrier transportation and injection abilities at high triplet energy level; and the DPSSi-hosted blue phosphorescent light-emitting diodes (PhOLEDs) show high current efficiency up to 36.6 cd A−1, power efficiency of 30.6 lm W−1, and external quantum efficiency of 20.8%, which are among the best results of blue PhOLEDs reported so far. The semi-conjugation feature of S-π interaction in selectively modulating electronic properties would offer exciting opportunities for settling intrinsic difficulties of organic optoelectronic materials and devices, promoting significantly the development of non-bonding interaction involved organic optoelectronic molecules.

Metal-free and purely organic phosphorescent light-emitting diodes using phosphorescence harvesting hosts and organic phosphorescent emitters

Pure organic based phosphorescent light-emitting diodes have been developed using bromine modified emitter and phosphorescence harvesting host.

Rational design of quinoxaline-based bipolar host materials for highly efficient red phosphorescent organic light-emitting diodes.

Two novel bipolar carbazole/diphenylquinoxaline-based host materials 3-(2,3-diphenylquinoxalin-6-yl)-9-phenyl-9H-carbazole (M1) and 3-(4-(2,3-diphenylquinoxalin-6-yl)phenyl)-9-phenyl-9H-carbazole (M2) have been rationally designed and synthesized. The phenyl spacer between the functionalized quinoxaline moiety and the carbazole moiety is also introduced to investigate its influence on their photophysical properties. The chemical structures, and thermal, photophysical and electrochemical properties of the two host materials were characterized and explored in detail. Red phosphorescent light-emitting diodes with M1 and M2 as hosts were prepared to explore their electroluminescent properties. Both M1 and M2 host-based red devices exhibit outstanding electroluminescent performance. For example, two red devices all realize good red emission with the maximum at 594 nm, the maximum external quantum efficiency and luminance can reach 14.66% and 28 619 cd m−2 for M1-based devices and 15.07% and 28 818 cd m−2 for M2-based devices, indicating compounds M1 and M2 designed in this work have potential applications in the development of high-performance monochrome and white OLEDs.

High-color-purity and efficient solution-processable blue phosphorescent light-emitting diodes with Pt(ii) complexes featuring 3ππ* transitions

A series of Pt(ii) complexes with narrow emission spectra were systematically investigated and Pt(ppzOczpy-4m), rationally designed with predominantly 3LC(3ππ*) character in T1, was fabricated into efficient blue OLEDs via a solution process.

Construction of Highly Efficient Carbazol-9-yl-Substituted Benzimidazole Bipolar Hosts for Blue Phosphorescent Light-Emitting Diodes: Isomer and Device Performance Relationships.

Four isomers of carbazol-9-yl-substituted 1,2-diphenylbenzimidazole at 7, 6, 5, and 4 positions, named as 1-CbzBiz, 2-CbzBiz, 3-CbzBiz, and 4-CbzBiz, respectively, have been synthesized. Instead of having an identical molecular weight, the CbzBiz's have their glass transition temperatures ( Tg) spanning a large range from 53 to 90 °C. Their Tg and melting point ( Tm) basically obey the Boyer-Kauzmann rule ( Tg = g· Tm with g ≈ 0.7) on the absolute temperature scale (in kelvins). However, while 1-CbzBiz and 4-CbzBiz demonstrate a small g value of 0.66, which is significantly smaller than other common organic glass molecules, 3-CbzBiz shows an unexpectedly high g value of 0.73, implying higher intermolecular interactions in the glass. These CbzBiz's are suitable hosts for bis[2-(4,6-difluorophenyl)pyridinato- C2, N](picolinato)iridium(III) (FIrpic) in phosphorescence organic light-emitting diodes (PhOLEDs). The optimized PhOLED of indium tin oxide/4,4'-cyclohexylidenebis[ N, N-bis(4-methylphenyl)benzenamine]/4Cbz/4-CbzBiz-FIrpic(15%)/diphenylbis(4-(pyridin-3-yl)phenyl)silane/LiF-Al shows a maximum brightness of 18 760 cd/m2, a current efficiency of 64.1 cd/A, and the external quantum efficiency of 30.9%.

Cyanopyridine based bipolar host materials for phosphorescent light-emitting diodes with low efficiency roll-off: Importance of charge balance

Two novel bipolar isomers, namely 3,5-bis(3-(9H-carbazol-9-yl)phenyl)isonicotinonitrile (m-DCzPyCN), and 3,5-bis(9-phenyl-9H-carbazol-3-yl)isonicotinonitrile (3-DCzPyCN), are designed and synthesized by selecting cyanopyridine as n-type unit and N-phenylcarbazole as p-type unit. Two peripheral carbazoles are conjugated through their C-3 atom with the central cyanopyridine in 3-DCzPyCN but are indirectly linked through the N-9 atom via meta-phenylene bridges in m-DCzPyCN. The linking strategy had significant effect on the physical properties of the isomers. The single-carrier device study reveals that 3-DCzPyCN has much better positive and negative charge balancing ability than m-DCzPyCN. The 3-DCzPyCN hosted green phosphorescent organic light-emitting diode exhibits superior performance over m-DCzPyCN, with the maximum current efficiency of 84.1 cd A−1 and external quantum efficiencies of 24.2% and low efficiency roll-off. At an extremely high brightness of 10000 cd m−2, the efficiency still remained at 78.4 cd A−1, which corresponds to a roll-off of 6.7% from the maximum value. Even at an ultra-high brightness of 100000 cd m−2, the efficiency is reduced by 47.0%. The superior performance of 3-DCzPyCN is ascribed to its remarkable charge balancing capability, which is the combined result of the cyanopyridine as n-type unit and the conjugated linking strategy between p- and n-type moieties.

Effects of Charge Balance and Exciton Confinement on the Operational Lifetime of Blue Phosphorescent Organic Light-Emitting Diodes

For 15 years, the rapid degradation of luminance over time has rendered blue phosphorescent light emitting diodes (PHOLEDs) too short-lived for many practical applications. To date, studies have concentrated on the degradation of the emitting materials, yet not the many other parts of these devices. The authors develop a method to measure charge and exciton leakage directly over time, and find that these effects are $n\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}t$ responsible for PHOLED degradation---an important revelation that shifts our attention toward the actual sources of failure.

Adjusting Nitrogen Atom Orientations of Pyridine Ring in Tetraphenylsilane-Based Hosts for Highly Efficient Blue Phosphorescent Organic Light-Emitting Devices.

Four wide bandgap host materials, namely, 9-(4-diphenyl(4-(pyridin-3-yl)phenyl)silyl-phenyl)-9H-carbazole (CSmP), 9-(4-diphenyl(4-(pyridin-2-yl)phenyl)silylphenyl)-9H-carbazole (CSoP), 9-(4-diphenyl(4-(pyridin-3-yl)phenyl)silylphenyl)-9H-3,9'-bicarbazole (DCSmP), and 9-(4-(diphenyl(4-(pyridin-2-yl)phenyl)silyl)phenyl)-9H-3,9'-bicarbazole (DCSoP), have developed by incorporation of pyridine with varied N atom orientation and carbazole/dimer carbazole units into the tetraphenylsilane skeleton for blue phosphorescent light-emitting diodes. These host materials all possess wide bandgap (3.54-3.64 eV) and high triplet energies (2.77-2.95 eV). As revealed by the absorption and emission spectra, theoretical calculations, and CV measurements, the N atom orientation exerts a strong influence on the LUMO energy level and electron-transportation behaviors without deterioring the photophysical properties. Among them, DCSmP with 3-pyridyl substituent manifests the best electron-transporting capability. The FIrpic-doped blue phosphorescent device using DCSmP as host material exhibits excellent electroluminescence performance with a maximum current efficiency of 40.1 cd A(-1) and a maximum external quantum efficiency of 20.0%. The current efficiency and external quantum efficiency are improved 3-fold, higher than those fabricated from DCSpP with 4-pyridyl as substituent, demonstrating an effective strategy for large improvement in device performance by a subtle change in molecular structure.

Highly Efficient Phosphorescent Light-Emitting Diodes by Using New Host Material.

An electron-transporting triazole (Tz) and hole transporting carbazole (Cz) moieties were introduced into the fluorene structure via a Suzuki coupling reaction. The resulting material, 9-(4'-(9-(4'-(4,5-diphenyl-4H-1,2,4-triazol-3-y)-[1,1'-biphenyl]-4-yl)-9H-fluoren-9-yl)-[1,1'-biphenyl]-4-yl)-9H-carbazole (Tz-FL-Cz), was synthesized and can be used as the host layer in phosphorescent OLEDs. The synthesized material exhibits wide energy gap (3.46 eV). The absorption and emission maximum of the Tz-FL-Cz was observed at 300 nm and 389 nm, respectively. Blue phosphorescent devices using Tz-FL-Cz as host material exhibited maximum current efficiency of 3.2 cd/A and power efficiency of 1.3 lm/W, respectively.

Extremely high external quantum efficiency of inverted organic light-emitting diodes with low operation voltage and reduced efficiency roll-off by using sulfide-based double electron injection layers

We developed efficiency roll-up blue fluorescent and green phosphorescent inverted light-emitting diodes with very-low energy consumption by using advanced double electron injection layers composing of metal sulfide.

A cationic iridium complex meets an electron-transporting counter-anion: enhanced performances of solution-processed phosphorescent light-emitting diodes.

An electron-transporting counter-anion, OXD-7-SO3-, was developed for a blue-green-emitting cationic iridium complex, which showed superior device performance in solution-processed phosphorescent light-emitting diodes over its counterpart complex with a PF6- counter-anion, due to the facilitated electron-injection/transport and more balanced carrier recombination imparted by OXD-7-SO3-.

Solution-processable bipolar hosts based on triphenylamine and oxadiazole derivatives: Synthesis and application in phosphorescent light-emitting diodes

Host material with enhanced and balanced charge injection/transport is essential for efficient phosphorescent organic light-emitting diodes (PhOLEDs). We synthesized two new bipolar hosts, TPA-2OXD and TPA-3OXD, composed of hole-transporting triphenylamine core linked with two and three electron-transporting aromatic 1,3,4-oxadiazole derivatives respectively, aiming to enhance and balance charge injection and transport. They revealed good thermal stability (Td>300°C, Tg>100°C) and homogeneous films were readily obtained by spin-coating process (roughness<0.9nm). Multilayer PhOLEDs (ITO/PEDOT:PSS/EML/BCP/LiF/Al) have been successfully fabricated using Ir(ppy)3-doped hosts as emitting layer (EML). The maximum luminance and maximum current efficiency of TPA-2OXD-based device were 8190cd/m2 and 4.50cd/A, much higher than 6299cd/m2 and 1.53cd/A respectively of TPA-3OXD-based one. This has been ascribed to more appropriate ratio (2/1) of electron-/hole-transporting moieties in TPA-2OXD. The TPA-2OXD also excels conventional host PVK in terms of device performance (5774cd/m2, 1.91cd/A). Moreover, the turn-on voltages of TPA-2OXD- and TPA-3OXD-based devices were 4.3V and 5.5V respectively, much lower than 7.4V of PVK-based one. Current results indicate that the bipolar TPA-2OXD and TPA-3OXD are promising host materials capable of being fabricated by wet processes.

A Solution-Processed Resonance Host for Highly Efficient Electrophosphorescent Devices with Extremely Low Efficiency Roll-off.

Solution-processible N-P=O resonance-host molecules are applied successfully in spin-coated phosphorescent light-emitting diodes (PHOLEDs) to selectively and self-adaptively tune the electrical properties for balanced charge transportation. The resonance-molecule-hosted PHOLEDs exhibit a high maximum quantum efficiency of 16.5% and a low efficiency roll-off down to 0.7%, highlighting the significant progress of these solution-processed PHOLEDs with high efficiency and flat efficiency roll-off achieved simultaneously.