High efficiency green phosphorescent organic light-emitting diodes with a low roll-off at high brightness

Highly efficient green phosphorescent organic light-emitting diodes (PHOLEDs) with low efficiency roll-off at high brightness have been demonstrated with a novel iridium complex. The host material 1,3-bis(carbazol-9-yl)benzene (mCP) with high triplet energy is also used as the hole transporting layer to avoid carrier accumulation near the exciton formation interface and reduce exciton quenching. It provides a new approach for easily fabricating PHOLED with high triplet energy emitter. Moreover, the hole blocking layer is extended into the light emitting layer to form a co-host, realizing better control of the carrier balance and broader recombination zone. As a consequence, a maximum external quantum efficiency of 20.8% and current efficiency of 72.9 cd/A have been achieved, and maintain to 17.4% and 60.7 cd/A even at 10,000 cd/m², respectively.     

Deep blue phosphorescent organic light-emitting diodes with excellent external quantum efficiency

Highly efficient deep blue phosphorescent organic light-emitting diodes (PHOLEDs) using two heteroleptic iridium compounds, (dfpypy)₂Ir(acac) and (dfpypy)₂Ir(dpm), as a dopant and 9-(3-(9H-carbazol-9-yl)phenyl)-9H-carbazol-3-yl)diphenylphosphine oxide as a host material have been developed. The electroluminescent device of (dfpypy)₂Ir(dpm) at the doping level of 3 wt% shows the best performance with external quantum efficiency of 18.5–20.4% at the brightness of 100–1000 cd/m² and the color coordinate of (0.14, 0.18) at 1000 cd/m².     

Soluble processed low-voltage and high efficiency blue phosphorescent organic light-emitting devices using small molecule host systems

We report low voltage driving and highly efficient blue phosphorescence organic light emitting diodes (PHOLEDs) fabricated by soluble process. A soluble small molecule mixed host system consisting of hole transporting 4,4’,4’’ tris(N-carbazolyl)triphenylamine (TCTA) and bipolar carrier transporting 2,6-bis(3-(carbazol-9-yl)phenyl)pyridine (26DCzPPy) exhibits high solubility with smooth surface properties. Moreover, this small molecule host shows the smoothest morphological property similar to a vacuum deposited amorphous film. A low driving voltage of 5.4 V at 1000 cd/m² and maximum external quantum efficiency 14.6% obtained in the solution processed blue PHOLEDs are useful for large area low cost manufacturing.     

Efficient green electroluminescent devices based on iridium complex with wide energy gap complexes as sensitizers

In this work, electroluminescent (EL) performances of a green iridium complex (tfmppy)₂Ir(tpip) were significantly improved by utilizing wide energy gap iridium complexes FK306 and FIrpic as sensitizers. Due to the low-lying energy levels, the co-doped FK306 or FIrpic molecules function as electron trappers, which are helpful in balancing holes and electrons on (tfmppy)₂Ir(tpip) molecules and in broadening exciton recombination zone. Consequently, the co-doped devices displayed high EL efficiencies and slow efficiency roll-off. Compared with FIrpic, FK306 acts as a more effective sensitizer because of its relatively lower energy levels. Consequently, highly efficient green EL device with maximum current efficiency, power efficiency and brightness up to 102.29 cd/A (external quantum efficiency (EQE) of 25.3%), 88.67 lm/W and 96,268 cd/m², respectively, was realized by optimizing the co-doping concentration of FK306. Even at the practical brightness of 1000 cd/m², EL current efficiency up to 92.93 cd/A (EQE = 23%) can still be retained.     

4, 6-Bis[3-(dibenzothiophen-2-yl)phenyl] pyrimidine bipolar host for bright,efficient and low efficiency roll-off phosphorescent organic light-emitting devices

A bipolar host 4, 6-Bis[3-(dibenzothiophen-2-yl)phenyl] pyrimidine (DBTPhPm) with small singlet-triplet splitting has been synthesized and confirmed through a series of photophysical and electrochemical properties. Monochromatic phosphorescent organic light-emitting devices (PHOLEDs) based on different hosts [(4,4′-N,N'-dicarbazole) biphenyl, 2,7-bis (diphenylphosphoryl)-9-[4-(N,Ndiphenylamino) phenyl]-9-phenylfluorene, (3,3'-bicarbazole) phenyl and DBTPhPm] and dopants are fabricated. Compared to other hosts, the DBTPhPm-based PHOLEDs exhibited high brightness, high efficiency and low efficiency roll-off. The maximum power efficiency of the DBTPhPm-based red (R), green (G), blue (B), yellow (Y), and orange (O) PHOLEDs are 12.2, 47.2, 17.6, 42.6 and 15.1 lm/W, respectively. The current efficiency roll-off of the R, G, B, Y, and O PHOLEDs are 29.8%, 8.6%, 18.2%, 5.9%, and 22.4% from the maximum current efficiency to the high brightness of 5000 cd/m². The detailed working mechanism of the DBTPhPm-based device is discussed.     

Color stable and low driving voltage white organic light-emitting diodes with low efficiency roll-off achieved by selective hole transport buffer layers

White organic light-emitting diodes (WOLEDs) showing high color stability, low operating voltage, high efficiency and low efficiency roll-off by adopting different hole transport buffer layers which also behaves as electron/exciton blocking layers (EBL) have been developed. The characteristics of WOLEDs based on blue–green and orange phosphors could be easily manipulated by hole transport buffer layer, which tailors charge carrier transportation and energy transfer. Our WOLEDs show low operating voltages, 100 cd/m² at 3.2 V, 1000 cd/m² at 3.7 V and 10000 cd/m² at 4.8 V, respectively, and achieve a current efficiency of 35.0 cd/A, a power efficiency of 29.0 lm/W at a brightness of 1000 cd/m², and a low efficiency roll-off 8.7% calculated from the maximum efficiency value to that of 5000 cd/m².     

Synthesis and device properties of mCP analogues based on fused-ring carbazole moiety

Novel mCP analogues consisting of blue phosphorescent host materials with fused-ring, 1,3-bis(5H-benzofuro[3,2-c]carbazol-5-yl)benzene (BFCz) and 1,3-bis(5H-benzo[4,5]thieno[3,2-c]carbazol-5-yl)benzene (BTCz) were designed and synthesized using benzofurocarbazole and benzothienocarbazole donor moieties. BFCz and BTCz exhibit high glass transition temperatures of 147 and 157 °C, respectively, and high triplet bandgaps of 2.94 and 2.93 eV, respectively. To explore the electroluminescence properties of these materials, multilayer blue phosphorescent organic light-emitting diodes (PHOLEDs) were fabricated in the following device structure: indium–tin-oxide (ITO)/PEDOT:PSS/4,4’-cyclohexylidene bis[N,N-bis(4-methylphenyl)aniline] (TAPC)/1,3-bis(N-carbazolyl) benzene (mCP)/host:FIrpic/diphenylphosphine oxide-4-(triphenylsilyl)phenyl (TSPO1)/LiF)/Al. The PHOLEDs with BTCz exhibited efficient blue emission with luminous and quantum efficiencies of 30.9 cd/A and 15.5% at 1000 cd/m², respectively.     

Efficient non-doped blue light emitting diodes based on novel carbazole-substituted anthracene derivatives

In this study, we synthesized three anthracene derivatives featuring carbazole moieties as side groups - 2-tert-butyl-9,10-bis[4-(9-carbazolyl)phenyl]anthracene (Cz⁹PhAnt), 2-tert-butyl-9,10-bis{4-[3,6-di-tert-butyl-(9-carbazolyl)]phenyl}anthracene (tCz⁹PhAnt), and 2-tert-butyl-9,10-bis{4′-[3,6-di-tert-butyl-(9-carbazolyl)]biphenyl-4-yl}anthracene (tCz⁹Ph₂Ant) - for use in blue organic light emitting devices (OLEDs). The anthracene derivatives presenting rigid and bulky tert-butyl-substituted carbazole units possessed high glass-transition temperatures (220 °C). Moreover, the three anthracene derivatives exhibited strong blue emissions in solution, with high quantum efficiencies (91%). We studied the electroluminescence (EL) properties of non-doped OLEDs incorporating these anthracene derivatives, with and without a hole-transporting layer (HTL). OLEDs incorporating an HTL provided superior EL performance than did those lacking the HTL. The highest brightness (6821 cd/m²) was that for the tCz⁹PhAnt-based device; the greatest current efficiency (2.1 cd/A) was that for the tCz⁹Ph₂Ant-based device. The devices based on these carbazole-substituted anthracene derivatives also exhibited high color purity.     

Efficient nondoped organic light-emitting diodes with Cu complex emitter

In Cu^I complex based organic light emitting diodes (OLEDs) a host matrix is traditionally thought to be required to achieve high efficiency. Herein, it is found that the device ITO/MoO₃ (1 nm)/4,4′-N,N′-dicarbazole-biphenyl (CBP, 35 nm)/[Cu(μ-I)dppb]₂ (dppb = 1,2-bis[diphenylphosphino]benzene, 20 nm)/1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBi, 65 nm)/LiF (1 nm)/Al (100 nm) with a vacuum thermal evaporated nondoped Cu^I complex emissive layer (EML) showed external quantum efficiency and current efficiency of 8.0% and 24.3 cd/A at a brightness of 100 cd/m², respectively, which are comparable to the maximum efficiencies reported in an optimized doped OLED with the same emitter, higher efficiency than the OLED with a [Cu(μ-I)dppb]₂:CBP EML, and much higher efficiencies than the nondoped OLED with a bis(2-phenylpyridine)(acetylacetonate)iridium [Ir(ppy)₂(acac)] EML. A series of reference films and single carrier devices were fabricated and studied to understand the difference between Cu^I and Ir^III complex based nondoped OLEDs.     

Stable efficiency roll-off in blue phosphorescent organic light-emitting diodes by host layer engineering

The device characteristics of blue phosphorescent organic light-emitting diodes (PHOLEDs) with mixed host structure were investigated by changing the combination and the composition of host materials in emissive layer. The distributed recombination zone and balanced charge carrier injection within emissive layer were achieved through mixed host optimization with a hole transport-type and an electron transport-type host materials, therefore the device performances were greatly enhanced, with external quantum and power efficiencies of 21.8% and 53 lm/W. Moreover, mixed host blue PHOLEDs exhibited a extremely low stable efficiency roll-off with quantum efficiencies of 20.3% and 18.6% at a luminance of 1000 and 10,000 cd/m².     

Efficient fluorescent white organic light-emitting devices based on a ultrathin 5,6,11,12-tetraphenylnaphthacene layer

White organic light-emitting devices (OLEDs) were fabricated using a ultrathin layer 5,6,11,12-tetraphenylnaphthacene as the yellow light-emitting layer and p-bis(p-N,N-diphenyl-aminostyryl)benzene (DSA-ph) doped in 2-methyl-9,10-di(2-naphthyl)anthracene (MADN) matrix as the blue light-emitting layer. The thickness of rubrene ultrathin layer will seriously affect the device performance, and the device with 1 nm rubrene achieves the best performance, with the maximum luminance of 33,152 cd/m² at 11 V and the maximum current efficiency of 8.69 cd/A at 7 V.     

Voltage independent white emission from all solution processed polymer light-emitting diode with dual emitting layers spaced by an alcohol soluble conjugated polymer as interlayer

Spatial control of recombination zone in multilayer white polymer light emitting diode (WPLED) is highly desirable for stable white-light emission. In this work, the utilization of 18-crown-6 (Cn6)-grafted polyfluorene (PFCn6) as an interlayer in between two emitting layers is demonstrated to control the recombination zones for the multilayer WPLED with β-phase and rubrene doped poly(9,9-di-n-octylfluorene) (PFO) as blue- and yellow-emitting layers, respectively. The device gives the maximum brightness of 15,695 cd/m² and maximum efficiency 5.43 cd/A, accompanying with voltage-independent electroluminescence spectrum having invariant Commission Internationale de L’Eclairage (CIE) coordinates of (0.32, 0.36). The performance with the luminance efficiency 5.43 cd/A and voltage independent white emission is the highest record among the reported multilayer WPLED.     

Towards High Efficiency and Low Roll‐Off Orange Electrophosphorescent Devices by Fine Tuning Singlet and Triplet Energies of Bipolar Hosts Based on Indolocarbazole/1, 3, 5‐Triazine Hybrids

Orange‐emitting phosphorescent organic light‐emitting diodes (PHOLEDs) are drawing more and more attention; however, high‐performance hosts designed for orange PHOLEDs are rare. Here, four indolocarbazole/1, 3, 5‐triazine hybrids are synthesized to optimize the singlet and triplet energies, as well as transporting properties, for ideal orange PHOLEDs. By introducing moieties with different electronegativity, a graded reduction of the singlet and triplet energies is achieved, resulting in minimum injection barrier and minimum energy loss. Besides, the charge transporting abilities are also tuned to be balanced on the basis of the bipolar features of those materials. The optimized orange PHOLED shows a maximum external quantum efficiency (EQE) of 24.5% and a power efficiency of 64 lm W^–1, both of which are among the best values for orange PHOLEDs. What is more, the efficiency roll‐off is extremely small, with an EQE of 24.4% at 1000 cd m^–2 and 23.8% at 10 000 cd m^–2, respectively, which is the lowest efficiency roll‐off for orange PHOLEDs to date, resulting in the highest EQE for orange PHOLEDs when the luminance is above 1000 cd m^–2. Besides the balanced charges, the small roll‐off is also attributed to the wide recombination zone resulting from the bipolar features of the hosts.     

High-efficiency phosphorescent organic light-emitting devices with low efficiency roll-off using a thermally activated delayed fluorescence material as host

Phosphorescent organic light-emitting devices (PHOLEDs) with high efficiency and low efficiency roll-off were fabricated. The emissive layer was composed of a thermally activated delayed fluorescence (TADF) material 4,5-bis(carbazol-9-yl)-1,2-dicyanobenzene (2CzPN) as host and an orange iridium complex bis(4-tert-butyl-2-phenylbenzothiozolato-N,C^2′)iridium(III)(acetylacetonate) [(tbt)₂Ir(acac)] as dopant. At a low dopant concentration of 1 wt%, a PHOLED without light extraction optimization achieved a maximum power efficiency of 42.1 lm/W, a luminance efficiency of 77.9 cd/A and an external quantum efficiency (EQE) of 26.8%, respectively. Meanwhile, the EQE maintained 26.6% at 1000 cd/m² and 25.8% at 5000 cd/m², respectively. Moreover, a critical current density of 300 mA/cm² was realized, indicating significantly improved efficiency roll-off. The efficient utilization of triplet excitons on 2CzPN for phosphorescence via reverse inter-system crossing of 2CzPN followed by Fӧrster resonance energy transfer from 2CzPN to (tbt)₂Ir(acac) is responsible for the superior performance.     

Improved property in organic light-emitting diode utilizing two Al/Alq3 layers

We reported on the fabrication of organic light-emitting devices (OLEDs) utilizing the two Al/Alq₃ layers and two electrodes. This novel green device with structure of Al(110 nm)/tris(8-hydroxyquinoline) aluminum (Alq₃)(65 nm)/Al(110 nm)/Alq₃(50 nm)/N,N′-dipheny1-N, N′-bis-(3-methy1phyeny1)-1, 1′-bipheny1-4, 4′-diamine (TPD)(60 nm)/ITO(60 nm)/Glass. TPD were used as holes transporting layer (HTL), and Alq₃ was used as electron transporting layer (ETL), at the same time, Alq₃ was also used as emitting layer (EL), Al and ITO were used as cathode and anode, respectively. The results showed that the device containing the two Al/Alq₃ layers and two electrodes had a higher brightness and electroluminescent efficiency than the device without this layer. At current density of 14 mA/cm², the brightness of the device with the two Al/Alq₃ layers reach 3693 cd/m², which is higher than the 2537 cd/m² of the Al/Alq₃/TPD:Alq₃/ITO/Glass device and the 1504.0 cd/m² of the Al/Alq₃/TPD/ITO/Glass. Turn-on voltage of the device with two Al/Alq₃ layers was 7 V, which is lower than the others.     

High-efficiency host free deep-blue organic light-emitting diode with double carrier regulating layers

A bright high-efficiency host-free deep-blue organic light-emitting diode (OLED) is demonstrated. Without the aid of any carrier regulating layer, the deep-blue OLED shows a power efficiency of 1.7 lm W⁻¹ with CIE coordinates of (0.143, 0.098) at 1000 cd m⁻². The respective power efficiency is increased from 1.7 to 2.1 and 2.2 lm W⁻¹ as a single- and double-carrier regulating layers were incorporated. The respective peak luminance also increases from 5250 to 7620 and 9130 cd m⁻², an increment of 45% and 74%. The marked brightness improvement may be attributed to the incorporated carrier regulating layers that effectively lead carriers to recombine in a wider zone. Moreover, the blue emission can be hypsochromic shifted by varying the incorporation position of the carrier regulating layer and the emissive layer thickness.     

Improved efficiency roll-off at high brightness in simplified phosphorescent organic light emitting diodes with a crossfading-host

We report a high efficiency and low efficiency roll-off green phosphorescent organic light emitting diode using both hole- and electron-transporting host materials in a crossfading profile. To eliminate the energy barrier and reduce the charge carrier accumulation, the host materials are used as transporting layers as well, which also simplifies the device fabrication. It is found out that the recombination zone of gradient doping host sample is not only wider but also extended at high current density, which contributes to the suppressed efficiency roll-off at high luminance. An external quantum efficiency of 21.0% at 1000 cd/m² is obtained, and maintains to 19.3% at 10,000 cd/m².     

Stacked inverted top-emitting green electrophosphorescent organic light-emitting diodes on glass and flexible glass substrates

Stacked inverted top-emitting green electrophosphorescent organic light-emitting diodes (OLEDs) are demonstrated on glass and flexible glass substrates. A single-unit OLED is shown to have a current efficacy of 46.8 cd/A at a luminance of 1215 cd/m². When two of these OLEDs are stacked, the double-unit OLED exhibits a current efficacy more than twice that of the single-unit OLED, with a current efficacy of 97.8 cd/A at a luminance of 1119 cd/m². With the addition of an optical outcoupling layer of N,N′-Di-[(1-naphthyl)-N,N′-diphenyl]-1,1′-biphenyl)-4,4′-diamine (α-NPD) on top of the semitransparent gold anode, the double-unit stacked OLED achieves a maximum current efficacy of 205 cd/A at a luminance of 103 cd/m², maintaining a high current efficacy of 200 cd/A at a luminance of 1011 cd/m². These stacked inverted OLED combine the advantages of inverted OLEDs with the benefits of having a stacked architecture.     

Aminoborane-based bipolar host material for blue and white-emitting electrophosphorescence devices

A novel aminoborane-based host material, 9-(dimesitylboryl)-9′-phenyl-9H, 9′H-3,3′-bicarbazole (BCzBMes) was developed for blue and white phosphorescent OLEDs (PHOLEDs). The thermal, photophysical and electrochemical properties were systematically investigated. BCzBMes not only has a high triplet energy but also shows a bipolar behavior. To validate the superior properties of BCzBMes, blue and white PHOLEDs were fabricated using BCzBMes as a bipolar host material. A blue PHOLED containing Bis(4,6-difluorophenylpyridinato-N,C2)picolinatoiridium (FIrPic) as a dopant exhibited excellent performance with a maximum external quantum efficiency (EQE) of 16.7%. In particular, the blue PHOLED exhibited an extraordinary low efficiency roll-off of 10.1% at a brightness of 5000 cd/m². Meanwhile, an all-phosphor near-white device hosted by BCzBMes was also fabricated, and a high EQE of 18.8% was achieved. This excellent performance suggests that BCzBMes is a potential bipolar host material for the PHOLEDs.     

Bright green organic light-emitting devices having a composite electron transport layer

A bright green organic light-emitting device employing a co-deposited Al-Alq₃ layer has been fabricated. The device structure is glass/indium tin oxide (ITO)/ N, N′-diphenyl-N, N′- (3-methylphenyl)-1, 1′-biphenyl-4, 4′-diamine (TPD)/tris(8-quinolinolato) aluminum (Alq₃)/ Al-Alq₃/Al. In this device, Al-Alq₃ is used as electron transport layer (ETL). The device shows an operation voltage of 6.1 V at 20 mA/cm². At optimal condition, the brightness of a device at 20 mA/cm² is 2195 cd/m² achieved a luminance efficiency of 5.64lm/W. The result proves that the composite Al-Alq₃ layer is suitable for the ETL of organic light-emitting devices (OLEDs).