Mixed Host Devices Research Articles

Improved carrier injection and balance in solution-processed blue phosphorescent organic light emitting diodes based on mixed host system and their transient electroluminescence

Solution processed blue phosphorescent organic devices based on mixed host system were fabricated in this paper. A set of blue phosphorescent organic light emitting diodes (PhOLEDs) utilizing two small molecular materials of 3,3-Di(9H-carbazol-9-yl) biphenyl (mCBP) and 4,4′,4′′-tris(N-carbazolyl)-triphenylamine (TCTA) as mixed host doped with phosphor bis[(4,6-difluorophenyl)-pyridina-to-N,C2](picolinate) iridium(III) (FIrpic) are fabricated. The results show that the performance of mixed host system devices is improved compared with two single host devices. The max luminance of 23,340 cd/m2 is achieved in the mCBP:TCTA (2:1) mixed host device while the 14,400 cd/m2 in mCBP single host devices and the 12,968 cd/m2 in TCTA, and the current efficiency is improved from 8.17 cd/A(mCBP), 9.61 cd/A (TCTA) to 13.62 cd/A at the luminance of 1000 cd/m2. The transient electroluminescence measurement was utilized to detect the carrier injection between two kinds of PhOLEDs and penetrate how the trap charges works in the mixed host devices. This presents that the mixed host system contributes to the hole injection and the equilibrium of the carrier transport, reduces trapped charges, thus brings about high efficiency devices.

Crosslinkable hole-transporting small molecule as a mixed host for efficient solution-processed red organic light emitting diodes

The fabrication of multilayer organic light-emitting diodes through solution processing presents numerous challenges, primarily concerning dissolution of the first layer during deposition of a second layer. Here, we demonstrate a highly efficient solution processed red organic light emitting diode utilizing an electron confining cross linkable small molecule, 9,9′-bis(4-vinylphenylmethylen)[3,3′]bi-carbazole, as a hole transporting and co-host material. Compared with the corresponding single host device, the luminance of the mixed host device is enhanced from 8351 cd m−2 to 15,200 cd m−2, an increment of 82%, in addition, power efficiency and current efficiency also show an increment of 25% and 20% at 100 cd m−2, respectively. Results show that use of the crosslinking material makes the hole injecting, poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate), layer more intact during the deposition of the emissive layer which improves the film quality and hence device brightness. The enhanced charge carrier balance and broadened exciton recombination zone due to the mixed host contribute to the improvement of efficiency and efficiency roll-off.

Design strategy of exciplex host for extended operational lifetime

Design strategy of exciplex hosts made up of a hole transport type host and an electron transport type host for extended lifetime was established by developing exciplex hosts with different energy level offsets. An examination of the lifetime of the exciplex mixed host devices clarified the design rule of the exciplex hosts for extended lifetime. It was revealed that the HOMO and LUMO level offset between the hole type host and electron type host was the lifetime determining factor in the exciplex host devices. Long lifetime was obtained in the exciplex host devices with large HOMO and LUMO level offset between the hole type host and electron type host.

Hole-transporting small molecules as a mixed host for efficient solution processed green phosphorescent organic light emitting diodes

We have demonstrated highly efficient and reduced efficiency roll-off solution processed green phosphorescent organic light emitting diodes (OLEDs) utilizing two hole-transporting small molecular materials of 1,3-bis(carbazol-9-yl)benzene (mCP) and 4,4′,4″-tris(N-carbazolyl)-triphenylamine (TCTA) as a mixed host. Compared with the corresponding two single host devices, the mixed host one shows a superior overall performance. A low driving voltage of 5.5 V at 1000 cd/m2 with a current efficiency of 39.5 cd/A is achieved in the mCP:TCTA (3:1) mixed host device. Even at the luminance of 10000 cd/m2, the efficiency still reaches 28.6 cd/A. The enhanced charge carrier balance and broadened exciton recombination zone due to the mixed host contribute to the improvement of efficiency and efficiency roll-off.

Light emission mechanism of mixed host organic light-emitting diodes

Light emission mechanism of organic light-emitting diodes with a mixed host emitting layer was studied using an exciplex type mixed host and an exciplex free mixed host. Monitoring of the current density and luminance of the two type mixed host devices revealed that the light emission process of the exciplex type mixed host was dominated by energy transfer, while the light emission of the exciplex free mixed host was controlled by charge trapping. Mixed host composition was also critical to the light emission mechanism, and the contribution of the energy transfer process was maximized at 50:50 mixed host composition. Therefore, it was possible to manage the light emission process of the mixed host devices by managing the mixed host composition.

High efficiency thermally activated delayed fluorescent devices using a mixed host of carbazole and phosphine oxide derived host materials

A mixed host of a carbazole type host and phosphine oxide type host materials was developed and used as the host for green thermally activated delayed fluorescent (TADF) device to obtain high quantum efficiency and to manage the emission color of the TADF device. Two mixed hosts, 1,3-bis(N-carbazolyl) benzene (mCP):diphenylphosphine oxide-4-(triphenylsilyl) phenyl (TSPO1) and mCP:9,9-spirobifluoren-2-yl-diphenylphosphine oxide (SPPO1) were used as the mixed hosts and the device characteristics of the mixed host devices were investigated. The electron transport type of the mixed host devices could manage the quantum efficiency and emission color of the TADF devices. High quantum efficiency of 27.5% was achieved in the green TADF OLEDs using the mixed host of mCP:TSPO1 and the emission peak wavelength could be shifted by more than 10nm without sacrificing the quantum efficiency of the TADF devices.

Controlled host mixture and doping profile for ideal electrophosphorescent devices

We report the high-efficiency and low-driving-voltage behavior of green and red phosphorescent devices with mixed and layered light emitting host profile. The pair of hosts, which are predominant for either hole- or electron-transporting, was selected based on the result of charge carrier mobility and initial screening test for device performance. Devices with hole- and electron-transporting mixed host profiles with narrower band-gap materials showed lower driving voltage behavior as well as improved efficiency, where the charge-trapping issue due to host-dopant energy level offset is not significant and efficient Förster energy transfer is satisfied. For the electron transporting host materials, either for green or red phosphorescence, beryllium complexes were efficient both for efficiency at high current (brightness) and low driving voltage. The uniform mixing of host was compared with the layered double host structure, where the thickness of hole- and electron-transporting host was varied. Precisely defined heterostructured mixed layer profile, composed of hole-transporting/mixed/electron-transporting host (three layer), showed further improvement of luminous efficiency at high brightness maintaining driving voltage as low as that of simple mixed host device.

High efficiency blue phosphorescent organic light-emitting diodes using 2-(1H-pyrazol-1-yl)pyridin-3-ol ligand based Be compound

An organometallic complex based on Be and pyridinepyrazole type ligand, bis(2-(1H-pyrazol-1-yl)pyridin-3-olate) beryllium (BePyPy), was synthesized as an electron transport type host material for blue phosphorescent organic light-emitting diodes. High triplet energy of 2.79 eV was obtained and the highest occupied molecular orbital/lowest unoccupied molecular orbital of BePyPy was −6.26 eV/−2.71 eV. The BePyPy host was effective as the host material for blue phosphorescent organic diodes and a high quantum efficiency of 19.7% was achieved. In addition, a mixed host device with the BePyPy host showed high quantum efficiency of 22.0% in the blue device.

Dependence of hole and electron current density of mixed host devices on mixed host composition

The hole and electron current densities of mixed host emitting layer were studied according to the composition of the mixed host to investigate the origin of high quantum efficiency of the mixed host devices. Hole only and electron only devices of mixed host emitting layer were fabricated and the current density of single charge devices was studied. The hole current density was greatly affected by the mixed host composition, while the electron current density was not greatly changed by the mixed host composition. Therefore, the change of hole current density by mixed host composition was critical to the device performances of the mixed host devices.

Enhanced efficiency in mixed host red electrophosphorescence devices

Enhanced efficiency of red phosphorescent organic light-emitting devices is observed by using a bis[2-(2′-benzothienyl)pyridinato-N,C3′] iridium(acetylacetonato) doped 4,4′-N,N′-dicarbazole-biphenyl (CBP) and 1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBI) mixed host emitting layer. The CBP:TPBI mixed host device shows a maximum external quantum efficiency of 9.1%, which is dramatically improved compared to that of the CBP (6.6%) and TPBI (5.4%) single host devices. Such a mixed host strategy can also be exploited in red phosphor dibenzo[f,h]quinoxaline iridium (acetylacetonate) doped devices. Investigations reveal that the position of charge carrier recombination zone of the mixed host devices predominantly locates in the electron blocking layer/emitting layer interface. The efficiency enhancement is attributed to the optimized hole and electron injection balance and hence increased charge carrier recombination rate in the emitting layer.

Recombination zone study of phosphorescent organic light-emitting diodes with triplet mixed host emitting structure

Recombination zone of green phosphorescent organic light-emitting diodes (PHOLEDs) with triplet mixed host was studied using red sensing layer. Recombination zone of triplet mixed host device with 4,4′,4″-tris( N-carbazolyl)triphenylamine and 1,3,5-tris( N-phenylbenzimidazole-2-yl)benzene as hosts was rather dispersed compared with that of single host device with recombination zone near charge transport layer. The recombination zone was shifted to hole transport side at high driving voltage.

Solution Processed Blue Phosphorescent Organic Light Emitting Diodes Using a Phosphine Oxide Host Material

Solution processed high efficiency blue phosphorescent organic light emitting diodes were developed using a phosphine oxide derivative (SPPO1) as a host material. The SPPO1 was mixed with a polyvinylcarbazole (PVK), and the optimization of the device gave a high efficiency of 11.9 cd/A at . The reduction in the aggregation of the dopant, a smooth surface morphology, and improved charge balance were responsible for the high efficiency in the PVK:SPPO1 mixed host device.

Effect of doping concentration on device performances of triplet mixed host devices

Device performances of triplet mixed host devices were correlated with host composition and doping concentration. A mixed host of (4,4′-N,N′-dicarbazole)biphenyl (CBP) and spirobifluorene type host (PH1) was used and quantum efficiency of mixed host devices was optimized in CBP:PH1 (90:10) devices at all doping concentration. Quantum efficiency was not greatly affected by host composition at high doping concentration, while host composition was critical to quantum efficiency at low doping concentration. In addition, doping concentration had no influence on quantum efficiency of CBP:PH1 (90:10) devices.

Relationship between dopant energy levels and device performances of triplet mixed host devices

Device performances of phosphorescent organic light-emitting diodes (PHOLEDs) with triplet mixed host structure were correlated with dopant energy levels and mixed host composition. A mixed host of 4,4′,4″-tris-( N-carbazolyl)triphenylamine (TCTA) and spirobifluorene type host (PH1) was used and three different dopants with different bandgap were doped in the mixed host. Optimum quantum efficiency was obtained in TCTA:PH1 (50:50) mixed host device in the case of green dopants, while the optimum host composition for red and orange dopants was TCTA:PH1 (25:75).

P‐225: Suppressed Efficiency Roll‐Off in Phosphorescent Organic Light‐Emitting Diodes

AbstractOrigin of efficiency roll‐off in phosphorescent organic light‐emitting diodes was investigated with triplet mixed host devices and stable devices with little efficiency roll‐off was developed. Efficiency roll‐off was significant in the device with narrow recombination zone and charge leakage out of emitting layer at high luminance was critical to efficiency roll‐off. Efficiency roll‐off could be reduced in triplet mixed host device with broad recombination zone and little charge leakage at high driving voltage. Triplet mixed host devices with an exciton blocking layer showed a quantum efficiency over 90 % of maximum quantum efficiency at a luminance of 20,000 cd/m2.

Triplet host engineering for triplet exciton management in phosphorescent organic light-emitting diodes

The device performances of green phosphorescent organic light-emitting diodes with a triplet mixed host emitting layer were correlated with the energy levels and composition of the host materials. Two hole-transport-type host materials, (4,4′-N,N′-dicarbazole)biphenyl (CBP) and 4,4′,4″-tris(N-carbazolyl)triphenylamine (TCTA), were combined with two electron-transport-type host materials, 1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBI) and PH1. The maximum quantum efficiency was obtained in the 5:5 mixed host in the case of TCTA:TPBI and TCTA:PH1, while CBP:PH1 showed the best performances in the 9:1 mixed host. The quantum efficiency of the green mixed host devices was improved by more than 50% compared with that of the corresponding single host devices.

Stable efficiency roll-off in phosphorescent organic light-emitting diodes

Origin of efficiency roll-off in phosphorescent organic light-emitting diodes was investigated with triplet mixed host devices and stable devices with little efficiency roll-off was developed. Efficiency roll-off was significant in the device with narrow recombination zone (RZ) and charge leakage out of emitting layer at high luminance was critical to efficiency roll-off. Efficiency roll-off could be reduced in triplet mixed host device with broad RZ and little charge leakage at high driving voltage. Triplet mixed host devices with an exciton blocking layer showed a quantum efficiency over 90% of maximum quantum efficiency at a luminance of 20000cd∕m2.

Relationship between host energy levels and device performances of phosphorescent organic light-emitting diodes with triplet mixed host emitting structure

Green phosphorescent organic light-emitting diodes (PHOLEDs) with a triplet mixed host emitting layer were developed and device performances were studied by changing host materials in light-emitting layer. Power efficiency of green PHOLEDs could be improved from 12.7to29.1lm∕W by using triplet mixed host emitting layer. Combination of hole-transport-type host with good hole injection properties and electron-transport-type host with good electron injection properties was effective to get high efficiency in triplet mixed host devices.