Blue Phosphorescent Dye Research Articles

High CRI and stable spectra white organic light-emitting diodes with double doped blue emission layers and multiple ultrathin phosphorescent emission layers by adjusting the thickness of spacer layer

The color rendering index (CRI) and Commission Internationale de L'Eclairage coordinate (CIE x,y ) are significant parameters for the white organic light-emitting diodes (WOLEDs). We fabricate a series of the phosphorescent WOLED devices, in which there are two doped phosphorescent layers (for blue emission) on both sides of the emitting layer (EML) and three ultrathin phosphorescent layers (for red, orange and green emission) separated by the spacer layers (mCP) in EML. One doped blue emission layer near the hole transport layer contributes to CRI enhancement by increasing the doping concentration (x %) of the blue phosphorescent dye because Dexter energy transfer increases from the blue emission to the red emission. When x is 20, CRI reaches to 90 at 5 V in device A3. Based on device A3, CRI can be further improved to 94 in device B1 with a thinner thickness of the spacer layer (y = 1 nm) between the green emission layer and orange emission layer in EML because Dexter energy transfer from the green emission layer to the orange and red emission layer becomes easier. Although CRI declines, the spectra become more stable by increasing the thickness of the spacer layer (y = 3 nm) in device B2 or decreasing the thickness of the green emission layer (z = 0.05 nm) in device C, in which the CIE x,y coordinate shift is only (0.0214, 0.0102) from 5 V to 8 V. • The double doped blue phosphorescent layers (EML1 and EML5) on both sides of EML can adjust CRI and spectra stability. • The EML1 near HTL is beneficial for CRI and the red emission by increasing the doping concentration of FIrpic. • The EML5 near ETL can improve spectra stability by increasing the thickness of the spacer layer (Spacer2). • The EML5 near ETL can improve CRI by decreasing the thickness of green ultrathin phosphorescence layer (EML4).

Enhancement of electroluminescent properties of organic optoelectronic integrated device by doping phosphorescent dye**Project supported by the National Natural Science Foundation of China (Grant No. 61675041) and the National Science Funds for Creative Research Groups of China (Grant No. 61421002).

Organic optoelectronic integrated devices (OIDs) with ultraviolet (UV) photodetectivity and different color emitting were constructed by using a thermally activated delayed fluorescence (TADF) material 4, 5-bis(carbazol-9-yl)-1, 2-dicyanobenzene (2CzPN) as host. The OIDs doping with typical red phosphorescent dye [tris(1-phenylisoquinoline)iridium(III), Ir(piq)3], orange phosphorescent dye {bis[2-(4-tertbutylphenyl)benzothiazolato-N, C2′]iridium (acetylacetonate), (tbt)2Ir(acac)}, and blue phosphorescent dye [bis(2, 4-di-fluorophenylpyridinato)-tetrakis(1-pyrazolyl)borate iridium(III), FIr6] were investigated and compared. The (tbt)2Ir(acac)-doped orange device showed better performance than those of red and blue devices, which was ascribed to more effective energy transfer. Meanwhile, at a low dopant concentration of 3 wt.%, the (tbt)2Ir(acac)-doped OIDs showed the maximum luminance, current efficiency, power efficiency of 70786 cd/m2, 39.55 cd/A, and 23.92 lm/W, respectively, and a decent detectivity of 1.07 × 1011 Jones at a bias of −2 V under the UV-350 nm illumination. This work may arouse widespread interest in constructing high efficiency and luminance OIDs based on doping phosphorescent dye.

Solution Processed WPLEDs with Good Color Stability and High Color Rendering Index via a Phosphor-Sensitized System

We present efficient, solution processed white polymer light emitting diodes (WPLEDs) with good color stability and high color rendering index (CRI) via a phosphor-sensitized system using blue phosphorescent dye Bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium(III) (FIrpic) as emitter and sensitizer, and the fluorescent dye Rubrene and 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) as yellow and orange/red emitter respectively. Two different fluorophores, Rubrene and DCJTB, were chosen due to their different emission zone and different charge trapping ability. WPLEDs, in which FIrpic with Rubrene or DCJTB is doped at different feed ratios into poly (N-vinylcarbazole) (PVK) host, were fabricated and the effect of their concentration on the current density v/s voltage (J−V) characteristics and the electroluminescence (EL) spectra were studied. WPLEDs made with FIrpic:Rubrene (10:1) and FIrpic:DCJTB (80:1) were found to give CIE coordinates of (0.30, 0.42) and (0.34, 0.40) with CRI value of 58 and 76 at luminous efficiencies of 4.91 cd A−1 and 2.79 cd A−1 respectively. Finally, FIrpic, Rubrene and DCJTB were doped into PVK to get high quality white light and CIE coordinates of (0.41, 0.43) with a CRI value of 78 at luminous efficiency of 4.73 cd A−1 was achieved for FIrpic:Rubrene:DCJTB feed ratio of (100:8:1).

Highly efficient orange and white phosphorescent organic light-emitting devices with simplified structure

We successfully developed efficient orange and white phosphorescent organic light-emitting devices based on simplified structure. The maximum power efficiency and current efficiency of the orange device with only 5nm emission layer are 34.1lm/W and 38.0cd/A. Performances of the simplified orange devices can be greatly improved to 62.6lm/W (79.7cd/A) by introducing one blue phosphorescent dye iridium(III)[bis(4,6-difluorophenyl)-pyridina-to-N,C2′]picolinate into the orange emission layer. The emission mechanism of the above orange devices was also discussed. White light emission with Commission International de L’Eclairage coordinates of (0.36, 0.45) and efficiency of 40.6lm/W (38.7cd/A) was obtained by exactly adjusting the doping concentration of orange phosphorescent dye.

White organic light-emitting diodes based on a single-emissive layer using electrophosphorescent dopants in a fluorescent host

A single-emissive fluorescent layer co-doped with red and blue phosphorescent dyes in a host [4, 4, bis-9 carbozyl-biphenyl] (CBP) was used to fabricate white organic light-emitting devices (WOLEDs). The electroluminescent spectrum of the generated white light was observed to have two spectral peaks at ∼495 and ∼590 nm and a dip at ∼ 536 nm, with different thicknesses of the emissive layer (EML). By optimizing the ratio of the dopants in the EML, the ratio of the maximum intensity of the two spectral peaks was brought to unity. The Commission Internationale de l'Eclairage (CIE) coordinates and correlated color temperature of the fabricated device were calculated, and the results are reported herein. No significant change in the spectral shape of the emission spectra and the CIE coordinates were obtained when the applied voltages were varied. It is easier to fabricate WOLEDs with a single EML as compared with their multilayer counterparts.

High-Efficiency Small-Molecule-Based Organic Light Emitting Devices with Solution Processes and Oxadiazole-Based Electron Transport Materials

We demonstrate high-efficiency small-molecule-based white phosphorescent organic light emitting diodes (PHOLEDs) by single-active-layer solution-based processes with the current efficiency of 17.3 cdA(-1) and maximum luminous efficiency of 8.86 lmW(-1) at a current density of 1 mA cm(-2). The small-molecule based emitting layers are codoped with blue and orange phosphorescent dyes. We show that the presence of CsF/Al at cathodes not only improves electron transport in oxadiazole-containing electron transport layers (ETLs), but also facilitates electron injection through the reacted oxadiazole moiety to reduce interface resistance, which results in the enhancement of current efficiency. By selecting oxadiazole-based materials as ETLs with proper electron injection layer (EIL)/cathode structures, the brightness and efficiency of white PHOLEDs are significantly improved.

Small-molecular blue phosphorescent dyes for organic light-emitting devices

Organic light-emitting devices (OLEDs) are on the lips of most electronic manufacturers currently. With good progress made in terms of production cost, efficiency and color output, OLEDs have found more applications recently as compared to those some years ago. Because of the possibility of obtaining long-lasting, durable and energy-efficient OLEDs, researchers devote much time and effort towards the improvement of OLED technology and development of advanced OLED products. Blue light-emitting materials, especially blue phosphorescent materials, are indispensable for full-color displays and white OLED lighting. Compared with green and red light-emitting materials and devices, the blue-emitting counterparts show a relatively inferior performance in terms of color purity, luminescence efficiency and device durability. In this perspective article, we highlight the recent progress and current challenges of blue-emitting metallophosphors based on small molecules and their applications in phosphorescent OLEDs.

High-efficiency conjugated-polymer-hosted blue phosphorescent light-emitting diodes

Highly-efficient blue phosphorescent light-emitting diodes were fabricated based on a conjugated-polymer host by doping bis(2-(4,6-difluorophenyl)-pyridinato-N,C2′) picolinate (FIrpic) into poly(9,9-dioctylfluorene) (PFO). Previously, conjugated polymers were not considered as potential hosts for blue phosphorescent dyes because of their low-lying triplet energy levels. Energy back transfer would occur and lead to poor luminescent efficiency in both photoluminescence (PL) and electroluminescence (EL) processes. However, by inserting a hole-transporting layer of poly(N-vinylcarbazole) (PVK), the energy back transfer was suppressed. At low FIrpic-doping concentrations, PFO emissions were completely quenched; with 8 wt% FIrpic, a maximum luminous efficiency of 11.5 cd/A was achieved.

Highly efficient pure white polymer light-emitting devices based on poly(N-vinylcarbazole) doped with blue and red phosphorescent dyes

Efficient white-polymer-light-emitting devices (WPLEDs) have been fabricated with a single emitting layer containing a hole-transporting host polymer, poly(N-vinylcarbzole), and an electron-transporting auxiliary, 1,3-bis[(4-tert-butylphenyl)-1,3,4-oxadiazolyl]-phenylene, codoped with two phosphorescent dyes: Iridium(III)bis (2-(4,6-difluorophenyl)-pyridinato-N,C2′) picolinate (FIrpic) and home-made Ir-G2 for blue and red emission, respectively. With the structure of ITO/PEDOT:PSS 4083(40 nm)/emission layer(80 nm)/Ba(4 nm)/Al(120 nm), the device showed a maximal luminous efficiency (LE) of 13.5 cd A−1(corresponding to an external quantum efficiency (EQE) of 6.8%), and a peak power efficiency (PE) of 6.5 lm W−1 at 6.0 V. Meanwhile, the device exhibited pure white emission with Commission Internationale de l’Eclairage (CIE) coordinates of (0.34, 0.35) at a current density of 12 mA cm−2, which is very close to the equi-energy white point with CIE coordinates of (0.33, 0.33). The device performance can be further optimized when more balanced hole/electron injection is achieved by incorporating a lower conducting type anode buffer layer (PEDOT:PSS) and incorporating poly[(9,9-bis(3′-(N,N-dimethylamino) propyl)-2,7-fluorenene)-alt-2,7-(9,9-dioctyfluorene)] (PFN) as an electron injection layer at the cathode. The optimized device showed an LE of 24.6 cd A−1 (with an EQE of 14.1%), while the peak power efficiency reached 12.66 lm W−1. Moreover, the WPLEDs showed good electroluminescence (EL) stability over a wide range of operating current density and luminance.

Enhanced Electroluminescence of CdSe/ZnS Quantum Dot Light–emitting Diodes with Phosphorescent Donors

ABSTRACTWe demonstrated the enhancement of electroluminescence (EL) from green CdSe/ZnS QDs in hybrid QD/organic light-emitting diodes (QD-LEDs) by employing blue phosphorescent dyes Bis(4,6-difluorophenylpyridinato-N,C2)picolinatoiridium (FIrpic) as efficient exciton harvesters and energy transfer donors. Precise control of the concentration of the FIrpic donors doped in a 4,4’-N, N’-dicarbazole-biphenyl (CBP) host and their distance from the QD layer led to complete triplet exciton energy transfer and EL enhancement by a factor of 2.5. The Förster distance between FIrpic molecules and green CdSe/ZnS QDs was determined to be ∼ 8 nm, which is in a good agreement with the value calculated using the Förster model. Our study shows that integrating colloidal QDs with phosphorescent organic dyes provides an effective means for improving the quantum efficiency of QD-based hybrid LEDs.

Efficient solution-processed phosphor-sensitized single-emitting-layer white organic light-emitting devices: fabrication, characteristics, and transient analysis of energy transfer

We present high-efficiency solution-processed WOLEDs with good color stability and high color rendering index using a phosphor-sensitized system. The devices with a single active layer in which the blue phosphorescent dye FIrpic as emitter and sensitizer, and the fluorescencent dye 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) as orange emitter, show a maximum luminance efficiency of 16.4 cd A−1 and a maximum luminance of 24300 cd m−2. By adding a hole-blocking layer, a substantially enhanced efficiency of 26.9 cd A−1 can be achieved. The origin and mechanism of the efficient white emission are also discussed by using the transient photoluminescence and electroluminescence techniques. Transient analyses indicate that white emission is from the residual blue phosphorescence of the sensitizer, the sensitized orange-red fluorescence due to triplet-to-singlet energy transfer, and the instantaneous fluorescence resulting due to singlet-singlet transfer and direct exciton formation caused by charge trapping in DCJTB molecules. It is also demonstrated that the direct triplet exciton formation on DCJTB due to charge trapping are the primary loss process. For the first time, the energy transfer efficiency of the incomplete energy transfer from FIrpic to DCJTB, and the ratios of instantaneous to sensitized fluorescence emissions are investigated.

Electroluminescence of green CdSe/ZnS quantum dots enhanced by harvesting excitons from phosphorescent molecules

We demonstrated the enhancement of electroluminescence from green CdSe/ZnS quantum dots (QDs) in hybrid QD/organic light-emitting diodes (LEDs) by employing blue phosphorescent dyes bis(4,6-difluorophenylpyridinato-N,C2)picolinatoiridium (FIrpic) as efficient exciton harvesters and energy transfer donors. Precise control over the position and concentration of the donors doped in a fluorescent host led to complete exciton energy transfer from FIrpic molecules located within the Förster distance of ∼8 nm from the QD layer, and a 2.5-fold increase in the quantum efficiency of the QD-LEDs.

Highly efficient white polymer light-emitting devices based on wide bandgap polymer doped with blue and yellow phosphorescent dyes

We realize high luminous efficiency (LE) in white polymer light-emitting devices by employing a wide bandgap polymer P36HCTPSi as host and introducing a blue-yellow two-color system for obtaining white emission. Phosphorescent dopants bis[(4,6-difluorophenyl)pyridinato-N,C(2)]-(picolinato)iridium(III) (FIrpic) and iridium(III) bis(2-(9,9'-spirobi[fluorene]-7-yl)pyridine-N,C(2')) acetylacetonate [Ir(SBFP)(2) (acac)] were used as blue and yellow emitters, respectively. Electroluminescent spectra and LE were both affected by Ir(SBFP)(2)(acac) concentration; the device with low concentration (0.4 wt.%) showed white emission with a LE of 24.1 cd/A at 1000 cd/m(2), while the one with high concentration (4 wt.%) emitted yellowish-white light with a higher LE of 47.3 cd/A at 1000 cd/m(2).

Highly efficient single-emitting-layer white organic light-emitting diodes with reduced efficiency roll-off

By codoping blue and orange phosphorescent dyes into a single host material, a highly efficient white organic light-emitting diode (WOLED) with Commission Internationale de L’Eclairage coordinates of (0.38, 0.43) at 12 V is demonstrated. Remarkably, this WOLED achieves reduced current efficiency roll-off, which slightly decreases from its maximum value of 37.3–31.0 cd/A at 1000 cd/m2. The device operational mechanism is subsequently investigated in order to unveil the origin of the high performance.

Efficient white organic light-emitting devices based on blue, orange, red phosphorescent dyes

We demonstrate efficient white organic light-emitting devices (WOLEDs) based on an orange phosphorescent iridium complex bis(2-(2-fluorphenyl)-1,3-benzothiozolato-N, C2′)iridium(acetylacetonate) in combination with blue phosphorescent dye bis[(4, 6-difluorophenyl)-pyridinato-N,C2)](picolinato) Ir(III) and red phosphorescent dye bis[1-(phenyl)isoquinoline] iridium (III) acetylanetonate. By introducing a thin layer of 4, 7-diphenyl-1,10-phenanthroline between blue and red emission layers, the diffusion of excitons is confined and white light can be obtained. WOLEDs with the interlayer all have a higher colour rendering index (>82) than the device without it (76). One device has the maximum current efficiency of 17.6 cd A−1 and a maximum luminance of 39 050 cd m−2. The power efficiency is 8.7 lm W−1 at 100 cd m−2. Furthermore, the device has good colour stability and the CIE coordinates just change from (0.394, 0.425) to (0.390, 0.426) with the luminance increasing from 630 to 4200 cd m−2.

White organic light emitting devices with hybrid emissive layers combining phosphorescence and fluorescence

We fabricated a white organic light-emitting diode (WOLED) by hybrid emissive layers which combined phosphorescence with fluorescence. In this device, the thin layer of 4-(dicyanomethylene)-2-(t-butyl)-6-(1, 1, 7, 7-tetramethyljulolidyl-9-enyl)-4H-pyran played the role of undoped red emissive layer which was inserted between two blue phosphorescence emissive layers. The blue phosphorescent dye was bis[(4, 6-difluorophenyl)-pyridinato-N, C2] (picolinato) Ir(III), which was doped in the host material, N, N′-dicarbazolyl-1, 4-dimethene-benzene. The WOLED showed stable Commission Internationale de L'Eclairage coordinates and a high efficency of 9.6 cd A−1 when the current density was 1.8 A m−2. The maximum luminance of the device achieved was 17 400 cd m−2 when the current density was 3000 A m−2.

Efficient Solution-Processed Blue Electrophosphorescent Devices Based on a Novel Small-Molecule Host

Efficient blue small molecular phosphorescent light-emitting diodes with a blue phosphorescent dye bis(3,5- difluoro-2-(2-pyridyl)-phenyl-(2-carboxypride) iridium (III) (FIrpic) doped into a novel small-molecule host 9,9- bis[4-(3,6-di-tert-butylcarbazol-9-yl)phenyl] fluorene (TBCPF) as the light-emitting layer have been fabricated by spin-coating. The host TBCPF can form homogeneous amorphous films by spin-coating and has triplet energy higher than that of the blue phosphorescent dye FIrpic. All the devices with different FIrpic concentration in the emitting layer give emission from FIrpic indicating complete energy transfer from TBCPF to FIrpic. The device shows the best performance with a peak brightness of 8050 cd/m2 at 10.2 V and the maximum current efficiency up to 3.52 cd/A, when the FIrpic doped concentration is as high as 16%.

Recent progress in solution processable organic light emitting devices

Organic light emitting devices (OLEDs) have been the subject of intense research because of their potential for flat panel display and solid state lighting applications. While small molecule OLEDs with very high efficiencies have been demonstrated, solution processable devices are more desirable for large size flat panel display and solid state applications because they are compatible with low cost, large area roll-to-roll manufacturing process. In this review paper, we will present the recent progress made in solution processable OLEDs. The paper will be divided into three parts. In the first part of the paper, we will focus on the recent development of fluorescent polymer OLEDs based on conjugated polyfluorene copolymers. Specifically, we will present results of carrier transport and injection measurements, and discuss how the charge transport and injection properties affect the device performance. In the second part of the paper, we will focus on the recent progress on phosphorescent dye-dispersed nonconjugated polymer OLEDs. Specifically, we will present our recent results on high efficiency green and blue emitting devices based on the dye-dispersed polymer approach. Similar to fluorescent conjugated polymer OLEDs, charge transport and injection properties in dye-dispersed polymer OLEDs also play an important role in the device performance. In the third part of this paper, we will present our results on white emitting phosphorescent OLEDs. Two approaches have been used to demonstrate white emitting OLEDs. First, white emitting OLEDs were made using blue emitting OLEDs with downconversion phosphors. Second, white emitting OLEDs were made by dispersing red, green, and blue phosphorescent dyes into the light emitting layer. High efficiency devices have been demonstrated with both approaches.

Energy Transfer Employing Europium Complex and Blue Phosphorescent Dye and Its Application in White Organic Light-Emitting Diodes

We investigated the energy transfer of a blue phosphorescent molecule, bis[(4,6-difluorophenyl)-pyridinato-N,C2'] (picolinate) iridium(III) (FIrpic), and a red europium complex of tris(dibenzoylmethane)-mono(4,7-dimethyphenanthroline) europium(III) [Eu(dbm)3phen] doped in poly(N-vinylcarbazole) (PVK). The photoluminescence (PL) spectral intensity of a PVK:Eu(dbm)3phen film was increased by FIrpic doping. Additionally, we demonstrated white organic light-emitting devices (OLEDs) employing the energy transfer from a host to dopants, showing the Commission Internationale de L'Eclairage (CIE) coordinates of (0.31, 0.38) at a current density of 0.5 mA/cm2. The advantage of using Eu complexes as red dopants is that it is easy to control the doping ratio and obtain a white emission because of their high triplet level.

High-efficiency white organic light-emitting devices using a blue iridium complex to sensitize a red fluorescent dye

We report the fabrication of high-efficiency white organic light-emitting devices (WOLEDs) by using a blue phosphorescent dye iridium (III) tris(5-(2,4-difluoro-phenyl)-10,10-dimethyl- 4-aza-tricycloundeca-2,4,6-triene) (Ir(F2-mppy)3) to sensitize the red dye[2-methyl-6- [2-(2,3,6,7-tetrahydro-1H c5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene]propane-din- itrile (DCM2). Ir(F2-mppy)3 and DCM2 were codoped into the 4,4′-N,N′-dicarbazole-biphenyl (CBP) host. The WOLEDs with 8wt% Ir(F2-mppy)3 and 0.5wt% DCM2 showed white emission with a color rendering index of 70. The maximum luminance and maximum current efficiency of the device are, respectively, 16220cd∕m2 and 9.28cd∕A.