Yellow-emitting Layer Research Articles

White organic light-emitting devices with high color purity and stability

A white organic light-emitting device (WOLED) with dual-emitting layers was presented, in which the blue fluorescent dye 2,5,8,11-terta-tertbutylperylene (TBPe) was doped in 2-methyl-9, 10-di(2-naphthyl)-anthracene (MADN) as a blue-emitting layer, while 5,6,11,12-tetraphenylnaphthacene (rubrene, Rb) was doped in the above-mentioned materials as a yellow-emitting layer. The fabricated monochromatic devices using the blue- and yellow-emitting layer have demonstrated that the direct charge trapping mechanism is the dominant emission mechanism in the yellow OLED. Studies on the WOLEDs with dual-emitting layers have shown that the performances of these devices are strongly susceptible to the thickness of the emitting layer and the stack order of two emitting layers. Structure of ITO(160 nm)/NPB(30 nm)/MADN: 5 wt%TBPe: 3 wt%Rb(10 nm)/MADN: 5 wt%TBPe(20 nm)/BCP (10 nm)/Alq3(20 nm)/Al(100 nm) was determined to be the most favorable WOLED. The maximum luminance of 16 000 cd cm−2 at the applied voltage of 13.4 V and Commission International de 1′Eclairage (CIE) coordinates of (0.3263, 0.3437) which is closer to the standard white light (CIE (0.33, 0.33)) than the most recent reported WOLEDs were obtained. Moreover, there is just slight variation of CIE coordinates (ΔCIEx, y = 0.0171, 0.0167; corresponding Δu′v′ = 0.0119) when the current density increases from 10 to 100 mA cm−2. It reveals that the emissive dopant Rb acts as charge traps to improve electron–hole balance, provides sites for electron–hole recombination and thus makes carriers distribute more evenly in the dual-emitting layers which broaden the recombination zone and improve the stability of the CIE coordinates.

Highly efficient and color-stable white organic light-emitting diode based on a novel blue phosphorescent host

Highly efficient phosphorescent white organic light-emitting devices (WOLEDs) have been fabricated by using two complementary blue and yellow emitting layers, in which the blue guest iridium(III) bis[(4,6-difluorophenyl)-pyridinato-N,C2′] picolinate (Firpic) was doped in a novel blue phosphorescent host. The efficiencies and spectra of WOLEDs can be easily tuned by inserting an ultrathin interlayer between the two emitters and adjusting the thickness of the yellow emitting layer (Y-EML). The device with 1nm thick interlayer and 3nm thick (Y-EML) obtains very high efficiencies of 42.4cd/A and 47.6lm/W and a luminance of 1144cd/m2 was realized at a low voltage of 3.6V. In addition, another device with 2nm thick interlayer and 5nm thick Y-EML exhibited nearly voltage-independent electroluminescent (EL) spectra. Commission International de L’Eclairage (CIE) coordinates of this device only changes from (0.333, 0.436) at a luminance of 100cd/m2 to (0.330, 0.434) at that of 10,000cd/m2, nearly independent of the driving voltage.

Polymeric White Organic Light-Emitting Diodes with Two Emission Layers

A white light-emitting diode was fabricated by preparing multilayer emitting films with an inserted buffer layer. The device structures are ITO/PEDOT:PSS/Emissive layer/LiF/Al. The emissive layer comprises a yellow-emitting layer of Poly[9,9-dioctylfluorenyl-2,7-diyl]-co-1,4-benzo-(2,1,3)-thiadiazole (F8BT), a blue-emitting layer of Poly[9,9-di-(2'-ethylhexyl)fluorenyl-2,7-diyl] (BEHF) and PEDOT:PSS as a buffer layer between the emission layers. The solution processed multi-layer polymer light-emitting diodes (PLED) were prepared by introduction of a water-soluble buffer layer between organic solvent soluble layers. We present white organic light-emitting diodes (WOLEDs) that has bilayer emission zones. This device exhibits a brightness of 280 cd/m2 and emission efficiency of 1.18 cd/A at 12.6 V. The device with a doped PEDOT:PSS layer and a thicker blue-emission layer exhibits CIE color coordinates of (0.30, 0.34), which is close to the white coordinates of (0.33, 0.33) used by the standard CIE color coordinates.

Combined host–guest doping and host-free systems for high-efficiency white organic light-emitting devices

Highly efficient white organic light-emitting devices (WOLEDs) with a four-layer structure were realized by utilizing phosphorescent blue and yellow emitters. The key concept of device construction is to combine host–guest doping system of the blue emitting layer (EML) and the host-free system of yellow EML. Two kinds of WOLEDs incorporated with distinct host materials, namely N,N'-dicarbazolyl-3,5-benzene (mCP) and p-bis(triphenylsilyly)benzene (UGH2), were fabricated. Without using light out-coupling technology, a maximum current efficiency (ηC) of 58.8cd/A and a maximum external quantum efficiency (ηEQE) of 18.77% were obtained for the mCP-based WOLED; while a maximum ηC of 65.3cd/A and a maximum ηEQE of 19.04% were achieved for the UGH2-based WOLED. Meanwhile, both WOLEDs presented higher performance than that of conventionally full-doping WOLEDs. Furthermore, systematic studies of the high-efficiency WOLEDs were progressed.

High-efficiency white organic light-emitting devices with a non-doped yellow phosphorescent emissive layer

Highly efficient phosphorescent white organic light-emitting devices (PHWOLEDs) with a simple structure of ITO/TAPC (40nm)/mCP:FIrpic (20nm, x wt.%)/bis[2-(4-tertbutylphenyl)benzothiazolato-N,C2′] iridium (acetylacetonate) (tbt)2Ir(acac) (y nm)/Bphen (30nm)/Mg:Ag (200nm) have been developed, by inserting a thin layer of non-doped yellow phosphorescent (tbt)2Ir(acac) between doped blue emitting layer (EML) and electron transporting layer. By changing the doping concentration of the blue EML and the thickness of the non-doped yellow EML, a PHWOLED comprised of higher blue doping concentration and thinner yellow EML achieves a high current efficiency of 31.7cd/A and Commission Internationale de l'Eclairage coordinates of (0.33, 0.41) at a luminance of 3000cd/m2 could be observed.

Effect of Charge Carrier Transport Property and Energy Level of Host Material on Phosphorescent White Organic Light-Emitting Device

Phosphorescent white organic light-emitting devices (PhWOLEDs) with a double emitting layer (EML) structure were fabricated. The EMLs were doped with blue and yellow phosphors of iridium(III) bis[4,6-(difluorophenyl)-pyridinato-N,C2'] picolinate and [2-(4-tert-butylphenyl)benzothiazolato-N,C2'] iridium (acetylacetonate). Comparing the performance of PhWOLEDs with the host of yellow EML varying from p- to n-type materials, the results showed that the PhWOLED fabricated with 4,7-diphenyl-1,10-phenanthroline (BPhen) as the yellow host had the highest performance, exhibiting a peak current efficiency of 31.7 cd/A, a peak power efficiency of 17.1 lm/W, and the Commission Internationale del'Eclairage coordinates of (0.33, 0.41) at a bias of 9 V. The enhancement was attributed to the improved charge carrier balance, broadened recombination region, and adequate energy levels between the BPhen host and the dopants, which made distinct roles for the dopants to effectively harvest the charge carriers.

Efficient bright white organic light-emitting diode based on non-doped ultrathin 5,6,11,12-tetraphenylnaphthacene layer

High-performance undoped white organic light-emitting diode (OLED) has been fabricated using an ultrathin yellow-emitting layer of 5,6,11,12-tetraphenylnaphthacene (rubrene) inserted at two sides of interface between two N,N′-bis-(1-naphthyl)-N,N′- biphenyl-1,1′-biphenyl-4,4′- diamine (NPB) layers as a hole transporting and blue emissive layer, respectively. The results showed that a maximum luminance of the device reached to as high as 21,500 cd/m 2 at 15 V. The power efficiencies of 2.5 and 1.6 lm/W at a luminance of 1000 and 10000 cd/m 2, respectively, were obtained. The peaks of electroluminescent (EL) spectra locate at 429 and 560 nm corresponding to the Commissions Internationale De L’Eclairage (CIE) coordinates of (0.32, 0.33), which is independent of bias voltage. The performance enhancement of the device may result from direct charge carrier trapping in rubrene. Energy transfer mechanism was also found in the EL process.

High-Efficiency White Organic Light-Emitting Diodes Based on Phosphorescent Iridium Complex and 4,4'-Bis[N-1-napthyl-N-phenyl-amino]biphenyl Emitters

High-efficiency white organic light-emitting devices (WOLEDs) based on phosphorescent dye bis[2-(4-tert-butylphenyl)benzothiazolato-N,C2']iridium (acetylacetonate) [(t-bt)2Ir(acac)] doped 4,4'-bis(carbazol-9-yl) biphenyl (CBP) as a yellow-emitting layer together with blue fluorescent emitter and hole-transporting material 4,4'-bis[N-1-napthyl-N-phenyl-amino]biphenyl (NPB) were fabricated. In those devices with a simple structure, 2,9-dimethyl,-4,7-diaphenyl,1,10-phenanthroline (BCP) and tris(8-quinolinolato) aluminum (Alq) were used as hole-blocking and electron-transporting layers, respectively. The purity of white light can be adjusted by doping concentration and bias voltage. The device shows a maximum luminance of 7580 cd/m2 with a luminance efficiency of 4.32 cd/A and Commission International De L'Eclairage (CIE) coordinates of (0.319,0.342) at 13.5 V. A maximum luminance efficiency of 10.2 cd/A at a current density of 0.026 mA/cm2 was achieved.

White organic light-emitting diodes with fine chromaticity tuning via ultrathin layer position shifting

Nondoped white organic light-emitting diodes using an ultrathin yellow-emitting layer of rubrene (5,6,11,12-tetraphenylnaphtacene) inserted on either side of the interface between a hole-transporting 4,4′-bis[N-(1-naphtyl)-N-phenylamino]biphenyl (α-NPB) layer and a blue-emitting 4,4′-bis(2,2′-diphenylvinyl)-1,1′-biphenyl (DPVBi) layer are described. Both the thickness and the position of the rubrene layer allow fine chromaticity tuning from deep blue to pure yellow via bright white with CIE coordinates (x=0.33, y=0.32), an ηext of 1.9%, and a color rendering index of 70. Such a structure also provides an accurate sensing tool to measure the exciton diffusion length in both DPVBi and NPB (8.7 and 4.9nm, respectively).

Non-doped-type white organic light-emitting devices based on yellow-emitting ultrathin 5,6,11,12-tetraphenylnaphthacene and blue-emitting 4,4′-bis(2,2′-diphenyl vinyl)-1,1′-biphenyl

Non-doped-type white organic light-emitting devices based on the ultrathin yellow-emitting layer (0.1 nm) of 5,6,11,12-tetraphenylnaphthacene (rubrene) and the thick blue-emitting layer of 4,4′-bis(2,2′-diphenyl vinyl)-1,1′-biphenyl (DPVBi) made from vacuum-deposited organic thin films are reported. The recombination zone of the devices can be adjusted by changing the thicknesses of DPVBi and the electron transport layers (tris-(8-hydroxyquinoline)aluminium, Alq3). In the device with 30 nm DPVBi and 30 nm Alq3, bright white light, over 10 000 cd m−2, was successfully obtained at a low drive voltage of ∼10 V. The highest power efficiency is 3.18 lm W−1 at 4 V, and stable 1931 Commision International de L'Eclairage coordinates are obtained for luminance ranging from 100 to 20 000 cd m−2 (at ∼12 V).