•Tandem OLEDs were demonstrated using both n/p-type and p/n-type CGLs.•The influence of MoOx and LiF on the charge generation process was investigated.•A doping layer Ir(ppz)3:MoOx was introduced in n/p-type CGL.High performance tandem organic light-emitting diodes (TOLEDs) were demonstrated for the first time using both buffer-modified n/p-type and p/n-type planar organic heterojunctions (OHJs) as charge generation layers (CGLs), and the optimized configurations of these two CGLs were “LiF/4,7-diphenyl-1,10-phenanthroline (Bphen)/tris(phenylpyrazole)iridium (Ir(ppz)3)/Ir(ppz)3:molybdenum oxide (MoOx)/MoOx” and “LiF/Ir(ppz)3/Bphen/MoOx”, respectively. Compared to the single-unit OLED, both n/p-type and p/n-type CGLs based TOLEDs exhibited about two folds enhancement in current efficiency. The influence of both MoOx and LiF as carrier extraction layers on the charge generation process of CGLs was conscientiously investigated. The working mechanism of two CGLs was discussed in details. It was also found that the novel design concept, a buffer-modified p/n-type planar OHJ could generate enough charges under a forward applied voltage and the carriers extraction was a tunneling process. Our results provide the great practicality of implementing planar OHJs as CGLs in high performance TOLEDs.Download high-res image (175KB)Download full-size image

•Efficient and simplified WOLEDs based on a novel host, 4,6-bis(diphenylphosphoryl)dibenzofuran (DBFDPO), are demonstrated.•The WOLEDs exhibit low turn-on voltage and excellent color stability.•The underlying mechanism that impact on color stability is finally clarified and discussed in detail.•A thin Ir(ppz)3 interlayer is introduced to obtain balanced white emission and improve the efficiency.Efficient phosphorescent white organic light-emitting devices (WOLEDs) based on a novel phosphine oxide host, 4,6-bis(diphenylphosphoryl)dibenzofuran (DBFDPO), with particularly low turn-on voltage and high color stability are reported. Without any electrical doping (p-doing or n-doping) strategies, the simplified optimal WOLEDs utilizing DBFDPO as the same host for blue/yellow emitters show low operating voltages, 1 cd/m2 at 2.5 V (turn-on voltage), 46 cd/m2 at 3 V and 2787 cd/m2 at 5 V, and 12840 cd/m2 at 7 V, respectively. A forward-viewing current efficiency (CE) of 41.3 cd/A (power efficiency (PE) of 34.0 lm/W) is achieved at the luminance of 673.4 cd/m2 without any out-coupling enhancement structures. Furthermore, improved color stability is achieved through adjusting the yellow emitting layer. The Commission Internationale de I’Eclairage coordinates (CIE) of the optimal WOLED merely shift from (0.330, 0.405) to (0.327, 0.400) over a wide range of luminance (10–10,000 cd/m2). We have revealed that the competition of the two mechanisms of energy transfer and carrier-trapping/-assist transport severely impact on the spectra stability.

•Different potential barrier layers PBLs are investigated in the OLEDs based on multiple emission layers.•The general selective rules for PBL are clarified for obtaining high device efficiency.•High efficiency yellow OLEDs are obtained based on PO-01 emission.High efficiency Iridium (III) bis (4-phenylthieno [3,2-c] pyridinato-N,C2′) acetylacetonate (PO-01) based yellow organic light-emitting devices are fabricated by employing multiple emission layers. The efficiency of the device using 4,4′,4″-tris(N-carbazolyl) triphenylamine (TCTA) as potential barrier layer (PBL) outperforms those devices based on other PBLs and detailed analysis is carried out to reveal the mechanisms. A forward-viewing current efficiency (CE) of 65.21 cd/A, which corresponds to a maximum total CE of 110.85 cd/A is achieved at 335.8 cd/m2 in the optimized device without any outcoupling enhancement structures.

•We demonstrate WOLEDs with a phosphorescent MEML architecture.•The blue guest FIrpic was doped in a novel blue phosphorescent host.•The efficiencies and spectra can be tuned by the thickness of interlayer and Y-EML.•High efficiencies of 42.4 cd/A and 47.6 lm/W were realized in the optimized device.•Device B1 exhibited nearly voltage-independent electroluminescent spectra.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 1 nm thick interlayer and 3 nm thick (Y-EML) obtains very high efficiencies of 42.4 cd/A and 47.6 lm/W and a luminance of 1144 cd/m2 was realized at a low voltage of 3.6 V. In addition, another device with 2 nm thick interlayer and 5 nm 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 100 cd/m2 to (0.330, 0.434) at that of 10,000 cd/m2, nearly independent of the driving voltage.

•TEWOLED with low operating voltage and broadband emission is achieved.•TEWOLED with high color stability and low efficiency roll-off is demonstrated.•TWOLED shows similar spectrum and comparable luminance from both sides.•TWOLED exhibits high efficiency, high CRI and ideal CCT.It is challenging to obtain broadband emission covering as much of the visible light spectrum as possible in top-emitting white organic light-emitting diodes (TEWOLEDs) due to the well known microcavity effects. In this work, we achieved TEWOLED with three separate peak and negligible angular dependence by employing a high transmittance stack cathode Al (2 nm)/Cu (18)/TcTa (60 nm). The TEWOLED shows an efficiency of 25.6 cd/A, 20.1 Lm/W at 1000 cd/m2, and low voltage of 4.2 V for 1222 cd/m2. Synchronously, we achieved transparent white organic light-emitting diode (TWOLED) using this high transmittance stack cathode, the TWOLED exhibits similar spectrum and comparable luminance from both sides, and the maximum total efficiencies of the TWOLED are 28.6 cd/A, 24.9 Lm/W.Graphical abstract

•Color stable all-phosphor WOLEDs are obtained utilizing Ir(ppz)3 interlayer.•High CRI of 92/90 at 100/1000 cd m−2 is obtained in a four-color device.•CIE coordinates of the best four-color device only change from (0.388, 0.455) to (0.387, 0.453) over 100–1000 cd m−2.The color stability of all-phosphor white organic light-emitting diodes (WOLEDs) is crucial and remains a challenge that must be overcome before the wide application of phosphor WOLEDs technology. Besides, color stable all-phosphor WOLEDs should also offer high color rendering index (CRI) and ideal correlated color temperature (CCT) simultaneously to make the technology competitive against other alternative technologies such as inorganic LEDs. In this work, we demonstrate a series of color stable all-phosphor WOLEDs with two emitters (blue and yellow), three emitters (blue, green/red, and yellow) and four emitters (blue, green, yellow and red) by introducing tris (phenylpyrazole) Iridium [Ir(ppz)3] as interlayer. The results show that appropriate thickness of Ir(ppz)3 interlayer not only can control exciton distribution in the emission zone, but also can improve the spectra stability. In particular, one efficient four-color device with double-interlayer yields fairly high CRI of 92 and 90, ideal CCT of 3703 K and 3962 K at illumination-relevant luminance of 100 cd m–2 and 1000 cd m–2, respectively, which is very appropriate to indoor lighting application. By further employing appropriate hosts to regulate the carrier injection, ultrahigh stable four-color devices with applicable CRI are finally achieved.

We report hybrid white organic light-emitting diodes (WOLEDs) based on yellow phosphorescence of Iridium (III) bis(4-phenylthieno[3, 2-c]pyridinato-N, C2′)acetylacetonate (PO-01) and blue fluorescence of p-bis (p-N,N-diphenyl-amino-styryl) benzene (DSA-Ph). By introducing appropriate thickness of tris (phenylpyrazole) Iridium [Ir(ppz)3] as interlayer between the adjacent emission layers, taking advantage of the assistance electron-transporting behavior of guest molecules, efficient WOLEDs with negligible efficiency roll-off and high color stability are obtained. The best device shows a peak current efficiency of 21.0 cd/A at 2, 300 cd/m2 and the value can be maintained as high as 20.1 cd/A at 9, 300 cd/m2. Furthermore, the Commission Internationale de L'Eclairage coordinate of the corresponding device only changes marginally from (0.41, 0.46) to (0.40, 0.46) over 103–104 cd/m2.Highlights► Hybrid WOLEDs are obtained based on efficient yellow phosphorescence PO-01 and blue fluorescence DSA-Ph. ► Balanced carrier injection is realized by employing appropriate emitters to build transporting-channels for electrons. ► The best device shows a peak current efficiency of 21.0 cd/A at 2, 300 cd/m2.

•LiF n-doping layer and MoOx p-doping layer were used as charge generating units.•The device performance was improved by optimizing the thickness of n-doping layer.•High luminance and efficiency were both achieved at a very low current density.•The device showed rather stable spectra over a wide range of luminance.We have demonstrated color-stable and efficient tandem organic light-emitting devices (OLEDs) using 4,7-diphenyl-1,10-phenanthroline (Bphen):LiF/4,4′,4″-tris(N-3-methylphenyl-N-phenyl-amino)triphenylamine (m-MTDATA):molybdenum oxide (MoOx) as charge generating unit (CGU), which has the advantages of air stability and ease of fabrication; the working mechanism of Bphen:LiF/m-MTDATA:MoOx is also discussed through analysis of the electrical and spectral emission properties of tandem devices with different CGUs. The performance of tandem white OLED, comprising blue and yellow phosphorescent EL units, can be improved by optimizing the thickness of Bphen:LiF layer. The device comprised of 30 nm Bphen:LiF layer has a maximum current efficiency of 38.7 cd/A and it can still maintain 24.6 cd/A at the luminance of 10,370 cd/m2. Moreover, the Commission Internationale de L'Eclairage (CIE) coordinates of the device are rather stable and the variation is only (± 0.003, ± 0.007) over a wide range of luminance (100–13,000 cd/m2).

•Hybrid white organic light-emitting diodes are demonstrated.•A detailed study is carried out to investigate the spectra variation in the devices.•The maximum current efficiency of the device reaches 24.7 cd/A at 1000 cd/m2.We have demonstrated white organic light-emitting diodes (WOLEDs) based on a yellow phosphorescence iridium complex and an efficient blue fluorescence. The property of the blue fluorescence is studied and the pure blue device can obtain a peak current efficiency (CE) of 8.5 cd/A. The best obtained WOLED shows a peak CE of 24.7 cd/A at 1000 cd/m2 with Commission Internationale De L'Eclairage coordinate of (0.44, 0.48), while WOLEDs based on different architectures exhibit good color stability.

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.Graphical abstractHighlights► We have investigated WOLEDs with different hole transport buffer layers. ► WOLEDs with low operating voltage and high color stability are demonstrated. ► WOLEDs with low efficiency roll-off are achieved utilizing an additional orange EML.

A wide band-gap polymer derived from 3,6-carbazole and tetraphenylsilane has been employed as a host for ternary phosphorescent iridium complexes in a white polymer light-emitting device (PLED). By studying both electroluminescence and photoluminescence of the doped polymeric light-emitting layer, it was found that both energy transfer and direct trapping attribute to the emission. Balanced white light with an external quantum efficiency of 4.26%, corresponding to a luminous efficiency of 8.55 cd/A and a power efficiency of 4.55 lm/W, as well as a color rendering index of 82 were achieved at 100 cd/m2.