Wenfa Xie

White organic light-emitting devices with a bipolar transport layer between blue fluorescent and orange phosphorescent emitting layers

White organic light-emitting devices based on an orange phosphorescent iridium complex, bis(2-(2-fluorphenyl)-1,3-benzothiozolato-N,C2′) iridium (acetylacetonate) [(F-BT)2Ir(acac)] and blue fluorescent 4,4′-bis(2,2′-diphenylvinyl)-1,1′-biphenyl are reported. By introducing a bipolar transport 4,4′N,N′-dicarbazole-biphenyl layer between the fluorescent and the phosphorescent emission layers, additional light emission from (F-BT)2Ir(acac) is observed. The authors attributed it to the elimination of the Dexter energy transfer between the two emitters. Pure white emission with Commission Internationale de I’Eclairage coordinates of (0.33, 0.34) and a maximum luminance of 40960cd∕m2 were obtained. The maximum current efficiency and the color rendering index of the device are 13.4cd∕A and 71, respectively.

Improvement of efficiency and color purity utilizing two-step energy transfer for red organic light-emitting devices

We present organic red light-emitting devices with improved optical and electrical characteristics utilizing the two-step energy transfer. N,N′-diphenyl-N,N′-bis(1-naphthyl)-(1,1′-biphenyl)-4,4′-diamine and tris-(8-hydroxyquinoline) aluminum (Alq) are used as hole and electron-transporting layers, respectively. Quinacridone (QAD) and 4-(dicyanomethylene)-2-t- butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) are codoped into the Alq emitting layer. Compared with devices where the emitting layer is only composed of Alq and DCJTB, the emitting characteristics such as emission efficiency and purity of emission color are greatly improved, and the turn-on voltage is decreased as much as 4 V. We attribute these improvements to the assistant dopant (QAD), which leads to the more efficient energy transfer from Alq to DCJTB.

Color-stable and efficient stacked white organic light-emitting devices comprising blue fluorescent and orange phosphorescent emissive units

We have demonstrated two kinds of stacked white organic light-emitting diodes (WOLEDs) employing tri(8-hydroxyquinoline) aluminum:20 wt %Mg/MoO3 as charge generation layer. White light emission can be obtained by mixing blue fluorescence and orange phosphorescence. Stacked WOLED with individual blue fluorescent and orange phosphorescent emissive units has better color stability and higher efficiency than that with double white emissive units, which is attributed to the avoidance of the movement of charges recombination zone and elimination of the Dexter energy transfer between blue and orange emission layers occurring in the latter. The efficiency of the stacked WOLED is 35.9 cd/A at 1000 cd/m2.

Iridium(III) complexes adopting 1,2-diphenyl-1H-benzoimidazole ligands for highly efficient organic light-emitting diodes with low efficiency roll-off and non-doped feature

Two novel iridium(III) complexes (pbi)2Ir(mtpy) (1) and (pbi)2Ir(pbim) (2) adopting 1,2-diphenyl-1H-benzoimidazole (Hpbi) as cyclometalated ligands were successfully synthesized and characterized. Strong emissions at 501 and 536 nm with high photoluminescence quantum yields of 48% and 91% in CH2Cl2 at 298 K were obtained for 1 and 2, respectively. The quantum chemical calculations and the photophysical properties indicated that the dominant 3MLCT (metal-to-ligand charge-transfer) state mixed with 3LLCT (ligand-to-ligand charge-transfer) and 3LC (ligand-centered 3π–π*) characters contributed to their phosphorescence emissions. Doped organic light-emitting diodes (OLEDs) based on 1 and 2 showed a peak current efficiency of 45.0 cd A−1 and power efficiency of 47.9 lm W−1 accompanied by very low efficiency roll-off values. In their non-doped OLEDs, high efficiencies of 24.4 cd A−1 and 26.3 lm W−1 were achieved as well. These appealing results reveal that complexes 1 and 2 open interesting perspectives for the development of high-performance OLEDs in the future.

Top-emitting white organic light-emitting devices with down-conversion phosphors: Theory and experiment

White top-emitting organic light-emitting devices (TEOLEDs) with down-conversion phosphors are investigated from theory and experiment. The theoretical simulation was described by combining the microcavity model with the down-conversion model. A White TEOLED by the combination of a blue TEOLED with organic down-conversion phosphor 3-(4-(diphenylamino)phenyl)-1-pheny1prop-2-en-1-one was fabricated to validate the simulated results. It is shown that this approach permits the generation of white light in TEOLEDs. The efficiency of the white TEOLED is twice over the corresponding blue TEOLED. The feasible methods to improve the performance of such white TEOLEDs are discussed.

Efficient non-doped phosphorescent orange, blue and white organic light-emitting devices

Efficient phosphorescent orange, blue and white organic light-emitting devices (OLEDs) with non-doped emissive layers were successfully fabricated. Conventional blue phosphorescent emitters bis [4,6-di-fluorophenyl]-pyridinato-N,C2′] picolinate (Firpic) and Bis(2,4-difluorophenylpyridinato) (Fir6) were adopted to fabricate non-doped blue OLEDs, which exhibited maximum current efficiency of 7.6 and 4.6 cd/A for Firpic and Fir6 based devices, respectively. Non-doped orange OLED was fabricated utilizing the newly reported phosphorescent material iridium (III) (pbi)2Ir(biq), of which manifested maximum current and power efficiency of 8.2 cd/A and 7.8 lm/W. The non-doped white OLEDs were achieved by simply combining Firpic or Fir6 with a 2-nm (pbi)2Ir(biq). The maximum current and power efficiency of the Firpic and (pbi)2Ir(biq) based white OLED were 14.8 cd/A and 17.9 lm/W.

High-efficiency and low-efficiency-roll-off single-layer white organic light-emitting devices with a bipolar transport host

High-efficiency single-layer (SL) white organic light-emitting devices (WOLEDs) with a bipolar transport host were fabricated. The SL WOLEDs were achieved by combining blue and orange emission. Compared with the corresponding multilayer WOLEDs, the SL WOLEDs alleviated the efficiency roll-off without compromise of current efficiency due to the broader exciton formation zone and balance of carrier injection and transport. For example, The power efficiency of the SL WOLED based on Iridium (III) bis(4-phenylthieno[3,2-c]pyridinato-N,C2′) acetylacetonate orange emission could reach 20.9 lm/W at 1000 cd/m2, which also could reach 14.5 lm/W at a very high brightness of 5000 cd/m2.

Influence of interlayer on the performance of stacked white organic light-emitting devices

Stacked white organic light-emitting devices (WOLEDs) comprising of blue fluorescent and orange phosphorescent emissive units employing tri(8-hydroxyquinoline) aluminum (Alq3):Mg/MoO3 as charge generation layer are fabricated. The working mechanism of Alq3:Mg/MoO3 is also discussed using a simple method. We demonstrate charge-carrier separation takes place only in MoO3 layer. Stacked WOLED with better performance was obtained by adjusting the thickness of MoO3. The stacked WOLED with efficiency of 39.2 cd/A has excellent color stability with the Commission Internationale de l’Eclairage coordinates only changing from (0.407, 0.405) to (0.398, 0.397) when luminance increases from 22 to 10 000 cd/m2.

Top-emitting quantum dots light-emitting devices employing microcontact printing with electricfield-independent emission

Recent breakthroughs in quantum dot light-emitting devices (QD-LEDs) show their promise in the development of next-generation displays. However, the QD-LED with conventional ITO-based bottom emission structure is difficult to realize the high aperture ratio, electricfield-independent emission and flexible full-color displays. Hence, we demonstrate top-emitting QD-LEDs with dry microcontact printing quantum dot films. The top-emitting structure is proved to be able to accelerate the excitons radiative transition rate, then contributing to stable electroluminescent efficiency with a very low roll-off, and preventing spectra from shifting and broadening with the electric field increases. The results suggest potential routes towards creating high aperture ratio, wide color gamut, color-stable and flexible QD-LED displays.

Improving efficiency roll-off in phosphorescent OLEDs by modifying the exciton lifetime

Studies on phosphorescent organic light emitting devices (PhOLEDs) with phosphorescent emitter, fac-tris (2-phenylpyridine) iridium (Ir(ppy)(3)), show that the lifetime of triplet exciton is modified by surface plasmon coupling of Au nanoparticles (NPs). Interactions between the triplet exciton and gold (Au) nanoparticles (NPs) lead to a decrease in the exciton lifetime and result in the spontaneous emission decay rate of triplet exciton faster as the distance between the phosphorescent material and the Au NPs becomes smaller. This interaction reduces the efficiency roll-off of Au NPs containing device. These results provide new guides for device design to improve efficiency performance.