Doping-free orange and white phosphorescent organic light-emitting diodes with ultra-simply structure and excellent color stability

We demonstrate simplified doping-free orange phosphorescent organic light-emitting diodes (OLEDs) based on ultrathin emission layer. The optimized orange device has the maximum current efficiency of 52.1 cd/A and power efficiency of 36.3 lm/W, respectively. Efficient simplified doping-free white OLEDs employing blue and orange ultrathin emission layers have excellent color stability, which is attributed to the avoidance of the movement of charges recombination zone and no differential color aging. One white device exhibits high efficiency of 33.6 cd/A (30.1 lm/W). Moreover, the emission mechanism of doping-free orange and white OLEDs is also discussed.     

High-efficiency phosphorescent organic light-emitting devices with low efficiency roll-off using a thermally activated delayed fluorescence material as host

Phosphorescent organic light-emitting devices (PHOLEDs) with high efficiency and low efficiency roll-off were fabricated. The emissive layer was composed of a thermally activated delayed fluorescence (TADF) material 4,5-bis(carbazol-9-yl)-1,2-dicyanobenzene (2CzPN) as host and an orange iridium complex bis(4-tert-butyl-2-phenylbenzothiozolato-N,C^2′)iridium(III)(acetylacetonate) [(tbt)₂Ir(acac)] as dopant. At a low dopant concentration of 1 wt%, a PHOLED without light extraction optimization achieved a maximum power efficiency of 42.1 lm/W, a luminance efficiency of 77.9 cd/A and an external quantum efficiency (EQE) of 26.8%, respectively. Meanwhile, the EQE maintained 26.6% at 1000 cd/m² and 25.8% at 5000 cd/m², respectively. Moreover, a critical current density of 300 mA/cm² was realized, indicating significantly improved efficiency roll-off. The efficient utilization of triplet excitons on 2CzPN for phosphorescence via reverse inter-system crossing of 2CzPN followed by Fӧrster resonance energy transfer from 2CzPN to (tbt)₂Ir(acac) is responsible for the superior performance.     

Suppression of efficiency roll-off in highly efficient blue phosphorescent organic light-emitting devices using novel iridium phosphors with good electron mobility

High-efficiency blue organic light-emitting diodes were reported by adopting two novel iridium phosphors. Due to phosphoryl moiety in ancillary ligands, both complexes (dfppy)₂Ir(ppp) and (dfppy)₂Ir(dpp) (dyppy = 2-(2,4-difluorophenyl)pyridine, ppp = phenyl(pyridin-2-yl)phosphinate, dpp = dipyridinylphosphinate) own high electron mobility which can balance the injection and transport of carriers. Furthermore, the double light-emitting layers with TcTa (4,4′,4″-tris(carbazol-9-yl)triphenylamine) and 26DCzPPy (2,6-bis(3-(carbazol-9-yl)phenyl)pyridine) hosts broaden the exciton formation zone and suppress efficiency roll-off. The optimized double light-emitting layers devices exhibited decent performances with peak current efficiency near 50 cd/A and external quantum efficiency above 20% as well as negligible efficiency roll-off.     

Improved efficiency roll-off at high brightness in simplified phosphorescent organic light emitting diodes with a crossfading-host

We report a high efficiency and low efficiency roll-off green phosphorescent organic light emitting diode using both hole- and electron-transporting host materials in a crossfading profile. To eliminate the energy barrier and reduce the charge carrier accumulation, the host materials are used as transporting layers as well, which also simplifies the device fabrication. It is found out that the recombination zone of gradient doping host sample is not only wider but also extended at high current density, which contributes to the suppressed efficiency roll-off at high luminance. An external quantum efficiency of 21.0% at 1000 cd/m² is obtained, and maintains to 19.3% at 10,000 cd/m².     

High efficiency green phosphorescent organic light-emitting diodes with a low roll-off at high brightness

Highly efficient green phosphorescent organic light-emitting diodes (PHOLEDs) with low efficiency roll-off at high brightness have been demonstrated with a novel iridium complex. The host material 1,3-bis(carbazol-9-yl)benzene (mCP) with high triplet energy is also used as the hole transporting layer to avoid carrier accumulation near the exciton formation interface and reduce exciton quenching. It provides a new approach for easily fabricating PHOLED with high triplet energy emitter. Moreover, the hole blocking layer is extended into the light emitting layer to form a co-host, realizing better control of the carrier balance and broader recombination zone. As a consequence, a maximum external quantum efficiency of 20.8% and current efficiency of 72.9 cd/A have been achieved, and maintain to 17.4% and 60.7 cd/A even at 10,000 cd/m², respectively.     

Efficient single layer RGB phosphorescent organic light-emitting diodes

We report efficient single layer red, green, and blue (RGB) phosphorescent organic light-emitting diodes (OLEDs) using a “direct hole injection into and transport on triplet dopant” strategy. In particular, red dopant tris(1-phenylisoquinoline)iridium [Ir(piq)₃], green dopant tris(2-phenylpyridine)iridium [Ir(ppy)₃], and blue dopant bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl)iridium [FIrpic] were doped into an electron transporting 1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBi) host, respectively, to fabricate RGB single layer devices with indium tin oxide (ITO) anode and LiF/Al cathode. It is found that the maximum current efficiencies of the devices are 3.7, 34.5, and 6.8 cd/A, respectively. Moreover, by inserting a pure dopant buffer layer between the ITO anode and the emission layer, the efficiencies are improved to 4.9, 43.3, and 9.8 cd/A, respectively. It is worth noting that the current efficiency of the green simplified device was as high as 34.6 cd/A, even when the luminance was increased to 1000 cd/m² at an extremely low applied voltage of only 4.3 V. A simple accelerated aging test on the green device also shows the lifetime decay of the simplified device is better than that of a traditional multilayered one.     

Efficient non-doped monochrome and white phosphorescent organic light-emitting diodes based on ultrathin emissive layers

Efficient red, orange, green and blue monochrome phosphorescent organic light-emitting diodes (OLEDs) with simplified structure were fabricated based on ultrathin emissive layers. The maximum efficiencies of red, orange, green and blue OLEDs are 19.3 cd/A (17.3 lm/W), 45.7 cd/A (43.2 lm/W), 46.3 cd/A (41.6 lm/W) and 11.9 cd/A (9.2 lm/W). Moreover, efficient and color stable white OLEDs based on two complementary colors of orange/blue, three colors of red/orange/blue, and four colors of red/orange/green/blue were demonstrated. The two colors, three colors and four colors white OLEDs have maximum efficiencies of 30.9 cd/A (27.7 lm/W), 30.3 cd/A (27.2 lm/W) and 28.9 cd/A (26.0 lm/W), respectively. And we also discussed the emission mechanism of the designed monochrome and white devices.     

Quenching in single emissive white phosphorescent organic light-emitting devices

To explore energy loss by diffusive triplet excitons in single emissive white phosphorescent organic light-emitting devices, the authors investigated collisional quenching between the electron transport materials 4,7-diphenyl-1,10-phenanthroline (Bphen), 2′,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi), or 1,3,5-tri(3-pyrid-3-yl-phenyl)benzene (TmPyPB) and the blue phosphorescent material, 3,5-difluoro-2-(2-pyridyl)-phenyl-(2-carboxypyridyl) Iridium III (FIrpic) spectroscopically in solution. The luminous efficiency and the external quantum efficiency (EQE) of an emissive white phosphorescent organic light-emitting device, in which TmPyPB acted as the electron transport material, was found to be greater than those of devices prepared using Bphen or TPBi due to the lack of collisional quenching. In addition, it was found that to prevent triplet exciton loss, an ETL material should have a low bimolecular quenching rate constant k_q of less than 1.458 × 10⁷ s⁻¹ M⁻¹, which is the k_q of TmPyPB.     

Low driving voltage white organic light-emitting diodes with high efficiency and low efficiency roll-off

Single emission layer white organic light-emitting diodes (WOLEDs) showing high color stability, low turn-on voltage, high efficiency and low efficiency roll-off by incorporating iridium(III) bis[(4,6-difluo-rophenyl)-pyridinato-N,C2] (FIrpic) and bis(2-phenylbenzothiazolato) (acetylacetonate)iridium(III) (Ir(BT)₂(acac)) phosphors dyes have been demonstrated. Our WOLEDs without any out-coupling schemes as well as n-doping strategies show low operating voltages, low turn-on voltage (defined for voltage to obtain a luminance of 1 cd/m²) of 2.35 V, 79.2 cd/m² at 2.6 V, 940.5 cd/m² at 3.0 V and 10 300 cd/m² at 4.0 V, respectively, and achieve a current efficiency of 40.5 cd/A, a power efficiency of 42.6 lm/W at a practical brightness of 1000 cd/m², and a low efficiency roll-off 14.7% calculated from the maximum efficiency value to that of 5000 cd/m². Such improved properties are attributed to phosphors assisted carriers transport for achieving charge carrier balance in the single light-emitting layer (EML). Meanwhile the host–guest energy transfer and direct exciton formation process are two parallel pathways serve to channel the overall excitons to dopants, greatly reduced the unfavorable energy losses.     

高效率非掺杂型白色有机电致发光器件

制备了基于rubrene超薄层和NPBX做激子阻挡层的高效率的非掺杂型白色有机电致发光器件.器件结构为:ITO/2T-NATA(20 nm)/NPBX(25-d nm)/rubrene(0.2 nm)/NPBX(d nm)/DPVBi(30 nm)/Alq(30 nm)/LiF(0.5 nm)/Al.器件的电致发光光谱依靠激子阻挡层NPBX厚度d的变化而变化,当NPBX厚度d为5 nm时,器件色坐标从7 V变化到16 V时均在白光的中心区域,有最大电流效率7.91 cd/A(V=7 V)和最大亮度13 540 cd/m2 (V=16 V).     

High-performance green phosphorescent top-emitting organic light-emitting diodes based on FDTD optical simulation

We have successfully applied finite-difference time-domain (FDTD) method in top-emitting organic light-emitting diodes (TOLEDs) for structure optimization, demonstrating good agreement with experimental data. A mixed host with both hole transport and electron transport materials is employed for the green phosphorescent emitter to avoid charge accumulation and broaden the recombination zone. The resulting TOLEDs exhibit ultra-high efficiencies, low current efficiency roll-off, and a highly saturated color, as well as hardly detectable spectrum shift with viewing angles. In particular, a current efficiency of 127.0 cd/A at a luminance of 1000 cd/m² is obtained, and maintains to 116.3 cd/A at 10,000 cd/m².     

一种新的多发光层白色有机电致发光器件

采用多发光层结构,将一种新型的黄橙色荧光染料2-溴-4-氟苯乙烯-8-羟基喹啉锌(BFHQZn,(E)-2-(2-bronw-4-fluorostyryl)quinolato-Zinc)与蓝色9,10-二-2-蔡蔥(ADN)组合在一起实现白光.研究了插入4,4-N,N-二咔唑联苯(CBP)对器件色度的影响,通过改变发光...     

High-efficiency blue and white organic light-emitting devices by combining fluorescent and phosphorescent blue emitters

High-efficiency blue and white organic light-emitting devices (OLEDs) combined fluorescent and phosphorescent blue emitters were reported. The hybrid blue OLED showed better color purity than that of all phosphorescent device without sacrificing efficiency. The maximum power efficiency of the blue device could reach 23.5 lm/W with the CIE coordinates of (0.163, 0.325). High-efficiency white OLED with maximum power efficiency of 50.6 lm/W was obtained by combined such hybrid blue device and ultrathin phosphorescent yellow emitter. At the practical brightness of 1000 cd/m², the power efficiency of the white device was 28.3 lm/W with a low voltage of 3.37 V and CIE coordinates of (0.40, 0.44). The excitons recombination zone was adjusted by the introduction of the fluorescent blue emitter which resulting a relative high color rendering index and power efficiency of the white device.     

Deep blue phosphorescent organic light-emitting diodes with excellent external quantum efficiency

Highly efficient deep blue phosphorescent organic light-emitting diodes (PHOLEDs) using two heteroleptic iridium compounds, (dfpypy)₂Ir(acac) and (dfpypy)₂Ir(dpm), as a dopant and 9-(3-(9H-carbazol-9-yl)phenyl)-9H-carbazol-3-yl)diphenylphosphine oxide as a host material have been developed. The electroluminescent device of (dfpypy)₂Ir(dpm) at the doping level of 3 wt% shows the best performance with external quantum efficiency of 18.5–20.4% at the brightness of 100–1000 cd/m² and the color coordinate of (0.14, 0.18) at 1000 cd/m².     

Improved efficiency of indium-tin-oxide-free flexible organic light-emitting devices

An indium-tin-oxide (ITO)-free flexible organic light-emitting device (OLED) with improved efficiency has been demonstrated by employing a template stripping process to create an ultrasmooth PEDOT: PSS anode on a photopolymer substrate. The device performance has been improved owing to lowered surface roughness of the PEDOT: PSS anode. A 38% enhancement in efficiency has been obtained. The ITO-free OLEDs on the polymer substrate have shown flexibility, and the device is free of cracks and dark spots under small bending radius. Moreover, the elimination of the H₂SO₄ residues on the surface of the H₂SO₄-treated PEDOT: PSS by the template stripping has demonstrated its beneficial effect on the device stability.     

Management of charge carriers and excitons for efficient and color-stable white phosphorescent organic light-emitting diodes with simplified structure

We have demonstrated color-stable and highly efficient simplified white phosphorescent organic light-emitting diodes. The key feature is the use of a novel approach to confine the distribution of charge carriers and excitons across the whole blue emission layer. The resulting two-color white device has the maximum power efficiency and current efficiency of 45.5 lm/W and 43.5 cd/A with a very low color shift over a wide range of luminance. By systematically investigating the working mechanisms, we found that the ambipolar charge carrier transport ability of co-host layer which ensures the distribution of excitons to form in the whole blue emission layer was the critical factors for constructing color-stable white devices. Our results show that simplified white devices based on two organic materials achieving excellent color stability are possible.     

通过引入电子阻挡层的高效率的有机磷光白光器件

以CBP作为母体材料,绿色磷光染料Ir(ppy)3作为敏化剂,以荧光染料rubrene作为受主,制备了结构为ITO/2T-NATA(25 nm)/ NPBX (25-d nm)/ CBP:5%Ir(ppy)3:0.5%Rubrene(8 nm)/NPBX(d nm)/DPVBi(30 nm)/TPBi(20 nm)/Alq(10 nm)/LiF(1 nm)/Al的白光器件.在器件中,敏化剂Ir(ppy)3、荧光染料rubrene的浓度分别为5.0 wt%和0.5 wt%,发光层的厚度选择8 nm,通过调整两层NPBX的厚度来改善器件的性能,得到了比较理想的白光发射.当d的厚度为10 nm 时,器件在7 V的电压下最大电流效率达到11.2 cd/A,在17 V的电压下其最大亮度达到28 170 cd/m2,色坐标为(0.37,0.42),处于白光区.     

High-efficiency orange and white phosphorescent organic light-emitting diodes based on homojunction structure

We demonstrate high-efficiency orange and white phosphorescent organic light-emitting diodes based on homojunction structure. Excellent performance is realized by using step-graded p- and n-type doping structure in orange homojunction device. The resulting orange homojunction device exhibits a maximum current efficiency of 30.0 cd/A and low efficiency roll-off. The improvements are mainly attributed to the utilization of step-graded doped profile, which facilitates balanced charge carrier injection and transport. Moreover, one optimized white homojunction device based on two complementary colors shows a maximum efficiency of 15.4 cd/A, and superior color-stability in a wide range of luminance.     

高效高亮度硅基顶发射有机电致发光器件的研制

以半透明超薄金属银作为阴极,紫外臭氧处理的厚金属银作为阳极,制备了高效率高亮度的黄光硅基顶发射有机发光器件。当电压为9V时,器件的最大电流效率为4.9cd/A,当电压为17V时,器件的亮度达到14 040cd/m2。通过增加掺杂浓度及阳极厚度对器件结构进一步优化后,器件性能显著提高,其电流效率在外加电压为10V时达到11cd/A,相应亮度为21 748cd/m2.顶发射器件中存在的微腔效应能有效提高器件的发光效率以及亮度,但是也会使器件的共振波长随着观察视角的增大而蓝移。由于采用合适的发光材料,本实验制备的器件的发光峰值在0°~75°视角范围内几乎没有变化。