高效叠层OLED白光器件进展

叠层有机发光二极管（Organic Light-Emitting Diode,OLED）白光器件具备低功耗、高亮度、高色域等性能优势。然而,由于效率、寿命及驱动电压等性能仍有较大改进空间,叠层结构的材料及电学结构仍需进一步优化。本文重点介绍叠层OLED白光器件的最新研究进展,总结了三类电荷产生层（Charge Generation Layer,CGL）在工程化应用中存在的问题以及其非破坏性检测方法;综述高效叠层OLED白光器件的“全磷光体系”、“并行通道激子收集”及“混合磷光热活性型延迟荧光（Thermally Activated Delayed Fluorescence,TADF）体系”最新研究成果,对器件寿命问题进行总结,探讨分析“分级掺杂”、“四色混合TADF体系”等从结构方面提出优化方案,并针对不同发光材料体系中的CGL材料及结构综述叠层OLED白光器件实现较低工作电压的技术方法,最后对叠层OLED白光器件的材料和结构提出改进建议。     

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.     

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.     

Efficient solution-processed blue phosphorescent organic light-emitting diodes with halogen-free solvent to optimize the emissive layer morphology

Efficient solution-processed blue phosphorescent organic light-emitting diodes (OLEDs) featuring with halogen-free solvent processing are fabricated in this study. The organic molecule 3,6-bis(diphenylphosphoryl)-9-(4′-(diphenylphosphoryl) phenyl)-carbazole (TPCz) that possesses good solubility in halogen-free polar solvents is selected to serve as the host of blue phosphorescent iridium(III) [bis(4,6-difluorophenyl)-pyridinato-N,C²]-picolinate (FIrpic) dopant. The morphology of the TPCz:FIrpic emissive layer prepared with different polar solvents including chlorobenzene (CB), n-butanol (ButA) and isopropanol (IPA) and the effect on their electroluminescent performance have been investigated in detail. It is found that the more polar halogen-free solvent IPA restrains the FIrpic aggregation and renders a more densely packed emissive layer as compared to the CB-processed counterpart, which results in the enhanced electroluminescent performance. The luminous efficiency and power efficiency of the blue phosphorescent OLEDs prepared with CB are merely 5.7 cd/A and 3.3 lm/W, respectively. When using more polar halogen-free solvent IPA, the efficiencies are enhanced to 22.3 cd/A and 15.6 lm/W, about 2.9 and 3.7-time increment, respectively. This work provides an approach to fabricate efficient solution-processed phosphorescent OLEDs with environmental-friendly solvents, which is highly required in large-scale solution-processed manufacturing.     

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.     

Color stable multilayer all-phosphor white organic light-emitting diodes with excellent color quality

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.     

高效率磷光与荧光相结合的白色有机发光器件

制作了一种白色有机电致发光器件(WOLED)。将红光[Ir(piq)2(acac)]及绿光[Ir(ppy)3]磷光掺杂染料分别掺入到母体CBP中,在2种磷光发光层间插入蓝光材料DPVBi,引入电子传输能力强的BPhen作为电子注入层和空穴阻挡层,通过改变蓝光发光层的厚度,得到了高效率的WOLED,最大电流效率可达17.6cd/A,最大功率效率达13.7lm/W,最大亮度达27525cd/m2,当电压从4V变化到12V时,色坐标从(0.54,0.35)变化到(0.30,0.31),基本处于白光区。器件的特点在于DPVBi的存在阻挡了2种磷光材料间的能量转移,色度可以通过简单地调整DPVBi的厚度,避免使用稀有的蓝光磷光材料和与其相匹配的母体材料,同时又可以保持较高的发光效率。     

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.     

High-efficiency fluorescent white organic light-emitting device with double emissive layers

The authors demonstrate a fluorescent white organic light-emitting device (WOLED) with double emissive layers. The yellow and blue dyes, 5,6,11,12-tetraphenylnaphthacene and N-(4-((E)-2-(6-((E)-4-(diphenylamino)styryl)naphthalen-2-yl)vinyl)phenyl)-N-phenylbenzenamine, are doping into the same conductive host material, N,N′-dicarbazolyl-4-4′-biphenyl). The maximum luminance and power efficiency of the WOLED are 14.6 cd/A and 9.5 lm/W at 0.01 mA/cm², with the maximum brightness of 20 100 cd/m² at 17.8 V. The Commission International de L’Éclairage coordinates change slightly from (0.27, 0.37) to (0.28, 0.36), as the applied voltage increases from 6 V to 16 V. The high efficiencies can be attributed to the balance between holes and electrons.     

Color temperature tunable white organic light-emitting diodes

In this work, we demonstrated color-tunable white organic light-emitting diodes by stacking upper orange transparent and lower blue bottom emission organic light-emitting diodes (OLEDs). By independently operating each OLED, it was possible to tune the color temperature in a range of 1500–10,000 K, which covers the full Planckian locus in the 1931 CIE space. In designing stable and efficient OLEDs, in addition to the electrical characteristics, the importance of internal microcavity was emphasized and implemented. In fabricating the upper transparent OLED, special attention was paid to the capping layer for enhancing the emission. Our results presented a general guideline that is practically useful in designing high-performance color-tunable OLEDs with transparent OLEDs.     

High efficiency green phosphorescent top-emitting organic light-emitting diode with ultrathin non-doped emissive layer

Ultrathin non-doped emissive layer (EML) has been employed in green phosphorescent top-emitting organic light-emitting diodes (TOLEDs) to take full advantages of the cavity standing wave condition in a microcavity structure. Much higher out-coupling efficiency has been observed compared to conventional doped EML with relatively wide emission zone. A further investigation on dual ultrathin non-doped EMLs separated by a special bi-layer structure demonstrates better charge carrier balance and improved efficiency. The resulting device exhibits a high efficiency of 125.0 cd/A at a luminance of 1000 cd/m² and maintains to 110.9 cd/A at 10,000 cd/m².     

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 and superior color-stability white phosphorescent organic light-emitting diodes based on double mixed-host emission layers

We demonstrate high-efficiency and superior color-stability white phosphorescent organic light-emitting diodes based on double blue mixed-host emission layers (EMLs) with different mixed ratios. The key feature of the concept is to introduce double blue mixed-host EMLs with an orange ultrathin layer sandwiched between them. The improved white device without spacer or interlayer achieves superior color-stability and reduced efficiency roll-off, which are consistent with the good ambipolar conductivity of the mixed-host layer. Moreover, peak efficiency of 40.8 lm/W and low turn-on voltage of 2.71 V are realized. The double mixed-host EMLs concept proves to be quite useful in achieving excellent device performance.     

具有不同掺杂层厚度的荧光与磷光材料相结合的高效白色有机发光器件

实验制作一种多层白色有机发光器件(WOLED)。将 绿光磷光材料和红光磷光材料 Ir(piq)₂(acac)共掺到母体BPhen中作为绿光和红光发光层;荧光材料DPVBi作为蓝光发 光层,通过改变掺杂层的厚度,得到了高效率的白色WOLED。器件的最大电流效 率可达4.55cd/ A,14 V时亮度达8489cd/m² ;当电压从4V变化到12 V时,色坐标从(0.52,0.34)变化到(0.34, 0.26),基本处于白光区。此器件的 特点,在于其性能可以通过简单地调整掺杂层的厚度来控制。     

Tailoring electronic structure of organic host for high-performance phosphorescent organic light-emitting diodes

We investigated highly efficient phosphorescent organic light-emitting diodes (PHOLEDs) based on three novel 1,3,5-triazine derivatives as the host materials and two kinds of iridium complexes as the guests, respectively. For comparison, the devices using a common phosphorescent host 4,4′-N,N′-dicarbazolebiphenyl (CBP) have also been fabricated. Results show that the devices using 9-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-9H-carbazole (PTC) and 4-(4,6-diphenoxy-1,3,5-triazin-2-yl)-N,N-diphenylaniline (POTA) as host have better performance than that of CBP. In comparison with the PHOLEDs based on CBP host, PTC- and POTA-based PHOLEDs show significantly lower driving voltages and higher power efficiencies. The high bipolar carrier mobility of the host is found to be critical to this kind of doping system, which would balance the injection of both carriers and improve efficiency.     

Highly efficient pure white phosphorescent organic light-emitting diodes using a deep blue phosphorescent emitting material

A high efficiency pure white phosphorescent organic light-emitting diode was developed by combining a deep blue emitting phosphorescent dopant material with red/green phosphorescent emitting materials. A simple stack structure of blue/red:green was used and tris((3,5-difluoro-4-cyanophenyl)pyridine) iridium was used as a deep blue emitting phosphorescent dopant. A pure white emission with a color coordinate of (0.29, 0.31) and a very high current efficiency of 28 cd/A was obtained after managing the device architecture of the all phosphorescent white devices.     

Simplified phosphorescent organic light-emitting diodes with a WO3-doped wide bandgap organic charge transport layer

The effects of p-type doping of wide bandgap ambipolar 4,4′-N,N′-dicarbazolebiphenyl (CBP) with WO₃ were investigated through detailed electrical device characterization. It was found that, to achieve effective doping for improved hole injection and transport, the doping level should be greater than 20 mol% and the doped layer should be at least 10 nm thick. A large downward shift of the Fermi level in WO₃-doped CBP causes band bending and depletion at the doped/undoped CBP interface, resulting in an additional energy barrier which hampers hole transport. Simplified green phosphorescent organic light-emitting diodes (PhOLEDs) with CBP as the hole transport and host material were fabricated. With a WO₃-doped hole transport layer, the PhOLEDs attained brightness of 11,163 cd/m² at 20 mA/cm², and exhibited an improved reliability under constant-current stressing as compared to undoped PhOLEDs.     

Orange phosphorescent organic light-emitting diodes with high operational stability

In this article we report on the performances of phosphorescent orange organic light-emitting diodes (OLEDs) having a high operational stability. The fabricated devices all consist of a “hybrid” structure, where the hole-injection layer was processed from solution, while the rest of the organic materials were deposited by vacuum thermal evaporation. A device stack having an emissive layer comprising a carbazole-based host TCzMe doped with the orange phosphor tris(2-phenylquinoline)iridium(III) [Ir(2-phq)₃] shows improved efficiencies compared to a the same device with the standard N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-benzidine (NPB) as host material. External quantum efficiency (EQE) up to 7.4% and a power efficiency of 16 lm/W were demonstrated using TCzMe. Most importantly, the operational stability of the device was largely improved, resulting in extrapolated values reaching lifetimes well above 100,000 h at initial luminance of 1000 Cd/m².     

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.     

Acridine derived stable host material for long lifetime blue phosphorescent organic light-emitting diodes

A host material having acridine as a hole transport moiety, 10-(3′-(9H-carbazol-9-yl)-[1,1′-biphenyl]-3-yl)-9,9-dimethyl-9,10-dihydroacridine (CZBPAC), was explored as the host material of phenylimidazole type Ir triplet emitter to realize both high quantum efficiency and stable operational lifetime. The acridine containing CZBPAC was superior to carbazole based host material with the same backbone structure in that it can improve driving voltage, quantum efficiency and lifetime of the blue phosphorescent organic light-emitting diodes simultaneously.