Novel scattering and color converting substrates for simple-structured white organic light-emitting diodes

Implementing rigorous standards for high‐quality white organic light‐emitting diodes (WOLEDs) demands further investigation. Herein, a novel and feasible color remedy strategy (CRS) is proposed in WOLEDs composed of binary‐emitters, to arouse the green‐emission, thereby complementing the spectral deficiency in white‐emission. Thus, the color rendering indexes (CRIs) of binary‐emissive WOLEDs can be boosted from 63 to 80 threshold, and the Commission International de I'Eclairage‐(x, y) coordinates are precisely located inside the American National Standard Institute quadrangles, which can rival the WOLEDs integrating ternary or more emitters. Moreover, it is more feasible for CRS‐based binary‐emissive system to tune white‐emission from cool white‐emission (correlated color temperature (CCT) ≈ 5000 K) to eye‐friendly warm white‐emission (CCT ≈ 2000 K). Meanwhile, benefiting from the reduced energy loss and low driving voltage of CRS zone, all of the CRS‐based WOLEDs with diverse CCTs can exceed 20% external quantum efficiency, and the highest approach 25%, as well as the highest power efficiency beyond 60 lm W−1, which is comparable with those reported employing light‐extracting techniques. In addition, it is evident that CRS‐based WOLEDs also exhibit outstanding color stability within the variation of luminance in several orders of magnitude (50–12 000 cd m−2). Thus, this novel CRS provides an innovative pathway to fabricate high‐quality WOLEDs composed of binary emitters.

Improving CRI of white phosphorescence organic light-emitting diodes by controlling exciton energy transfer in the planar heterojunction

Ultra-simple two color WOLEDs with CRI exceeding 90 based on electron-transporting Bepp2 simultaneously as blue emitter and exciplex acceptor

High efficiency white organic light-emitting diodes (WOLEDs) with simple device architecture are desirable for next-generation light sources. However, it is still challenging in the construction of high-performance WOLEDs with a simple device structure. Based on a thermally activated delayed fluorescence (TADF) blue emitter 2SPAc-MPM and a TADF yellow emitter TXO-TPA, high performance two-color WOLEDs with simple device architecture are demonstrated. Benefiting from efficient energy transfer and wide coverage over the visible spectrum, optimized WOLED devices that have a single emitting layer (s-EML) provide a maximum color rendering index (CRI) and maximum external quantum efficiency (EQE) of 78.1 and 21.8%, respectively. More importantly, with multiple emitting layers (m-EMLs), a maximum EQE of 14.5% and a high CRI of 90.7 can be achieved. These results are among the best for the two-color WOLEDs with two TADF emitters.

Improving power efficiency (PE) and reducing roll-off are of significant importance for the commercialization of white organic light-emitting diodes (WOLEDs) in consideration of energy conservation. Herein, record-beating PE of 130.7 lm W−1 and outstanding external quantum efficiency (EQE) of 31.1% are achieved in all-fluorescence two-color WOLEDs based on a simple sandwich configuration of emitting layer consisting of sky-blue and orange delayed fluorescence materials. By introducing a red fluorescence dopant, all-fluorescence three-color WOLEDs with high color rendering index are constructed based on an interlayer sensitization configuration, furnishing ultrahigh PE of 110.7 lm W−1 and EQE of 30.8%. More importantly, both two-color and three-color WOLEDs maintain excellent PEs at operating luminance with smaller roll-offs than the reported state-of-the-art WOLEDs, and further device optimization realizes outstanding comprehensive performances of low driving voltages, large luminance, high PEs and long operational lifetimes. The underlying mechanisms of the impressive device performances are elucidated by host-tuning effect and electron-trapping effect, providing useful guidance for the development of energy-conserving all-fluorescence WOLEDs.

High efficient and color stable WOLED using double white emissive layer

All non-dopant red–green–blue composing white organic light-emitting diodes

We demonstrate flexible white organic light-emitting diode (WOLED) with high color stability, which combines vacuum deposited blue flexible organic light-emitting diode (FOLED) with red surface color conversion material (CCM). Firstly, we utilize the novel multiple quantum well (MQW), which consists of the alternate layers of 2, 3, 5, 6-Tetrafluoro-7, 7, 8, 8-tetracyano-quinodimethane (F4-TCNQ) and 4, 4', 4"-tris-(N-3-methylphenyl-N-phenylamino) tripheny-lamine (m-MTDATA) as hole injection layer (HIL), through combining a new blue emitting material of N6, N6, N12, N12-tetrap-tolylchrysene-6, 12-diamine (NCA), to fabricate highly efficient blue FOLED. Then, the CCM of 4-(dicyanomethylene)-2-tert-butyl-6-(1, 1, 7, 7-tetramethyljul-oli-dyl-9-en-yl)-4H-pyran (DCJTB) is deposited on the outside of the ITO flexible substrate. Finally, the thickness of the color conversion film (CCF) is optimized to improve the color purity of flexible WOLED. The results demonstrate that at a driving voltage of 7 V, the CIE coordinates of (0.33, 0.27) which are very close to the white equal-energy area are obtained with the thickness of DCJTB of 120 nm. Moreover, the CIE coordinate migrations of the flexible WOLED are less than (±0.02, ±0.02) in a wide range of driving voltages (from 6 V to 11 V) for the device, indicating the flexible WOLED has excellent color stability.

Inexpensive and highly luminescent Cu(I) complexes exhibit great potential as emitters in organic light‐emitting diodes (OLEDs), especially in white OLEDs (WOLEDs). In this work, two compounds 9‐(3‐(quinolin‐4‐yl)phenyl)‐9H‐carbazole (CzPQ) and 9,9′‐(quinoline‐4,6‐diylbis(3,1‐phenylene))bis(9H‐carbazole) (2CzPQ) are designed and synthesized to form Cu(I) complexes in situ by codeposition with copper iodide (CuI). The corresponding OLEDs show an orange red emission with a maximum external quantum efficiency (EQE) of 6.7%. Based on this result, an “on‐and‐off” strategy is proposed to achieve WOLEDs with the combination of Cu(I) complex layer and the pure ligand layer by controlling the shutter of CuI source on and off. The optimized WOLEDs give a maximum EQE of 2.4% with excellent Commission Internationale de L'Eclairage (CIE) coordinates of (0.32, 0.33) and a remarkable color‐rendering index (CRI) of 94. It can be forecasted that the device efficiency would be improved by tuning the chemical structure of the ligand.

With the aim of endowing triplet emitters in the development of organic light-emitting devices (OLEDs) with electron-injection/-transporting (EI/ET) features, the phenylsulfonyl moiety was introduced into the phenyl ring of a 2-phenylpyridine (Hppy) ligand and the yellow phosphorescent heteroleptic iridium(III) complex 1 was developed. It was shown that the SO(2)Ph unit could provide EI/ET character to 1, as indicated from both electrochemical and computational data. Complex 1 is a promising yellow-emitting material for both monochromatic OLEDs and white OLEDs (WOLEDs). The outstanding electronic traits associated with 1, coupled with careful device design, afforded very attractive electroluminescent performances for two-element WOLEDs, including a low turn-on voltage of less than 3.7 V, a maximum brightness of 48,000 cd m(-2), an external quantum efficiency of 13.0%, a luminance efficiency of 34.7 cd A(-1), and a power efficiency of 24.3 Lm W(-1). In addition, a good color rendering index (CRI) of about 74, a stable white color with a Commission Internationale de L'Eclairage (CIE(x,y)) variation of Δ(x, y) < ±(0.02, 0.02), and a correlated color temperature higher than 5130 K were obtained. These encouraging results indicate the potential of these WOLEDs as good candidates for warm indoor lighting sources, as well as the critical contribution of such key EI/ET properties to triplet emitters to advance new OLED research.

Enhance efficiency of blue and white organic light emitting diodes with mixed host emitting layer using TCTA and 3TPYMB

Developing deep-blue emitters for organic light-emitting diodes (OLEDs) is critical but challenging, which requires a good balance between light color, exciton utilization, and photoluminescence quantum yield (PLQY) of solid film. Herein, a high-quality deep-blue emitter, abbreviated 2TriPE-CzMCN, is designed by introducing an aggregation-induced emission (AIE) group into a crossed long-short axis (CLSA) skeleton. Theoretical and experimental investigations reveal that the CLSA molecular design can achieve a balance between deep-blue emission and triplet-excitons utilization, while the high PLQY of the solid film resulting from the AIE feature helps to improve the performance of OLEDs. Consequently, when 2TriPE-CzMCN is used as the emitting dopant, the OLED exhibits a deep-blue emission at 430nm with a record-high maximum external quantum efficiency (EQE) of 8.84%. When 2TriPE-CzMCN serves as the host material, the sensitized monochrome orange and two-color white OLEDs (WOLEDs) realize high EL performances that exceed the efficiency limit of conventional fluorescent OLEDs. Moreover, high-performance three-color WOLEDs with a color rendering index (CRI) exceeding 90 and EQE up to 18.08% are achieved by using 2TriPE-CzMCN as the blue-emitting source. This work demonstrates that endowing CLSA molecule with AIE feature is an effective strategy for developing high-quality deep-blue emitters, and high-performance versatile OLEDs can be realized through rational device engineering.

We present highly power-efficient flexible organic light-emitting diodes by combining nanostructured metallic transparent conductor on plastic substrates, leading to a power efficiency over 160 lm/W with angular color stability. Flexible organic light-emitting diodes (OLEDs) hold great promise for future bendable display and curved lighting applications. One key challenge of high-performance flexible OLEDs is to develop new flexible transparent conductive electrodes (TCEs) with superior mechanical, electrical and optical properties. Indium-tin-oxide (ITO) electrode is the most widely used TCE in optoelectronic devices due to its excellent electrical conductivity and light transmission, but the high-temperature fabrication and the brittle nature under repeated bending condition hinder its application on flexible plastic substrates. An additional drawback of using ITO in flexible OLEDs on plastic substrates is the limited light outcoupling efficiency due to the severe trapping loss of the internally emitted photons. Various materials and structures have been proposed to function as flexible TCEs. Metal-dielectric composite electrode (MDCE) has been regarded as an effective TCE for flexible devices in terms of mechanical flexibility, electrical conductivity, optical transparency, and large-area film uniformity. Whilst MDCE may be an ideal candidate to replace ITO, particularly in flexible devices on plastic substrates, two key challenges should be overcome. First, forming an ultrathin metal film (≤ 10 nm) in a MDCE for improving the optical transmission is extremely difficult due to the dewetting problem with an isolated granular morphology, causing the adverse degradation in optical transmittance and electrical conductance with particle plasmon absorption and severe disconnection. Second, an optical microcavity effect is inevitable with the use of a planar MDCE structure, leading to the spectral and angular dependence of the emission characteristics. Herein, we demonstrate a new strategy to achieve a powerful transparent conductive electrode on plastic substrate that combines a quasi-random nanostructured optical coupling layer and an ultrathin metal alloy conduction layer [1,2]. The optimum electrical conductivity, optical manipulation capability, and high tolerance to mechanical bending are realized in this composite electrode, which is favorable for the fabrication of ITO-free flexible OLEDs with state-of-the-art performance on low-refractive-index plastic substrate. The angularly and spectrally independent boost in light outcoupling of white emission is obtained by minimizing the waveguide mode, metallic electrode-related microcavity effect and surface plasmonic loss due to the integrated quasi-random outcoupling structure in the composite electrode[3-5]. The resulting white flexible OLED exhibits the high enhancement in efficiency, e.g., external quantum efficiency ~70% and power efficiency over 160 lm/W. In addition, this composite electrode has a scalable manufacturing potential in large-area flexible electronic systems. Fig. 1. Device structure and performance of flexible white OLEDs. (a-c) Comparison of different transparent electrodes: (a) Optical properties. Inset is the schematic illustration of flexible OLED structure with NMDCE on plastic substrate. (b) Sheet resistance as a function of nominal film thickness. (c) Mechanical stability under continuous bending test. (d-f) Performance characteristics of white flexible OLEDs: (d) Current density and luminance as a function of driving voltage. (e) Current and power efficiencies as a function of luminance. (f) Normalized EL spectra. Inset is the photograph of white-emission flexible OLED. (g-i) Simulations of normalized cross-section intensity field distributions (at 520 nm) with propagation of OLEDs on (g) ITO, (h) MDCE and (i) NMDCE using FDTD method.

Organic white light-emitting diodes using a new DCM derivative as an orange-red doping molecule

Performance of white organic light-emitting diode for portable optical biosensor