Achieving High Efficiency and Improved Stability in ITO‐Free Transparent Organic Light‐Emitting Diodes with Conductive Polymer Electrodes

Efficient transparent organic light‐emitting diodes (OLEDs) with improved stability based on conductive, transparent poly(3,4‐ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) electrodes are reported. Based on optical simulations, the device structures are carefully optimized by tuning the thickness of doped transport layers and electrodes. As a result, the performance of PEDOT:PSS‐based OLEDs reaches that of indium tin oxide (ITO)‐based reference devices. The efficiency and the long‐term stability of PEDOT:PSS‐based OLEDs are significantly improved. The structure engineering demonstrated in this study greatly enhances the overall performances of ITO‐free transparent OLEDs in terms of efficiency, lifetime, and transmittance. These results indicate that PEDOT:PSS‐based OLEDs have a promising future for practical applications in low‐cost and flexible device manufacturing.     

Highly Transparent and Flexible Organic Light‐Emitting Diodes with Structure Optimized for Anode/Cathode Multilayer Electrodes

In this study, a dielectric layer/metal/dielectric layer (multilayer) electrode is proposed as both anode and cathode for use in the fabrication of transparent and flexible organic light‐emitting diodes (TFOLEDs). The structure of multilayer electrodes is optimized by systematic experiments and optical calculations considering the transmittance and efficiency of the device. The details of the multilayer electrode structure are [ZnS (24 nm)/Ag (7 nm)/MoO₃ (5 nm)] and [ZnS (3 nm)/Cs₂CO₃ (1 nm)/Ag (8 nm)/ZnS (22 nm)], as anode and cathode, respectively. The optimized TFOLED design is fabricated on a polyethylene terephthalate (PET) substrate, and the device shows high transmittance (74.22% around 550 nm) although the PET substrate has lower transmittance than glass. The TFOLEDs operate normally under compressive stress; degradation of electrical characteristics is not observed, comparable to conventional OLEDs with ITO and Al as electrodes. In addition, because the fabricated TFOLEDs show a nearly Lambertian emission pattern and a negligible shift of Commission International de l'Eclairage (CIE) coordination, it is concluded that the fabricated TFOLEDs are suitable for use in displays.     

Device Physics of White Polymer Light‐Emitting Diodes

The charge transport and recombination in white‐emitting polymer light‐ emitting diodes (PLEDs) are studied. The PLED investigated has a single emissive layer consisting of a copolymer in which a green and red dye are incorporated in a blue backbone. From single‐carrier devices the effect of the green‐ and red‐emitting dyes on the hole and electron transport is determined. The red dye acts as a deep electron trap thereby strongly reducing the electron transport. By incorporating trap‐assisted recombination for the red emission and bimolecular Langevin recombination for the blue emission, the current and light output of the white PLED can be consistently described. The color shift of single‐layer white‐emitting PLEDs can be explained by the different voltage dependencies of trap‐assisted and bimolecular recombination.     

Candle Light‐Style Organic Light‐Emitting Diodes

In response to the call for a physiologically‐friendly light at night that shows low color temperature, a candle light‐style organic light emitting diode (OLED) is developed with a color temperature as low as 1900 K, a color rendering index (CRI) as high as 93, and an efficacy at least two times that of incandescent bulbs. In addition, the device has a 80% resemblance in luminance spectrum to that of a candle. Most importantly, the sensationally warm candle light‐style emission is driven by electricity in lieu of the energy‐wasting and greenhouse gas emitting hydrocarbon‐burning candles invented 5000 years ago. This candle light‐style OLED may serve as a safe measure for illumination at night. Moreover, it has a high color rendering index with a decent efficiency.     

Surface‐Directed Phase Separation of Conjugated Polymer Blends for Efficient Light‐Emitting Diodes

The ability to control organic‐organic interfaces in conjugated polymer blends is critical for further device improvement. Here, we control the phase separation in blends of poly(9,9‐di‐n‐octylfluorene‐alt‐benzothiadiazole) (F8BT) and poly(9,9‐di‐n‐octylfluorene‐alt‐(1,4‐phenylene‐((4‐sec‐butylphenyl)imino)‐1,4‐phenylene) (TFB) via chemical modification of the substrate by microcontact printing of octenyltrichlorosilane molecules. The lateral phase‐separated structures in the blend film closely replicate the underlying micrometer‐scale chemical pattern. We found nanometer‐scale vertical segregation of the polymers within both lateral domains, with regions closer to the substrate being substantially pure phases of either polymer. Such phase separation has important implications for the performance of light‐emitting diodes fabricated using these patterned blend films. In the absence of a continuous TFB wetting layer at the substrate interface, as typically formed in spin‐coated blend films, charge carrier injection is confined in the well‐defined TFB‐rich domains. This confinement leads to high electroluminescence efficiency, whereas the overall reduction in the roughness of the patterned blend film results in slower decay of device efficiency at high voltages. In addition, the amount of surface out‐coupling of light in the forward direction observed in these blend devices is found to be strongly correlated to the distribution of periodicity of the phase‐separated structures in the active layer.     

Electron‐Enhanced Hole Injection in Blue Polyfluorene‐Based Polymer Light‐Emitting Diodes

It has recently been reported that, after electrical conditioning, an ohmic hole contact is formed in poly(9,9‐dioctylfluorene) (PFO)‐based polymer light‐emitting diodes (PLED), despite the large hole‐injection barrier obtained with a poly(styrene sulfonic acid)‐doped poly(3,4‐ethylenedioxythiophene) (PEDOT:PSS) anode. We demonstrate that the initial current at low voltages in a PEDOT:PSS/PFO‐based PLED is electron dominated. The voltage at which the hole injection is enhanced strongly depends on the electron‐transport properties of the device, which can be modified by the replacement of reactive end groups by monomers in the synthesis. Our measurements reveal that the switching voltage of the PLED is governed by the electron concentration at the PEDOT:PSS/PFO contact. The switching effect in PFO is only observed for a PEDOT:PSS hole contact and not for other anodes such as indium tin oxide or Ag.     

Improved Operational Stability of Polymer Light‐Emitting Diodes Based on Silver Nanowire Electrode Through Pre‐Bias Conditioning Treatment

For the first time, highly efficient and flexible polymer light emitting diodes (PLEDs) based on silver nanowire (AgNW) electrode, with improved operational stability by simply applying pre‐bias conditioning treatment, are demonstrated. Reverse bias conditioning performed before J–V–L measurement of the PLEDs enables the rough AgNW networks to function properly as a bottom electrode by stabilizing current characteristics, and the devices continue to show consistent operational performances. Conditions of applied bias and thicknesses of active layer are controlled for optimization and it is found that high reverse voltage is required to obtain current stabilization. Adequate thickness of polymer is also necessary to avoid breakdown induced by reverse bias. The essential effect of pre‐bias conditioning on the improved performances of PLEDs is investigated, and it is found that morphological change of AgNW networks contribute to the improvement in device performance. Some of the AgNWs that appear to be pathway of leakage current are deformed, and surface roughness (RMS) of the AgNW film is decreased while the sheet resistance of the film is maintained when the reverse bias conditioning is applied. It is also revealed that pre‐bias conditioning is independent from directionality of the applied bias when utilizing insulating polymer sandwiched between two electrodes.     

Barium Hydroxide as an Interlayer Between Zinc Oxide and a Luminescent Conjugated Polymer for Light‐Emitting Diodes

A study of hybrid light‐emitting diodes (HyLEDs) fabricated with and without solution‐processible Cs₂CO₃ and Ba(OH)₂ inorganic interlayers is presented. The interlayers are deposited between a zinc oxide electron‐injection layer and a fluorescent emissive polymer poly(9‐dioctyl fluorine–alt‐benzothiadiazole) (F8BT) layer, with a thermally evaporated MoO₃/Au layer used as top anode contact. In comparison to Cs₂CO₃, the Ba(OH)₂ interlayer shows improved charge carrier balance in bipolar devices and reduced exciton quenching in photoluminance studies at the ZnO/Ba(OH)₂/F8BT interface compared to the Cs₂CO₃ interlayer. A luminance efficiency of ≈28 cd A^?1 (external quantum efficiency (EQE) ≈ 9%) is achieved for ≈1.2 μm thick single F8BT layer based HyLEDs. Enhanced out‐coupling with the aid of a hemispherical lens allows further efficiency improvement by a factor of 1.7, increasing the luminance efficiency to ≈47cd A^?1, corresponding to an EQE of 15%. The photovoltaic response of these structures is also studied to gain an insight into the effects of interfacial properties on the photoinduced charge generation and back‐recombination, which reveal that Ba(OH)₂ acts as better hole blocking layer than the Cs₂CO₃ interlayer.     

Binaphthyl‐Containing Green‐ and Red‐Emitting Molecules for Solution‐Processable Organic Light‐Emitting Diodes

Strong intermolecular interactions usually result in decreases in solubility and fluorescence efficiency of organic molecules. Therefore, amorphous materials are highly pursued when designing solution‐processable, electroluminescent organic molecules. In this paper, a non‐planar binaphthyl moiety is presented as a way of reducing intermolecular interactions and four binaphthyl‐containing molecules ( BNCM s): green‐emitting BBB and TBT as well as red‐emitting BTBTB and TBBBT , are designed and synthesized. The photophysical and electrochemical properties of the molecules are systematically investigated and it is found that TBT , TBBBT , and BTBTB solutions show high photoluminescence (PL) quantum efficiencies of 0.41, 0.54, and 0.48, respectively. Based on the good solubility and amorphous film‐forming ability of the synthesized BNCM s, double‐layer structured organic light‐emitting diodes (OLEDs) with BNCM s as emitting layer and poly(N‐vinylcarbazole) (PVK) or a blend of poly[N,N′‐bis(4‐butylphenyl)‐N,N′‐bis(phenyl)benzidine] and PVK as hole‐transporting layer are fabricated by a simple solution spin‐coating procedure. Amongst those, the BTBTB based OLED, for example, reaches a high maximum luminance of 8315 cd · m⁻² and a maximum luminous efficiency of 1.95 cd · A⁻¹ at a low turn‐on voltage of 2.2 V. This is one of the best performances of a spin‐coated OLED reported so far. In addition, by doping the green and red BNCM s into a blue‐emitting host material poly(9,9‐dioctylfluorene‐2,7‐diyl) high performance white light‐emitting diodes with pure white light emission and a maximum luminance of 4000 cd · m⁻² are realized.     

Optically‐Pumped Lasing in Hybrid Organic–Inorganic Light‐Emitting Diodes

Here, the use of metal oxide layers both for charge transport and injection into an emissive semiconducting polymer and also for the control of the in‐plane waveguided optical modes in light‐emitting diodes (LEDs) is reported. The high refractive index of zinc oxide is used to confine these modes away from the absorbing electrodes, and include a nano‐imprinted grating in the polymer layer to introduce distributed feedback and enhance optical out‐coupling. These structures show a large increase in the luminescence efficiency over conventional devices, with photoluminescence efficiency increased by up to 45%. Furthermore, optically‐pumped lasing in hybrid oxide polymer LEDs is demonstrated. A tuneable lasing emission is also obtained in a single device structure by employing a graduated thickness of a zinc oxide inter‐layer. This demonstrates the scope for using such architectures to improve the external efficiency of organic semiconductor LEDs, and opens new possibilities for the realization of polymer injection lasers.     

Red,Green, and Blue Light‐Emitting Polyfluorenes Containing a Dibenzothiophene‐S,S‐Dioxide Unit and Efficient High‐Color‐Rendering‐Index White‐Light‐Emitting Diodes Made Therefrom

A series of blue (B), green (G) and red (R) light‐emitting, 9,9‐bis(4‐(2‐ethyl‐hexyloxy)phenyl)fluorene (PPF) based polymers containing a dibenzothiophene‐S,S‐dioxide (SO) unit (PPF‐SO polymer), with an additional benzothiadiazole (BT) unit (PPF‐SO‐BT polymer) or a 4,7‐di(4‐hexylthien‐2‐yl)‐benzothiadiazole (DHTBT) unit (PPF‐SO‐DHTBT polymer) are synthesized. These polymers exhibit high fluorescence yields and good thermal stability. Light‐emitting diodes (LEDs) using PPF‐SO25, PPF‐SO15‐BT1, and PPF‐SO15‐DHTBT1 as emission polymers have maximum efficiencies LE_max = 7.0, 17.6 and 6.1 cd A^?1 with CIE coordinates (0.15, 0.17), (0.37, 0.56) and (0.62, 0.36), respectively. 1D distributed feedback lasers using PPF‐SO30 as the gain medium are demonstrated, with a wavelength tuning range 467 to 487 nm and low pump energy thresholds (≥18 nJ). Blending different ratios of B (PPF‐SO), G (PPF‐SO‐BT) and R (PPF‐SO‐DHTBT) polymers allows highly efficient white polymer light‐emitting diodes (WPLEDs) to be realized. The optimized devices have an attractive color temperature close to 4700 K and an excellent color rendering index (CRI) ≥90. They are relatively stable, with the emission color remaining almost unchanged when the current densities increase from 20 to 260 mA cm^?2. The use of these polymers enables WPLEDs with a superior trade‐off between device efficiency, CRI, and color stability.     

Organometal Halide Perovskite Quantum Dot Light‐Emitting Diodes

Organometal halide perovskites quantum dots (OHP‐QDs) with bright, color‐tunable, and narrow‐band photoluminescence have significant advantages in display, lighting, and laser applications. Due to sparse concentrations and difficulties in the enrichment of OHP‐QDs, production of large‐area uniform films of OHP‐QDs is a challenging task, which largely impedes their use in electroluminescence devices. Here, a simple dip‐coating method has been reported to effectively fabricate large‐area uniform films of OHP‐QDs. Using this technique, multicolor OHP‐QDs light‐emitting diodes (OQ‐LEDs) emitting in blue, blue‐green, green, orange, and red color have been successfully produced by simply tuning the halide composition or size of QDs. The blue, green, and red OQ‐LEDs exhibited, respectively, a maximum luminance of 2673, 2398, and 986 cd m^?2 at a current efficiency of 4.01, 3.72, and 1.52 cd A^?1, and an external quantum efficiency of 1.38%, 1.06%, and 0.53%, which are much better than most LEDs based on OHP films. The packaged OQ‐LEDs show long‐term stability in air (humidity ≈50%) for at least 7 d. The results demonstrate the great potential of the dip‐coating method to fabricate large‐area uniform films for various QDs. The high‐efficiency OQ‐LEDs also demonstrate the promising potential of OHP‐QDs for low‐cost display, lighting, and optical communication applications.     

High‐Performance All‐Polymer White‐Light‐Emitting Diodes Using Polyfluorene Containing Phosphonate Groups as an Efficient Electron‐Injection Layer

We report an efficient non‐doped all‐polymer polymer white‐light‐emitting diode (PWLED) with a fluorescent three‐color, white single polymer as an emissive layer, an ethanol‐soluble phosphonate‐functionalized polyfluorene (PF‐EP) as an electron‐injection/electron‐transport layer, and LiF/Al as a cathode, respectively. The all‐polymer PWLED achieves a peak external quantum efficiency of 6.7%, a forward viewing luminous efficiency of 15.4 cd A^?1 and a power efficiency of 11.4 lm W^?1, respectively, at a brightness of 347 cd m^?2 with Commission Internationale d’Eclairage coordinates of (0.37, 0.42) and color rendering index of 85, which is the best results among the non‐doped PWLEDs. Moreover, this kind of PWLED not only shows excellent color stability, but also achieves high brightness at low voltages. The brightness reaches 1000, 10000, and 46830 cd m^?2 at voltages of 4.5, 5.4, and 7.5 V, respectively. The significant enhancement of white‐single‐polymer‐based PWLEDs with PF‐EP/LiF/Al to replace for the commonly used Ca/Al cathode is attributed to the more efficient electron injection at PF‐EP/LiF/Al interfaces, and the coordinated protecting effect of PF‐EP from diffusion of Al atoms into the emissive layer and exciton‐quenching near cathode interfaces. The developed highly efficient non‐doped all‐polymer PWLEDs are well suitable for solution‐processing technology and provide a huge potential of low‐cost large‐area manufacturing for PWLEDs.     

Merging Biology and Solid‐State Lighting: Recent Advances in Light‐Emitting Diodes Based on Biological Materials

Solid‐state lighting (SSL) is one of the biggest achievements of the 20^th century. It has completely changed our modern life with respect to general illumination (light‐emitting diodes), flat devices and displays (organic light‐emitting diodes), and small labeling systems (light‐emitting electrochemical cells). Nowadays, it is however mandatory to make a transition toward green, sustainable, and equally performing lighting systems. In this regard, several groups have realized that the actual SSL technologies can easily and efficiently be improved by getting inspiration from how natural systems that manipulate light have been optimized over millennia. In addition, various natural and biocompatible materials with suitable properties for lighting applications have been used to replace expensive and unsustainable components of current lighting devices. Finally, SSL has also started to revolutionize the biomedical field with the achievement of efficient implantable lighting systems. Herein, the‐state‐of‐art of (i) biological materials for lighting devices, (ii) bioinspired concepts for device designs, and (iii) implantable SSL technologies is summarized, highlighting the perspectives of these emerging fields.     

有机小分子掺杂的聚合物共混发光二极管

报道了用可溶性发光材料聚（２,５－二丁氧基苯）做发光材料,分别与母体聚合物聚乙烯基咔唑（ＰＶＫ）和聚甲基丙烯酸甲脂（ＰＭＭＡ）共混,并掺杂电子传输材料叔丁基联苯基苯基口恶二唑和空穴传输材料二胺衍生物作发光层,用铟锡氧化物和铝分别作正负电极,制作了两种蓝紫光有机／聚合物单层发光器件。通过比较两种器件的器件特性,发现以ＰＭＭＡ做母体的器件比用ＰＶＫ做母体的器件有更好的稳定性,器件开启电压为１０Ｖ左右,发光峰值波长均位于４２４ｎｍ,电致发光效率可达２．９％,比用ＰＶＫ做母体的器件效率高一倍多。     

Internal Field Screening in Polymer Light‐Emitting Diodes

We use electromodulation spectroscopy and modeling studies to probe the electric‐field distribution in polyfluorene‐based polymer light‐emitting diodes containing poly(3,4‐ethylenedioxythiophene) poly(styrene sulfonate). The bulk internal field is shown to be zero under ordinary operating conditions, with trapped electrons close to the anode fully screening the bulk semiconductor from the external field. The effect has far‐reaching implications for the understanding and optimization of organic devices.     

Highly Efficient Pure‐White‐Light‐Emitting Diodes from a Single Polymer: Polyfluorene with Naphthalimide Moieties

Light‐emitting diodes exhibiting efficient pure‐white‐light electroluminescence have been successfully developed by using a single polymer: polyfluorene derivatives with 1,8‐naphthalimide chromophores chemically doped onto the polyfluorene backbones. By adjusting the emission wavelength of the 1,8‐naphthalimide components and optimizing the relative content of 1,8‐naphthalimide derivatives in the resulting polymers, white‐light electroluminescence from a single polymer, as opposed to a polymer blend, has been obtained in a device with a configuration of indium tin oxide/poly(3,4‐ethylenedioxythiophene)(50 nm)/polymer(80 nm)/Ca(10 nm)/Al(100 nm). The device exhibits Commission Internationale de l'Eclairage coordinates of (0.32,0.36), a maximum brightness of 11 900 cd m^–2, a current efficiency of 3.8 cd A^–1, a power efficiency of 2.0 lm W^–1, an external quantum efficiency of 1.50 %, and quite stable color coordinates at different driving voltages, even at high luminances of over 5000 cd m^–2.     

Recent Progress in High‐Efficiency Blue‐Light‐Emitting Materials for Organic Light‐Emitting Diodes

Organic light‐emitting diodes (OLEDs) are increasingly used in displays replacing traditional flat panel displays; e.g., liquid crystal displays. Especially, the paradigm shifts in displays from rigid to flexible types accelerated the market change from liquid crystal displays to OLEDs. However, some critical issues must be resolved for expansion of OLED use, of which blue device performance is one of the most important. Therefore, recent OLED material development has focused on the design, synthesis and application of high‐efficiency and long‐life blue emitters. Well‐known blue fluorescent emitters have been modified to improve their efficiency and lifetime, and blue phosphorescent emitters are being investigated to overcome the lifetime issue. Recently, thermally activated delayed fluorescent emitters have received attention due to the potential of high‐efficiency and long‐living emitters. Therefore, it is timely to review the recent progress and future prospects of high‐efficiency blue emitters. In this feature article, we summarize recent developments in blue fluorescent, phosphorescent and thermally activated delayed fluorescent emitters, and suggest key issues for each emitter and future development strategies.     

Charge Transport and Recombination in Polyspirobifluorene Blue Light‐Emitting Diodes

The charge transport in blue light‐emitting polyspirobifluorene is investigated by both steady‐state current‐voltage measurements and transient electroluminescence. Both measurement techniques yield consistent results and show that the hole transport is space‐charge limited. The electron current is found to be governed by a high intrinsic mobility in combination with electron traps. Numerical simulations on light‐emitting diodes reveal a shift in the recombination zone from the cathode to the anode with increasing bias.