Baiquan Liu

Extremely high-efficiency and ultrasimplified hybrid white organic light-emitting diodes exploiting double multifunctional blue emitting layers

Numerous hybrid white organic light-emitting diodes (WOLEDs) have recently been developed. However, their efficiency is not comparable to that of their best all-phosphorescent WOLED counterparts, and the structures are usually complicated, restricting their further development. Herein, a novel concept is used to achieve a hybrid WOLED, whose crucial feature is the exploitation of double multifunctional blue emitting layers. The three-organic-layer WOLED exhibits a total efficiency of 89.3 and 65.1 lm W–1 at 100 and 1000 cd m–2, respectively, making it the most efficient hybrid WOLED reported in the literature so far. Significantly, the efficiencies of hybrid WOLEDs have, for the first time, been demonstrated to be comparable to those of the best all-phosphorescent WOLEDs. In addition, the device exhibits the lowest voltages among hybrid WOLEDs (i.e., 2.4, 2.7 and 3.1 V for 1, 100 and 1000 cd m–2, respectively). Such remarkable performance achieved from such an ultrasimplified structure opens a new path toward low-cost commercialization.

Efficient hybrid white organic light-emitting diodes with extremely long lifetime: the effect of n-type interlayer

The effect of n-type interlayer in hybrid white organic light-emitting diodes (WOLEDs) has been systematically investigated by using various n-type materials. A new finding, that the triplet energy rather than electron mobility or hole-blocking ability of interlayer plays a more positive role in the performance of hybrid WOLEDs, is demonstrated. Based on the new finding, a more efficient n-type interlayer bis[2-(2-hydroxyphenyl)-pyridine] beryllium has been employed to realize a high-performance hybrid WOLED. The resulting device (without n-doping technology) exhibits low voltages (i.e., 2.8 V for 1 cd/m2, 3.9 V for 100 cd/m2) and low efficiency roll-off (i.e., 11.5 cd/A at 100 cd/m2 and 11.2 cd/A at 1000 cd/m2). At the display-relevant luminance of 100 cd/m2, a total power efficiency of 16.0 lm/W, a color rendering index of 73 and an extremely long lifetime of 12596265 h are obtained. Such superior results not only comprehensively indicate that the n-type materials are effective interlayers to develop high-performance hybrid WOLEDs but also demonstrate a significant step towards real commercialization in WOLEDs.

Manipulation of exciton distribution for high-performance fluorescent/phosphorescent hybrid white organic light-emitting diodes

White organic light-emitting diodes (WOLEDs) are now approaching the lighting markets and also being aggressively investigated for display applications. In this review, the recent development of fluorescent/phosphorescent hybrid WOLEDs is highlighted. Since the key for the high performance is how to effectively harvest excitons (singlet and triplet), the manipulation of exciton distribution in hybrid WOLEDs is overviewed. In particular, the device structures, design strategies, working mechanisms and electroluminescent processes of the representative high-performance hybrid WOLEDs are systematically reviewed. It begins with the fundamental introduction of the types of hybrid WOLEDs, and next discusses how to manage the excitons for different types of high-performance hybrid WOLEDs. Then, it introduces the doping-free technology to realize hybrid WOLEDs. Lastly, the challenges and opportunities for further enhancement of the device performance are presented.

High-Performance Doping-Free Hybrid White OLEDs Based on Blue Aggregation-Induced Emission Luminogens

Doping-free white organic light-emitting diodes (DF-WOLEDs) have aroused research interest because of their simple properties. However, to achieve doping-free hybrid WOLEDs (DFH-WOLEDs), avoiding aggregation-caused quenching is challenging. Herein, blue luminogens with aggregation-induced emission (AIE) characteristics, for the first time, have been demonstrated to develop DFH-WOLEDs. Unlike previous DFH-WOLEDs, both thin (<1 nm) and thick (>10 nm) AIE luminogen (AIEgen) can be used for devices, enhancing the flexibility. Two-color devices show (i) pure-white emission, (ii) high CRI (85), and (iii) high efficiency. Particularly, 19.0 lm W1- is the highest for pure-white DF-WOLEDs, while 35.0 lm W1- is the best for two-color hybrid WOLEDs with CRI ≥ 80. A three-color DFH-WOLED shows broad color-correlated temperature span (2301-11628 K), (i) the first sunlight-like OLED (2500-8000 K) operating at low voltages, (ii) the broadest span among sunlight-like OLED, and (iii) possesses comparable efficiency with the best doping counterpart. Another three-color DFH-WOLED exhibits CRI > 90 at ≥3000 cd m-2, (i) the first DF-WOLED with CRI ≥ 90 at high luminances, and (ii) the CRI (92.8) is not only the highest among AIE-based WOLEDs but also the highest among DF-WOLEDs. Such findings may unlock an alternative concept to develop DFH-WOLEDs.

A host–guest system comprising high guest concentration to achieve simplified and high-performance hybrid white organic light-emitting diodes

Single-emitting-layer (single-EML) hybrid white organic light-emitting diodes (WOLEDs) have attracted a great deal of attention due to their simplified structures. However, the guest concentration is usually too low, which is quite difficult to control and reproduce in the coevaporation process. Herein, for the first time, N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine has been used as both the host and the blue emitter in single-EML WOLEDs. By dint of this multifunctional material, the concentration is found to be as high as 1.5%. This device exhibits a maximum total efficiency of 65.3 lm W−1, indicating a significant step towards the real commercialization. Besides, low voltages (i.e., the turn-on voltage is 2.4 V and 3.45 V at 1000 cd m−2) and a color rendering index (CRI) of 77 are obtained for this two-color WOLED. Unlike the working mechanisms in previous single-EML hybrid WOLEDs with low guest concentrations, devices comprising high concentrations exhibit more sophisticated engineering, in which the device smartly allows the utilization of both the fluorescence from the host itself, and the complementary phosphorescence from the guest by incomplete Forster energy transfer, Dexter energy transfer as well as direct exciton formation on the guest. Moreover, we have incorporated this unique host–guest system into a dual-EML hybrid WOLED. Maximum efficiencies of 17.2 lm W−1 and 10.2 lm W−1 at 1000 cd m−2 (3.85 V) with an ultrahigh CRI of 93 are achieved, providing a new opportunity to accomplish the simplified structure/low voltage/high efficiency/ultrahigh CRI trade-off.

Simultaneous achievement of low efficiency roll-off and stable color in highly efficient single-emitting-layer phosphorescent white organic light-emitting diodes

A highly efficient single-emitting-layer white organic light-emitting diode with low efficiency roll-off and great color-stability has been fabricated and characterized. The resulting device achieves a forward-viewing current efficiency of 45.2 cd A−1 and a power efficiency of 37.1 lm W−1 at a luminance of 100 cd m−2. Even at 1000 cd m−2, a current efficiency of 43.6 cd A−1 and a power efficiency of 31.3 lm W−1 can be obtained, indicating that the device exhibits low efficiency roll-off. In addition, only a slight color-shift Δ ≤ (0.025, 0.006) can be observed during a large range of luminance, revealing that the device shows stable color. It is found that these superior properties originate from the introduction of multifunctional dopants as well as the bipolar host. Moreover, it is demonstrated that the dopants at low concentrations have almost no influence on the electrical properties when negligible energy barriers exist between the dopants and charge transport layers.

High-Performance Hybrid White Organic Light-Emitting Diodes Comprising Ultrathin Blue and Orange Emissive Layers

Two novel high-performance hybrid white organic light-emitting diodes have been realized by the delta-doping method. The device comprising a single ultrathin emissive layer exhibits a luminance of 46923 cd/m2 and a low efficiency roll-off. To further simplify the device structures, another device comprising double ultrathin emissive layers achieves low driving voltages, a high color rendering index (75), and a high efficiency (8.9 lm/W). Moreover, it is found that these two devices not only exhibit fairly pure white emission but also show a rather stable color. Such superior properties reveal that the utilization of delta-doping technology provides a new way to achieve high-performance devices.

Strategies to Achieve High-Performance White Organic Light-Emitting Diodes

As one of the most promising technologies for next-generation lighting and displays, white organic light-emitting diodes (WOLEDs) have received enormous worldwide interest due to their outstanding properties, including high efficiency, bright luminance, wide viewing angle, fast switching, lower power consumption, ultralight and ultrathin characteristics, and flexibility. In this invited review, the main parameters which are used to characterize the performance of WOLEDs are introduced. Subsequently, the state-of-the-art strategies to achieve high-performance WOLEDs in recent years are summarized. Specifically, the manipulation of charges and excitons distribution in the four types of WOLEDs (fluorescent WOLEDs, phosphorescent WOLEDs, thermally activated delayed fluorescent WOLEDs, and fluorescent/phosphorescent hybrid WOLEDs) are comprehensively highlighted. Moreover, doping-free WOLEDs are described. Finally, issues and ways to further enhance the performance of WOLEDs are briefly clarified.

Room-Temperature Fabricated Thin-Film Transistors Based on Compounds with Lanthanum and Main Family Element Boron

For the first time, compounds with lanthanum from the main family element Boron (LaBx) were investigated as an active layer for thin-film transistors (TFTs). Detailed studies showed that the room-temperature fabricated LaBx thin film was in the crystalline state with a relatively narrow optical band gap of 2.28 eV. The atom ration of La/B was related to the working pressure during the sputtering process and the atom ration of La/B increased with the increase of the working pressure, which will result in the freer electrons in the LaBx thin film. LaBx-TFT without any intentionally annealing steps exhibited a saturation mobility of 0.44 cm2·V−1·s−1, which is a subthreshold swing (SS) of 0.26 V/decade and a Ion/Ioff ratio larger than 104. The room-temperature process is attractive for its compatibility with almost all kinds of flexible substrates and the LaBx semiconductor may be a new choice for the channel materials in TFTs.

Emergence of Nanoplatelet Light-Emitting Diodes

Since 2014, nanoplatelet light-emitting diodes (NPL-LEDs) have been emerged as a new kind of LEDs. At first, NPL-LEDs are mainly realized by CdSe based NPLs. Since 2016, hybrid organic-inorganic perovskite NPLs are found to be effective to develop NPL-LEDs. In 2017, all-inorganic perovskite NPLs are also demonstrated for NPL-LEDs. Therefore, the development of NPL-LEDs is flourishing. In this review, the fundamental concepts of NPL-LEDs are first introduced, then the main approaches to realize NPL-LEDs are summarized and the recent progress of representative NPL-LEDs is highlighted, finally the challenges and opportunities for NPL-LEDs are presented.