蓝光钙钛矿发光二极管:机遇与挑战

金属卤化钙钛矿由于具有优异的光电性能(如:高电子/空穴迁移率,高荧光量子产率,高色纯度,以及光色可调性等),成为应用于发光二极管(LED)的理想材料。近年来,钙钛矿LED的发展十分迅速,红光和绿光钙钛矿LED的外量子效率(EQE)均已超过20%。然而,蓝光(尤其是深蓝光)钙钛矿LED的EQE以及稳定性依然相对落后,这严重制约了钙钛矿LED在高性能、广色域显示领域和高显色指数白光照明领域的应用。因此,总结现阶段蓝光钙钛矿LED的发展,并剖析其机遇与挑战,对未来蓝光甚至整个钙钛矿LED领域的发展至关重要。本文将蓝光钙钛矿LED根据光色细分为天蓝光、纯蓝光、深蓝光三大部分进行总结,回顾了三种LED器件的发展历程,并详细阐述了现阶段实现他们的主要手段以及相关的基础原理,最后分析了它们各自的问题并提出了相应的解决思路。     

New Phosphors for White LEDs

White light-emitting diodes (WLEDs) have matched the emission efficiency of florescent lights and will rapidly spread as light source for homes and offices in the next 5 to 10 years. WLEDs provide a light element having a semiconductor light emitting layer (blue or UV LEDs) and photoluminescence phosphors. GaN-based highly efficient blue InGaN LEDs combined with phosphors can produce white light. These solid-state LED lamps have a number of advantages over conventional incandescent bulbs and halogen lamps, such as high efficiency to convert electrical energy into light, reliability, and long operating lifetime (about 100,000 hours). For the purpose of development of high energy-efficient white light sources, we need to produce highly efficient new phosphors, which can absorb excitation energy from blue or UV LEDs and generate emissions.In this paper, we investigate the development of blue or UV LEDs by the appropriate combination of new phosphors which can lead us to obtain high brightness white light. The criteria of choosing the best phosphors, for blue (380-450 nm) and UV (360-400 nm) LEDs, strongly depends on the absorption and emission of the phosphors. Moreover, the balance light between the light emission from blue LEDs and the yellow YAG:Ce,Gd phosphor is important to obtain white light with high color temperature. The phosphors with high efficiency which can be excited by UV LEDs are important to obtain the white light with high color rendering index.     

量子点发光二极管中电荷累积行为

量子点发光二极管(QLED)是不需要额外光源的主动发光技术,在显示领域中的应用前景被广泛看好。寿命较短是影响QLED商业化的重要因素之一,并且其老化机理尚不清晰。在本工作中,我们通过自主搭建电荷提取装置,证实红光QLED在恒流驱动过程中,存在显著的电荷累积。累积电荷量随着驱动电流密度增加而增加,但当超过阈值电流密度(对应于开启电压)后逐渐趋于饱和。随着器件老化,亮度下降伴随着累积电荷量进一步增加。本工作对QLED老化过程中电荷累积规律的理解,能为QLED材料和界面的优化设计提供直观判据。     

基于碳量子点的光电器件应用新进展

碳量子点(CQDs)作为一种新型荧光碳纳米材料,由于其较高的电子迁移率、较长的热电子寿命、极快的电子取出速度,可调的带隙宽度、较强的稳态荧光等独特的光电性质和可溶液加工、成本低廉的特点,使得CQDs在光电器件领域具有广阔的应用前景,近年来受到人们的广泛关注,重要的研究成果不断涌现。本文首先简要介绍了CQDs的合成方法、化学结构及其光电性质,然后总结了CQDs在发光二极管(LEDs)、太阳能电池(SCs)和光电探测器(PDs)等光电器件领域的研究进展,最后对CQDs的发展方向进行了展望。     

胶体量子阱发光二极管的研究进展

胶体量子阱(CQW)由于具有高色纯度、高光致发光量子效率、光色可调等优异的光电性能，近年来成为一种新型的光电材料，广泛用于制备发光二极管、激光、探测器、太阳能电池等半导体器件。其中，基于CQW的发光二极管(CQW-LED)因为具备极窄的光谱、极佳的色纯度、高效率、可溶液加工、可柔性化等优点，在显示和照明等领域展现出重要应用前景，受到学术界和工业界的广泛重视而成为研究热点。本文首先介绍了CQW-LED中的一些基本概念，包括CQW材料特性、LED器件结构、发光机理等；然后从CQW材料种类的角度出发，阐述了基于单核型、核/冠型、核/壳型、复杂异质结型、以及杂质掺杂型CQW-LED近年来的研究进展，并结合我们研究团队最近的工作详细的介绍了实现高性能CQW-LED的方法，包括对材料选取、设计策略、器件结构、器件性能、工作机理以及发光过程的分析；接着，介绍了CQW-LED的集成应用；最后探讨了CQW-LED目前面临的挑战及其未来的发展机遇。     

Photoselective excited state dynamics in ZnO-Au nanocomposites and their implications in photocatalysis and dye-sensitized solar cells

Improving the performance of photoactive solid-state devices begins with systematic studies of the metal-semiconductor nanocomposites (NCs) upon which such devices are based. Here, we report the photo-dependent excitonic mechanism and the charge migration kinetics in a colloidal ZnO-Au NC system. By using a picosecond-resolved F?rster resonance energy transfer (FRET) technique, we have demonstrated that excited ZnO nanoparticles (NPs) resonantly transfer visible optical radiation to the Au NPs, and the quenching of defect-mediated visible emission depends solely on the excitation level of the semiconductor. The role of the gold layer in promoting photolytic charge transfer, the activity of which is dependent upon the degree of excitation, was probed using methylene blue (MB) reduction at the semiconductor interface. Incident photon-to-current efficiency measurements show improved charge injection from a sensitizing dye to a semiconductor electrode in the presence of gold in the visible region. Furthermore, the short-circuit current density and the energy conversion efficiency of the ZnO-Au NP based dye-sensitized solar cell (DSSC) are much higher than those of a DSSC comprised of only ZnO NP. Our results represent a new paradigm for understanding the mechanism of defect-state passivation and photolytic activity of the metal component in metal-semiconductor nanocomposite systems.     

Highly Efficient Candlelight Organic Light-Emitting Diode with a Very Low Color Temperature

Low color temperature candlelight organic light-emitting diodes (LEDs) are human and environmentally friendly because of the absence of blue emission that might suppress at night the secretion of melatonin and damage retina upon long exposure. Herein, we demonstrated a lighting device incorporating a phenoxazine-based host material, 3,3-bis(phenoxazin-10-ylmethyl)oxetane (BPMO), with the use of orange-red and yellow phosphorescent dyes to mimic candlelight. The resultant BPMO-based simple structured candlelight organic LED device permitted a maximum exposure limit of 57,700 s, much longer than did a candle (2750 s) or an incandescent bulb (1100 s) at 100 lx. The resulting device showed a color temperature of 1690 K, which is significantly much lower than that of oil lamps (1800 K), candles (1900 K), or incandescent bulbs (2500 K). The device showed a melatonin suppression sensitivity of 1.33%, upon exposure for 1.5 h at night, which is 66% and 88% less than the candle and incandescent bulb, respectively. Its maximum power efficacy is 23.1 lm/W, current efficacy 22.4 cd/A, and external quantum efficiency 10.2%, all much higher than the CBP-based devices. These results encourage a scalable synthesis of novel host materials to design and manufacture high-efficiency candlelight organic LEDs.     

When Fluorescent Proteins Meet White Light‐Emitting Diodes

Over the last decades, fluorescent proteins (FPs) have been extensively employed for imaging and tracing in cell biology and medicine. However, their application for lighting devices like light‐emitting diodes (LEDs) and lasers has recently started. The interest of FPs is the result of their good photoluminescence features (high emission efficiency with a narrow spectrum and a high photon‐flux saturation), good photostability, sustainable production by bacteria, and eco‐friendly recycling. Their low stability at high temperatures as well as the need for an aqueous environment have, however, strongly limited their use in optoelectronics. This has recently been circumvented with new coating systems that are paving the way for the entrance of FPs into the LED field. In this Minireview, we summarize the first steps taken by a few groups towards the development of bio‐hybrid white LEDs (Bio‐HWLEDs) with a focus on using FPs as color down‐converters, highlighting the state of the art and challenges associated with this emerging field.     

魔幻尺寸ＣｄＳｅ纳米簇的连续生长与室温光学性质

以镉的脂肪酸盐和分散在三辛基膦中的硒单质分别作为镉和硒的前体,采用十二烷基硫醇做“成核剂”,十二烷基胺为活化剂,在非配位溶剂十八碳烯中,实现了连续尺寸高质量魔幻尺寸半导体纳米簇（ＣｄＳｅ）的合成。所得ＣｄＳｅ纳米簇的形态和光学性质分别通过透射电子显微镜、紫外可见吸收光谱、荧光激发光谱、荧光发射光谱及时间分辨的荧光光谱测试技术进行了表征和测定。该系列ＣｄＳｅ纳米簇具有良好的室温荧光性质,能够得到较强的以往只有低温（５～２００Ｋ）下才能探测到的荧光光谱。所得发射光谱大部分为表面缺陷导致的黄光发射。部分尺寸的纳米簇能够实现白光发射,其ＣＩＥ坐标为（０.３３９,０.３４０）,为白光光源的开发提供了备选材料。     

One‐pot synthesis of carbon dots@ZrO2 nanoparticles with tunable solid‐state fluorescence

In recent years, fluorescent carbon dots (CDs) have been developed and showed potential applications in biomedical imaging and light‐emitting diodes (LEDs) for their excellent fluorescent properties. However, it still remains a challenge to incorporate fluorescent CDs into the host matrix in situ to overcome their serious self‐quenching. Herein, a one‐pot hydrothermal method is used to prepare nano‐zirconia with CDs (CDs@ZrO₂) nanoparticles. During the reaction, CDs and nano‐zirconia are generated simultaneously and connected with silane coupling agent. The CDs@ZrO₂ nanoparticles exhibit tunable emission wavelength from 450 to 535 nm emission by regulating the content of citric acid in the feed. The quantum yield of the CDs@ZrO₂ is up to 23.8%. Furthermore, the CDs@ZrO₂ nanoparticles with regulable fluorescence emission can be used for the fluorescent material to prepare white LEDs. The prepared LED has significant white light emission with color coordinates of (0.30, 0.37) and its color rendering index (CRI) is 67.1. In summary, we have developed the solid‐state CDs@ZrO₂ nanoparticles with tunable emission by a valuable strategy, that is, one‐pot method, for white LEDs.     

High-efficiency,solution-processable,multilayer triple cation perovskite light-emitting diodes with copper sulfide–gallium–tin oxide hole transport layer and aluminum-zinc oxide–doped cesium electron injection layer

Light-emitting diodes with perovskite luminophores have great potential in next-generation displays because of their exceptional color purity with narrow emission bandwidth, broadband color tunability, and solution processability. However, their low luminescent efficiency is a critical drawback. Here, we report the first demonstration of a multicolor, large-area, perovskite display, which can be made flexible by using an optimized perovskite emissive layer sandwiched between inorganic metal oxide charge transport layers, all of which are coated via a facile solution process. We show that advanced interfacial engineering, especially the energy level alignment at the interface, plays a vital role in determining the device performance because of its effects on charge injection, transport, and recombination. These devices exhibit maximum current and power efficiencies of 74.25 cd A^?1 and 89.72 lm/w for green emission, 21.40 cd A^?1 and 25.84 lm/w for red emission, and 15.21 cd A^?1 and 15.84 lm/w for blue emission, respectively. Furthermore, with the introduction of inorganic charge transport layers, these devices exhibit high environmental stability, and the encapsulated devices have operating lifetimes exceeding 450 h with an initial brightness of 1000 cd/m².     

Polymer Light-emitting Diodes Based on End-capped Poly[9,9-di-(2'-ethylhexyl)fluorenyl-2,7-diyl]

We demonstrate polymer light‐emitting diodes (LEDs) based on poly[9,9‐di‐(2′‐ethylhexyl)fluorenyl‐2,7‐diyl] with end capper dimethylphenyl or N,N‐bis(4‐methylphenyl)‐N‐phenylamine. The introduction of end‐capper groups increased the device luminance and efficiency, while greatly depressing the green emission. For the devices constructed of poly[9,9‐di‐(2′‐ethylhexyl)fluorenyl‐2,7‐diyl] end capped with dimethylphenyl, the maximum luminance reached 381 cd/m² at 122 mA/cm². The maximum external quantum efficiency was 0.16% at 117 mA/cm², which is more than five times higher than that of the non‐end‐capped polymer LEDs. The electroluminescence (EL) maximum was at 485 nm, blue shifted by 52 nm with respect to that of the non‐end‐capped polyfluorene devices. It is proposed that efficient hole trapping at end capper and increased resistance of polyfluorene to oxidation are responsible for the improved device performance and color stability.     

Heterolayer light-emitting diodes based on poly-phenylene vinylene

In order to enhance the quantum efficiency of poly-p-phenylene vinylene (PPV) light-emitting diodes (LEDs), we have fabricated metal/insulator/polymer (MIP) LEDs and heterolayer LEDs based on PPV and oxadiazole polymers. The current–voltage (I–V) characteristics and electroluminescence (EL) intensity of the MIP structures display a pronounced dependence of the insulator thickness and we detect an increase in the quantum efficiency of more than a factor of 30 at an AlO_x layer thickness of 3–6 nm. The device characteristics are qualitatively understood within inorganic metal insulator semi-conductor (MIS) theory and can be explained by a voltage-dependent barrier for minority carrier injection in connection with a hole-blocking barrier at the PPV/insulator interface. Our oxadiazole polymers used in the heterolayer polymeric devices are characterized by a high thermal stability and excellent film-forming properties. These materials act as efficient hole-blocking, electron transport and injection layers in PPV-based LEDs and we measure a significantly improved device performance with external quantum efficiencies of more than 0.5%. Temperature-dependent investigations point to a relatively balanced charge carrier injection and reveal the influence of space charge limited currents on the device performance at low temperature. © 1997 John Wiley & Sons, Ltd.     

Optoelectronic and charge transport properties at organic-organic semiconductor interfaces: comparison between polyfluorene-based polymer blend and copolymer

We report detailed studies of optoelectronic and charge transport properties at the organic-organic semiconductor interfaces formed between polymer chains (interchain) and within a polymer chain (intrachain). These interfaces are fabricated using poly(9,9-di-n-octylfluorene-alt-N-(4-butylphenyl)diphenylamine) (TFB [f8-tfb]) (electron-donor) and poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) (F8BT [f8-bt]) (electron-acceptor) conjugated polymers, by blending them together or by covalently attaching them via a main polymer backbone (copolymer). For optoelectronic properties, when a bulky and twisted tfb molecule is incorporated into a rigid F8BT conjugated backbone, it disturbs the conjugation of F8BT polymer, leading to a blue-shift in the lowest absorption transition. However, by acting as an effective electron donor, it assists the formation of an intrachain singlet exciton that has a strong charge-transfer character, leading to a red-shifted and longer-lived emission than that of F8BT. An extremely efficient and fast energy transfer from tfb donor to bt acceptor is observed in the copolymer (<1 ps) compared to transfer from TFB to F8BT in the blend (tens of ps). This efficient energy transfer in the copolymer is found to be associated with its low fluorescence efficiency (40-45% vs 60-65% for blend) because of the migration of radiative singlet excitons to low-energy states such as triplet and exciplex states that are nonemissive or weakly emissive. The presence of molecular-scale tfb-f8-bt interfaces in the copolymer, however, does not hinder an efficient transport of charge carriers at high drive voltages. Instead, it provides a better balance of charge carriers inside the device, which leads to slower decay of the device efficiency and thus more stable light-emitting diodes with increasing voltage than the blend devices. These distinctive optoelectronic and charge transport properties observed at different organic-organic semiconductor interfaces will provide useful input for the design rules of conjugated polymers required for improved molecular electronics.     

High‐Performance Perovskite Light‐Emitting Diode with Enhanced Operational Stability Using Lithium Halide Passivation

Defect passivation has been demonstrated to be effective in improving the radiative recombination of charge carriers in perovskites, and consequently, the device performance of the resultant perovskite light‐emitting diodes (LEDs). State‐of‐the‐art useful passivation agents in perovskite LEDs are mostly organic chelating molecules that, however, simultaneously sacrifice the charge‐transport properties and thermal stability of the resultant perovskite emissive layers, thereby deteriorating performance, and especially the operational stability of the devices. We demonstrate that lithium halides can efficiently passivate the defects generated by halide vacancies and reduce trap state density, thereby suppressing ion migration in perovskite films. Efficient green perovskite LEDs based on all‐inorganic CsPbBr₃ perovskite with a peak external quantum efficiency of 16.2 %, as well as a high maximum brightness of 50 270 cd m^?2, are achieved. Moreover, the device shows decent stability even under a brightness of 10⁴ cd m^?2. We highlight the universal applicability of defect passivation using lithium halides, which enabled us to improve the efficiency of blue and red perovskite LEDs.     

多孔硅-导电聚酯材料复合发光器的光学特性研究

poly-4-dicyanomethylene-4H-cyclopenta\[2,1-b:3,4-b'\]dithiophene monolayer (PCDM)是一种导电、低导带聚酯材料.如果在多孔硅纳米结构中附上一层以自组方式生成的PCDM单分子层,就可以制成能够产生稳定电致发光的器件.发光器的结构是金/PCDM/多孔硅/硅/铝.发光器的电致发光,在白天可用肉眼观察到.有很宽的发光波长,几乎覆盖了整个可见光区域且峰值位于650 nm.发光器的面积为1 cm2,启动正向电压在14～30 V,电流约300 mA.经长时间测试,发光器的稳定性很好,在空气中放置3个月,在输入功率不变的情况下,发光强度也不发生变化.当施以反向电压时,样品仍可以发光而且稳定性较高,在250 h内I～V未发生明显变化.扫描电镜图像显示PCDM覆盖的表面要比多孔硅表面平整,而PCDM分子有可能进入到多孔硅纳米孔径当中去,起到了提高发光器稳定性和延长其寿命的作用.     

Hydration-Facilitated Fine-Tuning of the AIE Amphiphile Color and Application as Erasable Materials with Hot/Cold Dual Writing Modes

Hydration water greatly impacts the color of inorganic crystals, but it is still unknown whether hydration water can be utilized to systematically manipulate the emission color of organic luminescent groups. Now, metal ions with different hydration ability allow fine-tuning the emission color of a fluorescent group displaying aggregation induced emission (AIE). Because the hydration water can be removed easily by gentle heating or mechanical grinding and re-gained by solvent fuming, rewritable materials can be fabricated both in the hot-writing and cold-writing modes. This hydration-facilitated strategy will open up a new vista in fine-tuning the emission color of AIE molecules based on one synthesis and in the design of smart luminescent devices.     

Attaching perylene dyes to polyfluorene: three simple,efficient methods for facile color tuning of light-emitting polymers

The emission color of fluorene-based polymers can be facilely tuned across the whole visible spectrum by copolymerization with perylene dyes. Methods are demonstrated for incorporation of the dyes in the polymer mainchain, at the chain termini, or as side chains. Efficient energy transfer causes the emission to come solely from the dye units. Efficient LEDs have been made from the copolymers with dyes in the mainchain.     

Hydration‐Facilitated Fine‐Tuning of the AIE Amphiphile Color and Application as Erasable Materials with Hot/Cold Dual Writing Modes

Hydration water greatly impacts the color of inorganic crystals, but it is still unknown whether hydration water can be utilized to systematically manipulate the emission color of organic luminescent groups. Now, metal ions with different hydration ability allow fine‐tuning the emission color of a fluorescent group displaying aggregation induced emission (AIE). Because the hydration water can be removed easily by gentle heating or mechanical grinding and re‐gained by solvent fuming, rewritable materials can be fabricated both in the hot‐writing and cold‐writing modes. This hydration‐facilitated strategy will open up a new vista in fine‐tuning the emission color of AIE molecules based on one synthesis and in the design of smart luminescent devices.     

High-Performance Perovskite Light-Emitting Diode with Enhanced Operational Stability Using Lithium Halide Passivation

Defect passivation has been demonstrated to be effective in improving the radiative recombination of charge carriers in perovskites, and consequently, the device performance of the resultant perovskite light-emitting diodes (LEDs). State-of-the-art useful passivation agents in perovskite LEDs are mostly organic chelating molecules that, however, simultaneously sacrifice the charge-transport properties and thermal stability of the resultant perovskite emissive layers, thereby deteriorating performance, and especially the operational stability of the devices. We demonstrate that lithium halides can efficiently passivate the defects generated by halide vacancies and reduce trap state density, thereby suppressing ion migration in perovskite films. Efficient green perovskite LEDs based on all-inorganic CsPbBr₃ perovskite with a peak external quantum efficiency of 16.2 %, as well as a high maximum brightness of 50 270 cd m⁻², are achieved. Moreover, the device shows decent stability even under a brightness of 10⁴ cd m⁻². We highlight the universal applicability of defect passivation using lithium halides, which enabled us to improve the efficiency of blue and red perovskite LEDs.