Development of Highly Efficient Tandem White OLEDs
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摘要: 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。
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关键词:
- 叠层白光有机发光二极管 /
- 电荷产生层 /
- 有机发光单元 /
- 功能层结构 /
- 有机发光材料
Abstract: Tandem white OLEDs offer low power consumption, high brightness, and a high color gamut. However, the material and electrical structures of tandem white OLEDs still need to be optimized owing to the outstanding challenges in efficiency, lifetime, and driving voltage. In this study, we focused on the latest research on tandem white OLEDs and summarized the problems in engineering preparation and non-destructive detection method of 3 types of CGLs for high-efficiency tandem white OLEDs. We focused on the latest research on the "all-phosphorescent system, " "harvesting excitons via two parallel channels, " and the "mixed-phosphorescent-TADF system" simultaneously. We summarized the device lifetime problems and discussed structural solutions such as "graded doping" and "four-color mixed-phosphorescent-TADF system." From the aspect of CGL materials and structures in different systems, we reviewed the scheme of lower driving voltage for tandem white OLEDs. Finally, we provided suggestions for improving the materials and structures of tandem white OLEDs. -
0. 引言
短波红外波段作为大气光学窗口之一，近年来因其优良的光学特性被广泛应用于军事领域和民用领域。短波红外成像类似于可见光的反射式成像，具有穿透烟雾、雨雪、沙尘进行成像等可见光成像不具备的能力，另外与中波红外和长波红外对比，短波红外拥有更高的细节分辨能力[1]。工作在可见和近红外波段的传统微光夜视设备无法探测到波长处在1.1~1.7 μm内的短波红外激光，1.1~1.7 μm内的短波红外激光可在夜间作为辅助照明光源用来进行夜视成像[2]。加工工艺成熟的高响应度铟镓砷（InGaAs）探测器的峰值响应波长处于短波红外波段中，可用于制备工业化的短波红外成像器件，如短波红外相机等[3]。近年来短波红外成像的发展使得可见光与红外光谱之间可以无缝衔接地进行光学成像设计，这在对目标进行信息获取时能够全波段获取目标信息具有重要意义。在应用方面，短波红外成像不仅可以用于夜视侦察、空间遥感观测等领域，还可用于半导体表面特征分析、医疗、农产品检测等领域。未来短波红外成像将会向大视场、宽波段方向发展。
1. 短波红外成像系统简介
1.1 短波红外特性
除了可见光之外，还有众多人眼无法直接看到的射线，物理学界统一称为电磁波，将电磁波根据其波长的大小来排列，由大到小分为无线电波、微波、红外波段、可见光、紫外波段。红外波段的波长范围为0.7~12 μm，其中还可再细分为近红外、短波红外、中波红外和长波红外。短波红外的波长范围有多种定义，美国试验和材料协会（American Society for Testing and Materials，ASTM）将0.7~2.5 μm划分为短波红外，医用短波红外定义为0.76~1.5 μm，根据探测器响应可以定义为0.9~1.7 μm，根据大气光谱透过率可以定义为1~3 μm。一般来说，短波红外（short-wave infrared band, SWIR）通常是指波长范围在1~2.5 μm范围的电磁波。
短波红外介于近红外和热红外之间，是大气光学窗口之一，具有良好的大气传输特性，短波红外在0.9~1.1 μm、1.15~1.3 μm、1.4~1.8 μm三个波段内的大气透射率均在80%以上，如图 1所示，在1 μm、1.24 μm、1.64 μm处的大气透过率达到峰值，说明太阳光经过大气透射后，在自然环境中仍存在较高光照强度的短波红外辐射[4]。一般来说，探测器所接受到的短波红外辐射主要来自于高温物体自身辐射、目标对环境的反射以及人工制造的短波红外辐射。
1.2 短波红外成像原理
任何物体温度在非绝对零度时都会发出红外辐射，随着温度升高，物体所发出的红外辐射会越来越强。利用光电探测器获取目标的红外辐射并进行光电转换，可以将红外辐射转化为红外图像。
但是当目标物体是自身能发射足够强的短波红外辐射的高温物体时候，对该物体的短波红外成像原理就变成既接收高温目标自身短波红外辐射又接收反射周围环境内的短波红外辐射来进行成像。不同的地物目标具有各自的光谱特性，相同条件下，不同地物目标反射电磁波的能力是不同的，主要体现在反射强度和反射波谱的形态两个方面，这为地物目标的探测和识别提供了理论依据。
夜间大气光谱辐射通量大部分分布在0.5~2 μm波段范围内。在夜间有月光的条件下，在可见光范围内包括近红外辐射范围内的辐射通量随着波长增加都有所增加，短波红外成像在天气晴朗的夜间具有良好的夜视效果。如图 2所示，夜间天空辐亮度的大气辉光现象所发出的光照度在1.5~1.7 μm光谱范围内的辐亮度与满月光的辐亮度相比要强，故在夜间低照度的环境下依然可以进行成像[5]。
衍射是电磁波的固有属性，光线传播过程中当障碍物的尺寸和电磁波波长相近时，电磁波能够绕过障碍物传播，绕射能力和波长成正比。短波红外在进行大气传输时受到空气中漂浮物影响较小，传输距离较远。短波红外透烟、透雾成像的能力和在低照度环境下成像的能力使全天时、全天候对地观测成为可能。
短波红外成像和可见光成像都是接收来自地物目标反射的周围环境中的光辐射来进行成像，这种相似性使得短波红外图像具有丰富的细节特征，能够提供媲美可见光图像质量的短波红外遥感影像。
1.3 短波红外成像系统中的材料选择
常规光学玻璃与光学胶在可见光、近红外、短波红外3个波段内都具有良好的光学透过率，均可用于可见光、短波红外光学成像系统。不过由于材料的折射率会随工作波长的变化而变化，常规光学玻璃材料在短波红外波段内的色散特性与在可见光波段内的色散特性不同[6]。在可见光谱段内火石类玻璃具有较高的色散，在短波红外波段内高色散的火石玻璃的色散特性降低了。在可见光波段一些玻璃材料的阿贝数主要集中在20~80的范围之内，而在短波红外波段的阿贝数区间则会被压缩到40~60之间。
表 1给出了一些在短波波段内具有较高透射率的氟化物玻璃和其他光学晶体材料的光学特性。
表 1 不同材料折射率与透射范围Table 1. Refractive index and transmission range of different materialsMaterial Refractive index Abbe number Coefficient of thermal expansion（×10-6） Transmission band/μm MgF2 1.3777 106.22 9.4 0.2~7 CaF2 1.4338 94.996 18.9 0.23~9.7 BaF2 1.4655 81.608 18.4 0.23~10.3 ZnS 2.3672 15.305 6.6 0.42~18 F-SILICA 1.4585 67.821 0.51 0.21~3.71 H-K9L 1.5032 64.2 76 0.29~2.4 H-FK61B 1.4875 81.605 127 0.302~2.325 H-FK95N 1.4378 94.523 144 0.302~2.325 H-QF14 1.5955 39.22 78 0.36~2.4 光学设计过程中有时候也会采用双胶合、三胶合透镜组等方式来满足像差平衡的需要，而光学胶在0.4~1.7 μm也具有较高的透射率，可灵活运用搭配不同材料合成所需要的等效材料。此外，在进行短波红外光学系统消热差分析时，通常采用选择不同透镜材料搭配、分配光焦度的方法[7]。红外光学材料存在热特性差异，例如：衍射光学元件具有负色散、负的衍射热常量、正光焦度的特性。当温度变化时，镜筒材料的热膨胀也会导致光学系统的像面移动，故要将两者配合起来，以达到消热差的效果。
1.4 光学系统结构的选择
红外光学系统常使用反射式系统、折射式系统以及折反射式系统。反射式系统的优势是无色差，反射镜的口径不受红外透光材料限制，可以制作得较大，系统光能损失较小。但是反射式系统的最大缺点是轴外像差较大，一般只适用于小视场、小相对孔径的光学系统。
对于经典的卡塞格伦（Cassegrain）或里奇-克雷季昂（Ritchey-Chretien）双反射镜系统来说，当相对孔径增大，比如相对孔径大于1:2时，或者视场增大，比如视场大于1°时，其像质都将迅速恶化。此外，当通光口径大于20 mm时，像质也会变坏。国外研究机构常采用离轴三反射结构来设计大口径大视场反射式红外光学系统，但离轴三反射结构的光学系统加工和装调难度极大[8]。
折射式系统由多片不同材料的透镜组成，如图 3所示，可利用一个镜片的像差去平衡另一个镜片的像差，在满足大相对孔径、大视场的同时可得到良好的像质。但是，由于红外透光材料的尺寸受到较大的限制，折射式光学系统不能做成太大的口径。而且折射式光学系统通常只用于特定且范围有限的波段，例如1~3 μm波段、3~5 μm、8~13 μm波段。这是因为同时能透过很宽波段的红外光学材料种类很少，同时，由于光学材料色散的存在，同时消除两个波段的色差比较困难。
折反射式系统由反射镜与折射式校正透镜组组合而成，既具有反射式系统口径可以做得很大的优点，同时也具有折射式系统的大相对孔径、大视场的优点，但比较难实现大视场要求，并且存在中心遮拦。
1.5 短波红外与可见光谱段、中长波红外谱段成像优点比较
可见光作为传统的光学成像波段，具有波长短的特点，可以获得高分辨率图像，但在光照条件不好的情况下如夜晚，无法对目标实现有效观测。如图 4所示，短波红外可进行夜晚观测且穿透障碍物能力强，如图 5所示，短波红外还可以对伪装的目标进行识别。
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图 4 夜间可见光成像（左）和短波红外成像（右）Figure 4. Night visible light imaging (left) and short-wave infrared imaging (right)![]()
图 5 可见光（左）和短波红外（右）伪装识别图像对比Figure 5. Comparison of visible light (left) and short-wave infrared (right) camouflage recognition images相较于传统可见光成像光学系统，红外成像光学系统由于红外光热成像的特点，可以对空间目标实现全天时全天候的观测，且可以实现目标温度差异化观测，对其内部核心部件及其功能的推测提供有效依据，进而实现空间目标的精确识别。具有抗干扰性能好和作用距离远的特点。但红外光波长较长，分辨率较低，无法实现对目标的高分辨率观测。
短波红外光学系统所成的图像具有明显的阴影和强烈的反差对比，如图 6所示，这些丰富的细节特征信息为短波红外图像提供了远高于中波红外和长波红外图像的分辨率和动态范围，以及媲美可见光图像的成像质量。
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图 6 可见光、短波红外、中波红外、长波红外成像效果对比Figure 6. Comparison of visible light, short-wave infrared, medium-wave infrared and long-wave infrared imaging effect2. 短波红外探测器
2.1 短波红外探测器概述
短波红外对人眼不可见，其探测需要专门的探测器。短波红外成像技术起步较晚，但发展迅速，普通的硅基探测器值只覆盖了很少一部分短波波段，锗基探测器的红外噪声较高，目前常采用的有碲镉汞和铟镓砷两种材料所制成的探测器。碲镉汞探测器常需要制冷，故使用范围不广[9]。根据不同应用场景下对短波相机应用需求的不同，市面上主流的短波红外探测器以硅基（Si）、锗基（Ge）以及铟镓砷基（InGaAs）为主。常规的InGaAs短波红外探测器件的波长覆盖范围为0.9~1.7 μm，通过Inp衬底去除技术可以在短波方向将探测器的截止频率延伸至0.4 μm，通过调节In材料的组分，可以实现InGaAs材料的探测波长外延至2.5 μm，如图 7所示，分别为截止频率在1.7 μm、2.2 μm和2.5 μm的探测器的光谱响应和量子效率。当波长延伸后，铟镓砷探测器可更广泛地应用于短波红外夜视和探测领域。
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图 7 不同截止频率InGaAs探测器光谱响应（左）及量子效率（右）Figure 7. Spectral responsivity (left) and quantum efficiency (right) of InGaAs detectors with different cutoff frequencies铟镓砷探测器具有灵敏度高、小型化、低功耗、可室温下工作等特点，是近几年短波红外探测器的不二选择。InGaAs短波红外探测器从探测器的结构上来区分，主要分为单元探测器、线列探测器和面阵探测器。面阵短波红外探测器因其像素数量多、面阵覆盖范围大，适合做凝视型短波红外场景成像。相比于单元探测器和线列探测器而言，面阵短波红外探测器在进行短波红外场景成像时具有帧率高、无需扫描机械设备、便携性强等优点[10]。经过几十年的发展，国外已经实现大面阵、高像素的铟镓砷探测器的产业化。国内目前市面上所推出的铟镓砷探测器主要是15 μm中心距、像元数640×512规格的，更高规格的正在陆续推出[11]。上海技术物理研究所研发的面阵InGaAs短波探测器发展过程如图 8所示。
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图 8 上海技术物理研究所InGaAs探测器组件发展史Figure 8. History of InGaAs detector assembly at Shanghai Institute of Technical Physics未来铟镓砷探测器向着更加小型化、分辨率高、阵列规格更大的方面发展，且进一步降低成本，提高民用领域应用占比。为了向宽波段响应方向发展，国内外相关研究机构正在对铟镓砷探测器的新结构进行研究，例如：集成微纳人工结构的铟镓砷焦平面探测器。为了向减小像元中心距，增大阵列规格，提出胶体量子点短波红外探测技术，以及单像素成像技术[12]。
若为短波红外焦平面探测器设计相匹配的偏振光学系统，可实现分时偏振成像、分振幅偏振成像和分孔径偏振成像。这种传统的偏振成像系统组件过程简单对探测器加工工艺也没有特别高要求，但是光学部分光路较为复杂且光通量损失较高，整个偏振探测成像系统总体体积也会较大。
近年来，焦平面阵列（Focal Plane Array，FPA）技术的发展和微纳加工工艺的提高直接促进了新型偏振探测器的发展。具有特殊形态的微纳半导体结构，比如纳米片和椭圆形纳米线，可以具有高效光电转化和偏振探测功能直接实现偏振高效探测。将微纳阵列集成在短波红外探测器之上可以对动态目标的偏振图像信息进行处理时直接融合进行偏振成像。微纳金属光栅偏振片与探测器焦平面的光敏区的距离应尽可能近，即铟镓砷探测器的衬底应尽可能做薄或者去除InP衬底。偏振元件与焦平面探测器集成的短波红外探测器可以同时获取到接收到的红外辐射的强度和偏振的多维信息，同时具有体积小、重量轻、能耗低、可靠性强等优势，是未来新型红外探测器的热门发展方向[13]。
2.2 短波红外探测器国内外发展概况
随着短波红外探测技术的发展，铟镓砷探测器的使用潜力已经被世界各个行业所认可，全世界大多数国家和研发机构都在进行深入研究，以美国、法国为代表的许多发达国家在基础研究处于世界领先水平[14]。据报道，美国的Sensors Unlimited Incorporation（SUI）公司推出的面阵列的铟镓砷探测器的分辨率达到1920×1280，并推出了所对应的成像仪。2018年，应美国海军的战术开发光谱和侦察图像（SPRITE）计划要求，美国联合技术公司航空航天系统传感器业务部（UTC Aerospace Systems）开发出世界上分辨率最高的铟镓砷（InGaAs）近红外/短波红外（NIR/SWIR）成像传感器，该传感器像元间距5 μm，阵列规格为4 k×4 k的高分辨率铟镓砷探测器。
比利时Xenics公司产品覆盖0.9~1.7 μm红外谱段，部分产品更可扩展到0.4~2.35 μm谱段，推出的最新型T2SL短波红外相机，具有超低暗电流和ROIC噪声。
以色列Semi Conductor Devices（SCD）公司所研制开发的两种不同规格的短波红外探测器均采用铟镓砷焦平面阵列，像元中心距由15 μm降到10 μm，并有640×512和1280×1024两种规格。日本索尼公司研发的探测器产品响应波长为0.4~1.7 μm，在此响应波段内测得探测器的量子效率＞60%，但器件灵敏度和噪声未见报道。
除此之外，法国、德国、英国等许多国家也正在大力支持该国的短波红外光学成像技术的基础研究发展，并正在实现分辨率为640×512的铟镓砷焦平面探测的产业化[15-16]，如表 2所示。
表 2 国外代表性InGaAs探测器厂家及器件性能Table 2. Foreign representative InGaAs detector manufacturers and device performanceCountry Manufacturer Array specifications and pixel dimensions Response wavelength /μm Quantum efficiency QE% America SUI 1280×1024/12 μm 0.9(0.7)-1.7 μm ≥ 65% from 0.9 μm to 1.6 μm Japan HORIBA 1024×1
25 μm×500 μm0.8-1.7μm ＜80% from 1 μm to 1.6 μm Britain Raptorphotonics 1280×1024/10μm 0.4 - 1.7μm Peak＜85%(＜73% @1.064μm, ＜80% @1.55μm) Germany Allied Vision echnologies 640×512/15 μm 0.9 μm to 1.7 μm ＞75% France Sofradir 640×512/15 μm 0.9(0.4)~1.7 μm ＜70% from 1 μm to 1.6 μm 在国内领域，中国电子科技集团公司第四十四研究所、上海技术物理研究所、昆明物理研究所等都对铟镓砷探测器进行了相关研究。
中国电子科技集团公司第四十四研究所在红外焦平面领域研究颇深，在2015年期间，该研究所完成了可见短波红外铟镓砷探测器的技术开发，其单个像元尺寸做到了25 μm，像元规模为640×512，且该探测器的平均峰值探测率也可达5×1012 cmHz1/2W－1。
上海技术物理研究所在红外领域深耕多年，在短波红外波段的探测器研发成果颇多，近年来已经研制多个波长的线面阵探测器，还研制了平面型线阵的铟镓砷焦平面探测器，还研发了两种大面阵探测器，其像元规模分别为512×256元和1024×1280元，其截止波长扩展到2.5 μm，在国内同时期也属前沿。
国内其他公司也正在研制并生产规格为640×512的InGaAs焦平面探测器。表 3列出了一些国内主要InGaAs科研生产单位的生产情况。
表 3 国内主要InGaAs科研生产单位情况Table 3. Major domestic InGaAs research and production unitsUnit Characteristics Representative product Application direction Shanghai Institute of Technical Physics Professional infrared detector development unit, with the technical capability of spectral extension (-2.4 μm) Area array detector:
640×512@25 μm
1280×1024@15 μmAerospace application Chongqing photoelectric Technology Research Institute(CLP 44 Institute) Has a more mature unit detector and InGaAs - APD avalanche detector products Units &APD devices:
320×256@30 μm
640×512@15 μmPhotoelectric communication Kunming Institute of Physics Professional infrared detector development unit, took the lead in the development of digital, wide spectrum InGaAs detector, with strong scientific research, production and application promotion capabilities Area array detector:
640×512/15 μm,
1280×1024/10 μm
Line detector:
1024×2/12.5 μm、512×2/25 μmAerospace and civil fields Shanxi Guohui optoelectronic technology Co., LTD Preparation of high performance focal plane detector, movement design and application Area array detector:
320×256@30 μm
640×512@15-25 μmCivil domain Xi 'an Liding Optoelectronic Technology Co., LTD Movement development Area array detector:
320×256@30 μm
640×512@15-25 μmCivil domain Beijing Lingyun Light Technology Group With industrial detection sorting, spectrum analysis system development and application capabilities 640×512@15 μm
1024×2@12.5 μmSystem development applications, such as spectrometers 目前，国内市场上常见的铟镓砷短波红外探测器的规格是15 μm、640×512、1280×1024，2560×2048阵列规格的探测器还在陆续推出，像元间距缩小到10 μm及以下，器件响应波段向可见光波段拓展，各类探测器产品性能指标正在追赶国际水平。但国内的短波红外成像系统研究在民用和商用上还不够普遍，我国使用的短波红外探测系统和国外发达国家已经装备上系统相比还有很大差距，所以为了满足国内科技市场的需求，开展短波红外铟镓砷探测领域的研究显得十分必要。
3. 短波红外成像系统的应用
短波红外成像的应用领域非常广泛，不仅可以用于夜视、对地遥感探测等应用领域，还可用于半导体表面特征分析、医疗、农产品检测等民用应用领域。
短波红外相机配合短波红外激光，在夜间时进行主动照明成像，可以实现低照度成像且由于有辅助照明光源可提高夜间作用距离，如图 9所示。且短波红外激光作为辅助照明光源，传统微光夜视设备无法探测到。短波红外光也可用于激光的探测领域，作为激光器的光源，常用波段如1064 nm、1330 nm和1550 nm等。短波红外能实现主动和被动式成像，短波红外光具有高透性，能够增长探测的作用距离。
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图 9 雪夜对3.6 km处防火塔进行辅助照明成像Figure 9. Auxiliary lighting imaging was performed on the fire tower at 3.6 km on snowy night机载光学系统在侦察活动中是不可或缺的装备之一，相比其他侦察系统隐蔽性强，可对侦查目标实时成像。短波红外成像技术在机载侦察方面逐渐被广泛利用，这类装备了红外光学成像系统的机载系统可适应各种恶劣侦察环境的高隐蔽性和抗干扰能力，故可代替地面人员在各种恶劣的环境及高危险性的情况下进行侦察观测任务[17]。
短波红外成像技术在空间光学对地遥感中得到了长足的发展和广泛的应用。空间光学对地遥感技术的基础是光学成像技术，早期的空间光学对地遥感主要集中在可见光波段，对于电磁波谱特征分布较宽的地物目标，可见光不能准确、全面地反映其特征信息。空间光学对地遥感技术的光谱域从最早的可见光向近红外、短波红外、热红外、微波方向拓展。如图 10所示，神舟三号飞船搭载了由中国科学院上海技术物理研究所研制中分辨率成像光谱仪，实现了我国空间成像光谱仪零的突破，同时这也是全球第一款覆盖连续可见光近红外光谱、短波红外、热红外的成像光谱仪。
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图 10 神舟三号光谱仪整机结构和在轨影像图Figure 10. Overall structure and in-orbit image of Shenzhou 3 spectrometer短波红外成像可用于太阳能电池品质检测，也可根据不同塑料的不同的光谱特性来应用于塑料制品分拣。短波红外对100℃以上的高温物体热辐射敏感因此可用于高温物体监测。
由于生物体中不同成分对短波红外的吸收系数不同，生物医学成像也是短波红外的一个重要的应用方向。基于短波红外的光学相干断层扫描（SD-OCT）技术通过光学相干断层扫描可以获得血管以及神经纤维的分布图，通过短波红外血管造影甚至可以观测到血管中血液的流动，可以对病变组织进行监测和诊断。
由硅制作而成的半导体晶圆和芯片在短波红外光下是透明的。在对太阳能电池板上的裂缝和缺陷进行检测或者实现集成电路故障分析的时候采用短波红外照明的硅基设备成像采集系统可以有效实现对表面特征的观测。如图 11所示。
新鲜的水果表面无法看出损伤，通过短波红外成像可以透过表皮看见水果内部的伤痕，如图 12所示，方便鉴别水果的品质。这是因为短波红外在水分中的吸收率较高，以至水果受伤的部位对短波红外的吸收性较强。另外，不同的农产品的反射率也不同，尤其是当农产品中混合着其他杂质的时候，短波红外可以用来做品类筛检[18]。
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图 12 短波红外在农产品检测领域的应用Figure 12. Application of short-wave infrared in the field of agricultural product detection近年来，随着铟镓砷探测器的加工工艺成熟与焦平面探测阵列的发展，以及非制冷型探测器的发展，利用短波红外在大气中较强的透过率的特性，以及对烟雾较强的穿透力，在对粉尘和雾霾较多的城区、烟雾弥漫的火灾现场成像时，高空短波红外成像能获得相比于可见光和长波红外等波段更多的细节特征。在民用和商用的领域，也随处可见短波红外出现的身影。在民用的车载、测温枪和安防镜头等，在商用的船载、激光测距仪和无人机拍摄等使用了短波红外光的拓展。如图 13所示，在无人机拍摄方面，环境的复杂性限制了可见光拍摄的范围，在可见度不高的烟雾环境下，无人机使用可见光拍摄不能够很好地拍摄到物体的清晰影像，而使用短波红外成像拍摄无需借助外界的环境光照射，其拍摄距离也更远，像质更清晰[19]。
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图 13 可见光（左）和短波红外（右）透烟成像Figure 13. Visible light (left) and short-wave infrared (right) through smoke imaging4. 总结与展望
短波红外相机与中长波热成像相机捕获图像信息不同，短波红外相机接收物体反射的环境中的辐射，所得到的输出图像类似普通相机输出的可见光图像，图像中存在阴影和其他细节，具有高对比度。此外短波红外相机可放置在玻璃（车窗）后，但热成像相机目前不行。相比于中长波成像被动接收物体自身发出的红外辐射，短波红外成像相机可与激光距离选通成像技术结合，通过主动辅助照明光源，控制短波红外激光照明特定目标点，滤除非目标点的反射辐射，自主选择需要成像的目标进行成像。
短波红外光学成像相机设计过程中涵盖了光学设计、结构设计、电路设计、图像处理等众多关键技术。高分辨率的短波红外成像相机需要搭配先进的大规模的焦平面探测器，还要求焦平面探测器具有高灵敏度响应、响应光谱范围宽、均匀性好等高性能。在极限弱光照环境下，短波相机的图像增强技术占重要地位，图像增强方法一般有：灰度变化方法、直方图均衡方法、频域变换方法以及深度学习方法。
短波红外光学成像相机中光学设计是核心部分，光学系统的通光效率直接影响整个相机系统接收到的能量。与中长波的光学设计方式不同，短波红外光学设计的系统口径要小于中长波系统的光学口径，采用的红外材料也与中长波所用的光学材料不同，需采用镀专属的透射膜的光学镜片。
短波红外光学成像的主要优点有：1）当选用短波红外激光作为辅助照明时，具有隐蔽性；2）在低照度或环境情况恶劣时仍能维持工作；3）短波红外能透过常规光学玻璃材料成像，设计和优化的自由度被大幅提升。
随着民用商用和其他领域的需要，若想获得包含更多信息的图像，需要的不止可见光波段的图像，故现代光学成像工作波段向近红外、短波红外拓展，短波红外所具有的独一无二的光学成像特点使得短波红外光学成像在未来具有广阔的应用前景和巨大的开发潜力。由于空间目标的复杂与多样化，单一波段的成像已经无法满足，构建短波红外与可见光或和中波红外、长波红外一体化的成像光学系统，实现多个波段同时成像，可以大大提高适用性[20]。目前短波光学成像设计具有如下的发展趋势：
1）微光夜视条件下的短波红外成像及目标检测；
2）多波段的机载红外光学系统设计；
3）短波红外激光距离选通成像技术；
4）短波红外光探测系统和可见光望远系统共光路成像设计；
5）短波红外相机结合显微光学结构镜头，进行多聚焦图像融合设计等。
短波红外相比于中波、长波等红外相机能够更多地采集到被观测物体或者人员的细节信息，能够极大地提升被观测物体与人员的肉眼可识别性。短波红外光学成像相机在民用领域应用方面需要向低成本、易操作方向发展；在其他应用领域应用方面需要向高灵敏度、高性能方向发展，应用前景广阔。国内目前铟镓砷探测器的发展水平逐步上升，短波红外光学成像系统设计方面需要跟进发展，在保证光学系统的成像质量的基础上，实现更简单轻量、视场更大的多波段红外光学系统。
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图 3 OLED电致发光结构示意图 (a)单层白光OLED器件，(b)多层白光OLED器件，(c)叠层白光OLED器件
Figure 3. OLED electroluminescence structure diagram (a) Single EML white OLEDs, (b) Multiple EMLs white OLEDs, (c) Tandem white OLEDs
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图 4 CGL能级性质及CGL性能检测方法 (a)分别为Alq3/Mg: Alq3、Alq3/Yb: Alq3和Alq3-Ca: Alq3界面的能级图[75]，(b) CGL器件示意图，(c) CGL器件的C-V分析(@100Hz)[79]
Figure 4. CGL energy level properties and CGL performance testing methods (a) Energy level diagrams for Alq3/Mg: Alq3, Alq3/Yb: Alq3 and Alq3/Ca: Alq3 interfaces, respectively[75], (b) Schematic diagram of a CGL device, (c) C-V analysis of the CGL device (@100 Hz)[79]
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图 5 磷光叠层OLED白光器件的光电性能 (a)首个磷光叠层OLED白光器件的发光光谱随电压变化的趋势[61]，(b) 器件1和器件2的电流效率/外量子效率与电流密度之间的关系[92]
Figure 5. Electroluminescent properties of phosphorescent tandem OLEDs (a) Trending of luminescence spectrum with voltage in the first phosphorescent tandem OLEDs[61], (b) Relationship between current efficiency/external quantum efficiency and current density for device 1 and device 2[92]
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图 6 磷光叠层OLED白光器件的能级、结构及光电性能 (a) 采用Ir(ppy)3及PQIr共掺杂CBP主体材料红绿光以及FlzIr掺杂mCP主体材料蓝绿光的器件材料及能级结构示意图(插图：FlzIr结构式)[64]；(b) 采用蓝色磷光掺杂材料FIrpic以及橙色磷光掺杂材料(fbi)2Ir(acac)共掺杂mCP主体材料中的高效叠层有机磷光发光白光器件的功率效率、外量子效率(插图：电压-亮度及电压-电流密度图)[97]以及结构示意图[98]
Figure 6. Energy level, functional layer structure and electroluminescent properties of phosphorescent tandem OLEDs (a) The device material and energy level structure of Ir(ppy)3 and PQIr co-doped CBP main material red-green light and FlzIr doped mCP main material blue-green light are shown (Inset: FlzIr structure formula)[64]; (b) Power efficiency, external quantum efficiency (Inset: voltage-brightness and voltage-current density plots)[97] and structural schematic[98] of highly efficient tandem white OLEDs using FIrpic and (fbi)2Ir(acac) co-doped mCP bodies
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图 7 采用TCTA: 三[4-咔唑-9-基苯基]胺（TCTA）: 4, 6-双(3, 5-二(3-吡啶)基苯基)-2-甲基嘧啶(B3PYMPM: Ir(ppy)2(tmd)): 二[2-苯喹啉]四甲基庚二酸铱(Ir(mphmq)2(tmd))的橙色磷光EML以及mCP: B3PYMPM: FIrpic的蓝色磷光EML的叠层磷光OLED白光器件结构及优化 (a)功能层结构；(b) 基于经典偶极子模型对橙色磷光OLED器件以及叠层磷光OLED白光器件进行模拟优化[99]
Figure 7. Structure and optimization of tandem white OLEDs' structure with TCTA: B3PYMPM: Ir(ppy)2(tmd): Ir(mphmq)2(tmd) for orange phosphorescent EML and mCP: B3PYMPM: FIrpic for blue phosphorescent EML (a) Functional layer structure; (b) Optimization of devices based on the classical dipole model[99]
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图 8 TCTA: Bphen作为蓝色TADF EML以及TAPC: 3P-T2T作为橙色TADF EML的光学分析 (a)归一化发射光谱以及(b)在300 K下不同延迟时间下的归一化时间分辨光致发光光谱[113]
Figure 8. Optical analysis of tandem white OLED with TCTA: Bphen as blue TADF EML and TAPC: 3P-T2T as orange TADF EML (a) Normalized emission spectra and (b) normalized time-resolved photoluminescence spectra with different delay times (@300 K)[113]
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图 10 BRU+YRU混合叠层OLED白光器件能级结构及工作机理 (a) 能级结构示意图，以及蓝色TADF和黄色和红色磷光掺杂料的化学结构，(b)工作机制[122]
Figure 10. Energy level structure and working mechanism of BRU+YRU hybrid tandem OLEDs (a) Energy level structure, and structure formula of blue TADF, yellow and red phosphorescent dopants, (b) Working mechanism[122]
表 1 常见叠层OLED器件CGL的组成
Table 1 Composition of CGL for tandem white OLEDs
CGL结构及组成(n║p) Ref. LiF/Ca║Ag [43] LiF/Al║Ag [43] LiF/Al║Au [44] Al║WO3/Au [45] LiF/Al║WO3/Ag/MoO3 [46] LiF/Al║WO3/Au/MoO3 [47-48] LiF/Al║Dipyrazino[2, 3-f: 2', 3'-h]quinoxaline-2, 3, 6, 7, 10, 11-hex（HATCN） [49-50] LiF/Al║MoO3 [51-52] Lithium 8-Hydroxyquinolinolate（Liq）/Al║MoO3 [53] Zinc Phthalocyanine（ZnPc）║C60 [54] Copper(Ⅱ) phthalocyanine（CuPc）║C60 [54] Pentacene║C60 [55] Bathocuproine（BCP）: Cs║ITO [25, 56-57] BCP: Cs║V2O5 [58-60] BCP: Li║V2O5 [58, 61] Tris-(8-hydroxyquinoline)aluminum（Alq3）: Mg║WO3 [62-63] 4, 7-Diphenyl-1, 10-phenanthroline（BPhen）: Li║MoO3 [64] Alq3: Yb║WO3 [65] Alq3: Li║MoO3 [66] Alq3: Mg║WO3 [67] Alq3: Li║HATCN [68] 1, 3, 5-Tris(1-phenyl-1H-benzo[d]imidazol-2-yl)benzene（TPBI）: Li║NPB: FeCl3 [69] BPhen: Cs║NPB: 2, 3, 5, 6-Tetrafluoro-7, 7, 8, 8-tetracyanoquinodimethane（F4-TCNQ） [70] TPBI: Cs║4, 4', 4''-Tris(N-(aphthalene-2-yl)-N-phenylamino)triphenylamine（2-TNATA）: F4-TCNQ [71] BPhen: Rb2CO3║NPB: ReO3 [72] BPhen: Li║NPB: F4-TCNQ [68] BPhen: Cs2CO3║NPB: MoO3 [73] 
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