柔性光热电探测器研究进展
Flexible photothermoelectric (PTE) detectors have considerable research significance owing to their unique characteristics, including flexibility and PTE properties.
Flexible PTE detectors have the characteristics of lightness, flexibility, and softness, allowing them to be attached directly to irregular surfaces for continuous measurement of spatial information. They have considerable potential in the development and fabrication of miniaturized energy equipment, virtual-reality interactive systems, and implantable medical devices, which have application prospects in new energy, microelectronics, artificial intelligence, medical care, and other fields. They are also attractive for use in wearable devices, as they offer several advantages over traditional rigid sensors. These detectors can be easily bent or shaped to fit the contours of the human body, which allows comfortable and unobtrusive monitoring of physiological parameters.
Furthermore, the PTE properties of these detectors allow them to have ultra-broadband responses. In contrast to other types of detectors, which are typically limited to a specific wavelength range, PTE detectors can detect light across a wide range of wavelengths, from ultraviolet to terahertz. This makes them highly versatile and useful for various applications, including spectroscopy, imaging, and sensing. Another advantage of PTE detectors is their high speed. The PTE response breaks the limit of the low response speed of traditional thermal detectors by introducing hot carrier-assisted heat conduction. This fast response makes PTE detectors well-suited for applications that require rapid detection, such as high-speed imaging and sensing. Additionally, they can operate under zero-bias and room-temperature conditions, which makes them convenient and cost-effective to use. In contrast, other types of broadband detectors, such as bolometers, typically require a bias voltage to operate and may require cooling to achieve optimal performance.
Overall, the research into flexible PTE detectors has significant implications for the development and applications of novel electronic devices. In the past 20 years, the field has continued to advance, and there has been a large amount of research on new types of flexible PTE detectors. However, they face a series of challenges related to detection performance and manufacturing process improvement. Therefore, it is necessary to provide an overview of flexible PTE detectors to lay the foundation for the development of flexible optoelectronic technology.
In this review, we first describe the key parameters of flexible PTE detectors, including the responsivity, response time, cutoff frequency, noise equivalent power, and specific detectivity. Then, we summarize the research progress of flexible PTE detectors with detection wavelengths ranging from visible to terahertz and introduce the exploration, application, and optimization mechanism of carbon materials and inorganic and organic compounds with flexible properties in the field of PTE detection. Suzuki’s research group made significant contributions to the application of CNTs in flexible PTE detectors (Fig.4). They developed a variety of flexible CNT-based PTE detectors for different use scenarios (Fig.5) and applied them to detect terahertz light (Fig.7). In addition to CNTs, many other new materials, such as reduced graphene oxide (Fig.10), topological insulators (Fig.11), transition-metal halide (Fig.12), quasi-one-dimensional materials (Fig.13), MXenes (Fig.14), and PEDOT (Fig.15) have been studied and applied to flexible PTE detectors and have exhibited good performance. Combinations of conducting polymers and carbon materials for flexible PTE detectors have been widely studied in recent years. Studies on graphene/PANI, graphene/PEI (Fig.20), and PBI/MWCNTs (Fig.21) indicated that it is easier to prepare high-performance flexible PTE detectors by combining these materials than by using them alone. Finally, the problems faced and the ongoing research trends in this field are discussed, including methods for improving the detector performance, the evaluation criteria for flexibility, and the manufacturing and human compatibility problems in practical applications.
Flexible PTE detectors can revolutionize the field of photodetectors. We expect that they will become increasingly important—particularly in the development of wearable devices and other flexible electronics. We expect to see further advancements in these detectors, including improvements in sensitivity, response time, and reliability. To achieve these goals and promote the practical application of flexible PTE detectors, it is necessary to explore new materials, design the detector structure, and formulate unified evaluation standards.
1 引言
光电探测器是能够将光转换为电信号进行探测的器件［1-2］，在环境监测、生物传感、通信、雷达等工业或**领域得到了广泛应用。对于长波红外至太赫兹波段的低能量光子探测来说，常用的光子型光敏材料是窄带隙半导体和量子阱材料［3-4］，基于这些材料的探测器在室温下的暗电流通常很大，因此低温冷却单元是必不可少的，这无疑增加了探测器的尺寸、重量和复杂度［5-6］。因此，开发高性能非制冷光电探测器至关重要。
与传统的光子型光电探测器相比，基于热效应的光电探测器能够在室温下探测从红外到太赫兹波段的超宽带辐射，其工作原理是测量与温度相关的物理量，例如辐射热计测量电阻变化，热释电探测器测量由自发极化材料温度变化引起的极化电荷变化，光热电（PTE）探测器测量温差产生的电动势。其中：辐射热计在使用时需要加入外部偏压，这会引入额外的噪声；热释电探测器灵敏度较低、响应速度较慢，同时需要使用斩波器才能测量连续波辐射。这些问题限制了辐射热计与热释电探测器的进一步发展。光热电探测器工作时不需要施加偏压，从而可以忽略外加偏压带来的噪声，并且其响应速度快、室温灵敏度高，是未来发展宽带高性能探测器的重要方向。
传统光热电探测器的光敏和衬底材料主要通过高温外延生长方法在晶片上生长无机半导体材料来制备，采用这些材料制备的光热电探测器具有材料本身固有的刚性，并且制备工艺复杂、成本高昂。与刚性基板上的光热电探测器相反，柔性光热电探测器能够弯曲、折叠以及拉伸，可应用在可穿戴设备、电子皮肤、智能纺织品等领域［7］，因此将各种柔性热电材料制作成柔性光热电探测器，探究其物理机制并改进其探测性能逐渐成为了研究热点。
本文综述了近年来柔性光热电探测器的研究进展和应用前景。首先介绍了光热电探测器的原理和一些典型指标，之后介绍了基于不同柔性材料的光热电探测器，这些柔性材料包括碳纳米材料、无机化合物柔性材料、有机柔性材料以及无机-有机复合材料，其响应波段包括可见光到太赫兹波的多个波段。最后，简要总结和展望了柔性光热电探测器的发展趋势以及未来面临的挑战。
2 光热电探测器的原理和性能指标
2.1 光热电探测器的原理
光热电探测器的响应机制是光-热-电转换效应。如
式中：k是玻尔兹曼常数；T是绝对温度；e是元电荷；EF是费米能量。光热电探测器的灵敏度取决于光热转换效率和热电转换效率。通过天线、吸收体结构等增强入射光与光敏材料的相互作用，可以提高光热转换效率。热电转换效率可以通过材料的功率因子（PF=S2σ）和热电优值（ZT=S2σT/κ，其中κ是材料的热导率）来评价，功率因子和热电优值越大，材料的热电转换效率越高。为了实现高热电转换效率，材料应具有较大的Seebeck系数（以建立较高的开路电压）和较高的载流子迁移率（以提升导电性），同时还应具有较低的热导率（以建立较大的温差）。
目前适合用于柔性光热电探测器的光敏材料可以分为三类，如
2.2 光热电探测器的性能指标
与常规的光电探测器类似，光热电探测器的性能通常用光响应度、响应时间、截止频率、噪声等效功率、归一化探测率等指标来评价。
1）光响应度
光响应度衡量的是探测器对入射光信号的响应灵敏度。当功率为P的光入射到探测器上时，探测器输出的光生电流或光生电压表示为Ip=RIP或Vp=RVP，Ip和Vp分别表示光生电流和光生电压，RI和RV即被称为光电探测器的响应度，单位分别为A/W和V/W。
2）响应时间和截止频率
响应时间反映的是探测器对信号的响应速度，包括上升时间和下降时间。上升时间和下降时间通常分别定义为从光生电流的10%上升到90%以及从90%降低到10%所需要的时间。
光热电探测器的响应度随入射光调制频率f的升高而降低，其表达式为
式中：R0表示调制频率为0时的响应度。一般规定R（f）下降到
3）噪声等效功率
噪声等效功率（NEP，在公式中记为NEP）又称最小可测功率，是使探测器输出信号能够从噪声电压或电流中检测出所需的最小入射信号功率，是衡量光电探测器接收弱信号能力的参数。NEP最常用的定义如下：在1 Hz输出带宽中，信噪比为1时所需的输入信号功率［11］。对于光热电探测器，NEP标志着探测器的灵敏度，单位为W/Hz1/2。
4）归一化探测率（D*）
归一化探测率又称比探测率，也是表征光电探测器对弱信号探测能力的参数。
3 碳材料柔性光热电探测器
由于具有独特的晶体结构和优良的电学、光学、热学及力学性能，碳纳米管和石墨烯在光电探测器中展现出了巨大的应用潜力［12-13］。此外，一维碳纳米管和二维石墨烯作为低维材料，相比于传统的无机热电材料具有更小的尺寸和更大的表面体积比，在柔性电子方面展现出了更优异的性能［14-16］。随着对碳材料性能的逐渐了解以及碳材料制备工艺的改进，部分研究人员开始以碳材料为主体来制备柔性光热电探测器，这也是柔性光热电探测领域研究的一个新趋势。
3.1 碳纳米管柔性光热电探测器
碳纳米管可以看作是单层碳原子卷起形成的空心圆柱体，具有高的纵横比（可大于1000），因此具有一维纳米结构的特征，如
图 3. 碳纳米管示意图［17］。（a）单壁碳纳米管（SWCNT）；（b）多壁碳纳米管（MWCNT）
Fig. 3. Schematic diagram of CNT[17]. (a) Schematic diagram of an SWCNT; (b) schematic diagram of an MWCNT
碳纳米管光热电探测器的一个重要应用是太赫兹探测器。太赫兹探测器是太赫兹传感、成像、通信等系统中的核心器件［23-24］。传统的太赫兹探测器在灵敏度、带宽等方面存在局限性，因此探索全新的太赫兹探测器制备方法或材料体系具有重要意义。目前人们已经报道了在低温下使用天线耦合和单个金属单壁碳纳米管进行太赫兹探测的工作［25］，也研究了单管器件中太赫兹频率的电子传输现象［26-27］。碳纳米管的一个特殊优势是能够通过调节管径、长度、手性等参数对其光电性能进行优化，从而增强光响应。2014年，He等［28］开发了一种室温碳纳米管p-n结太赫兹光热电探测器，该器件无须供电，具有紧凑、柔性等特点，能够实现宽带响应且具有偏振选择性。如
图 4. 不同类型的碳纳米管柔性太赫兹光热电探测器。（a）偏振灵敏的碳纳米管太赫兹光热电探测器示意图［28］；（b）弯曲的碳纳米管薄膜照片与水平排列的碳纳米管的光学显微图像［28］；（c）对注射器进行多视图扫描拍摄的图像（在不使用大体积部件的情况下检测到注射器上的破损）［29］
Fig. 4. Different types of CNTs flexible THz PTE detectors. (a) Schematic diagram of polarization-sensitive carbon nanotube PTE detector[28]; (b) photograph of the curved CNT film and optical microscopy image of horizontally aligned CNTs[28]; (c) omnidirectional image taken by multi-view scan of a syringe (a breakage on the syringe is detected without bulky components)[29]
2016年，Suzuki等［29］开发了可弯曲的太赫兹相机，并演示了弯曲样品的360°多视角太赫兹成像，如
基于对碳纳米管材料的探索，Suzuki团队［33］近年来开始设计不同构型的柔性碳纳米管光热电探测器，以适应不同的使用场景，如
图 5. 应用于不同场景的碳纳米管柔性光热电探测器。（a）高度集成的
Fig. 5. CNT flexible PTE detectors for different applications. (a) Schematic diagram of high integrated π‑shaped pixel structure[34]; (b) photo-induced voltage response as a function of the number of series PN junction of the present CNT film-based stretchable device[35]; (c) non-destructive reflective multi-view stereoscopic photo-imaging inspection of capsules in a glass beverage bottle[36]; (d) photos of stretchable broadband optical sensor array sheet[37]
除了将柔性光热电材料应用在各种工业场景，人们也在探索将其与可穿戴技术结合，开发出具有更多功能的可穿戴设备。基于有机材料的柔性光电子器件在过去10年中取得了令人瞩目的进展，但它们的制造工艺复杂，且与通用编织技术兼容性不好，性能还有待提高，因此没有得到规模化应用。Zubair等［38］向纤维中掺杂高度定向的碳纳米管，制成了柔性光热电探测器。这种探测器能够对紫外线到太赫兹波段的宽带电磁辐射作出响应，并在整个光谱范围内具有低的噪声等效功率（约为nW·Hz-1/2）。之后，他们制造了一个由数十个p+-p-结组成的1 m长的器件，然后把它缝制到一件Polo衫上，如
图 6. 可缝制在Polo衫上的碳纳米管光热电探测器［38］。（a）缝制在Polo衫上的碳纳米管纤维探测器；（b）~（c）探测器的正面与背面，即Polo衫的外侧和内侧；（d）p+-p-连接处位于Polo衫外部，而p--p+连接点隐藏在Polo衫内部；（e）探测器的两端均连接到外部电路，用于测量感应光电压；（f）I-V特性曲线，曲线在照明下向上移动
Fig. 6. CNT PTE detectors that can be sewn on a Polo shirt[38]. (a) A carbon nanotube fiber detector sewn on the Polo shirt; (b)‒(c) the front and back of the detector, that is the outside and inside of the Polo shirt; (d) p+-p- junction is located outside the Polo shirt, while the p--p+ junction is hidden in the Polo shirt; (e) both ends of the detector are connected to an external circuit for measuring the induced light voltage; (f) I‑V characteristic curve, the curve moves upward under illumination
将柔性光热电探测器排成探测器阵列可以制成柔性光热电成像仪，从而能够在复杂条件下进行无损检测。如何实现探测器对准排列是制作探测器阵列的重要问题。Suzuki等［39］提出了一种自对准抽滤工艺，如
图 7. 不同对准方式制成的碳纳米管柔性光热电探测器阵列。（a）自对准抽滤工艺示意图［39］；（b）使用喷墨方式制成的碳纳米管成像仪照片［40］；（c）~（d）碳纳米管柔性光热电成像仪对刀片进行成像［40］
Fig. 7. CNT flexible PTE detector arrays fabricated with different alignment methods. (a) Schematic diagram of the self-aligned filtration process[39]; (b) photograph of the all-printable CNT film flexible PTE imager[40]; (c)‒(d) a knife image obtained by CNT flexible PTE imager[40]
3.2 石墨烯柔性光热电探测器
石墨烯是由碳原子以sp2杂化连接的二维原子晶体，具有特殊的电子能带结构［41］。石墨烯中的电子服从狄拉克方程［42］，这意味着它表现为无质量的狄拉克费米子，可以进行远距离弹道输运。在这种输运模式中，电子可以在不发生散射的情况下进行长距离移动，因此具有极高的载流子迁移率［43］。此外，石墨烯的硬度高，同时具有良好的韧性和延展性，可以弯曲，因此很多人将石墨烯引入柔性器件领域［44］。
2014年，Liu等［45］设计了一种制造大面积柔性、透明石墨烯光热电探测器的方法。他们利用化学气相沉积（CVD）技术生长石墨烯，并在生长过程中进行n型掺杂，从而在选定区域内得到大量的p-n结，如
图 8. 不同方法制成的石墨烯光热电探测器。（a）石墨烯柔性光热电探测器光响应测试示意图（左）和实物图（右），通道由p型（红色）和n型（蓝色）石墨烯组成［45］；（b）石墨烯探测器在应变下的电特性测量示意图［46］
Fig. 8. Graphene PTE detectors fabricated by different methods. (a) Schematic diagram (left) and experimental photo (right) show photoresponse test of graphene flexible PTE detector, where the channel is composed of p-type (red) and n-type (blue) graphenes[45]; (b) schematic illustration of the electrical characteristic measurement for the graphene detector under strain[46]
还原氧化石墨烯（rGO）也可以用来制作石墨烯柔性光热电探测器。通过化学氧化可以制备微米级厚度的氧化石墨烯（GO），其表面带有含氧官能团，具有良好的水溶性，可以通过印刷或喷涂方法制成薄膜。对GO进行高温退火处理，去除这些官能团，就可以获得rGO。相比于化学气相沉积、液相剥离等方法，这种方法不仅提高了石墨烯的产量，而且能够方便地制备光吸收率更高的微米级石墨烯薄膜，更适合用于制备光热电探测器。Wen等［47］制备了悬空的柔性rGO光热电探测器，这种器件具有较高的光热电转换效率，从而表现出了更高的光响应。此外，退火温度对探测器性能存在影响，如
图 9. 悬空的rGO光热电探测器以及退火温度对其光响应特性的影响［47］
Fig. 9. Schematic diagram of a suspended RGO photodetector and the effect of annealing temperature on its photoresponse[47]
三维（3D）石墨烯，即rGO纳米片交织形成的多孔网状结构，不仅具有单层石墨烯的光学和电学特性，而且表现出了更高的光吸收和更好的热学性能［48-49］，在光热电探测方面具有独特优势［50］。此外，将石墨烯与一些具有高Seebeck系数（如La0.9Sr0.1NiO3［51］）和超低热导率（如CH3NH3PbI3［52］）的材料复合可进一步提升探测性能。Li等［53］在激光刻蚀还原氧化石墨烯（LSG）活性层中引入CsPbBr3晶体并将其作为添加剂，同时采用聚对苯二甲酸乙二醇酯（PET）作为柔性衬底，制备了自供电的超宽带柔性光热电探测器，如
图 10. LSG/CsPbBr3光热电探测器的制作过程和结构示意图［53］。（a）~（e）制作过程；（f）结构示意图
Fig. 10. Manufacturing process and structure diagram of LSG/CsPbBr3 PTE detector[53]. (a)‒(e) Manufacturing process; (f) structure diagram
4 无机化合物柔性光热电探测器
传统的热电材料以碲化铋基无机物为主，这类块体材料一般具有较高的Seebeck系数和较低的热导率，但是由于电导率、Seebeck系数和热导率之间相互影响，很难独立地优化其中的一个参数，其室温热电优值一般在1以下［54］。近年来的研究发现，低维材料中的量子限制效应可以提高功率因子，而其界面及边界对声子的散射又可以降低热导率，因此低维无机热电材料的热电优值可以达到2以上［55］。对无机化合物热电材料进行纳米化处理，不仅能提高其热电优值，还能改变材料的内部结构，使得原本刚性的块体材料柔性化，从而能够制备以无机化合物半导体材料为主体的柔性光热电探测器。
近年来，人们不断探索用于制备柔性光热电探测器的无机材料。一般来说，具有宽带吸收、良好的热电性能并且易于柔性化的材料会被优先考虑。硒化锡（SnSe）是一种性能优秀的热电材料，具有超过500 μV·K-1的高Seebeck系数和约0.7 W·m-1·K-1的低热导率，已经被广泛应用于热电领域［56］。此外，SnSe在中红外波段还具有较高的光吸收［57］。这两个优点使得其成为构建高性能光热电探测器的理想候选材料。Zhong等［58］制成了基于SnSe多晶薄膜的自供电柔性光热电探测器。SnSe薄膜在550 K下的热电优值为0.15，功率因数为0.322 mW·m-1·K-2。与SnSe单晶相比，SnSe薄膜具有更大的带宽，并且表现出更高的响应度和更快的响应速度。
拓扑绝缘体（TI）由于体电子态的拓扑结构而表现出与一般金属和绝缘体不同的量子现象，因而具有独特的光学、热学和电学性质。拓扑绝缘体得益于其表面态的相对论性狄拉克色散关系，能够用于宽带电磁波探测，因此引起了广泛关注。例如，由Bi2Te3［59］制成的光热电探测器在紫外至近红外波段具有约1010 cm·Hz1/2·W-1的高比探测率和约4 mA·W-1的光响应度。Zhou等［60］利用化学气相沉积技术生长的铋薄膜制备了波长范围为405~1064 nm的柔性光热电探测器，如
图 11. 不同无机化合物柔性光热电探测器。（a）~（c）铋薄膜柔性光热电探测器示意图及柔性测试［60］；（d）处于弯曲状态的HfTe5光热电探测器［61］；（e）HfTe5柔性探测器在不同弯曲半径下的光电压［61］；（f）SnTe光热电探测器经过不同弯曲循环次数后的光响应［64］
Fig. 11. Flexible PTE detectors based on different inorganic compounds. (a)‒(c) Schematic diagram and flexibility test of Bi film flexible PTE detector[60]; (d) image of HfTe5 photodetectors in a flexed state［61］; (e) photovoltage of the flexible HfTe5 device at different bending radius[61]; (f) photoresponse of SnTe PTE detector after different bending cycles[64]
拓扑绝缘体与其他材料组成的异质结也可表现出优秀的光响应，比如：拓扑绝缘体/石墨烯异质结光电探测器在532 nm光照射下的光响应度高达35 A·W-1［62］，拓扑绝缘体/硅异质结光电探测器能响应从紫外波段到太赫兹波段的电磁辐射［63］。与拓扑绝缘体相比，拓扑晶体绝缘体（TCI）中的晶体对称性能够保护其拓扑表面态。例如，在SnTe中，由体缺陷和位错引起的散射可以自然地被抑制，这使得SnTe在开发无耗散光电器件方面具有巨大潜力。Liu等［64］将外延生长的SnTe薄膜转移到柔性基板上，以此制造了柔性光热电探测器，该探测器在数百个弯曲周期中表现出优异的机械灵活性，并且保持了1010 Jones的比探测率，如
金属卤化物以其简单的加工方法和独特的光电特性，近年来也被用作光热电探测器的光敏材料［65］。一些研究表明，CH3NH3PbI3可用于制作超宽带光热电探测器［66-67］，CH3NH3PbI3光热电探测器对太赫兹到紫外波段的光辐射具有纳秒级的快速响应时间以及102~105 mA·W-1的光响应度［68］。Gu等［65］使用双源共蒸发沉积技术制备了无铅Cs3Cu2I5柔性薄膜，该薄膜在320~2000 nm波长范围内具有80%以上的光学透明性。用Cs3Cu2I5薄膜制作的器件在零偏压下可以实现从可见光（532 nm）、近红外（980 nm）到太赫兹（119 μm）范围的自供电光热电探测。该探测器具有超过1 mA·W-1的光响应度和约106 cm·Hz1/2·W-1的比探测率。进一步，Gu等将15×15个探测单元组合成阵列，如
图 12. Cs3Cu2I5柔性光热电探测器［65］。（a）紫外光照射下柔性光热电探测器阵列的照片；（b）弯曲后柔性光热电探测器阵列的照片；（c）使用探测器阵列对物体成像的过程图；（d）三维图显示了光热电探测器阵列每个像素上的光电流
Fig. 12. Cs3Cu2I5 flexible PTE detectors[65]. (a) Photo of flexible PTE detector array under UV irradiation; (b) photo of flexible PTE detector array after bending; (c) process of imaging objects using detector array; (d) three-dimensional diagram shows the photocurrent on each pixel of the PTE detector array
相比于二维材料，准一维材料具有更小的尺寸和更大的表面体积比，更易于实现探测器的集成。根据渗流理论，准一维材料高纵横比的链状结构特征，降低了形成导电网络所需的临界密度，因此能够使用较少的材料实现高导电性［68］。此外，准一维结构减少了晶界或缺陷，具有优异的机械稳定性，可以应用于柔性电子器件［69］。在最近的研究中，准一维过渡金属硫族化合物，包括TiS3、ZrS3、ZrSe3、HfS3和HfSe3，已被证明是具有一定潜力的光热电材料［70-73］。Wu等［19］设计了一种基于NbS3的光热电探测器，其在室温下可以实现从紫外光到太赫兹波的超宽带光响应。该器件在零偏压下显示出了高响应度（大于1 V·W-1）和快速光响应（≈7 ms），响应时间远小于由毫米级石墨烯、三元硫系单晶等材料制成的超宽带光电探测器。此外，该器件还表现出了良好的柔性和环境稳定性，如
图 13. NbS3柔性光热电探测器［19］。（a）NbS3柔性光热电探测器弯曲测试示意图；（b）探测器实物图；（c）NbS3柔性光热电探测器的空气稳定性测量
Fig. 13. NbS3 flexible PTE detectors[19]. (a) Schematic of NbS3-based PTE detector in a flexed state; (b) photo of NbS3-based detector; (c) air stability measurement of NbS3-based device
MXene是一类新型的类石墨烯二维晶体材料——过渡金属碳/氮化物（化学式为Mn+1XnTx，其中
图 14. 用Ti3C2Tx墨水笔在不同的基材上书写［75］。（a）用笔在织物、PS泡沫、木材和PE泡沫表面绘制4种中国传统植物的图案；（b）基于该墨水制备的不同宽度的织物芯片
Fig. 14. Ti3C2Tx ink pen writes on different substrates［75］. (a) Use a pen to draw pictures of four traditional Chinese plants on the surface of fabric, PS foam, wood and PE foam; (b) photos of fabric chips based on the ink with different widths
5 有机材料柔性光热电探测器
有机热电材料具有柔性好、热导率低、安全无污染、成本低等优点［77-79］，但它们的Seebeck系数一般比较低，由此导致了较低的功率因子。目前提高有机热电材料性能的方法主要有两种：一是利用酸或有机溶剂等对其进行掺杂，通过调节掺杂比例使热电性能达到最优值；二是将有机热电材料与无机纳米半导体或者碳纳米材料进行复合，复合物一般兼具有机材料较高的电导率和无机纳米材料较高的Seebeck系数，其热电性能有较为显著的提高［80］。近年来，有机热电材料和器件已经在性能上取得了较大突破。例如，通过对聚（3，4-亚乙基二氧噻吩）：聚苯乙烯磺酸盐（PEDOT：PSS）进行掺杂，材料的热电优值达到了0.42［81］。Sun等［82］报道了一种n型热电材料1，1，2，2-乙烯四硫醚（ett）-金属配位聚合物——聚［Kx（Ni-ett）］，其具有0.2的热电优值。鉴于有机材料通常还具有极高的灵活性，利用有机热电材料制备柔性光热电探测器也是一个重要的方向。
有机导电聚合物，如聚吡咯（PPy）［83］、聚苯胺（PANI）［84］和PEDOT［85-86］，是有机热电材料的典型代表，具有柔性、易于聚合、制备简单和成膜性好等特点，易于进行结构设计，并且具有优异的热电性能，因此适用于制备柔性光热电器件［87-89］。其中，PEDOT具有出色的Seebeck效应和导电性［90］，而PPy在红外波段具有优异的光热转换性能，因此将PEDOT与PPy复合可开发出具有较高光热电转换能力的材料。如
图 15. 双壳结构热电织物的制备过程及其柔性和导电性演示［91］
Fig. 15. Preparation process of thermoelectric fabric with double shell structure and demonstration of its flexibility and conductivity[91]
对于掺杂的有机半导体聚合物，传统的块体供体/受体（D/A）异质结在平衡光载流子产生和电荷传输方面通常会受到限制，因此不能在光电探测器相关应用中实现高性能［93］。为了解决这一问题，人们开发了不同种类的多层结构，以促进光载流子的产生，从而赋予光电探测器更好的性能。Ji等［94］通过优化能级结构，制作了具有分层异质结的半导体聚合物PDPP4T，并实现了高效的光热电转换。这种异质结由一个
图 16. 基于PDPP4T的柔性光热电探测器［94］。（a）利用光热电探测器阵列对“PTE”字母进行成像；（b）阵列在100 mW·cm-2光强下的传感图像；（c）柔性光热电发电机原型的照片；（d）典型光热电发电机在无光照和100 mW·cm-2白光强度下的输出电压和功率
Fig. 16. Flexible PTE detectors based on PDPP4T[94]. (a) Imaging the letters “PTE” with PTE detector array; (b) sensing image of the array under 100 mW·cm-2 light intensity; (c) photo of flexible PTE generator prototype; (d) output voltage and power of a typical PTE generator under the dark environment and 100 mW·cm-2 white light intensity
为了获得最佳的光吸收，一种方法是通过调节D/A交替共聚物的分子轨道来调节其能隙［95-97］，另一种方法是将p型或n型半导体掺杂到聚合物薄膜中，利用电荷转移实现光吸收［98-99］。Hasegawa等［100］综合上述两种方法的优点，设计并合成了p型噻吩异靛蓝基聚合物PTII以及具有超低能隙的新型n型聚合物TzQI-TDPP，它们可以很好地吸收2200 nm红外光。PTII和TzQI-TDPP薄膜无须添加任何掺杂剂即可分别表现出p型和n型光热电效应。如
图 17. 基于PTII和TzQI-TDPP的柔性光热电探测器［100］。（a）p型沟道噻吩异靛蓝基均聚物PTII以及n型沟道聚合物TzQI-TDPP的合成；（b）光热电器件示意图；（c）1700 nm近红外激光照射下薄膜表面的温度
Fig. 17. Flexible PTE detectors based on PTII and TzQI-TDPP[100]. (a) p-channel thiophene isoindigo based homopolymer PTII and the synthesis of n-channel polymer TzQI-TDPP; (b) an illustration of PTE device configuration; (c) thin-film surface temperature under 1700 nm NIR laser irradiation
6 有机无机复合材料柔性光热电探测器
传统的无机热电材料，如碲（Te）、铋（Bi）基材料，具有较大的Seebeck系数和较低的热导率，因此具有较好的热电性能，这使得它们在热电领域被广泛应用。但是，这些传统的无机热电材料固有的刚性限制了其在柔性领域的发展。与之相比，包括导电聚合物在内的大部分有机材料具有较好的柔性，但是它们较低的Seebeck系数同样限制了其在热电领域的发展。因此，许多研究人员将有机热电材料与传统无机热电材料复合，综合二者优点制成了有机无机复合材料，这种复合材料具有良好的柔性和导电性、低导热性、优秀的成膜性及力学性能，是非常有前景的柔性光热电材料。将这种材料应用在柔性光热电探测器上已成为近年来的研究热点。
PEDOT：PSS具有电导率高、稳定性好和热导率低的优点［67］，是制备柔性光热电器件的理想材料。Liu等［101］在PEDOT：PSS中添加碲纳米线制造了复合材料，并详细研究了它的热电及光热电性能。结果表明，这种材料综合了碲纳米线优异的热电性能和PEDOT：PSS的柔性，能够很好地应用在光热电探测上。这项工作为基于光热电效应的太阳能收集开发提供了一种新途径。
有机热电材料的光热电性能取决于活性层的光热转换效率和热电性能，而具有强的光吸收是其成为光热电材料的重要条件。Huang等［102］证明了聚［Cux（Cu-ett）］在近红外区域具有强的光吸收以及较高的光热转换率，因而具有出色的光热电性能。如
图 18. 基于［Cux（Cu-ett）］：PVDF的柔性光热电探测器［102］。（a）聚［Cux（Cu-ett）］和PVDF的分子结构；（b）三种材料的吸收光谱；（c）~（e）光热电器件制造过程示意图
Fig. 18. Flexible PTE detectors based on [Cux(Cu-ett)]:PVDF[102]. (a) Molecular structure of poly [Cux(Cu-ett)] and PVDF; (b) absorption spectra of three materials; (c)‒(e) schematic diagram of manufacturing process of PTE device
等离激元效应可以增强光与物质的相互作用，实现更强的光吸收。与常规纳米晶体材料相比，金纳米颗粒可以激发等离激元，获得较高的光热转换效率［103］。因此，Xin等［104］首先将PEDOT：PSS与三种不同的无机材料（
图 19. 基于胶体等离激元金纳米粒子的柔性光热电探测器［104］。（a）Au NPs的TEM图像；（b）Au NPs涂层PEDOT：PSS/Ag2Se器件的光学图像；（c）~（d）在聚丙烯非织造布上制备的Au NPs涂层PEDOT：PSS/Ag2Se混合光电器件的电压输出
Fig. 19. Flexible PTE detectors based on colloidal plasmonic gold nanoparticles[104]. (a) TEM image of Au NPs; (b) optical image of the flexible Au NPs-coated PEDOT:PSS/Ag2Se hybrid PTE generators; (c)‒(d) voltage output of PEDOT:PSS/Ag2Se hybrid optoelectronic devices coated with photothermal Au NPs prepared on polypropylene nonwovens
聚合物与碳纳米复合材料综合了二者优异的热电性能与柔性，并且其制备工艺简单，适合应用于柔性光热电探测领域。Stokes等［105］报道了一项基于聚（3-己基噻吩）-嵌段聚苯乙烯（P3HT-b-PS）和MWCNT复合的研究，与纯MWCNT薄膜相比，复合薄膜的光响应度提升了164%。Kuriakose等［106］研究了PANI和CNT复合材料的光热电效应。Zhang等［107］将PEDOT：PSS与石墨烯复合得到了柔性、半透明和自供电的中红外探测器。他们在聚乙烯醇（PVA）基板上制备了石墨烯和PEDOT：PSS复合材料，并构建了高灵活性和高透明度的光热电探测器，其探测率比纯PEDOT：PSS高22倍。复合薄膜与PVA衬底之间接触良好，连接稳固，探测器可以弯曲成半径为1 mm的曲面，具有良好的柔性。此外，他们使用该器件测量了人体发射的辐射信号，证明了其用于穿戴设备的可能性。Wang等［108］将CNT与PEDOT：PSS复合制成的探测器在宽带黑体辐射下的最高比探测率为1.9×107 Jones。Jin等［109］将PEDOT：PSS/螺旋碳纳米管（HCNT）混合物和PEDOT：PSS连续滴涂到衬底上，制备了具有伪双层结构（PBA）的柔性PTE薄膜，该薄膜在166.01 mW·cm-2照射下获得了2.55 nW的最大输出功率，并且在10000次弯曲后依然保持良好的性能。Xie等［110］开发了石墨烯/PANI复合材料。PANI的加入增强了整个复合材料的导电性［111］，并由此改善了探测器的光热电转换能力。该探测器的峰值光响应度为2.5 V·W-1，比探测率为6.8×107 cm·Hz1/2·W-1。此外，如
图 20. 基于石墨烯/PANI复合材料的柔性光热电探测器。（a）石墨烯/PANI复合光热电探测器在手指自发辐射多次激励下的光电压响应（每个周期将指尖放在离光电探测器3~5 mm的地方，并垂直移动）［110］；（b）PET基底上的8 pixel×8 pixel柔性探测器阵列［110］；（c）石墨烯/PEI探测器示意图［112］；（d）石墨烯/PEI探测器在多次弯曲下光电流保持稳定［112］
Fig. 20. Flexible PTE detectors based on graphene/PANI composite. (a) Photovoltage response of graphene/PANI PTE detectors under multiple excitation of finger spontaneous radiation (place the fingertip 3‒5 mm away from the photodetector in each cycle and move it vertically)[110]; (b) flexible 8 pixel×8 pixel detector array on PET substrate[110]; (c) schematic of the graphene/PEI detectors[112]; (d) stable photocurrent of graphene/PEI detectors under multiple bending cycles[112]
聚苯并咪唑（PBI）是一类典型的芳香杂环聚合物材料，具有优异的热稳定性、化学稳定性和机械强度，已被用于保护纤维、涂层清漆和燃料电池膜［113-114］。Park等［115］研究了PBI光纤在三种可见光波长（405、520、635 nm）下的光热转换行为。PBI在高达500 ℃或更高温度下不会熔化，并且具有出色的光热电性能，因而被用作热电和光热电纳米复合材料的聚合物基质。为了开发具有更优异光热电性能且易于加工的先进材料，Park等［116］又制造了基于PBI的纳米复合薄膜，如
图 21. 基于PBI和MWCNT复合材料的柔性光热电探测器［116］。（a）PBI/MWCNT纳米复合膜在可见光照射下的光热电表征示意图以及PBI/MWCNT膜的TEM图像；（b）在波长为520 nm、功率为8.87 W·cm-2的可见光照明下，PBI/MWCNT膜边缘处的光热温升（ΔTPT）、光生电压（VPTE）和光生电流（APTE）随时间变化的曲线
Fig. 21. Flexible PTE detectors based on PBI/MWCNT[116]. (a) Schematic for the PTE characterization of PBI/MWCNT nanocomposite films under visible light irradiation and TEM image of PBI/MWCNT film; (b) time-dependent photothermal temperature increase (ΔTPT), voltage generation (VPTE), and electric current generation (APTE) changes of the PBI/MWCNT film at the edge under the visible light illumination with 520 nm and 8.87 W·cm-2
7 柔性光热电探测器性能分析
由于厚度较小，低维材料对光的吸收率通常较低，从而限制了其探测率，在进行探测器的结构设计和材料选择时需要考虑如何兼顾探测器的柔性和探测性能。为了解决这一问题，一方面可以进一步探索具有高光热电转换能力的材料，目前一些有机小分子已经被报道具有高Seebeck系数［117］，并且已经证明将一些有机小分子加入到低维材料中可以提高其热电性能［118-119］；另一方面，可以尝试使用不同的材料制成光热转换层和热电转换层，这样可以分别选择光吸收能力良好的材料和热电性能优异的材料，如PPy和PEDOT［88］，从而扩展材料的选择。
光敏材料不同的探测器面临着不同的挑战和发展方向。无机化合物虽然具有较大的热电优值，但其在柔性化过程中往往存在热电性能降低的现象［80］，需要优化柔性化方法，在保持其性能的同时增加其柔性。有机柔性光热电探测器的性能受限于其较低的Seebeck系数，相比于其他材料柔性探测器而言，其性能指标相对较低，因此还需要进一步探索有机光热电材料的物理机制并对新型材料进行结构设计［120］，以改善其探测性能。目前来看，有机-无机复合材料能够综合无机材料优异的热电性能和有机材料的柔性，制造出性能优异的柔性光热电器件，这也是未来柔性光热电材料的主要研究方向之一。
表 1. 部分代表性柔性光热电探测器的性能指标
Table 1. Key performances of some representative flexible PTEs
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8 结束语
本文综述了近年来国内外柔性光热电探测器的研究进展。光热电探测器有望用于低成本、非制冷、自供电、超宽带的紫外、可见光、红外和太赫兹波探测。随着低维材料和半导体技术的发展，光热电探测器的性能得到了显著提高，这为零偏压、室温下工作的超快、超宽带光电探测器的研制开辟了新的道路。此外，柔性材料的发展为光热电探测器在可穿戴设备、表面形状复杂物体无损检测方面的应用提供了新途径。然而，除了探测性能的提升之外，柔性光热电探测器的发展和实际应用仍然面临着许多挑战。
未来柔性光热电探测器的发展离不开其柔性性能的提高，但目前还没有统一的评价方法来定量比较不同器件的柔性。本文介绍的各种探测器分别使用了不同的方法来衡量其柔性，例如弯曲半径［46］或弯曲次数［53］，或者二者皆有［110］。因此，为了方便比较各类柔性探测器的性能，促进其在各个领域的应用，需要制定科学而全面的评价标准。一个合理的方法是对于每一种探测器，均测量其弯曲性能和拉伸性能，即在保证其性能不发生明显改变的情况下，测量探测器所能承受的最多弯曲次数、最大弯曲半径、最多拉伸次数和最大拉伸比例。例如，对于可穿戴设备，探测器在使用时根据人体的运动可能会时常发生各种形变，产生弯曲或拉伸变化，所以为了满足柔性光热电探测器的各类应用需求，需要分别对其弯曲和拉伸性能进行测试。
基于柔性光热电探测器的可穿戴设备可以准确、连续地测量人体信息，这在传统设备上是难以实现的。但是，尚存在一些未解决的问题限制了它的应用。例如：需要对材料或结构进行弹性设计，以提高穿戴时其在弯曲和拉伸下的稳定性；通过结构和功能设计提升皮肤表面光热电转换的输出功率；开发高性能柔性光热电纤维与织物，同时设计优化结构，使得可穿戴设备能够透气透湿，具有良好的人体兼容性。此外，尽管目前探测材料的柔性得到了明显提高，但探测器电源、电极、读出电路和数据处理器等器件的柔性还没有被充分研究。对于电源来说，虽然有研究使用基于太阳能的柔性电源［121］，但随着探测器阵列的集成化，其功耗也在增加，这种电源并不足以支撑整个系统的运行。此外，许多设备使用硅基刚性读出电路，这使得设备整体还是呈现为刚性。尽管已经有研究人员将CMOS电路与图像传感器集成以使读出电路变得具有柔性［122］，但仍未实现整个系统的完全柔性和小型化。因此，柔性电池和柔性集成电路技术的进一步发展至关重要。
总之，为了促进柔性光热电探测器的发展，仍需对探测材料和结构进行探索和优化以提高其探测性能，同时需要制定全面的评价标准以提高其柔性性能，从而推动柔性光热电探测器的实际应用。
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Article Outline
刘睿丰, 陈猛, 王迎新, 赵自然. 柔性光热电探测器研究进展[J]. 中国激光, 2023, 50(23): 2300001. Ruifeng Liu, Meng Chen, Yingxin Wang, Ziran Zhao. Research Progress of Flexible Photothermoelectric Detectors[J]. Chinese Journal of Lasers, 2023, 50(23): 2300001.
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