柔性可穿戴、可植入电子设备在生理参数监测、疾病治疗、动物行为研究等领域有着广泛的应用。传统电池仍然是这些设备最常用的供能方式，但其能量密度低、体积大、硬质、存在泄露的风险和需要手术替换等问题限制了电子设备长时间工作的能力。基于各种功能材料的柔性能量收集器可直接将环境中的机械能、热能等能源转换为电能，并用于给电子设备持续供能，实现设备的长时间工作。柔性热释电能量收集器（PyEH）可以从环境的温度波动中收集电能，柔性压电能量收集器（FPEH）可以从人体和动物的肢体、器官的运动中收集电能，这两种能量收集器在近年来受到了大量的关注。本文从高性能PyEH和FPEH的性能评估标准和结构优化设计出发，进行了如下研究：
    输出电压是评估各种柔性能量收集器性能的重要标准，以PyEH为例，许多文献通常用其在一定温度变化下的输出电压幅值来表征其器件的能量收集性能，有的也将其结果表述为“开路电压”。然而，即使在温度变化恒正的情形下，这些实验电压曲线通常呈现正负交替的波形，这与PyEH开路电压理论所预测的恒正电压波形相矛盾。基于这一问题，本文通过实验和理论分析对PyEH的输出电压测量进行了系统地分析，（1）实验发现电压表内阻和电容对PyEH输出电压的幅值和波形有着不可忽略的影响。当电压表内阻较低和较高时，输出电压幅值分别与温度变化速率和幅值相关；（2）建立了考虑电压表和PyEH自身内阻与电容的输出电压理论，提出了与测量电路无关的本征电压作为性能评估的新标准。
    基于压电基底薄膜的叠层结构是FPEH应用最广泛的结构设计。厚且硬的基底可使FPEH在弯曲时压电薄膜有更高的压电能量输出，但这也会显著提高器件变形刚度，降低能量效率。针对这一矛盾，本文（1）建立了考虑基底剪切的软基底三明治结构FPEH理论模型，分析发现当器件足够长时，采用弹性模量远低于压电层弹性模量的软基底可以显著降低器件的变形刚度且几乎不影响其压电能量输出，且存在最优解使能量效率达到极大值。（2）设计并制备了一种最优软基底三明治结构FPEH，其同时具有高面积能量输出密度、低变形刚度和高能量效率的特点。（3）探索了所制备FPEH从心跳、呼吸、鱼的游动和鸟的飞行中收集能量的可行性。
    本文所提出的输出电压理论和性能评估标准可进一步扩展至压电、摩擦电等其他能量收集器中。所提出的结构优化方法推进了FPEH的实际应用，并可进一步扩展至各种叠层结构FPEH和摩擦电等其他能量收集器中。

    Flexible Wearable/implantable electronics have a wide range of applications in human physiological parameter monitoring, disease treatment and animal behavior research et al. Conventional batteries remain the most commonly used energy source for these devices, but their low energy density, large size, rigidity, risk of leakage and need for surgical replacement limit the life-span of wearable/implantable electronics. Various flexible energy harvesters can convert the ambient mechanical and thermal energy into electrical energy and continuously power electronics. Flexible pyroelectric energy harvesters (PyEH) can harvest energy from temperature fluctuations, and flexible piezoelectric energy harvesters (FPEH) can collect electrical energy from the movement of human and animals. The both energy harvesters have received a great deal of attention in recent years. In this thesis, the performance evaluation standard and structural design of high-performance PyEH and FPEH are investigated as follows:
    The output voltage is an important criterion for evaluating the performance of various flexible energy harvesters. Taking PyEH as an example, many literatures usually use the output voltage under a certain temperature fluctuation to characterize the energy harvesting performance, and some of them express the result as “open-circuit voltage”. However, these experimental voltage curves usually show alternating positive and negative waveforms even under positive temperature fluctuation, which contradicts the constant positive voltage waveform predicted by the open-circuit voltage theory. Based on this problem, we systematically analyze the output voltage measurement of PyEH through experiments and theoretical analyses, with the following specific contents: (1) We found that the resistance and capacitance of the voltmeter used in the measurement have a non-negligible influence on both the amplitude and waveform of the output voltage curve. When the resistance of the voltmeter is low and high, the amplitude of the output voltage is correlated with the rate and the amplitude of temperature change, respectively. (2) The output voltage theory considering the resistance and capacitance of both the voltmeter and PyEH was established. The intrinsic voltage was proposed as a new criterion for the pyroelectric performance.
    In addition to the development of high-performance functional materials, electrodes, and energy storage circuits, structural design is also important for improving the overall performance of flexible energy harvesters. Laminated structures based on piezoelectric films and substrates are the most widely used structural designs for FPEHs. Thick and stiff substrate can make FPEH have higher piezoelectric energy output from piezoelectric film during bending, but this will also significantly increase the mechanical stiffness and reduce the energy efficiency. To address this contradiction, (1) we established a theoretical model of FPEH with soft substrate sandwich structure considering substrate shear deformation and found that when the FPEH is long enough, a soft substrate with elastic modulus much lower than that of piezoelectric layer can significantly reduce the mechanical stiffness without affecting the piezoelectric energy output, and an optimal value exists to maximize the energy efficiency. (2) We designed and fabricated an optimal soft-substrate sandwich FPEH which has high areal energy output density, low mechanical stiffness and high energy efficiency simultaneously. (3) We explored the application of the FPEH to harvest energy from heartbeat, breathing, fish swimming and bird flying.
    The theory of output voltage and performance evaluation criteria can be further extended to other energy harvesters such as piezo- and tribo-electric energy harvesters. The proposed structure optimization method advances the practical application of FPEHs and can be further extended to various laminated structure FPEHs and other energy harvesters such as triboelectric energy harvesters.