Near-infrared high-sensitivity photodetectors are the key component of wearable optical systems for noninvasive physiological monitoring, such as photoplethysmography (PPG) and near-infrared spectroscopy. Compared to the high-voltage driven avalanche photodiodes and photomultipliers, organic phototransistors based on a bulk heterojunction (BHJ) structure have a set of unique advantages including self-amplification (via a photoconductive gain mechanism), low operation voltage, lightweight, flexible, printable and CMOS compatible. By employing a bilayer dielectric design and an ultrathin encapsulation structure, we have realized a flexible/epidermal low-voltage (< 3V) driven BHJ phototransistor with ultra-high responsivity (3.5 ×10^5 AW^-1) and low noise equivalent power (1.2 × 10^−15 W Hz^−1/2). We combined the phototransistor with a high-efficiency III-V LED to realize a hybrid PPG sensor and demonstrated low-power and high stability continuous tracking of heart rate variability and pulse pressure. To reveal the fundamental correlations between the heterojunction morphology and the device’s figures-of-merits, such as responsivity, operation bandwidth, and noise, we carried out a systematical investigation combining morphological characterizations with photo-physics and charge transport studies. The results highlight the importance of optimizing interface charge separation and bulk charge transport through morphology control. This study not only reveals the physical mechanisms that govern the operation of organic phototransistors but also provides know-hows to realize highly flexible and stable photodetection systems.
Ni Zhao, Guodong Zhou, and Huihua Xu, "Device physics and biomedical applications of organic bulk heterojunction phototransistors (Conference Presentation)," Proc. SPIE 10529, Organic Photonic Materials and Devices XX, 1052913 (Presented at SPIE OPTO: February 02, 2018; Published: 15 March 2018); https://doi.org/10.1117/12.2289727.5752208483001.
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