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Guodong Zhou1 Quan Zhang2 3 Ni Zhao1

1, The Chinese University of Hong Kong, Hong Kong, , China
2, Massachusetts General Hospital, Boston, Massachusetts, United States
3, Harvard Medical School, Boston, Massachusetts, United States

Hemodynamic response (HR), which is associated directly with cardio-cerebrovascular diseases, such as hypertension, ischemia and Alzheimer’s disease, is a vital indicator of human health. In order to realize daily monitoring of HR with a compact, light-weight, and low-cost device, high-sensitivity optical sensing technologies have been developed in recent years. We have previously demonstrated that organic bulk heterojunction phototransistors can serve as an efficient light receiver in a flexible near-infrared (NIR) photoplethysmography (PPG) sensor [1]. The tunable and ultrahigh responsivity enables pre-amplifier-free, low-power tracking of HR, outperforming commercial PPG sensors. However, the reported single-wavelength phototransistor cannot separately extract physiologic parameters such as the concentration change of hemoglobin (Hb) and oxyhemoglobin (HbO2), which requires selective detection of more wavelengths. Also, the bias stress-induced instability limits the application of the device for long-duration monitoring. In this work, we introduce a novel device fabrication technique − blade-assisted spin coating (BAS), which enables dual-wavelength selective detection on a single chip (which contains two phototransistors) while maintaining high light responsivity to each individual wavelength. Furthermore, we greatly enhanced the device stability via a charge-selective electrode (CSE) that can suppress electron injection and thus minimize charge trapping at the semiconductor/dielectric interface. As a result, multiday stable monitoring of HR signals with dual-wavelength channels was achieved. The study shed light on a new application direction where organic transistors may have technological advantages and ultimately lead to performance improvement.
[1] H. Xu, J. Liu, J. Zhang, G. Zhou, N. Luo, N. Zhao, Adv. Mater. 2017, 29, 1700975. https://doi.org/10.1002/adma.201700975

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