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 . 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.
 H. Xu, J. Liu, J. Zhang, G. Zhou, N. Luo, N. Zhao, Adv. Mater. 2017, 29, 1700975. https://doi.org/10.1002/adma.201700975