Flexible and Robust 3D a‐SiGe Radial Junction Near‐Infrared Photodetectors for Rapid Sphygmic Signal Monitoring

Abstract Flexible near‐infrared (NIR) photodetectors (PDs) are desired for accurate heart rate monitoring, based directly on arterial‐blood‐volume‐change detection, instead of indirect oximetry technology. In this work, a robust 3D construction of flexible a‐SiGe:H p ‐ i ‐ n radial junction (RJ) PDs is explored directly upon soft Al foils, working at NIR wavelength 800 nm, which has the highest skin transparency and the least absorption difference from oxyhemoglobin and deoxyhemoglobin variation. The 3D a‐SiGe:H RJ‐PDs demonstrate excellent flexibility and mechanical stability that can undergo 1000‐times bending to 10 mm radius, with a high responsivity of ≈140 mA W −1 @800 nm and rise/fall time scales of 5.4 µs/17.6 µs (3.6 µs/13.2 µs before bending). Successful photoplethysmography (PPG) detection of sphygmic signals is achieved at the wrist with significant arterial blood volume changes, working in reflectance mode and using a single NIR source @800 nm. These unique capabilities, enabled by the a‐SiGe:H RJ, have a promising potential to establish a reliable and convenient PPG detection technology, which can help to extract the heartbeat via solely arterial blood volume change, as a complementary and potentially very useful new diagnostic dimension for efficient and accurate real‐time health monitoring.