Near-infrared Photodetectors Based on a Liquid Crystalline Organic Semiconductor for Photoplethysmography Applications

Highly responsive near-infrared (NIR) photodetectors are essential for the accurate measurement and monitoring of health parameters such as oxygen saturation level in blood, heart rate, and breathing rate. Photo detectors fabricated with emerging materials such as organic semiconductors must exceed responsivity of 0.5 A/W in order to compete with the commercial silicon photodiodes. In this study, NIR photodetectors with a liquid crystalline (LC) organic semiconductor, octaoctylphthalocyanine (8H2Pc), were realized exhibiting maximum responsivity of 0.7 A/W with -1 V at 740 nm. The photodetectors were prepared with a bulk heterojunction active layer with the organic phthalocyanine derivative acting as the donor and phenyl-C61-butyric-acid-methyl-ester (PC61BM) acting as the acceptor. The superior performance was attributed to the intermixed condition of the organic materials in the bulk heterojunction and the weak intermolecular interactions of the LC phthalocyanine material. High responsivity of these organic photodetectors in the NIR region enabled the monitoring of photoplethysmogram of a human subject with a very simple measurement setup.