Development of High Performance Infrared Printed Photodiodes for Biological Signal Detection

We report printed flexible optoelectronic biological sensors composed of red organic light emitting diodes (OLEDs) and red and infrared organic photodiodes (OPDs) for detection of various biological signals in a photo-plethysmograph (PPG) device. To achieve infra-red (NIR) light detection, we have developed low band gap conjugated polymers as electron donor and intermixed with various electron acceptors to form bulk heterojunction morphology for photoactive layer. The developed NIR OPD sensor showed high EQE over 30 % at 1000 nm wavelength and photosensitivity. The flexible RED OLEDs with conjugated polymer emitters has been achieved maximum luminance > 10,000 cd/m2 at 9 V, with peak at 640 nm. PPG signals were successfully detected using the developed flexible PPG sensor and conventional driving circuit. Human studies were conducted to evaluate the flexible PPG sensor performance in practical applications. Subject drowsiness was estimated from heart rate variability, extracted from the PPG signals, using machine learning algorithms. The flexible PPG sensor achieved 79.2% accuracy and 72.1% area under receiver (AUC) to predict drowsiness (60 sec window), which are meaningful results compared with conventional PPG sensors (83.3% accuracy and 69.0% AUC). Drowsiness estimation experiments using two PPG signals showed the flexible PPG sensor achieved similar or better performance to conventional PPG sensors.