Flexible Organic/Inorganic Hybrid Near‐Infrared Photoplethysmogram Sensor for Cardiovascular Monitoring

Abstract

Wearable photoplethysmogram (PPG) sensors offer convenient and informative measurements for evaluating daily physiological states of individuals. In this work, epidermal and flexible near-infrared (NIR) PPG sensors integrating a low-power, high-sensitivity organic phototransistor (OPT) with a high-efficiency inorganic light-emitting diode are developed. By exploiting an organic bulk heterojunction active layer and a bilayer gate dielectric design, a low voltage (<3 V) operated OPT with NIR responsivity as high as 3.5 × 10 A W and noise equivalent power of 1.2 × 10 W Hz is achieved, greatly surpassing commercial available silicon-based photodetectors. In addition, the ultrathin encapsulation structure renders the device highly flexible and allows transfer printing of the device directly onto human skin. It is demonstrated that the epidermal/flexible PPG sensors are capable of continuously monitoring heart rate variability and precisely tracking the changes of pulse pressure at different postures of human subjects with the aid of electrocardiogram monitoring, exhibiting more reliable performance than commercial PPG sensors while consuming less power. The study suggests that the hybrid PPG sensor design may provide a promising solution for low-power, real-time physiological monitoring.

Organic Field‐Effect Transistor for Energy‐Related Applications: Low‐Power‐Consumption Devices, Near‐Infrared Phototransistors, and Organic Thermoelectric Devices

“…[138][139][140][141][142] Organic polymers based on DPP have been extensively reported for solar cells and ambipolar transistors due to their planarconjugated structure and strong electron-accepting ability. [138][139][140][141][142] Organic polymers based on DPP have been extensively reported for solar cells and ambipolar transistors due to their planarconjugated structure and strong electron-accepting ability.…”

“…[139,142,166,167] Xu et al demonstrated a poly(N-alkyl diketopyrrolo-pyrrole dithienylthieno[3,2-b]thiophene) (DPP-DTT): [6,6]-phenyl-C61-butyric acid methylester (PCBM)-based phototransistor working at the NIR region, [138] in which the BHJ effectively trapped one type of charge and allowed the opposite charge transport to provide a high photocurrent gain. In both cases, the gate bias partially participates as a driving force.…”

Photocurrent Amplification in Bulk Heterojunction Organic Phototransistors with Different Donor–Acceptor Ratio

“…Organic photodetectors have attracted wide interest for their ability to detect a broad range of wavelengths, solution processability, and flexibility . While many interesting applications, such as photonic skin, image sensor arrays, and flexible near‐infrared sensor arrays, have been reported using organic photodiodes (OPDs), organic phototransistors (OPTs) have attracted interest for their large photocurrent and wide dynamic range . Photocurrents in OPTs are amplified by charge traps in the active layer and interface, in which either the majority carrier traps are accumulated in the channel region or the minority carrier trap lowers the injection barrier between the electrode and semiconductors .…”

“…At the same time, the contribution of a BHJ in faster response times for OPTs was confirmed. The understanding gained from this study should help with the design of the active layer in OPTs in order to improve photoresponse and speed, and can prove to be useful for future applications in wearables and robotics …”

“…In this paper, the amplification of the photocurrent in organic phototransistors (OPTs) employing bulk heterojunctions (BHJs) with different donor-acceptor ratios has been studied. As a model system, poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-p-phenylene vinylene] (OC 1 C 10 -PPV) and [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM) are used as donor and acceptor, respectively, combined with Al contact. It is found that for OPTs the mobility of the majority carrier is the dominating factor for maximizing the photocurrent.…”