Flexible Photovoltaic Cell Could be Incorporated into Textiles
Scientists at the RIKEN Center for Emergent Matter Science in Japan have developed a solar-powered, ultraflexible organic sensor that can act as an autonomously powered heart monitor taped to the skin. The technology could also catalyze high-performance ultraflexible organic photovoltaics (OPVs).
The team had previously developed a flexible photovoltaic cell that could be incorporated into textiles. Now, they have directly integrated a sensory device into a flexible organic solar cell. The so-called organic electrochemical transistor (OECT) can be used to measure a variety of biological functions. They were able to measure the heartbeat of rats and humans under bright light conditions.
The breakthrough: Integration of ultrathin solar cells with sensors
“The major finding in this paper is that we have developed a human-friendly, ultraflexible organic sensors powered by sunlight, which acts as a self-powered heart monitor,” says Kenjiro Fukuda, one of the corresponding authors of the paper « Self-powered ultra-flexible electronics via nano-grating-patterned organic photovoltaics,” published in Nature.
The key requirement for imperceptible sensors is a stable and adequate energy supply. “Although ultrathin solar cells with high power conversion efficiency and mechanical and thermal stability have been achieved, the integration of ultrathin solar cells with sensors has not yet been demonstrated because of their unstable output power under mechanical deformation and angular change,” Fukuda explains. “A key advance in this study is the use of a nano-grating surface on the light absorbers of the solar cell, allowing for high photo conversion efficiency and light-angle independency.”
Advantages of the new ultraflexible organic sensor
Flexibility and light weight can reduce discomfort when the device is attached to the human body. Improving a device’s flexibility means it can now withstand mechanical deformation, such as when it is attached to a flexing joint. Additionally, such thin devices are more comfortable to wear than conventional wearable devices, such as a smartwatch. As a result, the ultraflexible organic sensor can lead to better, more stable monitoring than conventional devices.
The future of organic photovoltaics
The research breakthrough could help to advance the development of the next generation of organic photovoltaics. One of the most important features of organic photovoltaics (OPVs) is flexibility. However, there is currently a significant gap between the power conversion efficiency of “ideal” OPVs fabricated on rigid glass substrates and flexible ones. “Our researches are devoted to fabricating ultraflexible OPVs with high performance, which are the ultimate form of OPVs,” Fukuda says, elaborating that “high performance,” in this context, includes higher PCE, better mechanical robustness and better environmental stability (i.e., in air, water, high temperature). “The PCE of our ultraflexible OPVs, 10.5%, is the highest value among all flexible OPVs. Furthermore, this value is almost comparable with the OPVs fabricated onto rigid glass substrate,” the researcher says. “In addition, we can reduce the light incident sensitivity, which is a very important feature if the OECT is attached to a complex curved surface.”
Key applications for the new ultraflexible organic sensor
Fukuda says this technology can be used as a heartbeat monitor while exercising. “If accuracy is further improved, this can also be used as imperceptible electrocardiogram system for healthcare and medical use,” he adds.
The researcher says the integrated organic electrochemical transistor can measure changes in the body, so it can potentially be used for ECG, EMG, EOG or EEG monitoring.
“We improved the light-angle dependence of the solar cells, but further improving is needed for a more stable power supply,” Fukuda says. His team also wants to further improve environmental stabilities (air, water, heating and bending). The integration of other sensor systems is also planned. Additionally, Fukuda deems the integration with wire transmission and memory devices an important next step.