Previously, the scientists developed a flexible photovoltaic cell that could be incorporated into textiles. In this study, they directly integrated a sensory device, called an organic electrochemical transistor—a type of electronic device that can be used to measure a variety of biological functions—into a flexible organic solar cell. Using it, they claim they were then able to measure the heartbeats of rats and humans under bright light conditions.
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 (PCE) and light angle independency. Thanks to this, the researchers say they were able to achieve a PCE of 10.5 percent and a high power-per-weight ratio of 11.46 watts per gram, approaching the “magic number” of 15 percent that will make organic photovoltaics competitive with their silicon-based counterparts. They demonstrated a PCE decrease of only 25 percent (from 9.82% to 7.33%) under repetitive compression test (900 cycles) and a higher PCE gain of 45 percent compared to non-grating devices under 60 degree light angle.
To demonstrate a practical application, sensory devices called organic electrochemical transistors were integrated with organic solar cells on an 1mm substrate to allow the self-powered detection of heartbeats either on the skin or to record electrocardiographic (ECG) signals directly on the heart of a rat. They found that the device worked well at a lighting level of 10,000 lux, which is equivalent to the light seen when one is in the shade on a clear sunny day, and experienced less noise than similar devices connected to a battery, presumably because of the lack of electric wires.
According to Kenjiro Fukuda of the RIKEN Center for Emergent Matter Science, “This is a nice step forward in the quest to make self-powered medical monitoring devices that can be placed on human tissue. There are some important remaining tasks, such as the development of flexible power storage devices, and we will continue to collaborate with other groups to produce practical devices.
“Importantly, for the current experiments we worked on the analogue part of our device, which powers the device and conducts the measurement. There is also a digital silicon-based portion, for the transmission of data, and further work in that area will also help to make such devices practical.”