The realization of thin, flexible self-powered electronics and optoelectronics has taken a step forward with the demonstration of a solar-powered cardiac sensor that wraps around a finger. The approach offers a route to the development of stand-alone, self-powered, wearable biomedical sensors.

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Springer Nature Ltd

To create the device, Sungjun Park and co-workers from Japan and South Korea fabricated an ultra-thin, flexible organic photovoltaic (OPV) cell (pictured, left) as a power source and integrated it with organic electrochemical transistors (OECTs) on an elastomer substrate (Nature 561, 516–521; 2018). The OPV was just 3 μm thick and consisted of a PBDTT-OFT:PC71BM organic heterojunction sandwiched between a ZnO nanoparticle layer that served as an electron-transport layer and an upper electrode of Ag/MoOx. The OPV operated with a power conversion efficiency of ~10% and weighed just 36.6 μg and was capable of generating 11.46 W g–1 of electrical power.

A key innovation for the success was the patterning of both the photoactive organic polymer layer and the ZnO layer with a 1D grating to improve the efficiency of the device. The 760-nm pitch grating pattern was taken from the surface of a blank DVD-R.

After fabrication, the devices were delaminated from a glass substrate and transferred to a thin layer of parylene. The OPVs were then integrated with OECTs to demonstrate self-powered, on-finger cardiac sensors (pictured, right) based on gate-bias-induced changes in output current when illuminated with light from an LED. The peak intensity and standard deviation of the recorded cardiac signal were 0.47 μA and 23.5 nA, respectively. Comparisons to the use of a commercial battery as power source, showed the benefit of no external power line noise, and no ground-loops due to the short interconnections.