Larger, flexible, and skin-mountable energy devices with graphene single layers for integratable, wearable, and health monitoring systems

In recent years, the development of weightless, ultrathin, and non-toxic devices has received tremendous interest owing to their potential applications as building blocks for various flexible, wearable, and skin-mountable systems. In this view, we explored the exceptional performances of flexible energy harvesters and storage devices along with wearable optoelectronic devices by adopting high-quality graphene monolayers and a user-friendly transfer methodology. Flexible and transparent energy generators with an active layer (AL) thickness of ∼20 μm and storage devices (AL≤ 1 μm) were developed by sandwiching the piezoelectric and solid electrolyte materials between two graphene monolayers, respectively. Wearable photosensors, with an AL thickness of ∼30 μm, were designed by integrating an ultrathin zinc oxide layer with a graphene monolayer. Under nominal mechanical movements, typical graphene monolayers-based piezoelectric energy generators exhibited very stable peak voltage and current density of 5.5 V and 0.2 nA/cm², respectively. Whereas the skin-mountable micro-supercapacitor (mSC) showed slightly lower areal and volumetric capacitances (6.3 μF/cm² and 91 mF/cm³@100 mV/s scan rate) than that of the flexible mSCs. Interestingly, these mSC devices also showed significant mechanical flexibility, stability, and durability. Further, the as-fabricated photosensors exhibited a strong response to visible light with an On/Off current density ratio of 1.8 and excellent wearability. Based on these demonstrated outcomes, we emphasize that the devices fabricated on different substrates by using graphene single layers could be adopted for various wearable, biocompatible, and skin-mountable devices that are widely being used in various health monitoring systems.