Initiating the Idea
Ultraflexible electronics have garnered immense attention because of their ability to seamlessly integrate with the human body, providing more accurate data acquisition and an enhanced degree of user comfort. The progress of wearable technology is closely tied to the development of flexible energy devices, which should offer high efficiency, durability, and constant power output and possess the capacity for effortless integration. Most commercial wearables rely on bulky coin cells or rechargeable batteries as power sources. These add significantly to the system’s overall rigidity, posing limitations to mechanical compliance and often necessitating frequent charging or replacement. The next frontier is to produce ultraflexible energy sources, especially flexible energy harvesting-storage systems (FEHSSs) that efficiently generate and store power and adapt to curved surfaces and moving objects under variable light conditions.
 Technical Challenges
Although there have been rapid developments in ultraflexible energy harvesters, integrating these with flexible energy storage devices into an efficient and robust power system presents several challenges. These challenges include achieving high power conversion and storage efficiency without sacrificing mechanical flexibility, creating a stable interface between components, and ensuring a user-friendly design. Achieving the desired flexibility in an integrated FEHSS requires carefully optimizing the thickness of each component to ensure a balance between electrochemical performance, mechanical properties, and durability.
 Our breakthrough Technology
We introduce a highly efficient and fully integrated flexible energy harvesting and storage system that is only 90 µm thick. This innovative system addresses the challenges by utilizing ultrathin, high-performance organic photovoltaic (OPV) modules combined with flexible zinc-ion batteries (ZIBs).
First, we developed high-performance, ultraflexible OPVs with excellent power conversion efficiency (PCE), shelf lifetime, and photostability. The enhancement of our OPVs’ performance was achieved through two main strategies: firstly, by utilizing a ternary blend toward optimal donor: acceptor phase separation, and secondly, by modifying the zinc oxide (ZnO) electron transport layer to passivate surface defects, which significantly improved charge dissociation and collection efficiency. Progressing towards our aim of designing an integrated energy harvesting and storage system, we engineered OPV modules capable of charging multiple zinc-ion batteries and powering small electronic gadgets. We meticulously adjusted the series and parallel connections within these arrays and tailored the overall voltage and current output to meet specific energy requirements. This adaptive approach allows us to customize OPV arrays for a variety of applications – from low-power devices such as miniaturized biosensors requiring microwatt (µW) power to high-power electronics like smartwatches and displays that demand milliwatt (mW) levels of power.
Addressing the typical constraint of bulky energy storage in wearable devices, our innovative approach introduced an ultrathin, highly flexible ZIB. Traditional wearables rely on bulky components such as coin cells or rechargeable batteries, which add to the system’s rigidity. Our approach significantly decreased the total thickness of the energy storage device, allowing for more user-friendly designs, safety, and comfort. Using a facile ‘cold lamination method’, we reduced the thickness of a hydrogel electrolyte without sacrificing its electrochemical performance. This ultrathin hydrogel, combined with thin anode and cathode materials and encapsulated with ultrathin parylene coating, resulted in an 85 µm-thick flexible ZIB compatible with human skin and textiles. This innovation is a leap forward in wearable device design, emphasizing safety and comfort while ensuring robust energy storage capabilities.
The final breakthrough was integrating ultraflexible OPV modules and ZIBs into a stacked back-to-back configuration to yield a FEHSS. A critical issue we encountered during this phase was the risk of current backflow from the battery to the OPV module under dark conditions—a scenario where the OPV’s voltage could drop below the charged batteries without a power management system, potentially damaging the module. To mitigate this risk, we incorporated ultraflexible blocking diodes between the battery and the OPV module. These diodes function as one-way valves that prevent any undesirable current reflux, ensuring the integrity and operational stability of the integrated unit under varying lighting conditions.
With these advancements in flexible OPV technology, storage batteries, and integrated smart power management, we envision our FEHSS paving the way for next-generation wearables that are more efficient, adaptable, and indulgent in user comfort needs.
Remark
For more details, please refer to the article “An Ultraflexible Energy Harvesting-Storage System for Wearable Applications”, https://doi.org/10.1038/s41467-024-50894-w.
https://www.nature.com/articles/s41467-024-50894-w
FEHSS Unfolded: The Journey to Ultraflexible Energy Solutions
