Transforming wearable technology with advanced ultra-flexible energy harvesting and storage solutions

Flexible OPVs and energy storage systems have profound implications for the future of wearable electronics. Researchers have made significant advancements in developing ultra-thin, flexible, and stretchable energy harvesting and storage systems. Specifically, the development of solution-processed ultrathin flexible photovoltaics, hydrogel electrolyte-based ultrathin flexible rechargeable batteries, and printed flexible/stretchable electronic circuits represents a substantial leap toward realizing commercial ultrathin flexible wearable electronics. However, several fundamental concerns must be addressed to achieve efficient and self-sustainable flexible wearable electronic systems^4,8,9.

To elucidate these issues, consider a basic and simplified model of a wearable device, depicted in Fig. 1a, which includes an energy harvesting-storage system, human performance monitoring sensors (such as body temperature, heartbeat rate, blood pressure, electrocardiogram, and breathing), and a low-power electronic circuit to manage input power and communicate sensor data to external devices like smartphones, computers, or IoT cloud systems. A self-sustainable wearable electronics system necessitates an efficient and continuous power supply to operate the electronic control unit circuits and sensors, sourced from an energy storage unit (battery). The energy harvesting unit, typically a photovoltaic module, must effectively generate power to recharge the battery before depletion by the electronic circuits and sensors. Power consumption by the electronic circuits and sensors can be optimized by periodically controlling the sleep/standby and active modes of sensor data collection. This strategy reduces the load on the battery, thereby extending the operational lifetime of the system.

a Schematic design of a simple flexible wearable device along with the integrated energy harvesting and storage system. b Powe density and power output of flexible OPV cells and modules under standard 1 Sun (100 mW/cm²) illumination conditions with respect to active area^{7,10,11,12,13,14}. c Powe density and power output of OPV cells and modules under indoor light (1000 lx, ~ 280 µW/cm²) illumination conditions with respect to the active area. The data incorporated in this graph is a mixed performance of both flexible and rigid indoor OPV cells and modules. There are insufficient reports of flexible indoor OPVs (active area > 1 cm²) for making a fair comparison^15,16,17,18. d A comparison of time required to charge a capacity of battery, under standard 1 Sun and indoor lighting conditions, along with the effect of light fluctuations.

A primary concern lies in the energy harvesting efficiency of flexible OPV modules when integrated into wearable electronics, particularly under varying light spectrums and illumination intensities. The charging rate of energy harvesting and storage systems is primarily linked to incident light intensities, which directly influence the output power generation of flexible OPV modules and subsequently affect the battery’s charging behavior.

To address this issue and devise practical solutions, it is essential first to examine the power generation capabilities of flexible OPVs. Survey graphs illustrating the power density and power output of flexible OPVs under standard 1 Sun (100 mW/cm²) (Fig. 1b) and indoor light (1000 lx, ~ 300 µW/cm²) (Fig. 1c) illumination conditions relative to device active area are presented^{7,10,11,12,13,14,15,16,17,18}. These graphs clearly show that the power density of organic photovoltaics drops dramatically with an increase in active area, particularly for active areas over 1 cm². Conversely, the power output of flexible OPVs is directly proportional to the active area. OPV modules with an active area of approximately 20 cm², which is reasonable given device dimensions of 5 × 5 to 5 × 7 cm² for wearable applications, exhibit power outputs of ~ 250–300 mW under standard 1 Sun (100 mW/cm²) and about 0.82–1 mW under indoor light (1000 lx, ~ 300 µW/cm²) illumination conditions.

Fluctuations in light spectrum and intensity can significantly affect the battery’s charging rate, which can range from a few hours under 1 Sun/outdoor conditions to several hundred hours under indoor lighting environments. Moreover, dramatic input power fluctuations can cause efficiency losses and potential thermal issues, leading to accelerated battery degradation due to mechanical and chemical stresses. Therefore, a well-balanced power management system between energy harvesting and storage units is essential to mitigate these effects and ensure stable and efficient energy storage.

A comparison of the time required to charge a battery under standard 1 Sun and indoor lighting conditions, including the effects of light fluctuations, is shown in Fig. 1d. These simulation results indicate that designing a self-sustainable flexible OPV-integrated wearable electronic system requires not only high-performance photoactive materials for efficient energy harvesting but also the development of ultra-low power electronic circuits and sensors.

The integration of all components of an ultrathin flexible wearable device, such as flexible energy harvesting-storage system (FEHSS), flexible electronic control unit, and ultralow power sensors into a singular ultrathin flexible substrate for wearable devices represents a substantial engineering challenge due to the necessity of combining various functionalities without compromising the overall flexibility and performance of the system. Saifi et al., have recently developed a fully integrated 90 µm ultrathin flexible energy harvesting and storage system that shows immense potential in addressing these challenges¹⁹. This system, which integrates ultrathin flexible OPVs and zinc-ion batteries, is a significant step forward in the development of wearable technology. Where 4 µm ultrathin flexible freestanding OPV modules have output power density above 10 mW cm⁻². In addition, a significant advancement in the integration of zinc-ion batteries, featuring a substantial reduction in the hydrogel electrolyte thickness from millimeters to 10 µm. This reduction in thickness is significant as it maintains high electrochemical performance while greatly enhancing the system’s overall flexibility. The ability to reduce the electrolyte thickness without sacrificing performance is a notable achievement, as it contributes to the ultrathin design of the energy harvesting and storage system. This ultrathin design ensures mechanical compliance, allowing the FEHSS to be comfortably attached to the human body or integrated into textiles, which is essential for wearable applications. The mechanical durability of the FEHSS is another significant strength. The system has demonstrated the ability to retain over 80% of its efficiency after being subjected to rigorous mechanical testing. Specifically, it retains its efficiency after being bent to a radius of less than 1 mm for 500 cycles and compressed to a strain of 10% for 100 cycles. These results underscore the robustness of the system under mechanical stress, which is crucial for wearable devices that undergo constant movement and deformation. The high mechanical durability ensures that the FEHSS can withstand the physical demands of wearable applications without significant degradation in performance.