Can an integrated flexible energy harvesting and storage system facilitate efficient and consistent power output for ultrathin, flexible wearable electronics applications? Wearable technology has evolved rapidly from novelty gadgets to essential tools across various sectors, including healthcare, fitness, and consumer electronics. The performance and usability of these devices are heavily reliant on their power supply systems. This commentary primarily addresses the transformative potential and challenges associated with integrating flexible organic photovoltaics into wearable devices as sources of energy harvesting.

Flexible energy harvesting-storage system and wearable electronics

Flexible electronics, initially characterized by flexible electrical connections and circuit boards, have undergone significant advancements due to the integration of nanotechnology. Modern developments have introduced highly miniaturized and intelligent components, including nanotech sensors, integrated circuits, radios, microprocessors, and systems on chips. This progression has enabled the creation of sophisticated, multifunctional flexible electronic devices, expanding their applicability in diverse domains such as biomedical devices, wearable technology, and smart textiles. The continuous progress in flexible electronics underscores their potential to revolutionize numerous industries by providing adaptable and high-performance solutions1,2.

To realize the vision of efficient, comfortable, and self-sustainable wireless flexible wearable electronics, it is imperative to develop a flexible, lightweight, efficient, and robust power source. Addressing the escalating energy demands of wearable electronics can be directly approached by enhancing the volumetric capacity of flexible energy storage devices, thereby increasing their energy and power densities. However, the trend toward integrating more functions into compact device designs presents significant challenges for long-life sustainable power supply, often leading to early battery replacement or frequent recharging. In addition, elevating the energy density of flexible energy storage devices raises safety concerns, especially in wearable applications subjected to repetitive mechanical stresses. Traditional power solutions, such as bulky coin cells and rigid rechargeable batteries, frequently undermine the mechanical compliance and convenience of wearable systems. In light of these challenges, a flexible self-sustainable system capable of harvesting ambient energy while simultaneously charging energy storage devices without relying on an external power source would represent a promising solution3,4.

Organic photovoltaics (OPVs) have garnered significant attention as a sustainable energy technology due to their lightweight nature, solution processability, and exceptional mechanical flexibility. These attributes render OPVs suitable for large-scale production techniques, such as roll-to-roll (R2R) manufacturing, which can lower production costs and enhance commercial viability. In recent years, the power conversion efficiency (PCE) of flexible OPV cells and modules has surpassed 17% and 14%, respectively. Notably, the advancement of non-fullerene acceptor (NFA) materials and the interfacial engineering of OPV devices have significantly improved performance under both outdoor and indoor illumination conditions. This progress makes OPVs particularly advantageous for applications in the Internet of Things (IoT) and wearable electronics5,6,7.

Flexible OPVs energy harvesting capabilities and wearable electronics

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 systems4,8,9.

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.

Fig. 1: Flexible OPV’s energy harvesting capabilities for wearable electronics.
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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 area7,10,11,12,13,14. 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 comparison15,16,17,18. 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.

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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 presented7,10,11,12,13,14,15,16,17,18. 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.

Future of flexible OPV integrated self-sustainable wearable electronics

Flexible organic photovoltaics integrated into self-sustainable wearable electronics hold significant promise, driven by ongoing advancements in material science, manufacturing processes, and system integration. These technologies are poised to revolutionize wearable electronics by offering lightweight, flexible, and efficient means of harvesting ambient energy, thereby reducing reliance on traditional power sources. Continued progress in non-fullerene acceptor materials and other photoactive compounds is crucial, enhancing power conversion efficiencies and spectral matching with diverse lighting sources. Particularly in low-light environments, the development of photoactive materials with adjustable light absorption properties is essential for maximizing energy extraction from low-intensity light sources. Innovations in interfacial engineering to minimize power losses under varying light conditions can further optimize OPV performance, ensuring consistent energy output necessary for wearable applications.

Establishing comprehensive standards to assess energy harvesting systems is imperative, encompassing various photovoltaic and energy storage parameters within a unified framework. Evaluating projected areas and volumes alongside total efficiency facilitates meaningful performance comparisons. While current metrics like bending radius gauge flexibility, incorporating figures of merit that consider yield strain provides a more holistic assessment of flexibility and stretchability. Addressing these challenges and establishing robust standards will facilitate the integration of flexible OPVs and energy storage units, transforming self-sustainable wearable electronics into efficient, dependable, and user-friendly devices applicable across diverse practical contexts.

In conclusion, integrating flexible OPVs with energy storage systems represents a transformative opportunity in wearable electronics. Overcoming challenges related to variable light conditions, ultralow power sensors, electronic integration, and energy management will enable the development of efficient, sustainable, and user-centric wearable devices. Future research should prioritize material innovations for enhanced PCE and reduced power losses under dynamic lighting conditions, advanced manufacturing techniques for consistency, and improved geometric fill factors to minimize device footprint. Real-world validation is essential to confirm performance and durability, paving the way for next-generation wearable electronics that seamlessly integrate into daily life, enhancing functionality and user experience across various applications.