An ultraflexible energy harvesting-storage system for wearable applications

Wearable technology is rapidly advancing, creating a significant demand for flexible and efficient power systems. Current commercial wearables often rely on bulky batteries that limit flexibility and require frequent charging. The development of ultraflexible energy harvesting-storage systems (FEHSSs) that can efficiently generate and store power while adapting to curved surfaces is a key challenge. This research addresses this challenge by integrating high-performance organic photovoltaics (OPVs) and zinc-ion batteries (ZIBs) into an ultrathin, flexible system. OPVs are particularly attractive for wearable applications due to their lightweight nature, biocompatibility, and short energy payback time. Recent advancements have significantly improved the power conversion efficiency (PCE) of flexible OPVs, exceeding 10%. However, creating large-area, high-power-output ultraflexible OPV modules remains a hurdle. The intermittent nature of solar energy necessitates integration with energy storage. Lithium-ion batteries, while prevalent, present safety and flexibility challenges in wearable applications. ZIBs offer a safer and more flexible alternative. Integrating these components while maintaining mechanical compliance, stable interfacing, and high efficiency poses significant challenges. Prior integrated FEHSSs typically exhibit thicknesses in the millimeter range and limited power output. This study aims to overcome these limitations by developing a highly efficient, ultraflexible FEHSS with significantly improved performance and flexibility.

Existing literature highlights the progress in both flexible energy harvesting and storage technologies. Research on flexible OPVs has focused on enhancing PCE and long-term stability, with some achieving efficiencies above 10%. However, scaling up these devices to large, high-power-output modules while maintaining flexibility remains a challenge. Studies on flexible energy storage have explored various battery chemistries, including lithium-ion and zinc-ion batteries. Lithium-ion batteries, while efficient, face safety concerns and flexibility limitations when miniaturized for wearable applications. Zinc-ion batteries are emerging as a safer alternative due to their inherent safety features and potential for flexible designs. Several attempts to integrate energy harvesting and storage have been documented, often using thermoelectric generators, bioenergy harvesters, and combinations of photovoltaic cells with supercapacitors or batteries. However, these systems typically suffer from limitations in flexibility, efficiency, and power output, often exceeding 1 mm in thickness. This study builds upon this existing research by addressing the limitations of previous FEHSS designs.

The researchers fabricated a 90 µm-thick FEHSS by integrating high-performance ultraflexible OPV modules with ultrathin zinc-ion batteries. The OPV modules were constructed using a ternary blend of PM6:O-IDTBR:Y6 active materials to enhance efficiency and stability. The electron transport layer (ETL) was modified with 1,2-ethanedithiol (EDT) to further improve stability and reduce trap-assisted recombination. The ultrathin zinc-ion batteries employed a 10 µm-thick poly(vinyl alcohol)/graphene oxide (PVA-GO) hydrogel electrolyte and thin graphite paper to minimize thickness while maintaining electrochemical performance. To prevent current reflux from the battery to the OPVs in dark conditions, ultrathin organic diodes with a high rectification ratio were integrated. The performance of the OPV modules and ZIBs was characterized using various techniques including current density-voltage (J-V) measurements, external quantum efficiency (EQE) measurements, grazing incidence wide-angle X-ray scattering (GIWAXS), and electrochemical tests. The integrated FEHSS was then evaluated for its overall efficiency, stability, and ability to power wearable electronics, such as an ECG sensor and a smartphone. Detailed descriptions of the fabrication processes and characterization techniques are provided in the methods section of the paper. The fabrication of the flexible OPV arrays involved cleaning glass substrates, depositing a sacrificial layer, sputtering ITO, depositing a ZnO ETL (modified with EDT in some cases), spin-coating the active layer, and depositing Ag and MoO3 electrodes. The active layers were PM6:Y6 (binary) and PM6:O-IDTBR:Y6 (ternary) blends. The flexible ZIBs were fabricated using a cold-lamination technique to create ultrathin PVA-GO hydrogel electrolytes, thin graphite paper as the cathode, Zn foil as the anode, and parylene encapsulation. The integration involved connecting the OPV and ZIB components using anisotropic conductive film (ACF) tape, incorporating organic diodes to prevent backflow, and adding copper contact tabs. Characterizations of the OPV involved J-V curves under AM1.5G illumination, EQE measurements, GIWAXS analysis for microstructure, and long-term stability tests under different conditions. Electrochemical characterizations of the ZIBs included cyclic voltammetry, rate performance tests, long-term cycling stability assessments, and mechanical durability tests (bending and compression).

The key findings demonstrate significant advancements in the development of an ultraflexible FEHSS: 1. **High-Performance OPVs:** The ultraflexible OPV cells achieved a power conversion efficiency (PCE) of up to 16.18%, with an areal power output exceeding 10 mW cm². The ternary blend (PM6:O-IDTBR:Y6) and EDT-modified ZnO ETL were crucial for achieving high performance and stability. Long-term stability tests showed the OPVs retained over 92% of their initial PCE after 500 hours of continuous illumination. Scaled-up modules maintained high areal power output, reaching 10.2 mW cm² for a 6.72 cm² module and generating more than 68 mW of power. 2. **Ultrathin, High-Performance ZIBs:** The ultrathin zinc-ion batteries reached a thickness of only 85 µm, with a high specific capacity of 3.88 mA h cm² and an energy density exceeding 5.82 mWh cm². This was achieved by using a 10 µm-thick PVA-GO hydrogel electrolyte and thin graphite paper, significantly reducing the overall battery thickness without compromising electrochemical performance. The batteries demonstrated excellent cycling stability, retaining over 86% of their initial capacity after 200 cycles, and good mechanical durability. 3. **Integrated FEHSS:** The integrated FEHSS, with an overall thickness of 90 µm, achieved an overall energy conversion and storage efficiency of up to 6.91%. The integration of organic diodes effectively prevented current reflux from the battery to the OPVs in dark conditions, ensuring smooth system operation. The FEHSS showed excellent operational and storage stability, retaining over 80% of its efficiency after 60 charge-discharge cycles and maintaining functionality for over two weeks in ambient conditions. The FEHSS exhibited exceptional mechanical durability, maintaining over 80% of its efficiency after extensive bending and compression tests. 4. **Wearable Applications:** The FEHSS successfully powered various wearable electronics, including a flexible ECG sensor and demonstrated the ability to charge a smartphone and smartwatch, highlighting its potential as a practical and sustainable power source for real-world applications.

The results demonstrate the successful integration of high-performance OPV modules and ultrathin ZIBs into a highly flexible and efficient FEHSS, addressing the limitations of previous systems. The high PCE of the OPVs, coupled with the high energy density and stability of the ZIBs, provide a reliable and sustainable power source for wearable electronics. The ultraflexible nature of the FEHSS enables seamless integration with various wearable platforms, opening avenues for a wide range of applications, including health monitoring, personal electronics, and human-computer interaction. The integration of organic diodes effectively manages current flow, preventing undesired backflow from the battery to the OPVs, contributing to the system's overall efficiency and stability. The exceptional mechanical durability ensures the FEHSS can withstand the stresses of daily wear and tear. This research represents a significant advancement toward self-powered wearable technology, paving the way for more versatile, comfortable, and long-lasting wearable devices. The high overall energy conversion and storage efficiency, combined with the system’s robustness, suggests its potential for practical applications in various wearable devices and offers promising prospects for diverse fields, including healthcare and personal electronics. The successful powering of an ECG sensor and partial charging of a smartphone underscores the system's practicality and potential impact on future wearable technology.

This study successfully developed a highly flexible and efficient energy harvesting-storage system (FEHSS) for wearable applications. The integration of high-performance OPV modules and ultrathin ZIBs, along with the inclusion of organic diodes for current management, resulted in an innovative power system with superior flexibility, efficiency, and stability. Future research could focus on further miniaturization, exploring alternative battery chemistries, and developing more sophisticated power management strategies to enhance the FEHSS's capabilities and expand its applications to a wider range of wearable devices.

While the FEHSS demonstrates significant advancements, some limitations exist. The efficiency of the FEHSS is still dependent on the incident light intensity; charging times increase under low-light conditions. Further research could explore optimization strategies for operation under varying light conditions. The long-term stability of the integrated system under continuous operation and various environmental conditions warrants further investigation. While the mechanical durability of the system was demonstrated, exploring more robust materials and integration techniques may further improve the system's resilience to mechanical stresses.