An ultraflexible energy harvesting-storage system for wearable applications

Wearable electronics demand thin, compliant, and safe power sources capable of continuous operation under deformation. Conventional coin or pouch lithium batteries add rigidity and safety concerns, while stand-alone organic photovoltaics (OPVs) suffer from intermittent output under variable light. The research addresses how to integrate ultraflexible, efficient solar energy harvesting with equally thin, safe energy storage to realize an autonomous, robust power system suitable for on-skin and in-textile use. Key challenges include achieving high PCE and energy density without sacrificing mechanical compliance, developing ultrathin yet safe electrolytes, ensuring stable interfaces, preventing current backflow in darkness, and delivering sufficient voltage/current for practical loads.

Prior integrated flexible energy harvesting–storage systems include thermoelectric, tribo/piezoelectric, and photovoltaic-supercapacitor/battery combinations, typically 1 mm–600 µm thick and with power output below 5 mWh cm⁻², indicating limited flexibility and efficiency. Ultraflexible OPVs have exceeded 10% PCE but are often small-area devices with few reports showing areal power output above 10 mW cm⁻². Integrated photo-rechargeable systems using silicon PVs with solid-state Li-ion stacks (≈5.4 V), homo-tandem OPVs to boost Voc to charge Li/Na batteries, and textile-based Li-ion batteries (~650 µm systems) have been reported. However, trade-offs in thickness, safety, mechanical compliance, and efficiency remain, and stable, highly compliant, thin electrolytes present persistent challenges.

OPV fabrication: Ultraflexible inverted OPVs were built on a 1.5 µm parylene substrate with sputtered 100 nm ITO, ZnO electron transport layer (ETL; sol–gel), optionally modified by 1,2-ethanedithiol (EDT) to passivate defects (EDT-ZnO), a PM6:O-IDTBR:Y6 ternary bulk heterojunction active layer deposited by dynamic spin-coating, and MoO3 (7 nm)/Ag (100 nm) top electrodes. Devices were encapsulated with a 1.5 µm parylene layer and released from glass to yield ~4 µm-thick freestanding OPVs. Single-cell and modules (various series/parallel configurations; e.g., 12S×14P; effective area 6.72 cm²) were characterized under AM1.5G and varying light intensities; EQE, J–V, GIWAXS, light-intensity dependence (α, n), and stability (shelf and photostability) were measured. OPV module design: Series/parallel configurations were tuned to meet application power requirements (µW–mW). Brittle ITO was placed near the neutral mechanical plane between parylene layers to enhance durability under bending and compression. Zinc-ion battery (ZIB) fabrication: A thin Zn/MnO2-graphite battery used: Zn metal anode (20 µm), MnO2/carbon/PVDF slurry on 25–50 µm graphite paper as cathode, and an ultrathin 10 µm PVA–graphene oxide (PVA-GO) hydrogel electrolyte prepared by cold lamination followed by freeze–thaw crosslinking and drying, then soaked in 1 M ZnSO4/0.3 M MnSO4/K2S2O3 solution. The stack was encapsulated with 1.5 µm parylene and sealed with 10 µm PET tape; copper tabs provided contacts. The overall battery thickness was minimized to ~85 µm. Electrochemical tests included EIS (ionic conductivity vs hydrogel thickness), cyclic voltammetry, rate tests (0.2C–5C), cycling (up to 200 cycles at 5C), and mechanical bending durability (1 mm radius, 500 cycles). Integration: OPV modules were back-to-back stacked with ZIB(s) using anisotropic conductive film (ACF) tape. To suppress current reflux from battery to OPV in the dark, ultraflexible organic blocking diodes (active area ~0.02 cm²) were connected in series at the positive terminal of each series string. The OPV-diode units were then connected in parallel with the ZIBs. System performance included photo-charging/discharging under varied light intensities, overall photo-electric conversion and storage efficiency calculation, cycling stability, storage stability, and mechanical durability of the integrated FEHSS. Applications: The FEHSS powered on-skin ECG electrodes via a flexible PCB and charged consumer devices (smartwatch/smartphone) through a DC–DC regulator to 5 V.

- Ultraflexible OPV single cells (PM6:O-IDTBR:Y6 with EDT-ZnO ETL) achieved PCE up to 16.18% with JSC 26.3 mA cm⁻², VOC 0.815 V, and FF 75.5%. Ideality factor n decreased to 1.08 and α approached 1.005, indicating reduced trap-assisted and bimolecular recombination. - Stability: Ternary OPVs retained ~98% PCE over 1500 h in inert/dark; in ambient/dark, τ80 > 2 months. Under 50 mW cm⁻² continuous illumination, EDT-ZnO devices retained >92% PCE after 500 h (vs τ80 ≈ 380 h for unmodified ZnO ETL), with minor VOC/JSC losses and a smaller FF decline. - GIWAXS indicated enhanced π–π stacking (peak shift from 1.70 to 1.73 Å⁻¹; spacing 3.70 → 3.63 Å) in the ternary blend, supporting improved charge transport. - OPV modules: A 6.72 cm² freestanding module (12S×14P) delivered PCE ≈ 10.5%, Pmax > 68.9 mW, and areal power output >10.2 mW cm⁻² at 1 sun. Other modules (1.28, 16 cm²) also showed high areal outputs. Under 0.5, 0.3, and 0.1 sun, the 6.72 cm² module produced ~35, 20, and 6 mW, respectively; under ~7000 lux and 700 lux, outputs were ~3.5 mW and ~0.2 mW. After 1000 bending cycles (1 mm radius) it retained ~90% power; after 1000 compression cycles (50% strain) it still provided ~50 mW (≈9.8 mA, 6.4 V). - ZIB electrolyte thickness: Reducing hydrogel from 1 mm to 10 µm decreased ohmic resistance (EIS) and yielded ionic conductivities of ~7.1, 6.7, and 6.2 mS cm⁻¹ for 10 µm, 100 µm, and 1 mm, respectively; discharge capacity decreased by ~4.5% at 10 µm due to reduced electrolyte volume/ion availability. - Ultrathin ZIB (≈85 µm): Active area 1.75 cm²; capacity 6.8 mAh; specific capacity 3.88 mAh cm⁻²; coulombic efficiency up to 98%; >86% capacity retention after 200 cycles at 5C; stored 10 days at ambient retained 94% voltage (>1.58 V). Energy density exceeded 5.82 mWh cm⁻². - Integration with blocking diode: Organic diodes (rectification ratio ~10⁴ at ±2 V) prevented current reflux; battery voltage remained >1.4 V in dark when connected to OPV, versus dropping <1.3 V without the diode. - Photo-charging: A 1.28 cm² OPV could fully charge one ZIB to ~1.8 V at low light (1 mW cm⁻²); three ZIBs in series exceeded 5.4 V under 50 mW cm⁻². Under 100 mW cm⁻², a ZIB paired with a 6.72 cm² OPV charged in ~37 min; at 50 mW cm⁻² in ~72 min; under indoor 75–100 µW cm⁻², charging to 1.8 V required several hours but partial charging to ~1.5 V within 2 h enabled low-power loads. - Overall photo-electric conversion and storage efficiency reached up to 6.91% (e.g., 10 mW cm⁻² light, 1.28 cm² OPV, 5.9 mAh in 10 h); at ~65 mW cm⁻² and 1.5 h to 5.5 mAh, efficiency was ~6.61%. - System durability: FEHSS retained >80% conversion-storage efficiency over >60 charge–discharge cycles, τ50 shelf lifetime >2 weeks, and >80% efficiency after 500 bends (1 mm radius) or 100 compression cycles (10% strain). Minimum bending radius down to ~120 µm. - Demonstrations: Powered on-skin ECG sensing (clear P, QRS, T waves) and charged a smartwatch (28%/1 h) and smartphone (6%/1 h) under 100 mW cm⁻² with a regulator yielding ~0.5 W output.

The study demonstrates that integrating ultraflexible, high-efficiency OPV modules with ultrathin, safe Zn-ion batteries overcomes the intermittency and rigidity issues of conventional wearable power solutions. The ternary OPV with EDT-ZnO ETL reduces trap-assisted and bimolecular recombination, delivering high PCE and photostability, while module-level engineering provides configurable voltage/current for diverse loads with excellent mechanical resilience. On the storage side, a 10 µm PVA-GO hydrogel electrolyte enables an ~85 µm-thick ZIB with high energy density and robust cycling/mechanical stability. A simple, ultraflexible blocking diode effectively prevents current backflow, enabling efficient photo-charging and maintaining battery voltage in darkness without complex power management. The integrated FEHSS achieves high overall conversion–storage efficiency (up to 6.91%), stable operation across many cycles, and sustained performance under bending/compression, thereby addressing the core challenge of continuous, safe power delivery for on-skin and textile wearables across variable lighting conditions.

This work delivers a 90 µm-thick, fully integrated ultraflexible energy harvesting–storage system comprising high-performance OPV modules (PCE up to 16.18%; module areal output >10.2 mW cm⁻²) and an ~85 µm-thick Zn-ion battery (energy density >5.82 mWh cm⁻²; 6.8 mAh capacity) connected via rectifying diodes to eliminate current reflux. The FEHSS shows overall conversion–storage efficiency up to 6.91%, long-term operational and shelf stability, and excellent mechanical compliance, successfully powering biosensors and consumer devices. Future work could focus on further improving indoor-light charging rates, scaling module area, optimizing interfacial adhesion within the battery stack to mitigate deformation under repeated bending, and refining power regulation and wiring to increase charging speed for larger devices.

- Electrolyte thinning to 10 µm slightly reduces capacity (~4.5%) due to limited ion reservoir; balancing thickness, conductivity, and short-circuit risk remains a challenge. - OPV modules under continuous illumination still experience fill-factor degradation over time; although EDT-ZnO improves stability, long-term outdoor exposure may cause further aging. - After extensive bending cycles, slight deformation and kinks on the battery side were observed, contributing to some performance loss; interfacial strength within the battery stack could be enhanced. - Indoor-light charging is slow (several hours to reach 1.8 V), requiring trade-offs between charging rate and overall efficiency. - Reported diode rectification (~10⁴ at ±2 V) is effective for ZIBs but improved rectification and lower forward loss could further enhance system efficiency.