Solar-driven self-powered systems are highly desirable for applications like wearable electronics and IoT devices. Traditional systems use separate solar cells and energy storage modules, leading to increased cost and size. While two-electrode bifunctional designs are compact, they suffer from poor efficiency and stability. Three-electrode designs, combining solar cells with batteries (like Li-ion or Li-sulfur) offer improved energy density but lack power performance. Aluminum-ion batteries are high-power but expensive and corrosive. Solar rechargeable capacitors (SRCs) show improved specific power, but compromise on specific energy and overall efficiency. The ideal system needs high specific energy and power, high overall efficiency, high safety, and low cost (4H1L). This study addresses this challenge by integrating a high-capacity, high-power aqueous zinc battery with a high-efficiency perovskite solar cell, aiming for a 4H1L device. The aqueous zinc battery system is chosen due to its cost-effectiveness, safety (using non-flammable water-based electrolytes), and potential for high energy and power densities when coupled with a suitable cathode material. The integration is facilitated by a novel sandwich joint electrode design that protects the water-sensitive perovskite solar cell while maintaining electrical contact.

Existing solar rechargeable systems (SRSs) face limitations. Four-electrode systems, while efficient, are bulky and expensive. Two-electrode systems, though compact, lack efficiency and long-term stability due to issues like poor spectral response, inefficient charge separation, and corrosion. Three-electrode systems combining solar cells with various battery types (LIBs, LSBs) improve energy density but fall short on power performance. High-power options like aluminum-ion batteries are costly and employ corrosive electrolytes. SRCs integrating solar cells and capacitors enhance specific power but compromise on energy and overall efficiency. This paper reviews these existing limitations to highlight the need for a system meeting 4H1L criteria (high specific energy and power, high efficiency, high safety, and low cost). The integration of aqueous zinc batteries and perovskite solar cells is proposed as a promising approach to address these issues.

The integrated solar rechargeable zinc battery (SRZB) was designed with a layered architecture. A sandwich joint electrode is central to the design, integrating the perovskite solar cell and aqueous zinc battery units. This electrode has a hydrophilic-hydrophobic-hydrophilic structure, protecting the perovskite from the aqueous electrolyte while enabling efficient charge transfer. Heterostructural Co₂P-CoP-NiCoO₂ nanoneedle arrays (NAs) were grown in situ on one hydrophilic side of the sandwich joint electrode to serve as the battery's positive electrode. The synthesis involved a phosphating process using PH₃ gas generated from NaH₂PO₂·H₂O, controlling the extent of phosphating by varying the NiCo₂O₄:NaH₂PO₂·H₂O ratio. This controlled the resulting material composition from pure NiCo₂O₄ to a Co₂P-CoP-NiCoO₂ heterostructure. The other hydrophilic side of the sandwich joint electrode acts as the counter electrode for the perovskite solar cell. A Cs₀.₁₅FA₀.₈₅PbI₃ perovskite absorber was chosen for its stability and performance. The perovskite solar cell module consisted of three cells connected in series. The aqueous electrolyte contained KOH and Zn(Ac)₂ in distilled water. The SRZB was assembled layer-by-layer, sealing the electrolyte within the battery unit while maintaining contact with the perovskite. Electrochemical testing was performed using a solar simulator and battery tester. Photocharging efficiency was assessed by varying the active area exposed to light. Specific energy and power were determined from galvanostatic discharge tests at varying current densities. Long-term stability was evaluated through repeated photocharge and discharge cycles. Cost analysis compared the SRZB with solar rechargeable LIBs, considering electrode materials, electrolyte solvents, and salts. Safety was assessed by comparing the flammability of the aqueous electrolyte with that of a typical LIB electrolyte.

The heterostructural Co₂P-CoP-NiCoO₂ cathode demonstrated superior electrochemical performance compared to undoped NiCo₂O₄, exhibiting a significantly higher capacity (~210 mAh/g vs. ~30 mAh/g). The cathode showed excellent capacity retention at high current densities (~170 mAh/g at 32 A/g), maintaining ~78% capacity after 25,000 cycles with a coulombic efficiency exceeding 95%. The HTM-free carbon-based perovskite solar cells (C-PVKs) achieved a power conversion efficiency (PCE) of 14.85%, among the best reported for this type of cell. The integrated SRZB demonstrated an overall energy conversion efficiency of up to 6.4%, maintaining ~5.9% even after 200 photocharge-discharge cycles. The device exhibited a high specific energy of 366 Wh/kg (at 2 A/g) and a high specific power of 54.01 kW/kg (at 32 A/g). The cost analysis revealed that the SRZB is significantly cheaper than solar rechargeable LIBs, with substantial reductions in the cost of metal electrodes, electrolyte solvents, and salts. The aqueous electrolyte provided enhanced safety compared to flammable organic electrolytes used in LIBs. The SRZB demonstrated good performance even at freezing temperatures (0°C), attributed to the photothermal effect.

The results demonstrate the successful integration of a high-performance aqueous zinc battery and a perovskite solar cell, achieving a 4H1L solar rechargeable device. The sandwich joint electrode design plays a crucial role in protecting the perovskite while facilitating efficient charge transfer. The heterostructural cathode material enhances both energy and power density, while the HTM-free perovskite cell simplifies fabrication and improves stability. The narrow voltage-matching mechanism maximizes energy conversion efficiency. The superior performance and low cost of the SRZB compared to existing technologies positions it as a strong candidate for large-scale, outdoor applications. The high cycle life and inherent safety of the aqueous system further enhance its appeal for practical use.

This study successfully demonstrates a solar rechargeable zinc battery (SRZB) that meets the 4H1L criteria. The integration of a high-performance aqueous zinc battery and an efficient perovskite solar cell, facilitated by a novel sandwich joint electrode, results in a device with high specific energy, high specific power, high efficiency, high safety, and low cost. Future research could focus on further optimizing the perovskite solar cell performance, exploring alternative cathode materials for the zinc battery, and investigating the scalability and manufacturing processes for large-scale production.

While the SRZB demonstrates excellent performance, some limitations exist. The efficiency of the HTM-free perovskite solar cell is lower than that of cells with hole transport materials. The long-term stability of the integrated device is currently limited to 200 cycles; longer-term studies are needed to assess its true lifetime. Further optimization of the electrolyte composition could improve coulombic efficiency and address any potential side reactions. Finally, the study mainly focuses on the laboratory-scale device performance, and further work is needed to explore its scalability for larger applications.