German scientists have developed a lightweight, metal-free solar battery that stores sunlight for over two days using only organic molecules and water.

Researchers in Germany created an all-organic "solar battery" that captures sunlight, stores energy for over two days, and releases it as electricity. It uses a porous, two-dimensional covalent organic framework (COF) from naphthalenediimide that combines photovoltaic and electrochemical functions in one light material. This metal-free design offers a more sustainable option for off-grid power.

The COF acts as both a light absorber and a charge reservoir. When illuminated, it generates electron-hole pairs and traps the resulting charges within its ordered pores. In aqueous conditions, those charges remain stable for at least 48 hours -- an unprecedented retention time for this class of materials -- and can be discharged on demand to run an external load. The team measured a specific capacity of 38 milliampere-hours per gram, outperforming comparable carbon nitrides, polymer semiconductors, and metal-organic frameworks.

Water molecules help preserve the stored energy. Rather than quenching charges, water rearranges around the COF structure and forms an energy barrier that prevents recombination. Studies confirm this interaction stabilizes trapped electrons without needing ions or metal centers.

Simulations led by spectroscopists at the Technical University of Munich explored several charge-stabilization scenarios and showed how molecular design, framework architecture, and the surrounding medium work together to lock charges in place. The simplicity of the mechanism -- organic building blocks plus water -- helps explain the material's robustness under cycling.

Practical testing underscored that robustness: the device retained more than 90 percent of its capacity after repeated charge-discharge cycles. That performance positions the COF platform as a promising candidate for integrated solar-energy storage, particularly where weight, sustainability, and material scarcity are critical concerns. Further work will focus on scaling the framework's synthesis, improving charge density, and integrating the material into complete photovoltaic systems.