Spectrally-Tailored Hygroscopic Hydrogels with Janus Interfaces for Hybrid Passive Cooling of Solar Cells

Energy-efficient thermal management is critical for addressing the inevitable heat generation and inefficient heat dissipation of conventional solar cells. Designing cooling systems that optimize optical windows, solar conversion, interfacial properties and aesthetic appeal for diverse photovoltaic technologies remains a significant challenge. Herein, passive cooling membranes are developed as "front surfaces" for both silicon-based (inorganic) and polymer (organic) solar cells. The LiCl-embedded hydrogel combines highly cross-linked and highly entangled polymers within a two-layered membrane. The salt-in effect of highly concentrated zwitterionic copolymers and large-volume swelling of hydrogels ensure high LiCl retention (max. 20 wt.%) with negligible leakage, leading to stable evaporative cooling. This membrane is also distinguished by its Janus-like surfaces and tailored optical windows, integrating high transmittance (≈90%) for the sunlight absorption and high emissivity (≈98%) for sky radiative cooling. This study represents one of the few examples of integrating hybrid cooling mechanisms on solar panels, achieving a maximized efficiency increase above 21% under natural sunlight while enhancing cooling power regeneration overnight. Moreover, its cooling power and durability enable ≈95% of efficiency retention over 10 000 bending cycles in flexible solar cells. This study shines a light on developing hierarchically aligned hydrogels for self-contained and user-defined thermal management for diverse electronics.