Photonic Crystal Fluorinated Polyimide Superstrate Enabling Radiative Cooling for Efficiency Enhancement in Flexible Thin-Film Solar Cells

Flexible thin-film solar cells, owing to their bendability and adaptability to diverse application scenarios, are promising for emerging energy electronics. However, conventional polymer superstrates lack sufficient near-infrared (NIR) reflection, causing heat accumulation and limiting photovoltaic efficiency. Here, a photonic crystal fluorinated polyimide (PC-FPI) superstrate with radiative cooling is developed to enable spectrally selective modulation. The structured PC-FPI exhibits a 76% relative enhancement in reflectance across 1.1–2.5 μm compared with unstructured films, effectively suppressing solar-induced heating. Combined with a high emittance of 88% in 8–13 μm, the PC-FPI superstrate enables efficient passive radiative cooling, while maintaining high transmittance of 83% in 0.3–1.1 μm for photon harvesting. The optimized one-dimensional grating (period 1.2 μm, duty cycle 0.7, sidewall angle 54.7°) is fabricated via a hard-template-assisted transfer process and exhibits antifouling behavior with a water contact angle of 138°. Integrated with flexible CIGS solar cells, the superstrate reduces operating temperature by 56 K, increases open-circuit voltage by 40 mV, and improves power conversion efficiency (PCE) by 7.28%. Compared with commercial superstrates, an absolute PCE gain of ∼2% is achieved, corresponding to ∼1000 kWh of additional annual energy output. This work highlights PC engineering as an effective strategy for spectrally selective cooling in high-performance flexible photovoltaics.