Flexible composite film utilizing VO self-adaptive photothermal and infrared radiative cooling for continuous energy harvesting.

Self-adaptive photothermal (PT) and radiative cooling (RC) based on insulation-metal phase transition vanadium dioxide (VO) are among the most promising continuous energy harvesting technologies recently. However, previous work relies on rigid substrates that cannot fit complex or non-planar surfaces. Here, we propose a flexible composite film by bonding a VO thin film and a polyimide (PI) substrate with polymethyl methacrylate (PMMA), which achieves efficient spectrally self-adaptive broadband absorption/emission and can convert between the daytime PT mode and nighttime RC mode. Because of the inherent absorption of VO and the intricate interplay within multi-layer structure, the solar absorptance of the film could to up to 0.886 in the PT mode with the incorporation of an AlO anti-reflection layer. On the other hand, due to the phase change properties of VO, this film exhibits a broadband infrared emissivity modulation from 0.32 to 0.82 and reaches a maximum RC power of approximately 244.59 W/m in the RC mode at night. Moreover, the film maintains the infrared spectrum switching capability and high emissivity in RC mode even after 10 bending cycles. Our work shows potential to broaden the applications of VO smart coatings, including tunable selective emitters, thermal management of spacecraft and smart skins.