Broadband-spectrally selective polymeric film for building integrated photovoltaic (BIPV) skylights cooling

Spectrally selective design has attracted increasing scientific attention in building integrated photovoltaic (BIPV) technologies for its high flexibility. This study introduces a spectrally selective strategy, aimed at optimizing BIPV skylights with three key features: (1) high above-bandgap transmittance to ensure electrical power generation; (2) high sub-bandgap reflectivity to reduce the negative solar heat gain, and (3) high emissivity in mid-infrared band to facilitate radiative sky cooling. The proposed strategy involves the development of a flexible PDMS/ITO/PET film. In such a multi-layered structure, the PDMS (Polydimethylsiloxane) layer provides high emissivity (0.93) in the atmospheric window, the ITO (Indium Tin Oxide) offers high sub-bandgap reflectivity for solar cells (> 1.1 μm), and the PET (Polyethylene terephthalate) provide structural support. Experimental tests were conducted on this novel spectral regulation strategy, and results indicated that retrofitting glazing with this film reduced indoor temperatures by 4.7 ℃ and indoor solar radiation by 20 %, and retrofitting a PV panel with the film lowered indoor temperatures by 2.6 ℃ whereas the power generation decreased by 9 %. After experiments, a transient model was developed to assess the performance of the spectral regulation across different seasons and climatic conditions. Simulations conducted in Hong Kong revealed that the PDMS/ITO/PET film can help to reduce the annual solar heat gain by 18.3 % (0.57 GJ/m²), increase the indoor radiative cooling gain by 35 % (0.08 GJ/m²), whereas it slightly decreases the power generation by about 0.03 GJ/m². This research underscores the potential of the energy-saving PDMS/ITO/PET film. With facile fabrication and scalability, it is foreseen that the proposed strategy will offer a sustainable solution for green buildings in hot regions.