Fig. 1. Self-cleaning and visibly transparent nanocomposite film for radiative cooling. Schematic of the composite film functioned with both radiative cooling and self-cleaning abilities. (a) On a sunny day, the film spontaneously cools the underlying solar cell by strong emission of thermal radiations whilst still transmitting sunlight. Meanwhile, the low-energy surfaces are less prone to adhere and accumulate dust, as the falling dust can be easily removed by a gentle wind. (b) On a rainy day, the superhydrophobic film shows excellent water repellence, so that the near-spherical rolling droplet picks up and removes the dirt. (c) Comparison of the penetrating AM 1.5 [5] solar irradiation and the output thermal radiation spectra for the composite film at a temperature of 50 °C. It was found that the composite film can transmit adequate solar light whilst dissipating heat passively through infrared radiation.

Fig. 2. Optimization of optical properties. (a) Spectral optical constants of TPX and SiO2 in the wavelength range of 0.3 to 20 μm. The optical constants for TPX are experimentally measured in the present work, while that for SiO2 is literature values from Ref. [10]. (b) Comparison of the calculated visible light (550 nm) transmissivity for the hybrid composite films using SiO2 spheres with different particle sizes. A sharp drop in transmissivity occurs, when the particle diameter matches with the wavelength of incident light and should be avoided inside the solar light range. (c) Comparison of the calculated solar reflectivity (left panel) and spectral emissivity (right panel) for the hybrid composite films using SiO2 spheres with different diameters of 20 nm and 10 µm (at a loading volume of ~22.6 vol.% and a film thickness of about 50 μm). (d) Comparison of the calculated solar transmissivity of the composite film with different volume fractions of the SiO2 nanospheres.

Fig. 3. The hydrophobic properties of the composite film. (a) The water contact angle (θ) as a function of SiO2 loading volume fraction fv, with different diameters of 20 nm and 10 µm. SEM surface morphologies of the composite films with (b) microspheres and (c) nanospheres as fillers, respectively; the inset shows the corresponding water contact angle of the film surface. (d) The AFM image of the nanocomposite film showing a nano-textured surface morphology.

Fig. 4. Visible light transmittance of the composite film and its impact on the performance of a coated silicon-based solar cell. (a) A photo showing the composite thin film which is 250 mm wide and a length of several meters. (b) Visible spectral transmissivity of a 50-μm-thick nanocomposite film. (c) Comparison of the J-V curves of the solar cells with and without the film coating at room temperature (25 °C); the inserted table shows the measured electrical parameters of the solar cells. (d) Comparison of the external quantum efficiency (EQE) for the silicon-based solar cells with and without the nanocomposite film coating.

Fig. 5. Radiative cooling performance of the composite film. (a) Schematic illustration of the spectral characteristics of the composite film. (b) Infrared thermal emissivity of a 50-μm-thick nanocomposite film. The emissivity spectrum of the glass was plotted for comparison purpose and is taken from Ref. [24]. (c) Schematic cross-sectional view of the experimental set-up for the radiative cooling of solar cells using the nanocomposite film. The structures from left to right are the solar cell coated with the nanocomposite film and the bare solar cell, respectively. (d) Comparison of the junction temperature of the solar cells with and without the film under direct sunlight. (e) Comparison of the real-time temperature of the 3-watt LED loads powered by the solar cell with and without the film coating.

Fig. 6. The robust self-cleaning property of the composite film. (a) A photo showing the dust-resistance of the nanocomposite film after exposure to a dusty environment. The left half of the solar cell which is coated with nanocomposite film is much cleaner with less dust on its surface compared to the right half without the coating. (b) The change of static contact angle with the increase of outdoor exposure time. (c, d) The self-cleaning performance evaluated by monitoring the removal of dust deposited on the solar cell surface via artificial 3 m s - 1 wind and 5 mm rainfall, respectively. The dust removal was quantified by measuring the weight of the remaining dust on the solar cell surface.