Simultaneously enhanced solar absorption and radiative cooling with thin silica micro-grating coatings for silicon solar cells

Recently, the idea of radiative cooling by dissipating infrared thermal energy to the cold space through the atmospheric window, especially from 8 to 13 mu m in wavelength, has become an attractive way to cool down outdoor devices. Here we show that thin silica (SiO2) micro-gratings as solar-transparent and radiatively cooling coatings for silicon solar cells. The well-designed silica micro-gratings were fabricated with plasma-enhanced chemical vapor deposition, photolithography, and reactive ion etching processes. Spectrometric measurements showed that the SiO2 micro-gratings atop doped silicon wafer could remarkably enhance the infrared emittance up to 100% within the atmospheric window and increase the solar absorptance with anti-reflection. Numerical modeling confirmed the measured optical and radiative properties and elucidated the underlying physical mechanisms for the anti-reflection in the solar spectrum and enhanced infrared thermal emission. The radiative cooling performance calculated based on a heat transfer model signified that by enhancing the radiative heat dissipation to the space, the grating structure could increase the radiative cooling power when the structure temperature is higher than 45 degrees C, and reduce the stagnation temperature by up to 20 degrees C depending on convective heat transfer coefficients. Furthermore, an outdoor field test has been conducted to experimentally demonstrate the cooling performance of the silica micro-gratings, where the grating covered sample showed a lower temperature than the bare silicon sample did under direct sunlight.