Standard solar cells heat up under sunlight, and the resulting increased temperature of the solar cell has adverse consequences on both its efficiency and its reliability. We introduce a general approach to radiatively lower the operating temperature of a solar cell through sky access, while maintaining its sunlight absorption. We present first an ideal scheme for the radiative cooling of solar cells. For an example case of a bare crystalline silicon solar cell, we show that the ideal scheme can passively lower the operating temperature by 18.3 K. We then show a microphotonic design based on realistic material properties, that approaches the performance of the ideal scheme. We also show that the radiative cooling effect is substantial, even in the presence of significant non-radiative heat change, and parasitic solar absorption in the cooling layer, provided that we design the cooling layer to be sufficiently thin.
We exploit the unique properties of electromagnetic waves in nanophotonic structures to enhance the capabilities for active control of electromagnetic thermal transfer at nanoscale. We show that the near-field thermal transfer between two nanospheres can exhibit thermal rectification effect with very high contrast, and with large operating bandwidth. In this system, the scale invariance properties of the resonance modes result in a large difference in the coupling constants between relevant modes in the forward and reverse scenarios. Such a difference in coupling constants provides a new mechanism for thermal rectification.