Passive Radiative Cooling using Randomly Micro-Structured Silicon Meta-Materials

Decarbonized and low-energy technologies for heat management and conversion are critical milestones for climate change mitigation¹. Cooling is one of the major fields where such technologies are required. Passive radiative cooling in its nighttime and daytime versions is therefore a promising technology to achieve those goals². We consider in this work a silicon-based nanostructured meta-material with outstanding visible and infrared radiation absorption and emission capabilities, alternatively called Black Silicon due to its black color, as a candidate for nighttime passive radiative cooling. Indeed, we have recently shown that randomly micro-structured silicon surfaces can exhibit near-unit emissivity over a wide spectral range from the visible to the mid-infrared³. We have also shown that this range can be significantly extended and tuned by varying the silicon doping and the micro-structures aspect ratio^4–6 to reach the spectral range of the atmospheric transparency window (8 to 13 µm). These interesting radiative properties of Black Silicon combined with the different tuning possibilities open an avenue for using such materials for different thermal radiation control applications such as enhanced solar radiation absorption⁷, infrared radiation sources⁸, selective emitters, and passive radiative cooling⁹. In this contribution, we numerically compare Black Silicon with its flat silicon counterpart with respect to their radiative cooling power. We consider Black Silicon samples with different nano structuration aspect ratios and use their experimentally measured radiative properties. The radiative properties are measured using Fourier Transform Infrared Spectroscopy at room temperature. We show that the computed BSi cooling power is significantly larger than that of flat silicon by a factor up to 1.8 at 30°C with a cooling power of 75 W /m² and 140 W/m² for flat and Black Silicon, respectively. We also numerically investigate the influence of several parameters such as the conductive and convective heat transfer, the atmospheric transmittance spectra (idealized: rectangular function or realistic: with multiple absorption bands), and the operating temperature. We show that Black Silicon exhibits a larger radiative cooling power than a material with an ideal unit emissivity between 8 and 13 µm. Indeed, the considered atmospheric transmittance greatly influences the radiative cooling power. Finally, we perform nighttime radiative cooling experiments using black and flat silicon and compare experimental results with the numerical predictions.<br/>1. A. Henry, R. Prasher, A. Majumdar, <i>Nature Energy</i>. <b>5</b>, 635–637 (2020).<br/>2. B. Zhao, M. Hu, X. Ao, N. Chen, G. Pei, <i>Applied Energy</i>. <b>236</b>, 489–513 (2019).<br/>3. S. Sarkar, A. Elsayed, E. Nefzaoui, J. Drevillon, P. Basset, F. Marty, M. Anwar, Y. Yu, J. Zhao, X. Yuan, others, (2019).<br/>4. S. Sarkar, A. A. Elsayed, F. Marty, J. Drévillon, Y. M. Sabry, J. Zhao, Y. Yu, E. Richalot, P. Basset, T. Bourouina, E. Nefzaoui, in <i>2019 25th International Workshop on Thermal Investigations of ICs and Systems (THERMINIC)</i> (2019), pp. 1–4.<br/>5. S. Sarkar, A. A. Elsayed, Y. M. Sabry, F. Marty, J. Drévillon, X. Liu, Z. Liang, E. Richalot, P. Basset, E. Nefzaoui, <i>Advanced Photonics Research</i>, 2200223 (2022).<br/>6. S. Sarkar, E. Nefzaoui, G. Hamaoui, F. Marty, P. Basset, T. Bourouina, <i>Appl. Phys. Lett.</i> <b>121</b>, 231703 (2022).<br/>7. L. Gao, E. Nefzaoui, F. Marty, X. Wei, S. Bastide, Y. Leprince-Wang, T. Bourouina, <i>Solar Energy Materials and Solar Cells</i>. <b>243</b>, 111793 (2022).<br/>8. A. Saeed, A. A. Elsayed, F. Marty, E. Nefzaoui, T. Bourouina, H. A. Shawkey, Y. M. Sabry, D. Khalil, in <i>Nanophotonics VIII</i> (SPIE, 2020), vol. 11345, pp. 105–110.<br/>9. A. Hervé, G. Hamaoui, T. Bourouina, P. Basset, E. Nefzaoui, (Thessaloniki, Greece, 2023).