Polydimethylsiloxane based transparent and reflective materials for passive daytime radiative cooling

Passive daytime radiative cooling (PDRC) dissipates heat by emitting infrared radiation through the atmospheric transparency window (8–13 μm), enabling sub-ambient temperatures under direct sunlight without energy input. Polydimethylsiloxane (PDMS) is widely used in PDRC because it combines high emissivity in the 8–13 μm range with high transmittance (low absorption) in the solar spectrum (0.3–2.5 μm), while also being easy to fabricate. This study presents a systematic investigation of flat PDMS-based PDRC materials with thicknesses from around 1 μm to around 1000 μm deposited on either highly-transparent fused silica substrates or highly-reflective commercial sunlight reflector substrates. These configurations target applications such as cooling for transparent roofs and outdoor electronic enclosures. For a 58-μm-thick PDMS layer on silica, the sample shows a solar spectrum transmittance of 92.8%, absorption of 1.1%, and an emissivity of 94.4% in the 8–13 μm range, resulting in a net cooling power of 78.5 W/m 2 . For this transparent configuration, the cooling power remains nearly unchanged for PDMS thicknesses from 1.7 μm to 121 μm, indicating a broad thickness tolerance for practical fabrication. For the PDMS-coated sunlight reflector sample with 60-μm-thick PDMS thickness, the absorption reaches 6.2%, which reduces the net cooling power to only 17.5 W/m 2 . For this reflective configuration, an optimal PDMS thickness of around 60 μm is identified, but the cooling performance is strongly limited by the increased solar absorption of the reflector-based structure. This work highlights the potential of PDMS-based transparent and reflective materials for simple and scalable PDRC systems, while clarifying the influence of thickness and substrate choice on their cooling performance.