A new type of transparent cooling film, containing crystallized SiO₂ nanoparticles in an acrylate resin, lowers the operating temperature of solar cells and improves efficiency
The half-emitter design achieved a higher cooling effect and reduced PV operating temperature by nearly 6°C, even though it used only half the nanoparticle content
The films increased the open-circuit voltage of silicon solar cells by approximately 0.012 V and improved power conversion efficiency by about 1.3% absolute
A team of researchers from Korea University and the Korea Institute of Science and Technology (KIST) has developed a new type of transparent cooling film that lowers the operating temperature of solar cells and improves their efficiency. The researchers have developed a so-called next-generation radiative cooling that can be applied to PV cells, releasing the heat without the need for mechanical cooling. Vehicle windows are another application of the concept.
Radiative cooling is a thermal management approach that enables materials to release heat to the sky without mechanical systems, and the team applied this principle to solar applications. As part of this effort, the researchers developed colloidal opaline composite films consisting of crystallized SiO₂ nanoparticles embedded in an acrylate resin. The nanoparticles used in the films measured about 80 nm in diameter. Unlike cooling films with randomly dispersed particles that scatter visible light and become hazy at high concentrations, the ordered opaline structures remain highly transparent across the visible spectrum while exhibiting thermal emissivity of around 95% in the mid-infrared.
KU-KIST’s scientists compared 2 film configurations: a full-emitter consisting entirely of the nanoparticle composite, and a half-emitter with a 50 µm opaline layer topped by a 50 µm clear polymer coating. Despite using only half the nanoparticle content, the half-emitter achieved superior performance, with an emissivity of about 94.8% and cooling power of 59 W/m² at 300 K, compared with 93.5% and 56 W/m² for the full-emitter.
The films also offer aesthetic flexibility. By increasing the nanoparticle diameter, the photonic stopband (a range of wavelengths strongly reflected by the ordered lattice) shifted into the visible range, enabling structural coloration. The team produced blue-colored films with 125 nm particles, green with 145 nm, and red with 180 nm. These colored films, developed by controlling particle size and density, demonstrated no performance degradation, combining aesthetics and functionality, according to the research group.
As a proof of concept for PV, the researchers applied this transparent film on the silicon solar cells and compared the emissivity, which is a metric for how effectively a surface emits thermal radiation, with values ranging from 0 to 1. The reference cells, identical devices without the film, showed poor mid-infrared emissivity of about 0.03 in the 8 to 13 µm range. In contrast, the cells with radiative cooling films reached an emissivity of 0.95. Under outdoor conditions, the half-emitter reduced cell temperature by an average of 5.77°C, leading to an increase in open-circuit voltage of about 0.012 V and a power conversion efficiency gain of around 1.3% absolute. Additional benefits included improved short-circuit current density due to enhanced light absorption in the 400 to 550 nm range.
Durability testing indicated that the films could withstand 10,000 bending cycles at a 1 mm radius without damage and maintained their optical and thermal properties after 200 hours of exposure at 85°C and 85% relative humidity under 1 Sun illumination.
Professor Lee Seung-woo from the Department of Integrative Energy Engineering, Korea University, who led the research, said: “Our research demonstrates the practicality and applicability of transparent radiative cooling films. We have opened up the possibility of new cooling materials that can be expanded into various fields such as automobiles, architecture, and solar power generation.”
The study, titled Colloidal Opaline Composites as Throughput-Scalable, Fully Transparent, and Color-Tunable Radiative Cooling Exterior Films for Outdoor Photovoltaics. The study was published online in Advanced Functional Materials, a global journal in the field of energy and materials science.