Colorless solar windows: Transforming architecture into clean power plants

A research team led by Nanjing University has introduced a transparent, colorless, and unidirectional solar concentrator that can be directly coated onto standard window glass. Utilizing cholesteric liquid crystal (CLC) multilayers with submicron lateral periodicities, this diffractive-type solar concentrator (CUSC) selectively guides sunlight toward the edge of the window where photovoltaic cells are installed. The study appears in PhotoniX.

Unlike conventional luminescent or scattering-based concentrators, which often suffer from visual distortion, low efficiency, and poor scalability, the new CUSC achieves broadband polarization-selective diffraction and waveguiding without compromising clarity. The device maintains a high average visible transmittance (64.2%) and color rendering index (91.3), enabling clean energy generation without altering the appearance of the window.

“By engineering the structure of cholesteric liquid crystal films, we create a system that selectively diffracts circularly polarized light, guiding it into the glass waveguide at steep angles,” said Dr. Dewei Zhang, co-first author. “This allows up to 38.1% of incident green light energy to be collected at the edge.”

Experiments showed that a 1-inch-diameter prototype could directly power a 10-mW fan under sunlight. Modeling suggests a typical 2-meter-wide CUSC window could concentrate sunlight by 50 times, significantly reducing the number of photovoltaic cells required by up to 75%. The system supports integration with high-performance PV cells such as gallium arsenide for enhanced power conversion.

The multilayered CLC films are fabricated via photoalignment and polymerization techniques and are scalable via roll-to-roll manufacturing. The design remains stable under long-term exposure and can be retrofitted onto existing windows for sustainable urban upgrades.

“The CUSC design is a step forward in integrating solar technology into the built environment without sacrificing aesthetics,” said Professor Wei Hu. “It represents a practical and scalable strategy for carbon reduction and energy self-sufficiency.”

Future work will focus on enhancing broadband efficiency, polarization control, and adapting the technology for agricultural greenhouses and transparent solar displays. Their vision: to turn passive glass into active, energy-generating surfaces worldwide.

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