A groundbreaking advance in solar energy harvesting promises to revolutionize the integration of renewable power generation with everyday architecture, ushering in a new era of aesthetic and efficient green buildings. Researchers at Nanjing University have unveiled a pioneering colorless, transparent, and unidirectional solar concentrator that can be seamlessly applied to standard glass windows. This innovation leverages the unique optical properties of cholesteric liquid crystal (CLC) multilayers to direct sunlight precisely to photovoltaic (PV) cells installed along window edges, enabling significant energy generation without compromising visual clarity. The research, published in the prestigious journal PhotoniX, charts a path toward energy-generating architecture that blends invisibly into urban environments.
Traditional solar concentrator technologies have long wrestled with the challenge of balancing efficiency and transparency. Luminescent solar concentrators and scattering-based devices frequently introduce visual distortions, tint, or opacity that affect building aesthetics and occupant comfort. In contrast, the newly developed diffractive-type unidirectional solar concentrator (CUSC) harnesses nanoscale cholesteric liquid crystal films engineered with submicron lateral periodicities. These films exhibit broadband polarization-selective diffraction characteristics that steer circularly polarized sunlight into the glass substrates with high angular precision, funneling light to the perimeter PV modules while maintaining an impressive 64.2% average visible transmittance. The resulting window coatings remain virtually colorless and free from haze, preserving both natural illumination and color fidelity, evidenced by a high color rendering index of 91.3.
Dr. Dewei Zhang, co-first author of the study, explained the core operational principle behind the technology: “By precisely tuning the microstructure of cholesteric liquid crystal films, we enable selective diffraction specifically targeting circularly polarized components of sunlight. This light is guided at steep angles through the window substrate waveguide, minimizing optical losses and effectively concentrating incident energy at the edges.” The device’s ability to harvest up to 38.1% of the incident green light energy illustrates remarkable wavelength-specific efficiency, addressing key limitations faced by prior transparent solar concentrators.
Proof-of-concept experiments demonstrated the potent energy harvesting capabilities of the system. A modest one-inch diameter prototype readily powered a 10-milliwatt fan using only ambient sunlight, showcasing practical energy output from a minimal footprint installation. Computational modeling further indicates that scaling the window to typical urban dimensions — for example, two meters wide — could achieve a concentration factor of up to 50 times. This enhancement directly correlates to a 75% reduction in the number of costly photovoltaic cells needed to achieve the same power output, drastically lowering systemic costs and supporting the deployment of state-of-the-art PV modules such as gallium arsenide cells renowned for superior power conversion efficiency.
Fabrication of the multilayer cholesteric films employs advanced photoalignment and polymerization techniques, enabling precise molecular orientation control and ensuring reliable optical performance. Remarkably, this manufacturing approach is compatible with roll-to-roll processes, vital for large-scale, cost-effective production. The films exhibit excellent environmental stability, maintaining structural integrity and optical properties under prolonged exposure to sunlight and ambient conditions. This durability facilitates retrofit applications on existing glass façades, supporting sustainable urban upgrades without necessitating full window replacement.
Professor Wei Hu, a senior researcher involved in the project, emphasized the wider implications: “The unidirectional diffractive solar concentrator bridges the gap between renewable energy generation and architectural design. It offers a scalable, practical approach to carbon footprint reduction while enhancing urban energy self-sufficiency. Importantly, the technology integrates invisibly into buildings, avoiding trade-offs between functionality and aesthetics that have historically hindered widespread adoption.”
Beyond architectural glass, the research envisions versatile adaptations of the technology. Future investigations intend to extend broadband efficiency and refine polarization control mechanisms to optimize light management across the solar spectrum. Prospective applications also include agricultural greenhouses where transparent energy harvesting must coexist with plant growth requirements, alongside the development of transparent solar displays and dynamic energy-generating windows for consumer electronics. Such innovations promise to transform passive glass surfaces into active contributors to energy grids globally.
The fundamental mechanism harnessed by this technology relies upon the helical molecular arrangement within cholesteric liquid crystals, which selectively reflects and diffracts circularly polarized light. This selective diffraction differs from conventional scattering, as it preserves the directionality and spectral composition of transmitted light. Harnessing these subtleties at the micro/nanoscale allows the concentrator to maintain high transmittance and excellent color rendering while effectively channeling light. The simultaneous achievement of these optical parameters represents a significant leap beyond previous luminescent or scattering-based concentrators that often compromised visual quality or efficiency.
Scalability and commercial viability are underscored by the ability to fabricate the CLC films using industrially relevant roll-to-roll manufacturing, which can drastically reduce costs and accelerate market penetration. The robustness of polymerized films further assures that these coatings can endure the rigorous environmental stresses inherent to building exteriors. Moreover, because the concentrator is designed as a coating, it offers a non-intrusive integration route that can be retrofitted to existing windows, offering a rapid upgrade path for urban environments seeking to decarbonize energy use and enhance sustainability.
The researchers also highlight the system’s compatibility with high-performance photovoltaic cells installed along window edges, where guided light converges. This architecture not only enhances the overall solar conversion efficiency but also allows for the use of cutting-edge PV materials. Particularly remarkable is the potential synergy with gallium arsenide cells, which exhibit power conversion efficiencies exceeding those of conventional silicon counterparts in the visible spectrum, thereby maximizing energy yield from the concentrated sunlight.
This advance has implications far beyond simple energy harvesting. By embedding energy generation directly into window surfaces, buildings can transition from passive consumers to distributed energy producers, contributing to decentralized grid architectures and improving resilience. The aesthetic invisibility of the coating ensures occupant comfort and building heritage preservation, critical factors for urban deployment. As regulatory and market pressures mount toward carbon neutrality, technologies such as the CUSC offer a viable pathway to integrate renewable resources organically within the urban fabric.
The vision set forth by the Nanjing University team is ambitious yet pragmatic: to transform typical glass panes worldwide into multifunctional interfaces that seamlessly combine transparency, high-fidelity lighting, and efficient solar energy harvesting. If realized at scale, such innovations could dramatically accelerate global decarbonization efforts by utilizing existing building infrastructure, hence leveraging untapped solar potential ubiquitously.
In conclusion, the introduction of the colorless and unidirectional diffractive-type solar concentrator represents a significant leap forward in the field of transparent photovoltaics and smart building materials. Its novel use of cholesteric liquid crystal multilayers to selectively guide circularly polarized light to photovoltaic cells marks a departure from conventional solar concentration approaches. With demonstrated experimental efficacy, scalable manufacturing strategies, and compatibility with future high-efficiency PV technologies, this innovation delivers a compelling solution that marries form and function in sustainable urban development. As research advances toward broader spectral coverage and diversified applications, the prospect of windows as active energy sites draws nearer, promising a future where clean electricity generation is embedded transparently within the very structures we inhabit.
Subject of Research: Not applicable
Article Title: Colorless and Unidirectional Diffractive-type Solar Concentrators Compatible with Existing Windows
News Publication Date: 28-Jul-2025
Web References: http://dx.doi.org/10.1186/s43074-025-00178-3
Image Credits: Center for Liquid Crystal and Photonics/ Nanjing University
Keywords
solar concentrator, cholesteric liquid crystal, transparent photovoltaics, unidirectional diffraction, solar energy harvesting, colorless solar window, architectural photovoltaics, waveguiding, circularly polarized light, sustainable buildings, roll-to-roll manufacturing, energy-efficient glazing
