Semitransparent solar cells with natural looking color and strong heat blocking offer a realistic way for windows to generate electricity and cut energy use in large buildings.
(Nanowerk Spotlight) Cities depend on glass more than most people realize. Windows shape how much sunlight enters a room, how much heat a building absorbs, and how much energy is needed to keep that building comfortable. As urban areas grow taller and denser, these glass surfaces now play an even larger role in determining how much electricity a city uses on any given day. That makes them an unexpected but powerful lever for change. A material that could control light, manage heat, and also generate electricity would turn every window into part of a city’s energy system.
Researchers have explored this idea through many forms of solar technology. Traditional silicon panels deliver strong performance but are opaque and suited mainly for rooftops. Other approaches try to add color or partial transparency, yet they often weaken the electrical output. Some experimental designs transmit light but distort the view or shift color indoors. The gap between what architects want and what solar materials typically offer has slowed progress.
Organic solar cells have attracted interest because their carbon-based molecules can be adjusted the way inks or coatings are adjusted. They can harvest energy from invisible wavelengths while letting visible light pass through. They can fit curved or irregular surfaces and support the look of a building rather than competing with it. But these strengths introduce a familiar problem. Improving appearance often reduces efficiency, and raising efficiency often alters brightness or color in ways that limit architectural use.
Recent advances have begun to shift this balance. New organic molecules support better charge movement, while improved conductive layers help charges travel more cleanly. Computer models can now scan millions of device designs, allowing researchers to search for structures that balance efficiency and appearance instead of forcing a choice.
Building simulation software can also test how these devices affect heating, cooling, and power use across an entire city. These tools raise a new possibility: solar windows designed not only for energy generation but also for the way humans perceive color.
A study published in Advanced Functional Materials (“Human Vision‐Adapted Semitransparent Organic Solar Cells for Multicolored Architectural Application”) presents semitransparent organic solar cells designed around that idea. The work aims to produce controlled color, high efficiency, and strong heat blocking in one device. It then evaluates how these devices could influence energy use across Shanghai.
a) Device structure of organic solar cells (OSCs). b) Chemical structures and electrostatic potential
(ESP) distribution for 2PACz and IMZ. c) Schematic energy-level diagram of the materials used in OSCs. d) J–V characteristics, e) External quantum efficiency (EQE) spectra, and f) Power conversion efficiency (PCE) distribution measured for the control device based on pristine PEDOT:PSS hole transport layer (HTL) and OSCs having hybrid HTLs with different additives. (Image: Reproduced from DOI:10.1002/adfm.202520191 with permission by Wiley-VCH Verlag) (click on image to enlarge)
The researchers begin with a highly efficient opaque organic solar cell. The active layer blends three molecules: one polymer that donates electrons, called PM6, and two small molecules that accept electrons, known as BTP eC9 and L8 BO. Together they absorb a broad range of sunlight and support the separation of charges.
The team then improves a layer called the hole transport layer, which collects positive charges. They use PEDOT:PSS, a mixture of a conductive polymer (PEDOT) and an acidic polymer (PSS). Although common, this material has moderate conductivity and imperfect alignment with the active layer. To enhance it, the researchers add two small molecules. The first, imidazole, interacts with PSS and loosens the network. The second, 2 PACz, creates a more ordered surface that guides charges more effectively.
These changes shift the work function of the hole transport layer, a measure of how easily charges can move, from 4.33 eV to 4.93 eV. Conductivity improves, and tests show cleaner charge extraction and lower charge loss. As a result, the efficiency of the opaque device rises from 19.0 ± 0.2% to 20.0 ± 0.1%. This strong base supports the creation of semitransparent versions.
To transmit visible light, the team replaces the thick silver electrode with a thin gold silver layer. By adjusting the thickness of the active layer, they produce a semitransparent reference device with 13.6 ± 0.1% efficiency and 27.1% average visible transmittance.
Color control comes from an optical cavity placed on top of the device. The cavity uses two thin silver mirrors separated by a transparent spacer. Light bounces between these mirrors, and at certain wavelengths the reflections reinforce each other. This creates a narrow transmission peak that produces a clear and stable color.
The researchers tie this design to human vision. The eye uses three types of cone cells that respond most strongly to short, middle, or long wavelengths. Their peak responses lie near 420 nm, 530 nm, and 560 nm. To describe how well a device matches one of the cone responses, the team introduces the Visual Match Index. It ranges from 0 to 1 and measures how closely the device’s transmission curve aligns with one of these sensitivity curves.
To find strong designs, they use the transfer matrix model, a calculation method that traces how light passes through thin layers. They supply measured refractive indices, which describe how light slows in each material, and test 4,632,264 possible cavity structures. For each they compute transmission, predicted current, infrared blocking, and the Visual Match Index. They select structures that combine high efficiency, strong heat blocking, and close alignment with a cone response.
Three designs emerge. One matches the S cone, one matches the M cone, and one matches the L cone. They use different transparent spacers—such as molybdenum oxide or zinc selenide—to set the transmitted color. These devices produce blue, green, or red tinted light aligned with the cone sensitivities.
Measurements support the simulations. The S matched device reaches 17.3 ± 0.2% efficiency and a Visual Match Index of 0.99. The M matched and L matched devices reach 16.6 ± 0.2% and 16.2 ± 0.2%, with Visual Match Index values of 0.93 and 0.90. All block more than 98.4% of infrared light, reducing heat gain while still transmitting visible light.
To demonstrate scale, the team fabricates a 4 × 4 cm² sample of the S matched design with a patterned logo. They use slot die coating, a method suitable for large area production, and show that the sample can power a small environmental sensor.
The study then connects device performance to citywide energy use. Using EnergyPlus simulation software, climate data for Shanghai, and a model with a 50% window to wall ratio, the researchers assign the device’s optical properties to all window and roof areas. With remote sensing data, they estimate that Shanghai has about 5.86 × 10⁸ m² of building area. After adjusting for shading and performance loss, the reference device could generate about 6.47 × 10¹⁰ kWh of electricity each year. This value is about 33% of the city’s projected 2024 electricity use. The devices would also reduce cooling demand by about 1.10 × 10¹⁰ kWh because of their strong infrared blocking. The color tuned versions perform even better.
This study shows how solar windows can be shaped around human vision and still deliver strong electrical and thermal performance. By merging materials design, optical engineering, and building modeling, the work provides a clear path for colored solar glass that could operate as part of a city’s energy system while still serving as a building material.
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