A reflection-based solar cell design inspired by periscopes achieves near-complete visible transparency and record light utilization efficiency, outperforming all previous transparent photovoltaic devices.
(Nanowerk Spotlight) Skyscrapers sheathed in glass dominate modern city skylines, their facades absorbing enormous quantities of solar radiation that mostly converts to unwanted heat. What if those vast expanses of glass could harvest that energy instead? This tantalizing prospect has driven research into semi-transparent organic photovoltaics (ST-OPVs), thin and flexible solar cells designed to absorb invisible near-infrared radiation while letting visible wavelengths pass through, preserving natural daylight for building occupants. The technology promises windows that double as power generators, applicable to everything from office towers to automobile sunroofs.
The physics, however, present a stubborn contradiction. Solar cells generate electricity by absorbing light, yet windows must transmit it. Engineers quantify this tradeoff using light utilization efficiency, or LUE, calculated by multiplying power conversion efficiency by average visible light transmittance. A highly efficient cell that blocks too much light scores poorly; a perfectly clear window generating negligible power fails equally.
Theoretical models suggest optimized devices could achieve a maximum LUE of about 20.6% by selectively harvesting ultraviolet and near-infrared photons. Experimental results have lagged far behind this ceiling, with the best laboratory devices struggling to exceed 6%.
The culprit lies in the transmission pathway itself. In conventional designs, light enters one side of the device, passes through multiple layers including glass substrates, transparent electrodes, charge-transport films, and active materials, then exits the other side. Even with materials engineered for visible transparency, each layer absorbs or reflects some photons. These parasitic losses accumulate, bleeding away energy that could otherwise reach viewers’ eyes or convert to electricity.
Researchers have attacked the problem through molecular engineering, electrode optimization, and sophisticated optical coatings, but each advance has yielded diminishing returns. The fundamental constraint remains: visible light must traverse absorbing materials to create the see-through effect.
A study published in Advanced Energy Materials (“Periscope‐Inspired High‐Performance Semitransparent Organic Photovoltaics With Dual Surface‐Reflection Optical Structure”)offers an elegant workaround by reimagining the optical pathway entirely. The research team drew inspiration from an unlikely source: the periscope. That familiar submarine instrument uses two angled mirrors to redirect light around obstacles, letting sailors observe the surface without exposing themselves. The researchers applied analogous logic to photovoltaics, creating what they term a reflection-mode semi-transparent organic photovoltaic.
Reflection ST-OPV design and concept. (a) Schematic diagram of ideal t-ST-OPV and r-ST-OPV windows. (b) Schematic diagrams of t-ST-OPV and r-OPV structures. (c) The chemical formula of the active layer materials (PM6, BTP-eC9, and L8-BO). (Image: Adapted with permission from Wiley-VCH Verlag) (click on image to enlarge)
The design inverts conventional transmission-mode architecture. Instead of passing through the device, incoming sunlight first strikes a specially engineered optical reflector at the surface. This reflector bounces visible wavelengths while transmitting infrared radiation into the underlying solar cell for energy conversion. The reflected visible light then hits a second mirror, positioned like the upper mirror in a periscope, which redirects it outward. The result mimics transparent transmission without visible photons ever penetrating the absorbing layers.
Optical simulations confirmed the advantages. Because visible light never enters the active material or electrodes, parasitic absorption drops substantially. The team fabricated reflective coatings from alternating thin films of tellurium dioxide (refractive index 2.15) and magnesium fluoride (refractive index 1.39). These materials create precise interference effects when stacked in calculated thicknesses. The optimized reflector, designated the A-layer, bounces more than 95% of visible light in the 300 to 655 nm wavelength range while transmitting 96% of near-infrared radiation between 693 and 1200 nm.
The researchers built devices using a multi-component organic blend as the active layer, pairing the polymer donor PM6 with non-fullerene acceptors BTP-eC9 and L8-BO. These materials absorb strongly in the near-infrared region. A baseline opaque device using this blend delivered a power conversion efficiency of 19.51%.
To create the reflection-mode structure, the team integrated the A-layer reflector with high-performance opaque solar cells. Testing at a 45-degree incident angle, the optimal geometry for dual-reflection pathways, produced notable results. The best device achieved 8.27% power conversion efficiency combined with 92.2% average visible transmittance.
Multiplied together, these figures yield a light utilization efficiency of 7.62%, surpassing all previously reported values for semi-transparent photovoltaics. Binary and ternary material variations reached comparable figures of 7.51% and 7.49%, respectively.
The improvement over transmission-mode devices stems from spectral separation at the surface. Earlier transmission-based designs pushed LUE to around 6% through molecular engineering and optical outcoupling structures, but visible photons still had to traverse absorbing layers. Reflection-mode architecture bypasses this limit entirely.
Practical deployment introduces additional considerations. Visible transmittance depends on sunlight angle, performing optimally at 45 degrees but declining as the sun moves overhead. Geometric analyses show transmittance remains high when light enters from below horizontal but decreases as incidence angles rise above 45 degrees. This angular dependence suggests louver-like configurations, where individual panels tilt to track sunlight or maintain viewing sightlines, could optimize both power generation and illumination throughout the day.
Color rendering also matters for architectural applications. The three-layer reflective coating scored 89.1 on the color rendering index, high enough that objects viewed through the window appear natural. Stability testing showed devices retained 80% of initial efficiency after about 50 hours at 85 °C, with operational lifetimes around 150 hours for A-layer-based configurations.
The reflection-mode approach establishes a fundamentally distinct strategy for transparent photovoltaics. It permits engineers to employ opaque, high-efficiency active layers while achieving exceptional visible transmittance, a combination previously unattainable through transmission-based designs. Building facades, vehicle glazing, and portable electronics all represent potential applications.
These devices demonstrate a viable path toward solar windows functioning nearly as clearly as ordinary glass while contributing meaningful power generation. The gap between experimental performance and theoretical limits remains substantial, but this architectural innovation opens new territory for closing it.
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