CNR tests a prototype bifacial solar tile using spectral splitting

On a ceramic-floored terrace in Catania, Italy, a small solar tile faced four sunny days of testing between late May and mid-July 2025. Researchers from the Italian National Research Council’s CNR-IMM laboratories were looking for a better way to use sunlight that would otherwise be lost or blocked inside a module. Their prototype is a four-terminal photovoltaic tile: a 4T design in which the two solar cells operate through separate electrical connections.

The tile sorts the sunlight before conversion. A wedge-shaped right-angled prism made from Schott N-BK7 glass guides the light, while dichroic mirrors — optical mirrors that reflect selected wavelengths and transmit others — send visible light to a 2.0 cm × 2.0 cm gallium arsenide cell and infrared radiation to a 6.8 cm × 2.0 cm bifacial silicon heterojunction cell. The silicon cell has approximately 24% efficiency and 90% bifaciality, meaning it can generate electricity from light reaching its rear side.

That orientation is the point of the design. The rear of the narrow-bandgap silicon cell faces south to collect albedo, or ground-reflected, light, while the GaAs cell stands at a 90-degree angle to reduce module self-shading and land use. The researchers set the mirrors’ cutoff wavelength at 805 nm, where the cells’ external quantum-efficiency curves cross, so each cell receives the part of the spectrum it is better suited to convert.

The outdoor measurements found a clear result for the silicon side: measured photocurrent rose by around 7% in the morning and late afternoon around the summer solstice at the test site’s latitude. The best increase in bifacial operation came at a 19 cm clearance height. The silicon output stayed relatively stable, while the GaAs cell varied more during the day because it accepts light over a narrower range of angles.

So what changes in practice? The architecture could help solar installations produce more electricity from a given area while using relatively low-impact supports, particularly where ground-reflected light is available and panels can sit close to the surface. But this remains a prototype, not a commercial module: its maximum optical power ratio was approximately 62%, compared with more than 90% in simulations of the ideal design, and air gaps between the prism, mirrors and cells added losses. The CNR measurements broadly followed IEC 60904-2 principles, though full compliance was not possible for the unconventional 4T format.