Researchers at the Italian National Research Council (CNR) have developed a four-terminal (4T) photovoltaic tile that uses spectral splitting to direct different portions of the solar spectrum to the cells best suited to convert them. The system directs visible light to a gallium arsenide (GaAs) cell and infrared radiation to a bifacial silicon cell.
“The innovation of the patented new design relies on the use of an optical core in the shape of a wedged right-angled glass prism,” lead author Floriana Morabito told pv magazine. “Overall, the optical core acts as a light guide through a combination of total internal reflection (TIR) at the top surface and dichroic reflections at the mirrors.”
“The optical design is such that the rear side of the bifacial narrow-bandgap (NBG) silicon cell faces south, boosting the collection of albedo irradiation,” added corresponding author Silvia Maria Pietralunga. “At the same time, the 90-degree orientation of the wide-bandgap (WBG) GaAs cell minimizes land use and module self-shading.”
Described in the study “Experimental validation and numerical assessment of 4 T spectral-splitting photovoltaic tile with unconventional bifacial capabilities and minimized shading,” published in Solar Energy, the dual-junction tile was built around a right-angled wedge prism made of Schott N-BK7 glass. It incorporates a 2.0 cm × 2.0 cm GaAs cell with 20% efficiency and a 6.8 cm × 2.0 cm bifacial silicon heterojunction (HJT) cell with approximately 24% efficiency and 90% bifaciality. The bottom and back surfaces of the prism were coated with anti-reflective material to reduce optical losses.
The researchers achieved spectral splitting with complementary long-pass and short-pass dichroic mirrors designed for a 45-degree angle of incidence and a cutoff wavelength of 805 nm. The selected wavelength corresponds to the crossing point of the external quantum efficiency curves of the silicon and GaAs cells, allowing the system to direct the appropriate portion of the spectrum to each cell.
The researchers also designed a custom two-part holder, fabricated using resin 3D printing, to integrate the prism, mirrors, silicon cell and packaged GaAs cell into a single structure. They said the assembled prototype differs from the ideal optical design because of unavoidable air gaps between the prism, mirrors and solar cells, resulting in additional optical losses.
The team characterized the 4T PV tile outdoors at the CNR-IMM laboratories in Catania, Italy, on a ceramic-floored terrace. The measurements broadly followed IEC 60904-2 principles, although the team said full compliance was not possible because the novel 4T design differs from conventional PV modules.
Tests were conducted on four sunny days between late May and mid-July 2025, from 9 a.m. to 5 p.m. Three pyranometers measured global horizontal irradiance, tilted front-side irradiance and reflected irradiance reaching the rear side at five-second intervals. Two spectroradiometers also measured global horizontal and direct normal spectral irradiance across the 300 nm to 1,100 nm wavelength range.
The researchers compared the performance of the prototype with simulations of an ideal device operating under optimal conditions. The ideal design maintained an optical power ratio of more than 90% across a broad range of incidence angles, while the fabricated prototype reached a maximum of approximately 62%.
Outdoor measurements, however, showed relatively stable normalized current from the silicon cell, with values peaking around solar noon and improving slightly as the tile was positioned closer to the ground. The GaAs cell was essentially unaffected by clearance height but showed greater variation throughout the day because of its more restrictive angular acceptance, with output also peaking around noon.
“A clear increase of around 7% in measured photocurrent is obtained in outdoor conditions for a bifacial Si cell with a 23% efficiency, in the morning and late afternoon around the summer solstice at 37°30’ N,” Morabito said. “The maximum photocurrent was constant with the distance from ground, and the best increase in bifacial operation was obtained at a clearance height of 19 cm.”
The results indicate that the 4T architecture can reduce self-shading and improve the use of ground-reflected radiation, with bifacial operation benefiting particularly from relatively low installation heights.
“The 4T solution presented here could prove economically sustainable from a global perspective, as it provides increased efficiency per unit area while requiring low-impact mechanical supports,” Pietralunga said. “The prospects for overcoming the current challenges are promising, as is the potential for further improvements, which could open up viable market opportunities in the near future.”
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