Researchers in Sweden developed a bifacial chalcoprite solar cell using a titanium-doped indium oxide (ITiO) back contact, achieving 68% bifaciality. The device is able to maintain strong front-side performance with around 15% efficiency, 595 mV open-circuit voltage, and 75% fill factor.”
Image: Jan Keller, Uppsala University
Researchers at Sweden’s University of Uppsala have fabricated a bifacial chalcoprite (ACIGS) solar cell that can reportedly achieve a power conversion efficiency of 15.1% and a bifaciality factor of 68%.
The main technical feature of the cell is the use of an titanium-doped indium oxide (ITiO) back contact, specifically designed to mitigate optical losses that are typically associated with conventional, highly doped transparent back contact (TBC) materials. These traditional materials tend to absorb a significant portion of the incoming near-infrared (NIR) light, thereby reducing the overall efficiency of the device.
In contrast, ITiO offers high electrical conductivity combined with superior transparency in the NIR region, enabling more effective light transmission to the active layers of the cell while still maintaining efficient charge collection. “This is not the first attempt to explore ITiO as a back contact for chalcopyrite solar cells,” the research’s lead author, Jan Keller, told pv magazine.
ITiO was preferred over simple indium tin oxide (ITO) as a back contact because its higher thermal and chemical stability during ACIGS deposition, combined with slightly better near-infrared transparency, enables improved rear-side light harvesting and overall solar cell efficiency.
The cell was built with a soda-lime glass (SLG) substrate, the ITiO-based TBC, a sodium fluoride (NaF) precursor layer, a 1 µm-thick ACIGS absorber, a cadmium sulfide (CdS) buffer layer, and a zinc oxide (ZnO) window layer stack.
Through hall-effect measurements, the scientists found that ITiO exhibits a significantly higher mobility and lower carrier density than conventional ITO, with the TBC maintaining its electrical properties after high-temperature ACIGS deposition. Optical characterization also demonstrated that ITiO has lower free-carrier absorption than ITO, shifting parasitic losses far into the infrared and resulting in only 5% absorption at the ACIGS band gap of 1.1 eV, compared with 25% for ITO.
As a result, rear-illumination short-circuit current density is significantly higher for ITiO (22.7 mA/cm²) than for ITO (19.7 mA/cm²), while the open-circuit voltage remains similar at around 595 mV. Under front illumination, the best cells show comparable efficiencies of 15.1% for ITiO and 15.4% for ITO, demonstrating that the new TBC supports excellent device performance. Rear-side illumination efficiency, however, benefits markedly from ITiO, reaching 10.2% versus 8.8% for ITO, primarily due to reduced optical losses and lower free-carrier absorption (FCA).
Internal quantum efficiency (IQE) analysis confirmed that the remaining losses originate mainly from recombination and incomplete absorption in the ACIGS absorber rather than the TBC.
“Our work demonstrates that with ITiO can replace opaque molybdenum (Mo) as a transparent back contact in bifacial ACIGS cells,” Keller said. “The new primary challenge now lies in minimizing electrical losses at the back-contact interface, which become the critical factor limiting performance. Future improvements should also focus on further enhancing rear-side carrier collection and reducing parasitic absorption in front contacts.”
The device was presented in “Titanium-Doped In₂O₃: A High-Mobility, Thermally Stable Back Contact for Bifacial Chalcopyrite Solar Cells,” published in RRL Solar. The research team also included academics from the University of Loughborough in the United Kingdom.
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