U.S. scientists build copper-contacted TOPCon solar cell with 24.3% efficiency

The TOPCon solar cells fabricated with screen-printed, fire-through copper rear contacts and silver front contacts, using laser-enhanced contact optimization (LECO) to significantly reduce contact resistivity. Optimized copper cells achieved 24.3% efficiency, comparable to Ag-contacted cells, with excellent stability.

April 10, 2026 Emiliano Bellini

Image: Georgia Institute of Technology, Solar Energy Materials and Solar Cells, CC BY 4.0

From pv magazine Global

A research team in the United States has fabricated TOPCon solar cells with screen-printed, fire-through copper (Cu) contact on the rear side and a silver (Ag) contacted boron emitter on the front side through the laser-enhanced contact optimization (LECO) process.

The LECO process consists of using a highly intense laser pulse on the front side of the solar cell at a constant reverse voltage of more than 10 V, with the resulting current flow of several amperes considerably reducing the contact resistivity between semiconductor and metal electrode.

“In this work, we demonstrate that LECO treatment is very effective in enabling screen-printed, fire-through Cu contacts to n-TOPCon on the rear side of a TOPCon cell,” the scientists explained. “We used a unique screen-printable, fire-through Cu paste from Bert Thin Films, Inc. (BTF) which helps in inhibiting Cu diffusion by forming a thin Cu-oxide around the Cu particles.”

Bert Thin Films launched its new copper paste in February, as reported by pv magazine. According to the manufacturer, the new paste can be screen printed and fired in air, and can also be co-fired with commercial silver pastes for frontside metallization.

The cells were fabricated using standard 242.32 cm² n-type wafers that underwent saw damage etching, surface texturing, and cleaning. TOPCon precursors were formed with a boron-diffused emitter passivated by an aluminum oxide/silicon nitride (Al₂O₃/SiNx) layer on the front side, and a full-area TOPCon stack on the rear. Ag paste was screen-printed on the front with 135 gridlines and fired at 700 C. Cu paste was then applied to the rear and fired at a lower temperature of 500–600 C to prevent copper migration.

“All the tools and processes we used in this study are already in use in the PV industry,” corresponding author Young Woo Ok told pv magazine. “It only requires replacing the Ag paste with the Cu paste. The process can be a plug-and-play alternative to Ag contacts in production.”

The LECO treatment was performed with varying reverse bias voltages to improve cell performance. Fully silver-contacted cells were fabricated as references. Electrical properties, including metal-induced recombination current density and contact resistivity, were characterized using the transfer length method (TLM) and electroluminescence (EL) imaging.

Schematic cross-sectional Cu contacted n-TOPCon cell structures and process sequences.Image: Georgia Institute of Technology, Solar Energy Materials and Solar Cells, CC BY 4.0

The researchers systematically optimized Cu printing and firing parameters, including screen design, firing temperature, belt speed, and LECO settings. Cells fired at 500–550 C were found to achieved stable open-circuit voltage and pseudo-fill factor up to 530 C, with degradation occurring at higher temperatures due to copper diffusion into the tunnel oxide. Moreover, short-circuit current density slightly was found to decrease at higher temperatures, while fill factor peaked around 530–535 C due to reduced series resistance (Rs). EL imaging confirmed improved contact quality with firing at 535 C, eliminating dark areas caused by poor contacts.

The LECO treatment was optimized for cells fired at 530 C, with reverse bias voltages of 17–19 V providing the best balance between series resistant (Rs) and pseudo fill factor. Contact resistivity decreased from around 300 mΩ·cm² to around 10 mΩ·cm² after LECO, indicating enhanced rear Cu contacts, while laser power had minimal effect. Microstructural analysis showed increased Cu colloids and crystallites confined to the poly-Si layer, improving contact properties without degrading pseudo fill factor and open-circuit voltage.

Comparisons with Ag-contacted cells revealed that the Cu-contacted cells achieved comparable open-circuit voltage and pseudo fill factor, with slightly lower short-circuit current and fill factor. Optimized Cu cells reached 24.3% efficiency, only 0.2–0.3% below Ag-contacted cells. Contact resistivity for Cu was higher than Ag, but could be mitigated by increasing rear contact coverage. Stability tests under thermal stress at 200 C in nitrogen also showed negligible changes in open-circuit voltage and pseudo fill factor over 1000 hours, which reportedly demonstrated Cu contact reliability.

“Such high efficiency screen printed Cu contacted n-TOPCon cells provide unique opportunity to replace very expensive Ag contact on n-TOPCon with cheaper screen printable Cu metal pastes,” the scientists emphasized.

The novel cell concept was presented in “>24% screen printed Cu contacted n-TOPCon solar cells with successful implementation of LECO process,” published in Solar Energy Materials and Solar Cells. The research team included academics from the US Department of Energy’s National Laboratory of the Rockies, the Georgia Institute of Technology, and US-based copper paste specialist Bert Thin Films Inc.

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