Tongwei-led team reaches 26.03% efficiency with high-resistance TOPCon emitter

A research group led by Chinese polysilicon and module manufacturer Tongwei has certified a TOPCon solar cell at 26.03% power conversion efficiency, with Germany's Institute for Solar Energy Research Hamelin (ISFH) confirming the result. The cell's defining feature is a selective emitter whose non-contacted regions reach a sheet resistance of 1,600 Ω/sq — a value the team describes as the highest reported for TOPCon to date.

Corresponding author Xiajie Meng framed the result as a statement about the technology's commercial runway. "The structure features a sheet resistance as high as 1,600 Ω/sq – to the best of our knowledge, the highest reported for TOPCon to date," Meng said. "This successful implementation confirms that TOPCon remains highly competitive for commercial applications."

How the cell was built

The work targets a known constraint in tunnel oxide passivated contact cells: the boron-doped emitter forces a trade-off between low contact resistance and reduced carrier recombination. The researchers resolved it by combining heavily boron-doped regions beneath the metal contacts with lightly doped regions between them.

"In our work, a non-destructive photoresist-based etching process was used to form a selective emitter structure for TOPCon solar cells," Meng said. The team included scientists from the School of Optoelectronic Science and Engineering at the University of Electronic Science and Technology of China (UESTC) and Jiangsu Inspiray Equipment Co., Ltd., a Chinese industrial equipment company. The findings appear in Solar Energy Materials and Solar Cells.

What the measurements showed

Characterisation covered the full chain from structure to device performance:

• Scanning electron microscopy and 3D laser microscopy examined etching depth and surface morphology.
• Four-point probe measurements determined sheet resistance in both doped regions; the transmission line method assessed contact resistivity.
• Electrochemical capacitance-voltage profiling mapped doping profiles and junction depths.
• Quasi-steady-state photoconductance, electron-beam-induced current and photoluminescence imaging evaluated lifetime, junction continuity and passivation.
• Current-voltage measurements delivered open-circuit voltage, short-circuit current density, fill factor, efficiency and series resistance, and Quokka3 simulations compared emitter architectures using experimentally derived parameters.

The analysis showed that high-temperature oxidation reduced surface boron concentrations while increasing junction depth, which suppressed recombination without degrading metal contacts. Raising sheet resistance in non-contacted regions from 800 Ω/sq to 2,000 Ω/sq improved minority carrier lifetime and passivation, but gains flattened beyond 1,600 Ω/sq as doping uniformity deteriorated — settling 1,600 Ω/sq as the fabrication sweet spot.

What does this change for cell makers?

The comparison of three architectures quantifies the payoff. The selective emitter, combining 450 Ω/sq beneath contacts with 1,600 Ω/sq elsewhere, raised average efficiency from 25.82% to 25.98% — an absolute gain of 0.16 percentage points. Open-circuit voltage rose from 742.6 mV to 745.5 mV, and fill factor improved from 83.54% to 83.76%.

The optimised cell pushed further, delivering the certified 26.03% efficiency at 746.5 mV. A locally boron-doped finger structure also improved passivation but lost ground on fill factor, reaching only 83.34%, partly due to higher series resistance and possible additional recombination.

Where the next gains sit

The Quokka3 simulations mapped the remaining design space. A contacted-region width of roughly 200 µm offered the highest efficiency among the configurations tested. Narrowing metal fingers to 7 µm to 9 µm would allow a denser grid with a pitch of 500 µm to 550 µm, potentially lifting efficiency further.

"For the locally boron-doped finger architecture, simulations indicate that using silicon wafers with higher bulk resistivity alongside more closely spaced front contacts could offer opportunities for further performance improvements, although experimental validation is still required," Meng said.

The study, she noted, demonstrates that photoresist-based selective etching can produce exceptionally high sheet resistance in non-contacted areas without compromising contact quality. "The findings could help manufacturers refine emitter doping profiles and contact designs, paving the way for further efficiency gains and potential industrial adoption of the technology."

What decides what happens next is transfer from lab to line: whether Tongwei and its equipment partner can carry the photoresist etching route, and the 1,600 Ω/sq emitter it enables, into volume cell production.