China: Tongwei Unveils TOPCon Solar Cell with 26.03% Efficiency and High Sheet Resistance

A research team led by Tongwei, a Chinese manufacturer of polysilicon and solar modules, has developed a TOPCon (Tunnel Oxide Passivated Contact) solar cell featuring an emitter with higher resistance. The design aims to reduce recombination losses, enhance surface passivation, and boost energy conversion efficiency.

“We employed a non-destructive etching process based on photoresist to create a selective emitter structure for TOPCon solar cells,” said corresponding author Xiajie Meng in an interview with pv magazine. “The structure achieves a sheet resistance of 1,600 Ω/sq, which, to our knowledge, is the highest value reported to date for TOPCon technology. The successful implementation confirms that TOPCon remains highly competitive for commercial applications.”

Researchers noted that standard TOPCon cells face a trade-off between achieving low contact resistance and minimizing carrier recombination within the boron-doped emitter. The proposed architecture overcomes this limitation by integrating heavily boron-doped regions beneath the metal contacts with lightly doped areas situated between them.

The team employed various characterization techniques to examine the structural, electrical, and passivation properties of the suggested cell design. Their aim was to pinpoint optimal design parameters that balance surface passivation, lateral charge transport, and losses resulting from metal shading.

Scanning electron microscopy and 3D laser microscopy were employed to inspect etch depth and surface morphology, while four-point probe measurements quantified the sheet resistance of the boron-doped emitter regions, both lightly and heavily doped. The team further assessed contact resistivity using the transmission line method and characterized doping profiles and junction depths via electrochemical capacitance-voltage measurements.

Minority carrier lifetime and emitter recombination were evaluated through quasi-steady-state photoconductance measurements. Beam-induced current measurements provided qualitative evidence of junction continuity, while photoluminescence imaging was utilized to assess passivation quality under varying illumination levels.

Current-voltage measurements were employed to establish the cell's critical performance metrics, including open-circuit voltage, short-circuit current density, fill factor, efficiency, and series resistance. The research team also utilized Quokka3 simulations to evaluate conventional uniform emitters, selective emitters, and locally boron-doped "finger" structures, incorporating parameters derived from experimental data.

The analysis revealed that high-temperature oxidation reduced surface boron concentrations while simultaneously increasing junction depth. This process helped suppress recombination and preserved effective metallic contacts.

The scientists further observed that raising the sheet resistance of unconnected emitter regions from 800 Ω/sq to 2,000 Ω/sq significantly enhanced minority carrier lifetime and surface passivation. However, improvements became marginal beyond 1,600 Ω/sq, as doping uniformity deteriorated. Consequently, this threshold was identified as the optimal value for cell fabrication.

The team evaluated three distinct architectures: a conventional uniform emitter, a boron-doped selective emitter, and a locally boron-doped “finger” structure. The selective emitter configuration exhibited a sheet resistance of 450 Ω/sq beneath the metallic contacts and 1,600 Ω/sq in the contact-free regions.

This design increased the average cell efficiency from 25.82% to 25.98%, representing an absolute gain of 0.16 percentage points. The open-circuit voltage rose from 742.6 mV to 745.5 mV, while the fill factor improved from 83.54% to 83.76%.

The optimized selective emitter cell also achieved a certified efficiency of 26.03%, with an open-circuit voltage of 746.5 mV. These results were verified by the Institute for Solar Research in Hamelin (ISFH). While the locally doped boron finger structure enhanced passivation, it incurred higher fill factor losses, which constrained its overall efficiency advantage. The cell recorded a fill factor of 83.34%, compared to 83.76% for the selective emitter design. This discrepancy partly reflects increased series resistance and potential additional recombination effects.

Quokka3 simulations evaluated how contact width, grid pitch, finger width, and wafer resistivity influence cell performance, identifying that a contact region width of approximately 200 µm yielded the highest efficiency among the configurations tested. The researchers noted that increasing the width of the metal fingers from 7 µm to 9 µm could enable a denser grid with a pitch ranging from roughly 500 µm to 550 µm, potentially offering further efficiency gains.

“For the locally boron-doped ‘finger’ architecture, simulations suggest that employing silicon wafers with higher bulk resistivity, combined with closer-spaced front contacts, could present opportunities for additional performance enhancements, although experimental validation remains necessary,” Meng stated. “More broadly, the study demonstrates the potential of photoresist-based selective etching to produce emitters with exceptionally high sheet resistance in non-contact areas, without compromising electrical contact quality.”

The new cell concept was detailed in a paper titled “High-efficiency TOPCon solar cells with extremely high sheet resistance emitters achieved by low-cost photoresist etching,” recently published in Solar Energy Materials and Solar Cells. The research 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 manufacturer of industrial equipment.

“The results could help manufacturers refine emitter doping profiles and contact design, paving the way for further efficiency gains and the potential industrial adoption of the technology,” Meng concluded.