A research group led by Chinese polysilicon and solar module manufacturer Tongwei has developed a tunnel oxide passivated contact (TOPCon) solar cell featuring an emitter with higher sheet resistance in a bid to reduce recombination losses, improve surface passivation and increase power conversion efficiency.
“In our work, a non-destructive photoresist-based etching process was used to form a selective emitter structure for TOPCon solar cells,” corresponding author Xiajie Meng told pv magazine. “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. This successful implementation confirms that TOPCon remains highly competitive for commercial applications.”
The researchers explained that conventional TOPCon cells face a trade-off between low contact resistance and reduced carrier recombination in the boron-doped emitter. The proposed architecture addresses this limitation by combining heavily boron-doped regions beneath the metal contacts with lightly doped regions between the contacts.
The scientists used a range of characterization techniques to investigate the structural, electrical and passivation properties of the proposed cell architecture, with the aim of identifying optimal design conditions that balance surface passivation, lateral charge transport and metal shading losses.
They used scanning electron microscopy and 3D laser microscopy to examine etching depth and surface morphology, while four-point probe measurements determined the sheet resistance of the lightly and heavily boron-doped emitter regions. They also evaluated contact resistivity using the transmission line method and characterized doping profiles and junction depths through electrochemical capacitance-voltage measurements.
The team assessed minority carrier lifetime and emitter recombination using quasi-steady-state photoconductance measurements. Electron-beam-induced current measurements provided qualitative evidence of junction continuity, while photoluminescence imaging was used to evaluate passivation quality under different illumination levels.
Current-voltage measurements were used to determine key cell parameters, including open-circuit voltage, short-circuit current density, fill factor, efficiency and series resistance. The researchers also conducted Quokka3 simulations to compare conventional uniform emitters, selective emitters and locally boron-doped finger structures using experimentally derived parameters.
The analysis showed that that high-temperature oxidation reduced surface boron concentrations while increasing junction depth, helping suppress recombination and maintain effective metal contacts.
The scientists also found that increasing the sheet resistance of non-contacted emitter regions from 800 Ω/sq to 2,000 Ω/sq significantly improved minority carrier lifetime and surface passivation, although improvements became marginal beyond 1,600 Ω/sq, while doping uniformity deteriorated, making this value preferable for cell fabrication.
The team then compared three architectures: a conventional uniform emitter, a boron-doped selective emitter and a locally boron-doped finger structure. The selective emitter combined a sheet resistance of 450 Ω/sq beneath the metal contacts with 1,600 Ω/sq in non-contacted regions.
This configuration increased average cell efficiency from 25.82% to 25.98%, representing an absolute improvement of 0.16 percentage points.
Open-circuit voltage increased from 742.6 mV to 745.5 mV, while 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, with the results being confirmed by Germany’s Solar Energy Research Hamelin (ISFH). The locally boron-doped finger structure also improved passivation but suffered greater fill factor losses, limiting its efficiency advantage. Its fill factor reached 83.34%, compared with 83.76% for the selective emitter, partly reflecting increased series resistance and potentially additional recombination.
The Quokka3 simulations examined the effects of contact width, grid pitch, finger width and wafer resistivity on cell performance and indicated that a contacted-region width of approximately 200 µm offered the highest efficiency among the investigated configurations. Reducing metal finger widths to 7 µm to 9 µm could enable a denser grid with a pitch of around 500 µm to 550 µm, potentially improving efficiency further, the scientists said.
“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 stated. “More broadly, the study demonstrates the potential of photoresist-based selective etching to produce emitters with exceptionally high sheet resistance in non-contacted areas without compromising electrical contact quality.”
The new cell concept was described in “High-efficiency TOPCon solar cells featuring super-high sheet resistance emitters via low-cost photoresist etching,” which was recently published in Solar Energy Materials and Solar Cells. The research team included scientists from the School of Optoelectronic Science and Engineering, University of Electronic Science and Technology of China (UESTC) and Chinese industrial equipment company Jiangsu Inspiray Equipment Co., Ltd.
“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,” Meng concluded.
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