Dual Oxide Barrier Lifts TOPCon Cell Efficiency by 0.07 Percentage Points

Researchers at Nanchang Hangkong University in China have developed a dual silicon oxide diffusion barrier strategy to enhance the efficiency of tunnel oxide passivated contact (TOPCon) solar cells. This technique combines post-plasma oxidation with tunnel oxide refinement to control the depth distribution of electrically active phosphorus and the microstructural integrity of the polysilicon (poly-Si) layer.

Wenhao Chen, the study’s corresponding author, explained to pv magazine that conventional industrial TOPCon optimization efforts have primarily focused on adjusting the overall phosphorus concentration or improving tunnel oxide quality, rather than independently managing the location of the electrically active phosphorus peak within the polysilicon. He highlighted that an accumulation of phosphorus at the surface increases parasitic optical absorption without providing additional reductions in contact resistance, creating a conflict between contact and optical losses in mass-produced devices.

Chen explained that the research demonstrates an industry-compatible dual-SiOx diffusion barrier technique based on post-plasma oxidation. Rather than physically redistributing phosphorus atoms, the method utilizes inward-diffusing oxygen to create depth-selective dopant deactivation through the formation of electrically inactive P-O complexes. Extending the post-plasma oxidation phase to 100 seconds shifts the peak concentration of activated phosphorus from the poly-Si surface to a depth of approximately 50 nm, a position that aligns closely with the silver paste burn-off depth encountered during screen-printing and co-firing processes.

According to Chen, integrating this approach with a densified bottom tunnel oxide barrier enables the dual-oxide confinement structure to maintain high dopant activation levels while ensuring strong passivation quality. Industrial batch trials involving nearly 1,000 TOPCon cells with dimensions of 210 x 182.3 mm2 yielded an absolute efficiency improvement of 0.07 percentage points. This performance gain is primarily attributed to reduced contact resistivity and suppressed parasitic absorption, accompanied by only minor degradation in open-circuit voltage. Chen noted that this simple process modification can be implemented on existing PECVD production lines to further enhance the efficiency of commercial TOPCon solar cells.

Cell Fabrication and Testing

Researchers developed TOPCon solar cells using n-type monocrystalline silicon wafers with resistivities ranging from 1 to 3 ohm-centimeters. The manufacturing process incorporated standard alkaline texturing, boron diffusion, rear-side TOPCon fabrication, passivation, screen printing, high-temperature co-firing, and laser-enhanced contact optimization. A bilayer phosphorus-doped polysilicon structure, featuring silicon oxide diffusion barriers, was deposited using plasma-enhanced chemical vapor deposition (PECVD) at approximately 435 C.

The study evaluated five post-plasma oxidation durations: 0, 66, 85, 100, and 120 seconds. The 66-second duration aligns with conventional industrial practices, while the 100-second duration represents the optimized condition. Following oxidation, all samples underwent annealing at 910 C to crystallize the silicon layers and establish the passivating contacts. The resulting polysilicon layer achieved a total thickness of approximately 100 nm.

Photoconductance measurements assessed the passivation quality, while the transmission line method quantified rear contact resistivity. Elemental distributions and electrically active phosphorus concentrations were probed using secondary ion mass spectrometry and electrochemical capacitance-voltage profiling. Additional insights into crystallinity, surface morphology, and chemical bonding were provided by X-ray diffraction, Raman spectroscopy, atomic force microscopy, and X-ray photoelectron spectroscopy. Electrical testing of the completed cells evaluated the performance of the various oxidation treatments.

Results and Limits

The study found that lengthening the oxidation step from the standard 66 seconds to 100 seconds shifted the electrically active phosphorus peak from approximately 20 nanometers to 50 nanometers below the polysilicon surface. Extending the process to 120 seconds, however, led to excessive phosphorus deactivation and a decline in passivation quality. By fine-tuning the bottom tunnel oxide barrier, the researchers maintained the phosphorus peak at around 50 nanometers while improving dopant activation, resulting in an implied open-circuit voltage of 742.4 millivolts.

Characterization of the materials indicated that inward-diffusing oxygen was responsible for deactivating the phosphorus by creating electrically inactive phosphorus-oxygen complexes, rather than physically relocating the phosphorus atoms. The extended oxidation period also improved crystallinity near the surface and slightly increased surface roughness, although the overall crystallinity of the polysilicon remained largely unchanged. Contact resistivity decreased from 1.54 to 1.00 milliohm-square-centimeters, a reduction the researchers noted could not be attributed exclusively to the shift in the phosphorus peak.

Industrial trials conducted on 966 TOPCon solar cells demonstrated an increase in average conversion efficiency from 27.15% to 27.22%, representing a 0.07 percentage point gain. The majority of this improvement was attributed to enhancements in fill factor and short-circuit current, while open-circuit voltage experienced a marginal decline. Quantum efficiency data revealed reduced parasitic absorption within the rear polysilicon contact layer, which contributed to improved performance in the long-wavelength spectrum.

The study’s authors determined that refining the oxidation process offers a viable strategy for enhancing the industrial performance of TOPCon cells. However, they noted that isolating the specific impacts of dopant deactivation, structural modifications, and optical phenomena remains a complex challenge. The research group comprised scientists from ZQi Solar Technology, a Chinese photovoltaic manufacturer.

Interactive table based on the Store Companies dataset for this report.