DAS Solar and Hebei University have identified two types of holes in TOPCon solar cells: harmful recombination holes, which lack oxygen and cause carrier recombination, and beneficial passivating holes, which trap oxygen to enable efficient tunneling while maintaining interface passivation. Their work shows that optimizing oxide layer formation, back surface polishing, and polycrystalline silicon deposition can increase hole passivation, thereby increasing device efficiency and guiding industrial cell design.
Researchers from Chinese module manufacturer DAS Solar, Hebei University and the German Forschungszentrum Jülich GmbH have discovered that there are two different types of pinholes at the interface of TOPCon solar cells, namely recombination pinholes and passivating pinholes.
The former refers to the conventionally recognized type reported in the literature, where direct contact between polycrystalline silicon and crystalline silicon gives rise to a large number of dangling bond defects, while the latter is a newly discovered category, in which sufficient oxygen is retained at the polycrystalline silicon-crystalline silicon (poly-Si/c-Si) interface to passivate dangling bond defects.
“This unique microstructure is absent in silicon heterojunction (HJT) or passivated emitter and back contact (PERC) solar cells, indicating that TOPCon solar cells are capable of achieving higher performance, consistent with theoretical predictions,” said the study’s lead author, Dengyuan Song, narrated pv magazine.
Unlike recombinational pinholes, which suffer from severe carrier recombination due to a high density of dangling bonds, passivating pinholes retain enough oxygen to effectively passivate these bonds, while still allowing efficient carrier tunneling. “This implies that holes do contribute to carrier transport, but they are not necessarily detrimental to passivation,” Song continued. “The key lies not in the pinholes themselves, but in whether they are passivated. This conclusion provides a clear direction for the subsequent efficiency improvement of TOPCon cells and further increases the technical application value of the passivating pinhole theory.”
In the newspaper “Passivating pinholes for large-area, high-efficiency silicon solar cells with tunnel oxide passivated contact”, published in communication about naturethe researchers explained that in the industrial manufacturing of TOPCon solar cells, alkaline polishing on the back side often produces uneven surfaces, leading to non-uniform thickness of the oxide layer.
This can result in three scenarios: a thick oxide layer greater than 1.7 nm provides excellent defect passivation but limits carrier tunneling; a thin oxide layer of less than 1.3 nm causes insufficient oxygen passivation, causing harmful recombination holes; and a thin interlayer can trap oxygen at lattice contacts, creating useful passivating holes.
The first two scenarios have been extensively studied using high-resolution transmission electron microscopy (HR-TEM), as well as etch and electron beam-induced current (EBIC) measurements. HR-TEM has revealed oxide thicknesses ranging from 1.0 to 2.2 nm and sub-nanometer features called nanopites, although true holes remain challenging to identify. The third scenario, in which holes are passivated, had not yet been observed in crystalline silicon photovoltaic solar cells before this new research.
For their experiments, the scientists used a high-resolution spherical aberration-corrected transmission electron microscope (AC-TEM) to perform atomically precise observations of the silicon oxide (SiOₓ)/PolySi interface, and obtained clear physical evidence of the two pinhole types.
Using an optimized low-pressure chemical vapor deposition (LPCVD) oxidation process, combined with custom back-polishing and poly-Si deposition techniques, the research team built 333.3 cm² TOPCon solar cells on 182 mm x 183.75 mm quasi-square silicon wafers.
The front of the solar cell features an industry-standard selective emitter (SE) structure consisting of boron-diffused and laser-doped regions, an aluminum oxide passivation layer, and an anti-reflective silicon nitride (SiNₓ) coating. This configuration produces an excellent junction doping profile, achieving a contact resistance as low as 1 mΩ·cm² and an emitter carrier recombination parameter lower than 5 fA/cm².
On the backside, a polycrystalline silicon compound is formed by embedding an ultra-thin silicon oxide (SiOx) insulating layer between the crystalline silicon wafer and the heavily doped poly-Si layer. Pinholes in the SiOx layer are classified as oxygen-poor or oxygen-rich, which correspond to recombination and passivating pinholes, respectively.
The microstructure of holes is determined by thermal oxidation during LPCVD polycrystalline silicon deposition. The oxygen content in cavities can be controlled via oxidation temperature, duration and atmosphere. A two-step oxidation method was used: an initial oxygen-rich oxidation to form a thin SiO₂ layer, followed by a low-oxygen treatment.
The academics identified areas of high contrast as pinhole locations. Mapping STEM electron energy loss spectroscopy (EELS) showed that highly efficient holes in devices trapped sufficient oxygen at the poly-Si/c-Si interface, creating passive holes with smaller, oxygen-poor valleys. In contrast, low-efficiency devices lacked oxygen in the holes, creating larger, oxygen-poor valleys and conventional recombination holes. STEM energy dispersive spectroscopy (EDS) cross-sectional analysis confirmed these findings.
Tested under standard lighting conditions, the champion cell with passivating holes was able to achieve an energy conversion efficiency of 25.40% and an open-circuit voltage of 38.7 mV.
“To increase the efficiency of the TOPCon cells, industrial optimization should focus on back surface polishing, oxide layer control, and polycrystalline layer processing to increase pinhole passivation, balance interface passivation with carrier tunneling, and achieve higher open-circuit voltage and fill factor,” Song concluded. “Future work could investigate the controlled formation of passivating holes via optimized oxidation and annealing, and apply these insights to TOPCon-based tandem cells, including TOPCon-BC and perovskite/TOPCon architectures.”
The same research team unveiled a new method in February to identify hotspots in TOPCon back-contact solar panels. Previously, in October 2025, it developed a silicon solar cell with a new hole transport layer (HTL), designed to simplify production and reduce costs.
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