Researchers from Chinese module manufacturer DAS Solar, Hebei University and Germany's Forschungszentrum Jülich GmbH have discovered that there are two distinct types of pinholes at the interface of TOPCon solar cells, namely recombinational 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 rear contact (PERC) solar cells, indicating that TOPCon solar cells are capable of achieving higher performance, which aligns with theoretical predictions,” the research's lead author, Dengyuan Song, told pv magazine.
Unlike recombinational pinholes, which suffer from severe carrier recombination due to a high density of dangling bonds, passivating pinholes retain sufficient oxygen to effectively passivate these bonds while still allowing efficient carrier tunneling. “This implies that pinholes do contribute to carrier transport, yet they are not necessarily detrimental to passivation,” Song wento on to say. “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 enhances the engineering application value of the passivating pinhole theory.”
In the paper “Passivating pinholes for large-area and high-efficiency silicon solar cells with tunnel oxide passivated contact,” published in nature communications, the researchers explained that, in the industrial fabrication of TOPCon solar cells, rear-side alkaline polishing often produces uneven surfaces, leading to non-uniform oxide layer thickness.
This can result in three scenarios: a thick oxide layer of over 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, forming harmful recombinational pinholes; and an intermediate thin layer may retain oxygen at lattice contacts, creating beneficial passivating pinholes.
The first two scenarios have been extensively studied using high-resolution transmission electron microscopy (HR-TEM) as well as etching 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, though true pinholes remain challenging to identify. The third scenario, involving passivating pinholes, had not been observed in crystalline silicon photovoltaics prior to this new research.
For their experiments, the scientists used a high-resolution spherical aberration-corrected transmission electron microscope (AC-TEM) to conduct atomically precise observations of the silicon-oxide (SiOₓ)/PolySi interface, and obtained clear physical evidence of the two pinhole types.
Using an optimized oxidation process in low-pressure chemical vapor deposition (LPCVD), combined with tailored rear-side polishing and poly-Si deposition techniques, the research team built 333.3 cm² TOPCon solar cells on quasi-square silicon wafers measuring 182 mm × 183.75 mm.
The front side of the solar cell features an industry-standard selective emitter (SE) structure, composed of boron-diffused and laser-doped regions, an aluminum oxide passivation layer, and a silicon nitride (SiNₓ) anti-reflection coating. This configuration produces an excellent junction doping profile, achieving a contact resistivity as low as 1 mΩ·cm² and an emitter carrier recombination parameter below 5 fA/cm².
On the rear, a polycrystalline silicon junction 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-depleted or oxygen-rich, corresponding to recombinational and passivating pinholes, respectively.
The microstructure of pinholes is determined by thermal oxidation during LPCVD polycrystalline silicon deposition. Oxygen content in pinholes can be controlled through oxidation temperature, duration, and atmosphere. A two-step oxidation method was adopted: an initial oxygen-rich oxidation to form a thin SiO₂ layer, followed by oxygen-deficient treatment.
The academics identified high-contrast regions as pinhole locations. STEM-electron energy loss spectroscopy (EELS) mapping showed that high-efficiency device pinholes maintained sufficient oxygen at the poly-Si/c-Si interface, forming passivating pinholes with smaller oxygen-depleted valleys. In contrast, low-efficiency devices lacked oxygen in the pinholes, producing larger oxygen-depleted valleys and conventional recombinational pinholes. STEM-energy dispersive spectroscopy (EDS) cross-sectional analysis confirmed these findings.
Tested under standard illumination conditions, the champion cell with passivating pinholes was able to achieve a power conversion efficiency of 25.40% and an open-circuit voltage of 738.7 mV.
“To boost TOPCon cell efficiency, industrial optimization should focus on back-surface polishing, oxide layer control, and polycrystalline layer processing to increase passivating pinholes, balancing interface passivation with carrier tunneling, and achieving higher open-circuit voltage and fill factor,” Song concluded. “Future work could explore controlled formation of passivating pinholes 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 in February a new method to identify hot-spots in TOPCon back-contact solar modules. Earlier, in October 2025, it developed a silicon solar cell featuring a novel hole transport layer (HTL) designed to simplify production and reduce costs.
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