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The Chinese manufacturer claims the device relies on dense passivating contacts that reduce stray light absorption and improve passivation. It used a streamlined manufacturing technique that is reportedly about a third more efficient than conventional TOPCon technology.
Chinese solar module manufacturer Longi has developed a back-contact (BC) heterojunction solar cell using a laser-enhanced contact optimization process that reportedly has a total effective processing time of about one-third compared to that of conventional technologies such as PERC and TOPCon.
The device design was presented in the scientific paper “ Silicon heterojunction back-contact solar cells by laser patterning ” recently published in nature . “This cell can be used in all silicon-based photovoltaic application scenarios,” Chaowei Xue, department manager at Longi Solar, told pv magazine , noting that the device relies on dense passivating contacts that contain less hydrogen compared to common contacts used in BC cells, which he says reduces stray light absorption and improves passivation.
The companys researchers explained that laser patterning is currently the cheapest technique for constructing BC cells and stressed that this process has, however, so far produced devices with an efficiency of no more than 22.5%. In addition, this technique can lead to laser-induced cell damage and reduced open circuit voltage and fill factor by causing degradation at the amorphous passivating contact or at the crystalline silicon interface.
To address this issue, they used three laser stamping steps known as P1, P2, and P3 to create interdigitated n/p polarity by selectively removing the N contact, P contact, and indium tin oxide (ITO) layers, respectively. Steps P1 and P3 are intended to isolate the back contact layers from neighboring cells, and step P2 creates an electrical path between the back contact of one cell and the front contact of an adjacent cell. Step P3, in particular, is often the source of undesired effects such as back contact delamination, peeling, or poor electrical insulation due to residues left in the trench.
“The hat-shaped laser beam was used in two modes,” the research group explains. “The overlapping mode, used in P1 and P3, completely removed one layer, while the single-shot mode used in P2 preserved the region between consecutive shots. The single-shot mode created a partial, rather than a complete, contact zone between the hydrogenated amorphous silicon film and the ITO layer.”
The research team constructed the 243.0 cm2 cell using an M6 phosphor-doped n-type Czochralski monocrystalline silicon wafer. They deposited the dense passivating contacts using plasma-enhanced chemical vapor deposition (PECVD) at 240 C. A picosecond pulsed green laser with a 250 µm spot was used to ablate the films.
Tested under standard lighting conditions, the teams champion cell achieved a power conversion efficiency of 27.3%.
Furthermore, the academics produced a device with lower indium content that achieved 26.5% efficiency. “We also demonstrated 26.2% efficiency for metallized HBC solar cells using low-temperature screen-printed copper (Cu) paste,” they noted, adding that the proposed fabrication technique aims to decouple the use of rare indium and precious silver from the cell’s heterojunction technology, as PERC and TOPCon have scalability limitations due to their reliance on silver contacts.
Longi has not specified whether these results have been confirmed by an independent third party. |
