Longi introduces heterojunction BC solar cells with 27.3% efficiency – pv magazine International

18 November 2024
Emiliano Bellini

Chinese solar module manufacturer Longi developed a heterojunction back contact (BC) solar cell using a laser-enhanced contact optimization process that is reported to have approximately one-third total effective processing time compared to mainstream technologies such as PERC and TOPCon.

The design of the device was “presented in the scientific article”Silicon heterojunction back-contact solar cells via laser patterningrecently published nature. “This cell can be used in all silicon-based PV application scenarios,” said Chaowei Xue, Longi Solar Department Director. pv magazineThe device relies on dense passivating contacts that contain less hydrogen compared to common contacts used in BC cells, he said, adding that this reduces parasitic light absorption and improves passivation.

The company’s researchers explained that laser patterning is currently the cheapest technique to create BC cells, emphasizing that the process has so far produced devices with efficiencies not exceeding 22.5%. Moreover, this technique may lead to laser beam-induced damage in cells as it causes degradation of the amorphous passivating contact or crystalline silicon interface and may reduce the open-circuit voltage and filling factor.

To solve this problem, they used three laser patterning steps known as: P1, P2 and P3 to form the intertwined n/p polarity by selectively removing the N-contact, P-contact, and indium tin oxide (ITO) layers, respectively. Steps P1 and P3 aim to isolate the back contact layers of neighboring cells, and step P2 creates an electrical path between the back contact of one cell and the front contact of an adjacent cell. The P3 phase in particular is often the source of undesirable effects such as separation of back contact layers, flaking or poor electrical insulation due to debris remaining in the trench.

“The top hat-shaped laser beam was operated in two modes,” the research group explained. “The overlapping mode used in P1 and P3 completely removed a layer, while the single-shot mode used in P2 preserved the region between successive shots. The single-shot mode created partial rather than full area contact between the amorphous silicon hydrogenated film and the ITO layer.”

The research team constructed the 243.0 cm2 cell with M6 phosphorus-doped n-type Czochralski monocrystalline silicon wafer. Deposited through dense passivating contacts plasma assisted chemical vapor deposition (PECVD) at 240°C. A pulsed green picosecond laser with a 250 micron spot was used to etch the films.

Tested under standard lighting conditions, the champion cell designed by the team achieved a power conversion efficiency of 27.3%.

Additionally, academics produced a device with less indium content and 26.5% efficiency. “We also demonstrated 26.2% efficiency for HBC solar cells metallized with screen-printed low-temperature copper (Cu) paste,” they said, adding that the proposed fabrication technique aims to separate the use of rare indium and precious silver from the structure of the cell. Heterojunction technology, where PERC and TOPCon have scalability limitations due to their reliance on silver contacts.

Longi did not specify whether these results were confirmed by an independent third-party organization.

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