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In a new paper published in Nature Communications, the Chinese solar cell manufacturer explains that the back-contact heterojunction cell it introduced in late 2023 achieved one of the highest open-circuit voltages ever recorded for this cell technology, as well as lower than usual short-circuit current losses. The results were confirmed by Germany’s Institute for Solar Energy Research (ISFH).
A group of scientists from Chinese solar module manufacturer Longi has described in a new scientific paper the 27.09% efficient heterojunction back-contact (HBC) solar cell, which it unveiled in December 2023.
At the time, Longi simply said the result was made possible by a new laser-graphics process that costs less than conventional, high-cost photolithographic processes, without giving further details. In the new document, the company explained that the laser-patterning technique had been developed by Longi itself. In addition, it not only provided a comprehensive technical description of the cell architecture, but also laid out a strategy for how to improve contact resistivity, series resistance and back-face pattern design.
The group specified that HBC solar cells suffer from carrier recombination losses, saying that these arise from the hole-selective contact region and polarity boundaries. “We propose solutions to these problems and establish a clear relationship between contact resistivity, series resistance, and back-face pattern design,” they added.
The research group built the 243.0 cm2 cell using a 274.3 cm2, 200 µm-thick Czochralski Longi M6 wafer based on n-type monocrystalline silicon. The front side features anti-reflective coatings (ARC) and the back side is divided into four regions: hole-selective contact (HSC), gap region, electron-selective contact (ESC), and HSC plus gap.
“We have used amorphous silicon as contact passivation layers and laser ablation as mass production technology to fabricate HBC solar cells,” the researchers explain. “This brings advantages such as low leakage in the region close to the np polarity boundary, low cost, low deposition difficulty and high uniformity.”
Using laser patterning, the researchers applied a picosecond pulsed green laser to ablate the ia-Si:H/pa-Si:H stack overlaid on the designed ESC region. The transparent conductive oxide (TCO) layer was deposited using the magnetron sputtering technique.
They also used picosecond pulsed ultraviolet laser to remove the TCO/pa-Si:H/ia-Si:H stack from the separation region designed to isolate it, which they claimed prevents leakage channels. The silicon nitride (SiNx) layer was used to prevent the passivation performance of the ia-Si:H/na-Si:H stack from being impaired.
Tested under standard lighting conditions, the device achieved a power conversion efficiency of 27.07% and an open-circuit voltage of 751 mV, results confirmed by the German Institute for Solar Energy Research (ISFH). According to Longi, the open-circuit voltage value is one of the highest ever recorded at the research level for this type of solar cell.
Their analysis also showed that the cell, like other HBC devices, suffers from electrical shadowing effects, as carriers of both polarities accumulate on the same side, with a consequent decrease in short-circuit current density. However, they also found that these losses are slightly lower than those observed in previous devices.
“This is mainly attributed to the additional current gain from the thicker silicon wafer and the designated illumination area used in the measurements, excluding the influences of recombination and leakage at the silicon wafer edge,” they further explained.
The group also found that contact resistivity plays a critical role in high-efficiency HBC solar cells and stated that minimizing the contact area of ??the HSC or ESC region was crucial to improving cell efficiency. “By optimizing the HSC region, we achieved the lowest contact resistivity using the pa-Si:H film,” he specified. “Furthermore, we determined that the contribution of contact resistivity to resistivity can be directly calculated from the coverage area ratio.”
Looking ahead, the researchers said they want to utilize wafer edge effects and conduct “meticulous” optimization of the front antireflective coating and rear reflector, which they believe could help the cell achieve efficiency of up to 27.7%.
The solar cell architecture is described in “ 27.09%-efficiency silicon heterojunction back contact solar cell and going beyond,” published in Nature Communications . “The research offers multiple strategies and guidelines to optimize the structural design and resolve key contradictions of back-contact solar cells,” the group notes. |
