Scientists in Germany have developed a new process for the deposition of silicon dioxide layers during cell production. Without the need for high pressure, flammable gases or vacuum conditions, the process could lead to cost savings for cell manufacturers, provided it can be developed and applied in a large-scale production environment.

June 11, 2021

From the PV magazine Global

The deposition of the various layers that make up a PV cell stack onto a silicon wafer is one of the most complex and expensive areas in PV cell and module production. The processes used here often require the creation of a vacuum, the use of combustible or otherwise dangerous raw materials, high processing temperatures and other challenges.

A group of scientists led by the technical university Ilmenau in Germany kept these factors in mind when separating silicon dioxide (SiO2), which can be used as a passivation layer or protective layer in various types of silicon PV cells. The group notes that plasma-enhanced chemical vapor deposition (PECVD) is the most common method used with this material, but it requires both high temperatures and a vacuum.

The group explored alternative procedures, including Spray pyrolysis, low pressure chemical vapor deposition (LPCVD), Sol-gel deposition and Atomic layer deposition, finally on Atmospheric pressure separation (APCVD) as the best option for further investigation. “APCVD has the advantage that, in contrast to liquids, gases are used as precursors,” explains the group. “In this way, crack-free, dense films with good step coverage can easily be obtained.”

3D printed thermoplastic

With APCVD, the group was able to demonstrate a low-temperature SiO2 deposition process that also eliminates the use of highly flammable hydrosilane as a precursor material. And since the equipment doesn’t have to withstand very high temperatures, the entire structure could be made of inexpensive thermoplastic material that is 3D printed and thus easily adaptable to different wafer shapes and sizes.

The group investigated two different applications for construction in solar cell manufacturing. First one-sided texturing – wafers were coated on one side with around 180 nanometers of SiO2 and then tempered at 180 degrees Celsius. Second, the process used to deposit a protective layer to prevent parasitic plating of metals as the cell fingers and bus bars are deposited.

In the first application, the wafer was then treated with an alkaline texturing solution, which etched about 100 nm of the SiO2 layer and left a smooth, even surface. And in the second application it could be shown that the SiO2 layer effectively protects against undesired metal deposition. “The areas of the cells coated with SiO2 were practically free of unwanted metal deposits,” the scientists said. “Whereby there were massive parasitic coatings in the uncoated areas.”

Cell efficiencies were measured between 19.3% and 19.8% – below what has already been achieved in PERC cell production. However, the group notes that their focus was on measuring the performance and reliability of the coating processes, which means that overall cell efficiency was less of a focus. All the details of the work can be found in the paper Application of hydrosilane-free chemical vapor deposition of SiOx layers at atmospheric pressure in the production of crystalline silicon solar cells, published in Thin Solid Films.

The newly developed APCVD setup offers a simple and tailor-made approach for the deposition of SiO2 films on practically any substrate material at room temperature, ”the group concludes. “Because of the non-flammable and inexpensive gases used, the costs for the separation system and operation are low. The simple APCVD-SiO2 process presented can find several applications in photovoltaics. “

As part of a cooperation with the Hanover-based coating company Alethia, which currently runs until May 2022, work is underway to bring the process to an industrial scale.

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