Scientists in Germany Develop Cost-effective Solar Cell Deposition Method

Researchers led by the Technical University of Ilmenau in Germany claim that if silicon dioxide (SiO2) films are deposited at room temperature without complex vacuum systems, they can be a good, cost-effective candidate for use in commercial c-Si solar cell production lines. SiO2 deposition is used as a passivation layer or a protective layer in various types of silicon PV cells.

In a paper entitled ‘Application of Hydrosilane-free Atmospheric Pressure Chemical Vapor Deposition of SiOx Films in the Manufacture of Crystalline Silicon Solar Cells’, published last year in the journal Thin Solid Films, the scientists 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 reductions for cell manufacturers, provided it can be developed and applied in a large-scale production setting.

“In this work we present SiOx films deposited in cost-effective laboratory scale three-dimensional printed atmospheric pressure chemical vapor deposition setup,” the scientists say. The group notes that plasma-enhanced chemical vapor deposition (PECVD) is the most common process used with this material, but requires both high temperatures and a vacuum.

The quality of the deposited films was investigated as to their integrity, conformity with various surfaces, and post-treatment resilience such as stability against etchants and annealing. Several applications of the SiOx film prepared with the atmospheric pressure chemical vapor deposition (APCVD) were discussed.

In one application, the APCVD SiOx was utilized to effectively promote single-side texturing of Float Zone and Czochralski Si wafers by coating only one side with SiOx and subsequently annealing prior to texturing in an alkaline aqueous solution. Another application was to exploit the APCVD SiOx as a plating mask for silicon heterojunction solar cells.

Two processing options prior to the oxide-film deposition were investigated: i) application of an Ag seed-layer, which promotes subsequent electroplating, and ii) printing of an organic grid, which, after stripping, creates openings in the SiOx that facilitate electroplating of the solar cell’s electrode on the underlying transparent conducting oxide.

In a different application, the APCVD SiOx films acted as protection against parasitic plating on the front side of passivated emitter and rear solar cells. The deposited films were characterised by ellipsometry, hemispherical reflectance measurements, scanning electron microscopy, energy dispersive X-ray spectroscopy and optical microscopy.

The scientists conclude, “The newly developed APCVD setup provides a simple and tailorable approach for the deposition of SiO2 films on virtually every substrate material at room temperature,” adding, “Because of the used non-flammable and inexpensive gases, the costs of the deposition equipment and of the operations are low. The presented simple APCVD SiO2 process can find several applications in photovoltaics.”

This work is still going on to bring the process to an industrial scale, as part of a collaboration with Hannover-based coatings company Alethia, which is currently set to run until May 2022. The paper was authored by Esmail Issa, Henning Nagel, Jonas Bartsch, Markus Glatthaar and Edda Rädlein.