N-type single-crystalline Si solar cells: Front side metallization for solar cells reaching 20% efficiency

作者：Kumar Praveen; Pfeffer Michael; Willsch Benjamin; Eibl Oliver; Koduvelikulathu Lejo J; Mihailetchi Valentin D; Kopecek Radovan

来源：Solar Energy Materials and Solar Cells, 2016, 157: 200-208.

DOI：10.1016/j.solmat.2016.05.027

• Single crystalline Si solar cells
• Microstructure
• Specific contact resistance
• Structure-property relationship
• Percolation model

Screen printed front side contacts were investigated in n-type, high efficiency, single-crystalline Si solar cells with B doped emitters. Cells were processed identically and two different front side pastes were used: paste Ag-Al yielded efficiencies of 20.0%, and paste Ag, without Al, yielded 16.1% efficiency only. Cells contacted with paste Ag-Al yielded a continuous glass layer, a significantly higher density of Ag colloids, and an Al mole fraction of similar to 1 at%. In contrast, cells contacted with paste Ag revealed a thinner but continuous glass layer as compared to cells processed with paste Ag-Al. It contained a lower density of Ag colloids and Al was below the detection limit of the EDX microanalysis. In n-type cells pyramidal shaped Ag nanocrystals did not appear, whereas for p-type cells they are formed at intersections of differently {111} oriented surface planes at the Si emitter. We did not find Ag/Al spikes at the Si emitter for cells contacted with paste Ag-Al.
Cells contacted with paste Ag-Al had a smaller series resistance as well as a significantly lower specific contact resistance (4 m Omega cm(2)). A higher series resistance and specific contact resistance > 100 m Omega cm(2) was found for cells contacted with paste Ag. It was concluded that the specific contact resistance decreases with increased wetting and increased density of Ag colloids in the glass layer. The series resistance of the Ag-Al alloy contact depended only weakly on temperature, whereas the series resistance linearly increased with temperature for contacts processed with paste Ag.
p- and n-type cells with maximum efficiency yield a similar microstructure of the front side contact, similar series and contact resistance, respectively. A high density of Ag colloids in the glass layer was found for cells with low specific contact resistance. This suggests that also for n-type cells a three dimensional percolative charge transport through the glass layer is assisted by the presence of metallic Ag colloids.

• 出版日期2016-12