Technology

Ag-on-Al Solar Cell Design 101

Industrially produced silicon solar cells have silver rear tabbing layers (A) which are directly printed on the silicon wafer (B). The conventional rear tabbing layer does not form a back-surface field (BSF) and lowers the open-circuit voltage and fill factor of the solar cell. Commercial solar cells typically lose between 0.05-0.15% (absolute) because of the rear tabbing layer. PLANT PV’s Ag-on-Al technology (C) is printed directly on the dry aluminum layer and enables the entire solar cell to have a continuous back-surface field. Ag-on-Al solar cells have cell efficiencies that are comparable to fully aluminized back silicon solar cells. The Ag-on-Al paste is a drop-in replacement for conventional rear tabbing pastes and only requires the PV cell maker to change the print order of the rear aluminum paste and tabbing paste during production.

 

How does Ag-on-Al product work?

PLANT PV has developed novel particles that are mixed with silver. During co-firing these particles prevent silver/aluminum interdiffusion, while strengthening the underlying aluminum layer. This novel system allows for the full formation of the back-surface field and aluminum-silicon eutectic layer.

Silver-on-Aluminum Demonstrated Efficiency Gains

PLANT PV has externally demonstrated an efficiency gain with the Ag-on-Al paste at multiple locations in Europe and the United States. In one example, multi-crystalline silicon solar cells with four continuous busbars  (Ag area = 16cm²) were fabricated using conventional rear tabbing, Ag-on-Al pastes and compared to fully aluminized rear wafers (Al area = 154 cm x 154 cm). In this external test we demonstrated greater than 0.15% efficiency gain over solar cells that used a commercially available conventional rear tabbing paste. The Ag-on-Al cells had an equivalent power conversion efficiency to fully aluminized cells (that contain no rear silver). The Ag-on-Al devices consistently had a >2mV increase in open-circuit voltage and >0.2% increase in the fill factor. The efficiency improvement is dependent upon the Ag area and wafer type; industrially solar cells typically use 8 cm² area and could expect >0.07% efficiency gain for multi-crystalline cells with this product. Greater gains can be achieved for mono-crystalline wafers, which lose more open-circuit voltage due to the rear busbars. If you are interested in more information regarding test results and cell advantages, please contact sales@plantpv.com to receive a white paper related to our Ag-on-Al BSF product.

Additional Products in PLANT PV Pipeline

Ag-on-Al for solar shingles: Conventional rear tabbing layers do not make ohmic contact to p-type silicon and therefore do not effectively collect current. For conventional solar cell designs this is not a problem because the front busbars shade the region above the rear tabbing layer and therefore no current is produced above the tabbing layer. However, for asymmetric cells designs, such as solar shingles, the rear tabbing layer can result in an additional current loss since there are no front busbars above the rear tabbing layer. Ag-on-Al technology solves this problem by allowing aluminum to contact the p-type solar wafer and form an ohmic contact across the entire backside of the solar cell. This allows for full collection of current across the entire wafer, which meaningfully improves the short-circuit current density (in addition to Voc and FF). If you are interested in testing our Ag-on-Al product for solar shingle designs please contact sales@plantpv.com.

PERC pastes:PLANT PV is developing low-Ag content Ag-on-Al pastes for PERC cells. The Ag-on-Al PERC paste will allow for lower Ag print area and greater ease in cell processing such as laser scribing. We anticipate testing with customers in early 2018.