Photovoltaics
RENA discusses process options for edge isolation
Wet chemical edge isolation for solar cells
Edge isolation is a key process for the manufacturing of solar cells and with the market experiencing rapid growth there is a need for manufacturers to have access to techologies that provide the best cost of ownership and yield bearing potential. Despite an understanding of the traditional methods with new options, little effort has been made to compare the newer processes. Here RENA discuss results comparing the laser to wet chemical isolation.
Until some years ago, the standard process for the edge isolation of solar cells was the plasma edge isolation. But in the last few years, this technique has been widely replaced by wet chemical isolation and by laser isolation.
While there were results of comparisons between the wet chemical isolation and the plasma edge isolation that showed a significant advantage to the wet chemical process, no data has yet been gathered comparing the laser to wet chemical isolation.
The reasons for the change to new techniques were some clear disadvantages of the plasma etching process. For plasma etching, the required continuous production line had to be interrupted and the wafers had to be stacked, which meant a greater amount of handling effort and a reduction in device output. Also the high mechanical force on the wafers during the plasma process resulted in higher breakage rates, creating another reason for the changes.
To help alleviate these concerns RENA developed an inline solution for wet chemical isolation processes, which they then combined in the RENA InOxSide tool with the PSG-etching, to provide a potential solution to these issues that were reducing manufacturing potentials.
The InOxSide wet chemical process not only etches the edges of the wafers, but also removes the emitter on the back side of the wafer by transportation through an etching bath that then electrically isolates the solar cell emitter from the back contact.
A key issue for this process is that the diffused front side of the wafer does not come into contact with the etching liquid material, so that the front emitter is not damaged.
This is achieved by very precise wafer transportation with further physical protection effects. The edge isolation process is combined with the PSG removal process in the RENA InOxSide tool. Since development the tool has gained a positive foothold and is widely used in the PV industry.
In the standard laser edge isolation process, a groove is cut close to the edge of the front side of the wafer. This groove has to be deeper than the thickness of the diffused layer, in order to isolate the emitter from the back side of the cell. This process is normally performed right after the firing of the contacts.
InOxSide Sequence
In two independent studies, these two processes were compared with each other. In both experiments, 156 mm wafers were separated into two groups, and processed under the same production conditions. The difference was in the edge isolation procedures. While for one group the edge isolation was done with the laser on the front of the wafers, the second group was processed with the wet chemical isolation process.
The results of these experiments showed an advantage to the wet chemical etching process, which showed an efficiency increase of up to 0,2% absolute. The main reason for this increase appears to be that on the laser processed cells, active emitter surface is lost due to the groove which was cut into the wafer.
This area loss does not occur with the edge isolation done by wet etching with the InOxSide process, therefore these cells have a larger active emitter surface, which results in an efficiency increase of these cells leading to better performing solar cells.
Also, the complete removal of the parasitic emitter on the rear of the wafer allows innovative cell concepts that will not require aluminium for future cell production.
The InOxSide process results suggest that this method provides the most suitable process for edge isolation of solar cells. Not only compared to the plasma etching, but also in comparison with laser edging, the wet chemical process showed positive advantages.
More than 40 RENA InOxSide tools are already in production worldwide, which demonstrates the industry has already taken advantage of the higher efficiency, the high process integration, the reduced braking rates and the simplified handling.
