Design of efficient silicone solar cells entails the selection of metal grid pattern. Different metal grid patterns affect the electrical performance of silicone solar cells. The power losses calculated for square and linear grids at different illumination levels have shown that of both grid patterns, the power losses of the square grid are less sensitive to the illumination levels compared to linear grid.The top contact design involves the minimization of the finger resistance and busbar resistance, and also the general reduction of all top contact losses. These losses include resistive losses in the emitter, resistive losses in the top metal contact and shading losses.
Top contact Design losses
The main characteristics of the top contact design that determine the magnitude of the losses are:- finger and busbar spacing- height-to-width aspect ratio of the metal- resistivity of the metal and the minimum metal line width.
Impact of Finger Spacing on Emitter Resistance
An important element in the top contact design is the resistive losses in the emitter. The power loss from the emitter is a factor of the cube of the line spacing, and thus a shorter distance between the fingers is required for low emitter resistance.
The grid resistance is defined by the resistivity of the metal used to create the metal contact, the metalization pattern and the aspect ratio of the metalization scheme. Although low resistivity and high metal height-to-width aspect ratio are required in solar cells, but in reality those are limited by the fabrication technology used in making the solar cell.
Shading losses are produced by the presence of metal on the solar cell’s top surface. This prevents light from entering the solar cell. The shading losses are determined by the transparency of the top surface layer that is defined as the ratio of the top surface covered by metal. The transparency depends on the width of the metal lines on the top surface and on the metal lines spacing. A key practical constraint is the minimum line width that is associated with a certain metalization technology.
While there are multiple top contacting schemes, practically most top surface metalization patterns are fairly simple and highly symmetrical. A symmetrical contacting scheme can be easily divided into unit cells and numerous design rules can be determined, as the following :
The optimum busbar width (WB) occurs when resistive loss in the busbar is equal to its shadowing loss
A tapered busbar losses are lower losses than constant width busbar losses
Losses are much smaller for a smaller unit cell, smaller finger width(WF), and smaller the finger spacings (S)
In fact, the front surface grid pattern can be readily designed and developed by computer software.