C.2 Analysis Phase

Before redesigning the solar cells, it is necessary to obtain a better understanding of PV technology. Therefore, PV modules are analyzed because solar cells are generally used in a PV module. Next to this, the PV market and target group are explored. The derived information from this analysis is used to determine the demands and wishes of the new solar cell and PV module.

The characteristics and production of solar cells provide information about the limitations and possibilities in cell design. An important aspect is the production of the cell, because the metal contact pattern is applied by screen printing and a redesigned contact pattern must be printable.

The screen-printing process of a solar cell is similar to that of textiles. The machine squeezes silver paste through a patterned screening mesh onto the cell surface. After the paste is applied, the paste is dried at low temperature and then fired at a higher temperature. This step drives off the remaining organics and allows the silver regions to combine.

The advantages of screen printing are in its simplicity and cost-effectiveness. The disadvantages of screen printing are the relative large line width, the high contact resistance, and the low aspect ratio of the metal lines (broad and flat), which results in large shadowing losses.

The limitations and possibilities of screen printing lead to boundaries in metal line widths and heights. Beside these boundaries, the shape of the lines must be taken into account. Lines with sharp corners are difficult to screen print, because the metal paste is not able to reach the triangular part of the corner. Metal lines in the squeegee direction are more easily printed and are straight lines as opposed to curved lines.

Many patterns can be screen printed; however, the efficiency will be influenced by the pattern design:

Many possibilities are available in terms of geometries and aesthetics. Radike and Sumhammer (1999) described in their paper that newly designed contact patterns are possible but that efficiency loss is not preventable. The losses are, however, small: 0.71% efficiency loss in the worst-performing cell with respect to an H-patterned cell. This should not hold back the application of appealing front contact patterns. Radike and Sumhammer designed and simulated 15 contact patterns. These patterns can be used in a PV module creating a decorative appearance. Figure C.2.1 shows the standard cell and 15 new contact patterns of their research.

Figure C.2.1 Sixteen different contact patterns by Radike and Sumhammer (1999)

A new pattern can be produced on a screen mesh. The screen mesh in the screen-printing machine can be easily replaced by this new designed screen mesh. The need to change the machine's settings is dependent on the mesh and the opening of the screen.

The appearance of the solar cell is dependent on the metal contact pattern and the surface color. The front surface of the solar cell reflects light while the cell is illuminated. This reflection has to be reduced as much as possible. Therefore, an antireflection coating or antireflective coating (ARC) is applied on the solar cell.

The color of the crystalline silicon cell can be altered by varying the thickness of the ARC on the cell's top surface. A standard solar cell has a blue color, while other colors are available, such as purple and green (Figure C.2.2).

Figure C.2.2 Colored solar cells

With the gathered information from the analyses, demands and wishes for a multicrystalline silicon cell with a dimension of 6 inches are formulated. This input is then used in the design phase.