Two very essential parts of a PV cell are the electrical contacts, because they are the bridges that connect the active semiconductor to the external load. The back contact of a cell (away from the sun) is relatively simple and usually consists of a layer of aluminum or molybdenum metal. But the front contact (facing the sun) is more complicated. When placed in sunlight, the cell generates current (flowing electrons) all over its surface. Attaching contacts just at the edges of a cell would not be adequate because of the excessive electrical resistance of the top layer in this configuration (only a small portion of the electrons would make it into the contact). So, the contacts must be made across the entire surface to collect the most current. This is normally done with a metal "grid." Unfortunately, placing a large grid on the top of the cell shades the active parts of the cell from the sun, effectively reducing the cell's conversion efficiency.

Therefore, in designing grid contacts, we must balance electrical resistance losses against shading effects. The usual approach is to design grids with many thin, conductive fingers spreading to every part of the cell's surface. The fingers of the grid must be thick enough to conduct well (with low resistance), but thin enough to block a minimum of incoming light. Such a grid keeps resistance losses sufficiently low, while shading only about 3% to 5% of the surface.

More . . . Metallic Grids