The role of the top contact is to pass light into the cell while transferring the electricity out. Semiconductors, such as silicon, are orders of magnitude less conductive (more resistive) than metals so a top grid pattern is essential to reduce the series resistance on all but the smallest solar cells. There are transparent conductors such as indium tin oxide but they are also much less conductive than metals and absorb light.
Ideally, the metal wires of the top contact would be extremely narrow and close together, but very fine contacts are too expensive for solar applications. For instance, photolithography achieves line widths below 1 µm and is used extensively by the integrated circuit industry but requires expensive chemicals and cumbersome alignment. Only laboratory demonstration cells can justify the cost of photolithography and it is rarely used in commercial cells. For the design of the top contact, there is a trade off between shading the cell, which primarily affects ISC, and the resistance of the metal contact, which primarily affects FF.
The metallic top contacts are necessary to collect the current generated by a solar cell. "Busbars" are connected directly to the external leads, while "fingers" are finer areas of metalization which collect current for delivery to the busbars. The key design trade-off in top contact design is the balance between the increased resistive losses associated with a widely spaced grid and the increased reflection caused by a high fraction of metal coverage of the top surface.