The top of the cell is where the junction is, just a few atoms under the surface. The cells are coated with a very thin ITO coating to collect this current, and the grid is there to reduce resistance, as the ITO is conductive, but not a good conductor. There is a trade off between collector that reduces cell resistance, and size of cell, as the current collectors reduce the cell area. The back of the cell has a much higher doping level, and thus is quite low resistance silicon, but this highly doped silicon is not good at being a solar cell, but is good at conducting electricity. You cannot put this on the front. As well the back will have a full metal layer on it, just a few spots where they put something you can solder to.