If you are thinking about using the solar cell as your power source, forget it. If you are wanting to charge your battery while riding, it may be possible but it is certainly unpractical. The typical state of the art rooftop solar cell is 3' x 4' and is rated at 300 watts. They never actually produce that much, and that is in IDEAL conditions, clear, cool and full overhead sun. You will get 240 volts, +/-, and 1.25 Amps, IN ONE HOUR. That is no where enough to do anything but add a few minutes of riding time over the course of a ride, unless you are letting the bike sit and charge. Even then, you would only add 1/10th capacity to a 12 Amp battery, IN AN HOUR. Keep in mind that that panel will act like a sail, and will be hugely cumbersome. Unless you can situate it perfectly parallel to the ground, it will also create lift (or down force), and A gust of wind may likely seriously impact your ride safety! I think it is not a likely prospect, yet...
Having said that, I have rooftop solar, and charge my bike (and car) with solar, for free!