There just isn't enough surface area. A Model S has a roof area of around 9m^2. The sun provides about 1000W/m^2 on a clear day at high noon when directly overhead, so with 100% efficient solar cells on a perfect day, you'd get around 9kW of electricity. A Model S goes about 4 miles per kWh when it's doing well, so in this ideal situation you'd top out at around 36MPH. In the real world, the sun isn't directly overhead, and real solar cells have more like 20% efficiency, so cut that number down by a factor of ten or more.

The best way to build a solar-powered car would be to have a battery-powered car that gets recharged from solar energy. Basically, a Model S and a home solar generating system.