Spray on solar cells (
ie sprayed onto the cars surface like paint) are making some progress. You can read about the plastic type (
here) and there are Ti based mineral ones being designed too.
With current (20% efficient) silicon based solar, a Prius could be given ~18 miles per day (6,500 miles per year) zero fuel consumption EV-range, though a better battery would be required.
More info on using solar energy to power vehicles here:
http://www.solarvan.co.uk/
http://mysite.wanadoo-members.co.uk/ecotech/socar1.htm
