Actually this would be a anti-green add on. Solar panels consume resources and energy to make and have payback periods in years if they are set up in optimal sunlight. A cell phone would normally only last 2 years and a good part of that time in a poor location for the solar panel to work, so very possibly you would never produce enough power from the solar panel to make up for the power that went into manufacturing it.
This is also an issue with the Prius solar panel IMHO, if it’s really green at all, as opposed to putting that panel in a safe non-mobile place (such as home rooftops) that would last longer then the typical life of a car, and can be tilted for optimum light.