Charging you phone with solar panels is not green. Really it is not. Photovoltaic panels take a lot of energy to make. This energy is paid back over many years of generating electricity when the sun is shining. If you only use the panels for a few hours every few days they are a waste of energy.
The solar cell on a calculator is a different matter. Putting the solar cell on eliminates the need for a battery. A phone uses far too much power to be only solar powered so there are not component savings.