solar panel + prolong charger?

No and not advisable.

 

Mounting the solar system such that its extra weight and air drag don't consume all possible gain, will be difficult.

 

Toyota says, the solar roof of the Prius IV will extend the range up to five kilometers per day.

That would save me max. 0,3 € per day. Due to the cheaper gas prizes in the US you will save less.

In the winter the sun has less power. And then the rainy days...
I think it would last three years for me to save 100 €. Panel and charger and installation will cost much more I think. Don't forget the possibility of an early damage.

In the summer usually I try to park in the shadow (underground garage or parking house (not sure about the right English words)). With the solar roof I should park in the sun. But then the car would heat up much more and the solar power would feed the A/C. I don't know, but I think the A/C would need additional power.

It's possible that the disadvantages could dominate both ecologically and economically.

Cheaper standard panels installed at home could be better, because
- they can be standard -> cheaper
- they can be bigger (more surface) -> more power
- they can be in the sun all day long -> more energy
- they cause no additional weight/wind resistance to the car -> less consumption