Calculating minimum trickle charge power requirements
You can't really trickle charge a cell phone through the microUSB interface that most use.  There is a minimum current capability, below which your phone just won't charge.  A 500 mAh charger will top of your phone, then your phone will stop charging and just draw running current from the USB line.

You are needing a 3-5W solar cell to do minimal charging, plus any losses in regulation.  You best bet would be to use a 5V buck converter to drop the voltage of a 12V cell.  You are still looking at a solar cell around 250x180mm.  This isn't very portable for using while out and about.  And full power is only good when angled directly into full sun.  Ambient light will give a much lower output.