Some calculations.

-From this post I found that the phone will run for two days with everything turned off. The shortest day in Holland is 8 hours, so for two days I have 16 hours of sunlight.

The phone has a 2100mAh, 3.8V, 7.98Wh battery, which holds 28728 Joules. This amount of energy has to be restored in 16 hours. The solar-charger thus has to have at least 0.5 Watts of power.

The panel I have in mind right now has 0.935 Watts of power. This panel has to be hooked up to a voltage regulator to ensure a steady 5 volts.

With an input of 5.5 volts from the projected panel, the step-up/down dc-dc regulator has a measured efficiency of +- 93%, which leaves a comfortable .87 watts for the phone.

This is about 175ma @ 5 volts. Your regular laptop usb2.0 does 500ma, so its about 1/3rd of that.