That charger takes any voltage between about 2V and less than 5V and boosts it to 5V.
You could use that circuit with a 2V to say 4.5V solar panel BUT you can instead just use a solar panel directly.
A solar panel with full sun loaded voltage of 6V or more will charge a USB cellphone either directly or with the addition of a series diode. (Diode needed ONLY if cellphone battery voltage appears on charging input. )
For 'safety' you should also connect a 5V1 zener-diode (5.1 Volt) across the solar panel output so that Vout can never be > 5.1V.
Zener Wattage = Wmp panel = max panel power in full sun.
Charge rate is related to panel Wattage.
Panel peak watts in full sun = Wmp.
The panel wattage for maximum charge needs be no more than :
Vpanel_loaded_at_full_sun / Ah_of_cellphone_battery
eg a panel with Vmp=6V and charging a 1000 mAh = 1 Ah cellphone battery need have a Wattage of no more than 6V/1Ah = 6 Watts. This would charge the battery at maximum possible rate. Higher wattage panels will heat the zener and lower wattage panels take longer to charge.