Photovoltaic/solar power supply for Raspberry Pi
using a solar panel directly to power the raspberry pi would not be a good idea. in the same manner where solar panels power houses, not by direction connection, but in conjunction with power electronic converters, and storage systems, so also would those be applicable here. Now what you would need, would be a good rechargeable battery, a simple DC-DC converter, and finally, a linear regulator as the  buffer between the battery, and the raspberry pi.


Solar panel to DC-DC converter
DC-DC converter to rechargeable battery
rechargeable battery to linear regulator
Linear regulator to raspberry pi.


NB: you can cascade the solar panels which you find to give you more current output, and have faster charge rate.