Simple Solar Circuit
A solar cell has a point at which it supplies maximum power. While you can get away with a switching regulator, it is not optimal for drawing power from the cell. 

One problem is you are using a switching regulator connected directly to the solar cell. To use sufficient power from the cell, an switching regulator with Maximum Peak Power Tracking (MPPT) needs to be used.

The last problem is you might not have enough power that is sourced from the cell in the first place. Calculate the max power by using 300W/m^2 (equator on a sunny day is 1000W/m^2) and the area of your panel. Then multiply that power number by the efficiency of the panel, that will give you the amount of watts the panel can supply.