There are times when you would like to harvest just a little bit of energy from the sun.  Unfortunately, there are not many miniature solar panels available.  Most panels are 10 to 50 times larger than needed.   You can make your own and wire in series.  Just as in the larger panels, be sure to insert a small signal diode between the photodiode array and any battery or super capacitor being charged.  The diode will prevent current from leaking back into the array at night.