I found this post very useful. I am trying to design something similar for my thesis.
“Instead of adding a single supercapacitor, you might notice that we’ve actually added two. That’s because the supercaps that we had on hand are rated for 2.75 V– not enough to handle the 4.5 V output of the panel when sunlight is present. To get around this limitation, we used two of the caps in series, for which the voltage ratings add, giving us a barely-okay total rating of 5.5 V. (Note: be careful adding capacitors of different values in series– the voltage ratings may scale in non-obvious ways.) When first exposed to the light, this circuit takes about 30 s to 1 minute to charge the capacitors enough that the LED can turn on. After it’s fully charged, the circuit can be removed from the sunlight and still drive the blue LED for about 30 s to 1 minute– a very effective flywheel for light duty applications. ”
Taken from Evil Mad Scientist.com