I put a 0.5v 1A monolithic silicon solar cell on the front end of the Joule Thief circuit. Added a schottky diode and 2000uf cap on the output. I wanted to see how long it would take the Joule Thief to charge the cap bank. There are two 1000uf/50V caps in parallel to get the total 2000ufs. In full sun it took 3 mins 34 sec. to charge to 21.8 V.
Took voltage measurements every 15 seconds. Here is the data:
Here is the Joule Thief circuit that shows the connections for this experiment (to see a full size image click image then click image on following page):