Working voltage (MPP) of a solar cell is about 2/3 of it's max open circuit voltage.
So if you want to charge a 5volt cap under any light condition, you should have a cell with an open circuit voltage of ~7.5volt, plus the forward voltage of the backflow diode.
Me thinks your 5.7volt solar cell is made to charge a 3.7volt LiPo battery (4.2volt max).
Leo..