charge extraction from solar cells in low light conditions
I'd say you want an energy harvesting boost converter. These work down to very low voltages on their input (some as low as 0.25V I believe) and can provide a trickle of current for keeping a battery charged or for charging up a capacitor.

Here's one from TI that might fit the bill: -



It's got some quite useful applications. You should also try linear technology and Analog devices too - search for energy harvesting.