Solar cell charging and battery chemistry for low power 2.4-3V supply?
20uA charging current and >750mAh equates to a charge rate of 0.00027C or more than 4 years to get a full charge (and that's assuming continuous illumination). 

Standard Nicad and NimH batteries are not suitable because they will discharge much faster than the solar panel can charge them. 

Low self discharge NIMH (eg. Eneloop) only looses about 10~20% per year, and so should store at least some of the charge. At this low current trickle charging is safe, so you can just connect the solar panel directly across the battery (possibly with a blocking diode in series if the panel has high dark leakage).   

A lithium battery would hold the charge better, but needs a complex charging circuit that may waste more current than it saves. 

If you really need >750mAh of rechargeable battery then I suspect your average current drain is much higher than the solar panel's output. In that case it it might be better to forget solar charging and just rely on the primary energy source. If that isn't enough then you may want to rethink your design criteria.