Does the voltage of a solar panel have to be greater than that of a battery pack to charge it?
Although this is an old question the answer may be useful to others.
Summary: The PV panel suggested is of too low a voltage and power rating to be more than very marginally useful in this application.
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To charge a battery the applied voltage must be at least equal to the highest voltage the battery reaches. In this case either the PV panel voltage must be as high as desired or you need to add a boost converter.  I'll deal only with the direct PV panel connection.
The maximum possible charge rate is 100 mA into a 2300 mAh battery so the maximum rate = 100/2300 = C/23.
A NiMH cell charged at such a low rate will have a fully charged voltage of about 1.4V, so 4 cells will require about 5.6V.
The PV panel has 12 cells. As Voc (V open circuit)is about 0.5 - 0.55
then V/cell Voc of the panels is ABOUT 6 to 6.6V.
Vmp (V max power) will be 80 to 85% of Voc = 4.8 to 5.6V - with the 5.6V being an optimistic high value.
The combination of needed Vmax_battery of about 5.6V and the panel Vmp of probably less than 5.6V tells you that while you'll get some degree of charging the battery would not fully charge in sensible timespans.
Even at 100 mA the 2300 mAh battery will take 23 hours of full sun to charge. Typical equivalent full sun days vary with geography but in eg the continental US not too far north you may get 1 to 2 hours a day in Winter and 4 to 6 hours/day in summer.
So with the panel clean and well aligned and IF the battery had say 3 cells you'd need 4 to 12 days for a full charge.
In reality the charge current will be less to much less. It could take several times longer than the above figures. As a keep-alive for very low powered or occasional use equipment the PV panel MAY be of some use but it is really grossly underpowered and of too low a voltage for the task.
Adding a series diode would make matters worse again.