If the open circuit voltage on an solar panel drops a lot, like from > 12 V to below 9 V, there is only very little power (e.g less than 0.1 % of rated power) left to be used. So usual it is not worth the effort. The more important point is to have good efficiency when you have a reasonable amount of light (e.g. 20%-90% of rated).
The usual way to get high efficiency in charging a battery from a solar panel is using a panel with a somewhat higher voltage (e.g. 18-25 V for a 12 V battery, that is actually more like 14 V when full) and have a step down converter, that keeps the solar panel at its maximum power point.