What exactly are you trying to achieve with this measurement?
A Solar Cell is a current source , and will produce 0.5 V per cell even with only a low light level.
So a Solar panel , even in a relatively low light environment will produce close to its full voltage.
The voltage wont change much , even though the light level may change a lot.
To measure the change in light level, you need to put some kind of load on the Solar Panel and then
measure the current.