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.
Ever measured a solar cell?
Indoors 0.7m from lamp: 0.27V
indoors near window: 0.37V
Outdoors in shade of bush: 0.47V
Outdoors in weak sun (breaking through clouds): 0.515V
A PV cell is not a current source. Its a current source in parallel with a forward biased semiconductor diode(*). When you measure the open-circuit voltage you are measuring the diode voltage with the current all flowing through the diode rather than external circuit, so it obeys a logarithmic law against light-flux.