What is the approximate open-circuit voltage from a fully illuminated silicon photovoltaic cell?
Why A photovoltaic cell is just a large silicon PN junction, and the open-circuit voltage it can develop is limited by the junction's barrier potential, roughly 0.5 volt for silicon. That is true regardless of the cell's physical size; area determines current, not voltage. This is why solar panels wire many cells in series, about 36 cells to charge a nominal 12 V battery.
Watch out The 1.38 V figure looks like a battery cell voltage (nickel or alkaline chemistry range), not a silicon junction; the 0.02 V and 0.2 V choices are far too low to be useful for charging anything.
A solar cell is a big silicon diode: about 0.5 V, same ballpark as a diode's forward drop. Current scales with area, voltage does not.