Solar panels are photodiodes. Their junction voltage is determined by the properties of the material, not by the light intensity, although of course at sufficiently low light intensity they will cease to be reverse-biased. More sunlight will just produce more current, not more voltage.
All I know is that I hooked a small panel up to my multimeter and if I put it in direct sun light it produced 5 volts and under indoor lighting conditions the voltage was around 3 volts. So I don't really know how they work but I know how to measure the voltage coming out of them.
You may be aware that direct sunlight is thousands of times more intense than indoor lighting conditions. Probably if you'd hooked up a resistive load to it while indoors, the measured voltage would have dropped to a fraction of a volt; and if you'd used an oscilloscope instead of a multimeter to measure the voltage, you would very likely have measured 5 volts.
