In the textbook : Electric Power Principles, in Chapter 1
on the working of a pv cell :
The source current Is is the result of absorption of photons in sunlight that cause separation of valence electrons from their atoms. The resulting hole/electron pairs fall across the high field gradients present at the diode junction. Because any voltage resulting from this current tends to forward bias the actual junction, the voltage available is limited
I don't understand the train of thought leading to a limited voltage, and I'd appreaciate a better explaination or a reading recomendation.