Why do we consider a p-n junction diode as a voltage source sometimes and how do we justify it's working?

 

Do you have an example?

 

I have only seen pn jusctions considered boltqage surces in solar cells and simulations dimulations sometimes use the rectifier diodes as sources.

 

When we do DC analysis we usually replace a forward biased diode with a 0.7V or so voltage source.
We can do this because diode voltage drop is almost constant versus the diode current.
http://en.wikipedia.org/wiki/Diode_modelling#Ideal_diode_in_series_with_voltage_source