A PV module will not start conducting from an external DC source
until the voltage goes above the modules open circuit voltage.
Then is starts to look like a forward biased diode (actually, a
bunch of diodes in series)...
So, a 12V module will take up to, and probably more than
around 21 Volts before it starts conducting. This voltage
of course depends on the temperature of the PV module and
how many cells it has in series. The Voc is a fairly
good estimate though for a typical mono/poly crystaline
module.
boB