  We investigated the photocurrent in poly(3-hexylthiophene-2,5-diyl)
(P3HT):[6,6]-phenyl-C$_{61}$ butyric acid methyl ester (PCBM) solar cells by
applying a pulsed measurement technique. For annealed samples, a point of
optimal symmetry (POS) with a corresponding voltage $V_\text{POS}$ of
0.52--0.64 V could be determined. Based on macroscopic simulations and results
from capacitance--voltage measurements, we identify this voltage with flat band
conditions in the bulk of the cell, but not the built-in voltage as proposed by
[Ooi et al., J. Mater. Chem. 18 (2008) 1644]. We calculated the field dependent
polaron pair dissociation after Onsager--Braun and the voltage dependent
extraction of charge carriers after Sokel and Hughes with respect to this point
of symmetry. Our analysis allows to explain the experimental photocurrent in
both forward and reverse directions. Also, we observed a voltage--independent
offset of the photocurrent. As this offset is crucial for the device
performance, we investigated its dependence on cathode material and thermal
treatment. From our considerations we gain new insight into the photocurrent`s
voltage dependence and the limitations of device efficiency.
