  We investigated poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C61 butyric acid
methyl ester bulk heterojunction (BHJ) solar cells by means of pulsed
photocurrent, temperature dependent current-voltage and capacitance-voltage
measurements. We show that a direct transfer of Mott-Schottky (MS) analysis
from inorganic devices to organic BHJ solar cells is not generally appropriate
to determine the built-in potential, since the resulting potential depends on
the active layer thickness. Pulsed photocurrent measurements enabled us to
directly study the case of quasi flat bands (QFB) in the bulk of the solar
cell. It is well below the built-in potential and differs by diffusion-induced
band-bending at the contacts. In contrast to MS analysis the corresponding
potential is independent on the active layer thickness and therefore a better
measure for flat band conditions in the bulk of a BHJ solar cell as compared to
MS analysis.
