  We report a charge based model to establish the analytical equation,
describing the nature of capacitance-voltage ($C$-$V$) characteristics of
organic solar cells under dark condition (or organic diodes) over a wide range
of voltage, extending from deep reverse bias to well above the built-in
potential. The present model enables to explain the unusual peak in $C$-$V$
characteristics of these devices, as observed experimentally by various groups.
The results are further validated using self-consistent numerical simulation.
Finally, we discuss the discrepancy and limitation of using Mott-Schottky
relation to extract the built-in potential, doping density etc. in organic
solar cells/diodes.
