  Electric field dependent capacitance and dielectric loss in
poly(3-hexylthiophene) are measured by precision capacitance bridge. Carrier
mobility and density are estimated from fits to current-voltage and capacitance
data. The capacitance varies largely at lower frequency, and it decreases at
higher electric fields. The negative capacitance at low frequency and high
field is due to the negative phase angle between the dipole field and the ac
signal. The intrinsic carrier density is calculated from fits to the
Mott-Schottky equation, and this is consistent with I-V data analysis. At
higher frequency, the carriers do not follow the ac signal and their density
drops; and the flatband potential increases mainly due to the build-in
potentials within ordered and amorphous regions in the sample.
