The current density–voltage (J–V) characteristics in stainless steel/poly(3,4-ethylenedioxythiophene)/Ag devices show the formation of the complete Lampert triangle (ΔABC) bounded by three limiting curves: Ohmic, trap-limited/filling space charge limited conduction, and trap-free/trap-filled space charge limited conduction. From the analysis of the Lampert triangle, values for carrier density (p0) ∼ 0.72 × 1013/cm3, mobility (μp) ∼ 77.47 × 10−4 cm2/V s, and transit time (tt) ∼ 10−12 s are obtained and also the transition voltages for different conduction mechanisms are estimated. The relaxation processes in bulk and interface are observed to be different from temperature-dependent impedance measurements. Estimated values of relaxation times are interface (τ1) ∼ 10−3 s and bulk (τ2) ∼ 10−6 s. Two parallel RQ (Q: constant phase element) circuits in series are used to fit the impedance data; however, the model varies for data at 110 and 120 K (two parallel CQ circuits in series). Since the samples have doped carriers, the activation energies are low (< 70 meV), and relaxation times follow Arrhenius behavior.
The semiconducting properties of regioregular poly(3-hexylthiophene) are characterized by impedance measurements, from 40 Hz to 100 MHz. X-ray diffraction shows the presence of both ordered and disordered regions. The analysis of impedance data by Nyquist plots show two semi-circular arcs, and its size is reduced by d.c. bias. Also, the carrier variation by light and chemical doping alters the shape and size of arcs. The fits to the data and equivalent circuits show considerable changes in the resistive, capacitive and constant-phase element parameters as the carrier density increases. The increase in carrier density reduces the relaxation time in ordered regions, and it does not alter much in disordered regions.
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 [Formula: see text]–[Formula: see text] 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.
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