Electro-optic charge transport in semiconducting polymers

PhD;2023;Physics

In this thesis, I have discussed the electric field dependence of photoconductivity of regioregular poly(3-hexylthiophene) [rr-P3HT] and its sensitization with Squaraine (SQ) dye. Even 1 wt % of SQ dye enhances the transport of photogenerated carriers as probed by electric field-dependent photovoltaic signal. This relative variation is compared with the electric field-dependent photovoltaic data in pristine P3HT film. The SQ dye modifies the transport by reducing the deep-level traps within the sample and improves the transport of photogenerated carriers. Electric field-dependent capacitance and dielectric loss in P3HT are measured by a precision capacitance bridge. Carrier mobility and density are estimated from fits to current voltage and capacitance data. The capacitance varies largely at lower frequencies and 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 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 frequencies, the carriers do not follow the AC signal and its density drops, and the flat band potential increases mainly due to the built-in potentials within ordered and amorphous regions in the sample. The semiconducting properties of rr-P3HT 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 shows that DC bias voltage reduces two semi-circular arcs and their size. Also, the carrier variation by light and chemical doping alter 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 does not alter much in disordered regions. In polymer (poly(3-hexylthiophene)-poly(methyl methacrylate) [P3HT-PMMA]) blend, the charge transport mechanisms are investigated from current density-voltage (J-V) characteristics and impedance spectroscopy (IS) studies. In the undoped sample, the J-V characteristic shows two different transport mechanisms for low voltage (Ohmic) and higher voltage (space charge limited) regions. However, after doping, the Ohmic transport dominates over space charge limited conduction (SCLC), as confirmed by J-V and Nyquist plot analysis. In the undoped sample, the difference in the value of tao, from low bias voltage (Ohmic) to high bias voltage (space-charge), shows the role of injected carriers, whereas tao remains unchanged in the doped sample. The temperature and electric field-dependent conductivity have been carried out for three different levels of doped PEDOT free-standing films. Raman spectroscopy shows structural variations at different doping levels. The highest doped sample (S5) is in the critical regime of metal-insulator transition, whereas the lower doping level two samples, S10 and S15, are in the insulator regime. S5 has an electric field dependency in a low-temperature regime due to weakened barriers present, whereas for S10 and S15, hardly any electric field dependent is observed. This study differentiates the charge transport mechanism in terms of conductivity ratio, R/Lc, and hopping energy for three different PEDOT films