Investigation of DC and AC Charge Transport in Semiconducting Polymers and Devices

Abstract

Semiconducting polymer materials have several applications in electronics and optoelectronic devices. The charge transport in these complex systems is yet to be fully understood to optimize the performance of the devices. In organic semiconductors, the molecular level organisation and the possibility of disorder affect electrical and optical properties. Several parameters like intra and inter-chain interactions, interfacial interaction with electrodes, processing procedures, etc. complicate the charge transport. The aim and goal of this Ph.D. work is to understand the charge transport phenomenon in semiconducting polymer devices and its dependence on carrier concentration and disorder. A comprehensive study requires control of crucial parameters like growth temperature, doping level, the work function of the electrodes, etc. This Thesis consists of experimental studies of charge transport in semiconducting polymers and random copolymers.

We have observed a complete Lampert triangle formation in poly(3,4-ethylenedioxythiophene) [PEDOT] in SS/PEDOT/Ag device structure bounded by three limiting curves: Ohmic, trap limited/filling space charge limited conduction (TFL-SCLC), and trap-free/trap-filled space charge limited conduction (TF-SCLC). Further, we explored the DC and AC conductivity in these devices, the Ac conductivity data are analyzed by using Jonscher's double power law. The DC conductivity shows a possible crossover to β = ¼ (VRH: Variable-range hopping) at low temperatures and to β = 1 (NHH: Nearest-neighbor hopping) at high temperatures. The AC conductivity also shows the crossover transport from the quantum mechanical tunneling model (QMT) at low frequency to the correlated barrier hopping model (CBH) at a higher frequency. Co-polymerisation is an important strategy of polymer synthesis which can lead to significant modification of desired properties compared to its pristine polymer. We have shown such modification in a random copolymer of Polypyrrole [PPy] and poly(3-hexylthiophene) [P3HT] in presence of light and dark.

In the last part of the work, disorder and scattering in platinum nanowires were investigated through resistance (Bloch-Grüneisen formalism) and magnetoresistance behavior (Kohler’s rule).