Polymer field effect transistor as a probe to study injection barriers and donor-acceptor interface

Abstract

The central theme of this thesis is on fabricating and studying (p-channel, n- channel and bilayer) field effect transistor (FET) devices under dark, steady state and transient illumination conditions. This thesis probes injection barriers at metal-semiconducting polymer interface that determine the overall charge injec- tion property of device. FET’s consisting of n-channel acceptor with a coating of optically active donor polymers is studied. Presence of a D-A interface in FET showing n-channel transport is used to study the process charge separation and charge transport occurring in bulk of the acceptor upon photoexciting the donor polymer. Inefficient charge injection/extraction process at the source/drain electrodes is responsible for contact resistance in polymer based FET’s. The presence of the con- tact resistance in devices affect the ideal operation of PFET’s especially in short channel devices, where contact resistance (CR) becomes comparable to channel resistance. CR in polymer FET’s are studied using transmission line method, four- probe measurement and kelvin probe potentiometry. Local doping of polymer near metal interface improves charge injection property, by locally introducing charge carriers near polymer-metal semiconductor interface through a photodoping mech- anism lowering of interfacial barrier is achieved. The approach to study injecting barrier present in a metal-semiconductor interface relies on selective illumination of polymer under the source and drain electrodes and monitoring the electrical characteristics. Observation of asymmetric threshold voltage shift in the electrical measurement for identical photon flux incident on the polymer under source and drain electrodes indicates the presence of higher injecting barrier present at the source in comparison to barrier present at the drain electrode required for charge collection. In order to study charge transfer and transport process at D-A interface, accep- tor based FET were fabricated and optically active donor species were introduced using non-solvent deposition technique without altering the n type electrical charac- teristics. The possibility of exclusively exciting the donor species is used to decouple charge generation and charge transport in the acceptor bulk system. Steady state and transient pulse illumination are used to observe Vg dependent and indepen- dent processes at the D-A interface. Modified version of transient-time of flight is introduced to study the dynamics of photogenerated charge transport at the donor- acceptor interface. Poly(3-hexylthiophene) P3HT as the donor species and [6,6]- phenyl-C61-butyric acid methyl ester (PCBM) and naphthalene-bis(dicarboximide) (N2200) polymer as the acceptor system is chosen for the studies. Simple drift- diffusion transport analysis for photogenerated charge carriers at the D-A interface is carried out to model the transport of charge carriers from the D-A interface to the conducting channel. The highlight of the work shows existence of spatially separated charge-generating layer from charge-transporting layer that is used to extract simultaneous bulk and FET mobility.