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.