Charge transport and excited states in organic semiconductors
Abstract
Organic semiconductors are of increasing technological interest in applications such
as light emitting diodes, field effect transistors, and photovoltaic devices In order to
reveal the basic principles behind these organic semiconductors, charge transport
theory in these organic materials has been introduced and has been receiving
increasing attention over the last few years. Although excitons are known to interact
with free charges, the effect that excited states may have on the charge transport is
not generally considered in the field of organic electronics. This occurs even though
organic light emitting diodes (OLEDs) are known to contain large numbers of triplet
states during operation. Indeed, it is quite possible that the mobility in working
devices may well be a function of drive current, as the excited state population will
change with operating conditions. This work is thus motivated by both technological
and fundamental scientific interest.
In this thesis, the hole mobilities in both poly-(3-hexylthiophene) (P3TH) and N,N’-
diphenyl-N,N’-bis(3-methylphenyl)-(1,1’-biphenyl)-4,4’-diamine (TPD) devices
( P HT cm Vs
Hole m 3 » 5.0´10-5 2 / , TPD cm Vs
Hole m » 5.0´10-4 2 / ) have been measured, and
observed a remarkable mobility reduction (~15%) in ambipolar samples (in both
P3HT and TPD) after applying a small DC offset bias. This correlated to the turn-on
voltage in I-V characterization, and the luminescence in the ambipolar TPD sample.
In the unipolar sample, however, there is no such behaviour. This strongly suggests
that the reduction of the hole mobility is due to site blocking/interacting caused by
the excited triplet states.
In further experiments in the presence of a magnetic field (500 mT), results an
increase in the mobility ( ~ 5%) and steady state current density in ambipolar
samples only, this is consistent with magnetically mediated inter-conversion of
4
(blocking/interacting) triplet states to the singlet states. The correlation between the
magnetic mobility increase and the steady state current increase offers direct
evidence for a microscopic mechanism behind organic magneto resistance (OMR).
Given the experimental evidence, we conclude that excitons (specifically triplet
states play a critical role in charge transport in organic semiconductors.
Authors
Song, JingyaoCollections
- Theses [4490]