| dc.description.abstract | Search for low cost electronic materials has led towards the synthesis and
the employment of organic semiconductors (OSCs), a class of materials
that combine the electronic advantages of semiconducting materials with
the chemical and mechanical benefits of organic compounds. Despite the
intense research effort, new OSCs have usually been discovered by trial and
error and, even retrospectively, it was not always possible to explain why
some materials exhibit better performances than others. A more efficient
approach is now required and, in this respect, the use of computer-aided
materials discovery can be highly beneficial. Increasing numbers of new
OSCs have already been designed and improved through computational
modeling, which requires the efficient simulation of charge transport (CT)
processes taking place in OSC-based devices.
In this thesis we study and compare the relative performances of differ-
ent models in the simulations of charge transport in OSCs.
In the first part we focus on the different properties of organic semicon-
ductors with respect to their inorganic counterpart, their benefits and their
drawbacks, restricting our analysis to organic crystalline semiconductors,
which show the highest mobilities among all OSCs. Then we describe some
of the most widely studied classes of OSC materials, showing some cases
in which theory-guided material design has already been applied leading
towards new materials with improved electronic performances.
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In the second part of this thesis we dwell on the unique physical prop-
erties of organic semiconductors and on the reasons that animates the still
topical debate about the most appropriate theoretical model for the CT de-
scription in these materials. Then, we briefly analyze strengths and draw-
backs of five theoretical models: the Marcus theory, the Fermi Golden Rule
(FGR), the Second Order Cumulant expansion of the density matrix (SOC),
the quantum dynamics, and a recently developed approach, the Transient
Localization Theory (TLT). In particular we describe some approximated
strategies that significantly speed up the computations still ensuring accu-
rate results.
In the third part we apply the abovementioned models to the description
of charge transport in some of the most studied OSCs, comparing their
predictions with experimental data and discussing the relative performances
of each method. Our results show that SOC and TLT predictions are
in good agreement with experimental data, the latter being the method
of choice because of its low computational cost and physically well-sound
assumptions.
In the last part of this thesis we focus on the simulation of CT in
DNA oligomers, a topical issue since long range charge migration makes
DNA a potentially well-suited material for nanoelectronics. Our analysis
reproduces in a quantitative way published experimental data and allows
us to reconcile experimental results disagreeing about the role of thymine
bridges in CT across DNA oligomers. [edited by Author] | it_IT |
