• cristalli organici
• Density Functional Theory
• OFET

Organic materials as polymers and molecular crystals are very important in
the modern society. We can find them in packaging processes or in
fondamental components of electronic devices like TV, computers,
mobilephones and so on. They are generally named plastics due to the
properties which make them easily modeled. Indeed the mechanical, optical or conductive properties of organic materials can be tuned modifing their
chemical components.
In the last 30 years organic compounds have attracted strong attention
because they are lighter, more flexible and less expensive than inorganic
materials. This does not mean that plastics can substitute inorganic
compounds like silicon. The organic conductors/semiconductors in fact can
be used in complementary applications where their peculiar properties, like
machanical flexibility, low cost, ease of fabrication and large area
applicability are relevant.
Charge transport in organic materials is of enormous interest. In 1963 Weiss
et al. reported high conductivity in iodine-doped oxidized polypyrrole.
This discovery together with the one made by Shirakawa et al. in 1977
marks the birth of the Organic electronics. In fact in 1977 Shirakawa et al.
showed that the conductivity in polyacetylene increases several orders of
magnitude in presence of dopants like halogens. Shirakawa, MacDiarmid,
Heeger received the Nobel Prize in Chemistry for this discovery.
There are many branches of the research and development where organic
materials are crucial: Flexible displays, electronic papers and organic solar
cells, Organic Field Eeffct Transistors (OFET), and Organic Light Emitting Diodes (OLED). Recently great interest has been
devoted to organics also for thermoelectric applications.
The present thesis is concentrated on organic molecular crystals. Among
them I present the Perilene Diimide family which has been recently
recognized of great importance as air stable n-type semiconductor. The
high performance in electron transport for such materials can be ascribed
both to high electron affinity and low energy of the lowest unoccupied
molecular orbitals to facilitate electron injection from the contacts, and the
formation of π − π molecular stacking, which enhances electron mobility due to orbital coupling. In this field it is thus of primary importance to
investigate theoretically the relationships between structural, electronic and
optical properties of the chosen organic semiconductors, to determine the
conditions for optimal charge transport in solid state devices.
In the first part of the thesis I start presenting the state of art of molecular
crystal: properties, fabrication and the application in OFET. Indeed this
application is also useful to study the fundamental physics behind the
charge transport. As I said I show the properties of the PDIF-CN2
molecular crystal. The main part of the work is the analysis of a layer of
PDIF-CN2 on a silicon surface. We have studied which would be the best
configuration of the silicon surface to simulate an epitaxial growth. After
that we have studied the electronic properties of the system
Silicon/PDIF-CN2. We also applied an electric field to simulate the gate
potential of an OFET.
To simulate our systems I used two different ab-initio codes: Quantum
ESPRESSO, CRYSTAL09. They implement the Density functional theory
(DFT), a fundamental theory to solve many-body problem. In the thesis I
explain the theoretical framework of the DFT and the main characteristics
of the codes.