Growth and physical properties of crystalline rubrene - BOA Bicocca ...

Introduction
Organic compounds showing semiconducting properties, whose study be-
gan in the last decades of the past century, are now moving from being a
pure research interest to their industrial application in the fabrication of a
wide range of electronic devices. Indeed, since the fabrication of the first
field effect transistors (FETs) based on polymers or small molecular weight
organic materials[1–3], these materials are now finding application, alone or
in conjunction with inorganic semiconductors, as the active layers in various
kinds of electronic devices: organic light emitting diodes (OLEDs) used for
displays or environmental illumination[4–7], organic photovoltaic cells[8–10],
radio frequency identification tags (RFIDs)[11], organic lasers[12] and sen-
sors[13]. The market deployment of some of these devices has already begun,
for example in the case of OLEDs.
The interest toward these materials is mainly motivated by the low-cost
techniques required for their processing, their mechanical properties (such
as their flexibility) and the possibility to tailor their electronic properties by
modifying their molecular structure.
A common feature of organic semiconductors is the presence of delocal-
ized π molecular orbitals leading to delocalization of the electronic charge
along the whole molecule. Apart from this common trait, molecules belong-
ing to this class of materials can show a wide range of different properties. A
first classification of these materials is that distinguishing between polymers
and small molecular weight materials, referring to the length of the conju-
gated system. In the case of polymers the active layer is generally deposited
by solution based methods (such as spin-coating), while small molecule ma-
terials are usually deposited by evaporation methods. The latter methods,
while being generally more complicated and requiring a more controlled en-
vironment, lead to greater control over the structure of the deposited layers
and are thus particularly indicated for the study of the fundamental proper-