Crystal structure of fiber structured pentacene thin films

This PhD thesis presents a technique based on the grazing incidence
crystal truncation rod (GI-CTR) X-ray diffraction method used to
solve the crystal structure of substrate induced fiber structured
organic thin films. The crystal structures of pentacene thin films
grown on technologically relevant gate dielectric substrates are
reported. It is widely recognized, that the intrinsic charge
transport properties in organic thin film transistors (OTFTs)
depend strongly on the crystal structure of the organic
semiconductor layer. Pentacene, showing one of the highest charge
carrier mobilities among organic semiconductors, is known to
crystallize in at least four polymorphs, which can be distinguished
by their layer periodicity d(001). Only two polymorphs (14.4 Å and
14.1 Å), grow as single crystals and their detailed crystal
structure has been solved with standard crystallography techniques.
The substrate induced 15.4 Å polymorph, the so called pentacene
thin-film phase, is the most relevant for OTFT applications, since
it grows at room temperature on technologically relevant gate
dielectrics. However, the crystal structure of the pentacene
thin-film phase has remained incomplete as it only grows as a fiber
structured thin film. In this thesis, the GI-CTR X-ray diffraction
technique is extended to fiber structured thin films. The X-ray
diffraction experiments were carried out at the synchrotron source
beamline W1 at HASYLAB in Hamburg, in order to obtain enough
diffraction data for the determination of the crystal structure as
pentacene thin films only grow as ultra thin films with crystal
grains as small as 0.4μm. Pentacene thin films are also known to be
sensitive to environmental conditions, such as light and oxygen.
For this reason, the X-ray synchrotron measurements were performed
in-situ. A portable ultra high vacuum growth chamber equipped with
a rotatable sample holder and a beryllium window was built in order
to perform X-ray measurements of up to four samples right after the
thin film growth process without breaking the vacuum. Parallel to
this, a versatile software package coded with Matlab in order to
simulate, analyze and fit the complex data measured at the
synchrotron source was developed. The complete crystal structure of
the 15.4 Å pentacene thin-film polymorph grown on four model types
of gate dielectric materials, amorphous silicon dioxide (a−SiO2),
octadecyltrichlorosilane-treated a−SiO2 (OTS), Topas
(“thermoplastic olefin polymer of amorphous structure”) and
polystyrene films, was solved. It was found, that the unit cell
parameters are identical within measurement precision on all
measured substrates. The crystal structure belongs to the space
group P-1 and was found to be triclinic with the following lattice
parameters: a = 5.958 ± 0.005 Å, b = 7.596 ± 0.008 Å, c = 15.61 ±
0.01 Å, alpha = 81.25 ± 0.04°, beta = 86.56 ± 0.04° and 2 gamma =
89.80 ± 0.10°. The unit cell volume V = 697 Å^3 is the largest of
all pentacene polymorphs reported so far. However, the molecular
arrangement within the unit cell was found to be substrate
dependent. Here, the following parameters are reported: The
herringbone angle is 54.3°, 55.8°, 59.4° and 55.1° for a−SiO2, OTS,
Topas and polystyrene, respectively. The tilts of the two molecular
axes (theta_A, theta_B) are (5.6°, 6.0°), (6.4°,6.8°), (5.6°, 6.3°)
and (5.7°, 6.0°) for a−SiO2, OTS, Topas and polystyrene,
respectively. To conclude, it was shown that the molecular
orientation in the unit cell differs among substrates while the
unit cell dimensions of the 15.4 Å pentacene polymorph are
identical. This indicates that substrate effects have to be
included if one aims on understanding the molecular structure of
the thin-film phase in detail. The crystal structures reported here
provide a basis to apply techniques such as density functional
methods to investigate intrinsic charge transport properties and
optical properties of organic thin film devices on a molecular
level. In previous studies it was observed that different
substrates vary the charge carrier mobility in OTFTs. The substrate
dependent crystal structures observed here could be one reason for
this variation. This topic may lead ultimatively to a controlled
finetuning of intrinsic charge transport properties. The
experimental approach to determine the crystal structure developed
here can be easily applied to a wide range of organic thin film
systems used in organic electronic devices.