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 A and 14.1 A), grow as single crystals and their detailed crystal structure has been solved with standard crystallography techniques. The substrate induced 15.4 A 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 A 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 A, b = 7.596 ± 0.008 A, c = 15.61 ± 0.01 A, alpha = 81.25 ± 0.04°, beta = 86.56 ± 0.04° and 2 gamma = 89.80 ± 0.10°. The unit cell volume V = 697 A^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 A 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.