Thickness‐Driven Orthorhombic to Triclinic Phase Transformation in Pentacene Thin Films

Abstract

Al; 70 wt.-% Al content; from Technanogy) components were physically mixed and dispersed in hexane. The dispersion was then ultrasonicated for 20 min, and thereafter the mixture was heated at about 100 C to vaporize the dispersant. The field of plastic electronics has generated significant recent interest due to the excellent electrical properties measured from several organic materials. Understanding the relationship between thin-film processing parameters and the resulting crystal structure will be critical in integrating organic materials into useful devices. The processing/structure/properties relationship in ordered organic materials for electronic applications has recently been reviewed, with focus on aromatic molecules. [1,2] The aromatic hydrocarbon pentacene is currently under investigation for use as the active layer in flexible electronic devices such as thin-film field-effect transistors.[3±5] Pentacene thin films have shown tendencies for polymorphism, and a unique ªthin filmº phase of pentacene has been characterized by many researchers.[3±10] In a thin-film field-effect transistor, charge-carrier transport through the active layer will take place in the region of the film nearest to the dielectric layer. It is likely that this region will consist solely of the thin-film phase polymorph. Due primarily to the small crystal sizes in these films, however, the detailed structure of this phase has remained elusive. Using electron diffraction (ED), low voltage electron microscopy (LVEM), lowdose high resolution electron microscopy (HREM) and molecular simulations we have determined that thin films of COMMUNICATIONS

X-ray Structural Investigation of Nonsymmetrically and Symmetrically Alkylated [1]Benzothieno[3,2-b]benzothiophene Derivatives in Bulk and Thin Films

“…Hence, these findings strengthen the idea that substrate induced phases may be surface selected polymorphs that can be stabilized only in the vicinity of the underlying substrate as proposed by previous studies. 18,22 Noticeably, in the present particular case of alkylated BTBTs, it appears that the equilibrium stable bulk phase is also the one favored in thin films close to the substrate surface. This is an important result as the presence of substrate induced phases is a quite commonly observed phenomenon for conjugated molecules, so that its nonobservation can be judged as an exception and even might be a reason for the unprecedented charge transport properties pointed out for this family of compounds.…”

“…11,12 Subsequently, numerous studies have been undertaken to investigate the crystal structure 13−17 and the origin 18−23 of these thin film phases considering the high relevance of pentacene for thin film organic electronic applications. 2−4 Surface energy 18,19 and structure, 20,21 substrate roughness, 22,23 as well as processing conditions 24−26 have been reported to play a role in the preferential crystallization of a given structural organization close to the substrate. Remarkably, recent theoretical studies based on Molecular Dynamics (MD) simulations 20−23 brought significant insight on the influence of substrate/molecules and molecules/molecules interactions on polymorph selection close to the substrate−molecule interface.…”

Real-time X-ray diffraction measurements of structural dynamics and polymorphism in diindenoperylene growth

“…While the b axis only changes slightly, the a axis expands by 2.6% during growth, which may be due to a compressively strained first monolayer, but can also be explained by a changing molecular tilt angle. Molecules in a more upright standing and therefore more densely packed first ML have been found to minimise the surface energy in pentacene films [30], and similarly one may hypothesise that DIP molecules adopt a more upright orientation in the first ML.…”

Enhancing 2D growth of organic semiconductor thin films with macroporous structures via a small-molecule heterointerface

“…The (00l) reflections in the q z direction (out-of-plane) indicate that pentacene molecules have stand-up orientation on TSB3. The two intense in-plane reflections, appearing vertically at a given q xy , can be indexed to {1, ± 1} and {0, 2}, respectively; each reflection index represents two distinct crystalline polymorphs of pentacene thin film, which are referred to as the pseudoorthorhombic 'thin-film' phase and the triclinic 'bulk' phase 28 . The multiple reflection spots in these vertical 'Bragg-rods' suggest that our pentacene films consist of ordered multi-stacked layers in both vertical and lateral directions 29 .…”