Anisotropic thermal expansion in pentacene and perfluoropentacene: Effects of molecular packing motif and fixation at the interface

Abstract

Thermal expansion coefficients of molecular solids are typically significantly larger than those of inorganic materials. Since they are furthermore highly anisotropic, the molecular arrangement and consequently the intermolecular orbital overlap strongly depend on temperature, hence also affecting the energetics of optoelectronic excitations and the efficiency of charge transfer processes. Here, we report on the precise determination of the anisotropic thermal expansion coefficients of the organic semiconductor pentacene in its solid state. We compare the thermal expansion coefficients of three different pentacene polymorphs and observe distinct differences between both pentacene bulk polymorphs and the interface-stabilized thin film phase. By comparing epitaxial films with films prepared on weakly interacting, amorphous substrates, we identify a notable influence of the substrate fixation on the thermal expansion in thin pentacene films. Furthermore, the results for pentacene are compared to the thermal expansion of perfluoropentacene, where an exceptionally large vertical thermal expansion coefficient is found in the substrate-mediated π-stacked polymorph. The present study underlines the importance of thermal expansion for the interpretation of temperature-dependent spectroscopic measurements and device characterizations since the notable changes in the unit cell geometries severely affect the intermolecular coupling and thus the excitonic energetics.

Perfluorinated Acenes: Crystalline Phases, Polymorph-Selective Growth, and Optoelectronic Properties

“…This must be taken into account when making a precise comparison of the different crystal structures, since the thermal expansion coefficient of molecular solids are typically one order of magnitude larger than that of covalently bound inorganic semiconductors (such as e.g. Si or GaAs). − In addition, due to the shape anisotropy of rod- or platelet-shape molecules also the expansion is rather anisotropic along different crystalline directions, which affects the interlayer distance and the relative molecular orientation (e.g., herringbone angle) and even can lead to negative expansion coefficients for specific directions. , This effect is often overlooked and carries the risk that crystal structures measured at different temperatures are interpreted as different polymorphs.…”

Tailoring the Optical Response of Pentacene Thin Films via Templated Growth on Hexagonal Boron Nitride

“…5 In thinner pentacene films such as those investigated here, the thermal expansion mismatch between the film and substrate can induce strain that changes the temperature dependence of the optical response. 9,28 The observed pentacene/hBN PL behavior closely follows that of pentacene/polyethylene terephthalate (PET), 28 in accord with the similar in-plane thermal expansion coefficients of 5−15 × 10 −5 K −1 for PET 39 and 2−4 × 10 −5 K −1 for hBN, 40 both of which are close to the bulk pentacene value of 1.1 × 10 −4 K −1 along the crystallographic a-direction and 2.9 × 10 −5 K −1 along the crystallographic b-direction. 28 The significantly lower coefficient of thermal expansion for SiO 2 (5 × 10 −7 K −1 ) 41 leads to a larger strain in the pentacene film at lower temperature, which shifts the PL to higher energy.…”

“…Pentacene is a technologically relevant organic semiconductor, both for its high mobility in thin-film transistors and as a model system for carrier multiplication in organic photovoltaics via singlet fission . Since the optoelectronic properties of pentacene thin films strongly depend on their morphology, − control over their growth microstructure has been an area of intense study. − However, the relationship between thin-film structure and optical properties of pentacene films has been investigated primarily on conventional inorganic substrates, most commonly amorphous SiO 2 . These studies have demonstrated some degree of control over optical anisotropy through constrained growth and tunable molecular orientation , but require fabrication or annealing procedures that are unique to the interface between pentacene and amorphous SiO 2 and do not exploit molecular templating that could be enabled by atomically well-defined substrates.…”

Advances in Functional Nanomaterials Science

“…[183,184] Furthermore, pentacene (PEN) and perylene were exploited for excitonic studies of the various crystalline phases, [185,186] with temperature-dependent studies revealing exciton energy shifts as a consequence of strain at interfaces. [187,188] Additionally, individual molecular domains were probed optically for hetero-epitaxial organic films of perfluoropentacene (PFP) on alkali halide substrates. [189] The correlation between molecular packing motifs and singlet-exciton fission processes was studied by Witte and co-workers via time-and polarization-resolved pump-probe experiments, [190] as part of a broader effort to understand crystalline packing motifs of organic semiconductors.…”