Structural order in perfluoropentacene thin films and heterostructures with pentacene

Excited States Dynamics at Pentacene/Perfluoropentacene Interfaces: A Femtosecond Time-Resolved Second Harmonic Generation Study

Understanding the dynamics of excited states after optical excitation at donor-acceptor (D/A) interfaces is of paramount importance for improving the efficiency and performance of optoelectronic devices. Here, we studied the ultrafast excited state dynamics after optical excitation at interfaces between the electron donor (D) pentacene (PEN) and the electron acceptor (A) perfuoropentacene (PFP) as well as within the single compounds (PEN and PFP) using femtosecond (fs) time-resolved second harmonic generation (SHG). In the single compounds singlet fission is observed on a time scale of around 200 fs. In the bilayer systems a huge SHG intensity rise is observed due to the creation of charge transfer states at the interface and accordingly to formation of a local electric field within tens of picoseconds. The local electric field and therefore the SHG signal intensity from the interface of PEN/PFP bilayer is much more intense compared to the PFP/PEN system because the PFP and PEN intermixing at the PEN/PFP interface is higher. Accordingly a population of defect states on a time scale of 55±12 ps has been proposed for PEN/PFP. Our study provides important insights into D/A charge transfer properties, which is needed for the understanding of the interfacial photophysics of pentacene-based organic compounds.

Molecular orientation-dependent energetic shifts in solution-processed non-fullerene acceptors and their impact on organic photovoltaic performance

The non-fullerene acceptors (NFAs) employed in state-of-art organic photovoltaics (OPVs) often exhibit strong quadrupole moments which can strongly impact on material energetics. Herein, we show that changing the orientation of Y6, a prototypical NFA, from face-on to more edge-on by using different processing solvents causes a significant energetic shift of up to 210 meV. The impact of this energetic shift on OPV performance is investigated in both bilayer and bulk-heterojunction (BHJ) devices with PM6 polymer donor. The device electronic bandgap and the rate of non-geminate recombination are found to depend on the Y6 orientation in both bilayer and BHJ devices, attributed to the quadrupole moment-induced band bending. Analogous energetic shifts are also observed in other common polymer/NFA blends, which correlates well with NFA quadrupole moments. This work demonstrates the key impact of NFA quadruple moments and molecular orientation on material energetics and thereby on the efficiency of high-performance OPVs.

'Molecular Beam Epitaxy' on Organic Semiconductor Single Crystals: Characterization of Well-Defined Molecular Interfaces by Synchrotron Radiation X-ray Diffraction Techniques

Epitaxial growth, often termed "epitaxy", is one of the most essential techniques underpinning semiconductor electronics, because crystallinities of the materials seriously dominate operation efficiencies of the electronic devices such as power gain/consumption, response speed, heat loss, and so on. In contrast to already well-established epitaxial growth methodologies for inorganic (covalent or ionic) semiconductors, studies on inter-molecular (van der Waals) epitaxy for organic semiconductors is still in the initial stage. In the present review paper, we briefly summarize recent works on the epitaxial inter-molecular junctions built on organic semiconductor single-crystal surfaces, particularly on single crystals of pentacene and rubrene. Experimental methodologies applicable for the determination of crystal structures of such organic single-crystal-based molecular junctions are also illustrated.

Spatial Anisotropy of Charge Transfer at Perfluoropentacene-Pentacene (001) Single-Crystal Interfaces and its Relevance for Thin Film Devices

Archetypal donor-acceptor (D-A) interfaces composed of perfluoropentacene (PFP) and pentacene (PEN) are examined for charge transfer (CT) state formation and energetics as a function of their respective molecular configuration. To exclude morphological interference, our structural as well as highly sensitive differential reflectance spectroscopy studies were carried out on PFP thin films epitaxially grown on PEN(001) single-crystal facets. Whereas the experimental data supported by complementary theoretical calculations confirm the formation of a strong CT state in the case of a cofacial PFP-PEN stacking, CT formation is energetically less favorable and thus absent for the corresponding head-to-tail configuration as disclosed for the first time. In view of technological implementations, the knowledge gained on the single-crystal references is transferred to thin-film diodes composed of either stacked PFP/PEN bilayers or mixed PFP:PEN heterojunction interfaces. As demonstrated, their electronic and electroluminescent behavior can be consistently described by the absence or presence of interfacial CT states. Thus, our results hint at the thorough design of D-A interfaces to achieve the highest device performances.

Molecular Semiconductors for Logic Operations: Dead-End or Bright Future?

The field of organic electronics has been prolific in the last couple of years, leading to the design and synthesis of several molecular semiconductors presenting a mobility in excess of 10 cm² V^-1 s^-1 . However, it is also started to recently falter, as a result of doubtful mobility extractions and reduced industrial interest. This critical review addresses the community of chemists and materials scientists to share with it a critical analysis of the best performing molecular semiconductors and of the inherent charge transport physics that takes place in them. The goal is to inspire chemists and materials scientists and to give them hope that the field of molecular semiconductors for logic operations is not engaged into a dead end. To the contrary, it offers plenty of research opportunities in materials chemistry.

Singlet exciton fission via an intermolecular charge transfer state in coevaporated pentacene-perfluoropentacene thin films

Singlet exciton fission is a spin-allowed process in organic semiconductors by which one absorbed photon generates two triplet excitons. Theory predicts that singlet fission is mediated by intermolecular charge-transfer states in solid-state materials with appropriate singlet-triplet energy spacing, but direct evidence for the involvement of such states in the process has not been provided yet. Here, we report on the observation of subpicosecond singlet fission in mixed films of pentacene and perfluoropentacene. By combining transient spectroscopy measurements to nonadiabatic quantum-dynamics simulations, we show that direct excitation in the charge-transfer absorption band of the mixed films leads to the formation of triplet excitons, unambiguously proving that they act as intermediate states in the fission process.

Widely Dispersed Intermolecular Valence Bands of Epitaxially Grown Perfluoropentacene on Pentacene Single Crystals

Strong intermolecular electronic coupling and well-ordered molecular arrangements enable efficient transport of both charge carriers and excitons in semiconducting π-conjugated molecular solids. Thus, molecular heteroepitaxy to form crystallized donor-acceptor molecular interfaces potentially leads to a novel strategy for creating efficient organic optoelectronic devices via the concomitance of these two requirements. In the present study, the crystallographic and electronic structures of a heteroepitaxial molecular interface, perfluoropentacene (PFP, C₂₂F₁₄) grown on pentacene single crystals (Pn-SCs, C₂₂H₁₄), were determined by means of grazing-incidence X-ray diffraction (GIXD) and angle-resolved ultraviolet photoelectron spectroscopy (ARUPS), respectively. GIXD revealed that PFP uniquely aligned its primary axis along the [11̅0] axis of crystalline pentacene to form well-crystallized overlayers. Valence band dispersion (at least 0.49 eV wide) was successfully resolved by ARUPS. This indicated a significant transfer integral between the frontier molecular orbitals of the nearest-neighbor PFP molecules.

Molecular Reorientation during the Initial Growth of Perfluoropentacene on Ag(110)

Perfluoropentacene (PFP) is an organic material that has been widely studied over the last years and has already found applications in organic electronics. However, fundamental physical questions, such as the structural formation and the preferential orientation of the molecules during deposition on metal surfaces, are still not fully understood. In this work, we report on a unique in-plane molecular reorientation during the completion of the first monolayer of PFP on the Ag(110) surface. To characterize the molecular alignment, we have monitored the deposition process in real time using polarization-dependent differential reflectance spectroscopy and reflectance anisotropy spectroscopy. Abrupt changes in the optical signals reveal an intricate sequence of reorientation transitions of the PFP molecules upon monolayer completion and during the formation of the second monolayer, eventually leading to a full alignment of the long molecular axis along the [001] direction of the substrate and an enhanced structural ordering. Scanning tunneling microscopy and low-energy electron diffraction confirm the observed molecular reorientation upon monolayer compression and provide further details on the structural and orientational ordering of the PFP monolayer before and after compression.

Crystallization of Tyrian Purple (6,6'-Dibromoindigo) Thin Films: The Impact of Substrate Surface Modifications

The pigment 6,6'-dibromoindigo (Tyrian purple) shows strong intermolecular hydrogen bonds and the film formation is, therefore, expected to be influenced by the polar character of the substrate surface. Thin films of Tyrian purple were prepared by physical vapor deposition on a variety of substrates with different surface energies: from highly polar silicon dioxide surfaces to hydrophobic polymer surfaces. The crystallographic properties were investigated by X-ray diffraction techniques such as X-ray reflectivity and grazing incidence X-ray diffraction. In all cases, crystallites with "standing" molecules relative to the substrate surface were observed independently of the substrate surface energy. In the case of polymer surfaces, additional crystallites are formed containing "lying" molecules with their aromatic planes parallel to the substrate surface. Small differences in the crystallographic lattice constants were observed as a function of substrate surface energy, the corresponding small changes in the molecular packing are explained by a variation of the hydrogen bond geometries. This work reveals that despite the limited influence of the surface energy on the molecular orientation, the crystalline packing of Tyrian purple within thin films is altered and slightly different structures form.

Surface-Induced Phase of Tyrian Purple (6,6'-Dibromoindigo): Thin Film Formation and Stability

The appearance of surface-induced phases of molecular crystals is a frequently observed phenomenon in organic electronics. However, despite their fundamental importance, the origin of such phases is not yet fully resolved. The organic molecule 6,6'-dibromoindigo (Tyrian purple) forms two polymorphs within thin films. At growth temperatures of 150 °C, the well-known bulk structure forms, while at a substrate temperature of 50 °C, a surface-induced phase is observed instead. In the present work, the crystal structure of the surface-induced polymorph is solved by a combined experimental and theoretical approach using grazing incidence X-ray diffraction and molecular dynamics simulations. A comparison of both phases reveals that π-π stacking and hydrogen bonds are common motifs for the intermolecular packing. In-situ temperature studies reveal a phase transition from the surface-induced phase to the bulk phase at a temperature of 210 °C; the irreversibility of the transition indicates that the surface-induced phase is metastable. The crystallization behavior is investigated ex-situ starting from the sub-monolayer regime up to a nominal thickness of 9 nm using two different silicon oxide surfaces; island formation is observed together with a slight variation of the crystal structure. This work shows that surface-induced phases not only appear for compounds with weak, isotropic van der Waals bonds, but also for molecules exhibiting strong and highly directional hydrogen bonds.

Multiple scattering in grazing-incidence X-ray diffraction: impact on lattice-constant determination in thin films

Dynamical scattering effects are observed in grazing-incidence X-ray diffraction experiments using an organic thin film of 2,2':6',2''-ternaphthalene grown on oxidized silicon as substrate. Here, a splitting of all Bragg peaks in the out-of-plane direction (z-direction) has been observed, the magnitude of which depends both on the incidence angle of the primary beam and the out-of-plane angle of the scattered beam. The incident angle was varied between 0.09° and 0.25° for synchrotron radiation of 10.5 keV. This study reveals comparable intensities of the split peaks with a maximum for incidence angles close to the critical angle of total external reflection of the substrate. This observation is rationalized by two different scattering pathways resulting in diffraction peaks at different positions at the detector. In order to minimize the splitting, the data suggest either using incident angles well below the critical angle of total reflection or angles well above, which sufficiently attenuates the contributions from the second scattering path. This study highlights that the refraction of X-rays in (organic) thin films has to be corrected accordingly to allow for the determination of peak positions with sufficient accuracy. Based thereon, a reliable determination of the lattice constants becomes feasible, which is required for crystallographic structure solutions from thin films.

Revealing nanoscale optical properties and morphology in perfluoropentacene films by confocal and tip-enhanced near-field optical microscopy and spectroscopy

Combining confocal and high resolution near-field optical microscopy and spectroscopy, we propose a sensitive method for determining the local morphology in organic semiconductor thin films.

Controlling Nanostructures by Templated Templates: Inheriting Molecular Orientation in Binary Heterostructures

Precise preparation strategies are required to fabricate molecular nanostructures of specific arrangement. In bottom-up approaches, where nanostructures are gradually formed by piecing together individual parts to the final structure, the self-ordering mechanisms of the involved structures are utilized. In order to achieve the desired structures regarding morphology, grain size, and orientation of the individual moieties, templates can be applied, which influence the formation process of subsequent structures. However, this strategy is of limited use for complex architectures because the templates only influence the structure formation at the interface between the template and the first compound. Here, we discuss the implementation of so-called templated templates and analyze to what extent orientations of the initial layers are inherited in the top layers of another compound to enable structural control in binary heterostructures. For that purpose, we prepared crystalline templates of the organic semiconductors pentacene and perfluoropentacene in different exclusive orientations. We observe that for templates of both individual materials the molecular orientation is inherited in the top layers of the respective counterpart. This behavior is also observed for various other molecules, indicating the robustness of this approach.

Complex Behavior of Caffeine Crystallites on Muscovite Mica Surfaces

Defined fabrication of organic thin films is highly desired in technological, as well as pharmaceutical, applications since morphology and crystal structure are directly linked to physical, electrical, and optical properties. Within this work, the directed growth of caffeine deposited by hot wall epitaxy (HWE) on muscovite mica is studied. Optical and atomic force microscopy measurements reveal the presence of caffeine needles exhibiting a preferable alignment in the azimuthal directions with respect to the orientation of the defined mica surface. Specular X-ray diffraction and X-ray diffraction pole figure measurements give evidence that the β-polymorphic form of caffeine forms on the mica surface. All results consent that caffeine molecules have an edge-on conformation i.e. minimizing their interaction area with the surface. Furthermore, the azimuthal alignment of the long caffeine needle axis takes place along the [11̅0], [100], and [110] real space directions of mica; needles are observed every 60° azimuthally. While mica has a complex surface structure with mirror planes and lowered oxygen rows, the slightly disturbed 3-fold symmetry dictates the crystal alignment. This is different to previous findings for solution cast caffeine growth on mica. For HWE the needles align solely along the mica main directions whereby solution cast needles show an additional needle splitting due to a different alignment of caffeine with respect to the surface.

Diffusion-controlled growth of molecular heterostructures: fabrication of two-, one-, and zero-dimensional C(60) nanostructures on pentacene substrates

A variety of low dimensional C60 structures has been grown on supporting pentacene multilayers. By choice of substrate temperature during growth the effective diffusion length of evaporated fullerenes and their nucleation at terraces or step edges can be precisely controlled. AFM and SEM measurements show that this enables the fabrication of either 2D adlayers or solely 1D chains decorating substrate steps, while at elevated growth temperature continuous wetting of step edges is prohibited and instead the formation of separated C60 clusters pinned at the pentacene step edges occurs. Remarkably, all structures remain thermally stable at room temperature once they are formed. In addition the various fullerene structures have been overgrown by an additional pentacene capping layer. Utilizing the different probe depth of XRD and NEXAFS, we found that no contiguous pentacene film is formed on the 2D C60 structure, whereas an encapsulation of the 1D and 0D structures with uniformly upright oriented pentacene is achieved, hence allowing the fabrication of low dimensional buried organic heterostructures.

Epitaxial growth of π-stacked perfluoropentacene on graphene-coated quartz

Chemical-vapor-deposited large-area graphene is employed as the coating of transparent substrates for the growth of the prototypical organic n-type semiconductor perfluoropentacene (PFP). The graphene coating is found to cause face-on growth of PFP in a yet unknown substrate-mediated polymorph, which is solved by combining grazing-incidence X-ray diffraction with theoretical structure modeling. In contrast to the otherwise common herringbone arrangement of PFP in single crystals and "standing" films, we report a π-stacked arrangement of coplanar molecules in "flat-lying" films, which exhibit an exceedingly low π-stacking distance of only 3.07 Å, giving rise to significant electronic band dispersion along the π-stacking direction, as evidenced by ultraviolet photoelectron spectroscopy. Our study underlines the high potential of graphene for use as a transparent electrode in (opto-)electronic applications, where optimized vertical transport through flat-lying conjugated organic molecules is desired.

Structure and morphology of coevaporated pentacene-perfluoropentacene thin films

The structural properties of coevaporated thin films of pentacene (PEN) and perfluoropentacene (PFP) on SiO(2) were studied using x-ray reflectivity and grazing incidence x-ray diffraction. Reciprocal space maps of the coevaporated thin films with different volume fractions reveal the coexistence of two different molecular mixed PEN-PFP phases together with the pure PEN and PFP crystallites. The crystal structure of PEN:PFP blends does not change continuously with volume fraction, instead the proportion of the appropriate phases changes, as seen from the diffraction analysis. Additional temperature dependent experiments reveal that the fraction of the two mixed PEN-PFP phases varies with growth temperature. The λ-phase (molecular plane parallel to the substrate) is metastable and induced by low growth temperature. The σ-phase (molecular plane nearly perpendicular to the substrate) is thermally stable and nucleates predominantly at high growth temperatures.