The latter characteristic is particularly important since charge carrier mobility and transport properties of crystalline π-conjugated molecular materials are typically strongly anisotropic, [5] which underlines the importance of a precise structural control of OSC films at the contact interfaces.Among the newly synthesized OSCs, dinaphthothienothiophene (DNTT) has received particular attention because it combines a remarkably high charge carrier mobility [6,7] with distinct chemical robustness, [8,9] hence making it superior to previous OSC benchmark systems based on pentacene (PEN) or rubrene. [10][11][12] DNTT is a planar polycyclic hydrocarbon (PAH) with similar shape and intermolecular interaction as PEN [13] and likewise adopts a layered crystal structure. In the (001) planes, which are the typical surface planes in DNTT crystals, the molecules are uprightly oriented and exhibit a face-on-edge herringbone packing arrangement. [6] Despite their structural similarity, both the materials exhibit, however, different growth characteristics. For example, a notable dewetting was found upon growth of DNTT films on SiO 2 and other dielectrics as well as on graphite substrates, [14] while smoother PEN films are formed on these substrates. [15,16] On the other hand, PEN (as well as many other PAH) films that are deposited onto crystalline metal substrates reveal a distinct dewetting and island formation beyond the first monolayer (ML), [17][18][19] which is attributed to the structural misfit between the flat-lying chemisorbed molecules in the seed layer and the herringbone motif in the bulk crystal structure. [20] Despite their importance for device applications, interfaces between DNTT and metals are hardly studied so far. The only exception is the previous work of Hasegawa et al. who studied the initial stage of DNTT film growth on Au(111) and reported a transition from an initially diluted phase into a densely packed adlayer which comprises a mixture of flat-lying and inclined molecules. [21] While this growth scenario is of particular interest, as it would allow adjusting the interface structure to the crystal bulk phase, no clear evidence for such a tilting was given and also the structure of thicker films was not analyzed. A similar mechanism has been identified before for the growth of PEN Dinaphthothienothiophene (DNTT) is a promising new organic semiconductor which combines high charge carrier mobility with chemical robustness. Although the properties of organic electronic devices are largely determined by the interfaces with electrodes, the interface between DNTT and metals is hardly studied so far. Here, the interface structures of DNTT on Ag(111) and multilayers are examined and they are compared with films grown on polycrystalline and (111) surfaces of silver and gold. In the seedlayer regime, two different interface structures formed by exclusively flatlying molecules and a herringbone arrangement are identified for increased coverages. Combining low energy electron diffraction, scanning tunneling micro...

Controlling the crystallinity of organic thin films is an important aspect in the improvement of organic electronic devices. However, because of high molecular mass, structural anisotropy, and weak intermolecular van der Waals bonding, crystalline ordering is not easily accomplished. While film preparation at elevated substrate temperature often improves the crystalline quality, this approach cannot be applied to temperature-sensitive materials such as plastic foils used as substrates for flexible electronics. Here, we examine in detail a low-temperature approach to improve film crystallinity by using ultrathin pentacene (PEN) buffer layers that allow crystalline growth of buckminsterfullerene (C 60 ) thin films while without such buffer layers, only amorphous fullerene films are formed upon room-temperature deposition on various support substrates. Remarkably, this effect depends critically on the thickness of the PEN buffer and requires a thickness of at least two monolayers to induce crystalline growth, whereas a buffer layer consisting of a monolayer of PEN again yields amorphous C 60 films. Combining crystallographic investigations by X-ray diffraction and atomic force microscopy measurements, we determine distinct nucleation sites on buffer layers of different thickness, which are correlated to the amorphous, respectively crystalline C 60 islands. Our microscopic analysis reveals distinct differences for the nucleation and diffusivity of fullerenes on the PEN monolayer and on thicker buffer layers, which are attributed to the molecular arrangement in the PEN monolayer. Finally, we show that the crystalline C 60 films are exclusively (111)-oriented and the fullerene islands are even heteroepitaxially aligned on the PEN buffer.

Heterostructures of pentacene (PEN) and buckminsterfullerene (C60) are frequently attracting scientific interest as a well-defined small-molecule model system for the study of internal interfaces between two organic semiconductors. They are prototypical representatives forming a donor–acceptor combination for studies of fundamental optoelectronic processes in organic photovoltaics. Despite their importance in exciton dissociation, the energetics of their interfacial charge-transfer (CT) states and their microscopic excitation dynamics are not yet clarified and still being discussed. Here, we present steady-state and time-resolved photoluminescence measurements on stacked heterostructures composed of these two materials. All experiments are performed in the visible and near-infrared spectral regions as CT states are expected at energies below the fundamental electronic transitions of the respective bulk materials. A characteristic, interface-specific emission at around 1.13–1.17 eV is found, which we attribute to an interfacial CT state. Its excitation energy dependence reveals the intricate relaxation dynamics of excitons formed in both constituent materials. Moreover, the analysis of the dynamics of the C60 excitons shows that the lifetime of this state is reduced in the presence of an interface with PEN. This quenching is attributed to a long-range interaction, i.e. the relaxation of excitations into the interfacial CT state.

Layered metal halides like BiI 3 are of current interest in connection with both 2D materials and photovoltaics.Here, we present a facile new method for the preparation of millimeter-sized BiI 3 single crystals. We use these crystals to study the surface reactivity of their (001) cleavage planes toward various environmental conditions by measuring morphological changes using atomic force microscopy and analyzing the formed species by means of X-ray photoelectron spectroscopy and X-ray diffraction methods. We find that freshly cleaved samples show atomically flat surface regions extending over several micrometers and reveal steps corresponding to single BiI 3 layers. However, we also find that the surface deteriorates in air on a time scale of hours. By studying samples cleaved and stored under different conditions, we identify water as the agent initiating the changes in surface morphology, while under inert gas and dry oxygen, the surface stays intact. On the basis of the analysis of deteriorated long-term-stored samples we identify BiOI as the main product of hydrolysis. We also observe a second long-term decomposition route for samples stored under dynamic vacuum, where formation of BiI whiskers occurs. Overall, our findings emphasize the challenges associated with the surface reactivity of BiI 3 but also demonstrate that well-ordered BiI 3 surfaces can be obtained, which indicates that preparation of extended, atomically smooth BiI 3 monolayers by exfoliation from bulk crystals should be possible.

Organic semiconductors combine flexible tailoring of their optoelectronic properties by synthetic means with strong light−matter coupling, which is advantageous for organic electronic device applications. Although spatially selective deposition has been demonstrated, lateral patterning of organic films with simultaneous control of molecular and crystalline orientation is lacking as traditional lithography is not applicable. Here, a new patterning approach based on surface-localized F-centers (halide vacancies) generated by electron irradiation of alkali halides is presented, which allows structural control of molecular adlayers. Combining optical and atomic force microscopy, X-ray diffraction, and density functional theory (DFT) calculations, it is shown that dinaphthothienothiophene (DNTT) molecules adopt an upright orientation on pristine KCl surfaces, while the F-centers stabilize a recumbent orientation, and that these orientations are maintained in thicker films. This specific nucleation results also in different crystallographic morphologies, namely, densely packed islands and jagged fibers, each epitaxially aligned on the KCl surface. Spatially selective surface irradiation can also be used to create patterns of F-centers and thus laterally patterned DNTT films, which can be further transferred to any (including elastomer) substrate due to the water solubility of the alkali halide growth templates.