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We studied the structural and electronic properties of 2,3,9,10-tetrafluoropentacene (F4PEN) on Ag(111) via X-ray standing waves (XSW), low-energy electron diffraction (LEED) as well as ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS). XSW revealed that the adsorption distances of F4PEN in (sub)monolayers on Ag(111) were 3.00 Å for carbon atoms and 3.05 Å for fluorine atoms. The F4PEN monolayer was essentially lying on Ag(111), and multilayers adopted π-stacking. Our study shed light not only on the F4PEN–Ag(111) interface but also on the fundamental adsorption behavior of fluorinated pentacene derivatives on metals in the context of interface energetics and growth mode.

A library of symmetrical linear oligothiophene was prepared employing decarboxylative cross-coupling reaction as the key transformation. Thiophene potassium carboxylate salts were used as cross-coupling partners without the need of co-catalyst, base, or additives. This method demonstrates complete chemoselectivity and is a comprehensive greener approach compared to the existing methods. The modularity of this approach is demonstrated with the preparation of discreet oligothiophenes with up to 10 thiophene repeat units. Symmetrical oligothiophenes are prototypical organic semiconductors where their molecular electrical doping as a function of the chain length can be assessed spectroscopically. An oligothiophene critical length for integer charge transfer was observed to be 10 thiophene units, highlighting the potential use of discrete oligothiophenes as doped conduction or injection layers in organic electronics applications.

A library of symmetrical linear oligothiophene was prepared employing decarboxylative cross-coupling reaction as the key transformation. Thiophene potassium carboxylate salts were used as cross-coupling partners without the need of co-catalyst, base, or additives. This method demonstrates complete chemoselectivity and is a comprehensive greener approach compared to the existing methods. The modularity of this approach is demonstrated with the preparation of discreet oligothiophenes with up to 10 thiophene repeat units. Symmetrical oligothiophenes are prototypical organic semiconductors where their molecular electrical doping as a function of the chain length can be assessed spectroscopically. An oligothiophene critical length for integer charge transfer was observed to be 10 thiophene units, highlighting the potential use of discrete oligothiophenes as doped conduction or injection layers in organic electronics applications.

A general indexing procedure for grazing-incidence X-ray diffraction patterns recorded from fibre-textured films is introduced for lattices of monoclinic and higher-symmetry systems. The analytical mathematical expressions are given and their use is demonstrated in detail for two cases (monoclinic and orthorhombic)., Grazing-incidence X-ray diffraction studies on organic thin films are often performed on systems showing fibre-textured growth. However, indexing their experimental diffraction patterns is generally challenging, especially if low-symmetry lattices are involved. Recently, analytical mathematical expressions for indexing experimental diffraction patterns of triclinic lattices were provided. In the present work, the corresponding formalism for crystal lattices of higher symmetry is given and procedures for applying these equations for indexing experimental data are described. Two examples are presented to demonstrate the feasibility of the indexing method. For layered crystals of the prototypical organic semiconductors di­indeno­perylene and (ortho-di­fluoro)­sexi­phenyl, as grown on highly oriented pyrolytic graphite, their yet unknown unit-cell parameters are determined and their crystallographic lattices are identified as monoclinic and orthorhombic, respectively.

This chapter reviews the fundamentals and recent progress in the understanding of the molecular electrical doping of organic semiconductors; that is, the doping of conjugated polymers (CPs) and conjugated molecules (COMs). In contrast to inorganic semiconductors, the potential of using doping to improve the performance of organic electronic devices has been established only in the last decade, however, with resounding success. Doping COMs and CPs allows both increasing conductivity by orders of magnitude and tuning their energy levels to reduce energy barriers for charge transport. In this chapter, we focus on the broad range of phenomena observed upon doping organic semiconductors and highlight two distinctly different scenarios: the immediate, pairwise formation of ions and the formation of ground-state charge transfer complexes between the semiconductor and the dopant through supramolecular hybridization of their respective frontier molecular orbitals. Doping-related modifications of the organic semiconductor density of states and conditions for carrier release after the initial charge transfer are discussed, spectroscopic evidence for the underlying processes at work are summarized and correlations between the respective doping scenario and the underlying microstructure are highlighted. This chapter is a carefully revised and updated version of Hase, H., Salzmann, I., 2019. Doping in organic semiconductors. In: Ostroverkhova, O. (Ed.), Handbook of organic materials for electronic and photonic devices, second ed., Woodhead Publishing, pp. 349–383, https://doi.org/10.1016/B978-0-08-102284-9.00011-5 .

The development of heterogeneous covalent adaptable networks (CANs) embedded with carbon nanotubes (CNTs) that undergo reversible dissociation/recombination through thermoreversibility has been significantly explored. However, the carbon nanotube (CNT)-incorporation methods based on physical mixing and chemical modification could result in either phase separation due to structural incompatibility or degrading conjugation due to a disruption of π-network, thus lowering their intrinsic charge transport properties. To address this issue, the versatility of a macromolecular engineering approach through thermoreversibility by physical modification of CNT surfaces with reactive multidentate block copolymers (rMDBCs) is demonstrated. The formed CNTs stabilized with rMDBCs (termed rMDBC/CNT colloids) bearing reactive furfuryl groups is functioned as a multicrosslinker that reacts with a polymaleimide to fabricate robust heterogeneous polyurethane (PU) networks crosslinked through dynamic Diels-Alder (DA)/retro-DA chemistry. Promisingly, the fabricated PU network gels in which CNTs through rMDBC covalently embedded are flexible and robust to be bendable as well as exhibit self-healing elasticity and enhanced conductivity.

Halogenation of conjugated molecules represents a powerful approach to tune the electronic structure of molecular thin films through inductive effects and long-range intermolecular electrostatic interactions. The mixing of halogenated molecules with their pristine counterparts has recently proven successful in altering the blend’s energy levels to adjust the open-circuit voltage of organic solar cells by the mixing ratio. Here, we show that the prevailing rationale for this effect is not equally valid for different molecular orientations. We provide a comprehensive experimental and theoretical analysis of the prototypical blend formed by pentacene and perfluoropentacene to relate structure with electronic properties. We find a mixed-stack structural motif in standing and lying orientations depending on the substrate nature. In the standing orientation, the ionization potential lies in between the values of the pure components, in line with the established picture of averaged molecular quadrupole moments. For the lying orientation, however, we experimentally observe an ionization potential lower than both pristine values, which seems at odds with this simple rationale. Electrostatic simulations based on the knowledge of the atomistic structure of the films capture the complex experimental scenario for both orientations. In particular, the ultralow ionization potential of films formed by lying molecules is identified as a signature of the monolayer structure, where quadrupolar interactions are responsible for a difference of ca. 0.4 eV in the highest occupied molecular orbital energy as compared to thicker films with the same molecular orientation.

We exploited the thermal annealing of poly(3-hexylthiophene) (P3HT) molecularly p-doped with the strong electron acceptor 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) as a tool for tuning the doping concentration as a quasi singular parameter. Via directed dopant desorption, we could unravel the complex microstructure of this semicrystalline system, leading to a detailed growth model solely based on complementary experimental evidence from scattering and spectroscopic techniques. We find the crystalline portion of p-doped P3HT to comprise regions, where dopant anions pack with the polymer chains in a metastable, cocrystalline structure with additional ionized dopants dispersed in the alkyl chain region of P3HT. Simultaneously, regions exist where the pristine polymer backbones closely pack. The dedoping via dopant desorption through thermal annealing reveals the dopants within the mixed crystalline phase to be thermally least stable. Notably, their initial desorption does not alter the th...

We employ a simple and reproducible methodology based on thermal gradient crystallization to access forms I and II of acetaminophen. This methodology provides insight into the role of heat flux in the control over polymorphism and phase transitions. We report the crystallization of different polymorphs of acetaminophen as a function of the thermal gradient parameters (magnitude of the gradient, sample velocity) in a thin film geometry. The thin-film samples were displaced at well-defined velocities (1 ≤ v ≤ 75 μm/s) to control both the direction and the rate of crystal growth. We carried out a detailed structural analysis combining polarized optical microscopy and X-ray diffraction (specular and grazing-incidence) to characterize the crystalline forms isolated by the thermal gradient technique. The resulting polymorphic forms have been found to exhibit high phase purity and remarkable stability over time.

For many years, comprehensive data demonstrate a persistent lack of interest in the field of science, technology, engineering and mathematics (STEM) among students in European and North American countries. This lack of interest is regarded as one reason causing underrepresentation of diversity, especially that of female students within the field of STEM. Obviously, stu-dents—although possibly highly talented—refrain from choosing STEM careers only because their attention is not sufficiently attracted to STEM at the right time at the right place. In particular, this fact significantly affects the field of material research and, therefore, identifying opportunities for sparking students’ interest in materials is a crucial challenge in the framework of a modern STEM education. Here, we present the outline of the novel project “Metal Matters”, which aims at establishing an interdisciplinary approach to foster the field of materials in education. In essence, our research focusses on the omnipresence of metal as material. By exploring the K-16 continuum, we aim to identify windows of opportunities for raising awareness of the relevance of materials. Our approach is to stimulate interest in the relevance of materials by explicitly promoting metal as a topic across the curriculum. Our project is deliberately not restricted to STEM, but also covers history, society, economy, health, sports, literature, and language. Here, we present one part of the project contrasting the scientific relevance of metal with the students’ ideas about metal. For the present exploration and data collection, we employed a mixed-method design consisting of linguistic frequency analysis of scientific publications as well as the qualitative analysis of students’ written responses and drawings.

Here, we study the temperature-dependent transport properties of OFETs with the prototypical OSC 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) co-processed with polystyrene (PS) as the active layer. The active layer is deposited directly on SiO2 using the bar-assisted meniscus shearing (BAMS) method. The co-processing with PS favors a consequential decrease in interfacial trap densities as previously reported. Furthermore, we demonstrate how this processing method leads to devices exhibiting activation energies well below the current state of the art for TIPS-pentacene on SiO2 or other dielectrics. Altogether, our study reports on TIPS-pentacene thin films exhibiting an activation energy (Ea) as low as 15 meV when the active material is blended with PS and processed via BAMS. Such an unprecedentedly low value originates not only from a decrease in the interfacial trap densities but also from trapping energies much shallower than previously reported elsewhere for the same material. This allows us to clarify the previously reported notion that significant passivation of interfacial traps occurs following the separation of PS from TIPS-pentacene into an individual layer at the interface with the insulator and to confirm that the enhanced transport originates from a synergistic effect wherein both trapping density and depth are reduced., E.O. and I.S. acknowledge the financial support of the Natural Sciences and Engineering Research Council of Canada (NSERC) (funding Reference Nos. RGPIN-201805092 and RGPIN-2017-06748), the Fonds de recherche du Québec-Nature et technologies (FRQNT). E.O. also acknowledges the financial support of the Canadian Foundation for Innovation (CFI) through the John R. Evans Leaders Fund (JELF). M.M.T. acknowledges funding by the Spanish Ministry with the Project No. GENESIS PID2019-111682RB-I00 and through the “Severo Ochoa” Programme for Centers of Excellence in R&D (No. FUNFUTURE CEX2019-000917-S) and the Generalitat de Catalunya (No. 2017-SGR-918). A.T. acknowledges his FPU fellowship and is enrolled in the UAB Materials Science Ph.D. program. M.B.R. cordially thanks NSERC for support through the Canada Graduate Scholarships–Master's program and the Centre québécois sur les matériaux fonctionnels (CQMF) for his summer scholarship., With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).

Earlier reports on rod-like conjugated molecules of similar shape and size such as α-sexithiophene (6T) and para-sexiphenyl (6P) indicated mixed crystal growth in equimolar blends. The spectral overlap between the 6P fluorescence and 6T absorption might there give rise to resonant energy transfer between the two species. In marked contrast to H-type aggregation found for 6T bulk crystals, isolated monolayers of 6T as well as 6T monolayers sandwiched between 6P multilayers have been reported to show pronounced green (instead of red) fluorescence, which has been attributed to J-type aggregation. Here, we investigate whether these altered optical properties of 6T translate from the monolayer to a bulk equimolar blend with 6P. Insight into the mixed crystal structure for vacuum co-deposited films of 6T + 6P is provided by using synchrotron grazing-incidence x-ray diffraction on different substrates. By correlating the optical properties of the pure and the mixed systems using absorption and photoluminescence spectroscopy we identify the green emission known from 6T monolayers to prevail in the blend. Our analysis indicates the formation of aggregates which are promoted by the molecular arrangement in the mixed crystal structure highlighting that the remarkable optical properties of 6T/6P heterostacks translate into mixed crystalline films. This study underlines that tuning the opto-electronic properties of organic semiconductors by blending species of similar shape but distinct opto-electronic properties is a promising pathway to achieve altered material properties.

We present an evaluation carried out on a large-scale teacher training activity, the festival “Science on Stage” (SonS) 2013. SonS is a European initiative offering teachers the opportunity to share ideas and teaching practices both on the national and international level. We evaluated the project’s effectiveness in pursuing clear objectives, in clearly defining the target group, and in internationally disseminating teaching projects. Our study documents the transfer of knowledge from the festival to the classroom, aims to identify the prerequisites thereof and, finally, allows formulating key factors for successful future large-scale science communication activities.

Using 3D imaging with time of flight secondary ion mass spectrometry ToF SIMS complemented by grazing incidence X ray diffraction GIXRD , we spatially resolve changes in both the composition and structure of CH3NH3I3 amp; 8722;xClx perovskite films on conducting polymer substrates at different annealing stages, in particular, before and after complete perovskite crystallization. The early stage of annealing is characterized by phase separation throughout the entire film into domains with perovskite and domains with a dominating chloride rich phase. After sufficiently long annealing, one single perovskite phase of homogeneous composition on the lateral micrometer scale is observed, along with pronounced film texture. This composition evolution is accompanied by diffusion of chloride from the perovskite layer towards the conducting polymer substrate, and even accumulation there. Photoelectron spectroscopy analysis further shows that perovskite films become increasingly n type with annealing time and upon full conversion, which correlates with the change of film composition. Our results accentuate the importance of chloride for the formation of crystalline and textured films, which are crucial for enhancing the PV performance of perovskite based solar cells

Biopolymer cellulose is investigated in terms of the crystallographic order within thin films. The films were prepared by spin-coating of a trimethylsilyl cellulose precursor followed by an exposure to HCl vapors; two different source materials were used. Careful precharacterization of the films was performed by infrared spectroscopy and atomic force microscopy. Subsequently, the films were investigated by grazing incidence X-ray diffraction using synchrotron radiation. The results showed broad diffraction peaks, indicating a rather short correlation length of the molecular packing in the range of a few nanometers. The analysis of the diffraction patterns was based on the known structures of crystalline cellulose, as the observed peak pattern was comparable to cellulose phase II and phase III. The dominant fraction of the film is formed by two different types of layers, which are oriented parallel to the substrate surface. The stacking of the layers results in a one-dimensional crystallographic order with a defined interlayer distance of either 7.3 or 4.2 A. As a consequence, two different preferred orientations of the polymer chains are observed. In both cases, polymer chain axes are aligned parallel to the substrate surface, and the orientation of the cellulose molecules are concluded to be either edge-on or flat-on. A minor fraction of the cellulose molecules form nanocrystals that are randomly distributed within the films. In this case, the molecular packing density was found to be smaller in comparison to the known crystalline phases of cellulose.

A method is described for indexing grazing-incidence X-ray diffraction data of epitaxially grown thin films comprising various crystal orientations and/or polymorphs by measuring reciprocal-lattice vectors., Crystal structure identification of thin organic films entails a number of technical and methodological challenges. In particular, if molecular crystals are epitaxially grown on single-crystalline substrates a complex scenario of multiple preferred orientations of the adsorbate, several symmetry-related in-plane alignments and the occurrence of unknown polymorphs is frequently observed. In theory, the parameters of the reduced unit cell and its orientation can simply be obtained from the matrix of three linearly independent reciprocal-space vectors. However, if the sample exhibits unit cells in various orientations and/or with different lattice parameters, it is necessary to assign all experimentally obtained reflections to their associated individual origin. In the present work, an effective algorithm is described to accomplish this task in order to determine the unit-cell parameters of complex systems comprising different orientations and polymorphs. This method is applied to a polycrystalline thin film of the conjugated organic material 6,13-pentacene­quinone (PQ) epitaxially grown on an Ag(111) surface. All reciprocal vectors can be allocated to unit cells of the same lattice constants but grown in various orientations [sixfold rotational symmetry for the contact planes (102) and (102)]. The as-determined unit cell is identical to that reported in a previous study determined for a fibre-textured PQ film. Preliminary results further indicate that the algorithm is especially effective in analysing epitaxially grown crystallites not only for various orientations, but also if different polymorphs are present in the film.

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.

Organic semiconductor charge transport layers are important constituents of perovskite-based solar cells. To assess the suitability of potential solvents for the deposition of the charge transport layers on perovskite surfaces, the effect of solvent exposure on the properties of methyl ammonium lead mixed halide CH3NH3PbI3–xClx films is investigated by grazing incidence X-ray diffraction, atomic force microscopy, ultraviolet–visible absorption spectroscopy, and photoelectron spectroscopy. While exposure to dimethylformamide (DMF) and water instantly dissolves the perovskite film, exposure to chlorobenzene (CB) and chloroform (CF) does not detectably affect the perovskite bulk properties. However, the electronic properties of the perovskite surface are substantially modified by the solvent exposure, resulting in an increased work function and less n-type appearance.

Energy-level alignment at organic-metal interfaces plays a crucial role for the performance of organic electronic devices. However, reliable models to predict energetics at strongly coupled interfaces are still lacking. We elucidate contact formation of 1,2,5,6,9,10-coronenehexone (COHON) to the (1 1 1)-surfaces of coinage metals by means of ultraviolet photoelectron spectroscopy, x-ray photoelectron spectroscopy, the x-ray standing wave technique, and density functional theory calculations. While for low COHON thicknesses, the work-functions of the systems vary considerably, for thicker organic films Fermi-level pinning leads to identical work functions of 5.2 eV for all COHON-covered metals irrespective of the pristine substrate work function and the interfacial interaction strength.

This chapter reviews recent progress in the understanding of the molecular electrical doping of organic semiconductors; that is, the doping of conjugated polymers (CPs) and conjugated molecules (COMs). In contrast to inorganic semiconductors, the potential of using doping to improve the performance of organic light-emitting devices (LEDs), to cite one example, has been established only recently. The method has met with resounding success, as it allows both increasing conductivity by orders of magnitude and tuning energy levels to reduce barriers for charge transport. In this discussion, we focus on the broad range of phenomena observed upon doping organic semiconductors and highlight two distinctly different scenarios: the immediate, pairwise formation of ions and the formation of ground-state charge transfer complexes between the semiconductor and the dopant through supramolecular hybridization of their respective frontier molecular orbitals. Doping-related modifications of the organic semiconductor density of states (DOS) and conditions for carrier release after the initial charge transfer are discussed, spectroscopic evidence for the underlying processes at work are summarized and correlations between the respective doping phenomenology and the underlying microstructure are highlighted.

Molecular p-doping allows for an increase in the conductivity of organic semiconductors, which is regularly exploited in thermoelectric devices. Upon doping, integer and fractional charge transfer have been identified as the two competing mechanisms to occur, where the former is desired to promote the generation of mobile holes in the semiconductor host. In general, high dopant electron affinity is expected to promote integer-charge transfer, while strong coupling between the frontier molecular orbitals of dopant and host promotes fractional charge transfer instead. Here, we investigate the role that the width of the density of states (DOS) plays in the doping process by doping the conjugated polymer poly(3-hexylthiophene) (P3HT) with tetracyanoquinodimethane (TCNQ) derivatives of different electron affinities at a 2% dopant ratio. Cyclic voltammetry confirms that only the electron affinity of F4TCNQ (tetrafluorotetracyanoquinodimethane) exceeds the ionization energy of P3HT, while TCNQ and FTCNQ (2-fluoro-7,7,8,8-tetracyanoquinodimethane) turn out to have significantly lower but essentially identical electron affinities. From infrared spectroscopy, we learn, however, that ca. 88% of FTCNQ is ionized while all of TCNQ is not. This translates into P3HT conductivities that are increased for F4TCNQ and FTCNQ doping, but surprisingly even reduced for TCNQ doping. To understand the remarkable discrepancy between TCNQ and FTCNQ, we calculated the percentage of ionized dopants and the hole densities in the P3HT matrix resulting from varied widths of the P3HT highest occupied molecular orbital (HOMO)-DOS via a semi-classical computational approach. We find that broadening of the DOS can yield the expected ionization percentages only if the dopants have significantly different tendencies to cause energetic disorder in the host matrix. In particular, while for TCNQ the doping behavior is well reproduced if the recently reported width of the P3HT HOMO-DOS is used, it must be broadened by almost one order of magnitude to comply with the ionization ratio determined for FTCNQ. Possible reasons for this discrepancy lie in the presence of a permanent dipole in FTCNQ, which highlights that electron affinities alone are not sufficient to define the strength of molecular dopants and their capability to perform integer-charge transfer with organic semiconductors.

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.

The formation of ion pairs (IPA) through integer charge transfer and ground-state charge-transfer complexes (CPXs) with only fractional charge transfer are the two known mechanisms in the molecular doping of conjugated polymers and molecules, which form the material class of organic semiconductors (OSCs). As it entails no immediate ionization of the OSC, CPX formation is regarded as detrimental to the doping efficiency. For IPA formation to occur, a match between the electron affinity (EA) of the p-dopant and the ionization energy (IE) of the OSC is expected as crucial, while their pronounced frontier molecular orbital overlap should promote CPX formation instead [1]. To explore these complementary scenarios, we first studied the thermal de-doping of the prototypical conjugated polymer poly(3-hexylthiophene) (P3HT), p-doped with the common strong electron acceptor tetrafluoro-tetracyanoquinodimethane (F4TCNQ) [2]. We combined grazing incidence X-ray diffraction (GIXRD) with Fourier-transform infrared spectroscopy (FTIR) to determine the microstructure, and absorbance spectroscopy (UV/vis/NIR) to observe the spectral fingerprint of the two doping scenarios. We found two microstructural environments through their distinguished thermal stabilities, where, the dopants were observed (i) alternatingly stacked with the polymer backbone as well as (ii) dispersed in its sidechain region. From FTIR and UV/vis/NIR we deduce that all dopants are fully ionized, although packing between dopant and polymer backbone would be expected to favor CPX formation instead. Notably, while characteristic shifts of the mid-infrared cyano-stretch modes of TCNQ derivatives are known to well indicate the degree of charge transfer, we show here that they allow further to assess the dopant site in the polymer environment. We further investigated the influence of EA with respect to IE by contrasting F4TCNQ with its derivatives of lower degree of fluorination, which translates into reduced EA while leaving the spatial aspects of the dopant largely intact. We find that using F2TCNQ and FTCNQ still results in IPA formation in spite of EA < IE, which we understand through the lower degree of spatial, and therefore energetic order in OSCs as compared to inorganic semiconductors, which therefore possess no sharp band edges. Surprisingly, at high doping concentrations (additional) CPX formation occurred for all the differently strong dopants, which highlights the complexity of these systems going beyond the sole impact of the energetics of the individual constituents. Finally, we juxtaposed the known tendency of small molecular OSCs to form CPXs and that of polymer OSCs to form IPAs. To maximize the comparability with P3HT, alkylated oligothiophenes of different length were synthesized in-house (from 4 to 10 thiophene repeat units) and doped with F4TCNQ. For the longest oligomer, we were able to observe the switch from the CPX regime into that of IPA, i.e., we approach the doping behavior polymer limit. Knowing this threshold is valuable for applications in organic electronics where vacuum processible small molecular OSCs are preferred over polymeric OSCs, which are only processible via solution-based methods. [1] I. Salzmann, G. Heimel, M. Oehzelt, S. Winkler, N. Koch, Acc. Chem. Res. 2016, 49, 370. [2] H. Hase, K. O’Neill, J. Frisch, A. Opitz, N. Koch, I. Salzmann, The Journal of Physical Chemistry C 2018, 122, 25893.

One key challenge in the field of organic semiconductors (OSCs) and their application in organic electronics is controlling the electronic properties of the OSC by p-/n-doping in a predictable manner. It has been established that upon dopant admixture either integer or fractional charge transfer can occur, where the parameters deciding one over the other are still under debate. In numerous studies integer charge transfer was found for conjugated polymers, while small molecular OSCs show a tendency towards fractional charge transfer forming ground state charge transfer complexes (1). The typical approach to assure integer charge transfer is using molecular dopants with a high electron affinity, exceeding the ionization energy of the OSC (for p-doping), as the dopant then exhibits an empty orbital lying lower in energy than the occupied frontier molecular orbital of the semiconductor. Recently, however, a transition between the two scenarios has been reported for polythiophene and the strong molecular p-dopant F4TCNQ, where aging of the samples appeared to promote the formation of charge-transfer complexes showing fractional charge transfer (2). Another study highlighted the key role of the microstructure for the doping process, where different polymorphs show either fractional or integer charge transfer (3). This lack of predictability of traditional doping employing strong donors/acceptors recently led to intense research efforts to find alternative approaches. There, using Lewis acids (LAs) as molecular p-dopants for OSCs has been found to be particularly promising, as these compounds react selectively with Lewis bases to form robust adducts. This makes LAs, in principle, chemically selective dopants that attack OSCs only at the sites where electron lone pairs are located. Interestingly, despite an electron affinity far below the ionization energy of polythiophene, it has been found that the LA BCF can readily ionize this material, which is incompatible with the conventional picture of integer charge transfer as detailed above (4). Very recently, it has been suggested that LA doping does not follow from the formation of an adduct, but instead, that the LA simply act as a protonating agent after forming a complex with water and, hence, becoming a Br∅nsted acid (5). In this presentation, I will briefly review these ideas and discuss how the concepts of electron affinity, ionization energy, integer, and partial charge transfer as used in the field relate to the doping using LAs. By correlating experimental data from optical and infrared spectroscopy with DFT calculations, I will provide alternative descriptors that allow to clarify the discussion and, more importantly, that have predictive power towards the use of Lewis acids as dopants for OSCs. Finally, I will show that the use of these descriptors, in the case of polythiophene doped with BCF discussed above (4), lift the apparent theoretical inconsistency, and as such, will be of high value for a comprehensive larger-scale exploration of OSCs doping using LAs. 1. Salzmann et al., Acc. Chem. Res. 2016, 49, 370 2. Watts et al., Chem. Mater. 2019, 31, 6986−6994 3. Jacobs et al., Materials Horizons. 2018, 5, 655 4. P. Pingel et al., Adv. Electron. Mater. 2016, 2:1600204. 5. B. Yurash et al., Nature Materials. 2019, 18, 1327–1334

Crystal structure solutions from fibre-textured crystals within thin films are frequently achieved by grazing-incidence X-ray diffraction experiments. In the present work, analytical mathematical expressions are derived for the indexing of experimental diffraction patterns., Crystal structure solutions from thin films are often performed by grazing-incidence X-ray diffraction (GIXD) experiments. In particular, on isotropic substrates the thin film crystallites grow in a fibre texture showing a well defined crystallographic plane oriented parallel to the substrate surface with random in-plane order of the microcrystallites forming the film. In the present work, analytical mathematical expressions are derived for indexing experimental diffraction patterns, a highly challenging task which hitherto mainly relied on trial-and-error approaches. The six lattice constants a, b, c, α, β and γ of the crystallographic unit cell are thereby determined, as well as the rotation parameters due to the unknown preferred orientation of the crystals with respect to the substrate surface. The mathematical analysis exploits a combination of GIXD data and information acquired by the specular X-ray diffraction. The presence of a sole specular diffraction peak series reveals fibre-textured growth with a crystallographic plane parallel to the substrate, which allows establishment of the Miller indices u, v and w as the rotation parameters. Mathematical expressions are derived which reduce the system of unknown parameters from the three- to the two-dimensional space. Thus, in the first part of the indexing routine, the integers u and v as well as the Laue indices h and k of the experimentally observed diffraction peaks are assigned by systematically varying the integer variables, and by calculating the three lattice parameters a, b and γ. Because of the symmetry of the derived equations, determining the missing parameters then becomes feasible: (i) w of the surface parallel plane, (ii) the Laue indices l of the diffraction peak and (iii) analogously the lattice constants c, α and ß. In a subsequent step, the reduced unit-cell geometry can be identified. Finally, the methodology is demonstrated by application to an example, indexing the diffraction pattern of a thin film of the organic semiconductor pentacene­quinone grown on the (0001) surface of highly oriented pyrolytic graphite. The preferred orientation of the crystallites, the lattice constants of the triclinic unit cell and finally, by molecular modelling, the full crystal structure solution of the as-yet-unknown polymorph of pentacene­quinone are determined.

In organic optoelectronics, order of conjugated molecules is required for good charge transport, but strong aggregation behavior may generate grain boundaries and trapping, opposing those benefits. Side chains on a polymer's backbone are major reason for and also tool to modify its morphological characteristics. In this report, we show on the example poly(9,9-dioctylfluorenyl-co-bithiophene) (F8T2) that by a combination of two types of side-chains on the backbone of equal number of carbons, one promoting crystallization, another hindering it, organization of the main chains can be controlled, without changing its major properties. We compare the traditional F8T2 derivative with octyl substituent with two modified species, one containing solely 2-ethylhexyl side-chains and another with both types randomly distributed. Thermal characteristics, photophysics and morphology are compared and effects on film formation and charge transport in bulk-heterojunction blends demonstrated on photovoltaic devices utilizing F8T2s as donor and the fullerene derivative ICBA as acceptor material., Comment: 11 pages

The crystallization of the nootropic drug piracetam in spin-coated thin layers on silicon oxide surfaces was analyzed by optical microscopy, as well as powder and synchrotron grazing-incidence X-ray diffraction. We observed the crystal structure formed to depend on the concentration of the solution used in the preparation of the samples and identified either the polymorphic form I (for concentrations 0.147 mol·dm-3). Although significant dewetting of the materials occurred over time, the transformation of form I, which is metastable at room temperature, to the more thermodynamically stable forms II or III was not observed, even at the long-term (9 months). Even when subject to prolonged solvent-vapor annealing, dewetting but no alteration of the polymorphic form occurred. Our results suggest that the metastable piracetam form I is stabilized upon crystallization from solution in thin films, which might generally pave the way to stabilizing metastable forms of drugs of increased dissolution rate., SCOPUS: ar.j, info:eu-repo/semantics/published

Molecular electrical doping of organic semiconductors has emerged as a valuable strategy to improve the performance of organic (opto-)electronic devices and to extend the range of achievable functionality. In the present review, a brief overview of typical applications employing molecular electrical doping is given, followed by a critical summary of experiments performed to elucidate the microscopic mechanisms involved. In providing an in-depth discussion of the models that have been established on the basis of these experiments, a comprehensive portray of the current state of affairs emerges, which allows deriving certain guidelines for the design of improved molecular dopants. At the same time, phenomenological differences in the effect of doping in different classes of organic semiconductors are identified. This precludes, at the moment, a unifying explanation of all underlying processes, but nevertheless serves to steer future research efforts.

Film forming properties of semiconducting organic molecules comprising alkyl-chains combined with an aromatic unit have a decisive impact on possible applications in organic electronics. In particular, knowledge on the film formation process in terms of wetting or dewetting, and the precise control of these processes, is of high importance. In the present work, the subtle effect of temperature on the morphology and structure of dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) films deposited on silica surfaces by spin coating is investigated in situ via X-ray diffraction techniques and atomic force microscopy. Depending on temperature, bulk C8-BTBT exhibits a crystalline, a smectic A and an isotropic phase. Heating of thin C8-BTBT layers at temperatures below the smectic phase transition temperature leads to a strong dewetting of the films. Upon approaching the smectic phase transition, the molecules start to rewet the surface in the form of discrete monolayers with a defined number of monolayers being present at a given temperature. The wetting process and layer formation is well defined and thermally stable at a given temperature. On cooling the reverse effect is observed and dewetting occurs. This demonstrates the full reversibility of the film formation behavior and reveals that the layering process is defined by an equilibrium thermodynamic state, rather than by kinetic effects., info:eu-repo/semantics/published

Pentacene is one of the most studied organic semiconducting materials. While many aspects of the film formation have already been identified in very thin films, this study provides new insight into the transition from the metastable thin-film phase to bulk phase polymorphs. This study focuses on the growth behavior of pentacene within thin films as a function of film thickness ranging from 20 to 300 nm. By employing various X-ray diffraction methods, combined with supporting atomic force microscopy investigations, one crystalline orientation for the thin-film phase is observed, while three differently tilted bulk phase orientations are found. First, bulk phase crystallites grow with their 00L planes parallel to the substrate surface; second, however, crystallites tilted by 0.75° with respect to the substrate are found, which clearly dominate the former in ratio; third, a different bulk phase polymorph with crystallites tilted by 21° is found. The transition from the thin-film phase to the bulk phase is rationalized by the nucleation of the latter at crystal facets of the thin-film-phase crystallites. This leads to a self-limiting growth of the thin-film phase and explains the thickness-dependent phase behavior observed in pentacene thin films, showing that a large amount of material is present in the bulk phase much earlier during the film growth than previously thought.

The previously unknown crystal structure of epindolidione has been determined from a thin film by combining diffraction data with calculations., Epindolidione, a hydrogen-bonded derivative of the organic semiconductor tetracene, is an organic pigment which has previously been used to produce stable OFETs with relatively high hole mobilities. Despite its use as an inkjet pigment and organic semiconductor, the crystal structure of epindolidione has proved elusive and is currently unknown. In this work, we report a crystal structure solution of epindolidione determined from vapor deposited thin films using a combined experimental and theoretical approach. The structure is found to be similar to one of the previously reported epindolidione derivatives and is most likely a surface-mediated polymorph, with a slightly different crystal packing compared to the bulk powder. The effect of substrate temperature on film morphology and structure is also investigated, where it is found that the crystallite orientation can be tuned by deposition at different substrate temperatures. The results also illustrate the possibilities for crystal structures to be solved from thin films.

The electrical conductivity of organic semiconductors can be enhanced by orders of magnitude via doping with strong molecular electron acceptors or donors. Ground-state integer charge transfer and charge-transfer complex formation between organic semiconductors and molecular dopants have been suggested as the microscopic mechanisms causing these profound changes in electrical materials properties. Here, we study charge-transfer interactions between the common molecular p-dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane and a systematic series of thiophene-based copolymers by a combination of spectroscopic techniques and electrical measurements. Subtle variations in chemical structure are seen to significantly impact the nature of the charge-transfer species and the efficiency of the doping process, underlining the need for a more detailed understanding of the microscopic doping mechanism in organic semiconductors to reliably guide targeted chemical design.

Crystalline and uniform nanocolumns of the organic semiconductor diindenoperylene (DIP) were fabricated by glancing-angle deposition and employed in organic photovoltaic cells (OPVCs) forming an interdigitated donor/acceptor heterojunction, with fullerene as electron acceptor. In comparison to reference bilayer devices the nanocolumn-based solar cells exhibit increased power conversion efficiency. Based on a comprehensive structural and morphological analysis, we identify three advantages of the interdigitated nanocolumn structures: (i) The active donor/acceptor interface area, crucial for exciton dissociation, is increased and the column diameter is in the range of the exciton diffusion length. (ii) The molecular orientation of DIP is such in the nanocolumns that light absorption is enhanced. (iii) The ubiquitous presence of vertical interfaces throughout nanocolumn-based devices is further beneficial to light absorption, as it fully compensates wavelength-dependent interference effects within the device structure. This work shows how the benefits of nanocolumns can go beyond simple interface area enlargement to improve the efficiency of OPVCs.

This work presents a method for solving the crystal structure of an organic thin-film prepared on an isotropic surface. Combining specular X-ray diffraction and grazing incidence X-ray diffraction, a full structure solution is exemplarily derived for crystals of the molecule ternaphthalene (NNN). The data are analysed following a classic approach, first, by indexing the Bragg peak pattern and, second, by solving the structure using the experimentally observed intensities. Direct space methods with a rigid-body refinement are applied. The result shows that the NNN crystallizes in a layered herringbone structure. The calculated peak intensities based on this structure are in excellent agreement with the experimentally observed intensities. Overall, following this approach, the crystal structure of the molecule NNN grown in a preferred orientation within thin-films could be unambigously solved providing a general pathway for future thin-film structure solutions.

Using 3D imaging with time-of-flight secondary ion mass spectrometry (ToF-SIMS) complemented by grazing-incidence X-ray diffraction (GIXRD), we spatially resolve changes in both the composition and structure of CH

Molecular doping: The standard model for molecular p-doping of organic semiconductors (OSCs) assumes integer charge transfer between OSC and dopant. This is in contrast to an alternative model based on intermolecular complex formation instead. By systematically varying the acceptor strength it was possible to discriminate the two models. The latter is clearly favored, suggesting strategies for the chemical design of more efficient molecular dopants.

This Letter reports the impact of the evaporation rate on the crystallographic phase formation of vacuum deposited α-sexithiophene thin films studied by X-ray diffraction methods. The experiments reveal the formation of two crystal phases, one of which is a thermodynamically stable phase occurring at low rates, while the second is favored by high rates. This second phase exhibits an increased layer spacing and diffraction features typical for two-dimensional crystals which are laterally ordered but without interlayer correlations of the molecular positions. This disordered layered phase comprises molecules of nonuniform conformations, and is kinetically induced.

The order in molecular monolayers is a crucial aspect for their technological application. However, the preparation of defined monolayers by spin-coating is a challenge, since the involved processes are far from thermodynamic equilibrium. In the work reported herein, the dynamic formation of dioctyl-benzothienobenzothiophene monolayers is explored as a function of temperature by using X-ray scattering techniques and atomic force microscopy. Starting with a disordered monolayer after the spin-coating process, post-deposition self-reassembly at room temperature transforms the initially amorphous layer into a well-ordered bilayer structure with a molecular herringbone packing, whereas at elevated temperature the formation of crystalline islands occurs. At the temperature of the liquid-crystalline crystal–smectic transition, rewetting of the surface follows resulting in a complete homogeneous monolayer. By subsequent controlled cooling to room temperature, cooling-rate-dependent kinetics is observed; at rapid cooling, a stable monolayer is preserved at room temperature, whereas slow cooling causes bilayer structures. Increasing the understanding and control of monolayer formation is of high relevance for achieving ordered functional monolayers with defined two-dimensional packing, for future applications in the field of organic electronics., SCOPUS: ar.j, FLWNA, info:eu-repo/semantics/published

We report on bending strain-induced changes of the charge carrier mobility in pentacene organic thin-film transistors employing a combined investigation of morphological, structural, and electrical properties. The observed drain current variations are reversible if the deformation is below 2%. The morphology and structure of the active pentacene layer is investigated by scanning force microscopy and specular synchrotron X-ray diffraction, which show that bending-stress causes morphological rather than structural changes, modifying essentially the lateral spacing between individual pentacene crystallites. In addition, for deformations >2% the rupture of source and drain gold electrodes is observed. In contrast to the metal electrodes, the modification of the organic layer remains reversible for deformations up to 10%, which suggests the use of soft and flexible electrodes such as graphene or conducting polymers to be beneficial for future strain sensing devices.