pubs.acs.org/cm Article Molecular Disorder in Crystalline Thin Films of an Asymmetric BTBT Derivative Sebastian Hofer, Johanna Unterkofler, Martin Kaltenegger, Guillaume Schweicher, Christian Ruzié, Adriań Tamayo, Tommaso Salzillo, Marta Mas-Torrent, Alessandro Sanzone, Luca Beverina, Yves Henry Geerts, and Roland Resel* Cite This: Chem. Mater. 2021, 33, 1455−1461 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information ABSTRACT: The molecule 2-decyl-7-phenyl-[1]benzothieno- [3,2-b][1]benzothiophene (Ph-BTBT-10) is an organic semi- conductor with outstanding performance in thin-film transistors. The asymmetric shape of the molecule causes an unusual phase behavior, which is a result of a distinct difference in the molecular arrangement between the head-to-head stacking of the molecules versus head-to-tail stacking. Thin films are prepared at elevated temperatures by crystallization from melt under controlled cooling rates, thermal-gradient crystallization, and bar coating at elevated temperatures. The films are investigated using X-ray diffraction techniques. Unusual peak-broadening effects are found, which cannot be explained using standard models. The modeling of the diffraction patterns with a statistic variation of the molecules reveal that a specific type of molecular disorder is responsible for the observed peak-broadening phenomena: the known head-to-head stacking within the crystalline phase is disturbed by the statistic integration of reversed (or flipped) molecules. It is found that 7−15% of the molecules are integrated in a reversed way, and these fractions are correlated with cooling rates during the sample preparation procedure. Temperature-dependent in situ experiments reveal that the defects can be healed by approaching the transition from the crystalline state to the smectic E state at a temperature of 145 °C. This work identifies and quantifies a specific crystalline defect type within thin films of an asymmetric rodlike conjugated molecule, which is caused by the crystallization kinetics. ■ INTRODUCTION The crystal structure is of fundamental importance to However, microscopy methods are often difficult to perform and give only selective results; therefore, the use of integral understand the properties of organic semiconductors. The methods is preferred. Therefore, X-ray scattering techniques long-range order of the molecules is a defined way to describe are preferred, but the nature of the structural defects is often molecular packing so that electronic and phonon band difficult to identify.13 structures can be calculated. The distribution of electrons in There are a number of effects in the XRD pattern, which can terms of their energies and their apparent momenta is the basis be assigned to defects. Diffuse scattering (the scattering apart to understand charge transport in these materials.1 However, from Bragg peaks) appears because defects break the long- the deviation from the ideal crystal lattice is also important to range periodicity of a crystal structure.14,15 The peak understand application-relevant properties of organic semi- broadening of Bragg peaks is one quite well-developed tool conductors.2 There exists a variety of structural defects that to characterize the deviation from the ideal infinitely extended cause variations from the ideality.3 Chemical defects arise from perfect crystal. The two contributions for peak broadening are substitutional molecules4 or physical defects because of the microstrains (root-mean-square of the variations in the lattice distortion of periodic lattices like vacancies, dislocation lines, parameters) and the crystal size (size of the ideal crystal in the or stacking faults.5,6 Optical properties as well as charge direction of the scattering vector). The separation of these two transport within organic semiconductors are highly affected by effects is possible with Williamson−Hall plots.16 This method defects.7−9 Structural defects at a considerably larger length scales are the crystal size and crystal mosaicity associated with grain boundaries between single-crystalline domains.10 Received: December 11, 2020 The solution of crystal structures is a widely used and highly Revised: January 27, 2021 advanced technique mainly based on X-ray diffraction Published: February 9, 2021 (XRD).11 The characterization of structural defects is considerably more difficult, and direct observations using microscopy methods are the most successful techniques.12 © 2021 The Authors. Published by American Chemical Society https://dx.doi.org/10.1021/acs.chemmater.0c04725 1455 Chem. Mater. 2021, 33, 1455−1461 Chemistry of Materials pubs.acs.org/cm Article has successfully been applied to thin films of organic sample from the hot end to the cold end at a constant translation semiconductors.17,18 velocity, chosen in between 1 and 20 μm/s. For this purpose, the thin The molecule 2-decyl-7-phenyl-[1]benzothieno[3,2-b][1]- film was covered using an additional glass slide. The stack (substrate/ benzothiophene (Ph-BTBT-10) is a recently developed thin film/top glass) was heated to 245 °C for the complete melting of organic semiconductor that exhibits excellent performance in the thin film. Crystallization takes place by a translational movement toward the low-temperature zone which was maintained at 75 °C, a thin-film transistors.19,20 The asymmetric nature of the temperature well below the crystallization temperature of Ph-BTBT- molecule, there is a phenyl ring at one terminal end of the 10. The used system is a Linkam GS350 temperature gradient heating aromatic benzothieno−benzothiophene core and a decyl chain stage combined with a Nikon Eclipse 80i polarized light microscope. at the other terminal end, causes a specific phase behavior as a The third sample series was prepared by shear crystallization27 on function of temperature. Starting from the elevated temper- oxidized silicon wafers by bar-assisted meniscus shearing, following atures, a phase transition from the isotropic liquid to the the previously reported methodology.28 Solutions of Ph-BTBT-10 and smectic A phase appears at 223 °C and subsequently a blends of Ph-BTBT-10 with polystyrene (PS) in a ratio Ph-BTBT- transition to the smectic E phase at 210 °C.19,21 The transition 10:PS 2:1 in chlorobenzene and o-xylene with a concentration of 2% between the liquid crystalline state and the crystalline state is and 2.5% w/w, respectively, were prepared. The addition of a binding accompanied with strong retardation effects: the onset of insulating polymer has been reported to promote the crystallization of crystallization appears at around 100 °C, while the melting of the organic semiconductor and improve thin-film homogeneity.29 the crystalline state occurs at a temperature of about 143 °C. Deposition was performed at elevated substrate temperatures of 105 Within the crystalline state, the molecules form a double-layer and 110 °C for chlorobenzene and o-xylene, respectively. Specular XRD was performed with an PANalytical Empyrean herringbone structure: the layers are formed from separated system using a sealed copper tube together with a multilayer mirror alkyl chains and the aromatic parts of the molecule (compare for generating a parallelized and monochromatized primary X-ray Figure 1).22 The molecules of neighboring layers are arranged beam. A wavelength of 1.542 Å was used. The scattered intensity was in a head-to-head arrangement so that the BTBT core together detected with a PIXcel detector operating in a one-dimensional (1D) with the terminal phenyl ring points toward each other. mode for long-range measurements and as a point detector for short- range measurements. The diffraction pattern was converted into a 4π reciprocal space using the equation q = λ sinθ , with q as the length of the scattering vector, λ as the used wavelength, and 2θ as the angle between the primary and the scattered X-ray beam. The peak parameters were evaluated by subtracting the experimental back- ground and fitting a Gaussian curve to determine peak positions and peak widths; the peak widths are given as full width at half maximum. In situ temperature-dependent measurements were performed using the high-temperature attachment DHS900,25 setting the temperature and waiting for 10 min to let the system equilibrate before taking the Figure 1. Packing of the molecule Ph-BTBT-10 within the crystalline measurements. Subsequently, the temperature is increased in steps of state. Bilayers are formed with head-to-head arrangement of the 2 °C close to the phase transitions and with a step size of 10 °C molecules. Additionally, the crystallographic (001) planes are drawn. elsewhere. The calculations of specular diffraction patterns were performed by the Fourier transforms of electron densities. The electron density was Thin film preparation for transistor applications is performed modeled by the number of electrons for each atom in a single at elevated temperatures because the moderately ordered molecule of Ph-BTBT-10 along the 001 direction, with a width based smectic E phase is expected as a prestage of crystallization.23,24 on the atomic form factor for each element. Based on the known The present work shows that a molecular disorder is obtained crystal structure of Ph-BTBT-10, a layered structure was assumed when the thin films of the molecule Ph-BTBT-10 are processed with a defined layer distance of 26.5 Å. The thickness of the layer from elevated temperatures. The obtained crystalline packing is results from the length of the molecules together with their molecular substantially disordered, and the type of defect could be packing of a herringbone type. The electron density distribution identified. The defects could be healed by heat treatment at across the layers was chosen as nonperiodic by molecular disorders. temperatures close to phase transition to the liquid crystalline To include the disorder, each layer is stacked on top of the previous state. layer with a certain probability of being either in a head-to-head or ■ head-to-tail fashion. The fraction of molecular disorder p is defined by EXPERIMENTAL SECTION the fraction of molecules with a head-to-head orientation. A molecular disorder parameter p = 0 appears, if the number of up-right standing One batch of the molecule Ph-BTBT-10 was synthetized according to molecules is equal to the downward oriented molecules. A number of the strategy recently published.19 The first sample series was repeating units is set at 30 to be in best agreement with the deposited on 20 × 20 mm2 Menzel glass substrates by drop-casting experimentally prepared samples. This modeling is repeated up to 200 from an 8 g/L solution. Toluene was used as the solvent. The thin film samples were heated up to 245 °C, a temperature well above the times, and the resulting Fourier signal is averaged to generate a melting point of the material.19 In a subsequent step, the samples smooth calculated diffraction signal. were cooled to room temperature; defined cooling rates between 0.8 Grazing incidence X-ray diffraction (GIXRD) was performed at the °C/min up to 30 °C/min were chosen. The heat treatment was XRD1 beamline, Elettra, Trieste. The primary X-ray beam with a performed with the commercially available domed hot stage DHS900, wavelength of 1.400 Å and a size of 500 × 500 μm2 enclosed an Anton Paar.25 incident angle of 1.5° with the sample surface. The diffracted beams The second sample series was also based on drop-casted thin films were detected with a stationary Pilatus 2 M detector. The diffraction from toluene solutions, but the heat treatment was performed by pattern was converted into the reciprocal space using the software thermal-gradient crystallization.26 The setup consists of two GIDVis;30 the calculation of peak positions and peak intensities from independent heating stages (hot end and cold end) separated by a the known crystal structure of Ph-BTBT-10 was performed by the gap of 2.5 mm. A mechanical arm allows the displacement of the module crystal available within the GIDVis package. 1456 https://dx.doi.org/10.1021/acs.chemmater.0c04725 Chem. Mater. 2021, 33, 1455−1461 Chemistry of Materials ■ pubs.acs.org/cm Article RESULTS peaks are broadened considerably in comparison to the even- All the three sample series were investigated by specular XRD. numbered 00L peaks. No clear dependence of peak broadening The first sample series is based on the drop-casted thin films. is observed as a function of the withdrawal velocity. The last The result of an untreated drop-casted film is shown in Figure sample series is prepared by bar coating at elevated 2a. The diffraction pattern reveals defined diffraction peaks temperatures from a solution using different solvents. Similarly to the other treatments, the odd-numbered 00L peaks are considerably broadened in comparison to the even-numbered 00L peaks. An observation of only 00L peaks within a specular diffraction pattern is a frequently observed case19,31 and reveals a highly defined preferential out-of-plane orientation of the crystallites. In our case, the crystals grow with the (001) plane parallel to the substrate surface. The appearance of more than a single 001 Bragg peak is referred to higher-order reflections arising from the (001) net plane. The peak width analysis can be performed with high accuracy on the basis of a peak series arising from a series of higher-order reflections. Broadening due to the crystallite size and due to the microstrain can be clearly separated.32 However, peak widths that alternate with the order of diffraction cannot be explained by classical diffraction models.33,34 In the next step, more comprehensive structural studies are performed to get more details on the crystallographic properties of Ph-BTBT-10 within thin films. The GIXRD patterns are measured in an extended range so that a characteristic peak pattern of Ph-BTBT-10 is observable. Figure 3 shows a reciprocal space map of a thermally gradient Figure 2. Specular XRD of thin films of the molecule Ph-BTBT-10. (a) an untreated film, (b) films obtained from the melt by defined cooling rates, (c) thermal-gradient crystallization from different withdrawal velocities, and (d) shear crystallization using different solvents with and without using polystyrene (PS) as the binding polymer. which could be indexed on the basis of the known crystal structure of Ph-BTBT-10.22 The peak located at 0.118 Å−1 can be assigned to the 001 Bragg peak, and higher-order reflections are observed up to the fifth order. The expected peak positions of the 00L (L = 1 ... 5) peaks are drawn by vertical lines. Figure 3. Reciprocal space map of a thermal-gradient crystallized thin film (translation velocity 2.5 μm/sec.) together with the indexation of The drop-casted films were then thermally annealed above the diffraction pattern based on the known bulk structure of the the melting point, and subsequent crystallization of Ph-BTBT- molecule Ph-BTBT-10. For indexation, only the strongest Bragg peaks 10 was performed at defined cooling rates (Figure 2b). are depicted by circles, where the center and areas of the circles give Similarly to the drop-casted films, all the 00L peaks from the the peak positions and peak intensities, respectively. annealed samples were clearly visible. However, an outstanding feature is the considerable broadening of the 001, 003, and 005 peaks, while the 002 and 004 peaks do not show broadening crystallized film. The map shows the characteristic fingerprint with respect to the untreated sample. Peak broadening is more of a herringbone packing typically for rodlike conjugated enhanced at larger cooling rates. The largest peak widths of the molecules: elongated intensity features are observed along the odd-numbered 00L peaks are observed for the sample qz direction at defined qxy values at 1.31 Å−1, 1.60 Å−1, and 1.91 prepared using the highest cooling rate of 20 °C/min; in this Å‑1.35,36 The reciprocal space map is indexed on the basis of the particular case also, the even-numbered 00L peaks start to known phase of Ph-BTBT-10, assuming a 001 preferred broaden. orientation of the crystallites. The comparison of the calculated The next sample series is prepared by thermal-gradient diffraction peaks with the experimentally observed peak pattern crystallization; the results are depicted in Figure 2c. The 00L reveals that the bulk phase of Ph-BTBT-10 is present within Bragg peaks are observed at the expected positions of the the thin film. The excellent agreement can be clearly seen not known crystal phase. Even in this case, the odd-numbered 00L only for the strongest diffraction peaks like 112, 115, and 117 1457 https://dx.doi.org/10.1021/acs.chemmater.0c04725 Chem. Mater. 2021, 33, 1455−1461 Chemistry of Materials pubs.acs.org/cm Article but also for 121 and 122. The known crystal structure could be each increase of the temperature, the system was held at the confirmed also for the samples prepared by defined cooling temperature for 5 min to equilibrate. A section of the rates and for the bar-coated samples (compare Figure S1). measurements is presented using a color contour plot in The specular XRD studies (Figure 2) as well as the GIXRD Figure 5a. The 00L peak series is clearly visible together with investigations (Figure 3) reveal that the known crystal structure of Ph-BTBT-10 is present within our thin films. Based on the known molecular packing within the bulk structure a possible explanation of the outstanding peak- broadening phenomena can be developed. We assume disorder of the molecules within the crystal structure as the reason for the unusual behavior. The molecular disorder breaks the crystallographic long-range order; therefore, it cannot be treated by classical diffraction theory. As a consequence, statistical simulations have to be performed. The simulation model starts with the crystal structure of Ph-BTBT-10 using double herringbone layers with head-to-head arrangements of the molecules. The molecular disorder is implemented by inverted molecules as deviation from the crystallographic periodicity. The molecular disorder parameter p describes the fraction of head-to-head aligned molecules; p = 0 represents molecules only with head-to-head arrangement while p = 1 represents a molecular packing based on head-to-tail arrange- ments. The results of the simulations are depicted in Figure 4. Specular diffraction patterns are plotted as a function of the Figure 5. Series of in situ specular XRD of a thermal-gradient crystallized thin film (translation velocity 2 μm/sec.) presented in a color contour plot. The measurements are given in the temperature range from 120 to 180 °C covering the phase transitions from the crystalline state to smectic E at 145 °C. The intensity scale is color- coded from yellow for high intensity via green to blue for low intensity (a). Peak width analysis of specular diffraction peaks (Laue indices 00L) of a thermal-gradient crystallized thin film (translation velocity 1.5 μm/sec.) as a function of temperature; additionally, the peak widths from the simulation are given for a molecular disorder parameter p = 0.15 (b). Figure 4. Simulation of a specular diffraction pattern of the (001) plane of Ph-BTBT-10 crystallites with different degrees of random their variable peak width (compare Figure 2). At a temperature molecular disorder p presented in a color contour plot. The intensity of 147 °C, the expected phase transition to a smectic E phase scale is color-coded from yellow for high intensity via green to blue takes place, clearly visible by the disappearance of the odd- for low intensity (a). The molecular disorder parameter p represents numbered 00L peaks and a clear shift in the peak position of the deviation from the ideal head-to-head packing present within the known bulk structure (b). the even-numbered 00L peaks. The peak widths (Δq) of the in situ studies are evaluated as a function of temperature and plotted in comparison to an ex- disorder parameter p in a waterfall plot. No peak broadening is situ measurement at room temperature (25 °C). The results observed for p = 0, but a slight increase of the disorder are shown in Figure 5b. The plotting of the peak width as a parameter immediately causes a peak broadening of the odd- function of the length of the scattering vector q (Williamson− numbered 00L peaks, while the even-numbered 00L peaks Hall plots) reveals a linear behavior as it is expected from the remain at their initial peak widths. The peak widths on the classical diffraction theory.33,34 Considering the peak width at odd-numbered 00L peaks increase by increasing the parameter 25 °C, the odd-00L peak series show a different dependence p and disappears at about p = 0.4. Simulations are performed than the even-numbered 00L peak series. Here, the peak by random distributions as well as by systematic distributions widths of the 00L peaks are plotted up to L = 14. In the next of the inverted molecules; even random sequences of layers step, the peak widths are considered at elevated temperatures. with identical molecular alignment have been considered. We Approaching the phase-transition temperature for the smectic E found that the molecular disorder parameter p is a reasonable phase, the peak width of the odd-numbered peaks decreases, parameter to describe the observed effect on peak broadening. and the width of the even-numbered peaks increases. At a In the next step, the influence of heat treatment to the temperature just below the phase transition at 145 °C, the peak observed peak-broadening phenomena is investigated. Tem- widths of all the 00L peaks are arranged in one line. The perature-dependent in situ XRD experiments are performed on variable peak width of the 00L peaks disappears just at the a sample prepared by thermal-gradient crystallization. Specular phase transition to the smectic E phase. For comparison, we scans were started at 120 °C and performed stepwise in plotted the peak width of our simulation with p = 0.15. In both temperatures up to the complete melting of Ph-BTBT-10 at the cases, odd- as well as even-based 00L peaks, we observe a 230 °C. The temperature was increased in steps of 3 °C close linear behavior. The peak widths are comparable only in the to the phase transitions and in steps of 10 °C otherwise. After low q-range (for the 001 and 002) because we do not include 1458 https://dx.doi.org/10.1021/acs.chemmater.0c04725 Chem. Mater. 2021, 33, 1455−1461 Chemistry of Materials pubs.acs.org/cm Article instrumental broadening in our calculations. Please note that solid state, flip-flop motions become possible but only close to our calculations were performed for a stack of 30 repeating the transition to a liquid state, as observed for the bilayers of units. phospholipids at the transition from the gel phase to the fluid In the final step, the Williamson−Hall plots are prepared for phase.44 ■ the sample series prepared at defined cooling rates. A clear correlation between the cooling rates and the molecular CONCLUSIONS disorder parameter p was found (Figure S2). The cooling rates of 10 and 2 °C/min result in p = 0.15 and 0.07, respectively; A series of thin films were prepared from the molecule Ph- intermediate rates result in disorder parameters in between. BTBT-10 using various heat-treatment-based methods and ■ investigated via XRD methods. At room temperature the bulk DISCUSSION crystal structure of the material is observed, but a specific anomaly is observed in the peak widths of the 00L Bragg peaks. The thin films were prepared using a variety of methods; in all A model is set up to include a molecular disorder, based on the the cases, a heat treatment procedure was involved. In the first molecules which are embedded in a reversed manner (flipped case, the films were obtained by defined cooling from the melt; molecules) into the bulk crystal structure. The unexplained in the second case, the films were crystallized from the melt peak-broadening phenomena can be explained satisfactorily by within a thermal gradient, and in the third case, the films were a disorder parameter, which quantifies the fraction of obtained by bar coating at elevated temperatures with disordered molecules. It was found that 7 to 15% of the subsequent cooling to room temperature. After full solid- molecules show a reversed molecular alignment because of ification, the films crystallize in the known bulk structure of Ph- crystallization from elevated temperatures. The temperature BTBT-10, as shown by specular XRD as well as by GIXRD treatment close to the liquid crystalline state allows the investigations. molecules to orient into the expected molecular alignment The outstanding experimental observation is that the peak determined by the crystal structure. ■ width of the specular diffraction shows a different peak- broadening behavior for the odd-numbered and even- numbered 00L peaks. For an explanation of this effect, a ASSOCIATED CONTENT molecular disorder was assumed. Simulations are started with * sı Supporting Information the layered structures of Ph-BTBT-10 with a double The Supporting Information is available free of charge at herringbone structure with head-to-head-oriented molecules. https://pubs.acs.org/doi/10.1021/acs.chemmater.0c04725. Gradually introducing defects by inverted molecules, the odd- numbered peaks start to broaden while the even-numbered Reciprocal space map of a thin film prepared by a peaks remain at their initial peak widths. This phenomenon defined cooling rate and that of a thin film prepared by starts already at small deviations from the ideal crystal bar coating; peak width analysis of thin films prepared by structure. It is found that the fraction of head-to-head aligned defined cooling rates compared with the results of peak molecules (molecular disorder parameter p) is a meaningful width simulations (PDF). ■ parameter which describes the observed effect. Different types of the molecular disorder are investigated. The systematic inversion of all the molecules within one layer or the inversion AUTHOR INFORMATION of random molecules lead to the same result. The assignment Corresponding Author of the observed peak broadening to the disorder parameter p Roland Resel − Institute of Solid State Physics, Graz reveals values of 0.15 for a gradient crystallized sample and University of Technology, Graz 8010, Austria; orcid.org/ values in between 0.15 and 0.07 for samples prepared at 0000-0003-0079-3525; Email: roland.resel@tugraz.at different cooling rates. An interesting effect is that peak broadening disappears as a Authors function of heat treatment. A change in the peak width is Sebastian Hofer − Institute of Solid State Physics, Graz observed when the thin film is heated (from room temper- University of Technology, Graz 8010, Austria ature) close to the phase-transition temperature of the smectic Johanna Unterkofler − Institute of Solid State Physics, Graz E phase. Peak broadening disappears fully at the phase- University of Technology, Graz 8010, Austria transition temperature, which means that the molecular Martin Kaltenegger − Institute of Solid State Physics, Graz disorder is strongly reduced. Taking our disorder model into University of Technology, Graz 8010, Austria; Laboratoire de account, the flip-flops of molecules have to appear close to the Chimie des Polymères, Faculté des Sciences, Université Libre transition temperature to the smectic E phase, which can be de Bruxelles, Bruxelles 1050, Belgium explained by a pure translational movement of molecules Guillaume Schweicher − Laboratoire de Chimie des across different layers.37,38 Polymères, Faculté des Sciences, Université Libre de Bruxelles, The ability of the molecules to reverse its molecular Bruxelles 1050, Belgium; orcid.org/0000-0002-6501- orientation at elevated temperatures is a known phenomenon. 0790 The flip-flop motions of individual molecules are frequently Christian Ruzié − Laboratoire de Chimie des Polymères, observed in the liquid crystalline state.39 Phase transitions (i.e., Faculté des Sciences, Université Libre de Bruxelles, Bruxelles nematic to isotropic) are analyzed on the basis of flip-flop 1050, Belgium motions together with the available space of the molecules Adrián Tamayo − Institut de Ciència de Materials de within the liquid crystalline phases.40,41 However, liquid Barcelona, ICMAB-CSIC, 08193 Bellaterra, Spain crystalline states with a higher degree of order provides less Tommaso Salzillo − Institut de Ciència de Materials de space for molecular motion, and as a consequence, the Barcelona, ICMAB-CSIC, 08193 Bellaterra, Spain; molecules are less prone to flip-flop transitions.42,43 Also, in the orcid.org/0000-0002-9737-2809 1459 https://dx.doi.org/10.1021/acs.chemmater.0c04725 Chem. Mater. 2021, 33, 1455−1461 Chemistry of Materials pubs.acs.org/cm Article Marta Mas-Torrent − Institut de Ciència de Materials de (8) Karl, N. In Charge-Carrier Mobility in Organic Crystals; Barcelona, ICMAB-CSIC, 08193 Bellaterra, Spain; Farchioni, R., Grosso, G., Eds.; Springer Series in Materials Science, orcid.org/0000-0002-1586-005X 41. Springer: Berlin, Heidelberg, 2001; Chapter 8 , 283−326. Alessandro Sanzone − Department of Materials Science, DOI: 10.1007/978-3-642-56425-3_8 (9) Izvekov, S. V.; Sugakov, V. I. Exciton and Electron Traps on University of Milano-Bicocca, Milano 20125, Italy Structural Defects in Molecular Crystals with Dipolar Molecules. Phys. Luca Beverina − Department of Materials Science, University Status Solidi 1995, 191, 449−453. of Milano-Bicocca, Milano 20125, Italy; orcid.org/0000- (10) Chapman, B. D.; Checco, A.; Pindak, R.; Siegrist, T.; Kloc, C. 0002-6450-545X Dislocations and Grain Boundaries in Semiconducting Rubrene Yves Henry Geerts − Laboratoire de Chimie des Polymères, Single-Crystals. J. Cryst. Growth 2006, 290, 479−484. Faculté des Sciences and Laboratoire de Chimie des (11) Bennett, D. W. Understanding Single-Crystal X-Ray Crystallog- Polymères, Faculté des Sciences, International Solvay raphy; Wiley-VCH: Weinheim, 2010. Institutes of Physics and Chemistry, Université Libre de (12) Haruta, M.; Kurata, H. Direct Observation of Crystal Defects in Bruxelles, Bruxelles 1050, Belgium an Organic Molecular Crystals of Copper Hexachlorophthalocyanine by STEM-EELS. Sci. Rep. 2012, 2, 2−5. Complete contact information is available at: (13) Jones, A. O. F.; Chattopadhyay, B.; Geerts, Y. H.; Resel, R. https://pubs.acs.org/10.1021/acs.chemmater.0c04725 Substrate-Induced and Thin-Film Phases: Polymorphism of Organic Materials on Surfaces. Adv. Funct. Mater. 2016, 26, 2233−2255. Notes (14) Chan, E. J.; Goossens, D. J. Study of the Single-Crystal X-Ray The authors declare no competing financial interest. Diffuse Scattering in Paracetamol Polymorphs. Acta Crystallogr. Sect. B ■ Struct. Sci. 2012, 68, 80−88. (15) Welberry, T. R.; Weber, T. One Hundred Years of Diffuse ACKNOWLEDGMENTS Scattering. Crystallogr. Rev. 2016, 22, 2−78. This work was supported by the Fonds de la Recherche (16) Williamson, G. K.; Hall, W. H. X-Ray Line Broadening from Scientifique (FNRS) and the Fonds voor Wetenschappelijk Filed Aluminium and Wolfram. Acta Metall. 1953, 1, 22−31. (17) Hardigree, J. F. M.; Ramirez, I. R.; Mazzotta, G.; Nicklin, C.; Onderzoek − Vlanderen (FWO) under EOS project 30489208 Riede, M. In-Situ Observation of Stacking Fault Evolution in Vacuum- and the Austrian Science Fund (FWF): [P30222]. YG is also Deposited C60. Appl. Phys. Lett. 2017, 111, No. 233305. thankful to FNRS for financial support through research (18) Brandt, H.-J.; Resel, R.; Keckes, J.; Koppelhuber-Bitschnau, B.; projects BTBT n° 2.4565.11, Phasetrans n° T.0058.14, Pi-Fast Koch, N.; Leising, G. Determination of Crystallite Size and Lattice n° T.0072.18, and 2D to 3D No. 30489208. Financial supports Strain in Hexaphenyl Thin Films by Line Profile Analysis. MRS Proc. from the French Community of Belgian (ARC n° 20061) and 1999, 561, No. 161. by the Walloon Region (WCS No. 1117306, SOLIDYE n° (19) Iino, H.; Usui, T.; Hanna, J. I. Liquid Crystals for Organic 1510602) are also acknowledged. G.S. acknowledges post- Thin-Film Transistors. Nat. Commun. 2015, 6, 6828. doctoral fellowship support from the FNRS. M.M. thanks the (20) Wu, H.; Iino, H.; Hanna, J. I. Scalable Ultrahigh-Speed Spanish Ministry project GENESIS PID2019-111682RB-I00, Fabrication of Uniform Polycrystalline Thin Films for Organic Transistors. ACS Appl. Mater. 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