2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim - PDF document

2016 WILEY-VCH Verlag GmbH & Co. KGaA, W
2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheimwileyonlinelibrary.comPredictive Model for the Meniscus-Guided Coating of High-Quality Organic Single-Crystalline Thin FilmsRobby Janneck,* Federico Vercesi, Paul Heremans, Jan Genoe, and Cedric RolinR. Janneck, F. Vercesi, Prof. P. Heremans, Prof. J. Genoe, Dr. C. Rolinrobby.janneck@imec.beESATtechniques. To this end, we zone-cast two different organic tures, and with different solvents.ferent solvents. leading to solute super-saturation and precipitation in that zone. Most MGC tech-niques, however, make use of an external shearing force, for casting. This provides enhanced control, uniformity, and coating speed . So far, as no methodology exists to evaluate Following the distinction between Landau–Levich and evapoollowing the distinction between Landau–Levich and evapo optimal values for vc published in the MGC literature can be divided in two groups: The fast-processing group with vc (1 mm s1)  vefe and important shearing forces applied to the meniscus cor-responds to the Landau–Levich regime. In this case, solvent evaporation is decoupled from the meniscus motion and a partially wet lm is deposited. Variations of can deliver difdeliver dif and/or different polymorphs of the semiconductor crystal.crystal. In contrast, the slow-processing group corresponds to the evaporative regime: it utilizes much lower vc (10–100 µm s1) vefe and reduced shearing forces. It is this type of slow processing in the evaporative regime that we address in this work by developing a predictive model for vefe.Jang et al. have shown that for slow processing, the optimal coating speed depends on the temperature and choice of soland choice of sol Pure solvent systems are therefore appropriate to study the Figureis applied to the drop. It is sustained at the edge of a structure structure We conducted edge-in Table S1 (Supporting Information). The results of this experFigureArrhenius plot is given in Figure S3, Supporting Information), geometry. We rst assume that the evaporative mass transfer ductors for efcient charge transport in high-performance cient charge transport in high-performance – a number of solution-based techniques have been recently developed to grow highly crystalline lms of organic small molecules on large areas.on large areas.– Among them, meniscus-guided coating (MGC) techniques, such as zone casting,[9–11] dip coating,[12,13] solution shearing,[14–17] hollow pen writing,[4] or modied edge casting,[18] can potentially be applied for large-area coatings. They all rely on a unidirectional displacement of a droplet of solution of the organic semiconductor across the the The growth by MGC, however, results from a complex combination of physical phenomena taking place at different length different organic solute/solvent systems and different coating techniques, there exist different process windows to achieve ferent process windows to achieve So far, however, optimization has magnitude. A unied understanding of the inuence of solvent c

hoice and surface temperature on the opt
hoice and surface temperature on the optimal coating speeds and process transfer to different material systems and coating a pinned drop of pure solvent is receding under the sole effect for a variety of solvents and substrate temperatures. Next, we demonstrate that slow processing at the Adv.Mater.www.advmat.dewww.MaterialsViews.comCOMMUNICATION2wileyonlinelibrary.com 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheimrate is limited by the diffusion of solvent vapor away from the surface of the liquid. As detailed in the Supporting Information, this assumption is supported by ample evidence from the ln1vapbpPSRTT is the boiling point of the solvent at atmospheric presthe compensation between vaporization enthalpy and entropy: vapbvapHTS, where vapH is the vaporization enthalpy. To establish a link between , we now turn to the uid Dash and Garimella,arimella, we show in the Supporting Information efeM00.7vpVDTeRPh is the diffusion constant of the solvent vapor, . The temperature dependence in Equation (2) results from the thermal activation of the diffusion of solvent vapor away from the meniscus. Finally, Equations (1) and (2) are combined expefeM0.7vapbvAVTSRTTIn Figure 2b, we replotted the 66 data points of Figure 2a data for the different solvents fall together along one single line, drawn by tting Equation (3) to the data, using /vapSRxed to 10.5 (the same entropy of evaporation for all solvents, following Trouton’s rule).’s rule).– This single parameter t delivers the intercept A  1870  66 µm s1 mol mL1 K0.7 with a R2  94%. The ability of model in Equation (3) to capture the behavior of most pure solvents over a broad temperature range is explained as follows: The use of Trouton’s rule with a unique vapShave similar diffusivities, reducing the inuence of the solvent Adv.Mater.edgestructureissolutionleSchematic illustration of the setups used in this study. a) Edge- the solvent. The line is a linear t using the intercept as only tting parameter. The single parameter is then used to recalculate the solvent lines in (a).3wileyonlinelibrary.com 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheimin Equation (2). Finally, all edge-cast solvents formed a shallow and ) factors in Equation (2), hence a similar factor in capture individual solvent behavior. First, Trouton’s rule is a remarkable approximation that has shown a broad validity over many liquids, especially for nonpolar, quasi-spherical , quasi-spherical Trouton’s rule, however, shows limitations when applied over a wide range of temperatures and a temperature-ting, especially in the high region. Second, the ideal gas tration in Equation (2). Further renement can be obtained der Waals equation. Third, we set the temperature dependence of the gas diffusivity to solvent-specic values for the exponent of . Finally, small variations of the contact angles of the different solvent lines. Using th

e exact convergence of all solvent curve
e exact convergence of all solvent curves when plotted in Figure 2b. valid for most solvents. To verify that the values of dicted by Equation (3) effectively dene a slow process window matically depicted in Figure 1b to prepare lms of organic semiconductors on Si substrates covered with thermally grown SiOtaining good control over other growth parameters. We prowith different solvents and two semiconductors (2,7-dioctyl[1]ferent solvents and two semiconductors (2,7-dioctyl[1](8-BTBT) and 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-P5)). For these different and electrical quality, using the normalized coating speed /ccefevvvWe use polarization light microscopy images (PLM) and atomic -BTBT in heptane on a subFigure-BTBT lms cast at 0.5cv, 1, 3, and 5 (91msefe1vlayers, as can be seen by the decreasing image brightness. The AFM pictures of the same lms in Figure 3e–h support 0.5cv to 15 nm at 5cv. Despite being slightly thicker, the 0.5vc presents a similar AFM morphology as the lm produced at 1cv (Figure 3e,f). to the casting direction. At a lower magnication in the PLM image, however, large aggregates are scattered on the lm cast at the slowest speed (Figure 3a). These originate from the paraAdv.Mater.Figure 3.Typical morphology of C-BTBT lms cast at different coating speeds, photographed by polarized light microscopy with constant light expo/ccefevvv increasing from left to right. 91msefe1vtions. Arrows represent the direction of coating. Best morphology corresponds to 1cv (red dashed box). Slower speeds lead to aggregates of small 4wileyonlinelibrary.com 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheimsupply. But this yields a rise of organic solute concentration in 1cvFigure 3c,d. But the parallel ribbons in the AFM images tend to narrow and thin down (Figure 3g) and eventually form discontinuous islands (Figure 3h). Clearly, the lms produced at (1)cv present the best morphology, characterized by a thin single-crystalline lm that almost fully covers the substrate -BTBT lms cast at different X-ray diffraction (XRD). The scans are overlaid in Figure S4 (Supporting Information). The peak positions are mostly insensitive to , delivering for all speeds and temperatures an average vertical inter layer spacing of -BTBT lms.T lms. The micro-structure of C8-BTBT lms processed with low shear force throughout the explored parameter set as well as with the other cating organic thin-lm transistors (OTFTs). A highly doped Si substrate with 125 nm thick SiO served as common gate and dielectric. Gold electrodes were evaporated through a shadow Figuretively, shows the saturation transfer curve and output curves -BTBT OTFT based on a lm cast in the conditions of Figures 3b,f, at 1cv. These characteristics are well behaved, with limited hysteresis, a high on/off ratio, a low subthreshold 5.5 V and a threshold voltage 11.4 V. Using the transconductance method to extract eff

ective mobility eff, Figure 4c shows tha
ective mobility eff, Figure 4c shows that eff stabilizes to range. This stable value reects the quality of the semiconductor/dielectric interface and the absence of adverse effects from nonlinear contact resistance.fects from nonlinear contact resistance. Furthermore, 75 OTFTs were measured on the same 2 cm 2 cm eff(Figure 4d) and 1.5 V. We suspect two main reasons behind this statistical spread: First, each channel contains only a limited number of grains, with intrinsically different charge carrier mobilities due to the anisotropy of transport -BTBT single crystals.T single crystals. Second, there is likely a spread in contact resistance, stemming from both energetic level mismatch and damage incurred during gold deposition. Both these problems can be minimized, but device optimization is beyond the scope of the present study.Similar electrical measurements were conducted on lms produced across a large zone-casting parameter set. For each eff was averaged over 50 different devices Figureevolution of eff for lms produced with different temperatures (Figure 5a), solvents (Figure 5b) and semiconductors (Figure 5c). The average eff of the lms imaged in Figure 3 is represented by the red triangles in Figure 5a. The highest eff is 1cvmorphology. At low speeds, 1cvcontact resistance. At high speeds, 1cvresulting in lower effective channel width, and are sometimes When testing the zone casting of C-BTBT in heptane with rising substrate temperatures in Figure 5a, larger are . Despite over two orders of magnitude (see Figure S5a, Supporting Information), the highest effatically obtained at 1cv for all tested substrate temperatures. Interestingly, the highest eff1cv show a non-monotonous evolution with substrate temperature. Films produced at 20 and C have a much better morphological and electrical quality C. This phenomenon is linked to the complex effects of temperature on the dynamics of lm formation, which remain to be elucidated. A similar trend is obtained when testing three different solvents for the zone -BTBT at 20 C in Figure 5b. Despite the different resulting in different and requiring different (see Figure S5b, Supporting Information), the best electrical performance is again obtained at 1cvindependently of the solvent. Only the knowledge of solvent Adv.Mater.Typical electrical characteristics of OTFT devices 1900/240 µm) prepared from a solution of 0.25 wt% Cture. a,b) Transfer and output characteristics, respectively. c) Extracted 75 OTFTs measured on the same sample.5wileyonlinelibrary.com 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim is required to predict optimal coating . Finally, the same holds true upon exchange of the organic semiconductor. Figure 5c compares the evolution of CBTBT and TIPS-P5 processed in the same conditions in heptane C. Similarly to C-BTBT, the best TIPS-P5 lm is obtained when cast at 1cv, with average eff 0.32 0.09 cm V sThe transfer and output curves of a typical TIPS-P5 transistor are shown in Figure S6 (Supporting Information). The morphologies o

f both semiconductors are very similar (
f both semiconductors are very similar (see Figure S7, Supporting Information), reinforcing the observation that the nature of the organic solute only has a secondary inuence on We further verify the predictive power of our model by Table the optimized coating speeds reported calculated using strate temperatures. We nd that in the seven investigated shows a good match with our calculated speeds, conrming that calculated denes a successful process window for a variety of MGC techniques such as dip hollow-pen writing,[4] solution shearing,[14] or Notably, one of the investigated papers and another one grows thin polymer The reason why vefe constitutes such a good predictor for vc in low-shear conditions is not obvious. is determined from shown in Figure 1b, solution drops are concave and extend over a short distance only. The concavity tends to reduce evaporareduce evapora Besides, the presence of a solute slows solvent evaporation due to colligation effects.fects. In consequence, differences cersolute dependent. Yet the good correspondence we observe First, the meniscus can dynamically “absorb” small speed varianegligible shearing forces (see Figure S8, Supporting Information, for a comparison of drop shapes at different processing in low shearing conditions at the equilibrium front Adv.Mater.Average effective charge carrier mobilities -BTBT dissolved in heptane. b) Coating at room temperature with C1Table 1Comparison of optimum speeds predicted in this work with experimental results obtained in previous publications. /ccefevvv is the reported OTFT effective mobility.TechniqueTemperature [cv2 V1 s1]Su[21]Zone castingTIPS-P5Chloroform20.053500.90.67Jang[13]Dip coatingFTES-ADTDCM27.51161501.31.50Jang[13]Dip coatingTIPS-P5Chloroform27.572500.71.50Hofmockel[14]Solution shearingC10-DNTTDCB130.0106830.83.1Wo[4]Hollow-pen writingTIPS-P5TolueneolueneModied edge-castingC10-DNBDTDCB80.018301.79.50Rogowski[12]Dip coatingTIPS-P5IPA/toluene20.017201.20.75Schuettfort[37]Zone castingPBTTTDCB100.039300.80.29This workZone castingC8-BTBTHeptane50.091911.07.0a)FTES-ADT: uorinated 5,11-bis(triethylsilylethynyl) anthradithiophene; C-DNTT: 2,9-di-decyl-dinaphtho-[2,3-b:2-f]-thieno-[3,2-b]-thiophene; DNBDT: : ,3-d]benzo[1,2-b:4,5-b]dithiophene; PBTTT: poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene]; DMCM: dichloromethane; 6wileyonlinelibrary.com 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheimvent driven by evaporation only. The relation is valid for a wide crystalline organic thin lms. Films zone-cast with different substrate temperatures, solvents, and organic semiconductors teristics when the coating speed is equal to the predicted -BTBT lms show effective mobilities up to 7 cmtaining low threshold voltages. Furthermore, a literature survey reveals that the predictive model is valid for several different meniscus-guided coating techniques. Therefore, our approach offers a simple but versatile starting point to determine the low shear forces. This is superior to the simple trial-and

-error method used so far: we eliminate
-error method used so far: we eliminate some of the unknowns in the complex dynamics underlying single organic crystal formation Materials and Substrate Preparation: Highly doped silicon substrates water, acetone, and isopropyl alcohol. After exposure to UV-ozone for C and pumped down to 15 mbar. This -BTBT was supplied by Nippon Kayaku Co. and puried using a tri-zone purication oven from Creaphys. TIPS-pentacene was bought from : Edge-casting experiments were performed as previously described by Uemura et al. on a sample kept in a horizontal A 60 µL of pure solvent was dropped on the edge of a silicon holding piece. Zone-casting experiments were done on a home-built slot-die coater schematically depicted in Figure 1b. The gap between the Teon blade and the substrate was xed at 250 µm. of solute in solvent) was injected into the slot-die blade by an automatic syringe pump in order to ll the dead space inside the blade and form the initial drop in contact with the substrate. Afterward the substrate was translated to . To keep the meniscus shape constant during the coating process, solution was added by the syringe pump at a resupply rate adjusted to the substrate temperature and the Morphological Characterization: The evaporation times of the edge Velocity was averaged from at least eight measured values. AFM studies XRD measurements were done on a PANalytical X’Pert Pro Materials Transistor Fabrication and Characterizationcontacts for OTFTs were vacuum deposited through a shadow mask, with a deposition rate of 0.1 nm s and a substrate temperature of 240 and 1900 µm, respectively. This value of the width was always used for the analysis, although the real effective width of the transistor may at a temperature of 50 C for 40 h. Electrical characterizations were done using an Agilent Agt1500 in dry air. Field-effect mobilities were evaluated in the saturation regime by conventional transconductance analysis (2/)(1/)/FETiDG2LWCIV40 V. A total of 50 OTFTs for each experimental condition (substrate temperature, from the author.This work has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement no. 320680 (EPOS CRYSTALLI) and from the Research Foundation Flanders (FWO Vlaanderen) under the (ORSIC-HIMA). The authors thank Nippon Kayaku Co. for supplying the -BTBT used in this study..   G.    Horowitz, M. E.    Hajlaoui, Synth. Met. 2001, 122, 185.[2]   S. S.    Lee, C. S.    Kim, E. D.    Gomez, B.    Purushothaman, M. F.    Toney, Wang, A.Hexemer, J. E.Anthony, Y. L.Adv. Mater..   J.    Rivnay, L. H.Toney, R.Lu, T. J.Facchetti, A.Nat. Mater..   S.    Wo, R. L.Anthony, ,    Y.Shaw, S. C. B.Energy Environ. Sci.Sci.   L.    Shaw, Z.Isr. J. Chem.. J. Chem.   H.    Sirringhaus, Adv. Mater..   T.Takeya, Sci. Technol. Adv. Mater..

  W.Tracz, Sirringhaus, T.Adv. M
  W.Tracz, Sirringhaus, T.Adv. Mater..   C. M.Duffy, J. W.Andreasen, D. W.Breiby, M. M.Nielsen, M.T.Chem. Mater..   Y.Xing, Y..   R. Z.    Rogowski, A.    Dzwilewski, M.    Kemerink, A. A.    Darhuber, ,    J.    Jang, S.    Nam, K.    Im, J.    Hur, S. N.Anthony, J. J.Adv. Funct. Mater..   R.    Hofmockel, U.    Zschieschang, U.Rödel, N. H.Würthner, K.Takimiya, K.Adv.Mater.7wileyonlinelibrary.com 2016 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimAdv.Mater.   G.    Giri, E.    Verploegen, S. C. B.Atahan-Evrenk, D. H.Kim, S. Y.Lee, H. A.Becerril, A.Aspuru-Guzik, M. F.Toney, ,    Y.Tee, G.R. M.Stoltenberg, T. H.Lee, G.Xue, S. C. B.Mannsfeld, Z.Nat. Mater..   M. R.    Niazi, R.    Li, M.    Abdelsamie, K.    Zhao, D. H.    Anjum, M. M.    Payne, J.    Anthony, D.-M.Adv. Funct. Mater..   J.    Soeda, T.Uemura, T.Okamoto, C.Mitsui, M.Yamagishi, Takeya, akeya,    D. M.    Smilgies, R.    Li, G.    Giri, K. W.Chou, Y..   G.    Giri, R.    Li, D.-M.    Smilgies, E. Q.    Li, Y.Chiu, D. W.S. T.Clancy, Z.Nat. Commun.ommun.   Y.Xing, Y..   H.    Hu, R. G.    Larson, J. Phys. Chem. B 2002, 106, 1334.[23]   Y. O.Popov, Phys. Rev. E. E   M.    Le Berre, Y..   T.Uemura, Y.Takimiya, J.Takeya, akeya,    N.    Murisic, L.    Kondic, J. Fluid Mech. 2011, 679, 219.[27]   S.    Semenov, A.Trybala, R. G.Kovalchuk, V.Starov, Velarde, Adv. Colloid Interface Sci.olloid Interface Sci.   E.    Sultan, A.    Boudaoud, M.    Ben Amar, ,    S.    Dash, S. V..   D.    Mackay, A.Bobra, D. W.Chan, W. Y.Environ. Sci. Technol.echnol.   L. K.    Nash, J. Chem. Educ. 1984, 61, 981.[32]   J. K.    Fink, Physical Chemistry in Depth, Springer-Verlag, Berlin, erlag, Berlin,    H.    Ebata, T.Miyazaki, K.Takimiya, M.T.Yui, ui,    T.Takimiya, Adv. Mater..   T.Rolin, T. H.Fesenko, J.Takeya, Adv. Mater..   H.    Kobayashi, N.    Kobayashi, S.    Hosoi, N.    Koshitani, D.    Murakami, R.    Shirasawa, Y.Hobara, Y.Tokita, M.okita, M.   T.Watts, L.atts, L.   M. E. R.    Shanahan, Langmuir 2002, 18, 7763.[39]   P.    Letellier, A.Turmine, Colloids Surf., Aolloids Surf., A   D.    Kondepudi, I.    Prigogine, Modern Thermodynamics: From Heat , John Wiley & Sons, New York, COMMUNICATIONwww.advmat.dewww.MaterialsViews.