Growth of Highly Oriented Ultrathin Crystalline Organic Microstripes

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Growth of highly oriented ultra-thin crystalline organic micro-stripes: effect of alkyl chain length Tao Zhu, Chengliang Xiao, Binghao Wang, Xiaorong Hu, Zi Wang, Jian Fan, Lizhen Huang, Donghang Yan, and Lifeng Chi Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b01349 • Publication Date (Web): 22 Aug 2016 Downloaded from http://pubs.acs.org on August 24, 2016

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Growth of highly oriented ultra-thin crystalline organic micro-stripes: effect of alkyl chain length Tao Zhu1, Chengliang Xiao2, Binghao Wang1, Xiaorong Hu1, Zi Wang1, Jian Fan1, Lizhen Huang1*, DonghangYan3, Lifeng Chi1*

1

Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory

for Carbon-Based Functional Materials & Devices, Soochow University, 199 Ren'ai Road, Suzhou, 215123, Jiangsu, P.R. China 2

School for Radiological and Interdisciplinary Sciences, Soochow University, 199

Ren'ai Road, Suzhou, 215123, Jiangsu, P.R. China 3

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of

Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P.R. China *corresponding author: Email: [email protected], [email protected]. Fax: +8651265880820, Tel: +8651265880725

Keywords: alkyl chain length, dip-coating, organic microcrystals, organic thin film transistors

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ABSTRACT Growth of organic semiconductor with controllable morphology is a crucial issue for achieving high performance devices. Here we present the systematic study of the effect of alkyl-chain attached to the functional entity on controlling the growth of oriented microcrystals by dip-coating. Alkylated DTBDT-based molecules with variable chain lengths from n-butyl to n-dodecyl formed into one-dimensional micro- or nano-stripe crystals at different pulling speeds. The alignment and ordering are significantly varied with alkyl chain length, as well as the transistor performance. Highly uniform oriented and higher molecular order crystalline stripes with improved field-effect mobility can be achieved with alkyl-chain length around 6. We attribute this effect to the alkyl-chain-length dependent packing, solubility and self-assembly behavior.


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1. Introduction Organic semiconductors have drawn extensive attentions over 30 years owing to their potential in low cost, large area manufacture and flexible electronics.1 The morphology of such semiconductor is one of the most important factor that determined the property and device performance.2-3 Highly oriented, low dimensional organic thin films fabricated from solution process are good candidates for devices such as high mobility transistor, high performance gas sensor.4-8 However, growth of thin film with controllable morphology remains a challenge. Many efforts from processing method, material design, solvent choice or substrate modification have been made aiming to achieve high quality films with controllable morphology.4-11 Dip-coating, a traditional solution approach, has been successfully utilized in fabricating thin films of organic materials such as sol-gel or block copolymers and so on, since it enables to obtain precisely-thickness-controllable films through tuning the interplay between the capillary and gravitational force on the three phase line, accompanied with the solvent evaporation or liquid flow effect12-16. Recently, dip-coating process was applied to grow organic semiconductor micro-stripes with unique one-dimensional morphology and good orientation, which could be a potential method for growing organic semiconductor structured thin films with controllable orientation and thickness17-22. Precise thickness control from monolayer to multiple layers accompanied with good alignment along the pulling direction of these micro-stripes was achieved19, 22-23, which enables to realize high performance organic gas sensors and transistors22.


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Although such highly oriented structured ultra-thin films can be achieved through dip-coating, only few materials were successfully applied and the detailed factors and mechanism are still unclear. Materials structure, as an intrinsic effect for thin film and crystal growth, is the key factor that determined the morphology23-36. Here, we systemically investigate the effect of alkyl chain length on the growth of organic micro-stripes or structured thin films by such dip-coating process. The DTBDT was selected as the backbone, and alkyl chain from n-butyl to n-dodecyl were attached. Microcrystal thickness, orientation and packing structure of the series molecules were investigated and effect of alkyl chain length on growing such oriented micro-stripes will be discussed, together with the correlated field effect transistor performance.

2. Experimental section Materials synthesis: As demonstrated in Scheme 1, the [2, 3-d; 2’, 3’-d’] benzo [1, 2-b; 4, 5-b’] dithiophene (DTBDT) derivatives of the symmetric structures bearing linear alkyl chains from n-butyl to n-dodecyl (indicated by C4 to C12) in the molecular long-axis direction were synthesized via two-step reactions of Stille coupling and intramolecular ring-closure according to previous reports17. The chemical structures were fully characterized by 1H NMR and mass spectroscopy (MS) analyses (see Supporting Information). The solubility of the series of molecule from C4 to C12 in toluene at room temperature are 19.3，13.8，11.0，7.6，6.4 and 4.8 mmol/L, respectively.


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H 3C

S

O S

Br

i)Pd(PPh3)4 , DMF

R Br

S

SnMe3

R

ii)P2O5, CF3SO3 H, Pyridine

R S

S O

S

S

CH3

R1=C4H9 R2=C6H13 R =C4H9 R3=C7H15 1R4=C H R =C6H813 17 R5=C9H19R2R =C 6 H12H25 3 =C 9 19 R4 =C12H25

Scheme 1. Synthesis of the four predesigned symmetric alkylated DTBDT derivatives. Single crystal growth and characterization: The single crystals were grown from the solution in CH2Cl2. Data collection was performed on a Bruker D8-Venture diffractometer with a Turbo X-ray Source (Mo–Kα radiation, λ = 0.71073 Å) adopting the direct-drive rotating anode technique and a CMOS detector at room temperature. The data frames were collected using the program APEX2 and processed using the program SAINT routine in APEX2. The structures were solved by direct methods and refined by the full-matrix least squares on F2 using the SHELXTL-97 program. Thin film and OFET fabrication: p-doped silicon wafers with 300 nm SiO2 (Ci=10 nF·cm-2) were used as the substrates. The structured films were prepared through dipping the substrates into the DTBDT solution with toluene as the solvent and pulling out at controlled speeds. The solution concentration was made for 6.4 mmol/L for all the molecules used. However, for longer alkyl chain, especially DTBDT-C12, the solution has to be heated to reach this concentration owing to their less solubility in toluene. So for longer alkyl chains, the effective concentration at room temperature is lower. The process was carried out in the dip-coating system with a sealed chamber at room temperature. Top contact OFETs were fabricated on the above dip-coating


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films. Gold source and drain electrodes (30 nm) were evaporated through a shadow mask on top of the semiconductor layers by using a Nano 36 thermal evaporator (Kurt J. Lesker Company, USA). The transistor performance was measured in air with a Keithley 4200SCS semiconductor parameter analyser integrated with a probe station of Model 8060 from Micromanipulator Co., Inc. Thin film characterization: Optical micrograph was obtained on a Leica optical microscopy. Atomic force microscopy (AFM) measurements were conducted with a Bruker Dimensional Icon in tapping mode. The electron diffraction was performed with JEOL JEM-1011 transmission electron microscope operated at 100 kV. Dark field was used for experiments to provide weaker-intensity beam and high contrast. The out-of-plane and in-plane XRD measurements were carried out at BL14B1 in Shanghai Synchrotron Radiation Facility (SSRF) with a incidence wavelength of 1.24 Å.

3. Results and discussion The dip-coating films, as suggested in many systems, presented pulling speed varied morphology23, especially the thickness, which result from the interplay between capillarity and draining force17, 23. In order to clearly understand the role of materials structure in growing oriented micro-stripes during dip-coating process, we fabricated a series of films based on the DTBDT derivatives at pulling speed of 60, 800 and 5000 µm/s respectively. Figure 1 shows the optical micrographics of the series of DTBDT films under different pulling speeds. All the molecules formed into long dendritic stripe crystals with preferred orientation, like an array of


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one-dimensional micro-or nano-crystals. The dendritic stripes mainly consisted of large long stems attached with many branches by certain angles. The length of such stripe stems can be up to several hundred micrometers, which makes it easy to transport charge along the long stripes and allows simple fabrication of devices. With increased pulling speed, the stripe width increased obviously for all the molecules from 5-6 µm at low speed to 20-30 µm at high speed as shown in Figure 1, with no relevance to alkyl chain length. However, the orientation, coverage and size of the dendritic grains varied with the alkyl chain length and pulling speed. Generally, the micro-stripe orientation maintained a good manner when alkyl chain is smaller than 8, but gradually changed to a random feature for longer chain. This orientation was mainly revealed in two aspects: one is the stem morphology; the other is the angle between stem and branches. Along with the increased alkyl chain, the stripe stems tended to be parallel to the pulling direction but gradually deviated when chain length longer than 6, and stem shape were obviously bend for long chain molecules. Meanwhile, the angles between stems and branches changed from more regular to random. Micro-stripes of C6 and C7 attached molecules exhibited the strongest alignment and the straightest stems. While such good crystal alignment gradually disturbed when alkyl chain length is larger than 9 or shorter than 6. For chain length up to C12, spiral even circle-type stripes were formed, no matter at low speed or high speed. Such spiral or circle stripes significantly disturb the stripe size and continuity along the pulling direction. Figure S1 (Supporting information) shows the statistical deviation angles of stems from pulling direction and the angles between stems and


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branches of the series of structure thin films , which further revealed the alkyl chain varied orientation. Small angle within a small scale of C6 and C7 molecules accompanied with nearly constant angles between stems and branches demonstrated the strongly oriented manner and uniform morphology of all the micro-stripes. While the molecules with C4 chain or chain longer than 9 present angles range from 0o to 90o, indicated relative random orientation and curl morphology, as well as the relative random arrangement between stems and branches. The results indicated the alkyl chain length significantly influences the tailor effect of dip-coating process to the molecules.

Figure 1. Optical micrographics of large-area stripes of (a-c) DTBDT-C4 (d-f)


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DTBDT-C6 (g-i) DTBDT-C7 (j-l) DTBDT-C8 (m-o) DTBDT-C9 (p-r) DTBDT-C12 dip-coated at a pulling speed of 60, 800, 5000 µm/s, respectively. The arrow indicated the pulling direction. Apparently, the substituted alkyl chains on the conjugated backbone influence the aggregation behavior during the fast dip-coating process from the morphology aspect. Aiming deeply realize such effect, AFM were used to further characterize the detailed morphology. Figure 2a-f shows the AFM images of DTBDT-C6 and DTBDT-C12 films under pulling speed of 60, 800 and 5000 µm/s (AFM images of other compounds were summarized in Figure S2). Highly alignment stripe crystals of DTBDT-C6 with layered feature can be observed, especially for films fabricated in high pulling speed. While in contrast, the DTBDT-C12 films consisted of spiral crystal with a relative poor orientation and continuity. The height of each monolayer (dmonolayer) can be captured from the cross section profile of the AFM images, and the stripe thickness will be equal to the dmonolayer multiplied by layer number (N). The monolayer height of the series of molecules (Figure 2g) is close to the long axis of the molecule and presents a lineal correlation with the alkyl chain length, indicative of an edge-on orientation for all the molecules relative to the substrates. The layer number of all the films at different pulling speeds was statistically counted from AFM images, as illustrated in Figure 2h. The statistical results revealed that except the relative thicker for C4, the film thickness present a not strong but slight variation of the alkyl chain length. Generally, at the same pulling speed, the longer the alkyl chain was, the less layer number and thinner thickness the film was. The thickness of micro-stripe


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for low speed ranged from 5 nm to 12 nm, while from 9 nm to 20 nm for speed of 5000 µm/s.

Figure 2. AFM images of DTBDT-C6 films fabricated at (a) 60 µm/s (b) 800 µm/s (c) 5000 µm/s and DTBDT-C12 films at (d) 60 µm/s (e) 800 µm/s (f) 5000 µm/s, respectively. (g) Monolayer height of the series of molecule micro-stripes captured from the AFM images. (h) Statistical data of the layer number for DTBDT derivatives in different speeds. As presented above, highly oriented ultra-thin film can be easily achieved, while the orientation, size and thickness are modulated by alkyl chain length. Considering the mechanism of dip-coating, such a chain length dependency should relate to the evaporation rate, viscosity, capillary and gravitational force. Since all the molecules


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used the same solvent, the evaporation rate should be the same during the dip-coating process at the same speed. From the molecular aspect, the variation of alkyl chain length on one hand will directly result in different dissolving capacity in the solvent, which would be reflected in the concentration and viscosity of the solution28-31, on the other hand will lead to different assemble behaviors. In our case, the long alkyl chain substituent such as C12 made it more difficult to dissolve than the short alkyl chain such as C4 in toluene for the same amount of solute. During the dip-coating process, the poor dissolving capacity will result in small amount of solute molecules entrained in the solution dragged along the substrates under the same condition. Therefore, thickness will be smaller for molecules with long alkyl chain owing to the lower effective concentration. While more importantly, the longer alkyl chain leads to more flexibility and steric effect that influence the assembly process.33-34 As a result, the counterbalance between capillary and gravitational force will lead to the derivation of meniscus or entrained liquid from the upright direction, leading to spiral or circle grains on the substrates, especially in low speed regime.


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Figure 3. Single crystal packing of a) DTBDT-C4, b) DTBDT-C9

Besides the morphology, such alkyl chain length varied assemble behavior will reflect on the molecular packing and also on the film ordering which are crucial in device performance. Single crystal structure and structured ultra-thin film packing information are therefore analyzed for clear understanding. Table 1. Single crystal parameters of the C4, C6 and C9 attached molecules DTBDT-C4

DTBDT-C6a

DTBDT-C9

Crystal class

triclinic

triclinic

triclinic

Space group

P-1

P-1

P-1

a (Å)

5.7733(18)

5.3344(2)

5.2882(12)

b (Å)

7.759(2)

6.3989(4)

6.4950(14)

c (Å)

11.727(3)

17.6924(9)

21.505(5)

α (o)

103.068(11)

98.056(3)

86.953(10)

β (o)

103.960(10)

95.789(3)

84.312(10)

γ (o)

97.664(10)

97.869(5)

79.907(9)

Cell volume

486.684

587.87(5)

723.135

Z

1

1

1

R factor(%)

5.60

9.30

7.57

a. Single crystal data of DTBDT-C6 are cited from ref.17.

Single crystals of C4 and C9 were grown with sufficient size for characterization,


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while previous reported DTBDT-C6 single crystal information are used for comparison17. Table 1 summarized the single crystal parameters of C4, C6 and C9, Figure 3 illustrated the molecular packing in different planes. Similar to the DTBDT-C6 single crystal17, all the molecules adopted similar packing that the DTBDT bone present planar structure while the alkyl chain lies outside the plane of skeleton with alkyl chain varied derivation. The longer alkyl chain deviated from the backbone plane with a larger angle. Layer-by-layer molecular stacking are formed along the c axis for all the single crystals. While in the ab plane, molecules arranged into a cofacial packing with a certain sliding with each other. The interplanar distance that directly reflect the π-π interaction are 3.59 Å and 3.60 Å for C4 and C9, which is close to the value of C6 (3.63 Å). However, there exist a significant difference on the short contacts between short chain and long chain. Besides the backbone π interaction, there exist S-S interaction between adjacent molecules for C4 and C6. The shorter the alkyl chain, the stronger of the S-S interaction (reflected on the S-S distance which is 3.58 Å in DTBDT-C4, 3.73 Å in DTBDT-C6) and such short contacts does not exist for long chain like in the case of DTBDT-C9, which might be attributed to the crowd long alkyl chain. Such difference brought in distinct π-π interaction. Short chain attached molecules could provide a little bit two channel interactions by a certain loss of the backbone overlap that such loss is quite obvious in DTBDT-C4. While long chain attached molecules possess more π-π overlap between DTBDT backbone, which might lead to different performance on the charge transport. The out-of-plane XRD and 2D-GIXRD were performed to characterize the


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structure information of the series of micro-stripe films. We first compared the molecular ordering at different speeds (use DTBDT-C4 as an example), as shown in Figure S3a (supporting information). The films deposited at 5000 µm/s exhibited the strongest diffraction intensity than that formed at low speed, indicated better ordering in this speed. Figure S3b demonstrates the out-of-plane XRD patterns of the C4, C6, C9 and C12 films deposited on bare Si/SiO2 substrate by dip-coating at 5000 µm/s. The presence of highly ordered (00l) reflection peaks ranging up to multiple orders suggested a long-range ordered, lamellar crystalline feature of the series of films with an up-right molecular orientation. The first (001) peak along the out-of-plane direction were clearly observed at 2θ=4.69 °, 3.93 °, 3.20 ° and 2.75 °, with corresponding d-spacing of 1.51 nm, 1.81 nm, 2.22 nm and 2.58 nm respectively. Except the C4 molecules, these spacing are close to the corresponding (001) spacing of their single crystals. The larger (001) of C4 micro-stripe might indicated a smaller tilt angle of the molecule axis relative to the normal of substrate, compared with single crystal. More detailed in-plane packing and ordering are presented on the 2D-GIXRD patterns (Figure 4). The two dimensional diffraction pattern and intensity clearly present similar tendency as their orientation in the morphology. Several strong diffraction spots indexed to (01l) plane were observed for uniformly aligned micro-stripes of C6 or C7, while such diffraction information is obviously weaker for films of long chain molecules. DTBDT-C4, although possess a larger thickness gave few weak in-plane diffractions indicative of relative weak ordering and random alignment. For C9 and C12, barely clear in-plane diffraction information can be


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observed suggested a poor in-plane ordering originated from the small thickness, random alignment and low coverages.

Figure 4. 2D-grazing-incidence X-ray diffraction (2D-GIXRD) of a) DTBDT-C4, b) DTBDT-C6,

c)DTBDT-C7,

d)

DTBDT-C8,

e)DTBDT-C9,

f)

DTBDT-C12

micro-stripes fabricated at 5000 µm/s.. In order to further obtain the in-plane packing and structure of micro-stripes especially for molecules with long alkyl chain that their diffraction are not clearly


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shown in XRD, we further performed the selected area electron diffraction (SAED) which can provide in-plane diffraction for even single domain to check their in-plane packing. Figure 5a and b shows the diffraction pattern and corresponding morphology of C9 micro-stripes at speed of 5000 µm/s, others are listed in Figure S3c-e. Two dimensional quadrilateral unit cells can be clearly observed from the patterns (Figure 5a), where we defined the two axes as a* and b* respectively. The diffraction pattern and corresponding morphology indicated that for the dip-coated micro-striped, the preferred aggregation direction is the a axis which is also the π-π stacking direction. Figure 5c and d illustrate the molecule packing of the micro-stripe relative to substrate. Such molecular stacking revealed that during such fast dip-coating process, the π-π interaction mainly drive the molecule aggregation result in the formation of anisotropy one-dimensional stripe crystals. Alkyl chain attached on the backbone as we suggested above could influence such π-π interaction and then lead to obvious morphology differences, as well as the charge transport ability. In addition, the calculated lattice spacing corresponding to the diffraction spot (indexed according to the single crystal structure) from SAED (Table S1) demonstrate similar parameters to single crystal except C4. A relative smaller quadrilateral unit cell are observed for C4-DTBDT than its single crystal, which is reasonable because of the enlarged c axis.


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Figure 5. (a) Selected area electron diffraction (SAED) pattern and corresponding electron micrograph of DTBDT-C9 micro-stripes obtained at speed of 5000 µm/s, the black circle are the selected area for diffraction; c)The molecular orientation in the micro-stripe relative to the substrate and (d) the in-plane molecular packing of the micro-stripe. The structure analysis that the DTBDT derivatives bearing different alkyl chains adopted a similar symmetry packing when forming oriented ultra-thin micro-stripe through dip-coating, but the ordering and packing varied with chain length. This should be related to the alkyl chain affected intermolecular interaction and the aggregation behavior during the fast film formation process on the three-phase line. The moderate alkyl chain C6 and C7 presented the best ordering and a relative intimated in-plane packing. The packing in the micro-stripe clearly demonstrated a good charge transport path


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along the long axis of the micro-stripe, indicative of a good candidate for field effect transistors. The alkyl chain varied orientation, packing and ordering of the series of oriented ultra-thin films will directly reflect on their device performance. Considering the one-dimensional morphology feature, we fabricated a series of field effect transistors (FETs) through depositing gold electrodes on top of the micro-stripes with transport channel perpendicular to the long axis of the stripe. Figure 6a-b shows the transistor configuration and the real device image. Transfer characteristics (measured in air) of the DTBDT films with different alkyl chain substituent are shown in Figure 6c-h. (The output characteristics are shown in Figure S4). The FET parameters are calculated with Equation 1 in the saturation regime.

I DS =

µCiW 2L

(VGS − VT )2 (1)

Where IDS is the drain-source current, µ is the carrier mobility, Ci is the dielectric capacitance per unit area (10 nF·cm-2), W and L are the channel width and length respectively, VGS is the gate-source voltage, and VT is the threshold voltage. As the OFETs consist of multiple micro-stripes, the effective channel width is normalized with the accumulation of width of micro-stripes. All the FET devices showed typical p-channel responses in the saturation regime, while the performance depend on the alkyl chain length, as shown in Figure 7a and b. Except a little deviation of C9 films, the mobility presented an initially increasing but then decreasing tendency ranging from 10-3 to 10-1 cm2/Vs along with the increase of alkyl chain, so as to the threshold voltage, detailed parameters were listed in Table 2. The short alkyl chain C6 or C7 attached molecules, could obtain mobility larger than


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0.1 cm2/Vs with small threshold voltage in the scale of -1V to -6V and high current on/off ratio of 106. While long chain C12 only give mobility on the scale of 10-3 cm2/Vs, meanwhile with large threshold voltage and small on/off ratio.


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Figure 6. (a) OFET devices configuration (b) real device image based on DTBDT films fabricated at 5000 µm/s. Transfer characteristics of the DTBDT films with (c) C4 (d) C6 (e) C7 (f) C8 (g) C9 (h) C12 alkyl chain substituents.


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Such a performance variation can be understood from their morphology, ordering and packing feature. The C6 possessed the better orientation, continuity and in-plane crystallinity, accompanied with good π interaction leads to best mobility. C7 attached molecule presented similar morphology and accordingly good performance on the field effect transistors. Shorter chain or longer chain will induce a certain content disorder in the film plane, such as the random orientation, curl morphology. Long alkyl chain of C12 gave a very poor alignment and in-plane ordering, hence resulted in the poor charge transport ability. The π interaction of DTBDT-C4 could be one factor affect its charge mobility, because the strong S-S interaction induce much loss of the backbone overlap. Meanwhile, the cofacial packing with good overlap of the backbone might contribute to the high mobility of DTBDT-C9 crystals36.

0 0.1

2

Threshold Voltage (V)

-5

Mobility (cm /Vs)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.01

1E-3

-10 -15 -20 -25 -30

1E-4 3

4

5

6

7

8

9

10

11

12

13

4

5

6

Alkyl chain length

7

8

9

10

11

12

Alkyl chain length

Figure 7. (a) Mobility and (b) threshold voltage distribution of the molecules with different alkyl chain length. The above results revealed that the alkyl chain length will significantly influence the oriented film growth and accordingly their device performance. An optimal


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materials design would benefit to achieve films with controllable morphology, for instance a moderate alkyl chain length here will give the best orientation, film ordering and charge mobility. Table 2. Performance properties of the solution-processed TFT devices. 2

Compounds

Mobility* (cm /Vs)

I /I

DTBDT-C4

0.040-0.048

10

DTBDT-C6

0.15-0.21

10

DTBDT-C7

0.10-0.15

10

DTBDT-C8

0.075-0.084

10

DTBDT-C9

0.11-0.13

10

DTBDT-C12

0.001-0.005

10

on

off

V (V) th

5

-13～-19

6

-1～-6

5

5

-11~-18

-17~-22

4

-20～-25

3

-20～-28

4. Conclusions In conclusion, we have investigated the alkyl chain effect on the morphology, crystalline orientation, molecular packing, ordering and accordingly the transistor performance of the oriented ultra-thin films grown via dip-coating. We found that the length of the alkyl chain will affect film orientation, ordering and molecular packing and charge mobility. The orientation and in-plane ordering presented a trend from initial rising to good level but then falling to poor, as well as the charge mobility. Too


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short or long chain will bring in disorder on the orientation or in-plane ordering. Long alkyl chains tended to form thin, non-continuous random oriented stripes with poor crystallinity, which is ascribed to the poor dissolving capacity and slow self-assemble behavior or crowd alkyl steric effect. Meanwhile the alkyl chain length affected the intermolecular interaction specially the π-π interactions. Too short chain will influence the backbone overlap in one molecular column because of a strong S-S interaction with adjacent molecule, while long chain will favor the backbone overlap. Among the alkyl chains ranging from C4 to C12, the molecule with C6 or C7 substituent is optimal to present well-defined orientation parallel to the pulling direction, highly ordering nature and relative intimated cofacial π-π packing, showing therefore the best field effect mobility up to 0.2 cm2·V-1·s-1 for this series of molecules. Therefore, a moderate alkyl chain length in coordination with conjugated cores could promote an oriented and highly ordered film and accordingly efficient charge transport, which raised the importance of molecule structure design for achieving high performance thin film with controllable morphology.

Acknowledgements We acknowledge the financial support from National Natural Science Foundation of China (91227201, 11304213, 51503138, 21527805), China Postdoctoral Science Foundation (2014M550304, 2015T80579), Suzhou Industrial Park (SUN-WIN project 2013) and open Research fund of State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. We are grateful for the supporting of K. Muellen and M. Baumgarten (Max Planck


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Institute for Polymer Research, Mainz, Germany) for the instruction of material synthesis. In addition, we thank the support from Collaborative Innovation Center of Suzhou Nano Science & Technology and the Priority Academic Program Development of Jiangsu Higher Education Institutions. The authors thank beamline BL14B1 (Shanghai Synchrotron Radiation Facility) for providing the beam time. Supporting Information. Details of the synthesis, statistical data of the optical micrography, AFM height images, part of XRD and SAED data, output characteristics of the transistors.

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Table of contents Graphic


Growth of Highly Oriented Ultrathin Crystalline Organic Microstripes

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