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Crystal-Domain Orientation and Boundary in Highly Ordered Organic Semiconductor Thin Film Chuan Qian, Jia Sun, Lei Zhang, Han Huang, Junliang Yang, and Yongli Gao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b03727 • Publication Date (Web): 16 Jun 2015 Downloaded from http://pubs.acs.org on June 23, 2015

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The Journal of Physical Chemistry

Crystal-Domain Orientation and Boundary in Highly Ordered Organic Semiconductor Thin Film Chuan Qiana,b, Jia Suna,b*, Lei Zhanga,b, Han Huanga,b, Junliang Yanga,b*, and Yongli Gaoa,b,c* a. Institute of Super-microstructure and Ultrafast Process in Advanced Materials (ISUPAM), School of Physics and Electronics, Central South University, Changsha, Hunan 410083, China b. Hunan Key Laboratory for Super-microstructure and Ultrafast Process, School of Physics and Electronics, Central South University, Changsha, Hunan 410083, China c. Department of Physics and Astronomy, University of Rochester, Rochester, NY 14627, USA * Corresponding authors: [email protected] (J. Sun) [email protected] (J.L. Yang) [email protected] (Y.L. Gao) Abstract Conduction of electric charges is often done in polycrystalline materials. Unavoidably, the crystallite size, orientation and domain boundaries (DBs) affect the transport of the charge carriers. It is particularly so for organic semiconductors known to be highly anisotropic and strongly dependent on DBs. Understanding those effects will have a strong impact on improving the performance of organic electronic and optoelectronic devices. Herein, we report our investigation on the crystal-domain orientation and boundary on the charge transport of operating device with copper phthalocyanine (CuPc) thin films grown on parasexiphenyl (p-6P) by kelvin probe force microscopy. In CuPc intra-domains, the voltage drop increases as the angle increases between the domain orientation and the source-drain electric field. In DBs, the potential wells and steep voltage drops were observed. The increase of the DBs width and the angle between the orientations of neighboring domains results in the raise of voltage drop across the DBs, which restrict the charge transport in DBs simultaneously. The mobility of CuPc thin films increases with the domain size, resulting from the reduction of the mismatched orientation degree and the number of DBs.

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Introduction With the advantages of low cost, flexibility and large-area process properties, organic semiconductors as the active layers in organic field-effect transistors (OFETs) have attracted considerable interest from the scientific and technological community due to their potential applications in displays, sensors and integrated circuits.1-6 The mobility of organic semiconductors has been greatly improved over the past decade and the performance of OFETs can now rival to the level of amorphous silicon devices. The organic semiconductor active layers have many domain boundaries (DBs) that are spatially sharp or abrupt and obviously inhibit the charge transport.7 It is generally assumed that there is an electrostatic barrier formed at the DBs for the charge transport due to the presence of kinetically trapped charge based on macroscopic measurements and numerical simulations.8,9 As a result, large variations in carrier mobility, threshold voltage and degradation rate have been reported in OFETs with different size of grains.10-12 Meanwhile, the potential barrier of DBs is dependent on the molecular orientations, and the oriented growth of grains would greatly reduce the barrier.13 All of these indicate that one of the main bottlenecks for charge transport in organic devices is the combination of DBs effects, grain sizes, and orientations.14-16 In previous studies, the research of the DBs effects is mainly focused on the polycrystalline films with relatively small grains ( 1 µm), the electrical anisotropy properties of the domains and its effects on DBs can not be neglected. Phthalocyanine copper (CuPc, Figure 1a) is one of the most promising organic optoelectronic materials due to its advantageous attributes such as the excellent charge mobility and light absorption in visible range as well as the chemical and thermal stability.17 Although CuPc thin films are usually polycrystalline with small grain sizes and many DBs that limit the performance of CuPc-based devices,17-19 weak epitaxy growth (WEG) has been developed and succeeded in fabricating high-quality CuPc thin films by introducing an ultrathin template molecular layer and elevating the growth temperature.20 The sizes of CuPc crystalline domains can be dramatically increased to an average diameters of over 10 µm, leading to a significant improvement in carrier transport.21 However, the fundamental charge transport mechanism of these thin films is unclear. Given the present of intra-domains and nanoscale DBs, the microstructure of CuPc thin films obtained by WEG is complicated. The non-controllable domain orientations and boundaries may lead to large variations in the trap densities of the thin films with different microstructures, leading to non-uniform 2 ACS Paragon Plus Environment

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performances for these devices. Understanding their individual contributions to macroscopic charge transport at a microscopic level will enable us to assess the performance of the devices and help to optimize the microstructure of organic semiconductors with superior transport properties.

(a)

(c)

(b)

Vtip

S

D

CuPc p-6P Insulator Gate

Vds

Figure 1. Molecular structures of (a) CuPc and (b) p-6P. (c) CuPc-based field-effect transistor configuration and the schematic setup of KPFM measurement under operating conditions.

Kelvin probe force microscopy (KPFM) has been used as a powerful tool to study the morphological and electronic properties of thin films with nanoscale resolution by recording the electrostatic-force interaction between the tip and the sample.22 In particular, KPFM can quantitative map the electronic properties of nanostructures and the potential of nano-objects. In the present communication, we report our investigation by in-situ KPFM on operating device with CuPc thin films fabricated by the WEG method on p-6P layer (Figure 1c). In-situ KPFM was used for the first time to study the potential of the intra-domains and the domain boundaries in crystalline CuPc/p-6P organic semiconductors. In operating conditions, the effects of domain sizes, orientations, and DBs on the charge transport of CuPc thin films were



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systematically investigated. This study provides a direction for designing high-quality organic semiconductor films for high-performance organic devices. Experimental Methods Film growth and device fabrication: CuPc and p-6P materials were purchased from SigmaAldrich and Jilin OLED Material Tech Company, respectively. CuPc was purified twice before the use and p-6P was used as received. The SiO2/Si substrate (Si-Mat, Silicon Materials) was ultrasonically cleaned in acetone, alcohol and distilled water for 15 min respectively, then dried by N2 flow and treated by ozone for 15 min. All organic thin films were deposited at 6.0 × 10-4 Pa at a rate of about 1 nm/min recorded by a quartz crystal oscillator and at a substrate temperature of 180℃. The p-6P layer with different thicknesses was deposited on SiO2 layer. After that, 30 nm CuPc was epitaxially grown on the surface of p-6P thin film. Subsequently, the source and drain electrodes were prepared by evaporating gold through a shadow mask on the top of the oriented CuPc thin film. The width and length of the channels were 1600 µm and 80 µm, respectively. Characterization: Electrical properties of the devices were measured by a Keithley 4200 semiconductor parameter analyzer in air and at room temperature. KPFM measurements were in-situ performed for operating devices with an Agilent Technologies 5500 AFM/SPM System (USA) operated in air at ambient temperature. A Keithley 2400 sourcemeter was used to apply the voltage between the sources and drain contacts. The morphology and surface potential line profiles were recorded by Single-Pass tapping-mode simultaneously. The potential sensing tip was the PPP-EFM-50 probe purchased from NanoWorld AG. For transmission electron microscope (TEM) measurements, CuPc/p-6P was first deposited on SiO2 substrate, and then a carbon film was deposited on CuPc/p-6P, which was used as the support layer. The film was separated from SiO2 surface by floatation in 10% HF solution. CuPc/p-6P film with carbon coating was transferred to a copper grid for measurement. The


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selected area electron diffraction (SAED) was imaged with a JEOL JEM-1011 TEM operated at 100 kV.

Results and discussion Figure 2a shows the morphologies of CuPc thin film grown on p-6P layer. CuPc molecules exhibit a stripe-like ordered structure and the domain size is about 30 µm2. The epitaxial relationship between CuPc and p-6P is proved by selected area electron diffraction, as shown in Figure S1 in the supporting information. High-quality CuPc thin film fabricated by the WEG method could remarkably reduce the number of the DBs and residual disorder, which are the restrictive factors in polycrystalline OFETs.23,24 The usual way to model charge transport in polycrystalline thin film should include two parts, i.e., high conductivity region (the intra-domains) and low conductivity region (the DBs).25 In CuPc OFETs fabricated by the WEG method, the intra-domain mobility is very close to the value of its single-crystal OFETs due to the large ordered domains. In order to get the relationship between the domain orientation, domain size, and carrier transport, KPFM experiments were carried out to study the surface potential and voltage drop, which is helpful to understand the remarkable difference between the intra-domains and DBs (Figure 1c).22



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Figure 2. (a) Topographic image of CuPc thin film grown on p-6P layer. (b) Corresponding voltage drop image (Vds = 6 V). The scan area is 20 µm × 20 µm. (c) Voltage drops for the section of line 1 and line 2 in (a). (d) The correlation between the voltage drop per micron and the angle forming between the domain orientation and the current direction along the a axis. The blue line shows the elliptical orientational dependence fitted in-plane transformation of the voltage drop tensor. The maximum and minimum voltage drop values occur along the b (90°) and a (0°) axis, respectively. The insert in (d) show the schematic illustration of CuPc molecules arranged with standing-up mode on the substrate, in which the π-π conjugated direction is parallel to the film plane. In the plane, the a axis is parallel to the current direction and the b axis perpendicular to it.

A voltage (Vds) of 6V between the source and drain electrodes was added during the KPFM measurements for studying the voltage drop in the intra-domains and domain boundaries, respectively. Figure 2b is the voltage drop image of highly ordered CuPc thin film 6 ACS Paragon Plus Environment

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grown on p-6P layer. One of the most notable features in the potential image is that the clear terrace structures with lots of voltage-drop regions are separated by the DBs, in which the data are filtered by Gaussian smoothing filter for data denoising. The values of height and surface potential imaged by KPFM are displayed from the black (the lowest value) to the white (the highest value), and the changes of the colors represent the voltage drop. The current direction is from the left side (high potential) to the right side (low potential). The voltage drop shows the same track to its topography. The steep voltage drop with different height can be clearly observed at the DBs, and the potential decays gradually along the intradomains. Figure2c is the line sections in Figure2b. For planar aromatic molecules, it has been proved in single-crystal OFETs that the charge carrier mobility is usually maximized along the molecular cofacial π-π stacking direction.26 However, it is still unclear about the relationship between the molecular arrangements and the carrier transport in thin-film OFETs, especially in the intra-domains. For the domain with Line 1 in Figure 2a, the angle θ between the domain orientation and the current direction is about 10°. It results in the voltage drop of 0.2 V in 5 µm and 40 mV/µm, as shown in Figure2c. For the domain with Line 2, the angle θ is about 75°, resulting in the voltage drop of 0.4V in 5 µm and 80 mV/µm. The voltage drop of Line 2 is much larger than that of Line 1. More statistical experimental data are shown in Figure 2d. It shows the relationship between the angle θ and the voltage drop per micron in each domain. Τhe data points are the intra-domain voltage drop obtained by KPFM, we have fitted the voltage drop by the elliptical orientational dependence of the angle θ. The blue line is a quarter of an ellipse centered on the origin and shows the elliptical orientational fitted inplane transformation of the voltage drop tensor. The voltage drop increases with the angle θ. The maximum and minimum voltage drop values occur along the b (90°) and a (0°) axis, respectively. The deduced long and short axes are 164 mV/µm and 84 mV/µm, respectively.



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By elliptical fitting the data, we assume that the voltage drop in the intra-domains is given by the following equation: n

∆VD = ∑ i =1

n

ALDi 1 + Bcos 2θ i

,

∑L

Di

= L ds

(1)

i =1

Where LDi is domain size, Lds is the length between source and drain electrodes, and A and B are fitting parameters denoting the magnitude and ellipticity of the voltage drop. The parameter A is equal to b (b is half of the long axis of this ellipse) and the parameter B is equal to (b-a)/a (a is half of the short axis of this ellipse), so the fitting produces A=82 mV/µm2 and B=2.81. The fitting is an approximation of the anisotropy of the mobility.27,28 When the angle θi is equal to 0°, the orientation of domain is parallel to the current direction, resulting the lowest voltage drop of 42 mV/µm. On the other hand, as the angle θi is increased to 90°, the orientation of domain is perpendicular to the current direction, which results in the highest voltage drop of 82 mV/µm. Hence, the angle θ between the orientation of domain and the current direction can greatly influence the charge transport during the intra-domains. The insert in Figure 2d schematically illustrate the standing-up arrangement of CuPc molecules on the substrate, and the π-π conjugated direction is parallel to the film plane. Charge carrier conductivity is usually maximized along the direction of π−π stacking of CuPc molecules. In previous study, the mobility of rubrene single crystalline FETs is dependent on the π-π stacking orientation.27 The topographic image shows that the DBs can be easily observed between two highly oriented domains, as shown in Figure 3a. Its surface potential image is also shown in Figure 3b. Although the system noise is inevitable,29 it does not affect the data analysis. The surface potential (SP) of intra-domain is close, especially for the π-π stacked molecules with parallel arrangement. There is small fluctuation in the junction of adjacent π-π stacked molecules mainly due to the existence of defects.30 The surface potential map (with Vds = 0 V) in Figure


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3b clearly shows the presence of negative potentials at the DBs, which matches well with the gap in the morphology image (Figure 3a). The line scan in Figure 3d indicates that the DB potential wells are approximately 0.24 V in depth and 69 nm in width. The width may have a little deviation because of the limited spatial resolution of KPFM.31 The physical size of the domain boundary is 110 nm (Figure 3c), which is larger than the width of the potential well of 69 nm (Figure 3d). Previous characterization of polycrystalline films also showed that the barrier width was smaller than the physical width of the boundary, as the space charge layers develop on both sides of the boundary.8

Figure 3. (a) Topographic image of CuPc thin film grown on p-6P layer. (b) Corresponding surface potential image (Vds = 0 V). The scan area is 5 µm×5 µm. (c) Topographic data for the line section in (a). (d) Surface potential data for the line section in (b).

The observed potential wells suggest that a downward band bending of the vacuum level at the DBs. The local band bending of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) follow the vacuum level. Thus, the space9 ACS Paragon Plus Environment


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charge region (SCR) of hole depletion are formed at the DBs. Figure 4a illustrates the positively charged domain boundary in CuPc/p-6P thin film. The downward band bending shows that there are bound charges and traps capturing hole at the DBs. The downward bending of the valence band indicates that the barriers are formed to restrict the hole transport at the DBs in CuPc thin film. Because the CuPc is p-type materials, the DBs introduce donorlike trapping levels within the band gap. As the carriers were trapped, the trapping states become positively charged and form the potential barriers at the DBs. For the DBs as viewed from insulator-semiconductor interface, more trap states are stay in the DBs. Because of the existence of traps, the back-to-back Schottky barriers are formed at both sides of DBs. The current flowing through a DB is limited by thermionic emission and the charge carriers have to move across the potential barrier by thermionic emission at room temperature. In van der Waals bonded CuPc thin film, there seems no Fermi level pinning at the surface and surface states lie outside the band gap.32

Figure 4. Schematic of the voltage barrier at the DBs in CuPc thin film without a bias (a) and with a forward bias (b).

As intra-domains and DBs are connected in series, the medium can be divided into the intra-domains with single-crystal-like properties and the DBs where all the traps are located. 10 ACS Paragon Plus Environment

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So there is a sharp decline of voltage drop at the DBs under a bias of source-drain electrodes, as shown in Figure 4b. The potential barriers at the DBs should be overcome to make sure the charge carriers can transport from one domain to another one, resulting in a significant voltage drop across the DBs. The potential drops of the DBs under the bias of source-drain electrodes are shown in Figure 5a and b, which is extracted from Figure 2a (line 3, 4 and 5) measured using in-situ KPFM. The topographic line profile 3 shows a DB width of about 89 nm, and the corresponding voltage drop is about 120.4 mV, in which the potential drops abruptly. It obviously suggests that the abrupt potential steps are present in the voltage drop images caused by the DBs in the film, which results from a structural discontinuity in the molecular packing. Line profile 4 indicates that the width of DB is about 60 nm and the corresponding voltage drop is 99.8 mV. Line profile 5 shows that the width of DB is about 32 nm, which is the narrowest width at the DBs and results in the smallest voltage drop of 66.6 mV. In other words, the voltage drop becomes higher with the width of the DBs increased. As far as the depth of DBs is concerned, the scanning signal of topography has a little distortion at DBs based on AFM mechanism.23,33 Because the CuPc with a thickness of 30 nm was epitaxially grown on the surface of second layer p-6P thin film with disconnected islands,34 the DBs of CuPc film are formed. So the depth of DBs is about 30 nm.



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Figure 5. (a) Line 3, 4, 5 cross-sections in Figure 2. The widths of the DBs are 89 nm, 60 nm and 32 nm, respectively. (b) The corresponding line cross-sections of voltage drop at each DB, which are 120.4 mV, 99.8 mV and 66.6 mV, respectively. (c) Scatter diagram for the correlation between the voltage drop per micron and the width of domain boundaries. Each point is the average of the five or more scans. All data within the angle of the orientation of two connected domains for the angle range of 75°±5°. (d) The correlation between the voltage drop per micron and the angle of the orientation of two connected domains. Each point is the average of the five or more data. All data within the width of domain boundaries for the range of 85 nm ± 5 nm. The blue line shows the elliptical orientational fitted transformation of the voltage drop tensor. The maximum and minimum voltage drop values occur along the vertical direction (90°) and parallel direction (0°), respectively.

Figure 5c presents a statistic characterization of the width of the associated potential drop per micron and the values of the voltage drop per micron change a little with the width of DBs range from 50 nm to 105 nm. All data are got from the angle range of 75°±5° between the orientations of two connected domains. Each point is the average of the five or more scans, 12 ACS Paragon Plus Environment

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and the standard deviation is also plotted. The voltage drop increases with the width of DBs. It confirms that the charge carriers are easy to go through a lower back-to-back Schottky barrier at the DBs with a narrower width by thermionic emission. By reducing the width of DBs, the effect of the DBs on the charge transport would be reduced, and the voltage drop is decreased accordingly. The nanoscale DBs system can be very complicated.35,36 However, we do observe that the voltage drop per unit length across the DB is only weakly dependent on the DB width, as the data in Figure 5c indicate. It remains to be explored if this effect depends on the p-6P template layer. Figure 5d presents a statistic characterization of the misorientation angle α between two connected domains of the associated potential drop per micron. All data are obtained from the range of 85 nm ± 5 nm of the DBs’ width, and each point is the average of the five or more data and the standard deviation is smaller or equal to 80 mV/µm. It can be seen that the voltage drop is increased with the α at the DBs. Chwang and Frisbie found that, as the angle of misorientation between the two grains increases, the carriers are harder to across the grain boundary.13 We would like to explain the possible reasons about the increase of the misorientation angle α between the orientations of two connected domains restrict the transport of charge carriers in the DBs. The domains can be assumed as big CuPc ‘molecules’ which are standing-up arrangement on the substrate. The π electron clouds of the neighboring ‘molecules’ are begin to overlap with the angle α decreasing from 90° to 0°. The overlap of electron clouds is convenient for the carriers transport. Another possible reason is related to the charge scattering. The carriers cross a back-to-back Schottky barrier at the interface between two CuPc domains with different orientations, electron scattering will occur at the interface.37 The increase of domain’s misorientation results in a large probability of electronic scattering and the mean free path become short, which lead to a large voltage drop. Just like phase transition at the interface, the more the phase angle α change, the greater the electron 13 ACS Paragon Plus Environment


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scattering. For the whole CuPc film, each orientation of domain is randomly spread, the film is in disorder. But, the domains are orderly arranged with all small angle α, the voltage drop of DBs can be neglected. As the organic semiconductor is anisotropic, we expect similar orientational dependence of the adjacent domains. For a first order approximation, we again assumed elliptical anisotropy for the orientational dependence. In Figure 5d, we have fitted the voltage drop by the elliptical orientational dependence of the α as in Eq. 1, and deduced fitting parameters for the magnitude A = 1024 mV/µm2 and ellipticity B = 1.41. The long and short axis thus obtained is 2047 mV/µm and 1318 mV/µm, respectively. The domain size LDi in Eq. 1 is replaced by LDB, the width of the DBs. As LDBj = 0, the neighboring domains are connected together with the absence of DBs and ∆VDB, the voltage drop across the DB, is close to zero. In our study, the voltage drop across the DBs increases with LDB and α. For the cross-section line 5 in Figure 5b, the voltage drop at this DB with a narrow width is about 469 mV/µm, which is much larger than 80 mV/µm for Line 2 in the intra-domain. It indicates that, compared with intra-domains, the DBs can greatly limit carrier transport in a CuPc/p-6P film. Horowitz and Hajlaoui have been demonstrated that the mobility of organic small molecule thin films is increased with grain size.38 They tried to analyze the property by a grain boundary trapping models, the carriers pass over back-to-back Schottky barriers that are generated by traps at grain boundaries via thermionic emission.8,10 As the grain size enlarged to micrometer scale, this models is also valid.39,40 In our work, the voltage drop of CuPc/p-6P thin film may be greatly influenced by the DBs. The CuPc/p6p thin film is composed of the high conductivity regions (crystal domain) and low conductivity regions (domain boundary) and the mobility of the films is obviously increased with the domain size. Therefore, each domain can be seen as big “grain”. The domain size is much larger than that of DBs and the main conduction mechanism through the DBs can be assumed that the thermionic emission 14 ACS Paragon Plus Environment

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over the DB energy potential barrier Vb. On the basis of the above assumptions and to explain the dependence of mobility on the domain sizes and DBs, the carrier mobility in the channel is given by the following:

µ =

qv LD qV exp(− b ) 8kT kT

（2）

LD is average domain size; Vb is the potential barrier and v represents the charge mean velocity. From the Equation 2, the mobility is increased with domain size. To better understand this relationship between thin film microstructure and mobility, more information about CuPc OFETs is indispensable. Figure 6 shows the field-effect mobility in CuPc OFETs with different size of domains. This was investigated by depositing a series of thin films with increasing p-6P layer thickness and characterizing them with AFM. The mobility increases with domain size, in good agreement with published data.38,41 A hole field-effect mobility as high as 0.324 cm2/Vs was calculated in CuPc/p-6P thin film with the large oriented domain size of 70 µm2. The value is three orders of magnitude higher than OFETs with CuPc grown on bare SiO2 substrate and is close to that of CuPc single crystals.42,43 The improved mobility is mainly due to the high-quality of CuPc/p-6P film composed of large-area and oriented domains with fewer nanoscale boundaries. The increase of domain size can reduce the number of DBs, which results in the great decrease of the number of potential barriers for charge transport, and weaken the misorientation degree and randomness of CuPc/p-6P film.



0

10

-1

10

2

Mobility (cm /Vs)

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-2

10

-3

10

-4

10

0

20 40 60 2 Size of Domains (µm )

80

Figure 6. The field-effect mobility in CuPc OFETs with different sizes of domains in surface topography. (The value of the size 0 is from CuPc thin film grown on bare SiO2 substrate with complete disorder)

Conclusion In summary, for better understanding the effect of intro-domains and DBs on charge transport of organic semiconductor thin films, in-situ KPFM and electrical measurements were performed. The voltage drop increases with the angle between the arrangement of domain and the current. When the angle equal to zero, the orientation of π-π stacking of the molecules is parallel to the current, the voltage drop is the minimum. For the DBs, more trap states are stay in the DBs. Because of the existence of traps, the back-to-back Schottky barriers are formed at both sides of DBs. The voltage drop across the DBs is influenced by the width of DBs and the angle of neighboring domains. The mobility of CuPc/p-6P increases with the domain size, which is mainly due to the reduction of the misorientation degree of the thin films and the number of DBs. These results provide a direction for designing high-quality organic semiconductor films with superior transport properties. 16 ACS Paragon Plus Environment

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Acknowledgements. The authors express thanks to Prof. D. H. Yan and Dr. T. Wang (CIAC) for their technique assistance in TEM and SAED experiments. This project was supported in part by the National Natural Science Foundation of China (11334014, 51173205, 51203192, 61306085), the Program for New Century Excellent Talents in University (NCET13-0598), the China Postdoctoral Science Foundation (2013M530357), and the Fundamental Research Funds for the Central Universities of Central South University (2015zzts013). Y. G. acknowledges support by National Science Foundation DMR-1303742 and CBET-1437656.

Conflict of Interest: The authors declare no competing financial interest. Supporting Information Available: Characterization (TEM, SAED, AFM) and the typical I-V curves with different size of domains. This information is available free of charge via the Internet at http://pubs.acs.org.



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