Influence of Deposition Pressure on the Film Morphologies, Structures

Article pubs.acs.org/JPCC

Influence of Deposition Pressure on the Film Morphologies, Structures, and Mobilities for Different-Shaped Organic Semiconductors Yi Li,† Shuang Chen,‡ Qi Liu,† Yun Li,§ Yi Shi,§ Xizhang Wang,*,† Jing Ma,*,† and Zheng Hu† †

Key Laboratory of Mesoscopic Chemistry of MOE, Jiangsu Provincial Lab for Nanotechnology, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China ‡ Department of Chemistry and Nebraska Center for Materials and Nanoscience, University of NebraskaLincoln, Lincoln, Nebraska 68588, United States § School of Electronic Science and Engineering, Nanjing University, Nanjing 210093, China S Supporting Information *

ABSTRACT: Four typical-shaped organic molecules including disk-, rod-, branch-, and sphere-like semiconductors are selected to investigate the influence of deposition pressure (Pdep) on the film morphologies, molecular packing, and mobilities. Different correlations of the microstructures and mobilities with Pdep are obtained, which are closely related with the corresponding molecular shapes. For disk-like F16CuPc and rod-like pentacene, higher Pdep leads to the lager interplanar spacing (D value) and grain sizes of the films which are beneficial to the charge transport and mobilities. For the branch-like TIPS-pentacene and sphere-like C60, the D values of the films keep unchanged and the grain sizes increase with increasing Pdep, presenting the unchanged or even decreased mobilities, respectively. The Pdep-dependence should be correlated with the interactions between the collisional N2 and organic molecules, the organic molecules and substrate, as well as among the organic molecules themselves, which is closely associated with the molecular shapes as partly understood by our theoretical simulations. This study suggests a convenient approach to optimize high-performance organic thin film transistors (OTFTs) according to the molecular shapes by regulating deposition pressure, and is also helpful for understanding the charge transport and performance of OTFTs.

1. INTRODUCTION Organic thin film transistors (OTFTs) have received considerable attention in recent decades because of their low cost, low temperature process, compatibility with plastic substrate, and wide potential applications in large-area, lightweight, flexible electronics devices such as electronic displays, organic memories, sensors, and identification tags.1−6 Great effort has been devoted to achieve the high performance of OTFTs comparable to that of amorphous silicon thin film transistors (TFTs) by optimizing the thin film growth process,7−12 and the interfaces of metal/organic and organic/ insulator.13−18 Despite the considerable progress in fabrication and performance, the correlation of the charge transport with the thin film microstructure is still not very clear, and the charge carrier mobilities of most OTFTs need to be further improved. Generally, the mobilities of OTFTs highly depend on the molecular packing and crystal grain sizes, which in turn rest on the chemical structures of the organic semiconductors,19−22 deposition parameters,23−29 and substrates.30−35 Vacuum deposition is the most common techniques for preparing the thin films of small-molecule and oligomer semiconductors with the advantages of high uniformity and © XXXX American Chemical Society

good reproducibility owing to the controllable deposition parameters, e.g., the substrate temperature (Ts), deposition rate (rdep), and deposition pressures (Pdep) (vacuum level). The influence of Ts and rdep on the growth of organic thin films has been intensively studied, and increasing Ts and/or decreasing rdep usually leads to a highly ordered film with larger grain sizes and fewer grain boundaries, which is the effective way to obtain high-performance devices.36−38 Recent progresses indicate that Pdep is also an important factor to influence the nucleation and growth of films, thereof the microstructures and performance of the corresponding OTFTs, as demonstrated by the growth of pentacene, copper phthalocyanine (CuPc), dicyanoquinonediimine, and tetracyanoquinodimethane films.39−41 Our previous study showed that the morphologies and molecular packing structures of the disk-like CuPc semiconductor thin films could be effectively regulated simply by tuning Pdep; thus, the performance of the resulting CuPc TFTs could be conveniently optimized.40 In addition to the general results that larger grain Received: April 12, 2014 Revised: June 8, 2014

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dx.doi.org/10.1021/jp5035954 | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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Figure 1. Molecular structures of the four different-shaped organic semiconductors used in this study.

morphologies of the films were characterized by X-ray diffraction (XRD) and atomic force microscopy (AFM). The performances of the devices were evaluated by semiconductor characterization system (Keithley model 4200-SCS) under vacuum condition at room temperature. To understand the influence of Pdep on the molecular packing of different-shaped molecules in organic films, the molecular dynamic (MD) simulations were performed by using the Discover module of Materials Studio package.42 The incident frequency ratios of different organic gaseous molecules (F 16 CuPc, pentacene, TIPS-pentacene, and C 60 ) to N 2 molecules under a certain Pdep for film growth were estimated by the collision theory according to the experimental condition. Correspondingly, the slab models with the periodic boundary condition were built in line with the estimated ratios with four different-shaped molecules and number-determined N2 molecules deposited on amorphous SiO2 surface (Figure S1 of Supporting Information). For the starting configurations of different models in Figure S1, we directly deposited N2 molecules on the SiO2 substrate for the low Pdep and otherwise for the high Pdep (especially for 10−1 Pa), we first mixed deposited molecules with N2 molecules and then placed the gas layer on the substrate. The size of the slab models was set as 5.7 nm × 5.7 nm × 6 nm. The consistent-valence force field (CVFF)43 was employed to investigate the Pdep dependence of different-shaped molecules on film morphologies. The applicability of MD simulations lies in the choice of suitable force field, and this CVFF have been widely used in our previous works on charge-transfer properties of orientationdifferent CuPc and F16CuPc films on HOPG and C8-SAM/ Au(111) substrates,44 deposition-pressure-optimized CuPc films on SiO2 and OTS/SiO2 substrates,40 and the film morphologies of functionalized pentacenes with size-different substituents.45 The 1 ns MD simulation was performed for each model in the canonical (NVT) ensemble with the temperature set to growth temperature by using the Nosé thermostat.46−48 The motion equations were integrated by the velocity Verlet method49 with the time step of 1 fs. The electrostatics interactions were calculated by the Ewald summation method.50 About 100 ps equilibrium stages were needed for the NVT simulations on the SiO2 substrate. Dynamic trajectories of the last 900 ps were recorded every 100 fs for the statistical analysis. On the basis of the Marcus theory and the Einstein equation (cf., eqs S1−S3 of Supporting Information), the charge carrier mobility was also estimated within density functional theory (DFT) framework for the

size favors the higher mobility, both our experimental and theoretical results revealed that the interplanar spacing (D value) of the film is also sensitive to the mobility of the CuPc TFTs, and the larger D value leads to the higher mobility. In this study, the Pdep influence on the morphologies, structures, and field-effect mobilities of the thin films has been systematically examined for four different-shaped organic semiconductors including disk-like copper hexadecafluorophthalocyanine (F16CuPc), rod-like pentacene, branch-like 6,13bis(tri-isopropy-silylethynyl)-pentacene (TIPS-pentacene), and sphere-like fullerene (C60). With increasing Pdep, the grain sizes of all the films increase, and the D values of the F16CuPc and pentacene films increase while those for TIPS-pentacene and C60 films keep unchanged, leading to the different evolutions of mobilities with Pdep for the corresponding OTFTs. This study suggests a convenient approach to tune the morphologies and molecular packing structures of the organic semiconductors films according to the molecular shapes by regulating Pdep, thereby to achieve the high-performance OTFTs.

2. EXPERIMENTAL AND COMPUTATIONAL DETAILS The substrate was the 300 nm thick SiO2 thermally grown on the heavily doped n-Si wafers, which were ultrasonically cleaned with acetone, isopropyl alcohol, and ultra purified water, successively. The organic semiconductor materials of F16CuPc (99%), pentacene (99.99%), TIPS-pentacene (99.9%), and C60 (99.9%) were purchased from Sigma-Aldrich and used without further purification. The molecular structures of these organic semiconductors are given in Figure 1. A bottom-gate, topcontact device configuration was adopted. First, the substrate was set in the chamber and maintained at 5 × 10−5 Pa for 4 h. Subsequently, organic films with specific thickness (50 nm for F16CuPc and pentacene, 30 nm for TIPS-pentacene, and 40 nm for C60), patterned through a shadow mask, were deposited by vacuum deposition under a specific Pdep (i.e., 10−4, 10−3, 10−2, 10−1, 2, and 10 Pa) at the same rate of about 0.02 (F16CuPc, pentacene, and TIPS-pentacene films) or 0.01 nm/s (C60 films) recorded by a quartz crystal oscillator. Pdep was adjusted by introducing N2 to the chamber. Ts was chosen as 125, 60, 70, and 120 °C for F16CuPc, pentacene, TIPS-pentacene, and C60 films, respectively. The evaporation temperatures of these materials under different Pdep are demonstrated in Table S1. Finally, gold electrodes with thickness of 50 nm were defined on the organic thin films by thermal evaporation through another mask. The channel length (L) and width (W) are 70 and 500 μm, respectively. The structures and surface B



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Figure 2. AFM images of the (a) F16CuPc (1.5 μm × 1.5 μm), (b) pentacene (18 μm × 18 μm), (c) TIPS-pentacene (10 μm × 10 μm), and (d) C60 (3 μm × 3 μm) films deposited under different Pdep.

dimeric F 16 CuPc and pentacene models with a fixed intermolecular distance, r, of 3.82 Å for F16CuPc and 6.06 Å for pentacene, respectively, as a function of various interplanar spacing (D value). These D values are taken from our following experimental estimation. All those calculations were done within the Gaussian 09 program at the level of M06-2X/631G(d).51

Table 1. Grain Sizes and Root-Mean-Square (RMS) Roughnesses for the F16CuPc, Pentacene, TIPS-Pentacene, and C60 Films Deposited under Different Pdep samples F16CuPc

3. RESULTS AND DISCUSSION 3.1. Surface Morphologies of the Films. The morphological evolutions of the F16CuPc, pentacene, TIPS-pentacene, and C60 films with Pdep are shown in Figure 2. Due to the different molecular shapes of organic semiconductors, the films show four types of morphologies.52 Specifically, disk-like F16CuPc gives rise to fiber-like crystals, rod-like pentacene adopts large domain islands, branch-like TIPS-pentacene tends to form plate-shaped morphologies, and sphere-like C60 leads to the spherical grains. The grain sizes and root-mean-square (RMS) roughnesses of all the samples are summarized in Table 1. Generally, higher Pdep leads to the larger grain sizes, due to the lower nucleation density (Figure S2, S3 of Supporting Information). For the F16CuPc and C60 films, the grain sizes keep almost unchanged for Pdep below 10−2 Pa and significantly increase for further increasing Pdep to 10−1 Pa (Figure 2a,d), with the RMS roughnesses of 2−3 and 4−7 nm, respectively. For the pentacene films, with increasing Pdep from 10−4 to 2 Pa, the grain sizes gradually increase (Figure 2b, Figure S4a of

pentacene

TIPS-pentacene

C60

Pdep (Pa) −4

10 10−3 10−2 10−1 10−4 10−3 10−2 10−1 2 10 10−4 10−3 10−2 10−1 10−4 10−3 10−2 10−1

grain size 180.5 ± 18.8 nm 178.4 ± 17.4 nm 183.1 ± 19.1 nm 238.6 ± 20.0 nm 5.1 ± 0.3 μm 5.5 ± 0.3 μm 6.7 ± 0.6 μm 13.6 ± 1.3 μm 17.1 ± 1.8 μm 0.5 ± 0.1 μm 1.2 ± 0.2 μm 1.2 ± 0.2 μm 2.3 ± 0.3 μm 0.5 ± 0.1 μm 101.5 ± 10.3 nm 98.8 ± 8.2 nm 103.5 ± 9.5 nm 220.6 ± 18.8 nm

RMS roughness (nm) 2.2 2.7 2.9 2.0 10.5 9.9 11.2 7.4 7.1 16.2 10.6 10.3 16.4 13.5 6.5 6.2 4.2 6.4

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.3 0.4 0.2 0.5 0.6 0.5 0.7 0.3 0.4 0.8 0.2 0.3 0.6 0.4 0.1 0.2 0.3 0.2

Supporting Information), with the RMS roughnesses of ca. 7− 11 nm. Further increasing Pdep to 10 Pa, the films with irregular small grain are obtained (Figure S4b of Supporting C



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Figure 3. (a, c, e, and g) Characteristic XRD peaks and (b, d, f, and h) corresponding D values of the (a and b) F16CuPc, (c and d) pentacene, (e and f) TIPS-pentacene, and (g and h) C60 films with different Pdep. The insets in (b, d, f, and h) are the schematic stacking structures of F16CuPc, pentacene, TIPS-pentacene, and C60 molecules, respectively.

Information). For the TIPS-pentacene films with Pdep of 10−4 or 10−3 Pa, the plate-like crystals possess the similar grain sizes with the length of ca. 1.2 μm. As Pdep increased to 10−2 Pa, the crystal grain size increased up to 2.3 μm. Further increasing Pdep to 10−1 Pa, the films with irregular small grain of ca. 0.5 μm are obtained (Figure 2c). The corresponding RMS roughnesses are 10.6, 10.3, 16.4, and 13.5 nm, respectively (Table 1). 3.2. Structures of the Films and Theoretical Analysis. Figure 3 shows the structural evolutions of the F16CuPc,

pentacene, TIPS-pentacene, and C60 films with Pdep. With increasing Pdep, the D values of the F16CuPc and pentacene films increase while those for TIPS-pentacene and C60 films keep unchanged, which has also been rationalized by our MD simulations as shown in Figure 4. For the F16CuPc films, the characteristic XRD peaks and corresponding D values indicate the film is composed of α-form F16CuPc with the (200) lattice planes, corresponding to the herringbone stacking structure (Figure 3a,b).53 The (200) D



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experimental error, reflecting the 2D π−π stacking structure (Figure 3e,f). The film with Pdep of 10−1 Pa shows the diffused diffraction peaks of the poor crystallinity (Figure 3e, Figure S7 of Supporting Information). Different from the clear increase of D value with increasing Pdep for the disk-like F16CuPc and rodlike pentacene, the influence of Pdep on D value is negligible for the branch-like TIPS-pentacene. Our MD simulations reveal that the TIPS-pentacene molecules prefer lying-down orientation (snapshots in Figure 4c) with increasing D value as Pdep increases (Figure 4c, Figure S5c of Supporting Information). The difference between the theoretical and experimental results comes from the limitation of our theoretical models. Our theoretical models only include one TIPS-pentacene in each slab, which is much smaller than the critical nucleus size of TIPS-pentacene growth on SiO2 surface.45 The theoretical estimation of increasing D values with Pdep reveals that the collision effect with N2 molecules also lets TIPS-pentacene molecules gradually stand up at the initial nucleation stage. The Pdep-independent packing in experiment may result from the strong π−π interaction between the cofacial stacking TIPSpentacene molecules on the substrate, which makes them stand upright and keep the unchanged D value. The collision effect between the branch-like TIPS-pentacene and N2 molecules is negligible during the continuous growth stage. The XRD patterns of the C60 films show weak (111) peaks and broadened (113) peaks (Figure S8 of Supporting Information),56 corresponding to the D values of ca. 1.54 nm (Figure 3h). The D values are about 1 Å larger than literature reports,56 which may result from poor crystallinity deviating from the standard face-centered cubic phase (inset in Figure 3h). With increasing Pdep, the (113) peak has little shifting with unchanged D value (Figure 3g,h), indicating the negligible effect of Pdep on C60 molecular packing. Owing to the spherical shape of C60, the collision effect of C60 with N2 is negligible for the molecular orientation regulation (Figure 4d, Figure S5d of Supporting Information). It is noticed that, from our results, the microstructures of C60 films with weak crystallinity ARE less dependent on Pdep. The effect of Pdep on the structural properties of highly crystalline C60 films needs further investigation. 3.3. Electronic Properties of the Films. Figure 5 shows the transfer characteristics of F16CuPc, pentacene, TIPSpentacene, and C60 TFTs prepared under different Pdep. The pentacene and TIPS-pentacene TFTs exhibit typical p-channel field effect behavior, operated under a negative source-drain voltage (Vd) and gate voltage (Vg), and the F16CuPc and C60 TFTs show typical n-channel field effect behavior, under a positive Vd and Vg (Figure 5a−d, Figure S9 of Supporting Information). The carrier mobility, threshold voltage, on/off current ratio and subthreshold slope are extracted for each device, and the relationship between the mobilities and Pdep are shown in Figure 6 (Table S2 of Supporting Information). For clearly understanding the correlation of the mobilities with the corresponding morphologies and packing structures of the films, the changes of the D values and grain sizes with Pdep are replotted here. It is seen that the mobilities of F16CuPc and pentacene TFTs increase with increasing Pdep (Figure 6a,b), while the mobilities of TIPS-pentacene and C60 TFTs keep unchanged or decreased with increasing Pdep (Figure 6c,d). These different changing tendencies result from the different evolutions of the morphologies and molecular packing structures for the corresponding organic films as demonstrated below.

Figure 4. Most probable D values of (a) F16CuPc, (b) pentacene, (c) TIPS-pentacene, and (d) C60 molecules obtained from the MD statistical results as a function of Pdep (10−4, 10−3, 10−2, and 10−1 Pa). The insets in (a−d) highlight side views of 1 ns snapshots of the corresponding molecules on the SiO2 substrates under Pdep of 10−4 and 10−1 Pa, respectively. The theoretical D value is defined as the sum of vertical height of molecule and van der Waals (vdW) radii of both the highest and lowest atoms in the molecule along the vertical direction.

peaks exhibit progressive shifting toward the low angle side with increasing Pdep (Figure 3a), indicating the increasing D value of the F16CuPc molecular stacks (Figure 3b). These results reveal that the F16CuPc molecules tend to stand up along the direction perpendicular to the substrate with increasing Pdep. Similar to the case for CuPc samples,40 with increasing Pdep, the collision probability between F16CuPc and N2 molecules is largely enhanced, which increase the up-right tendency of the F16CuPc molecule (Figure 4a, Figure S5a of Supporting Information). For the pentacene films, when Pdep IS below 2 Pa, the XRD patterns show four peaks of (001), (002), (003), and (004) diffraction (Figure S6 of Supporting Information), with the D values of 1.52−1.54 nm, suggesting the “thin film phase” of pentacene with herringbone stacking structure (Figure 3d).54 For better comparison, the (001) peaks are extracted, which presents a gradual shifting to the low angle side with increasing Pdep, indicating the increasing D value (Figure 3c,d). This result also means the increasing tendency of the perpendicular orientation to the substrate for the rod-like pentacene due to the enhanced collision probability between pentacene and N2 molecules with increasing Pdep (Figure 4b, Figure S5b of Supporting Information), similar to the effect of Pdep on the disk-like F16CuPc molecular packing. Further increasing Pdep to 10 Pa leads to the films with amorphous structure as revealed XRD characterizations (Figure 3c, Figure S6 of Supporting Information), which might be attributed to the over collision between pentacene and N2. For the TIPS-pentacene films with Pdep below 10−2 Pa, XRD patterns present quite good crystallinity with three well-defined diffraction peaks of (001), (002), and (003) (Figure S7 of Supporting Information), with the (001) peaks grouped in Figure 3e.55 The negligible shifting with increasing Pdep indicates the unchanged D value of ca. 1.65 nm within the E



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Similarly, the vertical stacking of pentacene with the large D value facilitates the π−π overlapping along the direction of the charge transport (Figure S10b of Supporting Information).62 And large pentacene crystalline grain with less grain boundary is advantageous to decrease the charge trap.63−65 Hence, the pentacene films with larger D values and grain sizes are expected to show the better electrical properties. Indeed, with the D values and grain sizes of the pentacene films increase with increasing Pdep from 10−4 and 2 Pa, the mobilities of pentacene TFTs increase accordingly, as shown in Figure 6b. The highest mobility of 0.70 cm2/(V s) is obtained for the film with Pdep of 2 Pa. As Pdep further increases to 10 Pa, the performance of the device becomes rather poor (Figure 6b), due to the amorphous structures and irregular grains of the pentacene film (Figure 3c, Figure S4b of Supporting Information). Because of the unchanged D value of TIPS-pentacene and C60 films in the entire range of Pdep from 10−4 to 10−2 or 10−1 Pa, the change of mobilities is determined by the evolution of the films’ grain sizes. For TIPS-pentacene films, the devices prepared under 10−4 and 10−3 Pa exhibit the similar mobilities, in agreement with their similar grain sizes (Figure 6c). For Pdep of 10−2 Pa, the mobility keeps unchanged despite the increased grain sizes, which may result from the increased films’ roughness from ca. 10 to 16 nm (Table 1).66 Further increasing Pdep to 10−1 Pa, the mobility becomes very low due to the poor crystallinity of the film (Figure 3e, Figure S7 of Supporting Information). For C60 films with Pdep below 10−2 Pa, the mobilities remain unchanged due to the similar grain sizes (Figure 6d). With increasing Pdep to 10−1 Pa, the C60 films with larger grain sizes present the lower mobilities, suggesting the different grain-boundary effect for the spherical C60 molecules from that for the planar molecules. For the planar molecules such as CuPc,40,67,68 F16CuPc,60,61 pentacene,63−65 TIPSpentacene,66,69 perylene bisimide,26 and oligothiophene,24 the grain boundary is a crucial barrier for the carrier transport, since the intermolecular charge hopping across the grain boundaries is less efficient than that within the grains. Thus, reducing the grain boundaries by increasing grain sizes is a promising approach to improve charge transport and mobility.70 As a contrast, for the spherical C60 molecules, carrier could be effectively transported through nearest-neighbor hopping

Figure 5. Transfer (|Id (drain current)| − Vg) characteristics of (a) F16CuPc, (b) pentacene, (c) TIPS- pentacene, and (d) C60 TFTs prepared under different Pdep.

For the F16CuPc TFTs, with increasing Pdep from 10−4 to 10−2 Pa, their grain sizes keep almost unchanged; thus, the increasing mobilities come from the increase of the corresponding D values, similar to our recent experimental and theoretical results for the CuPc films (Figure 6a).40 The increases of D value of F16CuPc films, suggesting the enhanced π−π overlapping between the F16CuPc molecules, could be validated by theoretical estimation of increasing the charge transfer integral, leading to the improved mobilities (Figure S10a of Supporting Information).57−59 The further increase of the mobility for the film with Pdep of 10−1 Pa could be attributed to the increases of both D value and grain size (Figure 6a).60,61 Mobilities as high as 0.021 and 0.027 cm2/(V s) are achieved for the samples with Pdep of 10−2 and 10−1 Pa, respectively, which are nearly the highest mobilities of polycrystalline F16CuPc films.53 So the mobility does not show steeper increase with increasing Pdep from 10−2 to 10−1 Pa.

Figure 6. Carrier mobilities, grain sizes, and D values of (a) F16CuPc, (b) pentacene, (c) TIPS-pentacene, and (d) C60 films as functions of Pdep. F



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deposited under different Pdep. (7) Output characteristics of F16CuPc, pentacene, TIPS-pentacene and C60 TFTs prepared under different Pdep. (8) Density functional theory calculations on carrier mobilities for model systems. (9) AFM images and mobilities of C60 films with substrate temperatures of 150 and 180 °C. (10) Summary of the electrical parameters for F16CuPc, pentacene, TIPS-pentacene, and C60 TFTs prepared under different Pdep. This material is available free of charge via the Internet at http://pubs.acs.org.

between molecules. Thus, the grain boundary is not a serious barrier for the charge transport and the grain size is not crucial to high mobility in C60 TFTs.56,71,72 In this study, the larger grain sizes of C60 films lead to the lower mobilities similar to the cases in literature,56,71 probably due to the enlarged gaps between grains and the enhanced gold diffusion into C60 layer during deposition of gold electrodes, which are disadvantageous to the charge transport.26,73,74 The effect of grain boundaries on the mobilities of spherical molecules is interesting and needs to be further investigated. From the preceding experimental and theoretical results, it is learnt that the morphologies, packing structures, and mobilities for the films of different-shaped organic semiconductors could be optimized simply by regulating Pdep. For disk-like F16CuPc and rod-like pentacene, higher Pdep leads to the lager D values and grain sizes of the films which are beneficial to the charge transport and mobilities. For the branch-like TIPS-pentacene and sphere-like C60, the D values of the films keep unchanged and the grain sizes increase with increasing Pdep, leading to the unchanged or decreased mobilities, respectively. These results are helpful to speculate the Pdep-dependence of some other organic semiconductors according to their molecular shapes and packing models, which is an interesting issue for further study.

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Corresponding Authors

*E-mail: [email protected]. Tel: (+86) 25-83593696. *E-mail: [email protected] Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was financially supported by NSFC (21173114, 61306021, 21290192 and 21273102), NSFJS (BK20130579) and National Basic Research Program of China (973 Program, No. 2013CB932902, 2011CB808600).

4. CONCLUSIONS Influence of deposition pressure (P dep ) on the film morphologies, structures, and mobilities has been systematically examined for four typical-shaped organic semiconductors including disk-like F16CuPc, rod-like pentacene, branch-like TIPS-pentacene, and sphere-like C60. We found that the evolutions of molecular packing and film morphologies with Pdep are quite different, resulting in the different Pdepdependence of the corresponding mobilities. Specifically, for disk-like F16CuPc and rod-like pentacene, higher Pdep leads to the larger D values and grain sizes of the films, which are beneficial to charge transport and mobilities. For the branchlike TIPS-pentacene, both the grain sizes and roughness of the films increase, while the D values keep unchanged with increasing Pdep, presenting the unchanged mobilities. For the sphere-like C60, the grain sizes of the films increase with Pdep and the D values remain unchanged, and the mobilities decrease correspondingly, which suggests that the grain size is not crucial to high mobility for C60 TFTs probably due to the nearest-neighbor hopping between molecules. The Pdepdependence should be correlated with the interactions between the collisional N2 and organic molecules, the organic molecules and substrate, as well as among the organic molecules themselves, which is closely associated with the molecular shapes as partly understood by our theoretical simulations. This study suggests a convenient approach to optimize highperformance OTFTs according to the molecular shapes by regulating deposition pressure.

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AUTHOR INFORMATION

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ASSOCIATED CONTENT

S Supporting Information *

(1) The evaporation temperatures of organic semiconductors under different Pdep. (2) Starting configurations for molecular dynamic simulations. (3) AFM images of submonolayer organic films. (4) AFM images of pentacene films deposited under 2 and 10 Pa. (5) Distributions of D values of F16CuPc, pentacene, TIPS-pentacene and C60 molecules under different Pdep. (6) XRD patterns of pentacene, TIPS-pentacene and C60 films G



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Influence of Deposition Pressure on the Film Morphologies, Structures

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