Positional Disorder-Induced Mobility Enhancement in Rapidly Cooled

Apr 21, 2010 - The study of the charge carrier transport mechanism in organic ... Universal Trap Effect in Carrier Transport of Disordered Organic ...
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J. Phys. Chem. C 2010, 114, 9056–9061

Positional Disorder-Induced Mobility Enhancement in Rapidly Cooled Organic Semiconductor Melts Liang Chen, Guifang Dong, Lian Duan, Juan Qiao, Deqiang Zhang, Liduo Wang, and Yong Qiu* Key Lab of Organic Optoelectronics & Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua UniVersity, Beijing 100084, China ReceiVed: NoVember 23, 2009; ReVised Manuscript ReceiVed: March 28, 2010

The study of the charge carrier transport mechanism in organic semiconducting films is a great challenge because of the weak interaction between organic molecules. Generally, high crystallinity is regarded as being conducive to carrier transport in organic films. However, the organic semiconducting films have been found to exhibit less crystallinity but improved mobility after being heated to melt and then rapidly cooled. In this paper, hole mobilities of the as-vacuum deposited and rapidly cooled films of two organic semiconductors, 4,4′,4′′-tris(N-3-methylphenyl-N-phenyl-amino)triphenylamine (mTDATA) and N,N′-diphenyl-N,N′-bis(3methyl)-benzidine (TPD), were measured by the time-of-flight (TOF) method at different temperatures, ranging from 260 to 320 K. The Gaussian disorder model (GDM) has been employed to investigate the carrier transport properties of the organic films, and the energetic and positional disorder parameters were calculated quantitatively. It is found that the higher positional disorder Σ in the rapidly cooled film gives rise to its high mobility. The positional disorder-induced improvement in hole mobility was explained in terms of the detour pathways for carriers through the film which were opened by the irregular intermolecular arrangement. Additionally, the effect of the chemical structure upon the molecular packing and the carrier transport was also discussed. 1. Introduction Charge carrier transport in the stable films of low molar mass organic compounds has been considered one of the most important research subjects, since the performance of many organic electronic devices, including organic light-emitting diodes (OLEDs), organic thin-film transistors (OTFTs), and photovoltaic cells, is strongly dependent on the charge carrier mobility in these organic films. The most common method to prepare organic films is vacuum deposition, and many efforts have been made to measure and discuss the hole- and electrontransporting properties of the organic vacuum-deposited films through various methods.1 Though the vacuum-deposited organic films are always considered to be amorphous, actually it is difficult to obtain a “completely” amorphous film, since the molecules would spontaneously aggregate to form a crystalline phase in the film. A number of studies about organic semiconducting compounds were focused on the relationship between the charge carrier transporting property and the molecular structure.2–5 Most of the organic semiconductors are based on π-electron systems. Hole-transporting molecules usually possess low ionization potentials and low electron affinities, and the compounds which have both high ionization potentials and high electron affinities can function as electron-transporting materials.6 The enhancement of the intermolecular interactions, such as π-π and dipole-dipole interactions, should benefit the charge transfer between molecules.7 Generally, experimental results show that organic crystalline materials exhibit larger carrier mobilities than those of amorphous materials.6 The crystallinity of organic films indicates the extent of order of the molecular arrangement. Thus, * To whom correspondence should be addressed. E-mail: qiuy@ mail.tsinghua.edu.cn. Tel.: (008610) 62788802. Fax: (008610) 62795137.

many efforts have been made to form a regular structure with high crystallinity, just as shown in the studies on OTFTs.8–11 However, our previous reports showed that a special kind of organic films obtained through rapidly cooling the melts exhibits both lower crystallinity and higher carrier mobility compared with the as-vacuum deposited films. In 2007, our group investigated the hole-transporting property and stability of the film of a widely used hole transporter N,N′-diphenyl-N,N′-bis(3methyl)-benzidine (TPD) which was fabricated by rapidly cooling its melt. Experimental results demonstrated that this “rapidly cooled” film with a crystallinity value of 20.7% exhibits an enhanced hole mobility of (1-1.2) × 10-3 cm2/(V s) at room temperature in comparison with the crystallinity of 35.9% and the hole mobility of (6-8) × 10-4 cm2/(V s) for the traditional vacuum-deposited film.12 A considerable electron mobility which is one order of magnitude higher than that of the vacuumdeposited film was also found in the rapidly cooled electrontransporting bathophenanthroline film.13 It seems that the comparison between this special rapidly cooled melt and conventional vacuum-deposited film provides a good way to study the influence of the molecular packing upon the carrier transport of organic films of the same molecules, and this mobility improvement phenomenon is worth further exploration. It should be pointed out that not all organic materials can form continuous, stable rapidly cooled films. Decomposition or evaporation would occur in the heating process of many materials before the melting, and sometimes cracks can be found in the rapidly cooled films of materials with extremely high melting points due to the large density difference between the solids at room temperature and the melt.12 In this paper, a system for carrying on time-of-flight (TOF) experiments was established to measure the hole mobility at different temperatures to obtain the values of disorder parameters which can be used to describe

10.1021/jp911111h  2010 American Chemical Society Published on Web 04/21/2010

Mobility Enhancement in Organic Semiconductor Melts

Figure 1. Chemical structures of mTDATA and TPD.

the molecular packing status in the organic films. Two holetransporting materials with high thermal stability and proper melting points, 4,4′,4′′-tris(N-3-methylphenyl-N-phenyl-amino)triphenylamine (mTDATA) and TPD, were chosen as target molecules. Figure 1 shows the chemical structures of these two materials. It is noted that both materials comprise 4-(N-3methylphenyl-N-phenyl-amino)phenyl groups. In the mTDATA molecule, three 4-(N-3-methylphenyl-N-phenyl-amino)phenyl groups are linked to a central nitrogen atom to form a “threebranched” structure, while two 4-(N-3-methylphenyl-N-phenylamino)phenyl groups are connected directly to form a “twobranched” structure in the TPD molecule. Two kinds of disorder, energetic disorder and positional disorder, which can be understood as the width of the fluctuations in site energies and the relative orientation/position of adjacent molecules or chain segments, respectively,1 were introduced to describe the intermolecular arrangement structure in these organic films and were calculated quantitatively according to the Gaussian disorder model (GDM) which was proposed by Ba¨ssler and co-workers.14,15 GDM was originally used to describe the charge transport in conjugated polymers and molecularly doped polymers.16 In some recent works, GDM was also employed to explain the energetic disorder changes in some organic small molecular semiconductors, such as triarylamines17 and host-dopant systems.18,19 It was concluded that the large energetic disorder would decrease the carrier mobility,20 but the discussion about the effect of positional disorder is still low. The great difference in crystallinity between the as-vacuum deposited film and the rapidly cooled film enables us to explore the effect of the positional disorder upon the mobility. It is demonstrated that the positional disorder-induced acceleration of hopping leads to an extra high hole mobility in the rapidly cooled film which possesses a more disordered intermolecular arrangement, that is, a larger positional disorder parameter Σ. Additionally, the comparison between mTDATA and TPD also indicated that molecular structure should have a great influence on crystallinity, density, and disorder of organic semiconductor films, which may further affect the carrier mobility. 2. Experimental Section All samples for TOF measurements have a sandwiched structure with the organic layer in between two electrodes,

J. Phys. Chem. C, Vol. 114, No. 19, 2010 9057 indium tin oxide (ITO) and Ag. ITO substrates were cleaned by ultrasonication in ethanol-acetone (1:1) and deionized water successively and were then dried under an IR lamp for 2 h before use. To obtain rapidly cooled melts, organic compounds were heated on a hot plate until melting and then cooled rapidly while being placed between two ITO-coated glass slides with spherical spacers (diameter ) 5.2 µm) dispersed in the samples. The spacers were used to control the film thickness.21 An 80 nm thick Ag layer was then deposited on the surface of organic layer after removing the top slide in a vacuum of around 8 × 10-4 Pa. The control devices with the vacuum-deposited organic film as the active layer have a structure of ITO/organic layer (5.2 µm)/Ag (80 nm), and the deposition rates of organic compounds and Ag were 0.3-0.5 and 0.03 nm/s, respectively. The enthalpy data were determined by differential scanning calorimetry (DSC) on a TA DSC 2910 modulated instrument. During the DSC scan, samples were placed in aluminum oxide pans and heated at a rate of 5 °C/min under N2 protection. In the case of the vacuum-deposited sample, organic material was vapor-deposited directly into the aluminum oxide pan.22 The critical thermal analysis data were determined according to the method reported by Naito and Miura.23 The densities of vacuumdeposited films were measured according to the method described by Lai and co-workers.24 mTDATA and TPD were deposited separately on the ITO-coated substrate by using a shadow mask (4.50 × 4.50 cm2) in a vacuum of around 8 × 10-4 Pa to form a homogeneous, flat film with a thickness of 350 nm. The substrates containing the vacuum-deposited films were dipped into solvent CHCl3, dissolved by sonication, and then diluted to an appropriate concentration. The absorbance A versus wavelength curve of the diluted solution was recorded by an Agilent 8453 UV-vis spectrophotometer. Standard curves of TPD and BPhen were determined by the absorbance at the maximum absorption wavelength λmax. The densities of rapidly cooled samples were measured by using traditional gravity bottles. The empirical simulation for the dependence of charge carrier mobility µ on field E and temperature T suggested by GDM is

2σ ) ] exp{C [( kTσ ) [ ( 3kT

µ ) µ0 exp -

2

2

0

] }

- Σ2 √E

(1)

where µ0 is a prefactor mobility, k is the Boltzmann constant, C0 is a constant, σ is the energetic disorder parameter which represents the width of the Gaussian distribution of the site energies, and Σ is the positional disorder parameter.14 In this paper, two disorder parameters were calculated based on the mobility data at different temperatures according to the simulation of GDM. 3. Results In TOF experiments, nondispersive signals with clear plateaus were obtained for both materials. The hole mobility value was calculated according to the formula µ ) d2/τV, where d is the sample thickness, τ is the drift time, and V is the applied voltage. Mobility comparison between the rapidly cooled film and the vacuum-deposited film of mTDATA at four different temperatures, 260, 280, 296 (room temperature), and 320 K, is shown in Figures 2 and 3, and the calculated TOF results of TPD films are illustrated in Figures 4 and 5. The mobility values follow a conventional relationship over a wide range of electric fields at a certain temperature25

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µ ) µ(0) exp(β√E)

Chen et al.

(2)

where µ(0) denotes the mobility at zero field and β is the slope of the ln µ(E) versus E1/2 plot. The critical data of TOF results are summarized in Tables 1 and 2. In all cases, the hole mobility value is both temperature and electric field dependent, increasing with temperature and field. In agreement with the prediction of GDM, the field dependence of the mobility, which is indicated by the value of β, is less sensitive as the temperature increases. A comparison between the hole mobilities of the rapidly cooled film and the vacuum-deposited film shows that the zero-field

Figure 5. Hole mobility of the vacuum-deposited film of TPD versus the square root of the applied electric field E at different temperatures. The straight lines are the best line fits to the data.

TABLE 1: Zero-Field Mobilities and the Slopes of the ln µ(E) versus E1/2 Plots of the Films of mTDATA at Different Temperatures rca

Figure 2. Hole mobility of the rapidly cooled film of mTDATA versus the square root of the applied electric field E at different temperatures. The straight lines are the best line fits to the data.

vdb

T (K)

ln µ(0)

β × 103 (cm/V)0.5

ln µ(0)

β × 103 (cm/V)0.5

260 280 296 320

-10.97 -9.92 -9.22 -8.43

1.89 1.24 1.01 0.484

-12.23 -11.30 -10.78 -10.03

2.17 1.74 1.48 1.19

a

rc ) rapidly cooled sample. b vd ) vacuum deposited sample.

TABLE 2: Zero-Field Mobilities and the Slopes of the ln µ(E) versus E1/2 Plots of the Films of TPD at Different Temperatures rca T (K)

ln µ(0)

260 280 296 320

-8.06 -7.39 -6.87 -6.38

a

Figure 3. Hole mobility of the vacuum-deposited film of mTDATA versus the square root of the applied electric field E at different temperatures. The straight lines are the best line fits to the data.

vdb

β × 10 (cm/V) 3

1.98 1.48 1.02 0.742

0.5

ln µ(0)

β × 103 (cm/V)0.5

-8.65 -8.10 -7.62 -7.07

2.55 1.99 1.69 1.38

rc ) rapidly cooled sample. b vd ) vacuum deposited sample.

mobility µ(0) of the rapidly cooled film is larger than that of the vacuum-deposited film for both materials, but the β value is smaller. The zero-field mobility µ(0) of the rapidly cooled film of mTDATA at room temperature is 9.90 × 10-5 cm2/(V s), and that of the vacuum-deposited film is only 2.08 × 10-5 cm2/(V s). However, the β value of the rapidly cooled film of mTDATA is 1.01 × 10-3 (cm/V)0.5, smaller than that of the vacuum-deposited film (β ) 1.48 × 10-3 (cm/V)0.5). Here, GDM was employed to explore the temperature and field dependence of the hole mobility in these two holetransporting materials. In accordance with the prediction of GDM presented by eq 1, the relationship among the temperature, field, and mobility is greatly affected by the energetic disorder σ and positional disorder Σ. A linear relation is expected in the plots of ln µ(0) versus 1/T2 as

ln µ(0) ) ln µ0 - (2σ/3kT)2

Figure 4. Hole mobility of the rapidly cooled film of TPD versus the square root of the applied electric field E at different temperatures. The straight lines are the best line fits to the data.

(3)

and the energetic disorder parameter σ can be evaluated from the slope of this linear relationship. The positional disorder Σ can also be calculated from the plot of β versus 1/T2, which is predicted by GDM as

Mobility Enhancement in Organic Semiconductor Melts

β ) C0[(σ/kT)2 - Σ2]

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(4)

The values of σ and Σ of different samples were calculated, and the results (including other parameters and coefficients) are listed in Table 3. Our results about TPD are in agreement with the data reported by other groups.17,26 In Table 3, we can see that for both mTDATA and TPD, the two disorder parameters of the rapidly cooled films become larger than the vacuumdeposited films, indicating a greater energy width of the hopping sites27 and more complex molecular packing structure. Previous studies showed that µ0 could be described as µ00exp(Σ2/2), where µ00 is the zero-field mobility at infinite temperature in the absence of disorder.26,28 The calculated results of µ00 were also listed in Table 3. It is seen that although the value of µ0 is larger in the rapidly cooled sample which has a larger positional disorder parameter Σ, a greater µ00 is obtained in the vacuumdeposited counterpart. Compared to TPD, mTDATA has a much larger energetic disorder and positional disorder in both two different aggregative states. This phenomenon might suggest the complicated threebranched molecular structure and irregular intermolecular arrangement of mTDATA in these films, which will be discussed in detail in the next section. 4. Discussions As seen in Table 3, for both mTDATA and TPD, the rapidly cooled films possess higher hole mobilities than their vacuum deposited counterparts. It is worth investigating the undesired high hole mobilities in these less crystalline quenched samples. According to eq 1, the extra high positional disorder in the rapidly cooled film plays a key role in the transporting process, resulting in a much enhanced hole mobility. It is clear that the fabrication method has affected the molecular packing structures in these two kinds of organic films and further influences their carrier-transporting properties. Crystallinity and density (F) of the films were measured to help us comprehend the microscopic intermolecular packing in different aggregative states and the origin of this positional disorder-induced enhancement of mobility. Crystallinity is a useful parameter which indicates the regularity of the intermolecular arrangement in the solid phase and can be calculated from the enthalpy data by (∆Hm + ∆Hc)/∆Hm, where ∆Hm is the enthalpy of fusion and ∆Hc is the enthalpy of recrystallization which can be obtained from the DSC curve. The enthalpy

data and crystallinities are listed in Table 4. The data of TPD have been reported in our previous paper.12 ∆Hm of the rapidly cooled sample and the vacuum-deposited sample are both equal to that of the polycrystalline sample within experimental errors, and the difference in absolute values of ∆Hc between the two states leads to a lower crystallinity in the rapidly cooled film, suggesting that the molecules in rapidly cooled film have less regular orientation/position, implying a more complex intermolecular arrangement and a higher structural disorder. The rapidly cooled films of organic compounds were prepared by cooling the melted compounds fast enough to avoid crystallization.29 This “supercooled liquid” is a nonequilibrium state because most of the molecular configurations cannot be rearranged adequately during the fast cooling process.29 As a result, the rapidly cooled film should possess a greater positional disorder which gives rise to a larger value of Σ. The density of film is another crucial parameter which can reflect the average intersite distance a in the pristine film. It is not easy to measure the thin film density because of the difficulty in the mass determination. Then, single-crystal density Fc measured by X-ray diffraction is sometimes used to discuss the intermolecular arrangement in organic films. However, the density of vacuum-deposited film should be different from the single-crystal density because of the completely different molecular aggregations.24 In 2007, Lai and co-workers reported an efficient method to find out the tiny mass of an organic small molecule thin film.24 On the basis of the density values which were obtained by this accurate method, we have discussed the density difference between the films of bathophenanthroline and TPD and then explained the extra high electron mobility in rapidly cooled bathophenanthroline.13 In this paper, densities of films of mTDATA and TPD prepared by vacuum deposition were determined by Lai’s method, and those of rapidly cooled sample were measured through traditional gravity bottles. All of the density data are shown in Table 4. In the rapidly cooled film, the density was found to decrease in comparison with the vacuum deposited film of the same compound, indicating larger average intersite distance and decreased interaction between the adjacent hopping sites. The interaction between sites would average the site energies over molecules within the radius of delocalization, causing the distribution of these site energies to be narrower.30 Thus, molecules in the rapidly cooled film with lower density should have a larger width of the Gaussian distribution of site energies and a greater σ. The smaller µ00 for the rapidly cooled film is also related to the weaker interaction

TABLE 3: Parameters Calculated According to GDM material

state

µRT c (cm2/(V s))

µ0 (cm2/(V s))

σ(meV)

C0 (cm/V)0.5

Σ

µ00 (cm2/(V s))

mTDATA

rca vdb rc vd

1.62 × 10-4 4.31 × 10-5 1.72 × 10-3 1.14 × 10-3

3.09 × 10-2 2.99 × 10-3 4.69 × 10-2 1.80 × 10-2

91.9 85.1 75.1 72.7

2.39 × 10-4 2.01 × 10-4 3.32 × 10-4 3.26 × 10-4

3.00 1.91 2.30 1.69

3.43 × 10-4 4.82 × 10-4 3.33 × 10-3 4.32 × 10-3

TPD a

rc ) rapidly cooled sample. b vd ) vacuum deposited sample.

TABLE 4: Physical Parameters of mTDATA and TPD material

molar mass

state

∆Hc (J/g)

mTDATA

788

ppa vdb rcc pp vd rc

n.o.d -45.6 -52.8 n.o. -49.8 -60.2

TPD

a

516

∆Hm (J/g) 66.9 66.4 63.7 78.0 77.7 75.9

crystallinity 31.3% 17.1% 35.9% 20.7%

density (g/cm3) n.m.e 0.954 0.784 n.m. 1.10 0.836

pp ) polycrystalline powder. b vd ) vacuum deposited sample. c rc ) rapidly cooled sample. d n.o. ) not observed. e n.m. ) not measured.

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between the neighboring hopping sites due to larger average intersite distance and less compact intermolecular packing.31 From eq 1, we can conclude that in the rapidly cooled film, a larger Σ benefits the carrier transport, but a greater σ and lower µ00 depress the mobility. The existence of detour pathway for holes would help us comprehend why the large positional disorder leads to an increased prefactor mobility µ0 and a decreased β. The low crystallinity of the rapidly cooled sample, which indicates a disordered intermolecular arrangement, causes a distribution of electronic couplings within the material that can open detour pathways for carriers through the film by avoiding structural defects.1,14,28 The holes prefer traveling through these favorable routes to the counter electrode, and this disorder-assisted transport would be faster than the “normal” jumping process without detour carrier paths. At high electric fields, the well-conducting paths whose direction is not aligned to the external applied filed are eliminated32,33 and cannot further contribute to the carrier transport. As a result, though the electric field, which is beneficial to the hole transport, is increased, the mobility enhancement in the sample with high positional disorder is not so remarkable, and an insensitive field dependence of mobility β is observed in the rapidly cooled film. Furthermore, Table 4 shows that the densities and crystallinities of the two materials are much different. Although mTDATA has a much larger molar mass, the densities of the rapidly cooled film and the vacuum-deposited film of mTDATA are smaller than those of TPD. The central sp3 hybridized nitrogen atom of mTDATA makes the whole molecule into a twisted tripyramid structure, and this three-branched unit further gives rise to an incompact molecular arrangement which decreases the densities of the aggregative thin films in comparison with two-branched TPD. The complex molecular structure of mTDATA molecules may also increase the vibrational and rotational degrees of freedom of molecules and restrict the regular molecular orientation and packing. The decrease of the degree of structural order in the solids leads to lower crystallinity and larger positional disorders in both the rapidly cooled film and the vacuum-deposited film of mTDATA in contrast to the counterparts of TPD. As seen in eq 1, when the energetic disorder parameter is higher, the mobility should be smaller. Higher values of σ in mTDATA films may be due to the high dipole moment.26 Borsenberger and co-workers have reported that the dipole moments also have a significant effect on the value of σ.26,34 In their work, the total energetic disorder parameter can be described as the sum of two independent parts,

σ ) (σd2 + σvdW2)1/2

(5)

where σd is the dipole contribution and σvdW is the contribution arising from van der Waals interaction between a charge carrier and its electronically polarizable random environment. The study about the dipole contribution further revealed that σd is proportional to the dipole moment p, and the total energetic disorder parameter σ increases with p.34 In this study, the density functional theory (DFT) was employed to calculate the dipole moments of mTDATA and TPD by using Gaussian 03 package. The calculated results show that the dipole moment of mTDATA is 0.6816 D, which is higher than that of TPD (p ) 0.4310 D). Then, it is the bigger dipole contribution in mTDATA film that causes larger σ and thus lower mobility in comparison with TPD.

5. Summary In conclusion, a study on the hole-transporting properties of the films of mTDATA and TPD fabricated by two different methods was carried out, and the results of the TOF measurement are found to agree well with the predictions of the GDM. It is demonstrated that energetic disorder parameter σ and positional disorder parameter Σ which are calculated by the simulation of GDM have great effect on the hole transport in these samples. Both higher σ and Σ, especially the extra higher Σ, were observed in the rapidly cooled films in comparison with the vacuum-deposited counterparts and are explained according to the irregular molecular packing suggested by the lowered density and lowered crystallinity. As a result, the positional disorder-induced detour pathways at low electric fields for holes through the film lead to an enhanced mobility and insensitive field dependence in the rapidly cooled films of these two materials. Acknowledgment. This research was supported by the National Key Basic Research and Development Program of China (Grant No. 2009CB930602) and the National Natural Science Foundation of China (Nos. 60877026 and 50990060). Supporting Information Available: The cooling rates of rapidly cooled samples of mTDATA and TPD, experimental details of the measurement of the densities of films, measurement of DSC curves of mTDATA films, and TOF transient for the rapidly cooled film of mTDATA. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Coropceanu, V.; Cornil, J.; da Silva, D. A.; Olivier, Y.; Silbey, R.; Bredas, J. L. Chem. ReV. 2007, 107, 926. (2) Hreha, R. D.; George, C. P.; Haldi, A.; Domercq, B.; Malagoli, M.; Barlow, S.; Bredas, J. L.; Kippelen, B.; Marder, S. R. AdV. Funct. Mater. 2003, 13, 967. (3) Thelakkat, M.; Schmidt, H. W. AdV. Mater. 1998, 10, 219. (4) Salbeck, J.; Yu, N.; Bauer, J.; Weissortel, F.; Bestgen, H. Synth. Met. 1997, 91, 209. (5) Thelakkat, M.; Schmitz, C.; Hohle, C.; Strohriegl, P.; Schmidt, H. W.; Hofmann, U.; Schloter, S.; Haarer, D. Phys. Chem. Chem. Phys. 1999, 1, 1693. (6) Shirota, Y.; Kageyama, H. Chem. ReV. 2007, 107, 953. (7) Munakata, M.; Wu, L. P.; Kuroda-Sowa, T.; Maekawa, M.; Suenaga, Y.; Ning, G. L.; Kojima, T. J. Am. Chem. Soc. 1998, 120, 8610. (8) Newman, C. R.; Frisbie, C. D.; da Silva, D. A.; Bredas, J. L.; Ewbank, P. C.; Mann, K. R. Chem. Mater. 2004, 16, 4436. (9) Yamashita, Y. Sci. Technol. AdV. Mater. 2009, 10, 024313. (10) Kitamura, M.; Arakawa, Y. J. Phys.: Condens. Matter 2008, 20, 184011. (11) Sun, Y. M.; Liu, Y. Q.; Zhu, D. B. J. Mater. Chem. 2005, 15, 53. (12) Chen, L.; Dong, G. F.; Duan, L.; Wang, L. D.; Qiao, J.; Zhang, D. Q.; Qiu, Y. J. Phys. Chem. C 2007, 111, 18376. (13) Chen, L.; Dong, G. F.; Duan, L.; Wang, L. D.; Qiao, J.; Zhang, D. Q.; Qiu, Y. J. Phys. Chem. C 2009, 113, 16549. (14) Borsenberger, P. M.; Pautmeier, L.; Ba¨ssler, H. J. Chem. Phys. 1991, 94, 5447. (15) Ba¨ssler, H. Phys. Status Solidi B 1993, 175, 15. (16) Schein, L. B.; Tyutnev, A. J. Phys. Chem. C 2008, 112, 7295. (17) Heun, S.; Borsenberger, P. M. Chem. Phys. 1995, 200, 245. (18) Fong, H. H.; Lun, K. C.; So, S. K. Chem. Phys. Lett. 2002, 353, 407. (19) Noh, S.; Suman, C. K.; Hong, Y.; Lee, C. J. Appl. Phys. 2009, 105, 033709. (20) Cheung, C. H.; Kwok, K. C.; Tse, S. C.; So, S. K. J. Appl. Phys. 2008, 103, 093705. (21) Kaafarani, B. R.; Kondo, T.; Yu, J. S.; Zhang, Q.; Dattilo, D.; Risko, C.; Jones, S. C.; Barlow, S.; Domercq, B.; Amy, F.; Kahn, A.; Bredas, J. L.; Kippelen, B.; Marder, S. R. J. Am. Chem. Soc. 2005, 127, 16358. (22) Swallen, S. F.; Kearns, K. L.; Mapes, M. K.; Kim, Y. S.; McMahon, R. J.; Ediger, M. D.; Wu, T.; Yu, L.; Satija, S. Science 2007, 315, 353. (23) Naito, K.; Miura, A. J. Phys. Chem. 1993, 97, 6240.

Mobility Enhancement in Organic Semiconductor Melts (24) Xiang, H. F.; Xu, Z. X.; Roy, V. A. L.; Che, C. M.; Lai, P. T. ReV. Sci. Instrum. 2007, 78, 034104. (25) Pai, D. M. J. Chem. Phys. 1970, 52, 2285. (26) Borsenberger, P. M.; Fitzgerald, J. J. J. Phys. Chem. 1993, 97, 4815. (27) Ioannidis, A.; Dodelet, J. P. J. Phys. Chem. B 1997, 101, 891. (28) Pautmeier, L.; Ries, B.; Richert, R.; Ba¨ssler, H. Chem. Phys. Lett. 1988, 143, 459. (29) Debenedetti, P. G.; Stillinger, F. H. Nature 2001, 410, 259. (30) Young, R. H.; Fitzgerald, J. J. J. Phys. Chem. 1995, 99, 4230.

J. Phys. Chem. C, Vol. 114, No. 19, 2010 9061 (31) Schein, L. B.; Borsenberger, P. M. Chem. Phys. 1993, 177, 773. (32) Shi, Q. M.; Hou, Y. B.; Jin, H.; Li, Y. B. J. Appl. Phys. 2007, 102, 073108. (33) Mohan, S. R.; Joshi, M. P. Solid State Commun. 2006, 139, 181. (34) Dieckmann, A.; Ba¨ssler, H.; Borsenberger, P. M. J. Chem. Phys. 1993, 99, 8136.

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