J. Phys. Chem. C 2007, 111, 18411-18416
18411
Optical and Conductive Properties of Large-Area Liquid Crystalline Monodomains of Terthiophene Derivatives Wojciech J. Grzegorczyk,*,† Tom J. Savenije,† Josue J.P. Valeton,‡ Silvia Fratiloiu,† Ferdinand C. Grozema,† Dago M. de Leeuw,§ and Laurens D.A. Siebbeles† Opto-electronic Materials Section, DelftChemTech, Faculty of Applied Sciences, Delft UniVersity of Technology, Julianalaan 136, 2628 JB, Delft, The Netherlands, Laboratory of Polymer Technology, Department of Chemical Engineering and Chemistry, EindhoVen UniVersity of Technology, P.O. Box 513, 5600 MB, EindhoVen, The Netherlands, and Philips Research, High Tech Campus 4, 5656 AE, EindhoVen, The Netherlands ReceiVed: August 2, 2007; In Final Form: September 26, 2007
The optical and conductive properties of the liquid-crystalline organic semiconductor 5,5′′-bis (5-hexyl-2thienylethynyl)-2,2′:5′,2′′-terthiophene (TR5-C6) spin-coated on rubbed polyimide (PI) were studied. The absorption of light linearly polarized parallel to the rubbing direction largely exceeds that in the perpendicular direction. In contrast, a small anisotropy in the photoconductance was found from time-resolved microwave conductivity (TRMC) measurements. From analysis of the optical data, it is inferred that the terthiophene chains in the TR5-C6 molecules are oriented along the PI rubbing direction with a tilt angle of 53 degrees with respect to the plane of the substrate. The absence of a strong anisotropy in the mobility of charge carriers can be understood on the basis of calculated charge transfer integrals, which were found to be comparable for charge-transfer steps in different directions. This is due to arrangement of the molecules in a herringbone structure. Due to the relatively small values of the charge transfer integrals, it is likely that charge transport in TR5-C6 occurs via polaron hopping between localized states.
1. Introduction Field-effect transistors (FETs) based on inorganic semiconductors are reliable and highly efficient in the most performancedemanding electronic devices such as computer processors. However, the high production costs of inorganic transistors can form a serious drawback for large-scale application in cheap electronics. Production of inorganic semiconductors and devices demands relatively large amounts of energy, high-temperature processing, and complicated semiconductor growth techniques.1,2 Organic-based field-effect transistors (O-FETs) offer the prospect of inexpensive alternatives which can be produced with relatively simple techniques.3 O-FETs are of interest for application in the niche of low performance electronic devices such as radio frequency identification tags,4 gas sensors,5 portable displays, electronic paper,6,7 pixel drivers and switching elements in displays,8,9 as well as information storage devices.7 Physical vapor deposition (PVD) and solvent casting are the two commonly used deposition techniques of which the latter bears the advantage of being a low cost, high throughput thinfilm fabrication technique. Unfortunately, solution processing leads often to the formation of less well-organized active organic layers. Structural disorder and grain boundaries negatively affect the charge carrier mobility and in turn the performance of an O-FET. Therefore, at present, effort is devoted to the synthesis of soluble materials, which can be applied as an active layer using solution processing and yet yield high performance devices.10 * Corresponding author. E-mail:
[email protected]. † Delft University of Technology. ‡ Eindhoven University of Technology. § Philips Research.
The active layer of an O-FET must meet several conditions to achieve a high on/off ratio and switching speed.11 First, the crystal structure of the molecules must be such that it ensures good overlap of the pz orbitals to allow fast transport of the charges between adjacent molecules. Second, the π-π stacking should ideally be parallel to the channel between source and drain electrodes. Third, the number of structural defects and impurities within the active layer must be minimized, since these hinder charge transport or can act as trapping sites for charge carriers. Fourth, ordered domains of the active-layer material must be sufficient to cover the entire area between electrodes. Charge mobilities of the order of 1 cm2/Vs have been realized in thin films of aromatic molecules such as anthracene, tetracene, and derivatives of these materials.12,13 Recently, O-FETs based on pentacene and rubrene yielded field-effect mobilities (µFET) for holes of 3 and 20 cm2/Vs, respectively.14-18 By functionalizing them, the groups of Brooks19 and Stingelin-Stutzman17 have demonstrated that these derivatives of pentacene and rubrene can also be solution processed. Solution processible conjugated polymers are under intensive study, since thin films of these materials can be made by spincoating, drop-casting or ink-jet printing.20 Field-effect transistor measurements have yielded mobilities of 0.1 cm2/Vs for derivatives of polyphenylene-vinylenes21,22 and 0.7 cm2/Vs polythiophene derivatives.23-32 Films of conjugated polymers exhibit structural disorder, which hinders charge transport over long distances in large area devices.20 Relatively ordered aligned molecular films have been produced by using rubbed substrates.33-37 There is a great interest in exploiting the self-assembling properties of liquid crystalline (LC) materials in order to realize large area ordered domains in which charge transport can occur
10.1021/jp076180h CCC: $37.00 © 2007 American Chemical Society Published on Web 11/16/2007
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Figure 1. Phase behavior of TR5-C6.40 (K: crystalline, Sm: smectic, N: nematic, I: isotropic.)
Figure 2. Overview of the angle-dependent measurements inside the integrating sphere. N denotes the surface normal.
Grzegorczyk et al. absorption occurring preferentially for light polarized along the rubbing direction. By contrast, the charge carrier mobility obtained from O-FET measurements only exhibits a modest anisotropy.40 In this work, the molecular organization of TR5-C6 films aligned on rubbed PI was investigated by means of angledependent optical density measurements using linearly polarized light. To avoid effects of charge injection at electrodes on charge transport, the conductive properties of TR5-C6 films were studied with the time-resolved microwave conductivity (TRMC) technique. The TRMC technique is suitable for electrodeless measurements of photogeneration of charge carriers,41 charge transport,42,43 and the anisotropy of the charge mobility.34,44-46 The experimental results for the charge carrier mobility are analyzed on basis of quantum chemical calculations of charge transfer integrals. 2. Methods
Figure 3. (A) Optical density spectra of sample S- with a polyimide layer rubbed along the short axis of the substrate, measured at an angle of incidence φ ) +10° using light polarized parallel (P-, solid line) or perpendicular (P+, dotted line) to the PI rubbing direction. The inset shows the molecular structure of a TR5-C6 molecule. (B) Optical density at λ ) 355 nm as a function of the angle of incidence (φ) for light polarized parallel (P-, squares) or perpendicular (P+, triangles) to the PI rubbing direction. The solid line is a fit of eq 5 to the experimental data for the S-P- configuration.
efficiently over long distances.11,38 Recent results on O-FETs with an LC material as the active layer are promising,27,35,39,40 with a charge carrier mobility as high as 0.7 cm2/Vs for an LC polymer containing thienothiophene groups in the backbone.27 Recently, van Breemen et al. studied O-FETs based on the LC material 5,5′′-bis(5-hexyl-2-thienylethynyl)-2,2′:5′,2′′-terthiophene (TR5-C6). The chemical structure of TR5-C6 is shown in Figure 3A. Aligned films of TR5-C6 with monodomains as large as 15 cm could be obtained, using a velvet cloth rubbed polyimide (PI) alignment layer. A maximum hole mobility of 0.02 cm2/ Vs was obtained from measurements on O-FETs with TR5-C6 as the active layer. The absorption of linearly polarized light was found to be highly anisotropic (dichroic ratio of 19) with
2.1. Sample Preparation. The synthesis and characterization of the TR5-C6 compound and preparation of thin aligned films has been described earlier by van Breemen et al.40,47 A PI layer was spin-coated on top of a rectangular quartz slide (1.2 × 2.5 cm2) and manually rub-aligned using a velvet cloth, either in the direction of the short (sample denoted as S-) or long edge (S+) of the quartz slide. A 1.0 wt % solution of TR5-C6 was prepared in toluene. Approximately 100 nm thick TR5-C6 films were spin-coated at 1000 rpm for 60 s on top of the PI alignment layer. The phase behavior of TR5-C6 is schematically depicted in Figure 1. To achieve high-order self-organization of the TR5C6 molecules, the samples were annealed at 418 K (nematic phase) and gradually (5 K/min) cooled to room temperature (crystalline phase). Prior to the measurements the samples were stored at room temperature in air in the dark. 2.2. Optical Measurements. The optical properties of the aligned TR5-C6 films were studied with a UV/vis/NIR spectrophotometer (Lambda 900, Perkin-Elmer) equipped with GlanThomson polarizers and an integrating diffusive sphere (Labsphere). Samples were mounted in the center of the sphere to record the spectrum (FSPHERE) consisting of the sum of the fractions of transmitted (FTRA) and reflected (FREF) light (Figure 2). In addition, by selective removal of the transmitted light, the reflection spectrum could be recorded separately. The optical density (OD) is given by the product of the light absorption coefficient (R) and the sample thickness (d) and is obtained using
{
OD(φ,λ) ) - log10
}
FSPHERE(φ,λ) - FREF(φ,λ) 1 - FREF(φ,λ)
(1)
where φ and λ are the angle of incidence and the wavelength of the incident light, respectively. By rocking the sample around the y axis (see Figure 2), the angle of incidence (φ), measured with respect to the sample normal (N), could by varied between -60 and +60°. Incident light with the polarization direction along the y axis is denoted as P+, whereas light polarized perpendicularly to the y-axis is denoted as P-. 2.3. Photoconductivity Measurements. A detailed description of the TRMC setup can be found elsewhere.42 Briefly, the TR5-C6 samples were placed in an X-band (8.4 GHz) microwave cavity and photoexcited with a short (3 ns) linearly polarized laser pulse at λ ) 355 nm from the third harmonic of a Q-switched Nd:YAG laser (Infinity, Coherent). Photogeneration of mobile charge carriers in the sample leads to an increase
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J. Phys. Chem. C, Vol. 111, No. 49, 2007 18413
of the conductance during time, ∆G(t), and consequently to absorption of microwave power by the sample. The timedependent change of the conductance is obtained from the change in reflected microwave power (∆P(t)/P) from the cavity, according to48
∆P(t) P
∆G(t) ) - K
(2)
The sensitivity factor K is determined by the geometrical and dielectric properties of the media in the microwave cavity. The polarization of the laser light could be aligned parallel (denoted as L-) or perpendicular (denoted as L+) to the fixed direction of the linearly polarized electric field vector of the probing microwaves (denoted as E-). To determine the incident laser light intensity (I0) a pyroelectric sensor (Labmaster, Coherent) was used. 2.4. Calculation of Charge Transfer Integrals. The rate of charge transfer between molecular units, and consequently the mobility of charge carriers, increases with the magnitude of the effective charge transfer integral, Jeff, also referred to as the electronic coupling.49-51 Computed values of Jeff can thus be used to provide understanding of measured charge carrier mobilities. Effective charge transfer integrals were performed using the local density approximation (LDA) with exchange and correlation functionals based on Vosko-Wilk-Nusair (VWN) parametrization of electron gas data.52 In this approach, the orbitals of a system of two molecules are expressed as a linear combination of the molecular orbitals of the individual molecules. In this way, the charge transfer integral, J, is directly obtained as the off-diagonal matrix element of the Kohn-Sham Hamiltonian matrix in terms of the molecular orbitals of the two molecules.49 The effective charge transfer integral takes into account effects of the spatial overlap between the molecular orbitals and is given by53Jeff ) J - 1/2S(1 + 2), where S is the spatial overlap matrix element and 1 and 2 are the siteenergies for molecules 1 and 2, respectively (the site-energy corresponds to the energy of a charge when it is localized on one molecule and is given by the diagonal matrix element of the Kohn-Sham Hamiltonian). For hole transport, the charge transfer integral is computed from the highest occupied molecular orbitals (HOMOs), whereas for electron transport, the lowest unoccupied molecular orbitals (LUMOs) are used. The density functional theorem-based calculations were performed with an atomic basis set of Slater-type orbitals (STOs) of double-ζ quality including one set of polarization functions on each atom (DZP basis set in Amsterdam density functional utilized in this work). The generalized gradient approximation (GGA)54 corrections by Becke55 (exchange functional) and Perdew56 (correlation functional) were included. The crystal structure of a TR5-C6 thin film has not been reported yet. Therefore, the atomic coordinates were taken from the bulk crystal structure of TR5-C4.57 It is assumed that crystallographic properties of a 100 nm TR5-C6 layer are not significantly different from those of bulk TR5-C4. We also assumed that the presence of butyl substituents in TR5-C4 rather than the hexyl groups in TR5-C6 does not significantly affect the material structure. In the calculations the butyl substituents were replaced by hydrogen atoms. This simplification is not expected to affect the values of the charge transfer integrals. 3. Results and Discussion 3.1. Optical Properties. Figure 3A presents the optical density (OD) spectra of the S- sample with the PI layer rubbed along the shorter edge of the substrate. A strong absorption
Figure 4. Schematic representation of light entering the sample at angle φ and refracted at angle β. The direction of the terthiophene backbone in the TR5-C6 molecules at tilt angle Θ is indicated as M. The component of the optical transition dipole moment along the light polarization vector P- is denoted as M|.
Figure 5. (A) Schematic cross-sectional view of the S- sample consisting of a single monodomain in which the molecules are tilted at an angle Θ with respect to the substrate. Dashed line is the sample normal. M and Mz indicates the transition-dipole moment of TR5-C6 and in-plane component of the transition dipole moment, respectively. (B) Hypothetical case of a sample with two domains in which the molecules are tilted at an angle Θ or π - Θ. (C) Schematic view of the S- sample. Bold sections refer to the in-plane component (Mz) of the TR5-C6 molecular axes aligned along the PI rubbing direction, i.e., along the dotted lines.
between 320 and 400 nm is observed for light polarized parallel to the PI rubbing direction (configuration P-S-), whereas light polarized in a perpendicular direction (configuration P+S-) is hardly absorbed. The high optical anisotropy indicates that the transition dipole moments of the TR5-C6 molecules are preferentially aligned along the PI rubbing direction within the plane parallel to the rubbing direction and perpendicular to the substrate. For the optical transition involved, the transition dipole moment of oligothiophenes is oriented along the terthiophene backbone.58 Therefore, it can be inferred that the alignment of the molecular backbones of the TR5-C6 molecules corresponds to the alignment deduced for the transition dipole moments. The orientation of the terthiophene backbones of the TR5C6 molecules was further investigated by measuring the OD at 355 nm and rocking the sample (S-) around the y axis, as shown in Figure 2. The results can be seen in Figure 3B, show that for light polarized parallel to the rubbing direction (configuration S-P-, squares) the OD decreases from a value near 0.9 at φ ) -60° to almost zero at φ ) +60°. From this it can be concluded that the projection of the transition dipole moment of the TR5-C6 molecules along the light polarization vector is the largest for φ ) -60° and decreases as φ goes to +60°. Thus, the backbones of the TR5-C6 molecules are oriented at a nonzero tilt angle Θ with respect to the substrate. The OD for light polarized perpendicularly to the PI rubbing direction (configuration S-P+, triangles) is very small, indicating that the molecular backbones of the TR5-C6 molecules are almost completely oriented within a plane that is perpendicular to the substrate. The tilt angle Θ between the TR5-C6 backbones and the substrate can be determined by analysis of the OD data in Figure 3B. The OD depends on the angle dependent refraction of the incident light at the air/TR5-C6 interface and the absorption within the TR5-C6 layer, as shown in Figure 4.
18414 J. Phys. Chem. C, Vol. 111, No. 49, 2007
Grzegorczyk et al.
According to Snell’s law59 light incident at an angle φ is refracted at an angle β, given by
β ) arcsin
(
n1 sin φ n2
)
(3)
where n1 and n2 are the refractive indices of air (n1 ) 1) and the TR5-C6 layer (n2), respectively. For a molecular tilt angle Θ, the angle between the light polarization vector (P-) and the molecular transition dipole moments equals β - Θ. Since the absorption coefficient is proportional to the square of the amplitude of the component of the transition dipole moment parallel to the light polarization vector, the OD is given by
OD ∝
cos2(β - Θ) cos β
(4)
Combing eqs 3 and 4 gives
[ ( ) ] [ ( )]
n1 sin φ - Θ n2 n1 cos arcsin sin φ n2
cos2 arcsin OD ∝
(5)
The full line in Figure 3B shows a fit of eq 5 to the measured data for the S-P- configuration with the optimal fitting parameters being 1.5 for the refractive index of TR5-C6 and a molecular tilt angle equal to Θ ) 53°. This tilt angle agrees very well with that obtained form X-ray diffraction by van Breemen et al.40 The above findings lead to the conclusion that the TR5-C6 molecules are to a very high degree oriented as shown in Figure 5A,C, within the entire probed film area of 1 cm2. The formation of smaller domains, in which the tilt angle of the molecules is either Θ or π - Θ, as illustrated in Figure 5B can be ruled out. Angle-dependent optical absorption measurements (data not shown) were also carried out on a sample with the PI rubbing direction parallel to the long axis of the substrate (S+). This yielded results in agreement with the conclusions based on the findings for the other (S-) sample discussed above, demonstrating formation of a monodomain consisting of oriented TR5-C6 molecules tilted at 53° with respect to the substrate. 3.2. Photoconductance and Charge Mobility. The conductivity of aligned films of TR5-C6 upon photoexcitation at 355 nm near the maximum of the absorption was studied, using the time-resolved microwave conductivity (TRMC) technique. The extent to which the mobility of charge carriers moving along the PI rubbing direction differs from the mobility for motion along the perpendicular direction was studied by measuring the anisotropy of the photoconductivity. The photoconductivity was determined from measurements on a sample S- for which the PI rubbing direction is parallel to the electric field of the microwaves and on a sample S+ with the rubbing direction perpendicular to the microwave field, see the inset in Figure 6A. To photogenerate an equal amount of charge carriers for the two sample configurations, the laser polarization vector was rotated so that it is parallel to the rubbing direction in both cases (in Figure 6A the laser polarization vector is denoted as Lfor sample configuration A and L+ for configuration B). In Figure 6A, the photoconductance transients are shown for sample configurations A and B. The photoconductance has been normalized to the number of absorbed photons, i.e., ∆G/I0FA with the fraction of absorbed photons given by FA ) 1 FSPHERE. The initial rise of the signal up to 100 ns originates
Figure 6. (A) Photoconductance transients normalized to the number of absorbed photons measured in configuration A (dotted) and B (solid) for a fluence I0 ) 1.2 × 10-4 J/cm-2. The inset in Figure 6A shows that for configuration A the laser polarization (L-) and the PI rubbing direction (S-) are parallel to the probing microwave field (E-). In configuration B the light polarization (L+) and the PI rubbing direction (S+) are mutually parallel in a direction perpendicular to the microwave field (E-). (B) Fluence dependence of the maximum values in the photoconductance transients for configurations A and B normalized to the number of absorbed photons.
from the finite response-time of the resonant microwave cavity used in the experiment. The decay of the photoconductance at t > 100 ns is due to trapping and/or recombination of charges and is seen to be similar for both sample configurations, as expected. Figure 6B shows the maxima in the photoconductivity transients normalized to the number of absorbed photons(∆Gmax/ I0FA) as a function of the incident light fluence. At low laser fluences (I0 < 1 × 10-5 J/cm2), a nearly intensity independent behavior is observed, so that second-order decay processes are negligible. The dependence of ∆Gmax/I0FA on laser fluence is similar for the two sample configurations, since the density of photoexcitations is the same, due to the alignment of the laser polarization vector along the PI rubbing direction. Hence, the difference of ∆Gmax/I0FA for configurations A and B in Figure 6 reflects directly the difference in the sum of the electron and hole mobilities for motion parallel and perpendicular to the PI rubbing direction, with the former being highest. This implies that charge carriers move faster along the PI rubbing direction than the perpendicular direction, with the anisotropy of the mobility being only 1.7 ( 0.3. This value for the anisotropy is much smaller than that found for other organic films aligned on rubbed PI layers.34,60 The anisotropy in the mobility found from the TRMC measurements differs from the results obtained by measurements on aligned TR5-C6 films in a FET, for which the charge
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mobility along the PI rubbing was slightly smaller than in a perpendicular direction.40 The difference between the results from TRMC and FET measurements may be due to the fact that the mobility in a FET is negatively affected by hindrance of charge transport between the Au electrodes and the TR5-C6 film. In addition, for the top-gate FET configuration used in the work of ref 40, charge transport occurs in the part of the TR5-C6 film furthest away from the PI aligning layer. Deposition of the top-gate dielectric may distort the structure of the TR5-C6 film near the gate electrode, which can affect the mobility. Despite the difference between the present result for the anisotropy in mobility and that from van Breemen’s work40 it can be concluded that anisotropy of the mobility in the aligned TR5-C6 layers is very modest. The product of the initial quantum yield for charge carrier formation, φ, and the sum of the electron and hole mobility, ∑µ, can be obtained from
∆Gmax ) eβφ I0FA
∑µ
(6)
where e is the elementary charge and β is a proportionality factor related to the dimensions of the microwave cavity.42 Application of eq 6 to the data at low laser fluence yields for charge motion parallel to the PI rubbing direction (configuration A in Figure 6) that φ∑µ ) 2 × 10-3 cm2/Vs. Both the saturated mobility, measured at 313 K in a top-gate FET device, and the hole mobility measured by the time-of-flight (TOF) technique for the SmB phase of TR5-C6 amount to 2 × 10-2 cm2/Vs.40 The FET mobility of holes may be limited by hindrance of charge transport at electrodes and can therefore be considered as a lower limit to the sum of the electron and hole mobility determined by the TRMC technique. The FET mobility and the value of φ∑µ from the TRMC measurements then yield an upper limit to the photogeneration quantum yield φ ) 10% for excitation at 355 nm. The photoconductance obtained with the laser polarization perpendicular to the rubbing direction was found to be much smaller than for the parallel orientation discussed above, which is due to the smaller optical attenuation for light polarized perpendicular to the rubbing direction, see Figure 3A. The photoconductance normalized to the amount of absorbed photons was found to be the same for the laser polarization parallel or perpendicular to the PI rubbing direction (while keeping the microwave electric field vector in a fixed direction). From this it can be concluded that φ is independent of the direction of the polarization of the excitation light with respect to the rubbing direction. 3.3. Computational Analysis of Mobility. Effective charge transfer integrals were calculated as described in section 2.4. with atomic coordinates obtained from the crystal structure of TR5-C4. According to the TR5-C4 crystal structure,57 earlier introduced characterization of aligned TR5-C6 films,47 the optical sample characterization of the present work, the terthiophene backbones arrange in a herringbone structure with a tilt angle Θ ) 53° with respect to the substrate, as indicated in Figure 7. Charge transfer integrals were calculated for the five possible charge-transfer steps indicated in Figure 6, and the results are given in Table 1. The values of Jeff are of comparable magnitude for the different charge-transfer steps, with the exception of C and E. For charge transfer via C the value of Jeff for holes is smaller than in other directions, whereas for electrons, the value of Jeff
Figure 7. Arrangement of the terthiophene backbones on the PI substrate. The five possible charge-transfer steps between adjacent molecules are denoted as A-E. Viewed along the direction of the arrow shown in the inset, the planes of the terthiophene units form a tilted herringbone structure. The TR5-C4 molecules are bent to yield a concave structure at the gray colored sides, which is due to the presence of the acetylene linkage between the terthiophene moiety and the outer thiophene rings. The dashed lines indicate the PI rubbing direction.
TABLE 1: Effective Charge Transfer Integrals (Jeff) for the Charge-Transfer Steps Indicated in Figure 7 charge-transfer step A B C D E
Jeff, HOMO (eV)
Jeff, LUMO (eV)
1.4 × 10-2 1.4 × 10-2 4.0 × 10-3 1.9 × 10-2 2.1 × 10-2
1.6 × 10-2 1.6 × 10-2 8.4 × 10-2 2.2 × 10-2 7.4 × 10-2
is relatively high for C and E. Hole transfer will occur mainly via steps A, B, E, and D. For these charge-transfer steps, the displacement of a hole has components both parallel and perpendicular to the rubbing direction (see Figure 6), which explains the absence of a strong anisotropy in the mobility. Electrons will mainly move via steps C and E with close to equal rates. Since these two directions are close to perpendicular to each other, electron transport will not exhibit a large anisotropy. The lack of a strong anisotropy in the measured photoconductance can thus be understood on the basis of the similar magnitude of the effective charge transfer integrals for charge motion along or perpendicular to the PI rubbing direction. The values of Jeff in Table 1 are significantly smaller than typical values of 0.5-1 eV for intrachain transport along conjugated polymer chains,51 and at the lower end of the typical range of 0.05-0.1 eV for organic crystals.61 It is to be expected that an excess charge gives rise to structural reorganization of the terthiophene moiety. The typical reorganization energy for conjugated oligomers of 0.1-0.2 eV exceeds the values of Jeff, so that charge transport most likely occurs via polaronic hopping between adjacent TR5-C6 molecules. It is interesting to note that the effective charge transfer integrals for electron-transfer steps C and E exceed those for hole transfer by more than a factor 3. For incoherent hopping transport the mobility scales with the square of the effective charge transfer integral.50 On the basis of this, the electron mobility can be expected to be an order of magnitude higher than the hole mobility, provided the reorganization energies for electrons and holes are similar. Hence, TR5-C6 has the potential to exhibit both p- and n-type conductivity. The fact that van Breemen et al.40 found that the electron mobility is approximately 1 order of magnitude lower than the hole mobility could be due to electron trapping at impurities or structural defects.
18416 J. Phys. Chem. C, Vol. 111, No. 49, 2007 4. Conclusions Optical and photoconductive properties of aligned TR5-C6 films on rubbed PI were studied. According to analysis of the angular dependence of the optical absorption, the TR5-C6 molecules exhibit a tilt angle of 53° with respect to the substrate. The optical absorption is highly anisotropic with light polarized along the rubbing direction being absorbed much stronger than for a perpendicular direction. The anisotropy in the mobility of charge carriers obtained from TRMC measurements is found to be 1.7 ( 0.3. The lack of a strong anisotropy of the mobility can be understood on basis of calculated charge transfer integrals, which are of comparable magnitude for charge-transfer steps in different directions. Acknowledgment. The authors acknowledge Albert van Breemen and Peter Herwig, both of TNO Industrial Technology, The Netherlands, for the synthesis of the liquid-crystalline material. This work is part of the research program of the “Stichting Voor Fundamenteel Onderzoek der Materie (FOM)”, which is financially supported by the “Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO)”. References and Notes (1) Brotherton, S. D. Semicond. Sci. Technol. 1995, 10, 721. (2) Adams, A. C. VLSI Technology; McGraw Hill: New York 1983. (3) Forrest, S. R.; Thompson, M. E. Chem. ReV. 2007, 107, 923. (4) Subramanian, V.; Chang, P. C.; Lee, J. B.; Molesa, S. E.; Volkman, S. K. IEEE Trans. Compon., Packag. Technol. 2005, 28, 742. (5) Peres, L. O.; Gruber, J. Mater. Sci. Eng.: C 2007, 27, 67. (6) Huitema, H. E. A.; Gelinck, G. H.; van der Putten, J. B. P. H.; Kuijk, K. E.; Hart, C. M.; Cantatore, E.; Herwig, P. T.; van Breemen, A. J. J. M.; de Leeuw, D. M. Nature 2001, 414, 599. (7) Gelinck, G. H.; Huitema, H. E. A.; Van Veenendaal, E.; Cantatore, E.; Schrijnemakers, L.; Van der Putten, J.; Geuns, T. C. T.; Beenhakkers, M.; Giesbers, J. B.; Huisman, B. H.; Meijer, E. J.; Benito, E. M.; Touwslager, F. J.; Marsman, A. W.; van Rens, B. J. E.; De Leeuw, D. M. Nat. Mater. 2004, 3, 106. (8) Kitamura, M.; Imada, T.; Arakawa, Y. Jap. J. Appl. Phys. Part 1 2003, 42, 2483. (9) Huitema, H. E. A.; Gelinck, G. H.; van der Putten, J.; Kuijk, K. E.; Hart, K. M.; Cantatore, E.; de Leeuw, D. M. AdV. Mater. 2002, 14, 1201. (10) Leising, G.; Stadlober, B.; Haas, U.; Haase, A.; Palfinger, C.; Gold, H.; Jakopic, G. Microelectron. Eng. 2006, 83, 831. (11) Facchetti, A. Mater. Today 2007, 10, 28. (12) Park, J. G.; Vasic, R.; Brooks, J. S.; Anthony, J. E. J. Low Temp. Phys. 2006, 142, 387. (13) Pflaum, J.; Niemax, J.; Tripathi, A. K. Chem. Phys. 2006, 325, 152. (14) Garnier, F. Chem. Phys. 1998, 227, 253. (15) Hulea, I. N.; Russo, S.; Molinari, A.; Morpurgo, A. F. Appl. Phys. Lett. 2006, 88. (16) Klauk, H.; Halik, M.; Zschieschang, U.; Schmid, G.; Radlik, W.; Weber, W. J. Appl. Phys. 2002, 92, 5259. (17) Stingelin-Stutzmann, N.; Smits, E.; Wondergem, H.; Tanase, C.; Blom, P.; Smith, P.; De Leeuw, D. Nat. Mater. 2005, 4, 601. (18) Sundar, V. C.; Zaumseil, J.; Podzorov, V.; Menard, E.; Willett, R. L.; Someya, T.; Gershenson, M. E.; Rogers, J. A. Science 2004, 303, 1644. (19) Brooks, J. S.; Eaton, D. L.; Anthony, J. E.; Parkin, S. R.; Brill, J. W.; Sushko, Y. Curr. Appl. Phys. 2001, 1, 301. (20) Sirringhaus, H. AdV. Mater. 2005, 17, 2411. (21) Tanase, C.; Wildeman, J.; Blom, P. W. M.; Mena Benito, M. E.; de Leeuw, D. M.; van Breemen, A.; Herwig, P. T.; Chlon, C. H. T.; Sweelssen, J.; Schoo, H. F. M. J. Appl. Phys. 2005, 97. (22) van Breemen, A. J. J. M.; Herwig, P. T.; Chlon, C. H. T.; Sweelssen, J.; Schoo, H. F. M.; Benito, E. M.; de Leeuw, D. M.; Tanase, C.; Wildeman, J.; Blom, P. W. M. AdV. Funct. Mater. 2005, 15, 872. (23) Bao, Z.; Dodabalapur, A.; Lovinger, A. J. Appl. Phys. Lett. 1996, 69, 4108. (24) Sirringhaus, H.; Tessler, N.; Friend, R. H. Science 1998, 280, 1741. (25) Ong, B.; Wu, Y. L.; Jiang, L.; Liu, P.; Murti, K. Synth. Met. 2004, 142, 49. (26) Ong, B. S.; Wu, Y. L.; Liu, P.; Gardner, S. J. Am. Chem. Soc. 2004, 126, 3378.
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