Charge Transport Properties of Perylene–TCNQ Crystals: The Effect of

Oct 1, 2014 - Crystal Growth, HOMO–LUMO Engineering, and Charge Transfer Degree in Perylene-FxTCNQ (x = 1, 2, 4) Organic Charge Transfer Binary ...
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Charge Transport Properties of Perylene−TCNQ Crystals: The Effect of Stoichiometry D. Vermeulen,† L. Y. Zhu,‡,§ K. P. Goetz,¶ Peng Hu,⊥ Hui Jiang,⊥ C. S. Day,# O. D. Jurchescu,¶ V. Coropceanu,‡ C. Kloc,⊥ and L. E. McNeil*,† †

Department of Physics and Astronomy, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3255, United States ‡ School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia 30332, United States § National Center for Nanoscience and Technology, Beijing, 100190, People’s Republic of China ¶ Department of Physics, Wake Forest University, Winston-Salem, North Carolina 27019-7507, United States ⊥ School of Materials Science and Engineering, Nanyang Technological University, Singapore, Singapore 639798 # Department of Chemistry, Wake Forest University, Winston-Salem, North Carolina 27109, United States S Supporting Information *

ABSTRACT: In this work we have revisited the charge-transfer crystal system perylene− TCNQ and found that this complex can crystallize with a 2:1 stoichiometric ratio in addition to the 1:1 and 3:1 stoichiometries previously observed. The vibrational and electronic properties of these perylene−TCNQ charge-transfer crystals have been investigated by means of Raman scattering measurements and density functional theory calculations. Electrical measurements were also performed by preparing organic field-effect transistors (OFETs) from the crystals. The Raman spectra in the low-frequency range (below 200 cm−1) are found to be unique to the specific crystal structure and can therefore be used to determine the stoichiometry. The Raman data and the X-ray diffraction measurements indicate that at room temperature the amount of charge transferred to the TCNQ molecule is less than 0.2e for all three compounds, and is nearly the same in P1T1 and P2T1 but is slightly larger in P3T1. The electronic structure calculations suggest good intrinsic charge transport properties for both holes and electrons in P1T1 and P2T1 and only for holes in P3T1. Ambipolar charge transport characteristics were found for P2T1 and hole and electron charge transport characteristics were found in P3T1 and P1T1, respectively. TCNQ).20,21 In contrast, systems with a mixed-stack packing motif, in which the donor and acceptor molecules alternate along the stacks (...-D-A-D-A-...), are in general semiconductors or insulators.19 Like one-component systems, binary CT crystals can exist in several polymorphic forms. For instance, bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF)−TCNQ crystallizes in two triclinic (β′ and β′′) phases and a monoclinic phase. The crystals with the triclinic β′ or the monoclinic phase exhibit semiconducting properties while those with the β′′ phase (characterized by a segregated-stack packing) exhibit metallic behavior.22−24 The variety of CT systems is further increased by the fact that the same donor and acceptor molecules can yield crystals with different stoichiometries. For example, TMPD (N,N,N′,N′-tetramethyl-p-phenylenediamine) with TCNQ25 and TMBD (N,N,N′,N′-tetramethylbenzidine) with chloranil26 can form crystals of 1:1 and 2:1 stoichiometries.

1. INTRODUCTION Organic semiconductors have attracted much attention in recent years due to their potential applications in optoelectronic devices.1−4 Although most studies to date have focused on crystals based on a single molecular building block such as pentacene or rubrene, there is now an increasing interest in binary charge-transfer (CT) organic crystals in which one component acts as an electron donor (D) and the other as an acceptor (A).2,5−17 For example, several groups have recently fabricated ambipolar field-effect transistors using CT compounds as active elements.7,12,13,16,18 In the case of organic molecular crystals there is a strong relationship between the crystal geometry and the electronic, optical, and charge transport properties of the system. This relationship is even more strongly manifested in the case of CT crystals. For example, binary CT compounds with a segregatedstack packing motif, in which donor and acceptor molecules form adjacent separated donor and acceptor (...-D-D-D-... and ...-A-A-A-...) stacks, usually display high electrical conductivity.19 A famous representative of this class of systems is tetrathiafulvalene−7,7,8,8-tetracyanoquinodimethane (TTF− © 2014 American Chemical Society

Received: August 22, 2014 Revised: September 30, 2014 Published: October 1, 2014 24688

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Single crystals of P1T1 and P3T1 were also grown from solution with toluene and benzene as solvents. The crystal growth and the dependence of the stoichiometry of the crystals on the solvent used are described elsewhere.34 All crystals were characterized by using X-ray powder and single-crystal diffraction. The P2T1 structure was confirmed independently in three laboratories (NTU, WFU, and UNC). For data collection at Wake Forest University, for which the structural file is included in the Supporting Information, measurements were made on a Bruker APEX CCD system with Bruker SMART and SAINT software was used for data collection and cell refinement. For data reduction, structure solution and refinement, and CIF file generation, Bruker APEX2 software was utilized. Raman Scattering. Polarized Raman spectra in the range 20−3000 cm−1 were obtained in the backscattering geometry with a Jobin XY triple spectrometer with a Coherent Genesis CX 532 nm solid state laser. The spectrometer was equipped with a microscope to focus the incident light on the sample to a spatial resolution of approximately 5 μm. Room temperature measurements were made in air. Temperature-dependent Raman measurements were made under vacuum using a Joule-Thomson refrigerator manufactured by MMR Technologies and capable of cooling the sample to 80 K. To make measurements in which the polarization direction of the incident light was controlled, the orientation of the molecular stacking axis (a-axis) was determined by X-ray diffraction for all crystals. Electrical Measurements. Electrical measurements were performed by preparing organic field-effect transistors (OFETs) from the crystals. Several device architectures were fabricated for each crystal stoichiometry; the combination that yielded the best performance will be discussed. The P2T1 and P3T1 crystals were thin, flat platelets, and were therefore amenable to a prefabricated bottom-gate bottom-contact OFET geometry. Here, highly n-doped Si wafers were employed as the gate electrode, with 200 nm thermally oxidized SiO2 as the gate dielectric. Gold source and drain contacts (60 nm, with a 10 nm Ti adhesion layer) were defined by photolithography and deposited by e-beam evaporation. These test beds were cleaned for 10 min in a hot acetone bath and 10 min in a hot isopropanol bath. This was followed by 10 min in a UV ozone cleaner and a thorough rinse with DI water. After cleaning, crystals were laminated by hand on the surface of the test beds, adhering electrostatically to the dielectric surface.35,36 The P1T1 crystals grew as needles and were too thick to laminate, so a bottom-gate top-contact OFET geometry was employed instead of the bottom-gate bottom-contact geometry used for the platelets. A similar structure was also tested for the case of P2T1. Here, the crystals were placed on heavily doped Si wafers (the gate electrode) with 200 nm of thermally oxidized SiO2 for the gate dielectric (cleaned in the same way as the previous substrates), and Ag epoxy was painted as the source and drain top contacts on the crystals. The results obtained for the P2T1 crystals in the two different device geometries will be compared below. The samples were electrically characterized in air at room temperature with use of an Agilent 4155C Semiconductor Parameter Analyzer. All crystals were evaluated for both electron and hole transport. The charge-carrier mobility of the transistors was calculated from the slope of the square root of the source-drain current √ID versus the gate-source voltage (VGS), using the equation

Another example is the combination of perylene (P) and TCNQ (T) molecules, which were previously reported to form crystals with 1:1 (P1T1) and 3:1 (P3T1) stoichiometric ratios.27 Although perylene−TCNQ compounds have received significant attention in the literature,14,27−31 a complete understanding of their structure−property relationships has not yet been achieved. Here we revisit this class of compounds, which we will refer to collectively as PnT1. In the course of this study we found that in addition to 1:1 and 3:1 stoichiometries this complex can crystallize with a 2:1 stoichiometric ratio (P2T1). The electronic, vibrational, and electrical properties of these crystals are evaluated and compared.

2. EXPERIMENTAL SECTION Crystal Growth and Crystal Structure. Powders of perylene and TCNQ (Sigma-Aldrich) were purified before use by sublimation in flowing argon gas under a pressure of a few torr. Single crystals of both the monomolecular parent compounds (perylene and TCNQ) and the three perylene− TCNQ charge-transfer complexes (in D:A ratios of 1:1, 2:1, and 3:1) were grown from the gas phase. The perylene and TCNQ crystals were grown using the standard PVT method (Figure 1).32 The charge-transfer compounds were grown in

Figure 1. A typical mixture of phases grown in an open PVT system with flowing argon gas.

the same way, but the powders of perylene and TCNQ were placed separately in two zones of the furnace that were set at different temperatures.33 The stoichometries of the perylene and TCNQ compounds were regulated by selection of the position of the source materials, the temperature gradient between the two zones, and the gas flow rate through the furnace. In each growth a mixture of monomolecular crystals and perylene−TCNQ binary compounds with different stoichometries was obtained. The specific growth parameters and the duration of the crystallization process determined the composition of the mixture. Precise control of the formation of a compound with a specific stoichiometry was impossible in this open system with flowing argon gas. However, the dark-colored crystals of the binary compounds were easily distinguished from the yellow crystals of monomolecular TCNQ or perylene. It was also possible to visually distinguish needle-like P1T1 crystals from platelet-shaped P2T1 and P3T1 crystals. However, P2T1 and P3T1 platelets could be distinguished from one another only by X-ray diffraction or Raman spectroscopy of the individual crystals. It is worth noting that P3T1 platelets were always numerous among the crystals grown while P2T1 crystals were found only occasionally; a specific growth protocol to produce P2T1 has not yet been determined. 24689

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W Ci μ(VGS − VT)2 L 2

Article

(1)

where W and L are the channel width and length, respectively, Ci is the dielectric capacitance per unit area, μ is the charge carrier mobility, VGS is the gate-source voltage, and VT is the threshold voltage. Electronic-Structure Calculations. The molecular geometries of the neutral and radical-ion states were optimized at the DFT level by using the B3LYP functional and the 631G(d,p) basis set, as implemented in the Gaussian 09 program.37 The geometry optimizations of the crystal structures were performed by using the B3LYP functional and 6-31G basis set as implemented in the CRYSTAL06 package.38 During the optimization the positions of the atoms in the unit cell were relaxed while the cell parameters were kept fixed at the experimental values. The electronic band structures and density of states (DOS) were calculated by using the optimized crystal structures. The inverse effective mass tensor, mji−1, is defined as39

1 1 ∂ 2E = 2 mij ℏ ∂kj∂ki

(2)

where subscripts i and j denote the Cartesian coordinates in reciprocal space, E is the band energy, ℏ is the Planck constant, and k is the electron wavevector. Subsequent diagonalization of mji−1 provides the principal components and their orientations. The inverse effective mass tensor was calculated by means of the centered-difference method with dk = 0.01/Bohr. The effective transfer integrals for nearest-neighbor donor− acceptor pairs (which we denote as tdirect) at the optimized crystal geometry were evaluated by using a fragment orbital approach in combination with a basis set orthogonalization procedure.40 The electronic couplings between D molecules or between A molecules were computed according to the procedure described elsewhere.8 These transfer integrals were also computed by using experimental crystal structures measured at various temperatures. The calculations of the transfer integrals were performed with the B3LYP functional and 6-31G(d,p) basis set, using the Gaussian 09 package.37

Figure 2. [Color online] Molecular packing of (a) P1T1, (b) P2T1, and (c) P3T1. To distinguish donor from acceptor molecules in these figures the carbon atoms in perylene (donor) are colored green and those in TCNQ (acceptor) are colored gray.

platelets, green in color, of millimeter lateral dimensions and a few hundred micrometers thick. The molecular structure of P2T1 consists of mixed-stack arrays along the a-axis as in P1T1, but with an additional perylene molecule inserted between adjacent stacks in the manner presented in Figure 2b. The CIF file is included in the Supporting Information. The single-crystal structure of P3T1 is also triclinic and belongs to the same space group as that of P2T1 with unit cell parameters a = 10.875 Å, b = 12.699 Å, c = 10.422 Å and α = 114.905°, β = 90.797°, γ = 112.615° at room temperature, as has been previously published.42 When grown from the gas phase, single crystals of P3T1 adopt a platelet shape very similar to that of P2T1 (Figure 3d). When grown from solution they have the same crystal structure but adopt a three-dimensional cube-like shape (Figure 3e). Both are green. The molecular structure of P3T1 consists of donor−acceptor stacks of the form (...-A-D-D-A-D-D-A-...) along the c-axis with a perylene molecule inserted between the stacks, as in P2T1 (see Figure 2c). Vibrational Properties. Some Raman spectra of P3T1 and P1T1 have previously been reported.43−45 Our measured spectra of all three PnT1 compounds in the low-frequency range (below 200 cm−1) at room temperature are compared in Figure 4. For all crystals the axis of molecular stacking lay in (or very near) the plane of the face upon which the light was incident, and we were therefore able to determine the dependence of the Raman intensities on the orientation of the electric vector with respect to that axis. Measurements made with the incident and scattered electric field oriented along the direction of molecular stacking are shown in Figure

3. RESULTS AND DISCUSSION Crystal Structure. The molecular packings of P1T1, P2T1, and P3T1 are shown in Figure 2. Single-crystal X-ray diffraction measurements reveal that the crystal structure of P1T1 is monoclinic, belonging to the P21/c (C2h5) space group with unit cell parameters at room temperature identical with those previously published:41 a = 7.32 Å, b = 14.55 Å, c = 10.88 Å and α = 90°, β = 90°, γ = 90.4°. Each molecule is located at an inversion center of the unit cell. The PVT-grown and solutiongrown P1T1 crystals have the same structure. The P1T1 crystals have a needle-like form typically a few millimeters in length by a few hundred micrometers in width and are dark green. They exhibit a mixed-stack packing along the a-axis (Figure 2a), which is the long axis of the needle In this work we have discovered a compound of perylene− TCNQ with a stoichiometry not previously reported in the literature. The crystal structure of P2T1 is triclinic and belongs to the P1̅ space group with one formula unit per unit cell and cell parameters a = 7.181(2) Å, b = 11.080(5) Å, c = 11.904(4) Å and α = 103.81(4)°, β = 102.91(3)°, γ = 101.32(1)° at room temperature. The crystals are in the form of long, narrow 24690

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Figure 3. Optical images of single crystals of (a) perylene, (b) TCNQ, (c) P1T1, and (d) P2T1 grown from the gas phase (P3T1 grown from the gas phase has a very similar appearance) and (e) P3T1 grown from solution. The insets of panels a and b are the corresponding molecular structures.

4a and measurements with the electric fields perpendicular to the stacking axis are shown in Figure 4b. The calculated frequencies at the Γ point are given in the Supporting Information, see Tables S1−S3. The Raman spectra of these three crystals are very different in this range of frequency, reflecting the fact that the low-frequency vibrations observed are either intermolecular modes or hybrids of intermolecular and intramolecular motions. This also means that the agreement between the calculated values (which are based on single molecules) and the measured values is less good, as expected. Consequentially, each spectrum is unique to the specific crystal structure and can be used to identify the crystal stoichiometry. The vibrations at higher frequencies are predominantly intramolecular modes and are thus quite similar among the three crystals, which all contain the same molecular species. This categorization of the modes is also supported by the results of DFT calculations of isolated molecules (see Tables S4 and S5, Supporting Information) which indicate that all the intramolecular vibrational modes of perylene (four modes) and TCNQ (eight modes) located below 200 cm−1 show marginal or no Raman activity. As Figure 4 shows, the frequency of the lowest-frequency Raman peak observed in P1T1 at room temperature is nearly identical with that in P2T1, whereas that of P3T1 is about 6 cm−1 higher in frequency. The results of DFT calculations are in line with this finding. The frequencies of the predominantly intermolecular vibrations in the 200 cm−1) the vibrational modes are of primarily intramolecular character. Some of the modes show interesting but as yet unexplained departures from the corresponding spectra of crystals of the parent compounds. For example, the strong CC in-plane stretching mode (agsymmetry) of the inner ring of the TCNQ molecule, with a frequency of 1454 cm−1 in pure crystalline TCNQ,46−48 appears as a doublet in the Raman spectra of all three PnT1 compounds. This vibration is of particular interest since it has been shown both experimentally and in our calculations to be particularly sensitive to the amount of charge residing on the TCNQ molecule, i.e. to charge transfer. The frequency of this vibration has been measured49−52 to downshift linearly by 60 ± 3 cm−1 between the neutral TCNQ molecule and the anion, as the result of a decrease in the C−C bond length and an increase in the CC bond length as the net charge on the molecule becomes more negative.49,53−56 This is supported by our calculations (see Table S5, Supporting Information). Similar behavior can be expected in PnT1, and since the 1454 cm−1 vibration is rather intense in TCNQ, the most intense peak of the doublet was used to estimate the degree of charge transfer between perylene and TCNQ at room temperature in the three PnT1 compounds. The results can be seen in Table 1, together with estimates obtained by considering the bond lengths 24691

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Figure 5. Temperature dependence of selected Raman frequencies in P1T1 and P3T1, normalized to their values at 80 K: (a) P1T1 and (b) P3T1.

molecule in P1T1 and P2T1 are nearly the same but that the charge transfer is slightly larger in P3T1. This is to be expected since only the perylene molecules in the π stack contribute to the charge transfer (the additional perylene molecules inserted between the stacks remain neutral), and there are two such perylenes per TCNQ molecule in P3T1 and only one in P2T1 and P1T1. The intramolecular vibrational modes in these materials are little affected by interactions between molecules and therefore remain nearly constant in frequency as the temperature is lowered and the lattice contracts. However, if the amount of charge transferred from donor to acceptor changes with temperature, the frequencies of modes sensitive to the amount of charge on the molecules will shift as the temperature is lowered. By comparing the temperature dependence of the CC stretching mode of the TCNQ molecule in P3T1 with our measurements of the corresponding mode in pure crystalline TCNQ (in which there is of course no charge transfer) we have been able to make a detailed analysis of the charge transfer that accounts for the effects of thermal contraction. We observe that the charge transfer increases

Table 1. Charge Transfer between Perylene and TCNQ in the Three Compounds at Room Temperature, Estimated from the Frequency of the CC Stretching Mode of TCNQ and from the Bond Lengths Derived from XRD compd

mode freq (cm−1)a

Raman CT estimate

bond length estimate

P1T1 P2T1 P3T1

1449 1448 1446

0.04 ± 0.02 0.13 ± 0.02 0.17 ± 0.02

0. 01 ± 0.07 0.12 ± 0.07 0.23 ± 0.06

a

1454 cm−1 in neutral TCNQ.

derived from X-ray diffraction measurements in the neutral molecule (in pure crystalline TCNQ) and the anion.57,58 The change from a quinonoid to a benzenoid structure with increasing negative charge affects only a few of the bond lengths, so it is possible to use those changes to determine the charge transfer. Variations in the bond length ratios chosen by various authors lead to uncertainty in the charge transfer calculated by this method. However, the two methods give similar values for the charge transfer at room temperature, and show that the amounts of charge transferred to the TCNQ 24692

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slightly as the temperature is lowered (see Figure 6). This is to be expected, as thermal contraction increases the coupling

Figure 6. Change in charge transfer with temperature, from Raman scattering measurements: squares, P1T1; circles, P3T1.

between the donor and acceptor molecules. Indeed, our calculations indicate that the donor−acceptor electronic couplings (tDA) that determine the amount of charge transfer increase only by about 6% when the temperature is lowered from 300 to 150 K (see Table S6, Supporting Information). Even at 80 K the molecules remain quasineutral and the compounds are expected to remain semiconducting.59−62 In addition, the mean-plane spacing between donor and acceptor molecules in the π stack in P3T1 (3.29 Å) is smaller than in the other two compounds (3.44 Å in P1T1, 3.46 Å in P2T1), which promotes intermolecular interaction. Electronic Structure. The electronic band structures calculated for all three systems are displayed in Figure 7. Large band dispersion for both the valence band (VB) and the conduction band (CB) is observed in the case of P1T1. The VB and CB widths of this crystal are 393 and 298 meV, respectively. The largest VB and CB dispersion occurs along the stacking directions. Large band dispersion for the VB is also found in P2T1 (440 meV). In P3T1 the VB width is about 210 meV, being thus smaller than in the other two crystals. A very small width of about 40 meV is found for the CB in P3T1. To obtain a clearer description of the electronic coupling pathways, we also computed the transfer integrals along different crystal directions. The results for the three compounds are shown in the Supporting Information, Figures S1−S3. The largest transfer integrals for both electrons and holes are obtained along the stacking directions and have values of about 60 meV. We note that such large transfer integrals occur despite the fact that each pair of donor (acceptor) sites along the stack is separated by an acceptor (donor) site; this can be explained by means of superexchange mechanism.8 As a consequence of the large transfer integrals, the effective masses along the stacking direction of both holes and electrons are very small and are comparable, as can be seen from Table 2 (see also Figure S1, Supporting Information). Therefore, the transport along donor−acceptor alternating stacks for electrons and for holes is expected to be comparable as well. In addition, significant couplings for holes are found along other directions. This means that holes might also show good intrinsic transport properties along other directions. In contrast to P1T1, the electronic coupling along the stacks in P3T1 for holes and electrons is asymmetric. The packing along the stacks is now of the form ...D-D-A-D-D-A-D-D... (see Figure 1c), therefore there are two transfer integrals for holes

Figure 7. B3LYP/6-31G electronic band structure and density of states (DOS) of (a) P1T1, (b) P2T1, and (c) P3T1 crystals. The points of high symmetry in the first Brillouin zone are labeled as follows: Γ = (0,0,0), X = (0.5,0,0), Y = (0,0.5,0), Z = (0,0,0.5), C,U = (0,0.5,0.5), D,V = (0.5,0,0.5), A,T = (0.5,0.5,0) and E,R = (0.5,0.5,0.5), all in crystallographic coordinates. The zero of energy is given at the top of the valence band.

along the stacks related to dimer DD (direct coupling) and triad DAD (superexchange coupling) sets, and only one superexchange-type transfer integral for electrons, described by the molecular fragment ADDA. The calculations show that the direct coupling and superexchange couplings for holes are similar (see Figure S2, Supporting Information); as a result the effective mass for holes along this direction is relatively small, about 1.3 m0. In contrast, the effective transfer integral for electrons along the stacking directions is only about 10 meV, leading to a relatively large effective mass of about 5.9 m0. P2T1 is characterized by the same donor−acceptor alternating structure along the stacks as in P1T1, and the 24693

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Table 2. Hole and Electron Effective Masses m (in units of the free electron mass at rest, m0) at the Valence Band and Conduction Band Edges of the P1T1, P2T1, and P3T1 Crystals P1T1

holes at Γ(0,0,0)

electrons at ΔΓZ(0,0,0.115)

P2T1

holes at Γ(0,0,0)

electrons at Y(0,0.5,0)

P3T1

holes at Γ(0,0,0)

electrons at V(0.5,0.5,0)

m/m0

parallel to

0.81 1.45 2.16 0.93 11.11 ∞ 1.26 2.24 6.01 1.12 18.84 74.32 1.311 2.60 8.02 5.90 7.46 ∞

a − 0.020b c b + 0.095a a + 0.008b c b − 0.018a a − 0.256b + 0.189c b + 0.844a + 0.189c c + 0.515a + 0.081b a − 0.002b − 0.002c b + 0.450a + 0.383c c + 0.332a − 0.138b c − 0.080a + 0.038b a + 0.309b + 0.254c b + 0.441c + 0.134a a + 0.560c + 0.081b c − 0.489a + 0.048b b + 0.448c + 0.415a

effective transfer integrals and effective masses for holes and electrons are similar (see Figure S3, Supporting Information). However, in the case of holes there exist strong electronic interactions also along molecules located on different stacks; as a result the hole transport is expected to have a twodimensional character. In summary, electronic structure calculations predict very small effective masses for holes along the stacking directions in all three compounds. Relatively small masses for holes are also computed along directions perpendicular to the stacks (twodimensional transport in P2T1 and P3T1, and three-dimensional in P1T1). In the case of electrons, small effective masses are found only for P1T1 and P2T1 and only along the stacking directions. We note, however, that the small effective masses do not yet guarantee high intrinsic charge carrier mobility since the latter also strongly depends on the strength of electronvibration interactions. This problem is still largely unexplored in the charge-transfer systems. Electrical Properties. As can be observed from the transfer curves depicted in Figure 8, we found that the electrical properties of the perylene−TCNQ charge-transfer compounds were strongly dependent on stoichiometry. Figure 8a presents the evolution of the drain current (ID) as a function of gate voltage (VGS) in the saturation regime (VDS = −40 V) for a representative P1T1 crystal (crystal dimensions, which also define the transistor channel: length L = 160 μm, and width W = 60 μm). We found that this crystal exhibited an electron mobility of μe = 1.2 × 10−3 cm2 V−1 s−1 when measured in air, with no detectable activity for hole transport. P2T1 crystals (Figure 8b, device dimensions L = 100 μm, W = 275 μm) measured using Ag contacts displayed ambipolar charge transport characteristics, with similar electron and hole mobilities: μe = 2.9 × 10−5 cm2 V−1 s−1 and μh = 7.4 × 10−5 cm2 V−1 s−1. Note that P2T1 exhibited balanced transport, but lower mobilities, when Au contacts were used (μe = 2.1 × 10−7 cm2 V−1 s−1 and μh = 7.5 × 10−7 cm2 V−1 s−1), probably because of contact resistance and lamination imperfections. Unfortunately, the limited number of P2T1 crystals available

Figure 8. Electrical characteristics of the PnT1 crystals. (a) Electrononly transport in P1T1 with silver contacts, (b) ambipolar transport in P2T1 with silver contacts, and (c) hole-only transport in P3T1 with gold contacts.

prevented us from undertaking extensive device optimization, thus the reported numbers likely represent lower limits of mobility. The P3T1 crystal (Figure 8c, device dimensions L = 80 μm, W = 65 μm) exhibited a hole mobility of μh = 5.5 × 10−5 cm2 V−1 s−1 and no electron conduction. With the exception of P2T1 the mobilities reported here are representative values for each stoichiometry and were measured in several crystals.

4. CONCLUSIONS We have investigated the electronic, vibrational, and chargetransport properties of perylene−TCNQ charge-transfer crystals. We have found that this complex can crystallize with a 2:1 stoichiometric ratio in addition to the 1:1 and 3:1 stoichiometries previously reported. The Raman scattering data indicate that in the low-frequency range (below 200 cm−1) each spectrum is unique to the specific crystal structure and can be used to identify the stoichiometry. The Raman data, along with the estimates derived from considering the bond lengths derived from X-ray diffraction measurements, also indicate that the amount of charge 24694

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transferred to the TCNQ molecule is small (0.2e or less). The charge transferred is nearly the same in P1T1 and P2T1, but is slightly larger in P3T1. The difference is perhaps because in P3T1 there are two perylene molecules per TCNQ molecule within the alternating stack, whereas in P1T1 and P2T1 there is only one. In addition, the mean-plane spacing between donor and acceptor molecules in the π stack in P3T1 is smaller than in the other two compounds, which promotes intermolecular interaction. The electronic structure calculations suggest good intrinsic transport properties for both holes and electrons in P1T1 and P2T1, but only for holes in P3T1. In agreement with this prediction, in field-effect transistor measurements only hole transport was detected in P3T1 and ambipolar charge transport characteristics were found for P2T1. However, although ambipolar charge transport properties were suggested by electronic structure calculations for P1T1, the crystal exhibited only electron transport.

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ASSOCIATED CONTENT

S Supporting Information *

Normal modes computed at the Γ point, illustration of the most important charge-transport pathways for holes and electrons in P1T1, P2T1, and P3T1; X-ray data (CIF format) of P2T1; and complete refs 37 and 38. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation under grant DMR-1105147. The WFU X-ray Facility thanks the National Science Foundation (grant CHE-0234489) for funds to purchase the single crystal X-ray instrument and computers. K.P.G. is supported by the NSF Graduate Research Fellowship Program under Grant No. DGE-0907738. The work in Beijing was supported by the National Science Foundation under grant No. 21473043. The research in Singapore was conducted under the Campus for Research Excellence and Technological Enterprise (CREATE), which is supported by the National Research Foundation, Prime Minister’s Office, Singapore. We are grateful to Dr. J.-L. Brédas for stimulating discussions.

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REFERENCES

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