Air-stable n-channel organic field-effect transistors based on charge-transfer complexes including dimethoxybenzothienobenzothiophene and tetracyanoquinodimethane derivatives

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Introduction
][3][4] Among various organic semiconductors, [1]benzothieno [3,2-b][1]benzothiophene (BTBT) derivatives are known to show high fieldeffect mobility and excellent stability. 5,6A strong tendency to form a highly ordered herringbone packing is characteristic of the BTBT derivatives. 7In particular, C8-BTBT with octyl groups shows improved solubility in organic solvents and very high mobility even in solution-processed devices. 80][11] The complex of the unsubstituted BTBT with an octahedral anion, (BTBT) 2 PF 6 , shows as high conductivity as 1500 S cm À1 at room temperature, though this complex undergoes a metal-insulator transition at low temperatures. 10This complex consists of BTBT columns arranged orthogonally in a windmill manner, so the electronic structure is highly one-dimensional.Alkyl substituted BTBT derivatives (CnBTBT) form CT complexes with 7,7,8,8-tetracyanoquinodimethane (TCNQ) and fluoro TCNQ (F n -TCNQ). 11Although these CT complexes have mixed stacks, the thin-film transistors show the electron mobility of the 10 À2 cm 2 V À1 s À1 order, and the single-crystal transistor achieves at most 0.4 cm 2 V À1 s À1 .Ambipolar transport has been also observed in the single-crystal transistor of the TCNQ complex.11a,12 Since mix-stacked CT complexes are composed of donor and accepter molecules, the transistors potentially exhibit p-channel, n-channel, and even ambipolar properties depending on the energy levels. 13However, many CT complexes including TCNQ show only n-channel transistor properties. 11,14Owing to the strong electron acceptor ability, single-component TCNQ shows air-stable n-channel transistor properties in the thin films and the crystals. 15,16lthough the single-crystal transistors of TCNQ show high performance, the thin-film transistors are not easily obtained due to the high vapor pressure. 15Because air-stable n-channel transistor materials are still limited, 17 it is interesting to investigate CT complexes as air-stable n-channel organic transistor materials.
Thin films of CT complexes (DMeO-BTBT)(F n -TCNQ) were deposited by vacuum evaporation of the complexes on a tetratetracontane (TTC)-modified SiO 2 /Si substrate, where TTC was used as a passivation layer. 202][23] For single-crystal transistors, crystals of CT complexes were grown by the recrystallization in acetonitrile.The single-crystal transistors were fabricated by using polystyrene (PS) as a passivation layer and carbon paste as source-drain electrodes. 24The channel direction was determined by the single-crystal X-ray diffraction to be parallel to the crystal long axis corresponding to the molecular stacking (crystallographic c) axis.The transistor characteristics were measured under vacuum (10 À4 Pa) and in an ambient atmosphere.The mobility was evaluated in the saturated region.

Electrochemical properties and crystal structure of DMeO-BTBT
The electrochemical properties were studied by cyclic voltammetry.DMeO-BTBT shows one irreversible oxidation wave, from which the HOMO energy level is estimated to be located at À5.57 eV below the vacuum level.DMeO-BTBT is a slightly stronger donor than the unsubstituted BTBT (À5.65 eV), 10a because the methoxy groups act as electron-donating groups similarly to alkyl groups; the HOMO level of 2,7-dialkyl-BTBT is located at ca. À5.5 eV.7a Single-crystals of DMeO-BTBT for the X-ray structure analysis were grown by recrystallization from the ethyl acetate solution.The molecular packing is depicted in Fig. 1.The crystal belongs to a monoclinic system with the space group P2 1 /c.The half molecule is crystallographically independent.Unlike the herringbone structure universally observed in BTBT derivatives, the DMeO-BTBT molecules form brickwork-like stacks similar to dimethyldicyanoquinonediimine (DMDCNQI). 25Along the c-axis, two different layers parallel to the DMeO-BTBT planes are alternately stacked with the approximate interplanar spacing of 3.53 Å.One layer consists of parallel molecules, but the molecular long axis of another layer is tilted by 701.

Crystal structures of the CT complexes
Electrochemical crystallization of DMeO-BTBT cation-radical salts with inorganic anions was attempted expecting the slightly longer DMeO-BTBT molecules alter the orthogonal molecular packing of (BTBT) 2 PF 6 , but the crystals were not obtained.Needle-like black crystals of the TCNQ complexes with the typical dimensions as large as 40 Â 1 Â 1 mm 3 were obtained by diffusion method from the acetonitrile solutions.
Single crystal X-ray structure analyses were carried out for (DMeO-BTBT)(TCNQ), (DMeO-BTBT)(F 2 -TCNQ) and (DMeO-BTBT)(F 4 -TCNQ).Table 1 lists the crystallographic data.The molecular arrangements and the intermolecular interactions are depicted in Fig. 2. All crystals belong to a triclinic system with the space group P% 1. Respective half donor and acceptor molecules are crystallographically independent; each molecule is located on an inversion center.The unit cell contains one donor and one acceptor molecules, so that the donor/acceptor ratio is 1 : 1.All complexes consist of mixed stacks, where the interplanar distances between the donor and acceptor planes are approximately 3.38, 3.38, and 3.36 Å, respectively; all interplanar distances are equivalent owing to the inversion center at the molecular center.In the (DMeO-BTBT)(F 2 -TCNQ), the fluorine atoms have positional disorder with occupancy 84% for the majority position and 16% for the minority position (Fig. S3, ESI †).The nonequivalent occupancy is related to the steric hindrance of the sulfur atoms.In the crystal of (DMeO-BTBT)(TCNQ), the molecular long axis of the donor is approximately perpendicular to the acceptor long axis.In the crystals of (DMeO-BTBT)(F 2 -TCNQ) and (DMeO-BTBT)(F 4 -TCNQ), however, the long axes of the donor and acceptor molecules are almost parallel.Crystals of (DMeO-BTBT)(F 2 -TCNQ) and (DMeO-BTBT)(F 4 -TCNQ) are isostructural as evident from the lattice constants (Table 1).The ac lattice is approximately the same as the ab lattice of the alkyl BTBT complexes (7.8 Å Â 7.1 Å), 11a to indicate the structure of the core stacks are identical.In contrast, (DMeO-BTBT)(TCNQ) has an obviously different lattice.
The charge-transfer degrees estimated from the bond lengths of the TCNQ molecules (ESI †) are small, so the present TCNQ complexes are essentially regarded as neutral complexes.
The effective transfer integrals for holes and electrons (t eff h and t eff e ) are estimated by an energy-splitting approach for the D-A-D (t eff h ) and A-D-A (t eff e ) triads (Table 2 and Fig. S6-S8, ESI †). 27The resulting values are 20-70 meV.In the TCNQ complex, t eff h and t eff e are comparable, whereas in the F 2 -TCNQ and F 4 -TCNQ complexes, t eff e is predominant.This is attributed to the difference of the parallel and perpendicular molecular arrangements.3a.This is associated with the disordered arrangement of the F 2 -TCNQ complex.The small peak of (DMeO-BTBT)(TCNQ) coming from the single-crystal phase (2y = 10.31) also shows a large FWHM, which suggests comparatively poor crystallinity of the crystal phase in the thin films.AFM images of the evaporated thin films are shown in Fig. 3b-d.In all thin films, needle-like microcrystals cover the substrate densely.The thin film of (DMeO-BTBT)(TCNQ) shows relatively larger roughness than (DMeO-BTBT)(F 2 -TCNQ) and (DMeO-BTBT)(F 4 -TCNQ).This may be related to the presence of the two phases.In contrast, the other thin films consisting of one kind of layer structure show comparatively smooth surface.

Transistor properties
Transfer and output characteristics of the single-crystal transistors are shown in Fig. 4. The device parameters are summarized in Table 3.The CT complexes show n-channel transistor properties both in vacuum and in air.It has been generally 2][13][14] In addition, we have to mention the comparatively weak donor ability of DMeO-BTBT (Fig. 5), where electron injection to F n -TCNQ is preferable to hole injection to DMeO-BTBT.13c Neutral DMeO-BTBT does not show any transistor properties in the form of either single crystals or thin films.The crystal structure of DMe-BTBT (Fig. 1) is entirely different from the herringbone structure of alkyl-BTBT, and the large molecular displacement and the resulting small intermolecular overlap are responsible for the absence of the transistor properties.Among the thin-film transistors, the highest electron mobility of 0.022 cm 2 V À1 s À1 is observed in (DMeO-BTBT)(F 4 -TCNQ) in vacuum.The mobility increases in the order of TCNQ o F 2 -TCNQ o F 4 -TCNQ.This order is potentially related to the   film quality (Fig. 3); the XRD intensity increases, and the AFM roughness decreases in this order.However, since the mobilities of the thin-film devices are to some extent reduced in air, we could not exclude the potential influence of the improved acceptor ability (Fig. 5).By contrast, the single-crystal transistors do not show a systematic increase, and all complexes exhibit similar mobilities in the order of 10 À2 cm 2 V À1 s À1 .The highest mobility is 0.097 cm 2 V À1 s À1 in (DMeO-BTBT)(F 2 -TCNQ), but other complexes show mobilities of the same order.This value is not largely different from 0.4 cm 2 V À1 s À1 reported in (CnBTBT)(TCNQ)based single-crystal transistors. 11The transistor properties of the single-crystal transistors do not change in air.This is obvious from Fig. 4 as well as the mobility values in Table 3.The performance in air is practically unchanged even after one-year storage (Table 3 and Fig. S10, ESI †); note that the mobility values listed in Table 3 even slightly increase after one-year storage.It should be also pointed out that the threshold voltage V th is large in the thin-film transistors (Table 3), and increases in air.In contrast, V th is small in the single-crystal transistors, particularly in the F 4 -TCNQ complex.In general, it is difficult to realize air-stable n-channel organic transistors because the presence of gaseous water and oxygen reduces the performance. 31is particularly applies to thin-film transistors, and among the present combinations, even F 4 -TCNQ is insufficient to entirely eliminate the drop of the mobility and the shift of V th in air.The present work demonstrates that the single-crystal transistors are almost free from such instabilities.This is probably because water and oxygen do not penetrate into the crystals.As a result, TCNQ is sufficient to achieve the air-stable performance.

Conclusions
We have prepared a new donor molecule DMeO-BTBT and investigated the crystal structure and transistor properties of the CT complexes (DMeO-BTBT)(F n -TCNQ) (n = 0, 2, and 4).The thin-film and single-crystal transistors show n-channel transistor properties even in air.Although the performance and stability of the thin-film transistors are considerably influenced by the acceptor strength, the single-crystal transistors are particularly stable in air, and even the TCNQ complex is sufficient to achieve the excellent air stability.Since it is still difficult to realize air-stable n-channel organic transistors, the use of the CT complexes is an important strategy, where these TCNQ complexes achieve fairly good performance in spite of the mixed stack structures.The present work demonstrates, in such a case, the single-crystal transistors show remarkably improved stability in comparison with the thin-film transistors.Fig. 5 HOMO level of DMeO-BTBT, LUMO levels of TCNQ derivatives, 28 and the Fermi levels of (TTF)(TCNQ), 29 and carbon. 30

Table 2
Effective transfer integrals