Electronic structure of stable n-type semiconducting molecular complex (diC8-BTBT)(TCNQ) studied by ultraviolet photoemission and inverse photoemission spectroscopy

Organic Electronics 39 (2016) 184e190

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Electronic structure of stable n-type semiconducting molecular complex (diC8-BTBT)(TCNQ) studied by ultraviolet photoemission and inverse photoemission spectroscopy Harunobu Koike a, *, Jun'ya Tsutsumi b, Satoshi Matsuoka b, c, Kazuma Sato a, Tatsuo Hasegawa b, d, Kaname Kanai a a

Department of Physics, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba, 278-8510, Japan National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki, 305-8565, Japan Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennoudai, Tsukuba, 305-8571, Japan d Department of Applied Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 July 2016 Received in revised form 29 September 2016 Accepted 5 October 2016

The electronic structure of a semiconducting mixed-stack charge transfer (CT) complex composed of a 2,7-dialkyl[1]benzothieno[3,2-b][1] benzothiophene (diC8-BTBT) electron donor and a tetracyanoquinodimethane (TCNQ) electron acceptor, (diC8-BTBT)(TCNQ), was studied by ultraviolet photoemission spectroscopy and inverse photoemission spectroscopy. Compared with its components, the frontier electronic states observed for the (diC8-BTBT)(TCNQ) complex showed a large stabilization that originates from the reconstruction of electronic states by intermolecular donor-acceptor CT interactions. We discuss how the frontier electronic states of the complex are formed from those of the individual component molecules, and clarify the origin of the air-stable n-type organic field-effect transistor characteristics that the material exhibits when it is used as a channel semiconductor. © 2016 Elsevier B.V. All rights reserved.

Keywords: Charge transfer complex Electronic structure Ultraviolet photoemission spectroscopy Inverse photoemission spectroscopy Air-stable n-type semiconductor

1. Introduction Organic semiconductor materials are solution processable and mechanically flexible; thus, they have been intensively investigated for the fabrication of flexible printed electronic devices. In particular, single-component organic semiconductors have been widely used in devices [1e3]. Organic charge transfer (CT) compounds composed of electron donor and acceptor molecules exhibit a wide variety of physical properties such as metallic, semiconducting, ferroelectric, or superconductive properties [4e10]. These unique characteristics are promising for functional organic electronics or optoelectronic devices. However, studies of device applications have been limited to single-crystal devices. This is mainly because many organic CT complexes tend to form needle-shaped crystals along the donoreacceptor molecular stacks [11e14]. This feature prevents the fabrication of uniform thin films and thus prevents the

* Corresponding author. E-mail addresses: [email protected] (H. Koike), [email protected] (K. Kanai). http://dx.doi.org/10.1016/j.orgel.2016.10.005 1566-1199/© 2016 Elsevier B.V. All rights reserved.

efficient two-dimensional carrier transport required for organic field-effect transistors (OFETs). Therefore, to use CT complexes in electronic devices, two-dimensional layered crystalline films must be grown on a substrate. Recently, it was reported that a series of CT complexes, consisting of 2,7-dialkyl [1]benzothieno [3,2-b] [1] benzothiophene (Fig. 1(a): diCn-BTBT; n ¼ 4, 8, and 12) donors and fluorinated tetracyanoquinodimethane (Fig. 1(b): FmTCNQ; m ¼ 0, 2, and 4) acceptors, form a layered crystalline structure where the twodimensional semiconducting layers created by p-conjugated donor and acceptor skeletons are separated by alkyl chain layers derived from the diCn-BTBT end groups [15]. This unique structure affords high-quality polycrystalline thin films and allows the fabrication of thin-film OFETs based on channel layers of CT complexes [16]. The complexes exhibit n-type field-effect characteristics with excellent air and thermal stabilities [15,16]. One of the complexes maintains high mobility of more than 4  103 min in air. Moreover, the n-type field effect mobility of the OFETs based on (diC8BTBT)(TCNQ) polycrystalline films was estimated as mn ¼ 1.9  102 cm2/V in the saturation regime [16]. The properties are different from the component of diC8-BTBT, which shows p-

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Fig. 1. Molecular structure of (a) a diC8-BTBT monomer, (b) a TCNQ monomer and (c) a (diC8-BTBT)(TCNQ) oligomer (N ¼ 7) containing seven diC8-BTBT and TCNQ molecules. (d) Crystal structure of a (diC8-BTBT)(TCNQ) complex containing nine diC8-BTBT and TCNQ molecules as seen from side and near top [15,16]. Hydrogen atoms are not shown for clarity.

type characteristics and thermal instability due to the crystal-liquid crystal transition at the relatively low temperature of 380 K. However, the origin of these unique physical properties of CT complex films has not been clarified [17,18]. It is necessary to determine the electronic structures of the films to understand the effect of the donor-acceptor CT interaction on their unique electronic properties. In this work, we explore the electronic structure of a (diC8BTBT)(TCNQ) polycrystalline film by ultraviolet photoemission spectroscopy (UPS) and inverse photoemission spectroscopy (IPES). The electrical characteristics of the (diC8-BTBT)(TCNQ) films are explained well by the electronic structure around the frontier orbitals of (diC8-BTBT)(TCNQ) directly observed by UPS and IPES. Our results provide information about the origin of the thermal stability and the air-stable n-type OFET characteristics in these semiconducting CT complexes. 2. Experimental 2.1. Sample preparation diC8-BTBT was provided by Nippon Kayaku Co. TCNQ was purchased from Tokyo Chemical Industry Co. and Sigma Aldrich. The (diC8-BTBT)(TCNQ) film for UPS/IPES measurements was prepared

on highly doped n-typed Si(100) substrates with a thin native oxide surface (SiOx) by vacuum co-evaporation of diC8-BTBT and TCNQ from separate sources at a typical base pressure of P z 5  105 Pa. The molar ratio of donor and acceptor was 1:1. The deposition rates (r) for the donor and acceptor were monitored independently by quartz microbalance and controlled by the temperatures (T) of the evaporation sources, and were rdiC8BTBT ¼ 1.57 ± 0.02 Å/s and TdiC8BTBT z 463 K for diC8-BTBT, and rTCNQ ¼ 0.99 ± 0.01 Å/s and TTCNQ z 413 K for TCNQ. The deposition conditions were maintained until the thickness (t) of TCNQ reached tTCNQ ¼ 1.5  102 Å. The densities of the diC8BTBT and the TCNQ are 1.13 g/cm3 and 1.40 g/cm3, respectively. The ratio of the deposition rates did not correspond to the 1:1 molar ratio because diC8-BTBT and TNCQ molecules have different densities and molecular weights. In addition, the location of the evaporation sources and the quartz microbalances also affects the deposition rates. The deposition rates were adjusted to obtain a molar ratio of diC8-BTBT to TNCQ of 1:1, and the molar ratio of the CT complex film was confirmed by absorption spectroscopy and XRD. The diC8-BTBT film for UPS/IPES measurements was deposited on a highly doped n-typed Si(100) substrate with a thin native oxide surface (SiOx) in a vacuum. The deposition conditions were rdiC8BTBT z 1.5 Å/s and tdiC8BTBT ¼ 1.0  102 Å. The TCNQ film for IPES measurements was vacuum deposited on an Au film, which was cooled to about 90 K.

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The deposition conditions were rTCNQ z 0.28 Å/s and tTCNQ ¼ 5.0  10 Å. The Au film was deposited on an Au substrate in a vacuum until the thickness of Au reached tAu ¼ 3.0  102 Å [19]. 2.2. Characterization The electronic structure of the samples was determined by UPS/ IPES measurements. UPS measurements were performed with an electron analyzer (SES 200, VG Scienta) by using the He I resonance line (hn ¼ 21.22 eV). IPES measurements were performed with an IPES spectrometer (ISIS3000, PSP Vacuum Technology Ltd.) in isochromat mode. During the UPS measurements of the diC8-BTBT film and the IPES measurements of all films, the region of the sample surface that was measured was changed to reduce damage from the photon or electron beam impact. The UPS/IPES measurements were performed in an ultra-high vacuum (P < 1  106 Pa). The origins of the energy for the UPS/IPES spectra were calibrated by measuring the Fermi level (EF) of a clean Au(111) surface and a clean polycrystalline Au film. In the UPS spectra, the energy resolution estimated from the full width at half-maximum (FWHM) was ~0.1 eV. In the IPES spectra, the energy resolution estimated from the bandpass filter by using SrF2 and NaCl was ~0.4 eV [20]. The uncertainty of the leading edges and peak positions of the UPS/ IPES spectra were calculated by fitting the spectra. Molecular orbital calculations were performed based on density functional theory by using the GAUSSIAN 09 package at the B3LYP/6-31G level [21]. Simulated UPS/IPES spectra were convoluted with a Voigt function, which was convoluted with a Gauss function and a Lorentz function. In the simulated UPS spectra, the FWHM of the Gauss function was about 0.7 eV, whereas in the simulated IPES spectra the FWHM was about 1.4 eV. The difference in the FWHM represents the difference in energy resolution. The simulated UPS/IPES spectrum of the diC8-BTBT film was calculated from seven diC8-BTBT molecules (N ¼ 7) in Fig. S1 (see Supplementary data), based on crystallographic data for the diC8BTBT crystal [22]. The simulated UPS/IPES spectrum of a (diC8BTBT)(TCNQ) complex (N ¼ 1) was calculated for a pair of a diC8BTBT molecule and a TCNQ molecule, and the simulated UPS/IPES spectra of (diC8-BTBT)(TCNQ) oligomers (N > 1) were calculated from stacks of (diC8-BTBT)(TCNQ) complexes (Fig. 1(c)), based on the crystallographic data for a (diC8-BTBT)(TCNQ) crystal (Fig. 1(d))(CCDC 1031369) [15]. The simulated UPS/IPES spectrum of the TCNQ film was calculated from a TCNQ molecule. Ultravioletevisibleenear-infrared (UVeViseNIR) absorption spectroscopy was performed on a spectrophotometer (U-3500, Hitachi). A (diC8-BTBT)(TCNQ) film was deposited on a quartz glass substrate until the thickness of TCNQ reached tTCNQ ¼ 5.0  102 Å. Structural characterization was performed by atomic force microscopy (AFM) and X-ray diffraction (XRD). The (diC8BTBT)(TCNQ) film was measured by AFM (SPI3700, Seiko Instruments) in non-contact mode under ambient conditions. The film was deposited on highly doped n-typed Si(100) substrates with a thin native oxide surface (SiOx) under conditions similar to those described for UPS/IPES sample preparation. The deposition conditions were maintained until the thickness of TCNQ reached tTCNQ ¼ 1.5  102 Å. The out-of-plane XRD measurement (Ultima IV, Rigaku) was performed in air with Cu Ka radiation (40 kV, 200 mA). The film was deposited on the Si substrate until the thickness of TCNQ reached tTCNQ ¼ 3.0  102 Å.

evaporation of the component molecules. The complex formation by co-evaporation was confirmed by measuring the photoabsorption of the film deposited on a quartz glass substrate (Fig. 2). The spectrum shows a characteristic double peaked structure at 1.64 ± 0.01 eV (CT1) and 2.20 ± 0.01 eV (CT2), which agree well with those reported for the single crystal of the complex [15.16]. This result indicates that the (diC8-BTBT)(TCNQ) complex is mainly formed in the co-evaporated film. To avoid charging of the (diC8-BTBT)(TCNQ) films during UPS/ IPES measurements, the films were deposited on highly doped ntyped Si(100) with a thin native oxide surface as a conductive substrate. The film structure on the substrate was characterized by XRD and AFM. Fig. 3(a) shows an XRD pattern measured for the (diC8-BTBT)(TCNQ) film on the Si substrate. Strong first- and higher-order diffractions with a d-spacing of 1.77 ± 0.01 nm were observed, indicating the high crystallinity of the (diC8-BTBT)(TCNQ) film. The d-spacing was consistent with the layer thickness (or lattice constant along the c-axis) of the layered crystal of (diC8BTBT)(TCNQ), as reported for the single crystals (d ¼ 1.7946 nm) [15,16]. This indicates that the film is composed of microcrystals with c-axes oriented perpendicular to the substrate surface. Fig. 3(b) shows an AFM image for the surface of the (diC8BTBT)(TCNQ) film. A step-and-terrace structure was observed on the surface of the polycrystalline domains, which indicates that two-dimensional crystals are formed in the film. The step height, which corresponds to the monolayer thickness, was 2.0 ± 0.4 nm. This step height was similar to the d-spacing of the (diC8BTBT)(TCNQ) film based on the XRD data. Tsutsumi et al. reported that transfer integrals between the diC8-BTBT molecules or the TCNQ molecules are much smaller than those between the diC8BTBT and the TCNQ molecules [15]. Therefore, two-dimensional crystal growth parallel to the substrate surface was dominant in the (diC8-BTBT)(TCNQ) film. 3.2. Observation of electronic structure Fig. 4 shows the observed and simulated UPS/IPES spectra of a diC8-BTBT film, a TCNQ film, and a (diC8-BTBT)(TCNQ) film (black lines, A, B, E, F, I, and J). The previously reported UPS spectrum (E) of the TCNQ film is shown for comparison [23]. Dotted lines C, D, G, H, K, and L show the simulated spectra for each sample. The simulated spectra were obtained by molecular orbital calculations based on density functional theory. Simulated spectra C and D for the diC8-

3. Results and discussion 3.1. Thin film and characterization Thin films of (diC8-BTBT)(TCNQ) were prepared by co-

Fig. 2. UVeViseNIR absorption spectrum of a (diC8-BTBT)(TCNQ) film.

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Fig. 3. (a) XRD pattern and (b) AFM image of a (diC8-BTBT)(TCNQ) film.

BTBT film were calculated for an oligomer containing seven diC8BTBT molecules to model the effects of the p-stacking of the molecules in the film (Supplementary data, Figs. S1 and S2). Previous works have shown that the diC8-BTBT film is in a polycrystalline state where the diC8-BTBT molecules exhibit herringbone stacking [1,22]. Simulated spectra G and H for the TCNQ film were calculated for the TCNQ monomer. The calculation results for the TCNQ monomer may be sufficient to examine the electronic structure of the TCNQ film [24,25], because the IPES measurements were conducted on a TCNQ film deposited on an Au film substrate cooled to 90 K to avoid desorption of TCNQ from the substrate surface. Films deposited on cooled substrates are usually amorphous [26e30]. In addition, the Au film prevented charging of the film. The UPS/IPES spectra of the diC8-BTBT and TCNQ films are reproduced by the simulation spectra, although there are discrepancies in the intensity of UPS spectrum owing to the background of the secondary electrons. In the comparison with the simulation spectra, the vertical bar labeled “a” represents the leading edge of the UPS spectrum, which corresponds to the valence band top (VBT), that develops from the highest occupied molecular orbital (HOMO) of the diC8-BTBT molecule. The energy of the leading edge, labelled “a”, is 0.89 ± 0.02 eV. The vertical bars labeled “b” and “c” represent the peak positions of the UPS spectrum. Fig. S2 shows that the simulated UPS spectra for a diC8-BTBT monomer have two peaks derived from the HOMO and second HOMO, respectively. The

187

Fig. 4. UPS/IPES spectra measured for a diC8-BTBT film, a TCNQ film, and a (diC8BTBT)(TCNQ) film. Black lines labeled A, B, E, F, I, and J show the UPS/IPES spectra of each film. Dotted lines labeled C, D, G, H, K, and L show the corresponding simulated UPS/IPES spectra of the films. Spectra are plotted as a function of the binding energy with respect to EF. Leading edges or peak positions of the UPS/IPES spectra are indicated by black vertical bars (ael). Gray vertical lines are simulated molecular orbital energy levels. The UPS spectrum of the TCNQ film was cited from Ref. [23]. The energy resolution of the film was ~0.25 eV [23].

simulated UPS spectra for a diC8-BTBT oligomer also have two peaks, arising from contributions from the HOMO and the second HOMO of the monomer, respectively. Therefore, peaks “b” and “c” are derived from the HOMO and the second HOMO of the diC8-BTBT molecule, respectively. The conduction band bottom (CBB) of the diC8-BTBT film derived from the leading edge of the IPES spectrum is at least above 2 eV. Spectra E and F represent the electronic structure of the TCNQ film. The simulated spectra G and H reproduce the observed spectra E and F well. The TCNQ film was deposited on the Au film to prevent charging of the film, although other films were deposited on the n-type Si substrate. There is a case that the hole and electron injection barriers depend on the work function of a substrate. Murdey et al. and Braun et al. reported that if the work function of a substrate is 4.8 eV or less, the Fermi level of a TCNQ film can be pinned and the film work function can be 4.8 eV [31,32]. The work function of the n-type Si substrate is 4.25 eV [33]. The work function of an Au film deposited in vacuum can be less than 4.8 eV, although the work function of a clean Au(111) surface is 5.3 eV. Therefore, the Fermi level of the TCNQ film on the Si substrate and the Au film can be pinned. If the work function of the Au film is more than 4.8 eV, the hole injection

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barrier of the TCNQ film on the Au film is smaller than that on the Si substrate, and the electron injection barrier of the TCNQ film on the Au film is larger than that on the Si substrate. Thus, as discussed below, the electron injection barrier of the TCNQ film is smaller than that of the (diC8BTBT)(TCNQ) and diC8BTBT films. Therefore, the magnitude relationship of the HOMO and LUMO energy levels of each film are preserved, even if the Fermi level is not pinned. In contrast to the diC8-BTBT film, the leading edge of HOMO “d” is at a deeper energy of 2.2 ± 0.1 eV and the leading edge of lowest unoccupied molecular orbital (LUMO) “f” is above the Fermi level (EF). The intensity of the IPES spectrum of the TCNQ thin film was not zero due to the low energy resolution of the photon detector. To distinguish the crystallized and amorphous films, the leading edges of the UPS/IPES spectra for the crystallized film were represented by the VBT and CBB, respectively, and those for amorphous film were represented by the leading edges of the HOMO and LUMO, respectively. The vertical bars labeled “e” and “g” represent the UPS/IPES peak positions, which are derived from the HOMO and the LUMO of the TCNQ molecule, respectively. The electronic structure of the (diC8-BTBT)(TCNQ) film is completely different from the individual spectra of the diC8-BTBT and TCNQ films. The observed UPS/IPES spectra labeled I and J cannot be reproduced by superimposing the spectra of the diC8BTBT and TCNQ films, which reflects the CT interaction between the diC8-BTBT and TCNQ molecules to form the complex. VBT “h” is at 1.90 ± 0.02 eV and CBB “k” is at 0.29 ± 0.11 eV. The vertical bars labeled “i”, “j”, and “l” represent the peak positions of the UPS/IPES spectrum. Simulated spectra K and L for the (diC8-BTBT)(TCNQ) film were calculated for an oligomer containing seven diC8-BTBT and TCNQ molecules (N ¼ 7, see Fig. 1(c)). The results for the oligomer with N ¼ 7 were sufficient to examine the electronic structure of the solid (diC8-BTBT)(TCNQ) film because the electronic structure remained almost the same for N > 4 (Supplementary data, Fig. S3). Based on the simulation of the (diC8-BTBT)(TCNQ) monomer and oligomers, the peaks arise mainly from the HOMO, the second HOMO, and the LUMO of the (diC8-BTBT)(TCNQ) molecule. The binding energies of all the observed electronic states are summarized in Table 1. Fig. 5 shows the energy diagram determined from the leading edges of the UPS/IPES spectra. Fig. 6 shows the results of a

Fig. 5. Energy diagrams of the diC8-BTBT film, (diC8-BTBT)(TCNQ) film, and TCNQ film determined from the leading edges of UPS/IPES spectra. The labels “a”, “d”, “f”, “h”, and “k” correspond to those in Fig. 4.

molecular orbital calculation of the diC8-BTBT monomer, TNCQ monomer, and (diC8-BTBT)(TNCQ) complex. The monomers and the (diC8-BTBT)(TCNQ) complex (N ¼ 1) were used to determine the character of the molecular orbital of the (diC8-BTBT)(TCNQ) complex. The insets show the shapes of the molecular orbitals. The front views of the molecular orbitals for each molecule and complex (Fig. 6 (a)) show that the HOMO, second HOMO, and LUMO of the (diC8-BTBT)(TCNQ) complex have the same orbital shape as the HOMO and second HOMO of a diC8-BTBT molecule and the LUMO of a TCNQ molecule, respectively. The side views of molecular orbitals of the complex (Fig. 6 (b)) show that the diC8-BTBT and TCNQ molecules interact in phase in the complex HOMO, whereas those in the complex LUMO interact out of phase. These results indicate that these orbitals in the (diC8-BTBT)(TCNQ) complex correspond to bonding and antibonding orbitals between the HOMO of a diC8BTBT molecule and the LUMO of a TCNQ molecule. The complex HOMO is on the diC8-BTBT molecule, whereas the complex LUMO is on the TCNQ molecule (Fig. 6). These changes in the molecular orbitals indicate a weak CT occurs from diC8-BTBT to TCNQ in the ground state, which is consistent with the CT degree of 0.2 estimated from the vibration mode analysis of CN stretching [15]. The VBT of (diC8-BTBT)(TCNQ) film “h” is at a higher binding energy

Table 1 Binding energy of the vertical bars in Fig. 4. UPS-IPES Film

Labels of vertical bars

diC8-BTBT

a b c

TCNQ

d e f g

(diC8-BTBT)(TCNQ)

h i j k l

*1: *2: *3: *4: *5:

Spectral Spectral Spectral Spectral Spectral

structure structure structure structure structure

Binding energy with respect to EF/eV VBT *1 *2 Work function Leading edge of HOMO HOMO Leading edge of LUMO LUMO Work function VBT *3 *4 CBB *5 Work function Electron affinity (¼ Work function þ CBB)

mainly derived from the HOMO of a diC8-BTBT molecule. mainly derived from the second HOMO of a diC8-BTBT molecule. mainly derived from the HOMO of a (diC8-BTBT)(TCNQ) complex. mainly derived from the second HOMO of a (diC8-BTBT)(TCNQ) complex. derived from the LUMO of a (diC8-BTBT)(TCNQ) complex.

0.89 ± 0.02 1.40 ± 0.02 1.92 ± 0.01 3.9 ± 0.1 2.2 ± 0.1 2.9 ± 0.1 0 0.66 ± 0.03 4.8e5.0 [23,31] 1.90 ± 0.02 2.19 ± 0.01 2.73 ± 0.01 0.29 ± 0.11 1.65 ± 0.03 4.74 ± 0.03 4.45 ± 0.12

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Fig. 6. (a) Energy diagrams of a diC8-BTBT monomer, a (diC8-BTBT)(TCNQ) complex (N ¼ 1), and a TCNQ monomer obtained by molecular orbital calculations. Insets show front views of molecular orbitals for each molecule and complex. Red and green represent positive and negative phases of the molecular orbitals, respectively. (b) Side views of the molecular orbitals of the complex.

than that of diC8-BTBT film “a”. In addition, the CBB of (diC8BTBT)(TCNQ) film “k” is at a lower binding energy than the leading edge of the LUMO of TCNQ film “f”. The energy shift for the VBT is as large as 1.01 ± 0.03 eV and that for the CBB (LUMO) is as large as 0.29 ± 0.11 eV. These energy shifts indicate that the weak CT between a diC8-BTBT molecule and a TCNQ molecule provides electronic stabilization comparable to a moderate hydrogen bond (15e65 kJ/mol) [34e37], although it cannot be compared directly with the bond energy. This is consistent with the structural robustness of the (diC8-BTBT)(TCNQ) complex. The (diC8BTBT)(TCNQ) complex has no phase transition up to its degradation temperature (443 K), although diC8-BTBT shows thermal instability owing to a crystalliquid crystal transition at 380 K and a liquid crystalliquid transition at 400 K [15]. The UPS results indicate that the hole injection barrier from the electrode into the diC8-BTBT film determined from the UPS results is smaller than that from the electrode into the (diC8-BTBT)(TCNQ) film. In addition, the (diC8BTBT)(TCNQ) film has a small electron injection barrier due to the low CBB. Thus, it is reasonable that the (diC8-BTBT)(TCNQ) film exhibits n-type behavior in OFETs. In contrast, a single layer of the diC8-BTBT film shows p-type behavior because the VBT is close to EF and the CBB is far from EF [1,22,38,39]. Furthermore, the electron affinity of the (diC8-BTBT)(TCNQ) film from the UPS/IPES spectrum is 4.45 ± 0.11 eV and that from the molecular orbital calculation is 4.17 eV. These electron affinities are larger than that of H2O [40]. Anthopoulos et al. suggest that molecules with an electron affinity of <4 eV should be unstable toward reduction by H2O via a redox reaction [40]. Therefore, the (diC8-BTBT)(TCNQ) film exhibits airstable n-type field effect characteristics because of its large electronic affinity.

4. Conclusions We have explained the air-stable n-type field effect characteristics and structural robustness of the CT complex, (diC8-

BTBT)(TCNQ) based on its electronic structure determined by UPS and IPES. These results also provide a good example of how the physical properties of CT complexes are affected by the electronic structure of their constituent molecules. Because it is possible to control physical parameters, such as the transfer integral between molecular orbitals of donor and acceptor molecules of (diC8BTBT)(TCNQ), by changing the alkyl chain length of diCn-BTBT and the degree of fluorination of FmTCNQ [15], clarifying the effects of molecular modification on the electronic structure is important. Understanding the relationship between the molecular structure and the electronic structure will pave the way to using molecular complexes in electronic devices. Acknowledgements We are grateful to Nippon Kayaku for providing diC8-BTBT and we thank Satoru Inoue for helpful discussions. This work was supported by a Grant-in-Aid for Scientific Research (Grant No.16K05956) from the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT) (16K05956). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.orgel.2016.10.005. References [1] H. Minemawari, T. Yamada, H. Matsui, J. Tsutsumi, S. Haas, R. Chiba, R. Kumai, T. Hasegawa, Nature 475 (2011) 364e367.
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