Band-Like Carrier Transport at the Single-Crystal Contact Interfaces

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Letter

Band-Like Carrier Transport at the Single-Crystal Contact Interfaces Between 2,5-Difluoro-7,7,8,8-Tetracyanoquinodimethane and Electron Donors Takuro Shimada, Yukihiro Takahashi, Jun Harada, Hiroyuki Hasegawa, and Tamotsu Inabe J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b03053 • Publication Date (Web): 03 Jan 2018 Downloaded from http://pubs.acs.org on January 3, 2018

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The Journal of Physical Chemistry Letters

Band-Like Carrier Transport at the Single-Crystal Contact Interfaces between 2,5-Difluoro-7,7,8,8tetracyanoquinodimethane and Electron Donors Takuro Shimada,1 Yukihiro Takahashi,*1,2 Jun Harada,1,2 Hiroyuki Hasegawa,3 and Tamotsu Inabe1,2 1

Graduate School of Chemical Sciences and Engineering, Hokkaido University, Sapporo 060-

0810, Japan 2

3

Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan Kobe Advanced ICT Research Institute, National Institute of Information and Communications

Technology, Kobe 651- 2492, Japan. AUTHOR INFORMATION *Y.T.: E-mail: [email protected].

ACS Paragon Plus Environment

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ABSTRACT

Some heterojunction interfaces formed with molecular solids show metal-like transport behavior. In order to clarify the requirement, interfaces are fabricated by lamination of single-crystal electron-accepting 2,5-difluoro-7,7,8,8-tetracyanoquinodimethane (F2TCNQ) and electrondonating molecules with a wide range of ionization potentials. Carrier injection between the acceptor and donor crystals leads to highly conducting interfaces, some of which exhibited bandlike charge transport behaviors. Combinations with weak donors also resulted in interfaces with band-like transport properties. Accordingly, band-like conduction was achieved for interfaces where the donor and acceptor crystals do not have well-matched band energies. The results indicate the wide range of candidates have great potential for the modification of the electronic structure of organic crystals. The present method is expected to enable control of the electronic properties of the interface.

TOC GRAPHICS

KEYWORDS. Molecular crystal, Semiconductor, Heterojunction, Carrier doping, Charge transfer.


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Given the recent prominent progress in organic electronics,1,2 heterojunction interfaces have become important basic elements in organic devices such as solar cells and field-effect transistors as they facilitate charge separation and generate injection layers in organic solids.3–5 Some heterojunction structures in organic devices enable the formation of organic crystal surfaces with high electrical conductivity. For example, Morpurgo et al.6 reported the formation of highly conductive interfaces exhibiting metal-like temperature dependence, i.e., their surface resistance decreases with decreasing temperatures, by simply laminating single crystals of electron donor and acceptor molecules. The considerable increase in electrical conductivity was attributed to the mutual carrier injection at the heterojunction interfaces between the donor and acceptor crystals. Well-matched energy levels of the bands involved in the carrier injection, i.e., the ionization potential (IP) of the donor crystals and the electron affinity (EA) of the acceptor crystals, were deemed essential for efficient carrier injections and metal-like carrier transport at the charge-transfer interfaces.7 The limitation to matching band energies would severely hinder the diversity and potential for future applications of these interfaces, which to this date are generally composed of relatively strong donor and acceptor crystals to achieve the desired outcomes. However, we have discovered that interfaces comprising weakly donating crystal of Ni(Pc) ((phthalocyaninato)nickel) can also exhibit metal-like transport properties.8 This finding indicates that carrier injection can also be achieved for combinations of donor and acceptor crystals that exhibit a large band energy difference. However, it is important to tune the electron structure at the heterojunction interface, as reported in a study involving interfaces formed from rubrene and 7,7,8,8-tetracyanoquinodimethane (TCNQ) derivatives.9 Presently, the conditions required to achieve metallic transport at heterojunction interfaces are yet to be determined.


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Herein, a range of contact interfaces formed between F2TCNQ crystals and organic donor crystals with a wide range of IP values were synthesized via lamination of the organic crystals. The conductivity of all contact interfaces examined was considerably high, and metal-like carrier transport behaviors were observed for some of the interfaces examined. The results clearly demonstrate that metal-like features, such as band-like transport at organic semiconductor layers, may be achieved much more easily than previously expected using the present simple technique. Seven donor molecules used in this study, i.e., picene (1), , (phthalocyaninato)cobalt (Co(Pc); 2), Ni(Pc) (3), pentacene (4), rubrene (5) bis(ethylenedithio)tetrathiafulvalene (ET; 6), and tetrathiatetracene (TTT; 7) (Figure 1a), cover a wide range of IP values (Table 1),10–15 some of which are substantially different from the EA value of the F2TCNQ acceptor (4.6 eV) (Figure 1b).16 The donor and F2TCNQ crystals were combined under ambient conditions in such a way that the elongated axes of the crystal were parallel to each other, in which the sheet resistance of the contact interfaces was measured (Figure 1c). The surface planes of these crystals for the lamination were confirmed by X-ray diffraction. The Miller indices shown in Table 1 are based on the cell parameters reported in a previous study.17 The typical roughness was measured by atomic force microscopy (AFM; Supporting Information (SI)). Transfer integral (tij) values of the transport measurement direction were calculated from the atomic coordinates. It is worth noting that F2TCNQ was reported as a two-dimensional, high-mobility material18. However, in the present paper, the low-mobility direction was used for the transport measurements as the elongated axis of the F2TCNQ single crystal correspond to hydrogen- and halogen bond direction (see SI).


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Figure 1. (a) Donor molecules and F2TCNQ acceptor molecules examined in the present study. (b) Schematic energy level band configurations of the interface obtained by Ultraviolet Photoelectron Spectroscopy (UPS). (c) Schematic representation of the crystal conjugation. Table 1. Miller indices of the surface planes examined, IP, EA, transfer integral (tij) and mean square roughness (RMS) values of the donors and F2TCNQ crystal surfaces

materials

contact plane

IP(eV)

tij (meV)*

RMS (nm)

1 2

(001) (-101)

5.7 5.2

31 3

0.69 0.37

3 4

(-101) (001)

5.0 4.9

3 24

0.48 21.64

5 6

(100) (011)

4.8 4.7

36 10

0.49 0.80

7

(010)

4.4

6

3.12

F2TCNQ

(110)

EA:4.6

1

0.66

* tij between HOMOs (donors) or LUMOs (F2TCNQ) of the nearest neighbor molecules along the transport measurements


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All interfaces formed by lamination of F2TCNQ and donor crystals exhibited considerable enhanced electrical conductivity relative to the as-grown crystals. The sheet resistance values (ρs at T = 300 K) of the interfaces and the as-grown ; i. e. before lamination, constituent crystals are shown in Figure 2. The sheet resistance of the interfaces varied within a narrow range of 106–107 Ω sq−1. Such a behavior was also observed for interfaces formed between rubrene and TCNQ derivatives.9 Both the electrons in the acceptor F2TCNQ crystal and the holes in the donor crystal, which are doped through charge transfer between the crystals and are present near the contact surfaces, can contribute to the enhanced electrical conduction at the conjugated interfaces. The tij values (Table 1) and the results of thermoelectric power measurements (see SI) suggest that the dominant carriers at all the interfaces examined are holes. In contrast to the lamination of tetrathiafulvalene (TTF) and TCNQ single crystals, which readily afforded nanocrystals of highly conductive charge-transfer complex TTF-TCNQ at the contact interfaces [19], the combinations of donors and acceptor examined in the present study do not form stable complex crystals under the present conditions studied.


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Figure 2. (a) The sheet resistance at room temperature of the interfaces formed between F2TCNQ and 1–7 donor crystals (■) and the as-grown crystals (▼). ρs of the as-grown F2TCNQ is also shown for comparison as indicated by the bold arrow.

The increase in surface conductivity originated from mutual charge doping, as revealed by electron spin resonance (ESR) spectroscopy. The ESR spectra of a F2TCNQ single crystal fixed on a Teflon block (Figure 3a) before lamination and the interface formed after lamination between the F2TCNQ crystal and a donor crystal (Figure 3a) were measured. For all interfaces, F2TCNQ crystal, which was larger than the donor crystal, was used. The area of contact of the interfaces was determined, with W and L corresponding respectively to the width and length of the donor crystal. The ESR spectra are depicted in Figure 3b. The spectrum of the F2TCNQ crystal before laminating displayed weak signals. As F2TCNQ is a strong acceptor, its surface is easily reduced even under ambient conditions. In contrast, the donor crystals exhibited negligible ESR responses (see SI). After lamination, the ESR signals of the donor components increased considerably. The shapes slightly varied depending on the donors. Because the signal intensity depends on the amount of paramagnetic spins, this result indicates the formation of electric charges at the contact interface. In general, in the ESR spectra of charge transfer materials, the signals of the donor are often coupled with those of the acceptor.20 Therefore, the signals observed were considered as overlaps of the spins on both the F2TCNQ and donor crystals, which might be involved in the shapes of the spectra Double integration of the spectra in Figure 3b is proportional to the number of carriers (the contribution from the F2TCNQ background was subtracted). The number of doped charge at the interface per unit area (1 mm2) and the ionization ratio (percentage of the ionized molecules in the contact monolayer of F2TCNQ crystal) are summarized in Figure 3c. For example, the ionization rate of 2 % at the first monolayer of the


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crystal corresponds to the amount of charge injection by applying ~30 V to the 300-nm thick SiO2 insulating film in typical transistors, wherein the relative permittivity, ε, of SiO2 is 3.9 [21]. Considering that the spin density determined by ESR measurements is accurate to within 10% or more, the number of the charges estimated by ESR is not largely different from that determined by the Hall effect measurement in the study therein.9 The ionization ratio shows a much weaker dependence on the IP values than that expected from the energy difference between the IP values and the EA value of F2TCNQ. This result indicates that charge transfer at the interface is influenced not only by the energy difference, but also by various factors such as overlapping or coupling of molecular orbitals and the smoothness of crystal surfaces at the contact interfaces. Carrier injection can therefore be based on band modulations at the contact interfaces, where the energy level of the valence band of the donor crystal is raised and that of the conduction band of the acceptor crystal is lowered to yield matched energy levels at the interface. Similar band modulations at organic heterojunction interfaces have previously been reported,22 and large shifts in the IP or EA values (>0.5 eV), arising from the pinning of Fermi levels or molecular orbitals, have been observed by UPS.23 The contact interfaces examined in the present study should include modulation of the band energy levels that results in carrier injection with much higher efficiency than those expected from the IP values of the donor crystals.


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Figure 3. (a) Sample setting for the ESR measurements of the F2TCNQ single crystal before and after joining with the donor crystal. (b) ESR spectra of F2TCNQ before (dashed line) and after (solid line) joining with donor crystals 1–7. (c) Number of carriers and their ionization ratio at the interfaces studied.

The temperature dependence of the sheet resistance revealed that the organic contact interfaces exhibited metal-like conduction behavior irrespective of the IP value of the donor crystal (Figure 4). Based on the analogy with metal-like conduction behavior of bulk CT crystals,24 metal-like behavior at the interfaces is usually assumed to be limited to strong donor crystals with IP values close to the EA of F2TCNQ (4.6 eV). However, metal-like behavior at T < 300 K was also observed at the interfaces formed between crystals of F2TCNQ and 1, 2, 3,5, and 6, whose IP values range from 4.7 to 5.6 eV. Especially noteworthy is the metal-like behavior observed for


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the interface formed between F2TCNQ and 1; the energy difference between these two crystals is as large as 1.1 eV. These results demonstrate that the IP level of the donor crystals is not the determining factor for the formation of metal-like interfaces in the present system. The metallike behavior was changed to the thermally activated one at low temperature for the interfaces formed between F2TCNQ and 1, 2, 3, 5, and 6. In contrast, the interfaces formed between F2TCNQ and donor 4 or 7 showed non-metallic behavior at 220 K < T < 300 K. As shown in the AFM images in the SI, the absence of metal-like transport in these interfaces can be attributed to the presence of rough donor crystal surfaces, which may hamper close and uniform contact with the F2TCNQ crystals. This also indicates that the lamination is the key to generate the band-like transport.

Figure 4. Temperature dependence of the sheet resistivity of the interfaces formed between F2TCNQ and donor crystals 1–7.


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According to these observations, the metal-like behavior observed at the heterojunctions may be attributed to the delocalized carriers in the crystals, though other mechanism could explain the transport behavior.25 Recently, band-like transport behaviors exhibited by some organic in highgate voltage regions have been reported.26,27 Their band-like transport behaviors are reported to originate from filling of their trapping sites in the semiconductor layer by gate voltage. The same phenomenon may happen at the heterojunction interface. The high density charge injection at the heterojunction interfaces was confirmed by ESR measurements. Thus, a possible requirement for metal-like behaviors, i.e., band-like transport at heterojunction interface, is that the amount of doped carriers must exceed the number of trapping sites in the semiconductor crystal. In conclusion, heterojunction interfaces were fabricated by laminating single crystals of electron-accepting F2TCNQ and various electron-donating materials. Many of the interfaces displayed metal-like transport behaviors irrespective of the IP value of the donor crystals. The observed efficient carrier injection between the donor and acceptor crystals can thus be interpreted in terms of band modulations at the contact interface. Herein, the results demonstrate that band-like transport can be easily achieved by simply laminating donor crystals and F2TCNQ crystals. The efficient carrier doping between the crystals examined herein can not only generate the high electrical conductivities observed in this work, but also produce magnetic properties that result from the radicals, as well as dielectric and ferroelectric properties that arise from electrical polarization of the radical cations and anions. The wide range of possible candidates for constituent crystals to be used in this contact interface fabrication method should have great potential for the modification of the electronic structure of organic crystals. Moreover, the


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present method is expected to enable control of the electronic properties of the interface for future applications.

EXPERIMENTAL METHODS Electron donors 1–6 were purchased from Tokyo Kasei Co., Ltd. (Tokyo, Japan). Electron donor 7 was synthesized according to the literature.28 F2TCNQ was synthesized from 2,5difluoro-1,4-diiodobenzene according to the previous report.29 All materials, except 6, were purified by vacuum sublimation before crystal growth. The single crystals were obtained by vapor transport with N2 gas flow in a 20-mm diameter glass tube in a temperature-gradient tube furnace. The single crystal of 6 was prepared by recrystallization in monochlorobenzene. Crystal structures and surface planes were determined by X-ray diffraction on an automated RIGAKU R-Axis Rapid X-ray diffractometer with graphite monochromated Mo Kα radiation at room temperature. Overlap integrals were calculated from their crystal structure using CAESER.30 For typical organic solids, the transfer integral (tij) in electronvolt is 10-fold higher than the overlap integral.31 The charge transport properties of the interfaces were measured by a direct current two-probe method with using a source meter (Keithley 2636A). The temperature dependence of them were measured in temperature cooling and heating processes. However, since the crystals or the laminations are often broken or separated at low temperature due to the heat shrink, the data in the cooling process were used in this work. The electrical contacts between the interface and electrodes (diameter of the Au wires = 20 µm) were made by aqueous carbon past. AFM images in the tapping mode were captured on an SII scanning probe


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microscope system (Nanocute). ESR spectra were recorded on an EMX electron paramagnetic resonance spectrometer (Bruker Biospin Co., Ltd.). ASSOCIATED CONTENT Supporting Information Roughness of crystal surfaces, intermolecular interactions in F2TCNQ, thermoelectric power, and ESR spectra of donors used in this work and the method used for estimation of the number of spins. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT The authors thank Professor Naoki Sato at Kyoto University for advising us on the selection of the appropriate IP and EA values in this work. This work was financially supported in part by JSPS KAKENHI Grant Number JP 16K17887 and Grant Program of SEI Group CSR Foundation.

Additionally,

we

thank

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Editing

Team

at

Edanz

Group

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Band-Like Carrier Transport at the Single-Crystal Contact Interfaces

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