Charge Conduction Properties at the Contact Interface between

Dec 14, 2013 - Single-component crystals of both (phthalocyaninato)nickel(II) (Ni(Pc)) and 2,5-difluoro-7,7,8,8-tetracyanoquinodimethane (F2TCNQ) are ...
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Charge Conduction Properties at the Contact Interface between (Phthalocyaninato)nickel(II) and Electron Acceptor Single Crystals Yukihiro Takahashi,*,†,‡,§ Kei Hayakawa,‡ Katsuya Takayama,‡ Seiya Yokokura,‡ Jun Harada,†,‡,§ Hiroyuki Hasegawa,†,§ and Tamotsu Inabe*,†,‡,§ †

Department of Chemistry, Faculty of Science, ‡Graduate School of Chemical Sciences and Engineering, and §JST, CREST, Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan S Supporting Information *

ABSTRACT: Single-component crystals of both (phthalocyaninato)nickel(II) (Ni(Pc)) and 2,5-difluoro-7,7,8,8tetracyanoquinodimethane (F2TCNQ) are typical band insulators. However, the contact interface between them demonstrates metallike transport properties. Although Ni(Pc) and F2TCNQ are an electron donor and an acceptor, respectively, the combination of these two components does not yield any charge transfer (CT) complex crystals. Infrared spectra show that the highly conductive feature originates from charge injection at the contact interface. The thermoelectric power of the mixed powder reveals that the transport at the contact interface is dominated by the holes in the Ni(Pc) crystal. KEYWORDS: carrier doping, organic crystals, contact interface

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INTRODUCTION Organic electronics have been attracting growing interest for various applications such as organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs), and organic solar cells. Despite their clear advantages over inorganic devices in flexibility and low-cost processing,1−5 organic devices often encounter rather poor performance. One of the reasons for this is the difficulty in carrier doping of organic materials that is required to tune the devices for optimal electronic properties.6,7 Although some doping methods, such as “band filling control”, have demonstrated substantial progress,8−10 the dopant concentrations are hard to control. Moreover, the application of the methods is fairly limited because the doping molecules should have molecular skeletons similar to the constituent molecules in the solid being doped. Therefore, new universal techniques for the carrier doping of organic solids are needed for further development in organic electronics. In 2008, an epoch-making doping technique for organic crystals was reported by Morpurgo, et al. 11 Metallic functionality was realized at the contact interface between a TTF (tetrathiafulvalene) single crystal and a TCNQ (7,7,8,8tetracyanoquinodimethane) single crystal. They suggested that electrons were transferred from the donor crystal to the acceptor crystal, which is analogous to the injection of an electron gas, and that carrier doping of organic solids is possible at the crystal contact interface. Such enhancement of the conductivity at the crystal contact interfaces has been reported in some other combinations of donor and acceptor crystals,12,13 which shows that this simple technique of the fabrication of organic crystals is quite versatile. Although the contact doping technique has great potential © 2013 American Chemical Society

values, the mechanism of the conductivity enhancement and the whole chemical processes on the contact interfaces have yet to be fully explored. Very recently, Morpurgo et al. have succeeded in constructing a Schottky-gated heterostructure based on extremely thin single crystals of rubrene and N,N′-bis(nalkyl)-(1,7 and 1,6)-dicyanoperylene-3,4:9,10-bis(dicarboximide) (PDIF-CN2).14 Using this device, they have demonstrated that the interfacial transport of these crystals was attributable to electrons whose mobility exhibited band-like behavior. Although the previous reports clearly showed that the conductivity of contact interfaces of single crystals were dramatically enhanced, not all the processes on the contact surfaces have been clarified. In addition to the charge doping (reduction and oxidation), organic solids usually undergo a variety of processes, such as chemical reactions, polymorphism, phase transitions and the formation of charge transfer crystals. To establish the contact doping technique as one of the prime methods of fabrication of organic crystals, further understanding of chemical phenomena that can occur at the contact interfaces is required. In the course of our investigations on the contact interface between TTF and TCNQ crystals, we have found that nanosized TTF-TCNQ complex crystals as well as TCNQ−1 radicals were produced on the surface of TCNQ single crystal, when the powder of TTF was placed on it.15 In this case, the Received: September 11, 2013 Revised: December 9, 2013 Published: December 14, 2013 993

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conduction path through the TTF-TCNQ complex crystals can also contribute to the high conductivity of the contact interface as well as the π−π network at the surface of the TCNQ crystal. This result clearly shows that we have to pay special attention to CT crystals that can develop as extrinsic phases on the contact surfaces, especially when the constituent molecules of the single crystals can form their CT crystals. Although the formation of CT crystals itself can be used for fabrication of conducting layers on organic crystals, different conductive paths generated on the surface of doped crystals perturb the designed conductive properties of the doped base crystals. The combination of donor and acceptor crystals that do not form their CT crystals is, therefore, preferable to avoid the unnecessary complication and to achieve the enhancement of conductivity genuinely by carrier doping. With those in mind, we have focused on (phthalocyaninato)nickel(II) (Ni(Pc)) as the electron donor crystal. This is mainly because Ni(Pc), despite its low ionization potential, does not form CT complex crystals with most of small molecules that can work as acceptors.16 In any polymorphs of Ni(Pc) crystals the π−π interactions are sufficiently large for band-like charge carrier transport. The low vapor pressure of Ni(Pc) is also beneficial to the formation of robust devices free from any degradation involving gas phase processes. In this study, we have selected 2,5-difluoro-TCNQ (F2TCNQ) as an acceptor crystal for the single-crystal conjugations with the Ni(Pc) crystal. The contact interface showed remarkably enhanced conductivity. The enhancement of conductivity was reversibly controlled by removing F2TCNQ powder, carrier source, contacted on a single crystal of Ni(Pc). We have demonstrated that the responsible carrier for the transport on the contact interfaces can be determined by thermoelectric power measurements, which showed that the holes in the Ni(Pc) crystal play the dominant role. We have confirmed that Ni(Pc) and F2TCNQ crystals do not form any new solid phases, including CT crystals, even after intensive grinding of their mixed powder. The results guarantee that the enhanced conduction was genuinely induced by carrier doping between the crystals of Ni(Pc) and F2TCNQ.

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Figure 1. (a) Photograph of the crystal conjugation of Ni(Pc) (purple crystal) and F2TCNQ (yellow crystal) with their molecular structures, and (b) the crystal orientations at the conjugated interface. The interface was formed between the (100) plane of the Ni(Pc) crystal and the (001) plane of the F2TCNQ crystal; the red, green, and blue arrows indicate the a-, b- and c-axes, respectively. measurements were formed at both ends of the Ni(Pc) crystal using aqueous carbon paste to avoid dissolution of the crystal into organic solvents. The measurement direction corresponded with the longitudinal direction of the crystals, which is the direction indicated by the purple and orange arrows in Figure 1b. The temperature dependences of the carrier transport properties for the single crystal conjugation samples and compaction pellets were measured using a direct four-probe and two-probe current method, respectively. Tapping mode atomic force microscope (TM-AFM) images were obtained with an SII scanning probe microscope system (Nanocute). X-ray diffraction measurements on the powder were performed with a Burker D8 Advance diffractometer with graphite monochromated Cu Kα radiation. The Ni(Pc) and F2TCNQ powder mixture was ground intensively. Infrared (IR) spectra of nondiluted powder were measured with a Jasco FT/IR-4100 spectrometer with an attenuated total reflectance (ATR) unit. Thermoelectric power (TEP) was measured following a Chaikin and Kwak’s method with using two voltmeters (Keithley 2182 nanovoltmeter).20

EXPERIMENTAL SECTION

Materials. Ni(Pc) was purchased from Tokyo Kasei Co., Ltd., and F2TCNQ was synthesized from 2,5-difuloro-1,4-diiodobenzene following a previously reported method.17 All of the materials were purified by vacuum sublimation before crystal growth. Single crystals of Ni(Pc) and F2TCNQ were obtained by vapor transport with N2 gas flowing through 20 mm diameter glass tubes. X-ray structural analyses confirmed that both of the crystal structures were the same as those given in previous reports.18,19 Conjugation of the Ni(Pc) and F2TCNQ crystals used for charge transport measurements was carried out manually under ambient conditions. Platelet crystals of F2TCNQ that were longer and wider than the Ni(Pc) crystals were selected for the conjugation. A Ni(Pc) crystal was conjugated on an F2TCNQ crystal fixed on a PDMS (polydimethylsiloxane) elastomer, so that both of the crystals’ elongated axes were parallel to each other, as shown in Figure 1a. The compaction of pellets for the electrical measurements were prepared as follows from a mixture of Ni(Pc) and F2TCNQ. The weighed out Ni(Pc) and F2TCNQ crystalline powders were ground in a mortar. The powder mixture obtained was pressed in a die with a diameter of ϕ = 3 mm. The thicknesses of the pellets were measured with a micrometer. Measurements. The current−voltage characteristics of the Ni(Pc) single crystal and the Ni(Pc)/F2TCNQ interface were measured using a direct current two-probe method. Electrodes for the sheet resistance

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RESULTS AND DISCUSSION Single-Crystal Conjugation. Figure 1 depicts the Ni(Pc) and F2TCNQ single-crystal conjugation and the molecular arrangements at the interface. In Figure 1a, the dark purple crystal is Ni(Pc), and the crystal needle axis corresponds to the b-axis. The Ni(Pc) crystal was assigned as the β-form of the phthalocyanine derivatives.18 This type of crystal forms onedimensional molecular columns with π−π intermolecular interactions, the direction of which corresponds with the crystal needle axis (Figure 1b). In the yellow F2TCNQ crystal, the crystal longitudinal direction corresponds to the [110] direction. The molecular arrangement of F2TCNQ has the π−π interactions expanded in two-dimensions, corresponding to the (001) plane,19 as shown in Figure 1b. The crystals of Ni(Pc) 994

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the sheet resistances at room temperature were also different in each sample. The sample dependence of the carrier transport properties indicates that the metal-like behavior near room temperature was not due to the stable metallic band structure and that the changes in the behavior at low temperature were not caused by phase transitions between the metallic and semiconductive states. The temperature dependence of resistances can be explained by the competing effects between the number of carriers (n) released from the shallow traps [n(T) ∝ exp(−Ea/ kT), where Ea is the activation energy] and the mobility (μ) of the band-like transport [μ(T) ∝ T−α, where α is a parameter usually between 1 and 2] in the semiconducting crystals.21,22 When Ea is sufficiently small, such metal-like behavior in a hightemperature region appears, even when the materials do not have metallic band structures. We believe that the observed sample dependences were caused by the differences in the surface conditions, degeneration or oxidation of the Ni(Pc) and F2TCNQ crystal surfaces and the difference in the efficient area of the conjugated interface. Powder and Crystal Conjugation. It is of interest to confirm whether the high conductive state remains or disappears after separating the conjugated crystals. However, in case of single crystal conjugation, it was difficult to peel off the Ni(Pc) crystals from the F2TCNQ crystal surfaces without damaging the sample. To disclose and observe the phenomena that occurred at the crystal interface, another contact surface was prepared by contacting powdery crystals. Figure 3a depicts the change of the sheet resistance of Ni(Pc) crystal surface before and under contacting F2TCNQ powder, and after removing the F2TCNQ powder. After the measurement of as-grown Ni(Pc) crystal (black), finely ground acceptor powder was put on the Ni(Pc) single-crystal surface. The current at the interface between the Ni(Pc) crystal and F2TCNQ powder enhanced 2 orders of magnitude (pink). After removing the powder using compressed air, this conductive state returned to the high resistance state identical with the as-grown crystal (red). Figure 3b shows an AFM image of the Ni(Pc) crystal surface after removing the F2TCNQ powder. This surface had been contacted with F2TCNQ powder for one week. Only step-andterrace structures of the Ni(Pc) crystal were observed. This image does not show significant differences from that of asgrown Ni(Pc) crystal and indicates no nanosized crystals. The results provide a distinct contrast to TTF powder treatment on TCNQ crystal surfaces, where many nanosized crystals of charge transfer complex TTF-TCNQ were observed.15 The results of the powder and crystal conjugations prove that the charge injection does occur at the contact interface between Ni(Pc) and F2TCNQ crystals. The carrier doping proceeds without forming CT complex crystals and the original solid state can be resumed after separating the conjugated crystals. Mixed Powder Contacts. The measurements using powder mixture of Ni(Pc) and F2TCNQ allowed us to gain further insight into what occurs at the interface. Figure 4a shows the resistivity of the mixed powder pellet, with various compositions. Even at a low concentration (20 wt %) of F2TCNQ the pellets showed highly enhanced conductivity. Although the lowest resistivity, 1.7 × 104 Ω cm, was obtained at 40 wt % of F2TCNQ, the resistivity was almost independent of the acceptor concentration in the 20−80 wt % range. If the carriers were transported through the bulk of conducting phase, such as CT complex crystals formed between

and F2TCNQ were laminated so that the crystal longitudinal directions were parallel. The conjugated interface was formed between the (100) plane of the Ni(Pc) crystal and the (001) plane of the F2TCNQ crystal. Thus, the interface along the longitudinal direction of the Ni(Pc) crystal consisted of Ni(Pc)’s one-dimensional π−π stacking directions and F2TCNQ’s π−π network plane. Figure 2a shows the current−voltage characteristics of the Ni(Pc) single crystal and those of the interface between the

Figure 2. (a) Current−voltage characteristics of the as-grown Ni(Pc) crystal (black) and the Ni(Pc) and F2TCNQ conjugated interface (pink) at room temperature. (b) the temperature dependence of the sheet resistance at the interfaces between Ni(Pc) and F2TCNQ. The four plots indicate the sample dependence of the contacts.

Ni(Pc) and F2TCNQ crystals. The sheet resistance was 1.2 × 1014 Ω sq−1 for Ni(Pc) crystal, which was reduced to 1.4 × 107 Ω sq−1 at the interface between Ni(Pc) and F2TCNQ. Thus, the electrical conductivity was enhanced by ∼107 times by the conjugation of Ni(Pc) and F2TCNQ single crystals. The temperature dependence of the carrier transport properties showed metal-like behavior in all four samples measured in this work (Figure 2b). Sample 1, for example, showed the metal-like behavior in the temperature range of 200−300 K (Figure 2b), while it exhibited thermally activated behavior below 200 K. Although all of the samples demonstrated apparent metal-like transport behavior, the resistance upturn temperatures depended on the samples and 995

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Figure 3. (a) Current−voltage characteristics of an as-grown Ni(Pc) crystal (black), the interface between the Ni(Pc) crystal and F2TCNQ powder (pink) and the Ni(Pc) surface after removing the F2TCNQ powder (red). (b) (Left) Schematic representation of the preparation of the bare interface for the Ni(Pc) crystal surface after treatment with F2TCNQ powder, and (right) its AFM image.

the donor and acceptor, the conductivity should be strongly dependent on the amount of the conducting phase and, therefore, on the concentrations of donor and acceptor in the pellets. The observed independence on the composition can be interpreted on the assumption that the carrier transport in the pellets takes place only through the network of the doped interfaces formed between the contact surfaces of crystalline particles. In such case only contact interfaces provide carrier transport paths that are networked throughout the conductive pellets. Thus, the high conductivity can be achieved even when the concentrations of donor or acceptor crystals are rather small. The temperature dependence of the transport properties showed thermally activated behavior even near room temperature, reflecting random orientation of the conductive interfaces and contact resistance at the grain boundaries (Figure 4a, inset). The powder X-ray diffraction of the mixed powder sample is illustrated in Figure 4b. The sample had 40 wt % of F2TCNQ. The diffraction of the mixed powder sample consists of the diffraction patterns of Ni(Pc) and F2TCNQ. No new peaks corresponding to charge transfer complex crystals were observed in the powder diffraction. IR spectroscopy revealed that F2TCNQ−1 was generated in the mixed powder of Ni(Pc) and F2TCNQ. The degree of iconicity of TCNQ and derivatives is known to be estimated from the wavenumbers of CN stretching vibrations in their IR and Raman spectra. Figure 5 shows IR spectrum of the mixed powder of Ni(Pc) and F2TCNQ, along with the spectra of crystalline powders of F2TCNQ and TTF-F2TCNQ complex. Only the wavelength region around 2200 cm−1 was

Figure 4. (a) Measured resistances with different concentrations of F2TCNQ in a Ni(Pc) matrix at room temperature and the temperature dependence of the resistivity (40 wt % sample) (inset). (b) Powder Xray diffraction patterns of the Ni(Pc) and F2TCNQ mixture (blue), Ni(Pc) (pink), and F2TCNQ (black).

Figure 5. Infrared spectra focused on the CN stretching vibrations for the mixed powder of Ni(Pc) and F2TCNQ (pink), F2TCNQ (black), and the F2TCNQ−1 radical in TTF-F2TCNQ (dark blue).

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shown in the figure, where the peaks of neutral F2TCNQ0 and anionic F2TCNQ−1 are to be observed. The spectrum of the mixed powder of Ni(Pc) and F2TCNQ showed two strong peaks (2231 and 2219 cm−1), a weak peak (2188 cm−1), and a broad shoulder (2164 cm−1). The former two strong peaks are due to the neutral F2TCNQ, the wavenumbers of which are identical with those in the spectrum of F2TCNQ powder (2231 and 2219 cm−1). The latter two weak peaks are assignable to F2TCNQ−1, because the observed wavenumbers are identical with those of TTF-F2TCNQ crystals (2188 and 2164 cm−1), where the degree of charge transfer is almost unity (TTF+1)(F2TCNQ−1).23 Although we were not able to definitely identify the peaks of Ni(Pc)+ because of the overlaps of other peaks, the results of IR spectroscopy are consistent with electron transfer from Ni(Pc) crystals to F2TCNQ crystals through contact interfaces. Taking into account of the results of powder X-ray diffraction that showed no formation of new solid phases, we can conclude that the electron doping occurred through the contact interfaces maintaining the lattices of constituent crystals and that the formation of extrinsic CT crystals was not involved in the process. The results of Ni(Pc) and F2TCNQ crystals are in stark contrast to those of TTF and TCNQ crystals, the Raman spectrum of which clearly showed a peak of the CT complex crystal (TTF-TCNQ) along with those of TCNQ0 and TCNQ−1.15 The high conductivity genuinely induced by the charge injection at the contact interface of Ni(Pc) and F2TCNQ crystals was attributable to the characteristics of Ni(Pc) molecule that does not usually form its CT complex crystals. The electron transfer from Ni(Pc) crystal to F2TCNQ crystal at the interface produces holes in Ni(Pc) as Ni(Pc)+1 and electrons in F2TCNQ as F2TCNQ−1. Both chemical species can contribute to the high conductivity at the conjugated interface. In this study, we show that the dominant component at the interface can be determined by “a traditional method”, thermoelectric power (TEP) measurements. The TEP corresponds to the Seebeck coefficient of the sample with temperature gradient, and it reflects the sign of the carriers: The positive values of TEP indicate that the charge carriers are the holes, whereas the negative values show that the charge carriers are the electrons. The measurements were performed using compacted pellets with a mixture of Ni(Pc) and F2TCNQ crystals, as measurements with single-crystal specimens were extremely difficult to perform. Figure 6 shows the temperature

dependence of the TEP of the compacted pellet. The plot (Figure 6) shows that the sign of the TEP is positive, indicating that the majority of the charge carriers are holes. The temperature dependence of TEP, which is almost constant or slightly increased with lowering temperature, indicates that the charge transport is not metallic, suggesting that the electronic structure may be considered a doped semiconductor. The overlap integral, sij, in each single-component crystal was evaluated using an extended-Hückel method.24 The sij between Ni(Pc)’s highest occupied molecular orbitals (HOMOs) is 3.5 × 10−3, whereas that between F2TCNQ’s lowest unoccupied molecular orbitals (LUMOs) is 1.1 × 10−3. The calculations indicate that the conduction path in Ni(Pc) is considered to be overwhelmingly dominant in comparison with that in F2TCNQ, which is consistent with the results of the TEP.

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SUMMARY The lamination of single crystals of the organic insulators Ni(Pc) and F2TCNQ resulted in the formation of a conducting contact interface without formation of CT complex crystals at the contact interface. The temperature dependence of the sheet resistance showed metal-like behavior between 200 and 300 K. We have proven that the metal-like conductivity can be reversibly controlled by demonstrating that the conductive state of the contact interface returned to the original insulating state after separating the conjugated crystals. IR spectroscopy detected F2TCNQ−1 in the mixed powder of Ni(Pc) and F2TCNQ, which provided evidence for electron transfer from Ni(Pc) to F2TCNQ. No trace of CT complex formation was detected by X-ray, IR, and AFM measurements. We have found that a conventional method, thermoelectric power measurement, is quite useful to determine the dominant carriers at contact interfaces, which indicates that the dominant carriers of the mixed powder compaction of Ni(Pc) and F2TCNQ are the holes generated in the Ni(Pc) crystals. This study confirms that pure charge injection (carrier doping) in organic solids is attainable by contacting single crystals of donors and acceptors without forming CT crystals. The contact interface including Ni(Pc) crystals is highly promising for the genuine carrier doping, because Ni(Pc) molecules do not usually yield their CT crystals. Although strong electron donating or accepting molecules often tend to form their CT crystals, some of them are known to be unlikely to produce CT crystals. It is necessary to select such molecular crystals in order to realize genuine carrier doping by contacting the crystals without being disturbed by CT crystal formation. In light of this point, we can expect the crystal of rubrene, which was used in the previous study,14 to be another good candidate, because rubrene molecules also do not form their CT crystals. The present study has demonstrated that the mechanical contact between donor and acceptor crystals will provide a convenient and fairly versatile method for carrier doping to the organic solids if we properly select the component solids not to form their CT crystals.

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ASSOCIATED CONTENT

S Supporting Information *

The cell parameters and atomic coordinates (CIF) of Ni(Pc) and F2TCNQ single crystal that were used in this paper. This material is available free of charge via the Internet at http:// pubs.acs.org.

Figure 6. Temperature dependence of the thermoelectric power (Q) of a compacted pellet consisting of a mixture of Ni(Pc) and F2TCNQ powders (40 wt % F2TCNQ). 997

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AUTHOR INFORMATION

Corresponding Authors

*E-mail:[email protected]. *E-mail: [email protected] . Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan. Dr. Kobayashi and Mr. Shigeta at Hokkaido University are acknowledged for support of X-ray powder diffraction measurements.

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