Electrical Transport through Single Nanowires of Dialkyl Perylene

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Electrical Transport through Single Nanowires of Dialkyl Perylene Diimide Beom Joon Kim, Hojeong Yu, Joon Hak Oh, Moon Sung Kang, and Jeong Ho Cho J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp400807t • Publication Date (Web): 30 Apr 2013 Downloaded from http://pubs.acs.org on May 8, 2013

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The Journal of Physical Chemistry

Electrical Transport through Single Nanowires of Dialkyl Perylene Diimide Beom Joon Kim†,§, Hojeong Yu‡, §, Joon Hak Oh‡,*, Moon Sung Kang,# ,*, and Jeong Ho Cho†,*

†

SKKU Advanced Institute of Nanotechnology (SAINT) and Center for Human Interface Nano

Technology (HINT), School of Chemical Engineering, Sungkyunkwan University, Suwon, 440746, Korea ‡

School of Nano-Bioscience and Chemical Engineering, KIER-UNIST Advanced Center for

Energy, Low Dimensional Carbon Materials Center, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798, South Korea #

Department of Chemical Engineering, Soongsil University, Seoul 156-743, Korea

ACS Paragon Plus Environment

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ABSTRACT We investigated electrical charge transport through individual strands of singlecrystalline dipentyl perylene tetracarboxylic diimide (PTCDI-C5) and dioctyl perylene tetracarboxylic diimide (PTCDI-C8) nanowires prepared by a solution-phase self-assembly method. Temperature-dependent mobility measurements (100–280 K) revealed distinct electrical transport characteristics in the two types of nanowires. The PTCDI-C8 nanowire having shorter intermolecular distances exhibited a transition in the electrical transport mechanism from a thermally activated process (the multiple-trap-and-release model) to a band-like transport (the signature of excellent electrical conduction), with increasing temperature. In contrast, the transport through the PTCDI-C5 nanowire was mostly determined by thermally activated behavior. The observation of band-like transport in PTCDI-C8 nanowire was attributed to the small number of charge traps in the constituent molecules. Meanwhile, band-like transport was hardly attainable in the PTCDI-C5 nanowire due to the presence of a large number of charge traps, which followed an exponential energy distribution. Unlike the case of single-crystal PTCDI-C8 nanowire, thin-films of polycrystalline PTCDI-C8 contained significant numbers of exponentially distributed charge traps. Therefore, band-like transport was not observed. Overall, our results presented here demonstrate the importance of attaining good molecular ordering and orientation within the electrically-active molecular layer with a high electronic purity for achieving superior electrical transport, i.e., band-like transport.

KEYWORDS: organic semiconductor, single crystal, solution-phase self-assembly, band-like transport, field-effect transistors


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1. INTRODUCTION Electrical transport through organic semiconductors relies heavily on achieving good molecular ordering, i.e. enhanced crystallinity, among semiconductor molecules.1-4 Accordingly, the synthesis and utilization of organic single crystal semiconductors with extended intermolecular interactions between the backbones of organic semiconductors have been studied intensively over the last decade.5-8 In consequence, thin-film field-effect transistors (FETs) based on organic single crystals have been prepared that exhibit excellent device performance, such as high carrier mobility, from various types of organic semiconducting materials including both polymers and single molecules. Examples include rubrene,9-11 a fluorocarbon-substituted dicyanoperylene-3,4:9,10-bis(dicarboximide),12-13 6,13-bis(triisopropylsilylethynyl)pentacene1415

and dioctylbenzothienobenzothiophene.16 The outstanding electrical properties of these

materials arise from the defect-free and well-ordered molecular packing enabled by strong intermolecular π-π interactions, which drive the formation of an organic single crystal.17-18 Beyond the goal of preparing high-performance electronic devices, single crystals provide an ideal platform for investigating charge transport physics in organic materials, because electrical conduction in a single crystal can occur without confounding factors due to disorder in the film.10,19-26 The preparation of organic semiconductor single crystals requires synthetic methods that differ from typical thin-film growth techniques. Thin-film growth techniques are generally not suitable for obtaining single crystals because the structure of the substrate, onto which the molecules are deposited, can influence the molecular ordering and induce disorders in the layer of molecules in direct contact with the substrate. Alternatively, solution-phase self-assembly offers a promising method of forming single-crystalline structures based on the precipitation-


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induced self-assembly of molecules at a good solvent/poor solvent interface. This method not only yields single-crystal nanowires (NWs) that display excellent structural qualities and high chemical purity, but the technique can also be used to prepare dispersions of single crystal NWs, which are useful precursors for simple solution-processing of materials. Solution processes can not only reduce the manufacturing costs, but they can also produce sparsely distributed NW strands on a solid substrate by varying the concentration of the dispersion. Therefore, solutionphase self-assembly methods offer a platform from which the intrinsic properties of an individual organic semiconductor single crystal can be analyzed systematically.14,27-28 Herein, we investigate electrical charge transport through an individual strand of a singlecrystalline dialkyl perylene tetracarboxylic diimide (PTCDI-Cn, where n=5 and 8, Figure 1a) NWs. PTCDI derivatives are representative n-type organic materials that exhibit a high electron mobility and tunable optoelectronic properties, where the properties can vary according to the substituent, either on the N-atom of the imide or on the perylene backbone. PTCDI derivatives are, therefore, heavily used in organic transistor and photovoltaic cell applications.29-33 Despite sharing a common backbone, the different alkyl chain lengths of these two molecules yield dramatic differences in charge transport properties through single-crystalline NWs. Temperaturedependent field-effect mobility measurements over the range of 100–280 K for PTCDI-C8 NW, in which conduction is determined by intra-nanowire transport, revealed a distinct mechanistic transition from band-like transport (increased mobility upon cooling) in the high-temperature regime to thermally activated transport (decreased mobility upon cooling) in the low-temperature regime. Interestingly, a similar transition was not observed for FETs based on polycrystalline PTCDI-C8 thin-films, in which inter-domain transport was expected to play an additional or even more important role. Meanwhile, the observation of band-like transport through the PTCDI-


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C5 NW was not apparent (or was noticeable only at high temperature window (260‒280 K)). Instead, transport through the PTCDI-C5 NW was influenced by carrier traps, which appeared to be exponentially distributed in energy. Transport appeared to be predominantly a thermallyactivated process, similar to the charge transport through polycrystalline PTCDI-C5 thin-films. Overall, these experimental results demonstrated the importance of acquiring good intermolecular ordering and orientation and minimizing the number of charge traps to achieve enhanced electrical transport in organic semiconductors.

2. EXPERIMENTAL SECTION Preparation and characterization of PTCDI-Cn single NW dispersions prepared via solution-phase self-assembly. PTCDI-Cn derivatives with two different dialkyl chains (n=5 and 8) were purchased from Aldrich and were used without further purification. The chemical structures of these molecules are shown in Figure 1a. A solution-phase interfacial self-assembly method was employed to prepare PTCDI-Cn NWs, taking advantage of the forceful π-π interactions between molecules within a poor solvent.27 Typically, PTCDI-Cn powder (10 mg) was first dissolved in chloroform (10 mL), a good solvent, followed by the addition of methanol, a poor solvent. After gentle swirling of the mixture, the PTCDI-Cn self-assembled at the good/poor solvent interface and formed PTCDI-Cn NWs. The solution mixture containing PTCDI-Cn NWs was vacuum-filtered through a porous anodized aluminum oxide (AAO) membrane with a pore diameter of 0.2 µm and then washed using an excess of ethanol. Finally, the as-prepared PTCDI-Cn NWs were collected and re-dispersed in a vial containing ethanol. The structures of these NWs were characterized by high-resolution transmission electron microscopy


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(HRTEM) with selected area electron diffraction (SAED) analysis using a JEOL JEM2100F operated at an acceleration voltage of 200 kV, allowing a point resolution of 0.102 nm. Preparation of single NW FETs and thin-film FETs. PTCDI-Cn single NW FETs were fabricated according to the following procedures. A highly n-doped Si wafer ( 0). This implies that a thermally-activated mechanism governs charge carrier transport through the single crystal NWs at these temperatures. The multiple-trapping-andrelease (MTR) mechanism is generally accepted as a plausible description of the charge transport through organic molecules.19-20 The MTR model considers i) that most carriers are trapped in localized shallow traps formed by chemical impurities, sites of structural disorder, and surface states; and ii) that charge transport occurs through extended states (transport level) when the carriers are thermally activated (released) from the traps. An Arrhenius plot over the lowtemperature regime yielded an activation energy for this process, i.e., the average depth of the shallow trap was 10 meV. In the high-temperature regime, on the other hand, the electron mobility exhibited a negative mobility temperature coefficient (dµ/dT < 0). The observation of a negative mobility temperature coefficient is a general signature of charge carrier delocalization over a few molecules, that is, the band-like transport.15-16 In the high-temperature regime, sufficient


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thermally energy was available such that the influence of trapping could be eliminated and the overall conduction was determined by the intrinsic transport through the extended tranport level within the PTCDI-C8 NWs. The transition temperature (T*) between the two distinct temperature-dependent behavioral regimes occurred at 210 K, and the highest mobility value, 0.76 cm2/Vs, was obtained at this temperature. In contrast, the temperature-dependent mobility for the PTCDI-C5 NW did not exhibit a band-like transport behavior over the nearly entire temperature range, while a negative mobility temperature coefficient was observed over a narrow range at high temperatures (260‒280 K). These results indicate that the charge transport is mostly determined by the thermally activated process. The larger activation energy of 21 meV was obtained which indicates a larger energy separation between the transport level and the average energy of the localized states as compared to the energy separation observed in PTCDI-C8. The small temperature window showing a negative mobility temperature coefficient may be a signature of the band-like transport in the PTCDI-C5 NW. The presence of band-like transport in the PTCDI-C5 NW at elevated temperatures compared to that in the PTCDI-C8 NW may be due to the presence of numerous carrier traps in the NW, as verified below. Meanwhile, electron transport in a polycrystalline thin-film PTCDI-Cn solely yielded a positive mobility temperature coefficient over the entire temperature range examined (Figure 3a inset). Consistent results were observed by Frisbie et al.29 An Arrhenius plot yielded activation energies of 55 and 28 meV for PTCDI-C5 and -C8, respectively. Note that these values were higher the values obtained from the corresponding single crystalline NWs. Given that electrical conduction through a polycrystalline thin-film is not solely determined by charge transport within a single crystalline domain (intra-domain transport), as it is in single crystal NWs, and


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that conduction depends on the charge transport between domains (inter-domain transport), we attributed the activation energy difference to the energy required to facilitate inter-domain transport. As the average shallow trap depth in the MTR model increased, band-like transport became more difficult or was only possible at higher temperatures. This reasoning may explain why only a positive mobility temperature coefficient was observed in the polycrystalline PTCDICn thin-films. We next examined the gate-voltage dependence of the transport activation energy. Because the induced charge density increased with gate-voltage, this analysis revealed that the carrier density influenced the transport activation energy. According to the MTR model, the activation energy is generally expected to decrease as additional carriers are induced, because the induced carriers fill the trap states and reduce the average depth of the shallow traps. Here, µ was estimated as a function of VG by applying the relation µ=2L/(WCi)(∂ID0.5/∂VG)2 to a transfer characteristic at different VG values, and the activation energy was extracted from the temperature-dependent mobility data set obtained at the same VG values. As displayed in Figure 3b, distinct differences in the gate voltage-dependence of the activation energy were observed in the PTCDI-C5 and -C8 NWs. The activation energy in the PTCDI-C8 NW was nearly independent of the gate voltage, whereas the activation energy in the PTCDI-C5 NW exhibited a pronounced reduction from 230 to 22 meV as VG was increased from 10 V to 80 V. Such a significant reduction in EA in the PTCDI-C5 NWs indicated that the density of the deep carrier traps, which became filled under an applied gate voltage, was much larger in the PTCDI-C5 NWs than in the PTCDI-C8 NWs. On the other hand, the negligible dependence of the carrier trap density on the gate voltage in the PTCDI-C8 NWs suggested that the PTCDI-C8 NWs contained relatively few deep electron traps. In addition, the bias-stress stability measurements35-36 were performed by


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monitoring the threshold voltage (Vth) shift for NW FETs over a period of time (t) under a constant gate voltage of 60 V. As shown in Figure 4, the PTCDI-C5 NW, the transport activation energy of which depended significantly on the gate voltage, yielded a more pronounced shift in Vth. This implied that larger numbers of traps were generated in the PTCDI-C5 NW under a gate bias than were generated in the PTCDI-C8 NW. Overall, these sets of data indicated that significantly fewer carrier traps were formed in the PTCDI-C8 NW than in the PTCDI-C5 NW. These results further support the proposed reason why only band-like transport was observed in the PTCDI-C8 NWs. For comparison, the gate voltage-dependent activation energies for PTCDI-C5 and -C8 polycrystalline films are shown in the inset of Figure 3b. Interestingly, although the activation energies of PTCDI-C8 single NWs were nearly independent of the gate voltage, the corresponding films were significantly dependent on the gate voltage. This suggested that the presence of grain boundaries in the polycrystalline structure created additional deep traps. These results are consistent with the proposal that band-like transport, which was observed from the PTCDI-C8 NW, was not observable from the PTCDI-C8 thin-film. Finally, the trap distributions in the PTCDI-Cn NWs and thin-films were analyzed by applying the Meyer-Neldel relation (MNR). The MNR is an empirical relation indicating that thermally activated physical quantities such as the mobility or conductance follow the relation3741

,


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particularly in the case in which an exponential distribution of trap levels can be assumed. Here,

µ0 and µ00 are the pre-exponential factors of the relations, and EMN is the so-called Meyer-Neldel energy, which reflects the width of the trap state distribution, EMN = kBT0. The validity of the MNR for transport through NWs can be confirmed through the following procedure. First, µ0 values obtained from the y-intercepts of the Arrhenius plots (µ vs. 1/T on a semi-log scale, e.g., Figure 5a) for different VG values were plotted as a function of corresponding EA values on a semi-log scale (Figure 5b). If MNR holds, the semi-log plot should yield a linear relationship, and the inverse of its slope should be equal to EMN. Also, the isokinetic temperature (T0) of the thermally activated process can be estimated from the EMN value (EMN = kBT0). This is the temperature at which the Arrhenius lines obtained from the different VG values should intersect (Figure 5a). Figure 5a and 5b show a clear linear dependence of log µ0 on EA and the presence of an isokinetic temperature for the PTCDI-C5 NW yielding an EMN value of 32 meV. The applicability of MNR to the PTCDI-C5 NW, but not to the PTCDI-C8 NW, indicated that the energy distribution of the trap states in the PTCDI-C5 NW followed an exponential relation. An exponential distribution of trap states agreed well with the results discussed above, showing that the charge transport activation energy for the PTCDI-C5 NW depended strongly on the charge carrier density. Meanwhile, a linear fit to the relation between logµ0 and EA for the PTCDI-C8 NW was difficult to construct, suggesting that the MNR was not the best model for describing thermally activated charge transport through a single crystalline PTCDI-C8. For comparison, plots of logµ0 vs. EA for the PTCDI-C5 and -C8 polycrystalline films are displayed in the insets of Figure 5b. Both the PTCDI-Cn films revealed distinct linear relations, indicating that the MNR and, thus, an exponential distribution of trap levels were appropriate for describing charge transport through both the polycrystalline films. These results are consistent with the absence of


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band-like transport and the noticeable gate voltage-dependent activation energies observed from these films.

4. CONCLUSIONS The temperature-dependent electrical transport properties were investigated in single crystalline NWs composed of PTCDI derivatives possessing different length alkyl chain (pentyl and octyl), fabricated via a solution-phase self-assembly method. Despite the similarities between the molecular structures of the two molecules, band-like transport was only achievable in the PTCDI-C8 NW which turned out to contain fewer charge traps. Band-like transport was not observed in the PTCDI-C5 NWs, which displayed a significant number of traps with an exponential energy distribution. The same PTCDI-C8 sample was used to form in polycrystalline thin-films did not yield the band-like transport. These results demonstrated the importance of attaining i) single crystalline structures composed of organic molecules (compare the NW and thin-film) and ii) crystals with a small number of charge traps (compare the PTCDI-C8 NW and the PTCDI-C5 NW) to achieve enhanced electrical transport through organic molecular semiconductors.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected], [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. §B. J. Kim and H. Yu contributed equally.


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ACKNOWLEDGMENT This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korean Government (MEST) (2012-011603, 2011-0017174 and 2009-0083540). H.Y. acknowledges financial support from the Global Ph.D. Fellowship funded by National Research Foundation of Korea (NRF).


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Kim, I.; Haverinen, H. M.; Wang, Z.; Madakuni, S.; Li, J.; Jabbour, G. E. Effect of Molecular Packing on Interfacial Recombination of Organic Solar Cells Based on Palladium Phthalocyanine and Perylene Derivatives. Appl. Phys. Lett. 2009, 95, 023305. Hardin, B. E.; Hoke, E. T.; Armstrong, P. B.; Yum, J. H.; Comte, P.; Torres, T.; Fréchet, J. M. J.; Nazeeruddin, M. K.; Grätzel, M.; McGehee, M. D. Increased Light Harvesting in Dye-Sensitized Solar Cells with Energy Relay Dyes. Nat. Photonics. 2009, 3, 406-411. Chen, Z.; Zheng, Y.; Yan, H.; Facchetti, A. Naphthalenedicarboximide- vs Perylenedicarboximide-Based Copolymers. Synthesis and Semiconducting Properties in Bottom-Gate N-Channel Organic Transistors. J. Am. Chem. Soc. 2008, 131, 8-9. Lemaur, V.; Steel, M.; Beljonne, D.; Brédas, J. L.; Cornil, J. Photoinduced Charge Generation and Recombination Dynamics in Model Donor/Acceptor Pairs for Organic Solar Cell Applications: A Full Quantum-Chemical Treatment. J. Am. Chem. Soc. 2005, 127, 6077-6086. Horowitz, G. Organic Field-Effect Transistors. Adv. Mater. 1998, 10, 365-377. Suemori, K.; Uemura, S.; Yoshida, M.; Hoshino, S.; Takada, N.; Kodzasa, T.; Kamata, T. Threshold Voltage Stability of Organic Field-Effect Transistors for Various Chemical Species in the Insulator Surface. Appl. Phys. Lett. 2007, 91, 192112. Gomes, H.; Stallinga, P.; Dinelli, F.; Murgia, M.; Biscarini, F.; De Leeuw, D.; Muck, T.; Geurts, J.; Molenkamp, L.; Wagner, V. Bias-Induced Threshold Voltages Shifts in ThinFilm Organic Transistors. Appl. Phys. Lett. 2004, 84, 3184-3186. Yelon, A.; Movaghar, B.; Branz, H. Origin and Consequences of the Compensation (Meyer-Neldel) Law. Phys. Rev. B 1992, 46, 12244. Wang, J.; Chen, Y. The Meyer–Neldel Rule in Fullerenes. Appl. Phys. Lett. 1998, 73, 948-950. Meijer, E.; Matters, M.; Herwig, P.; de Leeuw, D.; Klapwijk, T. The Meyer–Neldel Rule in Organic Thin-Film Transistors. Appl. Phys. Lett. 2000, 76, 3433-3435. Meyer, W.; Neldel, H. Relation between the Energy Constant and the Quantity Constant in the Conductivity–Temperature Formula of Oxide Semiconductors. Z. tech. Phys 1937, 18, 588-593. Fishchuk, I.; Kadashchuk, A.; Genoe, J.; Ullah, M.; Sitter, H.; Singh, T. B.; Sariciftci, N.; Bässler, H. Temperature Dependence of the Charge Carrier Mobility in Disordered Organic Semiconductors at Large Carrier Concentrations. Phys. Rev. B 2010, 81, 045202.


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Table 1. Summary of the parameters obtained from temperature-dependent transport studies of PTCDI-C5 and -C8 NWs and thin-films. µRT(cm2/Vs)

µ*(cm2/Vs)

T*(K)

EA(meV)

EMN(meV)

Nanowire

0.09

-

-

21

32

Film

0.19

-

-

55

51

Nanowire

0.39

0.76

210

10

-

Film

0.56

-

-

28

34

PTCDI-C5

PTCDI-C8


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Figure 1. (a) Chemical structures of PTCDI-C5 and -C8 (b) A schematic diagram of a single NW FET. The inset shows an optical micrograph of a device channel connected by a NW. (c) TEM image of a PTCDI-C5 NW on a copper grid. The inset shows the SAED pattern of the PTCDI-C5 NW. (d) TEM image of a PTCDI-C8 NW on a copper grid. The inset shows the SAED pattern of the PTCDI-C8 NW.


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Figure 2. Transfer characteristics of PTCDI-C5 and -C8 NW FETs obtained at VD=80 V. The inset shows the transfer characteristics of PTCDI-C5 and -C8 thin-film FETs obtained at VD=80 V.


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Figure 3. (a) An Arrhenius plot of the temperature-dependent mobility in PTCDI-C5 and -C8 NWs. The inset shows an Arrhenius plot of the temperature-dependent mobility for PTCDI-C5 and -C8 thin-films. (b) Gate voltage-dependent activation energy in PTCDI-C5 and -C8 NWs. The inset shows the gate voltage-dependent activation energy in PTCDI-C5 and -C8 thin-films.


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Figure 4. Threshold voltage shift as a function of time upon application of a gate-bias stress at 60 V to the PTCDI-C5 and -C8 NW FETs as a function of time.


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Figure 5. (a) Arrhenius plots of the temperature-dependent mobilities obtained at different gate voltages for the PTCDI-C5 NW. An isokinetic point was obtained at 371 K. (b) A Plot of µo vs. EA, obtained from the Arrhenius relations for the mobilities of the PTCDI-C5 and -C8 NWs. Only the PTCDI-C5 NW exhibited a linear relation between µo and EA, confirming the suitability of applying the MNR for describing charge transport. The inset shows the same plot for the PTCDIC5 and -C8 thin-films.


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Table of Contents Image


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Electrical Transport through Single Nanowires of Dialkyl Perylene

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