9374
J. Phys. Chem. B 2005, 109, 9374-9378
Correlation of the Number of Thiophene Units with Structural Order and Carrier Mobility in Unsubstituted Even- and Odd-Numbered r-Oligothiophene Films Shuichi Nagamatsu*,† and Keiichi Kaneto Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology (KIT), 2-4 Hibikino, Wakamatsu-ku, Kitakyushu 808-0196, Japan
Reiko Azumi and Mutsuyoshi Matsumoto Nanotechnology Research Institute (NRI), National Institute of AdVanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba 305-8565, Japan
Yuji Yoshida and Kiyoshi Yase Photonics Research Institute (PRI), National Institute of AdVanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba 305-8565, Japan ReceiVed: December 20, 2004; In Final Form: March 17, 2005
We investigate the correlation of the number of thiophene units with the structural order and carrier mobility of the films through studies on thin-film transistors (TFTs) based on R-quinquethiophene (5T), R-sexithiophene (6T), and R-septithiophene (7T). The X-ray diffraction (XRD) data of the nT films deposited at low substrate temperatures present obviously different structural orders depending on the parity of the number of thiophene units. Although even-numbered nT films present well-ordered structures and large carrier mobilities, oddnumbered nT films present two different crystalline polymorphs and vastly low carrier mobilities reflecting the coexistence of two crystalline polymorphs. However, the XRD data of both even- and odd-numbered nT films deposited at high substrate temperatures indicate that the nT molecules form single well-ordered structures. Those ordered TFTs exhibit large carrier mobilities accompanying an increase in the number of thiophene units, 0.05, 0.08, and 0.13 cm2 V-1 s-1 for 5T, 6T, and 7T, respectively. The parity of the number of thiophene units affects the structural order intrinsically in grown thin films, and affects carrier mobilities extrinsically in their TFTs.
Introduction Organic semiconducting compounds consisting of conjugated oligomers have been studied as promising materials for electronic and optoelectronic devices.1-5 Drastic improvement of organic light-emitting diodes1,2 has been achieved and brought into the commercial stage. Similarly, organic solar cells3 and organic thin-film transistors (TFTs)4,5 are also being studied intensively to elevate the performances to the level of practical use. TFTs based on conjugated oligomers have now been reproduced and have proven to be promising devices with carrier mobilities of 1 cm2 V-1 s-1, which is comparable to that obtained with amorphous silicon.5 Oligothiophenes and polythiophenes are promising candidates for applications because of their excellent electronic properties, stability under ambient conditions, and ease of chemical modification.6 Numerous studies on oligothiophenes and their derivatives have been reported so far. Unsubstituted R-oligothiophenes (nT, with n being the number of thiophene units), the simplest compounds in the family, have been studied intensively since Horowitz and co-workers demonstrated high hole mobility in sexithiophene (6T) film in TFT.7 With the exception of septithiophene (7T), nTs have been applied to TFT and characterized up to octithiophene (8T).7-13 * To whom correspondence should be addressed. E-mail: s.nagamatsu@ aist.go.jp. † Present address: PRI, AIST.
The carrier mobility in nT-based TFT increases along with the chemical purity of the layer, the number of thiophene units (molecular length), and the structural order. In particular, the structural order in deposited film has a crucial importance on the TFT performances. Good nT-based TFT performance has been achieved by improving the structural order in deposited films, where all of the molecules adopt a stand-up orientation.8 When deposited by thermal evaporation onto sufficiently smooth substrates, linear small organics tend to form well-ordered films in which the molecules are typically arranged with their long axes nearly perpendicular to the substrate surfaces. The structural order in deposited films depends strongly on the deposition conditions. The optimal conditions for stand-up orientation of the molecules include low deposition rates or high substrate temperatures. The highest value of carrier mobility in nT-based TFTs has been obtained in an ordered 8T film deposited at a high substrate temperature.12 Most studies on nT-based TFTs use even-numbered nTs such as quaterthiophene (4T), 6T, and 8T.6-12 However, oddnumbered nTs such as quinquethiophene (5T) have been studied much less because of their synthetic difficulty.9,11,13 The longest synthesized odd-numbered nT, 7T, has not been applied to TFT so far. Furthermore, the carrier mobilities in 5T-based TFT, obtained by several research groups, show interesting results. Hajlaoui and co-workers obtained a lower carrier mobility compared to that in 4T-based TFT, although the molecular
10.1021/jp044222l CCC: $30.25 © 2005 American Chemical Society Published on Web 04/15/2005
Correlation of Length with Order and µ in R-nTs
J. Phys. Chem. B, Vol. 109, No. 19, 2005 9375
SCHEME 1: Structure of r-Oligothiophenes
TABLE 1: Crystal Structures of r-Oligothiophene in Single Crystals 5T 6 crystal system a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) a
monoclinic 6.00 7.77 39.00 90 97.7 90
6T 6
a
monoclinic 6.03 7.85 44.71 90 90.8 90
7T 18 triclinic 5.95 7.77 53.02 91.21 91.89 89.92
Low-temperature polymorph.
length is longer.9 Hong and co-workers obtained a carrier mobility between those in 4T- and 6T-based TFTs.11 Recently, Melucci and co-workers obtained a carrier mobility equivalent to that in 6T-based TFT, although the molecular length is shorter.13 Therefore, odd-numbered nT-based TFTs show carrier mobilities spanning 1 and 2 orders of magnitude. On the contrary, in even-numbered nT-based TFTs, the carrier mobilities with almost the same order of magnitude were obtained by a large number of research groups.6-12 In addition, although an improvement of carrier mobility with an increase in molecular length was demonstrated in even-numbered nT-based TFTs, the deviation from this improvement phenomenon occurred in 5Tbased TFT. Here, interest arises from the analogy of the “even-odd effect” of crystal packing of n-alkanes. The molecular packing of n-alkanes depends strongly on the parity of the number of carbon atoms in the methylene chains, which affects their physical properties such as the melting point.14-17 This feature was also observed in the molecular packing of nTs in single crystals.18 The molecular packing of odd-numbered nTs, 5T and 7T, in single crystals resembles that reported for even-numbered nTs, although the former molecules are in more random conformations. The calculated densities of nT single crystals show the alternation with respect to the parity of the number of thiophene units. The odd-numbered nTs have interesting features, random conformations, which are not present in evennumbered nTs. Thus, the slight difference in molecular conformation may lead to large differences in molecular properties similar to that of methylene chains. It is important to understand the correlation of molecular structure and physical properties as the information on the design of molecular conformation for organic semiconducting compounds. In this study, we investigate a series of nT-based TFTs including odd-numbered nTs and describe the correlation of the number of thiophene units with the structural order and carrier mobility through studies on 5T, 6T, and 7T, the last of which has not been applied to TFT so far. We have found that the structural order in grown thin films and the carrier mobilities in nT-based TFTs depend on the number of thiophene units. Experimental Section Film Preparation. Oligothiophenes, 5T, 6T, and 7T, were synthesized according to the literature18,19 and purified by
Figure 1. XRD pattern of nT films deposited at a low substrate temperature of 24 °C. (a) 5T, (b) 6T, and (c) 7T films.
sublimation. A polycrystalline oligothiophene film was deposited on a substrate surface by thermal evaporation at a base pressure of 10-6 Torr at a deposition rate of ca. 0.03 nm s-1. During the evaporation, the substrate was held at controlled temperatures. The resulting film thickness was approximately 100 nm. X-ray Diffraction. The structure of deposited oligothiophene was revealed by X-ray diffraction (XRD) analysis (Rigaku, RU200), which was carried out using the powder method with focused Cu KR radiation. The crystal structures of nTs in single crystals have been reported and are summarized in Table 1.5,20-24 The 6T molecules form the typical two polymorphs, the “lowtemperature polymorph” and the “high-temperature polymorph”, in single crystals. In grown thin films, it was reported that the even-numbered oligothiophene molecules form only a lowtemperature polymorph.5,20 Device Fabrication. Top contact type TFTs were constructed on highly doped n-type silicon wafers covered with 300-nmthick silicon dioxide with a capacitance per unit area (Cins) of 10 nF cm-2. The oxidized silicon wafers provide convenient substrates, gate electrodes, and gate dielectrics for fabricating TFT test structures.10 Prior to the deposition of the oligothiophene, the surface of the silicon dioxide was treated with hexamethyldisilazane. The oligothiophenes were deposited on the treated substrate surfaces. To complete the devices, we evaporated gold source-drain electrodes on top of the oligothiophene films through a shadow mask, defining the channel length, L ) 20 µm, and channel width, W ) 5 mm. The TFT devices were characterized using computer-controlled twosource measure units (Keithley6430 and 2400 sourcemeters) and were measured under vacuum (
