J. Phys. Chem. 1996, 100, 12631-12637
12631
Vacuum-Deposited Thin Film of Linear π-Conjugated Poly(arylene)s. Optical, Electrochemical, and Electrical Properties and Molecular Alignment Takakazu Yamamoto,*,† Takaki Kanbara,† Chiaki Mori,† Hiroshi Wakayama,† Takashi Fukuda,† Tetsushi Inoue,† and Shintaro Sasaki‡ Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226, Japan, and School of Materials Science, Japan AdVanced Institute of Science and Technology, 1-1 Asahidai, Tatsunokuchi, Ishikawa 923-12, Japan ReceiVed: March 8, 1996; In Final Form: April 30, 1996X
Vacuum deposition of linear rod-like π-conjugated poly(arylene)s [poly(p-phenylene) PPP, poly(thiophene2,5-diyl) PTh, and poly(2,2′-bipyridine-5,5′-diyl) PBpy] gives thin films of poly(arylene)s on various substrates. IR spectra of the films are essentially the same as those of original polymers. UV-visible and photoluminescence spectra of the films also show peaks near those observed with the original polymers. Vacuum deposited (vd-) PPP molecules stand upright on carbon and metal substrates as revealed by regular electron diffraction spots observed with the thin film of PPP. The degree of crystallinity of the PPP thin film depends on the kind of the substrate and increases in the order Au < Ag < Cu < Al < C, which is elucidated by the strength of interaction between PPP and the substrate. Similar orientation is observed with vd-PTh and -PBpy. In addition to the regular electron diffraction pattern, Debye-Scherrer rings are observable with the vd-PPP and -PTh films, and all the electron diffraction data are consistent with reported powder X-ray data of PPP and PTh. The vd-PPP and -PTh films are susceptible to p-doping with Epa of 1.34 and 0.71 Vs Ag/Ag+, respectively, whereas the vd-PBpy film receives n-doping with Epc of -2.30 V Vs Ag/Ag+. vdPTh film forms an ohmic contact with Au, whereas Al/vd-PTh/Au and Al/vd-PTh/ITO devices show rectification of electric current with rectification ratios of 200 and 22, respectively. The rectification effect is accounted for by assuming an injection barrier between PTh and Al. Source-drain electric current of a field effect transistor using the vd-PTh film as an active layer can be controlled by gate bias.
CHART 1. Typical π-Conjugated Poly(arylene)s
Introduction π-Conjugated poly(arylene)s such as poly(p-phenylene) PPP, poly(thiophene-2,5-diyl) PTh, and poly(2,2′-bipyridine-5,5′-diyl) PBpy exhibit interesting electrochemical, optical, and electric properties, and various studies have been made on the chemical and physical properties of the poly(arylene)s.1-9 Brown and his co-workers reported that poly(arylene)s were sublimable up to molecular weights of about 1500-2000,10,11 and we recently reported that vacuum-deposited films of the poly(arylene)s (Chart 1) showed interesting alignment on the substrates.8a,b,9 Here, we report (a) controlling factors for the alignment, (b) optical and electrochemical properties of the vacuum-deposited film, and (c) behavior of electric devices using the vacuum-deposited film. Poly(arylene)s (Ar)n used in this study are prepared by organometallic polycondensation of dihaloaromatic compounds X-Ar-X with a zero-valent nickel complex8,9 or magnesium,12 and they have linear rod-like structures. Ni catalyst
nX-Ar-X + M 98 (Ar)n
(1)
where M is a zero-valent nickel complex Ni0Lm or Mg. Experimental Section Materials. Poly(p-phenylene) PPP,9,12 poly(thiophene-2,5diyl) PTh,8,13,14 and poly(2,2′-bipyridine-5,5′-diyl) PBpy8 were prepared as previously reported. †
Tokyo Institute of Technology. Japan Advanced Institute of Science and Technology. X Abstract published in AdVance ACS Abstracts, June 15, 1996. ‡
S0022-3654(96)00724-1 CCC: $12.00
n
PPP
S
n
PTh
N
N
n
PBpy
Results and Discussion Structure of Vacuum-Deposited Polymer. IR Spectra. The vacuum-deposited PPP gives essentially the same IR absorption pattern as the original PPP (cf. Supporting Information). An out-of-plane δ(C-H) peak of the original PPP (originated from the p-phenylene unit) at 805 cm-1 is shifted to 808 cm-1 in vd-PPP. The IR spectrum of vd-PPP does not show δ(C-H) peaks of oligo-p-phenyls (Pn, n ) 3-6), which appear in a range of 837 (P3) through 811 cm-1 (P6).15,16 In addition, the IR spectrum shows a much weaker δ(C-H) peak of the terminal phenyl group15,16 compared with that of oligo-p-phenyls. Although the average degree of polymerization of the original PPP is not known because of the insolubility of PPP, the vdPPP film is most likely constituted of PPP molecules with about 10-25 p-phenylene units based on the above-described results and the previously reported mass spectrum of sublimable PPP (cf. Introduction). The vd-PBpy and -PTh also show IR spectra similar to those of the original PBpy8a and PTh,9 respectively. These results indicate that vd-poly(arylene)s have essentially the same chemical structures as the original poly(arylene)s, although the poly(arylene) molecules with relatively low molecular weights seem to be preferentially evaporated on the substrate. The following UV-visible and photoluminescence data give further informations about vd-poly(arylene)s. © 1996 American Chemical Society
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Figure 1. (a) Photoluminescence (irradiated with 325 nm light) and (b) excitation (monitored at 540 nm) spectra of vd-PBpy film on a quartz plate.
UV-Visible and Photoluminescence Spectra. The vd-PPP film on a quartz plate shows a lowest energy π-π* absorption peak at 363 nm17a and a photoluminescence peak at 448 nm. The peak position of the photoluminescence agrees with the onset position of the π-π* absorption band, as usually observed with photoluminescent aromatic compounds. The vd-PBpy film on a quartz plate exhibits a π-π* absorption peak at 332 nm. A cast film of PBpy (with molecular weight of 3100 or 40 pyridine units) on the quartz plate gives the π-π* absorption peak at 350 nm.17b Figure 1 depicts fluorescence and excitation spectra of the vd-PBpy film on the quartz plate. As depicted in Figure 1, the vd-PBpy film gives a strong photoluminescence band at 540 nm. A peak position of an excitation spectrum (monitored at 540 nm) agrees with the absorption peak of the vd-PBpy film. However, the peak position of the photoluminescence deviates considerably to a longer wavelength from the onset position (ca. 440 nm) of the absorption and excitation spectra. The peak position of the photoluminescence agrees with that of previously reported excimer-type photoluminescence from PBpy.8,17b Films of rigidly linear π-conjugated polymers often exhibit similar excimer-type photoluminescence, and such photochemical processes are the subject of recent interest.8,17b-22 The UV-visible spectrum of vd-PTh on a quartz plate exhibits an absorption band with λmax at about 410 nm. For vd-oligothiophene Thn, it has been reported that it gives the absorption peak at a shorter wavelength than Thn in solution. H
S
n
H
Thn
For example, vd-Th4 shows a π-π* absorption peak at 345 nm,23 which is shifted from that (391 nm24) of Th4 in benzene. The λmax of vd-Thn levels off at about n ) 6 (for example, both vd-Th6 and -Th8 exhibit λmax at about 400 nm at 296 K).23 The vd-PTh film on the quartz plate emits light with two peaks at 681 and 622 nm (peak height ratio ) 1.2:1). The two photoluminescence (PL) peak positions are at a considerably longer wavelength than that of Th4 in solution (λmax ) 450 nm),25 are comparable to those of (a) Th6 in a solid state (λmax ) 603 nm)26 and (b) end-capped Th6 in the solid at 15 K (two peaks at 633 and 582 nm in a peak hight ratio of 1.4:1),27 and agree with those of an electrochemically prepared PTh
Figure 2. Electron diffraction patterns of (a) PPP, (b) PTh, and (c) PBpy films (thickness ≈ 100 nm) vacuum-deposited on an amorphous carbon substrate at 150 °C. The a* axis is oriented in the horizontal direction in this figure, while the b* axis is in the vertical direction.
film (two peaks at 681 and 626 nm with about 1:1 intensity ratio).28 All the IR, UV-visible, and PL data support that PBpy and PTh with normal molecular structures are vacuumdeposited. Alignment of Poly(arylene). 1. Perpendicular Alignment. A. Alignment of PPP. Figure 2a exhibits an electron diffraction pattern of vd-PPP film formed on an amorphous carbon substrate at a substrate temperature of 150 °C. The PPP film has a thickness of about 100 nm, corresponding to the stacking of about 10 layers of the PPP molecules if the molecules have an average degree of polymerization of about 20 (molecular length ) about 9 nm) and are arranged perpendicularly to the surface of the substrate. All the spots shown in Figure 2 can be reasonably explained by assuming that PPP molecules are arranged essentially perpendicularly to the surface of the substrate (Figure 3).9 Table
π-Conjugated Poly(arylene)s
J. Phys. Chem., Vol. 100, No. 30, 1996 12633 CHART 2: Order of Orientation of PPP on Substrates order of orientation (eq 2)
C>
no. of observable diffraction spots (150 °C)a no. of observable diffraction spots (room temp)a
66
Al > typical metal 26
37
(12b)
Cu > Ag > transition metal 10 10
Au
7c
3c
9
10
a Substrate temperature for the preparation of vd-PPP. b Separation of spots from aluminum oxides is difficult. c Only dim spots were observed.
Figure 3. (A) Alignment of PPP molecules perpendicular to the surface of the carbon substrate. (B) Packing of PPP molecules studied by powder X-ray crystallography.29 The bar indicates the p-phenylene ring seen from the c axis. a ) 0.78 nm. b ) 0.56 nm.
TABLE 1: Electron Diffraction Data of PPP spacing/nm no.
obsd
calcda
indices hkla
(1 (2 3 4 5 6 (7 8 9 10 11 12 (13 14 15 16 17 (18 19 20 21 22 23 24 25 26
0.78 0.56 0.46 0.39 0.32 0.28 0.27 0.26 0.24 0.23 0.20 0.19 0.19 0.19 0.18 0.17 0.16 0.16 0.16 0.15 0.14 0.14 0.13 0.13 0.12 0.12
0.78 0.56 0.46 0.39 0.32 0.28 0.26 0.26 0.24 0.23 0.20 0.20 0.19 0.19 0.18 0.17 0.16 0.16 0.15 0.15 0.14 0.14 0.13 0.13 0.12 0.12
100) 010) 110 200 210 020 300) 120 310 220 400 320 030) 410 130 230 420 500) 510 330 520 430 600 610 530 620
the carbon substrate, revealing a lower order of the orientation of vd-PPP on the metal substrates. The order of orientation estimated from the number of observed electron diffraction spots is in the order shown in Chart 2. As for orientation or alignment of liquid crystalline molecules on a substrate, it is generally recognized that the liquid crystalline molecules stand upright on the substrate when the surface energy or adhesive energy between the liquid crystalline molecules and the substrate is smaller than that between the liquid crystalline molecules.30 However, when the former energy is larger than the latter energy, the liquid molecules tend to be oriented in parallel with the surface of the substrate. The trend observed for the order of the perpendicular orientation of PPP (eq 2) also seems to be accounted for on the basis of surface energy or strength of interaction between PPP and the substrate. It has been reported that the heat of wetting Hw of carbon, aluminum, and copper in benzene, which corresponds to the monomer unit of PPP, increases in the following order:31
C(Hw ≈ 110 erg cm-2) < Al(Hw ≈ 200 erg cm-2) < Cu(Hw ≈ 880 erg cm-2) The large Hw value of Cu seems to originate from π-coordination of benzene to the transition metal,
a Calculated by assuming an orthorhombic crystal system with a ) 0.78 nm and b ) 0.56 nm (cf. Figure 3B).
1 summarizes the assignment of the spots shown in Figure 2. As shown in Table 1, the reflection spacings observed for the PPP film agree within experimental error with the calculated values with the lateral unit cell dimensions of a ) 0.78 nm and b ) 0.56 nm. Essentially, the same ab parameters have been reported29 mainly based on powder X-ray analysis of PPP. It is interesting that the electron diffraction pattern gives some h00 and 0k0 reflections with odd numbers of h and k (cf. Figure 2 and Table 1). They should be systematically absent for a PPP crystal with the pgg two-dimentional symmetry. Actually, the powder X-ray diffraction pattern of PPP does not show such reflections. These results suggest that the additional spots parenthesized in Table 1 are due to the multiple diffraction effect of the electron beam. Effect of Substrate. vd-PPP films formed on metal substrates also show regular electron diffraction spots. However, in these cases, the number and clarity (or strength) of the spots are considerably decreased compared with those from vd-PPP on
and a number of π-benzene complexes of various transition metals including Cu and Ag have been isolated.32 Thus, the higher order of the perpendicular orientation of PPP on the carbon substrate seems to be attributable to the weaker interaction of the PPP molecule with carbon. As shown in Chart 2, the number of the electron diffraction spots decreases at the lower temperature (room temperature) of the substrate. Full rearrangement of the vd-PPP molecules to give the oriented crystalline structure on the substrate seems to require a higher temperature. B. Alignment of PTh and PBpy. PTh and PBpy (Chart 1) films vacuum-deposited on the amorphous carbon substrate at 150 °C also give clear regular electron diffraction spots (parts b and c of Figure 2), although the number of the electron diffraction spots observed with the vd-PBpy film is small presumably because of the lower symmetry of the PBpy molecule. The regular diffraction spots indicate a similar orientation of the PTh and PBpy molecules perpendicular to the surface of the substrate. The electron diffraction pattern of
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CHART 3: ab Parameters of Polymers polymer a × 10/nm b × 10/nm
PBpy 8.1 5.7
PPP 7.8 5.6
PTh 7.8 5.6
poly(acetylene) 7.6 4.9
poly(ethylene) 7.6 4.4
the vd-PTh film agrees well with the known X-ray crystallographic data of PTh.33 If the vd-PBpy film has the packing similar to those of PPP and PTh, the dimensions estimated from the electron diffraction pattern are a ) 0.81 nm and b ) 0.57 nm, respectively. These values are larger than the parameters of PPP, PTh, poly(acetylene),34 and poly(ethylene),35 all of which take the orthorhombic or similar crystal system. The order of magnitude of the ab parameters appears to reflect the volume of the repeating unit. The crystalline structure of the vacuum-deposited π-conjugated poly(arylene) films is stable at room temperature. However, it is not so stable against the electron beam, and the electron diffraction spots disappear on continued irradiation with the electron beam at room temperature. Debye-Scherrer Ring in Electron Diffraction. The electron diffraction pattern of PPP on the carbon substrate (Figure 2a, obtained at substrate temperature of 150 °C) shows a weak Debye-Scherrer ring corresponding to the 110 diffraction (the strongest principal diffraction in the powder X-ray diffraction), and the Debye-Scherrer ring passes through the four equivalent electron diffraction spots. When the substrate (amorphous carbon) temperature is lower (room temperature), the vd-PPP film has a lower degree of orientation (cf. Chart 2) and three Debye-Scherrer rings (hkl ) 110, 200, 210; these rings pass through the corresponding electron diffraction spots) are observed.36 In the case of PTh with lower molecular symmetry and less rigid linear structure, the PTh film shows three Debye-Scherrer rings corresponding to the 110, 200, and 210 diffractions (Figure 2b)33 even when the vacuum deposition was made on the carbon substrate at 150 °C. The electron diffraction data and reported X-ray diffraction data29,33 indicate that the present vd-PPP and -PTh have the isomorphous structure as the original PPP and PTh. That the unit cell parameters of the present vd-PTh are somewhat different from those of R-sexithiophene (a ) 0.7851 nm, b ) 0.6029 nm)37 but agree with those of PTh (Chart 3) also supports that vd-PTh resembles PTh rather than oligothiophene. 2. Parallel Alignment. As previously reported,8 when PBpy is deposited on a glass plate, PBpy molecules near the surface of the glass substrate are oriented along the direction of the surface of the glass plate. Vacuum deposition of PTh on a rubbed polyimide film, which is now widely used for orientation of liquid crystals along the direction of rubbing, leads to another type of orientation of PTh. Irradiation of the rubbed polyimide film containing vd-PTh with polarized light shows the π-π* absorption peak of PTh at 430 nm and reveals dichroism for the absorption peak. The absorbance of the π-π* absorption peak varies with the angle (θ) between the direction of the oscillating electric field of the polarized light and the direction of rubbing of the polyimide. The absorbance of the peak at θ ) 0° (a||) is larger than the absorbance (a⊥) observed at θ ) 90° (a||/a⊥ ) 1.5), indicating that PTh is oriented to some extent along the direction of the rubbing. Electrochemical Response. Figure 4 shows the cyclic voltammogram of the vd-PTh on an ITO (indium-tin oxide glass) electrode. The cyclic voltammogram shows the p-doping and undoping peaks at 0.71 and 0.27 V vs Ag/Ag+, respectively, and the potentials of the peaks38 and change in the UV-visible
Figure 4. Cyclic voltammogram of vd-PTh film (thickness ) 26 nm) on ITO electrode (1.0 × 1.0 cm) in an acetonitrile solution of [Et4N]ClO4(0.1M). Scan rate ) 10 mV s-1. Inset shows the change of UVvisible spectrum on p-doping: (a) at 0 V vs Ag/Ag+ (-); (b) 0.65 V (- - -); (c) 0.70 V (-); (d) 0.73 V (-‚-‚); (e) 0.80 V (-).
TABLE 2: Redox Potential for the vd-PTh, -PPP, and -PBpy Thin Films on ITOa (A) p-Type Doping poly(arylene) (thickness/nm)
Epa/Vb (p-doping)
xc
Epc/Vd (p-undoping)
PTh (26) PPP (35)
0.71 (blue) 1.34 (bluish yellow)
0.15 0.15
0.27 (orange red) 1.02 (colorless)
(B) n-Type Doping poly(arylene) (thickness/nm)
Epc/Ve (n-doping)
xc
Epa/Vf (n-undoping)
PBpy (23)
-2.30 (reddish purple)
0.25
-2.09 (yellow)
a
Color after the electrochemical reaction is shown in parentheses, e.g., p-doped PTh is blue whereas p-undoped PTh is orange red (color of original PTh, cf. Figure 4). Potential is given Vs Ag/Ag+. b p-Doping (oxidation) potential. c x is doping level (number of charges stored per each arylene unit (one pyridine for PBpy)). d p-Undoping (reduction of oxidized poly(arylene)) potential. en-Doping (reduction potential). f n-Undoping (oxidation of reduced poly(arylene)) potential.
spectrum (Figure 4)39 are almost the same as those reported for electrochemically prepared PTh.
S orange red
n
+ nxClO4–
0.71 V 0.27 V
+x
S
• xClO4–
+ nxe– (3) n
blue
Five samples of vd-PTh with a thickness range 26-120 nm on the ITO glass as well as vd-PTh’s on Au and C substrates give essentially the same CV curve. The thin films of vd-PPP and -PBpy are also electroactive, and the electrochemical data are summarized in Table 2. As shown in Table 2, PTh and PPP are susceptible to p-doping (oxidation), whereas PBpy receives n-doping (reduction).8 Electrical Conductivity and Device. vd-PTh. The following devices have been prepared by the vacuum deposition technique: (i) Au/vd-PTh/Au, (ii) Al/vd-PTh/Au, (iii) Al/vdPTh/Al, (iv) Al/vd-PTh/ITO, and (v) Mg(Ag)/vd-PTh/ITO (ITO ) indium-tin-oxide glass) devices. For the Au/vd-PTh/Au and Au/vd-PTh/ITO devices, a linear correlation is observed between the electric current (i) and
π-Conjugated Poly(arylene)s
J. Phys. Chem., Vol. 100, No. 30, 1996 12635
Figure 6. i-V curve for the Al/vd-PTh/Au device at 283 K. Thickness of PTh ) 89 nm. Area ) 0.7 cm × 0.7 cm.
Figure 5. i-V relationship for (a) Au/vd-PTh/Au (thickness of PTh ) 130 nm) and (b) Al/vd-PTh/Al (thickness of PTh ) 138 nm) devices at room temperature. Area ) 0.7 cm × 0.7 cm. Electric resistance calculated from the slope of the line in Figure 5a includes the electric resistance of other parts of the circuit, and the electrical conductivity of the vd-PTh film is calculated after subtracting the electrical resistance that originated from the other part of the circuit.
applied voltage (V), indicating that the electric contact between Au and vd-PTh is ohmic. The electrical conductivity (σ) of the vd-PTh in the Au/vd-PTh/Au device is estimated to be about 1.5 × 10-5 S cm-1 from the linear i-V relations (e.g., Figure 5a) obtained with four samples with various thickness of vdPTh (cf. Supporting Information). The electrical conductivity is considerably larger than that of the nondoped compressed powder of the original PTh (σ ) ca. 1 × 10-10 S cm-1),9 which may be attributed to the alignment of the PTh molecules along the direction connecting Au(+) and Au(-) electrodes and/or more facile p-doping (oxidation) of the vd-thin film of PTh (e.g., by electron transfer to O2) than the powder of the original PTh. In contrast to the Au/vd-PTh/Au device, the Al/vd-PTh/Au device using a metal electrode with a small work function (Al) shows good rectification of the electric current as shown in Figure 6. The rectification ratio calculated from electric currents at (4.3 V is about 200. Preparation of such an Al/vd-PTh/Au device is reproducible. The great difference between the Au/vd-PTh/Au and Al/vdPTh/Au devices is accounted for by assuming the presence of an injection barrier between Al and vd-PTh. If one assumes a Schottky-type barrier40 (although recently another type of barrier, tunneling barrier,41 has also been proposed), the forward electric current in Figure 6 is considered to follow the following equation.
i ) io exp(qV/nkT)
(4)
which has been applied to rectifying diodes composed of not only the usual inorganic semiconductors but also organic polymer semiconductors.40d-f In eq 4, q, k, and T stand for the charge of the electron, the Boltzmann constant, and the temperature, respectively. Actually, as shown in Figure 7, a plot of log i Vs V gives a linear line9c following the correlation of eq 4, and the factor n in eq 4 is calculated as 10.4 from the slope of the line. From the analysis of the reverse electric current,40 the Schottky barrier
Figure 7. Log i-V plot of the data shown in Figure 6.
between Al and vd-PTh is estimated as 0.95 eV, which roughly agrees with the reported barrier between Al and electrochemically prepared poly(3-methylthiophene-2,5-diyl).40e Almost the same n value (n ) 10.5 ( 0.6) is observed in a temperature range 10-62 °C, whereas the io value in eq 4 increases with temperature, from 1.33 × 10-4 µA cm-1 at 10 °C to 15.9 × 10-4 µA cm-2 at 62 °C. The presence of the injection barrier between Al and vdPTh is also supported by the flow of only a small electric current in the Al/vd-PTh/Al device (Figure 5b) compared with that in the Au/vd-PTh/Au device (Figure 5a). Such a symmetrical nonlinear correlation is often observed with inorganic diodes.40b It seems reasonable that PTh, which usually shows p-type electrically conducting properties,1-9 forms the ohmic contact with Au having a large work function (5.1 eV) and has a Schottky-type barrier against Al having a smaller work function (4.28 eV). All of the devices described above are stable during the measurement. The Al/vd-PTh/ITO device gives a similar rectification curve with the rectification ratio of 22 at (7 V. The Mg(Ag)/vd-PTh/ITO device show an electroluminescent property (peak position ) 600 nm).42 Other Devices. An Al/vd-PPP/Au (vd-PPP: 85 nm) device also shows a rectifying curve with a smaller rectification ratio of 14 at (7 V. Similar to the case of the Al/vd-PTh/Au device, a (-)Al/vd-PPP/Au(+) direction is the forward direction in accord with the p-type electrical conducting properties (Table 2) of PPP. Log i-V plot also gives a linear line, which gives an n value of 32.4 and an io of 1.0 × 10-3 µA cm-2 (eq 4). An Au/vd-PPP/Au device (vd-PPP: 85 nm) obeys the ohmic rule and gives an electrical conductivity (σ) of 1 × 10-6 S cm-1 for vd-PPP. The lower σ value of vd-PPP than that of vd-PTh may
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Figure 8. Structure of the field effect transistor using vd-PTh. Source, drain, and gate are Au coated on Ti film (Au ) 30 nm, Ti ) 10 nm). The gap between source and drain is 10 µm × 2 mm. The thickness of SiO2 is 300 nm, and the thickness of vd-PTh is 100 nm.
be attributed to the lower degree of natural p-doping (possibly related to abstraction of an electron of PPP by O2) of vd-PPP than that of vd-PTh due to the lower electron-donating ability of PPP than PTh as seen from the higher p-doping potential of PPP (Table 2). An Au/vd-PBpy/Au (vd-PBpy: 80 nm) device gives a linear i-V correlation from which the electrical conductivity of 4 × 10-7 S cm-1 is estimated for vd-PBpy. Field Effect Transistor. Recently, it has been reported43 that electrochemically prepared PTh and vd-oligothiophenes serve as active layers of field effect transistors, where its source-drain (or anode-cathode) electric current can be controlled by gate bias applied through an insulating layer (most commonly SiO2 layer; cf. Figure 8). The source-drain electric current of a field effect transistor prepared by using vd-PTh (Figure 8) also can be controlled by the gate bias as exhibited in Figure 9. The electrical conductivity of vd-PTh in the field effect transistor, which is roughly estimated from the electric current (about 2 × 10-8A) observed at a source-drain voltage of 10 V without application of gate bias (VG ) 0 V, Figure 9), is 1 order of magnitude (σ ≈ 1 × 10-6 S cm-1) smaller than that measured with the Au/vd-PTh/Au device (Vide ante). The difference between the two σ-values may be ascribed to the difference in the packing mode of the vd-PTh molecules along the direction of the electric current. As shown in Figure 9, the source-drain electric current increases on applying negative gate bias in accord with p-type conducting properties of PTh.1-9 However, the electric currentvoltage curve (in Figure 9) is considerably different from those of field effect transistors made of inorganic semiconductors40a and electrochemically prepared PTh or oligothiophene. Although such field effect transistors often show saturation of the source-drain electric current, such saturation is not observed with the present field effect transistor up to VSD of 30 V. Formation of a unique electronic state between SiO2 and vdPTh, which causes a different type of modulation of the sourcedrain electric current by the gate bias, is suggested. Conclusion and Scope Vacuum evaporation of linear rod-like π-conjugated PPP, PTh, and PBpy on various substrates gives thin films in which the π-conjugated poly(arylene)s are oriented perpendicularly to
Figure 9. An i-V curve for the field effect transistor shown in Figure 8 at room temperature. VSD is the source-drain voltage, iSD is the source-drain electric current, and VG is the gate bias.
the surface of the substrate or in parallel with the surface of the substrate. The degree of crystallinity of the vacuum-deposited film is considered to be controlled by the strength of the interaction between the π-conjugated poly(arylene) and the substrate. The vd-poly(arylene) films are active for the electrochemical doping and reveal basic electrochemical properties of PPP and PTh. The vd-PTh film forms an ohmic contact with Au. However, it has an injection barrier toward Al and the Al/vdPTh/Au device exhibits rectification of electric current. The field effect transistor made of the vd-PTh film gives a unique electric current-gate bias curve. The vacuum deposition technique provides new ways not only to reveal the basic properties of poly(arylene)s but also to expand the utility of poly(arylene)s. Acknowledgment. We are grateful to Dr. K. Sasaki (present position: Associate Professor at Kanazawa University) and Professors K. Tsutsui and S. Furukawa of our university for allowing us to use an apparatus (cf. Supporting Information) to obtain electric current-gate bias characteristics of the field effect transistor and for helpful discussion about the behavior of the transistor. The authors are indebted to Mr. R. Ooki of our university for measurement of the electron diffraction patterns of the vd-poly(arylene) films. Supporting Information Available: Experimental details of vacum deposition, preparation of devices, and i-V measurement of the Au/vd-PTh/Au device as well as IR spectra of vdPPP, -PTh, and -PBpy (3 pages). Ordering information is given on any current masthead page. References and Notes (1) Skotheim, T. A., Ed. Handbook of Conducting Polymers; Marcel Dekker: New York, 1986; Vols. I and II.
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