- Email: [email protected]
PSS as electrode
Synthetic Metals 139 (2003) 457–461
Flexible and transparent organic film speaker by using highly conducting PEDOT/PSS as electrode C.S. Lee a,b,d , J.Y. Kim a , D.E. Lee a , J. Joo a,∗ , B.G. Wagh c , S. Han d , Y.W. Beag d , S.K. Koh d a
Department of Physics and Center for Electro and Photo Responsive Molecules, Korea University, Seoul 136-701, South Korea b Thin Film Technology Research Center, Korea Institute of Science and Technology, Seoul 130-650, South Korea c K.K.W. College, Pimpalgaon (B), Tal. Niphad. Dist. Nasik (MS) 422 209, India d P&I Corporation, Seoul 131-221, South Korea Received 18 February 2003; received in revised form 14 April 2003; accepted 14 April 2003
Abstract Flexible organic film speaker (FOFS) was fabricated with ion-assisted-reaction (IAR) treated poly(vinylidene fluoide) (PVDF) as active layer and poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) (PEDOT/PSS), indium tin oxide (ITO), or copper (Cu) as electrode. The PEDOT/PSS materials were screen printed on the PVDF surface. The dc conductivity (σ dc ) of pristine PEDOT/PSS increases from 0.8 to ∼80 S/cm by adding various organic solvents. The sound pressure level (SPL) of PVDF-based FOFS with a highly conducting PEDOT/PSS (dimethyl sulfoxide, DMSO) electrode is 27 dB higher than that with a low conducting PEDOT/PSS (H2 O) at 1 kHz. The highly conducting PEDOT/PSS (DMSO) as electrode is more suitable to PVDF-based FOFS compared to Cu or ITO because of the stable and more flat SPL, the flexibility, easy patterning, and coating process. © 2003 Elsevier Science B.V. All rights reserved. Keywords: PEDOT/PSS; PVDF; Sound pressure level
1. Introduction Conducting poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) (PEDOT/PSS) material has been widely used in applications of electrodes of capacitors and photodiodes [1], antistatic coating [2], electrochromic windows [3], field effect transistors [4], and hole transport material, because of optically transparent and well spin-coated properties [5,6]. The increase and control of conductivity of PEDOT/PSS samples through adding organic solvents or melting solvents provides various uses for commercial applications such as transparent and flexible electrode. Since the discovery of the piezoelectric property of poled poly(vinylidene fluoide) [PVDF, (CH2 -CF2 )n ] in 1969 [7], electrical and structural properties of PVDF materials have been studied for commercial applications [8–10]. Ferroelectric PVDF is a semi-crystalline polymer with a degree of crystallinity of ∼50%. Stretched PVDF has the dipole moment of CF2 molecules aligned normal to the surface of the film after poling. The piezoelectric and pyro-
∗ Corresponding author. Tel.: +82-2-3290-3103; fax: +82-2-927-3292. E-mail address: [email protected] (J. Joo).
electric response occurs from dimensional changes of molecules in PVDF [11]. PVDF has advantages such as low density, mechanical flexibility, thermal stability, and high dielectric constant [12,13]. In order to improve the stability and reliability of electrical properties of PVDF for commercial applications, various derivatives and copolymers of PVDF materials such as PVDF-trifluoroethylene (TrFE) [14], PVDF-tetrafluoroethylene (TeFE) [15], and PVDF-conducting polymers [16–18] have been studied. Active control of sound radiations from PVDF volume displacement sensor has been reported earlier [19]. Acoustic transducers such as microphones, tweeters, and headphones using the piezoelectric constant were produced in 1975 [20]. Lotton et al. proposed the model to describe the dynamic behavior of laterally radiating piezoelectric loudspeakers [21]. In this study, we fabricated flexible organic film speakers (FOFSs) with ion-assisted-reaction (IAR) treated PVDF as active layer and PEDOT/PSS materials with various organic solvents, ITO, or Cu as electrode. In audible band of 20 Hz to 20 kHz, the acoustic properties of PVDF-based FOFS with various electrodes were measured. We observed that the sound pressure level (SPL) of PVDF-based FOFS with highly conducting PEDOT/PSS electrode is more stable than
0379-6779/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0379-6779(03)00199-1
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that with ITO or Cu electrode, and is higher than that of lower conducting PEDOT/PSS electrode.
2. Experimental The  phase of the PVDF was purchased from Kureha (Japan). The thickness of the PVDF film was ∼45 m and was cut into various sizes, such as 50 mm × 100 mm, 100 mm × 100 mm, 300 mm × 100 mm, and 150 mm × 150 mm. The film was cleaned in a solution of ethyl alcohol and de-ionized water. Since the highly hydrophobic surface of PVDF showed poor adhesion with other materials, the films were modified by IAR before the fabrication of film speaker. A cold hollow cathode type ion source was used for the IAR treatment. The argon (Ar) ion energy and blowing oxygen (O2 ) gas rate for IAR treatment was 1 keV and 8 standard cubic centimeter per minute (sccm), respectively. The Ar ion dose varied from 5 × 1014 to 1 × 1017 ions/cm2 . The base pressure inside chamber was 2.5 × 10−5 Torr and the working pressure was ∼1.0 × 10−4 Torr during the IAR treatment [22]. The wetting angles of the samples with water and formamide were measured by a contact anglometer (Tantec— CAM Micro). Surface free energy of the sample was calculated by Young’s formula. The X-ray photoelectron spectroscopy (XPS, Perkin-Elmer PHI5700 ESCA) measurements were performed in order to analyze the chemical bindings on the IAR-treated PVDF surface. PEDOT/PSS (Baytron P, Bayer) samples with various organic solvents were screen printed on the IAR-treated PVDF surface. In order to obtain highly conducting PEDOT/PSS samples, various organic solvents such as dimethyl sulfoxide (DMSO), N,N-dimethyl formamide (DMF), and tetrahydrofuran (THF) were added to the PEDOT/PSS (H2 O) solution and stirred for 8 h. The detailed process for the synthesis of PEDOT/PSS with organic solvents was reported earlier [23]. For metal electrodes, Cu was thermally evaporated and ITO was sputtered onto the PVDF film. The SPL of PVDF-based FOFS was measured by using an audio analyzer (B&K 2012). To eliminate interference between forward and backward sound from the speaker, an enclosing box made by wood was used, and a sound absorber was filled in the enclosure. The SPL of the speaker was measured in an anechoic room, and the distance from the speaker to microphone was 1 m. Because sound absorption materials were attached to the walls of the room, there was no reflection sound during the measurement. The width, length, and height of the room were 2, 3, and 1.8 m, respectively.
3. Results and discussion The surface of PVDF film was modified with 1 keV Ar ions without O2 gas flow and with O2 gas flow at the rate of 8 sccm. The wetting angle of de-ionized water on the surface
Fig. 1. (a) Variation of total surface energy, dispersion energy, and polar energy of the surface of PVDF film modified by 1 keV Ar ions with O2 gas flow (8 sccm) as a function of ion dose. (b) Comparison of XPS C1s core level spectra of pristine and 1 keV Ar ion beam treated PVDF samples in an O2 environment at the ion dose of 5 × 1014 , 1 × 1015 , 1 × 1016 , and 1 × 1017 ions/cm2 .
of pristine PVDF was 75◦ . When the Ar ion beam was irradiated on the PVDF surface without O2 gas environment, the wetting angle decreased to 51◦ . The wetting angle was minimum at the ion dose of 1 × 1015 ions/cm2 . When Ar ions were irradiated on surface of PVDF sample with an O2 gas environment, the minimum value of the wetting angle was 31◦ at the ion dose of 1 × 1015 ions/cm2 . Fig. 1(a) shows the variation of surface free energy of pristine and IAR-treated PVDF films as a function of ion dose. For pristine PVDF samples, the dispersion energy, the polar energy, and the total surface energy is 33, 3, and 36 erg/cm2 , respectively. After the IAR treatment, the surface energy increases up to 64 erg/cm2 at the ion dose of 1×1015 ions/cm2 . The polar force mainly contributes to the increase of the surface energy, while the contribution of dispersion force is relatively weak, as shown in Fig. 1(a). Fig. 1(b) compares carbon 1s (C1s) core level spectra on the surface of pristine and IAR-treated PVDF samples based on XPS experiments. The XPS spectra show that new chemical bonds are formed on the surface due to the IAR treatment in ambient O2 gas. For pristine PVDF samples, the –CF2 – and –CH2 – main peaks appear at 290.8 and 286.2 eV, respectively, which were reported as typical binding energies of the C1s core level of the PVDF samples [24]. After the IAR treatment
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Table 1 The dc conductivity (σ dc ) of the PEDOT/PSS film at room temperature prepared from various organic solvents (volume ratio of PEDOT/PSS:solvent = 3:1) [23] σ dc (S/cm)
Materials PEDOT/PSS PEDOT/PSS PEDOT/PSS PEDOT/PSS
Fig. 2. Photographs of adhesion test between PEDOT/PSS and (a) pristine PVDF sample and (b) IAR-treated PVDF sample.
on the PVDF samples, the hydrophilic groups containing oxygen such as –C–O–, –(C=O)–, and –(C=O)–O– are observed, which induce the increase of surface energy and the adhesion strength on the PVDF surface. The main peaks of the IAR-treated PVDF samples with O2 gas vary with the ion dose, as shown in Fig. 1(b). The intensity of the –CF2 – peak decreases with increasing ion doses from 5 × 1014 to 1 × 1017 ions/cm2 . This result implies that F atoms and its related bindings were sputtered selectively [25]. The intensity of the –CH2 – peak decreases with increasing the ion dose. However, the intensity of the –C–C– peak due to carbonization increases as the ion dose increases. Fig. 2 show photographs for adhesion test by using 3 M scotch tape for the pristine and IAR-treated PVDF-based FOFS with PEDOT/PSS electrodes, respectively. For the pristine PVDF sample with the PEDOT/PSS electrode, the PEDOT/PSS layer is easily detached from the surface of PVDF active layer by the scotch tape as shown in Fig. 2(a), while for the IAR-treated PVDF film with the PEDOT/PSS electrode, the PEDOT/PSS layer does not detached from the PVDF film by the scotch tape as shown in Fig. 2(b). The hydrophilic surface of the PVDF containing –C–O–, –(C=O)–, and –(C=O)–O– through IAR treatment shows the enhanced adhesion between the PEDOT/PSS electrode and the PVDF active layer and high durability. In order to increase dc conductivity (σ dc ) of PEDOT/PSS, we used various organic solvents such as DMSO, DMF, and THF. The σ dc of PEDOT/PSS (DMSO) is 80 ± 30 S/cm, while that of pristine PEDOT/PSS (H2 O) is 0.8 ± 0.1 S/cm. The enhanced conductivity of PEDOT/PSS with various organic solvents as listed on Table 1 was explained by the screening effect between counter ions and charge carriers [23]. As the result, Coulomb interaction between PEDOT and PSS dopants is reduced, and the charge carriers can transport easily along main chain. The enhanced σ dc of PEDOT/PSS (DMSO) plays an important role for electrode of the speaker, because it provides relatively high induced current on active PVDF layer.
(pristine, H2 O) with THF with DMF with DMSO
0.8 4 30 80
± ± ± ±
0.1 1 10 30
Fig. 3(a) presents schematic diagram of PVDF-based FOFS. Fig. 3(b) shows photograph of the PVDF-based FOFS consisting of PEDOT/PSS (DMSO)|PVDF (IAR treatment)|PEDOT/PSS (DMSO). It is flexible and transparent. Fig. 3(c) shows the photograph of FOFS consisting of Cu|PVDF (IAR treatment)|Cu, which is not transparent and is semi-flexible. When the input voltage of 150 Vrms was applied to the speaker with thermally evaporated Cu electrodes on both side of pristine PVDF, the lifetime of the speaker was only several minutes.
Fig. 3. (a) Schematic diagram of PVDF-based FOFS and photographs of FOFS consisting of (b) PEDOT/PSS (DMSO)|PVDF (IAR treatment)|PEDOT/PSS (DMSO), and (c) Cu|PVDF (IAR treatment)|Cu.
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Fig. 4. Frequency response of SPL of PVDF-based FOFS with different conducting PEDOT/PSS electrodes prepared by using DMSO, THF, and H2 O solvents.
Fig. 5. Frequency response of sound pressure level (SPL) of PVDF-based FOFS with various electrodes [Cu, ITO, and PEDOT/PSS (DMSO)]. Inset: magnification of the SPL (1.5 kHz ≤ f ≤ 10 kHz).
The SPL is defined as following equation in the decibel (dB) unit [26]: P2 I SPL (dB) = 10 log10 = 10 log10 2 Iref Pref P = 20 log10 , Pref
SPL of the speakers with ITO or Cu electrode is higher than that with the PEDOT/PSS (DMSO) one because of the high conductivity of the Cu and ITO electrodes. However, the SPL of PDVF-based FOFS with PEDOT/PSS (DMSO) electrode shows a more flat and stable frequency response over 400 Hz compared to that with ITO or Cu electrode, implying a better quality of speaker. For example, from 1.5 to 10 kHz, the differences between the maximum and minimum SPLs of the speakers with Cu and PEDOT/PSS (DMSO) electrodes are 22 and 12 dB, respectively. The distribution of the SPL of the speakers with PEDOT/PSS (DMSO) and ITO electrodes is 75.1±3.2 and 85.6±4.8 dB, respectively, in the frequency range from 1.5 to 10 kHz. The flat and stable frequency response of the SPL results from a moderately induced current and the mechanical flexibility of the PEDOT (DMSO) electrode compared to the metal electrodes and hydrophilic property on the active PVDF layer. It is noted that in the case of an ITO electrode, highly induced current for high SPL causes an easy breakdown of the speaker. Fig. 6 presents the induced current as a function of frequency for the speakers with various electrodes. The applied input voltage was 100 Vrms . Because the size of the speaker was the same, the resonance frequency ( fc ) of the induced current is the same at 4 kHz. We observe that the induced current at fc increases with increasing the conductivity of electrodes as shown in Fig. 6. The increase of the SPL of the speaker with Cu or ITO electrode (1.5–10 kHz) in the results of Fig. 5 agrees with the result
where I and P is the sound intensity and sound pressure, respectively. The sound intensity is proportional to the square of sound pressure. The standard threshold of the hearing intensity and pressure is Iref = 10−12 W/m2 and Pref = 20 Pa, respectively. Fig. 4 shows the frequency response of the SPL of PVDF-based FOFS (100 mm×100 mm) with a PEDOT/PSS electrode. The radius of curvature of the FOFS was 200 mm. Between 100 Hz and 10 kHz, the SPL of PVDF-based FOFS with a highly conducting PEDOT/PSS (DMSO) electrode is relatively higher than that with low conducting PEDOT/PSS (THF) or pristine PEDOT/PSS (H2 O) electrode. For example, in the frequency range from 1 to 10 kHz, the average value of SPL of the speaker with a highly conducting PEDOT/PSS (DMSO) electrode is 80 dB, while that with a low conducting PEDOT/PSS (H2 O) electrode is 44 dB. We observe that the SPL of the speaker with a PEDOT/PSS electrode increases as σ dc of the PEDOT/PSS electrode increases. The increase of the SPL of the systems results from the relatively highly induced current on the active PVDF layer due to the highly conducting electrode. The SPL spectra of the speaker as a function of frequency show different trends because of a different thickness of the electrode. The thickness of the PEDOT/PSS (DMSO), PEDOT/PSS (THF), and pristine PEDOT/PSS was 1500, 2000, and 2000 Å, respectively. The frequency responses of the SPL of PVDF-based FOFS (150 mm × 150 mm) with Cu, ITO, and PEDOT/PSS (DMSO) electrodes are compared in Fig. 5. The inset of Fig. 5 shows the magnified SPL spectra in the frequency range from 1.5 to 10 kHz. The applied voltage across the PVDF film was 100 Vrms . The radius of curvature of all the speakers was 400 mm. Below 1 kHz, all PVDF-based FOFS with different electrodes show a similar frequency response of the SPL. In the frequency range from 1.5 to 10 kHz, the
Fig. 6. Frequency response of total induced current of PVDF-based FOFS with various electrodes. Inset: resonance frequency (fc ) as a function of the size of the speaker.
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as shown in Fig. 6. The results imply that a higher conducting electrode induces higher induced current. The inset of Fig. 6 shows the variation of fc as a function of size of the speaker. With an increase in size of the speaker, the fc decreases. We also observe the increase of the SPL as the area of the speaker increases. The induced current and its resonance frequency of the speaker by an applied electric field are directly related to the vibration of molecules in PVDF. 4. Conclusion We fabricated flexible and transparent speaker by using IAR-treated PVDF as active layer and highly conducting PEDOT/PSS (DMSO) as electrode. The IAR treatment on the PVDF film revealed high adhesion strength between the PVDF and various electrodes. The PVDF-based FOFS with a PEDOT/PSS (DMSO) electrode showed flat and stable SPL over 400 Hz compared to that with the Cu or ITO electrode. This was due to the moderately induced current and mechanical flexibility of the PEDOT/PSS (DMSO) electrode and hydrophilic property of IAR-treated PVDF layer. The SPL of PVDF-based FOFS with PEDOT/PSS (DMSO, THF, or H2 O) increased with an increased σ dc of the electrode in audible band. Acknowledgements This work was supported in part by P&I Corporation, the CRM-KOSEF, the project of Industrial Use of Ion Beam (KISTEP), and the Brain Korea 21. One can compare the sound of various PVDF-based FOFSs from the following authors’ group website: http:// smartpolymer.korea.ac.kr.
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References [1] L. Groenendaal, F. Jonas, D. Freitag, H. Pielartzik, J.R. Reynolds, Adv. Mater. 12 (2000) 481. [2] A. Dkhissi, F. Louwet, L. Groenendaal, D. Beljonne, R. Lazzaroni, J.L. Bredas, Chem. Phys. Lett. 359 (2002) 466. [3] H.W. Heuer, R. Wehrmann, S. Kirchmeyer, Adv. Funct. Mater. 12 (2002) 89. [4] A.J. Epstein, F.-Ch. Hsu, N.-R. Chiou, V.N. Prigodin, Curr. Appl. Phys. 2 (2002) 339. [5] Q. Pei, G. Zuccafrello, M. Ahlskog, O. Inganas, Polymer 35 (1994) 1347. [6] F. Jonas, T. Morrison, Synth. Met. 85 (1997) 1397. [7] H. Kawai, Jpn. J. Appl. Phys. 8 (1969) 975. [8] D. Setiadi, H. Weller, T.D. Binnie, Sens. Actuators 76 (1999) 145. [9] C. Chee, L. Tong, G. Steven, Comput. Struct. 77 (2000) 253. [10] R. Bhatia, P.A. Lewin, Q. Zhang, Ultrasonics 34 (1996) 567. [11] D. Broussoux, F. Micheron, J. Appl. Phys. 51 (1980) 2020. [12] R. Spohr, Ion Tracks and Microtechnology, Vieweg, 1990. [13] W. Hollander, W. Dunkhorst, R. Brant, P. Vater, Ferroelectrics 115 (1991) 209. [14] M. Dali-Ali, Ph. Benech, J.P. Perrier, Ultrasonics 33 (1995) 321. [15] J. Datta, A.K. Nandi, Polymer 37 (1996) 5179. [16] L.H.C. Mattoso, L.F. Malmonge, Polymer 40 (1999) 513. [17] G. Boiteux, A. Ho-Hoang, F. Fache, M. Lemaire, I. Glowaski, J. Ulanski, Synth. Met. 69 (1995) 487. [18] B. Xu, Y. Ovchenkov, M. Bai, A.N. Caruso, A.V. Sorokin, S. Ducharme, Appl. Phys. Lett. 81 (2002) 4281. [19] F. Charette, A. Berry, C. Guigou, J. Acoust. Soc. Am. 103 (3) (1998) 1493. [20] M. Tamura, T. Yamaguchi, T. Oyaba, T. Yoshibi, J. Audio Eng. Soc. 23 (1975) 21. [21] P. Lotton, M. Bruneau, Z. Skvor, A.-M. Bruneau, Appl. Acoust. 58 (1999) 419. [22] S. Han, W.-K. Choi, K.H. Yoon, S.K. Koh, J. Appl. Polym. Sci. 72 (1999) 41. [23] J.Y. Kim, J.H. Jung, D.E. Lee, J. Joo, Synth. Met. 126 (2002) 311. [24] J.P. Duraud, A.L. Moel, Nucl. Instrum. Methods B 105 (1996) 71. [25] M.D. Duca, C.L. Plosceanu, T. Pop, Polym. Degrad. Stabil. 61 (1995) 65. [26] C.E. Speaks, Introduction to Sound: Acoustics for the Hearing and Speech Sciences, Chapman & Hall, London, 1992.
Flexible and transparent organic film speaker by using highly conducting PEDOT/PSS as electrode C.S. Lee a,b,d , J.Y. Kim a , D.E. Lee a , J. Joo a,∗ , B.G. Wagh c , S. Han d , Y.W. Beag d , S.K. Koh d a
Department of Physics and Center for Electro and Photo Responsive Molecules, Korea University, Seoul 136-701, South Korea b Thin Film Technology Research Center, Korea Institute of Science and Technology, Seoul 130-650, South Korea c K.K.W. College, Pimpalgaon (B), Tal. Niphad. Dist. Nasik (MS) 422 209, India d P&I Corporation, Seoul 131-221, South Korea Received 18 February 2003; received in revised form 14 April 2003; accepted 14 April 2003
Abstract Flexible organic film speaker (FOFS) was fabricated with ion-assisted-reaction (IAR) treated poly(vinylidene fluoide) (PVDF) as active layer and poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) (PEDOT/PSS), indium tin oxide (ITO), or copper (Cu) as electrode. The PEDOT/PSS materials were screen printed on the PVDF surface. The dc conductivity (σ dc ) of pristine PEDOT/PSS increases from 0.8 to ∼80 S/cm by adding various organic solvents. The sound pressure level (SPL) of PVDF-based FOFS with a highly conducting PEDOT/PSS (dimethyl sulfoxide, DMSO) electrode is 27 dB higher than that with a low conducting PEDOT/PSS (H2 O) at 1 kHz. The highly conducting PEDOT/PSS (DMSO) as electrode is more suitable to PVDF-based FOFS compared to Cu or ITO because of the stable and more flat SPL, the flexibility, easy patterning, and coating process. © 2003 Elsevier Science B.V. All rights reserved. Keywords: PEDOT/PSS; PVDF; Sound pressure level
1. Introduction Conducting poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) (PEDOT/PSS) material has been widely used in applications of electrodes of capacitors and photodiodes [1], antistatic coating [2], electrochromic windows [3], field effect transistors [4], and hole transport material, because of optically transparent and well spin-coated properties [5,6]. The increase and control of conductivity of PEDOT/PSS samples through adding organic solvents or melting solvents provides various uses for commercial applications such as transparent and flexible electrode. Since the discovery of the piezoelectric property of poled poly(vinylidene fluoide) [PVDF, (CH2 -CF2 )n ] in 1969 [7], electrical and structural properties of PVDF materials have been studied for commercial applications [8–10]. Ferroelectric PVDF is a semi-crystalline polymer with a degree of crystallinity of ∼50%. Stretched PVDF has the dipole moment of CF2 molecules aligned normal to the surface of the film after poling. The piezoelectric and pyro-
∗ Corresponding author. Tel.: +82-2-3290-3103; fax: +82-2-927-3292. E-mail address: [email protected] (J. Joo).
electric response occurs from dimensional changes of molecules in PVDF [11]. PVDF has advantages such as low density, mechanical flexibility, thermal stability, and high dielectric constant [12,13]. In order to improve the stability and reliability of electrical properties of PVDF for commercial applications, various derivatives and copolymers of PVDF materials such as PVDF-trifluoroethylene (TrFE) [14], PVDF-tetrafluoroethylene (TeFE) [15], and PVDF-conducting polymers [16–18] have been studied. Active control of sound radiations from PVDF volume displacement sensor has been reported earlier [19]. Acoustic transducers such as microphones, tweeters, and headphones using the piezoelectric constant were produced in 1975 [20]. Lotton et al. proposed the model to describe the dynamic behavior of laterally radiating piezoelectric loudspeakers [21]. In this study, we fabricated flexible organic film speakers (FOFSs) with ion-assisted-reaction (IAR) treated PVDF as active layer and PEDOT/PSS materials with various organic solvents, ITO, or Cu as electrode. In audible band of 20 Hz to 20 kHz, the acoustic properties of PVDF-based FOFS with various electrodes were measured. We observed that the sound pressure level (SPL) of PVDF-based FOFS with highly conducting PEDOT/PSS electrode is more stable than
0379-6779/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0379-6779(03)00199-1
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that with ITO or Cu electrode, and is higher than that of lower conducting PEDOT/PSS electrode.
2. Experimental The  phase of the PVDF was purchased from Kureha (Japan). The thickness of the PVDF film was ∼45 m and was cut into various sizes, such as 50 mm × 100 mm, 100 mm × 100 mm, 300 mm × 100 mm, and 150 mm × 150 mm. The film was cleaned in a solution of ethyl alcohol and de-ionized water. Since the highly hydrophobic surface of PVDF showed poor adhesion with other materials, the films were modified by IAR before the fabrication of film speaker. A cold hollow cathode type ion source was used for the IAR treatment. The argon (Ar) ion energy and blowing oxygen (O2 ) gas rate for IAR treatment was 1 keV and 8 standard cubic centimeter per minute (sccm), respectively. The Ar ion dose varied from 5 × 1014 to 1 × 1017 ions/cm2 . The base pressure inside chamber was 2.5 × 10−5 Torr and the working pressure was ∼1.0 × 10−4 Torr during the IAR treatment [22]. The wetting angles of the samples with water and formamide were measured by a contact anglometer (Tantec— CAM Micro). Surface free energy of the sample was calculated by Young’s formula. The X-ray photoelectron spectroscopy (XPS, Perkin-Elmer PHI5700 ESCA) measurements were performed in order to analyze the chemical bindings on the IAR-treated PVDF surface. PEDOT/PSS (Baytron P, Bayer) samples with various organic solvents were screen printed on the IAR-treated PVDF surface. In order to obtain highly conducting PEDOT/PSS samples, various organic solvents such as dimethyl sulfoxide (DMSO), N,N-dimethyl formamide (DMF), and tetrahydrofuran (THF) were added to the PEDOT/PSS (H2 O) solution and stirred for 8 h. The detailed process for the synthesis of PEDOT/PSS with organic solvents was reported earlier [23]. For metal electrodes, Cu was thermally evaporated and ITO was sputtered onto the PVDF film. The SPL of PVDF-based FOFS was measured by using an audio analyzer (B&K 2012). To eliminate interference between forward and backward sound from the speaker, an enclosing box made by wood was used, and a sound absorber was filled in the enclosure. The SPL of the speaker was measured in an anechoic room, and the distance from the speaker to microphone was 1 m. Because sound absorption materials were attached to the walls of the room, there was no reflection sound during the measurement. The width, length, and height of the room were 2, 3, and 1.8 m, respectively.
3. Results and discussion The surface of PVDF film was modified with 1 keV Ar ions without O2 gas flow and with O2 gas flow at the rate of 8 sccm. The wetting angle of de-ionized water on the surface
Fig. 1. (a) Variation of total surface energy, dispersion energy, and polar energy of the surface of PVDF film modified by 1 keV Ar ions with O2 gas flow (8 sccm) as a function of ion dose. (b) Comparison of XPS C1s core level spectra of pristine and 1 keV Ar ion beam treated PVDF samples in an O2 environment at the ion dose of 5 × 1014 , 1 × 1015 , 1 × 1016 , and 1 × 1017 ions/cm2 .
of pristine PVDF was 75◦ . When the Ar ion beam was irradiated on the PVDF surface without O2 gas environment, the wetting angle decreased to 51◦ . The wetting angle was minimum at the ion dose of 1 × 1015 ions/cm2 . When Ar ions were irradiated on surface of PVDF sample with an O2 gas environment, the minimum value of the wetting angle was 31◦ at the ion dose of 1 × 1015 ions/cm2 . Fig. 1(a) shows the variation of surface free energy of pristine and IAR-treated PVDF films as a function of ion dose. For pristine PVDF samples, the dispersion energy, the polar energy, and the total surface energy is 33, 3, and 36 erg/cm2 , respectively. After the IAR treatment, the surface energy increases up to 64 erg/cm2 at the ion dose of 1×1015 ions/cm2 . The polar force mainly contributes to the increase of the surface energy, while the contribution of dispersion force is relatively weak, as shown in Fig. 1(a). Fig. 1(b) compares carbon 1s (C1s) core level spectra on the surface of pristine and IAR-treated PVDF samples based on XPS experiments. The XPS spectra show that new chemical bonds are formed on the surface due to the IAR treatment in ambient O2 gas. For pristine PVDF samples, the –CF2 – and –CH2 – main peaks appear at 290.8 and 286.2 eV, respectively, which were reported as typical binding energies of the C1s core level of the PVDF samples [24]. After the IAR treatment
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Table 1 The dc conductivity (σ dc ) of the PEDOT/PSS film at room temperature prepared from various organic solvents (volume ratio of PEDOT/PSS:solvent = 3:1) [23] σ dc (S/cm)
Materials PEDOT/PSS PEDOT/PSS PEDOT/PSS PEDOT/PSS
Fig. 2. Photographs of adhesion test between PEDOT/PSS and (a) pristine PVDF sample and (b) IAR-treated PVDF sample.
on the PVDF samples, the hydrophilic groups containing oxygen such as –C–O–, –(C=O)–, and –(C=O)–O– are observed, which induce the increase of surface energy and the adhesion strength on the PVDF surface. The main peaks of the IAR-treated PVDF samples with O2 gas vary with the ion dose, as shown in Fig. 1(b). The intensity of the –CF2 – peak decreases with increasing ion doses from 5 × 1014 to 1 × 1017 ions/cm2 . This result implies that F atoms and its related bindings were sputtered selectively [25]. The intensity of the –CH2 – peak decreases with increasing the ion dose. However, the intensity of the –C–C– peak due to carbonization increases as the ion dose increases. Fig. 2 show photographs for adhesion test by using 3 M scotch tape for the pristine and IAR-treated PVDF-based FOFS with PEDOT/PSS electrodes, respectively. For the pristine PVDF sample with the PEDOT/PSS electrode, the PEDOT/PSS layer is easily detached from the surface of PVDF active layer by the scotch tape as shown in Fig. 2(a), while for the IAR-treated PVDF film with the PEDOT/PSS electrode, the PEDOT/PSS layer does not detached from the PVDF film by the scotch tape as shown in Fig. 2(b). The hydrophilic surface of the PVDF containing –C–O–, –(C=O)–, and –(C=O)–O– through IAR treatment shows the enhanced adhesion between the PEDOT/PSS electrode and the PVDF active layer and high durability. In order to increase dc conductivity (σ dc ) of PEDOT/PSS, we used various organic solvents such as DMSO, DMF, and THF. The σ dc of PEDOT/PSS (DMSO) is 80 ± 30 S/cm, while that of pristine PEDOT/PSS (H2 O) is 0.8 ± 0.1 S/cm. The enhanced conductivity of PEDOT/PSS with various organic solvents as listed on Table 1 was explained by the screening effect between counter ions and charge carriers [23]. As the result, Coulomb interaction between PEDOT and PSS dopants is reduced, and the charge carriers can transport easily along main chain. The enhanced σ dc of PEDOT/PSS (DMSO) plays an important role for electrode of the speaker, because it provides relatively high induced current on active PVDF layer.
(pristine, H2 O) with THF with DMF with DMSO
0.8 4 30 80
± ± ± ±
0.1 1 10 30
Fig. 3(a) presents schematic diagram of PVDF-based FOFS. Fig. 3(b) shows photograph of the PVDF-based FOFS consisting of PEDOT/PSS (DMSO)|PVDF (IAR treatment)|PEDOT/PSS (DMSO). It is flexible and transparent. Fig. 3(c) shows the photograph of FOFS consisting of Cu|PVDF (IAR treatment)|Cu, which is not transparent and is semi-flexible. When the input voltage of 150 Vrms was applied to the speaker with thermally evaporated Cu electrodes on both side of pristine PVDF, the lifetime of the speaker was only several minutes.
Fig. 3. (a) Schematic diagram of PVDF-based FOFS and photographs of FOFS consisting of (b) PEDOT/PSS (DMSO)|PVDF (IAR treatment)|PEDOT/PSS (DMSO), and (c) Cu|PVDF (IAR treatment)|Cu.
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Fig. 4. Frequency response of SPL of PVDF-based FOFS with different conducting PEDOT/PSS electrodes prepared by using DMSO, THF, and H2 O solvents.
Fig. 5. Frequency response of sound pressure level (SPL) of PVDF-based FOFS with various electrodes [Cu, ITO, and PEDOT/PSS (DMSO)]. Inset: magnification of the SPL (1.5 kHz ≤ f ≤ 10 kHz).
The SPL is defined as following equation in the decibel (dB) unit [26]: P2 I SPL (dB) = 10 log10 = 10 log10 2 Iref Pref P = 20 log10 , Pref
SPL of the speakers with ITO or Cu electrode is higher than that with the PEDOT/PSS (DMSO) one because of the high conductivity of the Cu and ITO electrodes. However, the SPL of PDVF-based FOFS with PEDOT/PSS (DMSO) electrode shows a more flat and stable frequency response over 400 Hz compared to that with ITO or Cu electrode, implying a better quality of speaker. For example, from 1.5 to 10 kHz, the differences between the maximum and minimum SPLs of the speakers with Cu and PEDOT/PSS (DMSO) electrodes are 22 and 12 dB, respectively. The distribution of the SPL of the speakers with PEDOT/PSS (DMSO) and ITO electrodes is 75.1±3.2 and 85.6±4.8 dB, respectively, in the frequency range from 1.5 to 10 kHz. The flat and stable frequency response of the SPL results from a moderately induced current and the mechanical flexibility of the PEDOT (DMSO) electrode compared to the metal electrodes and hydrophilic property on the active PVDF layer. It is noted that in the case of an ITO electrode, highly induced current for high SPL causes an easy breakdown of the speaker. Fig. 6 presents the induced current as a function of frequency for the speakers with various electrodes. The applied input voltage was 100 Vrms . Because the size of the speaker was the same, the resonance frequency ( fc ) of the induced current is the same at 4 kHz. We observe that the induced current at fc increases with increasing the conductivity of electrodes as shown in Fig. 6. The increase of the SPL of the speaker with Cu or ITO electrode (1.5–10 kHz) in the results of Fig. 5 agrees with the result
where I and P is the sound intensity and sound pressure, respectively. The sound intensity is proportional to the square of sound pressure. The standard threshold of the hearing intensity and pressure is Iref = 10−12 W/m2 and Pref = 20 Pa, respectively. Fig. 4 shows the frequency response of the SPL of PVDF-based FOFS (100 mm×100 mm) with a PEDOT/PSS electrode. The radius of curvature of the FOFS was 200 mm. Between 100 Hz and 10 kHz, the SPL of PVDF-based FOFS with a highly conducting PEDOT/PSS (DMSO) electrode is relatively higher than that with low conducting PEDOT/PSS (THF) or pristine PEDOT/PSS (H2 O) electrode. For example, in the frequency range from 1 to 10 kHz, the average value of SPL of the speaker with a highly conducting PEDOT/PSS (DMSO) electrode is 80 dB, while that with a low conducting PEDOT/PSS (H2 O) electrode is 44 dB. We observe that the SPL of the speaker with a PEDOT/PSS electrode increases as σ dc of the PEDOT/PSS electrode increases. The increase of the SPL of the systems results from the relatively highly induced current on the active PVDF layer due to the highly conducting electrode. The SPL spectra of the speaker as a function of frequency show different trends because of a different thickness of the electrode. The thickness of the PEDOT/PSS (DMSO), PEDOT/PSS (THF), and pristine PEDOT/PSS was 1500, 2000, and 2000 Å, respectively. The frequency responses of the SPL of PVDF-based FOFS (150 mm × 150 mm) with Cu, ITO, and PEDOT/PSS (DMSO) electrodes are compared in Fig. 5. The inset of Fig. 5 shows the magnified SPL spectra in the frequency range from 1.5 to 10 kHz. The applied voltage across the PVDF film was 100 Vrms . The radius of curvature of all the speakers was 400 mm. Below 1 kHz, all PVDF-based FOFS with different electrodes show a similar frequency response of the SPL. In the frequency range from 1.5 to 10 kHz, the
Fig. 6. Frequency response of total induced current of PVDF-based FOFS with various electrodes. Inset: resonance frequency (fc ) as a function of the size of the speaker.
C.S. Lee et al. / Synthetic Metals 139 (2003) 457–461
as shown in Fig. 6. The results imply that a higher conducting electrode induces higher induced current. The inset of Fig. 6 shows the variation of fc as a function of size of the speaker. With an increase in size of the speaker, the fc decreases. We also observe the increase of the SPL as the area of the speaker increases. The induced current and its resonance frequency of the speaker by an applied electric field are directly related to the vibration of molecules in PVDF. 4. Conclusion We fabricated flexible and transparent speaker by using IAR-treated PVDF as active layer and highly conducting PEDOT/PSS (DMSO) as electrode. The IAR treatment on the PVDF film revealed high adhesion strength between the PVDF and various electrodes. The PVDF-based FOFS with a PEDOT/PSS (DMSO) electrode showed flat and stable SPL over 400 Hz compared to that with the Cu or ITO electrode. This was due to the moderately induced current and mechanical flexibility of the PEDOT/PSS (DMSO) electrode and hydrophilic property of IAR-treated PVDF layer. The SPL of PVDF-based FOFS with PEDOT/PSS (DMSO, THF, or H2 O) increased with an increased σ dc of the electrode in audible band. Acknowledgements This work was supported in part by P&I Corporation, the CRM-KOSEF, the project of Industrial Use of Ion Beam (KISTEP), and the Brain Korea 21. One can compare the sound of various PVDF-based FOFSs from the following authors’ group website: http:// smartpolymer.korea.ac.kr.
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