Fully mass printed loudspeakers on paper

Organic Electronics 13 (2012) 2290–2295

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Fully mass printed loudspeakers on paper Arved C. Hübler a, Maxi Bellmann a, Georg C. Schmidt a,⇑, Stefan Zimmermann b, André Gerlach b, Christian Haentjes b a

Institute for Print and Media Technology, Chemnitz University of Technology, Reichenhainer Str. 70, 09126 Chemnitz, Germany Robert Bosch GmbH, Corporate Sector Research and Advance Engineering, Department Structural and Contact Dynamics – Acoustics (CR/ARU1), P.O. Box 10 60 50, 70049 Stuttgart, Germany

b

a r t i c l e

i n f o

Article history: Received 24 May 2012 Received in revised form 25 June 2012 Accepted 27 June 2012 Available online 16 July 2012 Keywords: Printed electronics Paper Loudspeaker P(VDF-TrFE) PEDOT:PSS Flexography

a b s t r a c t We report on the first fully mass printed large-area piezoelectric loudspeakers on paper. All functional layers were printed by means of flexography, alternatively screen and stencil printing, on conventional paper without any surface modification. We used polymers for the electrodes (poly(3,4-ethylendioxythiophene)/poly(4-styrenesulfonate, PEDOT:PSS)) as well as the piezoelectric layer (poly(vinylidene fluoride-trifluorethylene), P(VDF-TrFE)). It could be demonstrated that low-cost devices for mass markets can be realized successfully with the developed technology and process. Besides technical challenges, electrical and acoustic properties of printed speakers are investigated, taking the mechanical properties of the substrate and size of the active piezoelectric area into account. A sound pressure level up to 80 dB could be achieved. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Paper becomes more and more popular in the field of printed electronics besides its traditional usage as substrate in the graphic arts industry. Recent progress in the development of electronic devices on paper substrates was summarized in a review published by Tobjörk et al. [1]. Since the material has a much lower price than polymeric substrates, like poly(ethylene terephthalate) (PET) foil, it is especially interesting for low-cost applications. Furthermore, it is environmentally friendly and recyclable. Hence, in combination with mass printing technologies as highly productive methods for the realization of functional layers, it is a promising candidate for the production of cheap electronic devices with unique properties [2–7]. Nevertheless, paper as substrate is an extremely challenging basis because of its very rough and porous surface.

⇑ Corresponding author. Tel.: +49 371 531 35175; fax: +49 371 531 835175. E-mail address: [email protected] (G.C. Schmidt). 1566-1199/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.orgel.2012.06.048

Besides traditional loudspeaker concepts with magnetic stimulated vibration of a diaphragm, distributed mode loudspeakers (DML) and electrostatic loudspeakers, research was focused on flexible thin film in the last years. Often these concepts use electro active polymers (EAP) [8], for example dielectric EAPs in electrostatic speakers [9] and electrostrictive polymers [10]. Another type of thin film loudspeakers used the thermoacoustic effect in carbon nanotube (CNT) films [11]. A further approach made use of ferroelectric polymers to realize thin film piezoelectric loudspeakers [12]. In this case the piezoelectric material deforms according to an applied field strength in direction of x-, y- and z-axis and the whole loudspeaker follows this motion and interacts as diaphragm. In many cases piezoelectric loudspeakers are made of poly(vinylidene fluoride) (PVDF), provided as foil [12]. Such PVDF foils are available in different thicknesses, but they are very expensive due to the complex production process. Furthermore, it is difficult to print on top of these hydrophobic foils. Therefore, Lee et al. treated the surface of PVDF foil with an ion-assisted-reaction (IAR) and tested different electrode materials, like indium tin oxide (ITO) and screen printed

A.C. Hübler et al. / Organic Electronics 13 (2012) 2290–2295

poly(3,4-ethylendioxythiophene)/poly(4-styrenesulfonate) (PEDOT:PSS) [13]. Yu et al. developed a transducer consisting of a surface modified PVDF thin film, coated with compliant CNT based transparent conductors [14]. Sugimoto et al. proposed a PVDF foil pasted over a polyethersulfone (PES) film and found out that the use of PES foil as substrate had a low distortion in comparison to a loudspeaker made of PVDF only [15]. Printing of all necessary functional layers including the active component offers the potential of using new substrate materials and working without time-consuming surface treatment. The preparation of layers made of solved PVDF or its copolymers has been investigated in numerous studies [16]. But in most of them the active layer was spin coated. Printing technologies were only used in a few publications. For instance the usage of screen printed piezoelectric polymer, a copolymer of PVDF, was shown in a printed sensor network [17]. The use of the copolymer poly(vinylidene fluoride-trifluorethylene) (P(VDF-TrFE)) offers the advantage of crystallization in the b-phase [18]. In contrast to PVDF, the material does not need mechanical stretching during the polarization process to increase the piezoelectric effect, which is principally done by corona or electrode poling [18–23].

2. Experimental Here we demonstrate a flexible loudspeaker on the basis of standard coated paper substrate. In contrast to former investigations of printed electronics on paper it was prepared without additional smoothing layer [24] to keep the process as simple as possible and to reduce supplementary material consumption which is essential for low cost electronics. As substrate for flexographic printed samples matt coated wood-containing illustration printing paper (HighSpeed matt opak, IGEPA) with a density of 50 g cm 2 and a thickness of 55 lm was used. For stencil printed samples 90 g cm 2 paper was used. For our purpose the P(VDF-TrFE) was sandwiched between two electrodes, like a capacitor with P(VDF-TrFE) as dielectric material. The bottom and top electrodes were made of PEDOT:PSS, a conductive polymer well-known for its good

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printability in the form of commercially available waterbased dispersions. Previous investigations have shown that high conductivity could increase the sound pressure level (SPL) in piezoelectric speakers [12,13]. We attribute this to the lower impedance caused by lower ohmic resistance. As a result the power acting to the piezoelectric layer increases. The conductive polymer PEDOT:PSS was used especially in view of printing homogenous and smooth layers and preventing high leakage currents through the dielectric. Hence our all printed loudspeaker was composed of the paper substrate, the bottom electrode (PEDOT:PSS), the active layer P(VDF-TrFE) and the top electrode (PEDOT:PSS), as depicted in Fig. 1. In the first experiment all layers were printed by means of flexography on top of paper with a surface roughness Rz of 6.03 lm on a flexographic test printing machine (Flexiproof 100, RK Print) with a process speed of about 0.83 m s 1. Flexo printing offers extremely high productivity and the possibility for large area processing. The layout of all printed layers is shown in Fig. 1a. At first the bottom electrode was printed with a commercially available PEDOT:PSS ink suitable for flexographic printing (Clevios S HT, Heraeus Clevios GmbH). The mean thickness d of this electrode was approx. 140 nm. On paper substrate the sheet resistance RS was about 1100 X h 1. The roughness RZ on top of the electrode decreased slightly after flexographic printing to a value of 4.68 lm, i.e. PEDOT:PSS smoothed the paper surface. Lower conductivity on paper is a well-known phenomenon, possibly caused by penetration of the ink into the paper surface. To improve the conductivity on the paper substrate screen printing (Clevios S V4, Heraeus Clevios GmbH) at the semi-automatic screen printer EKRA X1-SL was used as alternative method for realization of the bottom electrodes. This effect is caused by an increased PEDOT:PSS layer thickness. With screen printing the measured sheet resistance RS on paper substrate was 70 X h 1 at a mean thickness d of 440 nm and a roughness RZ of 5.68 lm. In the next step the piezoelectric polymer P(VDF-TrFE) was printed on top of the bottom electrode. The P(VDF-TrFE) formulation was based on the copolymer VDF:TrFE (75:25 wt.%) provided by Solvay Solexis and was dissolved in Cyclopentanone. The measured viscosity was 0.7 Pa s at room temperature and a shear rate of 100 s 1. To ensure good insulating behavior

Fig. 1. Process steps of printed paper loudspeaker: (a) buildup of printed paper loudspeaker and (b) photograph of printed paper loudspeaker demonstrating its flexibility.

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Table 1 Summary of geometrical and electrical properties of each single printed layer. Substrate

Layer

Paper Bottom electrode P(VDF-TrFE) Top-electrode

Process

Rz/lm

d/lm

Unprinted Flexo printing Screen printing Flexo printing Flexo printing

6.03 ± 2.18 4.68 ± 1.68 5.68 ± 1.92 1.65 ± 0.37 1.76 ± 0.57

55 0.14 ± 0.02 0.44 ± 0.1 16.27 ± 0.77 0.18 ± 0.04

despite the large area and the rough surface three successive printing runs of P(VDF-TrFE) and curing steps at 130 °C for 2 h for each layer were realized, resulting in a total layer thickness of about 16 lm for the piezoelectric material. Flexographic printing produced smooth P(VDFTrFE) layers with a surface roughness RZ of 1.65 lm on paper. The surface energy was 34.2 mN m 1. In the last step the second PEDOT:PSS electrode was printed on top of the P(VDF-TrFE) layer. The surface tension of the applied PEDOT:PSS ink was 34.5 mN m 1. Hence the formulation was able to wet the P(VDF-TrFE) without any surface treatment as proposed by Lee et al. [13]. The top electrode had a thickness d of 180 nm and a sheet resistance RS of 200 X h 1. Both PEDOT:PSS layers were cured at 130 °C for 10 min. The properties of each single layer are summarized in Table 1. All mean values were calculated out of 10 different devices for each technology approach. After the printing process the samples were polarized by a contact-poling at room temperature in accordance with Setiadi et al. [25]. To achieve the required polarization field strength at the piezoelectric layer a DC supply voltage was connected to the electrode areas. During the stepwisepoling the field strength was applied for 4 min followed by a break of 2 min for relaxation. The field strength was increased in steps of 20 MV m 1, stopping at 80 MV m 1. The effective loudspeaker area is defined as the overlapping area between the electrodes. In our speaker the effective area was 16 cm2. Surface tension and surface energy were measured with OCA-20 contact angle meter from DataPhysics Instruments GmbH with the pendant and the sessile drop method, respectively. Viscosity was measured with the rotational rheometer MCR301 from Anton Paar. Film thickness and roughness values were measured using the profilometer Dektak 8 from Veeco. Elastic stiffness was measured with the static tensile test machine Zwick/Roell Z010 in accordance with DIN ISO 1924-3 and DIN 527-3. A four point probe unit with a Keithley 2400 was used to measure sheet resistance. All acoustical measurements were done in a non anechoic chamber with the low-noise measuring system G.R.A.S. type 40HH (microphone 40AH, microphone-preamplifier 26HH). The loudspeakers were clamped at two ends of the substrate in an angle of 45° and a center angle of 90°. All measurements were done from a distance of 1 m from the highest point of the bent sample and at an AC voltage of 50 V peak-to-peak. Signals were generated by the signal generator Agilent 33120A, amplified by Scientific Instruments SI HVA 3/450 and recorded with DATaRec 4 6-channel module DIC6B. For measuring the sound pressure level a sinus signal was induced. A linear frequency

Rs/X h

1

1113 ± 114 69 ± 2.8 208 ± 19

sweep (1 Hz–20 kHz) for 20 s was induced for measuring the frequency response of the whole system. The vibration analysis and the measurement of the displacement were done with a laser scanning vibrometer PSV-400 from Polytec at a pseudorandom noise with amplitude of 10 V.

3. Results and discussion The paper loudspeaker could be produced successfully solely by means of mass printing technologies. The sound quality measurements of our printed speaker devices showed a typical behavior for thin film speakers [17]. The speakers were able to reproduce high frequencies very well, but the reproduction of frequencies below 1 kHz was comparably weak (Fig. 2). In operation at a peak-topeak AC voltage of 50 V the printed paper loudspeakers reached SPLs of more than 50 dB for a frequency range starting at approx. Two kilohertz, measured at a distance of 1 m. The predicted influence of the conductivity of the electrodes on the SPL could be confirmed. The sound radiation of samples made by screen printing was up to 15 dB louder in comparison to speakers with flexo printed bottom electrode. The frequency response showed a resonant manner for all speakers, which is attributed to the geometry of the loudspeakers, a parallel clamped rectangular area. This influence of sample mounting of a thin film speaker was investigated by Toda et al. [26,27]. For optimal exploitation of the piezoelectric effect the loudspeaker was clamped at two ends of the substrate to form a cylindrical curve [26]. Main variations between the paper speakers resulted from the fiber direction of the paper substrate. Considering the whole frequency response, a significant increase in SPL could be measured for paper speakers curved across fiber direction. At a frequency of 5000 Hz, the flexographic printed paper speaker curved across fiber direction reached 53.1 dB while the speakers curved along fiber direction reached 40.1 dB. The screen printed samples acted similar with 63.8 dB for across curved and 56.4 dB for along curved speakers. We attribute these differences to the respective mechanical properties of paper. Variations of Young’s Moduli resulted in a variable stiffness of the loudspeakers. Despite the same setup of all printed layers Young’s Moduli were measured to be 2.1 GPa for paper across fiber direction and 6.9 GPa for paper along fiber direction, respectively. To determine the influence of these mechanical properties Fig. 3a and b show the displacement profiles of the whole paper speaker areas at a frequency of 5000 Hz. The distribution of the vibration modes including standing waves seems to be similar for both types. But the

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Fig. 2. Frequency response of printed paper loudspeakers in a range of 1000–20000 Hz.

Fig. 3. Displacement profile of printed paper loudspeakers at 5000 Hz with (a) paper along the fiber direction and (b) paper across the fiber direction.

mean displacement is 9.62 nm V 1 ± 6.80 nm V 1 along fiber direction and 11.14 nm V 1 ± 7.46 nm V 1 across fiber direction, respectively, in a range of 1000–10000 Hz. Video sequences showing the movement of the whole devices can be found in the Supporting information. These first results could demonstrate that a fully printed paper speaker acts like other piezoelectric thin film speakers. In a next step our aim was to improve the performance regarding SPL. One way to increase the SPL of printed speakers is to enlarge their piezo-active area. A major challenge of printing large area speakers is the realization of a well insulating, pinhole free piezoelectric layer. A well-known approach is printing thicker layers. With the help of stencil printing the total layer thickness could be increased to approx. 30 lm. Bottom and top electrodes

were printed with PEDOT:PSS. All other production steps were done in the same way as described above. The large speaker had an effective area of 128 cm2, i.e. eight times larger than the first version. The frequency responses of the speakers are plotted in Fig. 4. The paper speaker with an area of 128 cm2 shows a significant increase of SPL in the frequency range of 500 Hz–6000 Hz with a mean value of 19.7 dB ± 6.6 dB. Towards 6000 Hz SPL is in the range of 45 dB to 70 dB. The resonant manner of the speakers decreases for larger speaker areas, so the frequency response is smoothed especially in the range of 500 Hz–6000 Hz, which improves the sound quality significantly. Furthermore, these loudspeakers are stable for at least six months in air, although they are not encapsulated (see Fig. 4, dotted line). Former stability investigations of PEDOT:PSS

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Fig. 4. Frequency response of 128 cm2 and 16 cm2 paper speaker.

films corroborate this result, supporting the chance of producing low-cost organic electronics on paper substrates [28,29]. 4. Conclusions To sum up, fully mass printed loudspeakers on extremely rough paper substrate could be demonstrated successfully. Variations of the sound quality could be attributed to the anisotropic properties of the paper substrate, the conductivity and homogeneity of the electrodes, as well as the size of the piezo-active area. These results lead to the possibility of producing highly flexible and very thin low-cost acoustic systems on the basis of cheap and eco-friendly paper substrates. Acknowledgements The reported work was financially supported by the German Federal Ministry of Education and Research (BMBF) under Grant No. 16SV3952. We acknowledge support by Heraeus Clevios GmbH, X-SPEX GmbH and Fraunhofer ENAS. We further acknowledge technical support by W. Förster. References [1] D. Tobjörk, R. Österbacka, Paper electronics, Adv. Mater. 23 (2011) 1935–1961. [2] A.C. Huebler, F. Doetz, H. Kempa, H.E. Katz, M. Bartzsch, N. Brandt, I. Hennig, U. Fuegmann, S. Vaidyanathan, J. Granstrom, S. Liu, A. Sydorenko, T. Zillger, G. Schmidt, K. Preissler, E. Reichmanis, P. Eckerle, F. Richter, T. Fischer, U. Hahn, Ring oscillator fabricated completely by means of mass printing technologies, Org. Electron. 8 (5) (2007) 480–486. [3] M. Hambsch, K. Reuter, M. Stanel, G. Schmidt, H. Kempa, U. Fügmann, U. Hahn, A.C. Hübler, Uniformity of fully gravure printed organic field-effect transistors, Mater. Sci. Eng. B 170 (2010) 93–98. [4] A.C. Hübler, B. Trnovec, T. Zillger, M. Ali, N. Wetzold, M. Mingebach, A. Wagenpfahl, C. Deibel, V. Dyakonov, Printed paper photovoltaic cells, Adv. Energy Mater. 1 (6) (2011) 1018–1022.

[5] M. Jung, J. Kim, J. Noh, N. Lim, C. Lim, G. Lee, J. Kim, H. Kang, K. Jung, A.D. Leonard, J.M. Tour, G. Cho, All-printed and roll-to-roll-printable 13.56-MHz-operated 1-bit RF tag on plastic foils, IEEE Transac. Elec. Dev. 57 (3) (2010) 571–580. [6] G.C. Schmidt, M. Bellmann, B. Meier, M. Hambsch, K. Reuter, H. Kempa, A.C. Hübler, Modified mass printing technique for the realization of source/drain electrodes with high resolution, Org. Electron. 11 (10) (2010) 1683–1687. [7] D. Zielke, A.C. Hübler, U. Hahn, N. Brandt, M. Bartzsch, U. Fügmann, T. Fischer, J. Veres, Simon ogier, polymer-based organic field-effect transistor using offset printed source/drain structures, Appl. Phys. Lett. 87 (12) (2005) 123508–123510. [8] Y. Bar-Cohen, Electroactive Polymer (Eap) As Artificial Muscles, Reality, Potential, and Challenges, SPIE Press, Bellingham, 2001. [9] J. Borwick, Electrostatic Loudspeakers, Loudspeaker and Headphone Handbook, vol. 3, Focal Press, Oxford, 2001. [10] R. Heydt, Acoustical performance of an electrostrictive polymer film loudspeaker, J. Acoust. Soc. Am. 107 (2000) 833–839. [11] L. Xiao, Z. Chen, C. Feng, L. Liu, Z.-Q. Bai, Y. Wang, Flexible, stretchable, transparent carbon nanotube thin film loudspeakers, Nano Lett. 8 (12) (2008) 4539–4545. [12] C.S. Lee, J.Y. Kim, D.E. Lee, J. Joo, B.G. Wagh, S. Han, W.Y. Beag, S.K. Koh, Flexible and transparent organic film speaker by using highly conducting PEDOT/PSS as electrode, Synth. Met. 139 (2003) 457– 461. [13] C.S. Lee, J.Y. Kim, D.E. Lee, J. Joo, S. Han, Y.W. Beag, S.K. Koh, An approach to durable poly(vinylidene fluoride) thin film loudspeaker, J. Mater. Res. 18 (12) (2003) 2904–2911. [14] X. Yu, R. Rajamani, K.A. Stelsyon, T. Cui, Carbon nanotube-based transparent thin film acoustic actuators and sensors, Sens. Actuators A 132 (2006) 626–631. [15] T. Sugimoto, K. Ono, A. Ando, K. Kurozumi, A. Hara, Y. Morita, A. Miura, PVDF-driven flexible and transparent loudspeaker, Appl. Acoust. 70 (2009) 1021–1028. [16] T. Furukawa, Ferroelectric properties of vinylidene fluoride copolymers, Phase Trans. 18 (1989) 143–211. [17] M. Zirkl, A. Sawatdee, U. Helbig, M. Krause, G. Scheipl, E. Kraker, P.A. Ersman, D. Nilson, D. Platt, P. Bodö, S. Bauer, G. Domann, B. Stadlober, An all-printed ferroelectric active matrix sensor network based on only five functional materials forming a touchless control interface, Adv. Mater. 23 (2011) 2069–2074. [18] J.S. Harrison, Z. Ounaies, Piezoelectric polymers, ICASE, Report 43 (2001). [19] H. Kawai, The piezoelectricity of poly (vinylidene fluoride), Jpn. J. Appl. Phy. 8 (1969) 975. [20] T. Kaura, R. Nath, M.M. Perlman, Simultaneous stretching and corona poling of PVDF films, J. Phys. D: Appl. Phys. 24 (1991) 1848–1852. [21] A. Kumar, M.M. Perlman, Simultaneous stretching and corona poling of PVDF and P (VDF-TriFE) films II, J. Phys. D: Appl. Phys. 26 (1993) 469–473.

A.C. Hübler et al. / Organic Electronics 13 (2012) 2290–2295 [22] G. Eberle, Polarization dynamics of VDF-TrFE copolymers, IEEE Trans. Electr. Insul. 26 (1) (1991) 69–76. [23] S. Bauer, Poled polymers for sensors and photonic applications, J. Appl. Phys. 80 (10) (1996) 5531–5558. [24] B. Trnovec, M. Stanel, U. Hahn, A.C. Hübler, H. Kempa, R. Sangl, M. Forster, Paper for printed polymer electronics, Professional Papermake. 1 (2009) 48–51. [25] D. Setiadi, P.M. Sarro, P.P.L. Regtien, A 3  1 integrated pyroelectric sensor based on VDF/TrFE copolymer, Sens. Actuators A 52 (1996) 103–109. [26] M. Toda, Theory of Curved Clamped PVDF Acoustic Transducers, Ultrasonic Symposium IEEE (1999).

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[27] H. Wang, Curved PVDF airborne transducer, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 46 (1999) 1375–1386. [28] J. Liu, M. Agarwal, K. Varahramyan, E.S. Berney IV, W.D. Hodo, Polymer-based microsensor for soil moisture measurement, Sens. Actuators B 129 (2008) 599–604. [29] A.M. Nardes, M. Kemerink, M.M. de Kok, E. Vinken, K. Maturova, R.A.J. Janssen, Conductivity, work function, and environmental stability of PEDOT:PSS thin films treated with sorbitol, Org. Electron. 9 (2008) 727–734.