P1-108 / B. T. Bae
IMID 2009 DIGEST •
Abstract
To realize the flexible audible display (FAD), polymer dispersed liquid crystal(PDLC) is deposited on PVDF(Poly-vinylidene fluoride) film with piezoelectric property. With applied audio signal, it makes sound more than 70 dB in 700 Hz-10kHz. In case of transparency it is opaque and transmittance is less than 10% at 550nm wavelength without bias voltage. It is turned to be transparent and show transmittance of 66% with driving voltage of 60V.
1. Introduction
As flat display devices become lighter and smaller, the flexible display attracts our attention as a next generation display. Here, in addition to the simple conventional visible functionality, we suggest the “Flexible audible display(FAD, VizDioTM)” that combines flexible speaker and flexible display into one functional polymer film.
Poly-vinylidene fluoride (PVDF) is one of very famous piezoelectric polymer substrates for vibration sensor, actuator, and film speaker due to its piezoelectricity.[1,2] PVDF film is consist of polymer chains, -(CH2CF2)-, and is generally in non-polar form
called as α phase. After poling process α phase PVDF film is converted to β phase with polarity.[3] In β phase PVDF film, an external voltage is able to change the alignment of dipoles in polymer chains and the shape of PVDF film. When an altering electric field is applied in thickness direction of PVDF film, dipoles of PVDF film is re-oriented and polymer chain structure is vibrated to generate an acoustic wave. This wave propagates in the air and is in the range of human audible sound, usually 20Hz~ 20kHz, and this is the reason why PVDF film with conducting electrode is called as the film speaker.
In this study, it is tried to combine this piezoelectric property with polymer dispersed liquid crystal(PDLC) for realizing the flexible audible display. PDLC is used as a display layer because it changes its transparency with external electric field.[4]
2. Experimental
With roll-to-roll sputtering 160 nm thick indium-tin-oxide(ITO) layer is deposited on 180 ㎛ thick β phase PVDF film of 10×10cm2. Working pressure is kept about 1mTorr with argon/oxygen mixed gas and DC sputtering power is 400V-2kW. Its transmittance of ITO/PVDF is 76% at the wavelength of 550 nm(bare PVDF has transmittance of 85% at 550nm) and its surface resistance drops down to 170 Ω/sq when is process at optimized oxygen partial ratio of 0.2%. Since PVDF film is deformed even at the 70 ℃, temperature of PVDF film is carefully stabilized less than 30℃ by cooling system. In comparison to ITO/PET, which its film temperature is high than 80℃ at sputtering process, ITO/PVDF shows good electrical resistivity and transmittance.[5]
ITO is deposited on both sides of PVDF since two conducting layer is needed to vibrate PVDF film. This ITO/PVDF/ITO layer is used as film speaker and base film for PDLC deposition. After sputtering ITO on PVDF film, PDLC composite is sprayed on ITO/PVDF/ITO film. Another ITO/PVDF film for driving PDLC is laminated on PDLC/ITO/PVDF/ITO film. Finally PDLC layer is cured by UV irradiation. Upper PVDF/ITO/PDLC/ITO layer is used as display in this FAD.
Fig 1. shows the structure of FAD. It consists of 6 layers and has a structure of ITO(160nm)/PVDF(180
Flexible audible display(VizDio
TM) using polymer
dispersed liquid crystal on film speaker
Byeong Taek Bae
1,2, Dong Hee Park
1, Kwang Ho Jeong
2,
and Won Kook Choi
11Thin film materials research center, KIST, Seoul, 136-791, Korea
Tel.:82-2-958-5562, E-mail:
[email protected]
2Department of Physics, Yonsei University, Seoul, 120-749, Korea
P1-108 / B. T. Bae
• IMID 2009 DIGEST
μm)/ITO(160nm)/PDLC/ITO(160nm)/PVDF(180μ m). Upper two ITO layers are for driving PDLC and bottom two ITO layers are for vibrating PVDF as film speaker. All ITO layers have thickness of 160nm and surface resistance of 170 Ω/sq.
Fig. 1. Structure of FAD
3. Results and discussion
Two applying voltages are needed to turn on FAD. One is direct bias voltage for driving PDLC and another voltage is altering bias voltage for speaker layer. When audio signal (AC voltage) is applied on ITO layer, it makes sound more than 70 dB in 700 Hz-10kHz. Even it has small area of 10×10cm2, its sound
is enough to hear at 1m distance with normal environmental noise about 40~50dB.
10 100 1000 10000 50 60 70 80 90 100 Sound pr essure level [dB] Frequency [Hz]
Fig. 2. Sound pressure of FAD sample
When audio layer is making sound, transparency of PDLC layer is varied by external DC driving electric field simultaneously. While it is opaque and shows transmittance less than 10 % at 550nm wavelength, it is changed to transparent film with transmittance of
66% with driving voltage of 60V. When it is in mechanically bended form, PDLC layer and speaker layer works well.
400 600 800 0 20 40 60 80 T rans m itta nce [ % ] Wave length [nm] On Off
Fig. 3. Transmittance of FAD with/without
driving voltage
4. Summary
Under the optimized sputtering condition, good transparent conducting layer is deposited on PVDF film. FAD with one pixel of 10×10cm2 makes sound
and varies its transparency even with vibration of speaker layer. This means feasibility of functional piezoelectric base film and its application for all-in-one flexible display.
Acknowledgement
This work was supported by both KIST Future-Resource program (2E21093) and Seoul R&BD program (10848).
5. References
1. R.A. Betz, Proceedings of the 6th International Symposium on Electrets, p.214, (1988)
2. J. A. Paradiso, IBM systems journal, 35[3&4], p.473 (1996)
3. R. G. Kepler and H.S. Nalwa (eds.), Ferroelectric Polymers, Marcel Dekker, New York, p.183(1995) 4. J.Y. Kim, H.Y. Woo, J.W. Baek, T.W. Kim, E.A. Song, S.C. Park, and D.W. Ihm, Appl. Phys. Lett., Vol. 92, p.183301 (2008)
5. J.H.Kim, D.H.Park, J.B.Kim, D.J. Byun, and W.K. Choi , Kor. J. Mater. Res., 17(10), p.45, (2007)
