Proceedings of 2nd International Symposium for Color Science and Art 2020

Powder electroluminescent device based on paper substratefor designing multi-functional digital signage
Shota Tsuneyasu, Naoki Takeda, Yusuke Numata, Toshifumi SatohTokyo Polytechnic Univ., 1583 Iiyama, Atsugi, Kanagawa 243-0297, Japan
2. EXPERIMENTAL
2.1 Materials
Tracing paper applied as a substrate was purchasedfrom Sakae Technical Paper Co., Ltd. Zinc sulfide(ZnS)-type particles (GG65) used as a phosphor layer
were purchased from Osram Sylvania. Barium titaniumoxide (BaTiO3) utilized as a dielectric layer was purchasedfrom Kishida Chemical Co., Ltd. Cyanoresin (CR-V)
applied as a high dielectric polymer was purchased fromShinetsu Chemical Co., Ltd. Cyclohexanone used as asolvent was purchased from Fujifilm Wako Pure Chemical
Co. PEDOT:PSS applied as a transparent electrode waspurchased from Sigma-Aldrich Co., LLC. The
encapsulated TC materials utilized as a TC layer werepurchased in a slurry form from Kirokusozai Co., Ltd.
Finally, Ag-paste for application as a back electrode waspurchased from Mino Group Co., Ltd.
2.2 Preparation of the EL devices
The high-dielectric polymer paste was prepared bymixing cyanoresin and cyclohexanone at a weight ratio of3:7. The phosphor particles and BaTiO3 were dispersed ina high-dielectric polymer paste at a weight ratio of 4:6.
These functional materials were laminated using an
automatic screen printing machine (TU2020-C, SeritechCo., Ltd.) (Fig. 1). Each material was sintered in aconstant-temperature oven (FS-405, Yamato ScientificCo., Ltd.) at 150 °C for 6 min. The transparent electrodewas sintered for 40 min.

ABSTRACTWe demonstrated the emissions of multi-colored lightfrom a powder electroluminescent device using athermochromic material through a temperature control. At
room temperature, the emission of a sapphire blue lightfrom the device was observed under the application of analternative current voltage. With an increase in thetemperature, the color of the thermochromic materials
changed from orange to white, leading to sky blue
emissions from the device owing to the decrease inenergy transfer from the excited state of the phosphor tothe thermochromic material.
1. INTRODUCTION
To realize a Super Smart Society, namely, Society 5.0,
many different types of electronic devices must beconnected to the global network at any time and in any
location. In such a society, the devices must not only
achieve flexibility with the ability to easily attach to thesurface of a curved object such as paper, they must also
have a workability allowing them to be cut into anarbitrarily shape. Display devices with a human machineinterface also require such functionalities. However, aconventional liquid crystal display device uses a hardglass substrate and a light-emitting diode backlight. Inaddition, an organic electroluminescent (EL) display
device must block the oxygen and water to prevent anunfavorable reaction. Thus, such displays cannot achieve
the above requirements.
To overcome this, we focused on a powder EL device
that achieves numerous advantages, including a highthroughput, workability, weatherability, and stablebendability. With regard to the bending durability, wedemonstrated powder EL devices on an Ag grid laminatedwithPoly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) that achieves asuccessfully demented bending durability of 20,000
cycles [1, 2]. Despite such advantages, the commercial
applications of this device remain a niche market in its
present state. To realize new applications, a powder EL
device with a multiple functionalities should be achieved.
For this reason, we introduced a thermochromic (TC)material that can enable a reversible color change in a
powder EL device through an external heat stimulus,
thereby achieving a novel powder EL device that exhibits
multi-color light emissions from a change in temperature.
In this study, we introduced a TC layer that enables a color
change in a device from orange to white at 40 °C, andevaluate the electrical and optical behaviors.

Back electrode
Dielectric layer
Phosphor layer

TC layerTransparent electrode
Substrate

Fig. 1 Structure of powder EL devices

2.3 Photophysical and EL measurements
Ultraviolet-visible (UV-vis) absorption spectra of thefilms were measured using a spectrophotometer(OP-TR/RF-GONIO-MN,OceanPhotonics).
Photoluminescence spectra were obtained using aspectrofluorometer (FluoroMax-3, Horiba). The excitationwavelength of the films was 350 nm. The voltage
dependences of the current and the luminance of theprepared devices were measured using an EL
measurement system (SX-1152, Iwatsu Electric CO.,Ltd.).

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Proceedings of 2nd International Symposium for Color Science and Art 2020

5. ACKNOWLEDGMENTS
This work was supported by “FY2016 MEXT Private
University Research Branding Project” and the Ebara
Hatakeyama Memorial Foundation.
6. REFERENCES[1] M. Ohsawa, N. Hashimoto, N. Takeda, S. Tsuneyasu,and T. Satoh, “Printed Invisible Silver-Grid
Transparent Electrode on Flexible Epoxy Film andApplication to Powder Electroluminescent Device,”
Proc. IDW ’19, pp. 1558-1561 (2019).
[2] M. Ohsawa, N. Hashimoto, N. Takeda, S. Tsuneyasu,and T. Satoh, “Flexible powder electroluminescentdevice on transparent electrode of printed invisible
silver-grid laminated with conductive polymer,” Flex.
Print. Electron. (under review).
1.0Phosphor
Initial state
Heated state

0.5

0.5

0.0
400

500600
Wavelength / nm

0.0700Fig. 2 PL spectrum of the phosphor layer
and absorbance spectra of the TC layer inthe initial/heated statesLuminance / cd m-2

Current / mA

1.0

Frequency:1 kHz
Without TC
0.5
With TC
0.0
60
30050

100

150200Voltage / V

250300Fig. 3 Current (top) and luminescence(bottom) dependence of the voltage inpowder EL devices with/without TC layer

1.0

Initial state
Heated state

0.50.0400

500600
Wavelength / nm

700Fig. 4 Normalized EL spectra of the device in
the initial/heated states under the applicationof ±170 V at AC voltage of 1.2 kHz

32AbsorbanceNormalized PL intensity

1.0Normalized EL intensity

3. RESULTS AND DISCUSSION
Fig. 2 shows the normalized photoluminescence (PL)spectrum of the phosphor layer and the absorbance
spectra of the TC layer in the initial and heated states. The
PL spectrum of the phosphor showed a peak at 450 nm.
By heating the TC layer to 40 °C, the absorbance within
the visible light region was decreased. This change in theabsorbance band in the TC layer and the PL spectrum of
the phosphor overlap well. Such an overlap with theabsorption and emission bands facilitates an energy
transfers between the phosphor and TC layers.
Fig. 3 shows the current and luminescencedependence of the voltage in powder EL devices
with/without a TC layer at room temperature. With theintroduction of the TC layer, the current of the powder EL
device decreased to 0.36 mA, which is 0.45-times lower
than that of the powder EL device without a TC layer (0.80mA). Corresponding to this result, the luminance is alsolower. It is thought that the electrode distance increases by
the introduction of the TC layer.
To demonstrate the multi-color EL by utilizing a change
in the external temperature, we measured the EL spectraof the device in the initial and heated states under theapplication of an AC voltage of ±170 V at 1.2 kHz(Fig. 4).
In the initial state, a sapphire blue EL band with threeemission peaks at 450, 490, and 550 nm was observed
because the device was in a colored state. By contrast, ina heated state, the color of the device changed fromorange to white, and the sky blue EL band with anemission peak 450 nm is almost identical to that of the PLspectrum (Fig. 2). The intensity at 450 nm of the device ina heated state was 2.5-times higher than that in the initialstate. These results indicate that the EL color of the EL
device can be controlled through the energy transfer
between these materials.