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Effect of halohydrocarbon solvents on the dielectric performance of a polymer film and its application to inorganic electroluminescent devices
Materials Chemistry and Physics 162 (2015) 162e165
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Effect of halohydrocarbon solvents on the dielectric performance of a polymer film and its application to inorganic electroluminescent devices Jin-Young Kim Department of Mechanical Engineering and the Materials Science and Engineering Program, The University of Texas at Austin, Austin, TX 78712, USA
h i g h l i g h t s Polymer films with halohydrocarbon solvents were studied. The film with chlorobenzene (CRS/CB) showed the high dielectric performance. Increase in dielectric constant is due to the high polarizability of CeCl bonds. An EL device with CRS/CB was fabricated. High performance of an EL device was obtained.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 January 2015 Received in revised form 13 April 2015 Accepted 24 May 2015 Available online 31 May 2015
Polymer films with halohydrocarbon solvents as additive solvents were fabricated to investigate the solvent effect on dielectric performance. Bromobenzene, chlorobenzene (CB), chloroform, and fluorobenzene (FB) were used as halohydrocarbon solvents. The cyanoethyl pullulan (CRS) polymer film with CB had a dielectric constant of 21.5 with a dielectric loss of 0.017 at 100 Hz, which exhibited a 37% increase in dielectric constant compared to the neat CRS polymer film. Increment of the dielectric constant is attributed to the high polarizability of carbonechlorine bonds originated from CB absorbed in CRS. For the case of the CRS polymer film with FB, the dielectric constant did not increase due to the low polarizability of carbonefluorine bonds. In order to verify the dielectric performance of the CRS polymer film with CB (CRS/CB), AC-driven inorganic powder electroluminescent (EL) device was fabricated using the green emission ZnS:Cu,Cl phosphor particles. An EL device with CRS/CB polymer film as a dielectric layer exhibited a 37% and 55% increase in luminance and efficiency, respectively compared to that with neat CRS polymer. This increase in EL performance was attributed to the increase of charge carrier density at the interface between the emitting and dielectric layers, resulting in the increased possibility of the charge carrier tunneling into phosphor particles. © 2015 Elsevier B.V. All rights reserved.
Keywords: Composite materials Dielectric properties Electronic characterization Insulators Optical materials Polymers
1. Introduction Polymer-based dielectric films have attracted tremendous attention for potential applications in electronic devices, such as gate dielectrics, touch panels, and EL devices due to their simple fabrication process and high flexibility [1e3]. However, the polymer dielectric film yields low performance in electronic devices due to a low dielectric constant. In general, the hydrophobicity, polarizability, and free volume of polymer materials can strongly affect
E-mail address: [email protected]. http://dx.doi.org/10.1016/j.matchemphys.2015.05.052 0254-0584/© 2015 Elsevier B.V. All rights reserved.
the dielectric performance. Low polarizability and high hydrophobicity (or free volume) induce a low dielectric constant [4]. Lots of efforts have been poured into developments of the polymerbased dielectric films. Carbonechlorine (CeCl) bonds or Cl atoms in the dielectric film can increase a dielectric constant due to the increased polarizability [5]. In addition, incorporating conductive fillers in the polymer matrix can increase the capacitance due to the induced interfacial polarization [5,6]. The polymer material containing CeCl bonds may be one of strategies to increase a dielectric constant of the polymer-based film due to the high polarizability. A dielectric layer in an AC-driven inorganic powder electroluminescent (EL) device is one of application for the polymer-based
J.-Y. Kim / Materials Chemistry and Physics 162 (2015) 162e165
dielectric film [3,7,8]. In order to prevent the catastrophic dielectric breakdown for the EL device, a dielectric layer is needed. An EL can be defined as producing the light emission by applying a high electric field without thermal generation [7,8]. An EL device is consisted of the top electrode/the emitting layer/dielectric layer/ bottom electrode. In an emitting layer, phosphor powders should be suspended in the polymer matrix, which acts as a binder as well since phosphor powders cannot form a film by themselves. In addition, the polymer can protect the phosphor powder from the humidity and oxygen. This EL device has attracted interest for the potential applications such as flat panel display, backlight for cellphone, and e-poster due to its simple fabrication process, low cost, and mass production [7e10]. In this paper, the effect of the Cl-containing solvent on dielectric performance in polymer films was investigated. Cyanoethyl pullulan (hereafter CRS) was used as polymer material. Bromobenzene (BB), chlorobenzene (CB), chloroform (CF), and fluorobenzene (FB) were used as additive solvents. The CRS polymer film with CB additive solvent (hereafter CRS/CB) showed the enhanced dielectric performance compared to the neat CRS, CRS/BB, CRS/CF, and CRS/FB films. The increase in dielectric constant was attributed to the high polarizability of CeCl bonds originating from the Cl absorption in polymer matric. The mechanism for enhancing the dielectric constant was investigated by the measurement of the dielectric characterization. In addition, the increased dielectric performance of the CRS/CB polymer film was verified by applying an EL device. 2. Experimental details In order to investigate the solvent effect on dielectric performance, four types of polymer films were prepared (see Fig. 1). The
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fabrication process of the polymer film is as following. In the case of the reference film, 15 wt% CRS (Shin-Etsu Chemical) was dissolved into 85 wt% N,N-dimethylformamide (DMF), then this mixture was stirred for 4 h. For the case of the polymer film with BB, CB, CF, or FB (see Fig. 1), 15.0 wt% CRS was dissolved into 77.5 wt% DMF. Then 7.5 wt% BB (CB, CF, or FB) was added to the CRS/DMF mixture, and this mixture was stirred for 4 h. The fabrication procedure of the polymer film with BB, CB, CF, or FB is as following. The mixtures of the CB (BB, CF, or FB)/CRS were spin-coated onto indium tin oxidecoated polyethylene terephthalate substrate and dried at 130 C for 30 min. Cr(5 nm)-Au(50 nm) electrodes were deposited onto the films by evaporation. The thickness of all films was maintained to be around 2.0 mm. The dielectric performance of films was measured using an LCR meter (Agilent, E4980A) and an electrometer (Keithley, 6517A) at 0.1 V and frequencies from 100 Hz to 1 MHz. The polymer film with CB was applied to AC-driven inorganic powder electroluminescent device using the green emission ZnS:Cu,Cl phosphor particles to verify the dielectric performance. The fabrication process of the inorganic EL devices is as follows: An Al electrode as a bottom electrode was deposited onto a glass substrate by evaporation. The polymer film with CB as a dielectric layer was spin-coated onto the Al electrode and then dried at 130 C for 30 min. An emitting layer composed of the ZnS:Cu,Cl phosphor particle and CRS polymer was deposited onto the dielectric layer by screen-printing and then dried at 130 C for 30 min. Finally, ITO electrode as a top electrode was deposited onto an emitting layer by DC sputtering. The EL properties of devices were measured by applying a sinusoidal AC voltage (peak to peak value). The details of the measurement for the EL device can be found in the previous work [3,5,6]. 3. Results and discussion
Fig. 1. The schematic structure of the polymer film with the bromobenzene (BB), chlorobenzene (CB), fluorobenzene (FB), and chloroform (CF) as additive solvents. The chemical structures of BB, CB, FB, and FB solvents.
In order to investigate the solvent effect on dielectric performance, the CRS polymer film with BB, CB, or FB as an additive solvent was made, as shown in Fig. 1. The dielectric performance of polymer films was measured at 0.1 V and the frequencies were from 100 Hz to 1 MHz (see Fig. 2(a) and (b)). The dielectric constant of all films decreases with increasing frequency while the dielectric loss increases. This is due to the dielectric relaxation based on Debye equation [11]. As shown in the inset of Fig. 2(b), the dielectric losses of all films were under 0.05 for frequencies less than 8 kHz. The CRS polymer film with CB showed a dielectric constant of 21.7 ± 0.56 with a dielectric loss of 0.017 ± 0.0053 at 100 Hz while that without halohydrocarbon solvent (e.g., Cl) exhibited a dielectric constant of 15.8 ± 0.60 with a dielectric loss of 0.029 ± 0.0042. The CRS
Fig. 2. (a) Dielectric constant and (b) dielectric loss of the cyanoethyl pullulan (CRS) polymer films with BB, CB, and FB, and without halohydrocarbon solvents as a function of applied frequencies ranging from 100 Hz to 1 MHz. Inset is the enlarged graph of dielectric loss as a function of applied frequencies ranging from 100 Hz to 10 kHz.
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polymer film with CB showed a 37% increase in the dielectric constant compared to the CRS polymer only. This increment is attributed to the high polarizability of CeCl bonds by CB [5]. Cl from CB would be possibly absorbed in CRS polymer, leading to the induced CeCl bonds. In the case of the CRS polymer film with BB or FB, the dielectric constant of the film with BB (FB) was 15.9 ± 0.59 (15.7 ± 0.71) with a dielectric loss of 0.016 ± 0.0061 (0.022 ± 0.0055) at 100 Hz. The dielectric constant of the polymer with FB was lower than that without the additive solvent. The low dielectric constant was caused by a lowly electronic polarizability of the CeF, as well as the increased free volume [12]. For the case of the CRS with BB, the dielectric constant was slightly higher than the neat CRS film. However, the CRS/BB film exhibited a poor increase in the dielectric constant compared to the CRS/CB film. This means that electronegativities of the elements (e.g., BB, CB, and FB) are not responsible for enhancing of the dielectric constant in the films. In order to further investigate the effect of the CeCl bonds on the dielectric performance, the CRS polymer film with CF as an additive solvent was made. The measurement conditions were same with the previous experiment. As shown in Fig. 3, the CRS polymer film with CF exhibited a dielectric constant of 17.9 ± 0.57 with a dielectric loss of 0.015 ± 0.0072 at 100 Hz. The CRS/CF showed a 13% increase in the dielectric constant compared to the neat CRS polymer. This increment could be also explained by the high polarizability of CeCl bonds by CF. In order to investigate the effect of an increased dielectric constant on electroluminescence (EL) performance, AC driven inorganic powder EL device was fabricated, as shown in Fig. 4(a). Fig. 4(b) and (c) show the luminance and efficiency of an EL device with CRS and CRS/CB as a function of the applied voltage. All the thicknesses for the EL devices were maintained at approximately 30 mm, where the thickness for the dielectric layer was tuned to be
Fig. 5. An applied electric field to phosphor layer (Ep) as a function of a dielectric constant of a dielectric layer (εi). Ep values of the EL device with CRS and CRS/CB were 4.9 and 5.1 V/mm, respectively at 150 V.
2 mm. The luminance of an EL device with CRS/CB was 198 ± 2.98 cd/m2 while that with CRS was 145 ± 5.12 cd/m2 at 150 V and 400 Hz. An EL device with CRS/CB exhibited a 37% increase in luminance compared to that with CRS. This increase in luminance for an EL device with CB could be interpreted by the increase of charge carrier density. In general, the mechanism of producing a light emission of EL device can be explained by a recombination of the electronehole which is from the external source by tunneling [13]. The charge carrier density at the interface between the
Fig. 3. (a) Dielectric constant and (b) dielectric loss of the cyanoethyl pullulan (CRS) polymer films with CB and CF, and without halohydrocarbon solvents as a function of applied frequencies ranging from 100 Hz to 1 MHz.
Fig. 4. (a) The schematic structure of an EL device using the ZnS:Cu,Cl phosphor particles. (b) the luminance and (c) efficiency of the EL device with the neat CRS and CRS/CB dielectric layers as a function of applied voltages at 400 Hz.
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Fig. 6. (a) The luminance of the EL device with the neat CRS and CRS/CB films as a function of the frequencies at the fixed voltage of 150 V. (b) The EL spectra the EL device with the CRS/CB film at 400 Hz and 10 kHz. The emission color changed from 400 Hz to 10 kHz. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
emitting and dielectric layers could be increased with increasing a dielectric constant of the dielectric layer [14]. This would induce an increase in the probability of tunneling charge carrier into the phosphor particles leading to the increased electronehole recombination. In addition, an efficiency of an EL device was calculated by the equation, h ¼ p$L/V$J, where L is a luminance, V is an applied voltage, and J is a current density. The efficiency of the EL device with CRS/CB was 0.98 ± 0.012 lm/W while that with CRS was 0.63 ± 0.010 lm/W at 150 V and 400 Hz (see Fig. 4(c)). The EL device with CRS/CB showed a 55% increase in efficiency compared to that with neat CRS. In order to support the above mechanism, an electric field applied to phosphor particles (Ep) was calculated in ranges from 3 to 90 at the driving voltage of 150 and the dielectric constant of a phosphor layer (εp) of 20, as shown in Fig. 5. The Ep value can be expressed by this equation, Ep ¼ εiVtot/(εidp þ εpdi), where εi (εp) is the dielectric constant of a dielectric (phosphor) layer, Vtot is an applied voltage to the EL device, and di (dp) is a thickness of the dielectric (phosphor) layer [7,14,15]. Ep was gradually increased with increasing of the dielectric constant of a dielectric layer. The Ep value of the EL device with CRS and CRS/CB can be found to be 4.9 and 5.1 V/mm, respectively at 150 V. This fact indicates that the high Ep value could induce a high probability of tunneling charge carriers into the phosphor particles. In general, the luminance of the EL device shows the frequency dependence (see Fig. 6), because the light emission of the EL device occurs at the moment of the field reversal [7,8]. The luminance of the EL device with CRS/CB was 2647 ± 50.23 cd/m2 while that with CRS was 2081 ± 24.50 cd/m2 at 150 V and 20 kHz, as shown in Fig. 6(a). The EL device with CRS/CB exhibited a 27% increase in luminance compared to that with neat CRS. The luminance of the EL device increases with increasing of the frequency due to its emission mechanism. Fig. 6(b) shows the color shift of the EL device depending on the frequency. The emission color can be changed from green to blue at the high frequency (e.g., greater than 10 kHz). This is due to the difference of emission transition between the low and high frequencies [16,17]. The transition of a green emission occurred from Cl on an S site to Cu on a Zn site while that of a blue emission generated from Cl on an S site to interstitial Cu [8,17]. 4. Conclusions The CRS polymer film with CB exhibited a high dielectric performance, such as dielectric constant of 21.5 with the dielectric loss of 0.017 at 100 Hz. This was attributed to the high polarizability of CeCl bonds from CB solvent. However, the CRS polymer film with FB showed a low dielectric constant due to the low polarizability of
CeF bonds. In addition, an inorganic powder EL device was fabricated to verify the dielectric performance of the polymer film with CB. An EL device with the CRS/CB showed a 37% and 55% increase in luminance and efficiency, respectively compared to that with the neat CRS. This increase in EL performance was due to the increased charge carrier density at the interface between the emitting and dielectric layers leading to the increased possibility of charge carrier tunneling into the phosphor particles. References [1] S.Y. Yang, M. Park, H.H. Lee, Cooperative polymer gate dielectrics in organic thin-film transistors, Appl. Phys. Lett. 85 (2004) 2283e2285. [2] S. Takamatsu, T. Takahata, M. Muraki, E. Iwase, K. Matsumoto, I. Shimoyama, Transparent conductive-polymer strain sensors for touch input sheets of flexible displays, J. Micromech. Microeng. 20 (2010) 075017. [3] J.-Y. Kim, S.H. Park, T. Jeong, M.J. Bae, Y.C. Kim, I. Han, S. Yu, High electroluminescence of the ZnS: Mn nanoparticle/cyanoethyl-resin polymer/singlewalled carbon nanotube composite using the tandem structure, J. Mater. Chem. 22 (2012) 20158e20162. [4] Y.-S. Ye, W.-Y. Chen, Y.-Z. Wang, Synthesis and properties of low-dielectricconstant polyimides with introduced reactive fluorine polyhedral oligomeric silsesquioxane, J. Polym. Sci. Part A: Polym. Chem. 44 (2006) 5391e5402. [5] J.-Y. Kim, W.H. Lee, J.W. Suk, J.R. Potts, H. Chou, I.N. Kholmanov, R.D. Piner, J. Lee, D. Akinwande, R.S. Ruoff, Chlorination of reduced graphene oxide enhances the dielectric constant of reduced graphene oxide/polymer composites, Adv. Mater. 25 (2013) 2308e2313. [6] J.-Y. Kim, T. Kim, J.W. Suk, H. Chou, J.-H. Jang, J. Lee, I.N. Kholmanov, D. Akinwande, R.S. Ruoff, Enhanced dielectric performance in polymer composite films with carbon nanotube-reduced graphene oxide hybrid filler, Small 10 (2014) 3405e3411. [7] Y.A. Ono, Electroluminescent Displays, World Scientific, Singapore, 1995. [8] S. Shionoya, W.M. Yen, Phosphor Handbook, CRC Press, Boca Raton, 1999. [9] J.-Y. Kim, S.H. Park, T. Jeong, M.J. Bae, S. Song, J. Lee, I.T. Han, D. Jung, S. Yu, Paper as a substrate for inorganic powder electroluminescence devices, IEEE Trans. Electron Dev. 57 (2010) 1470e1474. [10] J.-Y. Kim, M.J. Bae, S.H. Park, T. Jeong, S. Song, J. Lee, I. Han, J.B. Yoo, D. Jung, S. Yu, Electroluminescence enhancement of the phosphor dispersed in a polymer matrix using the tandem structure, Org. Electron. 12 (2011) 529e533. [11] I.I. Perepechko, An Introduction to Polymer Physics: Polymer Physics, Mir Publishers, USSR, 1981. [12] Y. Zhang, L. Yu, Q. Su, H. Zheng, H. Huang, H.L.W. Chan, Fluorinated polyimidesilica films with low permittivity and lo dielectric loss, J. Mater. Sci. 47 (2012) 1958e1963. [13] A.G. Fischer, Electroluminescent lines in ZnS powder particles. II. Models and comparison with experience, J. Electrochem. Soc. 110 (1963) 733e748. [14] P.D. Rack, P.H. Holloway, The structure, device physics, and material properties of thin film electroluminescent displays, Mat. Sci. Eng. R. 21 (1998) 171e219. [15] Y.A. Ono, H. Kawakami, M. Fuyama, K. Onisawa, Transferred charge in the active layer and EL device characteristics of TFEL cells, Jpn. J. Appl. Phys. 26 (1987) 1482e1492. [16] K. Manzoor, S.R. Vadera, N. Kumar, T.R.N. Kutty, Multicolor electroluminescent devices using doped ZnS nanocrystals, Appl. Phys. Lett. 84 (2004) 284e286. [17] J. Stanley, Y. Jiang, F. Bridges, S.A. Carter, L. Ruhlen, Degradation and rejuvenation studies of AC electroluminescent ZnS: Cu,Cl phosphors, J. Phys. Condens. Matter 22 (2010) 055301.
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Effect of halohydrocarbon solvents on the dielectric performance of a polymer film and its application to inorganic electroluminescent devices Jin-Young Kim Department of Mechanical Engineering and the Materials Science and Engineering Program, The University of Texas at Austin, Austin, TX 78712, USA
h i g h l i g h t s Polymer films with halohydrocarbon solvents were studied. The film with chlorobenzene (CRS/CB) showed the high dielectric performance. Increase in dielectric constant is due to the high polarizability of CeCl bonds. An EL device with CRS/CB was fabricated. High performance of an EL device was obtained.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 January 2015 Received in revised form 13 April 2015 Accepted 24 May 2015 Available online 31 May 2015
Polymer films with halohydrocarbon solvents as additive solvents were fabricated to investigate the solvent effect on dielectric performance. Bromobenzene, chlorobenzene (CB), chloroform, and fluorobenzene (FB) were used as halohydrocarbon solvents. The cyanoethyl pullulan (CRS) polymer film with CB had a dielectric constant of 21.5 with a dielectric loss of 0.017 at 100 Hz, which exhibited a 37% increase in dielectric constant compared to the neat CRS polymer film. Increment of the dielectric constant is attributed to the high polarizability of carbonechlorine bonds originated from CB absorbed in CRS. For the case of the CRS polymer film with FB, the dielectric constant did not increase due to the low polarizability of carbonefluorine bonds. In order to verify the dielectric performance of the CRS polymer film with CB (CRS/CB), AC-driven inorganic powder electroluminescent (EL) device was fabricated using the green emission ZnS:Cu,Cl phosphor particles. An EL device with CRS/CB polymer film as a dielectric layer exhibited a 37% and 55% increase in luminance and efficiency, respectively compared to that with neat CRS polymer. This increase in EL performance was attributed to the increase of charge carrier density at the interface between the emitting and dielectric layers, resulting in the increased possibility of the charge carrier tunneling into phosphor particles. © 2015 Elsevier B.V. All rights reserved.
Keywords: Composite materials Dielectric properties Electronic characterization Insulators Optical materials Polymers
1. Introduction Polymer-based dielectric films have attracted tremendous attention for potential applications in electronic devices, such as gate dielectrics, touch panels, and EL devices due to their simple fabrication process and high flexibility [1e3]. However, the polymer dielectric film yields low performance in electronic devices due to a low dielectric constant. In general, the hydrophobicity, polarizability, and free volume of polymer materials can strongly affect
E-mail address: [email protected]. http://dx.doi.org/10.1016/j.matchemphys.2015.05.052 0254-0584/© 2015 Elsevier B.V. All rights reserved.
the dielectric performance. Low polarizability and high hydrophobicity (or free volume) induce a low dielectric constant [4]. Lots of efforts have been poured into developments of the polymerbased dielectric films. Carbonechlorine (CeCl) bonds or Cl atoms in the dielectric film can increase a dielectric constant due to the increased polarizability [5]. In addition, incorporating conductive fillers in the polymer matrix can increase the capacitance due to the induced interfacial polarization [5,6]. The polymer material containing CeCl bonds may be one of strategies to increase a dielectric constant of the polymer-based film due to the high polarizability. A dielectric layer in an AC-driven inorganic powder electroluminescent (EL) device is one of application for the polymer-based
J.-Y. Kim / Materials Chemistry and Physics 162 (2015) 162e165
dielectric film [3,7,8]. In order to prevent the catastrophic dielectric breakdown for the EL device, a dielectric layer is needed. An EL can be defined as producing the light emission by applying a high electric field without thermal generation [7,8]. An EL device is consisted of the top electrode/the emitting layer/dielectric layer/ bottom electrode. In an emitting layer, phosphor powders should be suspended in the polymer matrix, which acts as a binder as well since phosphor powders cannot form a film by themselves. In addition, the polymer can protect the phosphor powder from the humidity and oxygen. This EL device has attracted interest for the potential applications such as flat panel display, backlight for cellphone, and e-poster due to its simple fabrication process, low cost, and mass production [7e10]. In this paper, the effect of the Cl-containing solvent on dielectric performance in polymer films was investigated. Cyanoethyl pullulan (hereafter CRS) was used as polymer material. Bromobenzene (BB), chlorobenzene (CB), chloroform (CF), and fluorobenzene (FB) were used as additive solvents. The CRS polymer film with CB additive solvent (hereafter CRS/CB) showed the enhanced dielectric performance compared to the neat CRS, CRS/BB, CRS/CF, and CRS/FB films. The increase in dielectric constant was attributed to the high polarizability of CeCl bonds originating from the Cl absorption in polymer matric. The mechanism for enhancing the dielectric constant was investigated by the measurement of the dielectric characterization. In addition, the increased dielectric performance of the CRS/CB polymer film was verified by applying an EL device. 2. Experimental details In order to investigate the solvent effect on dielectric performance, four types of polymer films were prepared (see Fig. 1). The
163
fabrication process of the polymer film is as following. In the case of the reference film, 15 wt% CRS (Shin-Etsu Chemical) was dissolved into 85 wt% N,N-dimethylformamide (DMF), then this mixture was stirred for 4 h. For the case of the polymer film with BB, CB, CF, or FB (see Fig. 1), 15.0 wt% CRS was dissolved into 77.5 wt% DMF. Then 7.5 wt% BB (CB, CF, or FB) was added to the CRS/DMF mixture, and this mixture was stirred for 4 h. The fabrication procedure of the polymer film with BB, CB, CF, or FB is as following. The mixtures of the CB (BB, CF, or FB)/CRS were spin-coated onto indium tin oxidecoated polyethylene terephthalate substrate and dried at 130 C for 30 min. Cr(5 nm)-Au(50 nm) electrodes were deposited onto the films by evaporation. The thickness of all films was maintained to be around 2.0 mm. The dielectric performance of films was measured using an LCR meter (Agilent, E4980A) and an electrometer (Keithley, 6517A) at 0.1 V and frequencies from 100 Hz to 1 MHz. The polymer film with CB was applied to AC-driven inorganic powder electroluminescent device using the green emission ZnS:Cu,Cl phosphor particles to verify the dielectric performance. The fabrication process of the inorganic EL devices is as follows: An Al electrode as a bottom electrode was deposited onto a glass substrate by evaporation. The polymer film with CB as a dielectric layer was spin-coated onto the Al electrode and then dried at 130 C for 30 min. An emitting layer composed of the ZnS:Cu,Cl phosphor particle and CRS polymer was deposited onto the dielectric layer by screen-printing and then dried at 130 C for 30 min. Finally, ITO electrode as a top electrode was deposited onto an emitting layer by DC sputtering. The EL properties of devices were measured by applying a sinusoidal AC voltage (peak to peak value). The details of the measurement for the EL device can be found in the previous work [3,5,6]. 3. Results and discussion
Fig. 1. The schematic structure of the polymer film with the bromobenzene (BB), chlorobenzene (CB), fluorobenzene (FB), and chloroform (CF) as additive solvents. The chemical structures of BB, CB, FB, and FB solvents.
In order to investigate the solvent effect on dielectric performance, the CRS polymer film with BB, CB, or FB as an additive solvent was made, as shown in Fig. 1. The dielectric performance of polymer films was measured at 0.1 V and the frequencies were from 100 Hz to 1 MHz (see Fig. 2(a) and (b)). The dielectric constant of all films decreases with increasing frequency while the dielectric loss increases. This is due to the dielectric relaxation based on Debye equation [11]. As shown in the inset of Fig. 2(b), the dielectric losses of all films were under 0.05 for frequencies less than 8 kHz. The CRS polymer film with CB showed a dielectric constant of 21.7 ± 0.56 with a dielectric loss of 0.017 ± 0.0053 at 100 Hz while that without halohydrocarbon solvent (e.g., Cl) exhibited a dielectric constant of 15.8 ± 0.60 with a dielectric loss of 0.029 ± 0.0042. The CRS
Fig. 2. (a) Dielectric constant and (b) dielectric loss of the cyanoethyl pullulan (CRS) polymer films with BB, CB, and FB, and without halohydrocarbon solvents as a function of applied frequencies ranging from 100 Hz to 1 MHz. Inset is the enlarged graph of dielectric loss as a function of applied frequencies ranging from 100 Hz to 10 kHz.
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polymer film with CB showed a 37% increase in the dielectric constant compared to the CRS polymer only. This increment is attributed to the high polarizability of CeCl bonds by CB [5]. Cl from CB would be possibly absorbed in CRS polymer, leading to the induced CeCl bonds. In the case of the CRS polymer film with BB or FB, the dielectric constant of the film with BB (FB) was 15.9 ± 0.59 (15.7 ± 0.71) with a dielectric loss of 0.016 ± 0.0061 (0.022 ± 0.0055) at 100 Hz. The dielectric constant of the polymer with FB was lower than that without the additive solvent. The low dielectric constant was caused by a lowly electronic polarizability of the CeF, as well as the increased free volume [12]. For the case of the CRS with BB, the dielectric constant was slightly higher than the neat CRS film. However, the CRS/BB film exhibited a poor increase in the dielectric constant compared to the CRS/CB film. This means that electronegativities of the elements (e.g., BB, CB, and FB) are not responsible for enhancing of the dielectric constant in the films. In order to further investigate the effect of the CeCl bonds on the dielectric performance, the CRS polymer film with CF as an additive solvent was made. The measurement conditions were same with the previous experiment. As shown in Fig. 3, the CRS polymer film with CF exhibited a dielectric constant of 17.9 ± 0.57 with a dielectric loss of 0.015 ± 0.0072 at 100 Hz. The CRS/CF showed a 13% increase in the dielectric constant compared to the neat CRS polymer. This increment could be also explained by the high polarizability of CeCl bonds by CF. In order to investigate the effect of an increased dielectric constant on electroluminescence (EL) performance, AC driven inorganic powder EL device was fabricated, as shown in Fig. 4(a). Fig. 4(b) and (c) show the luminance and efficiency of an EL device with CRS and CRS/CB as a function of the applied voltage. All the thicknesses for the EL devices were maintained at approximately 30 mm, where the thickness for the dielectric layer was tuned to be
Fig. 5. An applied electric field to phosphor layer (Ep) as a function of a dielectric constant of a dielectric layer (εi). Ep values of the EL device with CRS and CRS/CB were 4.9 and 5.1 V/mm, respectively at 150 V.
2 mm. The luminance of an EL device with CRS/CB was 198 ± 2.98 cd/m2 while that with CRS was 145 ± 5.12 cd/m2 at 150 V and 400 Hz. An EL device with CRS/CB exhibited a 37% increase in luminance compared to that with CRS. This increase in luminance for an EL device with CB could be interpreted by the increase of charge carrier density. In general, the mechanism of producing a light emission of EL device can be explained by a recombination of the electronehole which is from the external source by tunneling [13]. The charge carrier density at the interface between the
Fig. 3. (a) Dielectric constant and (b) dielectric loss of the cyanoethyl pullulan (CRS) polymer films with CB and CF, and without halohydrocarbon solvents as a function of applied frequencies ranging from 100 Hz to 1 MHz.
Fig. 4. (a) The schematic structure of an EL device using the ZnS:Cu,Cl phosphor particles. (b) the luminance and (c) efficiency of the EL device with the neat CRS and CRS/CB dielectric layers as a function of applied voltages at 400 Hz.
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165
Fig. 6. (a) The luminance of the EL device with the neat CRS and CRS/CB films as a function of the frequencies at the fixed voltage of 150 V. (b) The EL spectra the EL device with the CRS/CB film at 400 Hz and 10 kHz. The emission color changed from 400 Hz to 10 kHz. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
emitting and dielectric layers could be increased with increasing a dielectric constant of the dielectric layer [14]. This would induce an increase in the probability of tunneling charge carrier into the phosphor particles leading to the increased electronehole recombination. In addition, an efficiency of an EL device was calculated by the equation, h ¼ p$L/V$J, where L is a luminance, V is an applied voltage, and J is a current density. The efficiency of the EL device with CRS/CB was 0.98 ± 0.012 lm/W while that with CRS was 0.63 ± 0.010 lm/W at 150 V and 400 Hz (see Fig. 4(c)). The EL device with CRS/CB showed a 55% increase in efficiency compared to that with neat CRS. In order to support the above mechanism, an electric field applied to phosphor particles (Ep) was calculated in ranges from 3 to 90 at the driving voltage of 150 and the dielectric constant of a phosphor layer (εp) of 20, as shown in Fig. 5. The Ep value can be expressed by this equation, Ep ¼ εiVtot/(εidp þ εpdi), where εi (εp) is the dielectric constant of a dielectric (phosphor) layer, Vtot is an applied voltage to the EL device, and di (dp) is a thickness of the dielectric (phosphor) layer [7,14,15]. Ep was gradually increased with increasing of the dielectric constant of a dielectric layer. The Ep value of the EL device with CRS and CRS/CB can be found to be 4.9 and 5.1 V/mm, respectively at 150 V. This fact indicates that the high Ep value could induce a high probability of tunneling charge carriers into the phosphor particles. In general, the luminance of the EL device shows the frequency dependence (see Fig. 6), because the light emission of the EL device occurs at the moment of the field reversal [7,8]. The luminance of the EL device with CRS/CB was 2647 ± 50.23 cd/m2 while that with CRS was 2081 ± 24.50 cd/m2 at 150 V and 20 kHz, as shown in Fig. 6(a). The EL device with CRS/CB exhibited a 27% increase in luminance compared to that with neat CRS. The luminance of the EL device increases with increasing of the frequency due to its emission mechanism. Fig. 6(b) shows the color shift of the EL device depending on the frequency. The emission color can be changed from green to blue at the high frequency (e.g., greater than 10 kHz). This is due to the difference of emission transition between the low and high frequencies [16,17]. The transition of a green emission occurred from Cl on an S site to Cu on a Zn site while that of a blue emission generated from Cl on an S site to interstitial Cu [8,17]. 4. Conclusions The CRS polymer film with CB exhibited a high dielectric performance, such as dielectric constant of 21.5 with the dielectric loss of 0.017 at 100 Hz. This was attributed to the high polarizability of CeCl bonds from CB solvent. However, the CRS polymer film with FB showed a low dielectric constant due to the low polarizability of
CeF bonds. In addition, an inorganic powder EL device was fabricated to verify the dielectric performance of the polymer film with CB. An EL device with the CRS/CB showed a 37% and 55% increase in luminance and efficiency, respectively compared to that with the neat CRS. This increase in EL performance was due to the increased charge carrier density at the interface between the emitting and dielectric layers leading to the increased possibility of charge carrier tunneling into the phosphor particles. References [1] S.Y. Yang, M. Park, H.H. Lee, Cooperative polymer gate dielectrics in organic thin-film transistors, Appl. Phys. Lett. 85 (2004) 2283e2285. [2] S. Takamatsu, T. Takahata, M. Muraki, E. Iwase, K. Matsumoto, I. Shimoyama, Transparent conductive-polymer strain sensors for touch input sheets of flexible displays, J. Micromech. Microeng. 20 (2010) 075017. [3] J.-Y. Kim, S.H. Park, T. Jeong, M.J. Bae, Y.C. Kim, I. Han, S. Yu, High electroluminescence of the ZnS: Mn nanoparticle/cyanoethyl-resin polymer/singlewalled carbon nanotube composite using the tandem structure, J. Mater. Chem. 22 (2012) 20158e20162. [4] Y.-S. Ye, W.-Y. Chen, Y.-Z. Wang, Synthesis and properties of low-dielectricconstant polyimides with introduced reactive fluorine polyhedral oligomeric silsesquioxane, J. Polym. Sci. Part A: Polym. Chem. 44 (2006) 5391e5402. [5] J.-Y. Kim, W.H. Lee, J.W. Suk, J.R. Potts, H. Chou, I.N. Kholmanov, R.D. Piner, J. Lee, D. Akinwande, R.S. Ruoff, Chlorination of reduced graphene oxide enhances the dielectric constant of reduced graphene oxide/polymer composites, Adv. Mater. 25 (2013) 2308e2313. [6] J.-Y. Kim, T. Kim, J.W. Suk, H. Chou, J.-H. Jang, J. Lee, I.N. Kholmanov, D. Akinwande, R.S. Ruoff, Enhanced dielectric performance in polymer composite films with carbon nanotube-reduced graphene oxide hybrid filler, Small 10 (2014) 3405e3411. [7] Y.A. Ono, Electroluminescent Displays, World Scientific, Singapore, 1995. [8] S. Shionoya, W.M. Yen, Phosphor Handbook, CRC Press, Boca Raton, 1999. [9] J.-Y. Kim, S.H. Park, T. Jeong, M.J. Bae, S. Song, J. Lee, I.T. Han, D. Jung, S. Yu, Paper as a substrate for inorganic powder electroluminescence devices, IEEE Trans. Electron Dev. 57 (2010) 1470e1474. [10] J.-Y. Kim, M.J. Bae, S.H. Park, T. Jeong, S. Song, J. Lee, I. Han, J.B. Yoo, D. Jung, S. Yu, Electroluminescence enhancement of the phosphor dispersed in a polymer matrix using the tandem structure, Org. Electron. 12 (2011) 529e533. [11] I.I. Perepechko, An Introduction to Polymer Physics: Polymer Physics, Mir Publishers, USSR, 1981. [12] Y. Zhang, L. Yu, Q. Su, H. Zheng, H. Huang, H.L.W. Chan, Fluorinated polyimidesilica films with low permittivity and lo dielectric loss, J. Mater. Sci. 47 (2012) 1958e1963. [13] A.G. Fischer, Electroluminescent lines in ZnS powder particles. II. Models and comparison with experience, J. Electrochem. Soc. 110 (1963) 733e748. [14] P.D. Rack, P.H. Holloway, The structure, device physics, and material properties of thin film electroluminescent displays, Mat. Sci. Eng. R. 21 (1998) 171e219. [15] Y.A. Ono, H. Kawakami, M. Fuyama, K. Onisawa, Transferred charge in the active layer and EL device characteristics of TFEL cells, Jpn. J. Appl. Phys. 26 (1987) 1482e1492. [16] K. Manzoor, S.R. Vadera, N. Kumar, T.R.N. Kutty, Multicolor electroluminescent devices using doped ZnS nanocrystals, Appl. Phys. Lett. 84 (2004) 284e286. [17] J. Stanley, Y. Jiang, F. Bridges, S.A. Carter, L. Ruhlen, Degradation and rejuvenation studies of AC electroluminescent ZnS: Cu,Cl phosphors, J. Phys. Condens. Matter 22 (2010) 055301.