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Study on ZnGa2O4:Cr3+ a.c. powder electroluminescent device
Materials Letters 61 (2007) 401 – 404 www.elsevier.com/locate/matlet
Study on ZnGa2O4:Cr 3+ a.c. powder electroluminescent device B. Qiao ⁎, Z.L. Tang, Z.T. Zhang, L. Chen State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China Received 15 February 2006; accepted 12 April 2006 Available online 19 May 2006
Abstract An a.c. powder electroluminescent (EL) device using ZnGa2O4:Cr3+ phosphor was fabricated by the screen printing method. Optical and electrical properties of the device were investigated. The fabricated device shows a red emission at 695 nm driven by the a.c. voltage. The emission is attributed to the energy transfer from hot electrons to Cr3+ centers via self-activated Ga–O groups. Luminance (L) versus voltage (V) matches the well-known equation of L = L0exp(− bV − 1 / 2) and luminance increases proportionally with frequency due to the increase of excitation probability of host lattice or Cr3+ centers. The diagram of the charge density (Q) versus applied voltage (V) is based on a conventional Sawyer– Tower circuit. At 280 V and 1000 Hz, the luminance and the luminous efficiency of the fabricated powder EL device are about 1.0 cd/m2 and 13 lm/W, respectively. And under the high field, the device fabricated with the oxide-based phosphor of ZnGa2O4:Cr3+ shows excellent stability in comparison with the conventional sulfide powder EL device. © 2006 Elsevier B.V All rights reserved. Keywords: Phosphor; Electroluminescence; ZnGa2O4:Cr3+
1. Introduction Electroluminescence (EL) was first discovered in 1936 by George Destriau [1]. According to the phosphor configuration, EL devices could be categorized into two types: powder EL and thin film EL (TFEL). Powder EL device is usually fabricated by screen printing which is an available and low-cost way compared with the fabricating method of the TFEL device. An advantage of powder EL devices is that the emission colors could be readily controlled by mixing different phosphors with different emission colors. And the characteristics, such as uniform light emission, thin profile and low power consumption provide the powder EL device infinitely applied potential. In virtue of the abovementioned features, powder EL devices are widely used as the backlight of liquid crystal displays (LCD) such as cell phone, personal digital assistant (PDA) and palmtop computer [2]. The basic structure of the powder EL device consists of a transparent electrode layer on a substrate, a phosphor layer, a dielectric layer and a back electrode layer [3]. Conventionally, sulfide phosphors with various emission colors ⁎ Corresponding author. E-mail address: [email protected] (B. Qiao). 0167-577X/$ - see front matter © 2006 Elsevier B.V All rights reserved. doi:10.1016/j.matlet.2006.04.070
are employed as the phosphor layer in powder EL devices, such as ZnS:Cu, Cl (blue or green) [4], and ZnS:Mn (orange) [5]. However, the biggest problem of the sulfide phosphors used for commercially available EL devices is the instability and the consequent short-term reliability, which greatly inhibits the development of the EL display. Oxide phosphors have recently attracted much attention because of the variety of materials available and the high chemical stability. Using oxide phosphor as the emitting layer of the EL device could overcome the inferiority of instability and satisfy the requirement for long lifetime. Since the first report of a TFEL device using Mnactivated Zn2SiO4 phosphor, high-luminance multicolor-emitting TFEL devices have been fabricated using various oxide phosphors activated with Eu or Mn [6]. However, there are few papers on the ZnGa2O4:Cr3+ phosphor applied for thick powder EL devices. In this work, the powder EL device using the ZnGa2O4:Cr3+ phosphor was fabricated by screen printing and the optical and electrical properties were investigated. 2. Experimental The ZnGa2O4:Cr3+ phosphor was synthesized by a citric-gel method using citric acid as the chelate agent. After being pre-
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B. Qiao et al. / Materials Letters 61 (2007) 401–404
calcined at 650 °C and eliminating the organic groups, the powders as synthesized were calcined at 1000 °C for 3 h in an air ambient. The a.c. powder electroluminescent devices were fabricated by a screen printing method. The devices consist of a PET substrate, a transparent ITO electrode, a ZnGa2O4:Cr3+ phosphor layer, a BaTiO3 dielectric layer, and an Ag rear electrode. The ZnGa2O4:Cr3+ phosphor powder was dispersed in a dielectric organic binder (cyanoethyl cellulate) with a weight ratio of 2:1 and deposited thrice on the ITO substrate by screen printing. The formed phosphor layer is about 60 μm thick. And the high dielectric layer (BaTiO3 mixed with the organic binder) was printed twice upon the phosphor layer to avoid a catastrophic dielectric breakdown. The thickness of the dielectric layer is about 40 μm. The Ag paste was printed upon the dielectric layer as a rear electrode. The fabricated thick film EL device is driven by a sinusoidal voltage. The phase of the synthesized phosphor was identified with a Rigaku D/max-RB X-ray diffraction spectrometer. EL emission spectrum and luminance were measured by Cary Eclipse fluorospectrometer and ST-86LA luminance meter, respectively. Hitachi S-450 scanning electron microscope (SEM) was employed to show the cross section of the device. The electrical properties were measured with Textronix oscilloscope. 3. Results and discussion Fig. 1 depicts the XRD pattern of the ZnGa2O4:Cr3+ powder synthesized by the citric-gel method. There are no ZnO-phase or Ga2O3-phase peaks discernible in the pattern and only the ZnGa2O4 single spinel phase is observed. The SEM image of the formed device in its cross sectional view is shown in Fig. 2(1). It is found that the interface between the phosphor layer and the dielectric layer cannot be apparently distinguished in the image. EDS analysis was performed along the line perpendicular to the cross section to detect the element ingredients. EDS of regions (a) and (b) on the cross section are shown in Fig. 2(2) and (3), respectively. According to the distribution of elements revealed in the EDS, the thickness of the BaTiO3 dielectric layer (region (a)) and the ZnGa2O4: Cr3+ phosphor layer (region (b)) could be concluded to be ∼ 40 μm and ∼60 μm, respectively.
Fig. 1. XRD pattern of the ZnGa2O4:Cr3+ powder phosphor.
Fig. 2. (1) Cross sectional SEM image, (2) EDS of region (a), (3) EDS of region (b).
The emission band of 695 nm is attributed to the 2E–4A2 transition of Cr3+ in octahedral sites [7]. In the EL process, Cr3+ ions may not be excited directly by the collision of hot electrons. The energy transfer from the host lattice to the luminescent centers may be involved which is favorable to the improvement of the excitation efficiency. Due to the energy transfer, the self-activated blue emission of the host originating
Fig. 3. Emission spectra of the ZnGa2O4:Cr3+ phosphors with different Cr3+ concentrations.
B. Qiao et al. / Materials Letters 61 (2007) 401–404
403
Fig. 6. Charge density versus voltage at 280 V and 1000 Hz. Fig. 4. Luminance versus various applied voltages from 70 to 300 V at a fixed frequency of 1000 Hz.
from the charge transfer between Ga3+ and O2− is greatly suppressed and the 695 nm emission of Cr3+ is dominant. As shown in Fig. 3, driven at a fixed voltage, the emission intensity varies with the concentration of doped Cr3+, and the optimal concentration could be determined to be 0.01 mol. With the increasing concentration of Cr3+, the emission band performs a redshift due to the effect of dopants on the crystal field. The fabricated powder EL device was driven by a sinusoidal power at varied applied voltages and frequencies. The threshold voltage (Vth) of the device could be determined at about 80 V. The electric field in the phosphor layer could be calculated as 105 V/cm. As known, the luminance versus applied voltage characteristics for the conventional EL devices can be fitted to the following equation: L ¼ L0 expðbV 1=2 Þ Where L is luminance, V is applied voltage and L0 and b are constants which are determined by the phosphor material, device structure and exciting condition [8]. The equation reveals that the logarithm of L is directly proportional to V − 1 / 2. Luminance (L) versus applied voltage (V − 1 / 2) varying from 70 to 300 V at a fixed frequency of 1000 Hz is plotted in Fig. 4. The experimental results shown in the figure are roughly in accordance with the above equation, which suggests that the
excitation mechanism of the powder EL of the ZnGa2O4:Cr3+ phosphor is just similar with that of the conventional sulfide phosphors. Driven at 280 V and 1000 Hz, a luminance of 1 cd/m2 could be obtained. As shown in Fig. 5, driven a fixed voltage, the luminance is linearly dependent on the frequency. It can be explained by an increase of the excitation probability with increasing frequency. However, high driven frequency may accelerate the aging of the device. And as depicted in the inlet, the shape and locality of EL spectra are independent of the frequency. Thus, oxide phosphor ZnGa2O4:Cr3+-based powder EL devices possess higher stability in an operation condition compared with the conventional sulfide-based devices. Charge density (Q) versus applied voltage (V) under 280 V and 100 Hz is depicted in Fig. 6. The Q–V diagram is based on a conventional Sawyer–Tower circuit. The remaining charge density (Q′) is 0.75 nC/cm2 when the applied voltage is 0 V. The charge density reaches a maximum of 1.7 nC/cm2 at 240 V in the falling phase of the applied voltage. The area encompassed within the Q–V diagram gives the energy density delivered to an EL device per cycle, Ein, i.e., input power density per cycle. Therefore, the input power density delivered to the EL device, Pin, is just the area times the driven frequency. Assuming an approximate parallelogram form for the Q–V diagram, Ein and Pin are given by [8]: Ein ¼ 4 Vth Q V Pin ¼ f Ein ¼ 4 f Vth Q V According to the above equations and Fig. 6, the input power density could be approximately calculated as 0.24 W/m2. As known, the luminous efficiency η could be expressed in terms of L and Pin as [8]: g ½lm=W ¼ p
L ½cd=m2 Pin ½W=m2
Thus, the luminous efficiency of the fabricated powder EL device is calculated as 13 lm/W.
4. Conclusion
Fig. 5. Luminance versus various frequencies from 60 Hz to 1 kHz at a fixed voltage of 240 V. EL spectra with increasing frequency are depicted in the inlet.
An a.c. powder EL device using a ZnGa2O4:Cr3+ phosphor was fabricated by the screen printing method. According to the EDS of the cross section, the thickness of the phosphor layer
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B. Qiao et al. / Materials Letters 61 (2007) 401–404
could be concluded as 60 μm, and accordingly the electric field in the phosphor layer could be calculated as 105 V/m. Above the threshold voltage (80 V), the ZnGa2O4:Cr3+ EL device gives a red emission at 695 nm due to the 2E–4A2 transition of the Cr3+ centers. The excitation could be attributed to the energy transfer from hot electrons to Cr3+ via a self-activated host lattice. The relationship between luminance (L) and applied voltage (V) is compatible with the equation of L = L0exp(− bV − 1 / 2). Luminance is linearly dependent on the frequency and the emission color is not affected by the operation condition with a high stability. According to the Q–V circuit, driven at 280 V and 100 Hz, the luminance and luminous efficiency could be determined to be 1 cd/m2 and 13 lm/W, respectively. Acknowledgements This project was supported by the National Natural Science Foundation of China (No. 59872016).
References [1] G. Destriau, J. Chim. Phys. (France) 33 (1936) 587. [2] J.S. Kim, S.G. Lee, H.L. Park, J.Y. Park, S.D. Han, Mater. Lett. 58 (2004) 1354–1357. [3] C.F. Yu, P. Lin, J. Mater. Sci. Lett. 17 (1998) 555–557. [4] P.D. Rack, A. Naman, P.H. Holloway, S.S. Sun, R.T. Tuenge, MRS Bull. 21 (1996) 49. [5] N.A. Vlasenko, Iu.A. Poplov, Opt. Spectrosc. 8 (1960) 39. [6] T. Minami, Solid-State Electron. 47 (2003) 2237–2243. [7] J.S. Kim, J.S. Kim, H.L. Park, Solid State Commun. 131 (2004) 735–738. [8] Y.A. Ono, Electroluminescence Displays, World Scientific, Singapore, 1995.
Study on ZnGa2O4:Cr 3+ a.c. powder electroluminescent device B. Qiao ⁎, Z.L. Tang, Z.T. Zhang, L. Chen State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China Received 15 February 2006; accepted 12 April 2006 Available online 19 May 2006
Abstract An a.c. powder electroluminescent (EL) device using ZnGa2O4:Cr3+ phosphor was fabricated by the screen printing method. Optical and electrical properties of the device were investigated. The fabricated device shows a red emission at 695 nm driven by the a.c. voltage. The emission is attributed to the energy transfer from hot electrons to Cr3+ centers via self-activated Ga–O groups. Luminance (L) versus voltage (V) matches the well-known equation of L = L0exp(− bV − 1 / 2) and luminance increases proportionally with frequency due to the increase of excitation probability of host lattice or Cr3+ centers. The diagram of the charge density (Q) versus applied voltage (V) is based on a conventional Sawyer– Tower circuit. At 280 V and 1000 Hz, the luminance and the luminous efficiency of the fabricated powder EL device are about 1.0 cd/m2 and 13 lm/W, respectively. And under the high field, the device fabricated with the oxide-based phosphor of ZnGa2O4:Cr3+ shows excellent stability in comparison with the conventional sulfide powder EL device. © 2006 Elsevier B.V All rights reserved. Keywords: Phosphor; Electroluminescence; ZnGa2O4:Cr3+
1. Introduction Electroluminescence (EL) was first discovered in 1936 by George Destriau [1]. According to the phosphor configuration, EL devices could be categorized into two types: powder EL and thin film EL (TFEL). Powder EL device is usually fabricated by screen printing which is an available and low-cost way compared with the fabricating method of the TFEL device. An advantage of powder EL devices is that the emission colors could be readily controlled by mixing different phosphors with different emission colors. And the characteristics, such as uniform light emission, thin profile and low power consumption provide the powder EL device infinitely applied potential. In virtue of the abovementioned features, powder EL devices are widely used as the backlight of liquid crystal displays (LCD) such as cell phone, personal digital assistant (PDA) and palmtop computer [2]. The basic structure of the powder EL device consists of a transparent electrode layer on a substrate, a phosphor layer, a dielectric layer and a back electrode layer [3]. Conventionally, sulfide phosphors with various emission colors ⁎ Corresponding author. E-mail address: [email protected] (B. Qiao). 0167-577X/$ - see front matter © 2006 Elsevier B.V All rights reserved. doi:10.1016/j.matlet.2006.04.070
are employed as the phosphor layer in powder EL devices, such as ZnS:Cu, Cl (blue or green) [4], and ZnS:Mn (orange) [5]. However, the biggest problem of the sulfide phosphors used for commercially available EL devices is the instability and the consequent short-term reliability, which greatly inhibits the development of the EL display. Oxide phosphors have recently attracted much attention because of the variety of materials available and the high chemical stability. Using oxide phosphor as the emitting layer of the EL device could overcome the inferiority of instability and satisfy the requirement for long lifetime. Since the first report of a TFEL device using Mnactivated Zn2SiO4 phosphor, high-luminance multicolor-emitting TFEL devices have been fabricated using various oxide phosphors activated with Eu or Mn [6]. However, there are few papers on the ZnGa2O4:Cr3+ phosphor applied for thick powder EL devices. In this work, the powder EL device using the ZnGa2O4:Cr3+ phosphor was fabricated by screen printing and the optical and electrical properties were investigated. 2. Experimental The ZnGa2O4:Cr3+ phosphor was synthesized by a citric-gel method using citric acid as the chelate agent. After being pre-
402
B. Qiao et al. / Materials Letters 61 (2007) 401–404
calcined at 650 °C and eliminating the organic groups, the powders as synthesized were calcined at 1000 °C for 3 h in an air ambient. The a.c. powder electroluminescent devices were fabricated by a screen printing method. The devices consist of a PET substrate, a transparent ITO electrode, a ZnGa2O4:Cr3+ phosphor layer, a BaTiO3 dielectric layer, and an Ag rear electrode. The ZnGa2O4:Cr3+ phosphor powder was dispersed in a dielectric organic binder (cyanoethyl cellulate) with a weight ratio of 2:1 and deposited thrice on the ITO substrate by screen printing. The formed phosphor layer is about 60 μm thick. And the high dielectric layer (BaTiO3 mixed with the organic binder) was printed twice upon the phosphor layer to avoid a catastrophic dielectric breakdown. The thickness of the dielectric layer is about 40 μm. The Ag paste was printed upon the dielectric layer as a rear electrode. The fabricated thick film EL device is driven by a sinusoidal voltage. The phase of the synthesized phosphor was identified with a Rigaku D/max-RB X-ray diffraction spectrometer. EL emission spectrum and luminance were measured by Cary Eclipse fluorospectrometer and ST-86LA luminance meter, respectively. Hitachi S-450 scanning electron microscope (SEM) was employed to show the cross section of the device. The electrical properties were measured with Textronix oscilloscope. 3. Results and discussion Fig. 1 depicts the XRD pattern of the ZnGa2O4:Cr3+ powder synthesized by the citric-gel method. There are no ZnO-phase or Ga2O3-phase peaks discernible in the pattern and only the ZnGa2O4 single spinel phase is observed. The SEM image of the formed device in its cross sectional view is shown in Fig. 2(1). It is found that the interface between the phosphor layer and the dielectric layer cannot be apparently distinguished in the image. EDS analysis was performed along the line perpendicular to the cross section to detect the element ingredients. EDS of regions (a) and (b) on the cross section are shown in Fig. 2(2) and (3), respectively. According to the distribution of elements revealed in the EDS, the thickness of the BaTiO3 dielectric layer (region (a)) and the ZnGa2O4: Cr3+ phosphor layer (region (b)) could be concluded to be ∼ 40 μm and ∼60 μm, respectively.
Fig. 1. XRD pattern of the ZnGa2O4:Cr3+ powder phosphor.
Fig. 2. (1) Cross sectional SEM image, (2) EDS of region (a), (3) EDS of region (b).
The emission band of 695 nm is attributed to the 2E–4A2 transition of Cr3+ in octahedral sites [7]. In the EL process, Cr3+ ions may not be excited directly by the collision of hot electrons. The energy transfer from the host lattice to the luminescent centers may be involved which is favorable to the improvement of the excitation efficiency. Due to the energy transfer, the self-activated blue emission of the host originating
Fig. 3. Emission spectra of the ZnGa2O4:Cr3+ phosphors with different Cr3+ concentrations.
B. Qiao et al. / Materials Letters 61 (2007) 401–404
403
Fig. 6. Charge density versus voltage at 280 V and 1000 Hz. Fig. 4. Luminance versus various applied voltages from 70 to 300 V at a fixed frequency of 1000 Hz.
from the charge transfer between Ga3+ and O2− is greatly suppressed and the 695 nm emission of Cr3+ is dominant. As shown in Fig. 3, driven at a fixed voltage, the emission intensity varies with the concentration of doped Cr3+, and the optimal concentration could be determined to be 0.01 mol. With the increasing concentration of Cr3+, the emission band performs a redshift due to the effect of dopants on the crystal field. The fabricated powder EL device was driven by a sinusoidal power at varied applied voltages and frequencies. The threshold voltage (Vth) of the device could be determined at about 80 V. The electric field in the phosphor layer could be calculated as 105 V/cm. As known, the luminance versus applied voltage characteristics for the conventional EL devices can be fitted to the following equation: L ¼ L0 expðbV 1=2 Þ Where L is luminance, V is applied voltage and L0 and b are constants which are determined by the phosphor material, device structure and exciting condition [8]. The equation reveals that the logarithm of L is directly proportional to V − 1 / 2. Luminance (L) versus applied voltage (V − 1 / 2) varying from 70 to 300 V at a fixed frequency of 1000 Hz is plotted in Fig. 4. The experimental results shown in the figure are roughly in accordance with the above equation, which suggests that the
excitation mechanism of the powder EL of the ZnGa2O4:Cr3+ phosphor is just similar with that of the conventional sulfide phosphors. Driven at 280 V and 1000 Hz, a luminance of 1 cd/m2 could be obtained. As shown in Fig. 5, driven a fixed voltage, the luminance is linearly dependent on the frequency. It can be explained by an increase of the excitation probability with increasing frequency. However, high driven frequency may accelerate the aging of the device. And as depicted in the inlet, the shape and locality of EL spectra are independent of the frequency. Thus, oxide phosphor ZnGa2O4:Cr3+-based powder EL devices possess higher stability in an operation condition compared with the conventional sulfide-based devices. Charge density (Q) versus applied voltage (V) under 280 V and 100 Hz is depicted in Fig. 6. The Q–V diagram is based on a conventional Sawyer–Tower circuit. The remaining charge density (Q′) is 0.75 nC/cm2 when the applied voltage is 0 V. The charge density reaches a maximum of 1.7 nC/cm2 at 240 V in the falling phase of the applied voltage. The area encompassed within the Q–V diagram gives the energy density delivered to an EL device per cycle, Ein, i.e., input power density per cycle. Therefore, the input power density delivered to the EL device, Pin, is just the area times the driven frequency. Assuming an approximate parallelogram form for the Q–V diagram, Ein and Pin are given by [8]: Ein ¼ 4 Vth Q V Pin ¼ f Ein ¼ 4 f Vth Q V According to the above equations and Fig. 6, the input power density could be approximately calculated as 0.24 W/m2. As known, the luminous efficiency η could be expressed in terms of L and Pin as [8]: g ½lm=W ¼ p
L ½cd=m2 Pin ½W=m2
Thus, the luminous efficiency of the fabricated powder EL device is calculated as 13 lm/W.
4. Conclusion
Fig. 5. Luminance versus various frequencies from 60 Hz to 1 kHz at a fixed voltage of 240 V. EL spectra with increasing frequency are depicted in the inlet.
An a.c. powder EL device using a ZnGa2O4:Cr3+ phosphor was fabricated by the screen printing method. According to the EDS of the cross section, the thickness of the phosphor layer
404
B. Qiao et al. / Materials Letters 61 (2007) 401–404
could be concluded as 60 μm, and accordingly the electric field in the phosphor layer could be calculated as 105 V/m. Above the threshold voltage (80 V), the ZnGa2O4:Cr3+ EL device gives a red emission at 695 nm due to the 2E–4A2 transition of the Cr3+ centers. The excitation could be attributed to the energy transfer from hot electrons to Cr3+ via a self-activated host lattice. The relationship between luminance (L) and applied voltage (V) is compatible with the equation of L = L0exp(− bV − 1 / 2). Luminance is linearly dependent on the frequency and the emission color is not affected by the operation condition with a high stability. According to the Q–V circuit, driven at 280 V and 100 Hz, the luminance and luminous efficiency could be determined to be 1 cd/m2 and 13 lm/W, respectively. Acknowledgements This project was supported by the National Natural Science Foundation of China (No. 59872016).
References [1] G. Destriau, J. Chim. Phys. (France) 33 (1936) 587. [2] J.S. Kim, S.G. Lee, H.L. Park, J.Y. Park, S.D. Han, Mater. Lett. 58 (2004) 1354–1357. [3] C.F. Yu, P. Lin, J. Mater. Sci. Lett. 17 (1998) 555–557. [4] P.D. Rack, A. Naman, P.H. Holloway, S.S. Sun, R.T. Tuenge, MRS Bull. 21 (1996) 49. [5] N.A. Vlasenko, Iu.A. Poplov, Opt. Spectrosc. 8 (1960) 39. [6] T. Minami, Solid-State Electron. 47 (2003) 2237–2243. [7] J.S. Kim, J.S. Kim, H.L. Park, Solid State Commun. 131 (2004) 735–738. [8] Y.A. Ono, Electroluminescence Displays, World Scientific, Singapore, 1995.