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Hybrid organic–inorganic light-emitting diodes
Microelectronic Engineering 69 (2003) 208–212 www.elsevier.com / locate / mee
Hybrid organic–inorganic light-emitting diodes a b, b b b O.N. Ermakov , M.G. Kaplunov *, O.N. Efimov , I.K. Yakushchenko , M.Yu. Belov , b M.F. Budyka b
a Joint-Stock Scientific and Technological Co. « Sapphire», Moscow, Russia Institute of Problems of Chemical Physics, Russian Academy of Sciences, Chernogolovka, Moscow Region, Russia
Abstract We demonstrate the hybrid organic–inorganic LEDs based on GaN and InGaN blue emitters with some organic luminophores providing blue, green, red and white emission colors with high luminous efficiency. The color of the emitted light from the GaN-based LED can be changed by varying the current through the device due to change in the relation between the UV and visible components of GaN electroluminescence. 2003 Elsevier B.V. All rights reserved. Keywords: Luminescence; Lighting; Colors; White light
1. Introduction Semiconductor light-emitting diodes (LEDs) are widely used today [1,2]. Recent progress in Group III compound technology has enabled the development of bright LEDs with high luminous efficiency which are prospects for solid state lighting [2,3]. However, while blue, green and red LEDs are available, separate structures, materials and growth processes are required to achieve the different colors thus resulting in completely different devices. Besides, relatively narrow spectral distribution makes it difficult to obtain white light sources. A prospective way to obtain a wide range of colors from such a system is a hybrid organic– inorganic LED in which the electroluminescence from Group III nitride crystals provides the blue component and, simultaneously, serves as the pump for exciting the green and red photoluminescence *Corresponding author. E-mail address: [email protected] (M.G. Kaplunov).
(PL) of organic luminescent substances. A model of such LEDs was demonstrated in Ref. [4] where conjugated polymer films located outside the blue InGaN LED were used to convert blue light into green and red colors. In this work we demonstrate true hybrid LEDs based on GaN and InGaN systems with organic luminophores located inside the LED and providing blue, green, red and white colors with high luminous efficiency. The luminophores were selected so as to comply with the technological process of LED fabrication. The color of the emitted light from the GaN-based LED can be changed by varying the current through the device due to change in the relation between the UV and visible components of GaN electroluminescence.
2. Experimental A MOCVD grown InGaN quantum well LED was used as a blue pump source. The emission peaked at
0167-9317 / 03 / $ – see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016 / S0167-9317(03)00298-3
O.N. Ermakov et al. / Microelectronic Engineering 69 (2003) 208–212
450 nm with corresponding CIE chromaticity coordinates of (0.151, 0.036). The LED chip is placed in a metallic reflector and encapsulated in a transparent epoxy lens (Fig. 1). The LED was supplied by direct current of 15 mA at about 3.5 V. To prepare a hybrid LED, a small amount of organic substance was dissolved in epoxy, placed in a reflector together with the LED chip (Fig. 1) and polymerized before forming the epoxy lens. Aromatic amines with high PL efficiency [5] were used as organic substances for obtaining green light. A typical example is 4-dimethylaminochalcone (DMAC) which absorbs at 450 nm and emits at 510 nm. Red dyes were used together with aromatic amines to obtain red light. The typical example is Nile Red (NR) absorbing at about 500 nm and emitting at about 600 nm. The additional stage of introducing an organic substance complies well with the standard technique of LED fabrication. The organic substances were selected due to their high chemical durability and high luminous efficiency. A MOCVD grown GaN-based LED with twoband emission spectrum was used for experiments with regulated emission. The emission peaked at 360 and 420 nm. The LED was supplied by rectangular pulses of current with 10 ms pulse duration and 100 ms period. The current in the pulse can be varied from 15 to 150 mA which resulted in a change of the
Fig. 1. The scheme of the hybrid LED: 1, semiconductor emitting element; 2, organic luminescent substance; 3, reflector; 4, epoxy lens.
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relative intensity of 360 and 420 emission bands. The oligomer of triphenylamine (PTA) [6] absorbing at 370 nm and emitting at 470–490 nm was used as an organic luminous substance for these experiments. The emission spectra were measured with an integrating sphere connected by a fiber optic light guide to the plug-in spectrometer PC1000 (Ocean Optics).
3. Results and discussion Fig. 2 shows some examples of the hybrid LED emission spectra. Curve 1 represents the spectrum of original InGaN LED without organic luminous substance. Curve 2 represents the spectrum of InGaNbased hybrid LED containing a layer of DMAC dissolved in epoxy at a concentration of about 0.5
Fig. 2. Emission spectra of InGaN LEDs: 1, original InGaN LED; 2–5, hybrid LEDs (see text).
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g / l covering the InGaN emitting element. The spectrum exhibits a large amount of original InGaN blue emission and a sufficient part of the green PL of DMAC. The emission is characterized by CIE coordinates of (0.197, 0.420) and visually looks blue–green. The integrated visual light intensity for this LED (measured in lumens) is 3.4 times that of the original InGaN LED. Curve 3 represents the spectrum of a similar hybrid LED containing DMAC at much higher concentration of about 5 g / l. The emission is characterized by CIE coordinates (0.400, 0.590) and visually looks yellow–green. The visual light intensity is 1.6 times that of the original InGaN LED. Curves 4 and 5 represent the spectra of hybrid LEDs in which the organic luminous region has a two-layered structure. In LED 4, first the DMACcontaining layer covers the InGaN emitting element and second the NR-containing layer covers the DMAC layer. In LED 5, the sequence of layers is vice versa. Red dye absorbs the green emission of aromatic amines and partially the blue emission of InGaN and transforms them to red emission. The resulting light is a mixture of blue, green and red emissions. The emission of LEDs 4 and 5 is characterized by CIE coordinates of (0.268, 0.357) and (0.580, 0.435) correspondingly. Visually the emission looks light-yellow (nearly white) for LED 4 and orange–red for LED 5. The visual light intensity is 2.6 and 0.15 times that of original InGaN LED for LEDs 4 and 5 correspondingly. In Fig. 3, color characteristics of our hybrid LEDs are shown as points on the CIE chromaticity diagram. Each point represents one LED. All the LEDs contain DMAC and NR in different concentrations and proportions. It may be seen that the points cover a large range of colors from blue to red and some points are very close to the white light point (open circle in the center of the diagram). Thus the described technique provides a simple means of obtaining LED of any desired color including white. The visible luminosity of hybrid LEDs is normally higher than that of original InGaN LED especially in the green region. This is demonstrated in Fig. 4 where relative luminosity (InGaN is unity) versus wavelength of emission is given. Each point corresponds to a point in Fig. 3. Relative luminosity reaches a maximum in the green region.
Fig. 3. CIE chromaticity diagram. Points correspond to the emission of hybrid LEDs; open circle, the point of white light.
Figs. 5 and 6 show the emission spectra of GaNbased LEDs at different driving currents. For GaN LED (Fig. 5), the 420 nm band originating from transitions involving impurity levels is saturated at high currents while the 360 nm band which is due to intrinsic interband transition is not saturated. This is demonstrated by current–intensity dependence shown in Fig. 7. For a hybrid GaN-based LED containing a small amount of PTA (Fig. 6), the spectra exhibit the PL band of PTA at 470 nm and the residual 420 nm band of GaN emission. The 320
Fig. 4. The dependence of relative visual luminosity of hybrid LEDs on their dominating emission wavelength.
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nm emission of GaN is completely absorbed by PTA. At high currents the GaN emission band at 420 nm is saturated similar to the original GaN while the PTA PL band at 470 nm is not saturated. This results in changing of the intensity relation between the 420 and 470 nm bands (Fig. 7) with changing of the current. Such a property is due to the fact that PL of PTA is excited to a great extent by the non-saturating GaN UV emission band. Thus the resulting color of the hybrid LED emission can be changed by changing the driving current. Fig. 5. Emission spectra of GaN LED at currents 15 (a), 30 (b), 60 (c), 90 (d) and 150 (e) mA.
Fig. 6. Emission spectra of the hybrid GaN–PTA LED at currents 15, 45, 75, 105 and 150 mA.
4. Conclusions We have demonstrated the universal technique of producing bright LEDs of different colors using only one type of semiconductor electroluminescent element emitting blue light. Any desired color (including white) can be obtained by introducing organic luminophores into the LED. Examples of appropriate luminophores are proposed. The additional stage of introducing an organic substance complies well with the usual process of LED fabrication. The visual brightness of hybrid LEDs is normally higher than that of original blue LED especially in the green spectral region. The application of this universal technique can sufficiently lower the cost of bright color LEDs appropriate for solid state lighting because in this case there is no need for high-cost development and production of separate semiconductor electroluminescent emitters for each new color. In addition, an example is given of regulatedcolor LED where the emitted color can be changed by varying the current through the device
References
Fig. 7. The dependence of relative emission intensity on current: 1, 420 nm line for GaN LED (open circles) and GaN–PTA LED (solid squares); 2, 320 nm line for GaN LED; 3, the relation of 490 and 420 nm line intensities for GaN–PTA LED.
[1] O.N. Ermakov, V.P. Sushkov, Semiconductor Alphabet Displays, Radio i Svyas, Moscow, 1990. [2] A. Berg, G. Craford, A. Duggal, R. Haitz, The promise and challenge of solid-state lighting, Phys. Today 4 (2001) 42– 47. [3] S.J. Pearton, C.R. Abernathy, M.E. Overberg, G.T. Thaler, A.H. Onstine, B.P. Gila, F. Ren, B. Lou, J. Kim, New applications for gallium nitride, Mater. Today (2002) 24–31, June.
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[4] F. Hide, P. Kozodoy, S.P. DenDaars, A.J. Heeger, White light from InGaN / conjugated polymer hybrid light emitting diodes, Appl. Phys. Lett. 70 (1997) 2664–2666. [5] M.F. Budyka, M.V. Alfimov, Photochemical reactions of complexes of aromatic amines with polyhalomethans, Russ. Chem. Rev. 64 (1995) 705–717.
[6] I.K. Yakushchenko, M.G. Kaplunov, O.N. Efimov, M.Yu. Belov, S.N. Shamaev, Polytriphenylamine derivatives as materials for hole transporting layers in electroluminescent devices, Phys. Chem. Chem. Phys. 1 (1999) 1783–1785.
Hybrid organic–inorganic light-emitting diodes a b, b b b O.N. Ermakov , M.G. Kaplunov *, O.N. Efimov , I.K. Yakushchenko , M.Yu. Belov , b M.F. Budyka b
a Joint-Stock Scientific and Technological Co. « Sapphire», Moscow, Russia Institute of Problems of Chemical Physics, Russian Academy of Sciences, Chernogolovka, Moscow Region, Russia
Abstract We demonstrate the hybrid organic–inorganic LEDs based on GaN and InGaN blue emitters with some organic luminophores providing blue, green, red and white emission colors with high luminous efficiency. The color of the emitted light from the GaN-based LED can be changed by varying the current through the device due to change in the relation between the UV and visible components of GaN electroluminescence. 2003 Elsevier B.V. All rights reserved. Keywords: Luminescence; Lighting; Colors; White light
1. Introduction Semiconductor light-emitting diodes (LEDs) are widely used today [1,2]. Recent progress in Group III compound technology has enabled the development of bright LEDs with high luminous efficiency which are prospects for solid state lighting [2,3]. However, while blue, green and red LEDs are available, separate structures, materials and growth processes are required to achieve the different colors thus resulting in completely different devices. Besides, relatively narrow spectral distribution makes it difficult to obtain white light sources. A prospective way to obtain a wide range of colors from such a system is a hybrid organic– inorganic LED in which the electroluminescence from Group III nitride crystals provides the blue component and, simultaneously, serves as the pump for exciting the green and red photoluminescence *Corresponding author. E-mail address: [email protected] (M.G. Kaplunov).
(PL) of organic luminescent substances. A model of such LEDs was demonstrated in Ref. [4] where conjugated polymer films located outside the blue InGaN LED were used to convert blue light into green and red colors. In this work we demonstrate true hybrid LEDs based on GaN and InGaN systems with organic luminophores located inside the LED and providing blue, green, red and white colors with high luminous efficiency. The luminophores were selected so as to comply with the technological process of LED fabrication. The color of the emitted light from the GaN-based LED can be changed by varying the current through the device due to change in the relation between the UV and visible components of GaN electroluminescence.
2. Experimental A MOCVD grown InGaN quantum well LED was used as a blue pump source. The emission peaked at
0167-9317 / 03 / $ – see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016 / S0167-9317(03)00298-3
O.N. Ermakov et al. / Microelectronic Engineering 69 (2003) 208–212
450 nm with corresponding CIE chromaticity coordinates of (0.151, 0.036). The LED chip is placed in a metallic reflector and encapsulated in a transparent epoxy lens (Fig. 1). The LED was supplied by direct current of 15 mA at about 3.5 V. To prepare a hybrid LED, a small amount of organic substance was dissolved in epoxy, placed in a reflector together with the LED chip (Fig. 1) and polymerized before forming the epoxy lens. Aromatic amines with high PL efficiency [5] were used as organic substances for obtaining green light. A typical example is 4-dimethylaminochalcone (DMAC) which absorbs at 450 nm and emits at 510 nm. Red dyes were used together with aromatic amines to obtain red light. The typical example is Nile Red (NR) absorbing at about 500 nm and emitting at about 600 nm. The additional stage of introducing an organic substance complies well with the standard technique of LED fabrication. The organic substances were selected due to their high chemical durability and high luminous efficiency. A MOCVD grown GaN-based LED with twoband emission spectrum was used for experiments with regulated emission. The emission peaked at 360 and 420 nm. The LED was supplied by rectangular pulses of current with 10 ms pulse duration and 100 ms period. The current in the pulse can be varied from 15 to 150 mA which resulted in a change of the
Fig. 1. The scheme of the hybrid LED: 1, semiconductor emitting element; 2, organic luminescent substance; 3, reflector; 4, epoxy lens.
209
relative intensity of 360 and 420 emission bands. The oligomer of triphenylamine (PTA) [6] absorbing at 370 nm and emitting at 470–490 nm was used as an organic luminous substance for these experiments. The emission spectra were measured with an integrating sphere connected by a fiber optic light guide to the plug-in spectrometer PC1000 (Ocean Optics).
3. Results and discussion Fig. 2 shows some examples of the hybrid LED emission spectra. Curve 1 represents the spectrum of original InGaN LED without organic luminous substance. Curve 2 represents the spectrum of InGaNbased hybrid LED containing a layer of DMAC dissolved in epoxy at a concentration of about 0.5
Fig. 2. Emission spectra of InGaN LEDs: 1, original InGaN LED; 2–5, hybrid LEDs (see text).
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g / l covering the InGaN emitting element. The spectrum exhibits a large amount of original InGaN blue emission and a sufficient part of the green PL of DMAC. The emission is characterized by CIE coordinates of (0.197, 0.420) and visually looks blue–green. The integrated visual light intensity for this LED (measured in lumens) is 3.4 times that of the original InGaN LED. Curve 3 represents the spectrum of a similar hybrid LED containing DMAC at much higher concentration of about 5 g / l. The emission is characterized by CIE coordinates (0.400, 0.590) and visually looks yellow–green. The visual light intensity is 1.6 times that of the original InGaN LED. Curves 4 and 5 represent the spectra of hybrid LEDs in which the organic luminous region has a two-layered structure. In LED 4, first the DMACcontaining layer covers the InGaN emitting element and second the NR-containing layer covers the DMAC layer. In LED 5, the sequence of layers is vice versa. Red dye absorbs the green emission of aromatic amines and partially the blue emission of InGaN and transforms them to red emission. The resulting light is a mixture of blue, green and red emissions. The emission of LEDs 4 and 5 is characterized by CIE coordinates of (0.268, 0.357) and (0.580, 0.435) correspondingly. Visually the emission looks light-yellow (nearly white) for LED 4 and orange–red for LED 5. The visual light intensity is 2.6 and 0.15 times that of original InGaN LED for LEDs 4 and 5 correspondingly. In Fig. 3, color characteristics of our hybrid LEDs are shown as points on the CIE chromaticity diagram. Each point represents one LED. All the LEDs contain DMAC and NR in different concentrations and proportions. It may be seen that the points cover a large range of colors from blue to red and some points are very close to the white light point (open circle in the center of the diagram). Thus the described technique provides a simple means of obtaining LED of any desired color including white. The visible luminosity of hybrid LEDs is normally higher than that of original InGaN LED especially in the green region. This is demonstrated in Fig. 4 where relative luminosity (InGaN is unity) versus wavelength of emission is given. Each point corresponds to a point in Fig. 3. Relative luminosity reaches a maximum in the green region.
Fig. 3. CIE chromaticity diagram. Points correspond to the emission of hybrid LEDs; open circle, the point of white light.
Figs. 5 and 6 show the emission spectra of GaNbased LEDs at different driving currents. For GaN LED (Fig. 5), the 420 nm band originating from transitions involving impurity levels is saturated at high currents while the 360 nm band which is due to intrinsic interband transition is not saturated. This is demonstrated by current–intensity dependence shown in Fig. 7. For a hybrid GaN-based LED containing a small amount of PTA (Fig. 6), the spectra exhibit the PL band of PTA at 470 nm and the residual 420 nm band of GaN emission. The 320
Fig. 4. The dependence of relative visual luminosity of hybrid LEDs on their dominating emission wavelength.
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nm emission of GaN is completely absorbed by PTA. At high currents the GaN emission band at 420 nm is saturated similar to the original GaN while the PTA PL band at 470 nm is not saturated. This results in changing of the intensity relation between the 420 and 470 nm bands (Fig. 7) with changing of the current. Such a property is due to the fact that PL of PTA is excited to a great extent by the non-saturating GaN UV emission band. Thus the resulting color of the hybrid LED emission can be changed by changing the driving current. Fig. 5. Emission spectra of GaN LED at currents 15 (a), 30 (b), 60 (c), 90 (d) and 150 (e) mA.
Fig. 6. Emission spectra of the hybrid GaN–PTA LED at currents 15, 45, 75, 105 and 150 mA.
4. Conclusions We have demonstrated the universal technique of producing bright LEDs of different colors using only one type of semiconductor electroluminescent element emitting blue light. Any desired color (including white) can be obtained by introducing organic luminophores into the LED. Examples of appropriate luminophores are proposed. The additional stage of introducing an organic substance complies well with the usual process of LED fabrication. The visual brightness of hybrid LEDs is normally higher than that of original blue LED especially in the green spectral region. The application of this universal technique can sufficiently lower the cost of bright color LEDs appropriate for solid state lighting because in this case there is no need for high-cost development and production of separate semiconductor electroluminescent emitters for each new color. In addition, an example is given of regulatedcolor LED where the emitted color can be changed by varying the current through the device
References
Fig. 7. The dependence of relative emission intensity on current: 1, 420 nm line for GaN LED (open circles) and GaN–PTA LED (solid squares); 2, 320 nm line for GaN LED; 3, the relation of 490 and 420 nm line intensities for GaN–PTA LED.
[1] O.N. Ermakov, V.P. Sushkov, Semiconductor Alphabet Displays, Radio i Svyas, Moscow, 1990. [2] A. Berg, G. Craford, A. Duggal, R. Haitz, The promise and challenge of solid-state lighting, Phys. Today 4 (2001) 42– 47. [3] S.J. Pearton, C.R. Abernathy, M.E. Overberg, G.T. Thaler, A.H. Onstine, B.P. Gila, F. Ren, B. Lou, J. Kim, New applications for gallium nitride, Mater. Today (2002) 24–31, June.
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[4] F. Hide, P. Kozodoy, S.P. DenDaars, A.J. Heeger, White light from InGaN / conjugated polymer hybrid light emitting diodes, Appl. Phys. Lett. 70 (1997) 2664–2666. [5] M.F. Budyka, M.V. Alfimov, Photochemical reactions of complexes of aromatic amines with polyhalomethans, Russ. Chem. Rev. 64 (1995) 705–717.
[6] I.K. Yakushchenko, M.G. Kaplunov, O.N. Efimov, M.Yu. Belov, S.N. Shamaev, Polytriphenylamine derivatives as materials for hole transporting layers in electroluminescent devices, Phys. Chem. Chem. Phys. 1 (1999) 1783–1785.