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Polymeric light-emitting diodes of submicron size — structures and developments
ELSEVIER
Synthetic Metals 76 (1996) 141-143
Polymeric light-emitting diodes of submicron size - structures and developments M. Granstrijm, M. Berggren, 0. Inganiis Laboratory
of Applied
Physics, Department
of Physics (IFM),
Lintiping
University,
S-581 83 Linkiiping,
Sweden
Abstract Micron- andsubmicron-sized light-emittingdiodes(LEDs) madeusingconjugatedpolymersaselectroluminescent layersandcontact materialsarepresented. Two differentroutesto makearraysof suchsmalllight sources havebeendeveloped.Thebenefitsanddrawbacksof the useof the conjugatedpolymerpoly(2,3-ethylene-dioxythiophene) (PEDOT) asholeinjectorin polymerLEDsarealsodiscussed. Keywords:
Diodes; Electroluminescence
1. Introduction The field of organic materials as active materials in lightemitting diodes has gained an incredible amount of interest since the first discoveries with molecules [ 1,2] and conjugated polymers [ 31. The use of organic materials makes it possible to use different approaches when designing and processing the devices, resulting in a wide range of possible geometries and applications. In this context we present two different methods of producing sub-wavelength-sized polymeric light-emitting diodes (LEDs) , using the same conjugated polymer as emitting material in both cases,but radically different methods of obtaining the small size.
2. Experimental
stop the polymerization when the polymer fibres reach the upper surface of the membrane. The electrode size is also expected to define the size of the light sources since the upper limit of the exciton diffusion length in conjugated polymers has been estimated to be 5 nm [ 10,111. Polycarbonate membranes (Nuclepore@) with pore diameters of 100 nm (thickness 6 mm) were attached to Au contacts on top of glass substrates [ 121. PEDOT was polymerized from a water solution of 0.1 M monomer and 0.1 M NaPSS (sodium polystyrenesulfonate) using a Bioanalytical Systems BAS 1OOAelectrochemical analyser, A standard three-electrode setup was used with Ag/AgCl and Pt as reference and counter electrodes, respectively, and the polymerization potential was 1.3 V versus Ag/AgCl. The Llght
ia>
2.1. Type I
In both types of structures, poly(2,3-ethylene-dioxythiophene) (PEDOT) [ 4-61 was used as hole-injecting contact and poly [ 3- (4-octylphenyl) -2,2’-bithiophene] (PTOPT) [7] as light-emitting polymer. However, the ways of producing the small light sources are very different. In type I, we polymerized the doped and conducting polymer PEDOT electrochemically in the randomly distributed pores of commercially available micro filtration membranes [ 8,9] to define the size of the light sources. The pore sizes in such membranes span from 10 nm to 14 pm. Electrosynthesis of conjugated polymers inside these pores results in electrodes of the same size as the pores, if care is taken to 0379-6779/96/$15.00
0 1996 Elsevier Science S.A. All rights reserved
Light Fig. 1. Diode structures: (a) type I; (b) type II.
142
M. Granstrijrn
et al. /Synthetic
Metals
76 (1996)
141-143
*5,00
.2.50
H
.n
Fig. 2. SFM pictures of PTOPT/PMMA
IJn w blends with (A) 25% PTOPT and (B) 5% PTOPT. The bright spots are the PTOPT phases,
polymerization was interrupted when the conductive polymer fibres reached the upper surface of the membrane to achieve a membrane surface with electrical contacts defined by the doped polymer fibres that had grown in the pores. The electroluminescent polymer PTOPT was spin-coated in its undoped form from a warm xylene solution (5 mg/ml, 50 “C) on top of the membrane/contact structure. The thickness of this layer was estimated from optical spectra to be 400 A. As electron-injecting contact, a thin layer of the low work function metal calcium [ 13 ] was evaporated on the PTOPT surface. The calcium layer was then covered with aluminium to protect the calcium from the ambient atmosphere. The total thickness of the metal layer was 180-250 A. The resulting structure is shown in Fig. 1 (a). 2.2. Type II
The second type of nano-LED is shown in Fig. 1 (b). In this type, we have taken advantage of the fact that polymer blends normally phase-separate. Using this it was possible to find a stoichiometry between PTOPT and the insulating polymer poly (methyl methacrylate) (PMMA) that gives small ‘islands’ of PTOPT that have the appropriate size. To find this stoichiometry, scanning force microscopy was used to identify the phases and sizes [ 14,151 (see Fig. 2). PEDOT was used as hole-injecting contact in this type of structure as well, although a different polymerization method was used to obtain a transparent, sky-blue film through which the light could be emitted. For this purpose, chemically polymerized PEDOT (poly (3,4-ethylene-dioxythiophene) ) films were made on glass substrates using the method in [ 161. The monomer was kindly donated by Bayer AG and the iron( III) tris-p-toluenesulfonate salt was prepared as described in the literature [ 171. The films were thoroughly rinsed in ethanol after preparation. Solutions of PMMA and PTOPT were prepared in chloroform (5% PTOPT and 95% PMMA by weight) and the polymer mixture was spin-coated (5 mg./ ml, 1000 rpm) on the PEDOT contact to form a 600-700 A
thick film. On top of this (see Fig. 1 (b)) a thin layer of calcium was vacuum-evaporated (P < 10T6 Torr), and was subsequently covered by an aluminium layer, as in type I. For this type of diode there was no need for the metal layer to be transparent, so the total thickness was chosen to be about 1500 A. 2.3. Measurements Current and light emissions as functions of voltage were measured using a computerized setup with a photodiode (Hamamatsu lOlOBR), a programmable electrometer and a picoamperometer (Keithley 617 and 417). Scanning force microscopy (SFM) was performed with a NanoScope III (Digital Instruments Inc.) run in tapping mode and an Olympus BHSP equipped with a 35 mm camera was used for the optical microscopy. 3. Results
In Fig. 3 typical current-voltage and light output curves are shown for the two types of small polymer LEDs. As can
0
5
10
15
20
VaIlage(v) Fig. 3. Current-voltage (solid lines) and light-voltage (dotted lines) char-
acteristicsof the hvotypesof polymerLEDs:typeII ismarked withcircles.
M. GranstrBm
et al. /Synthetic
Metals
76 (1996)
141-143
143
that the polymeric devices are far from optimized for efficiency, for instance, the current densities could be increased and pulse techniques [21] could be used to increase the photon emission by two orders of magnitude. This would put the polymer LEDs in the low-mid range of available subwavelength light sources when comparing the light intensities. The polymer nano-LEDs will thereby rather find their use in applications where their unique properties are necessary or give advantages compared to other techkiques. Such properties are the possibility of making a large number of light sources simultaneously and of putting the diodes on a flexible substrate [ 151.
Acknowledgements The iron( III) tris-p-toluenesulfonate salt was prepared by Dr Xiwen Chen. This work was supported by the Micronics Program of the Swedish National Board for Industrial and Technical Development (NUTEK) .
References
Fig. 4. Photographs of emitting nano-LEDs: (A) type I; (B) type II. Magnification 1000 X (reduced in reproduction by 62%) ; exposure time 15 min.
be seen from the figure, the behaviour is very similar for both types, although with one clear difference: the light output from type II is higher. This is mainly explained by the fact that in type I diodes the light is viewed through a metal layer, whereas in type II it comes through a transparent contact. Compared to LEDs made with IT0 as hole-injecting contact, the efficiency is about two orders of magnitude lower, possibly due to a larger mismatch in the position of the band edges. From the values of measured light in Fig. 3, the number of photons emitted per second from individual LEDs can be estimated. Such a calculation, including corrections for the surface area being contacts (for type I) and PTOPT phase (type II) and number of working diodes (as determined from pictures taken in the optical microscope, see Fig. 4)) gives a number in the range of IO* photons per second for both types of diodes. This should then be compared to other sub-micronsized light emitters that range from lo3 to 10” photons per second [ 18-201. In such a comparison it must be remembered
[ 11 C.W. Tang and S.A. VanSlyke, Appl. Phys. I.&t., 51 (1987) 913. [2] C. Ada&i, T. Tsutsui and S. Saito, Appl. Phys. Lea., 55 (1988) 1489. [3] J.H. Burroughes et al., Nature, 347 (1990) 539. [4] G. Heywang and F. Jonas,Adv. Mater., 4 (1992) 116. [5] Q. Pei, G. Zuccarello, M. Ahlskogand 0. Inganb, Polymer, 35 (1994) 1347. [6] M. Dietrich, J. Heinze, G. Heywang and F. Jonas,J. Hectrochem. Sot., 369 (1994) 87. [7] M. Berggren, G. Gustafsson,0. Inganti, M.R. Andersson, T. Hjertberg and 0. Wennerstriim, J. Appl. Phys., 76 (1994) 7530. [8] Z. Cai and CR. Martin, J. Am. Chem. Sot., Ill (1989) 4138. [9] M. Granstriim and 0. Inganb, Polymer, (1995) in press. [lo] A.R. Brown et al., Chem. Phys. Lett., 200 (1992) 1. [ 111 M. GranstrGm, M. Berggren and 0. Inganb, Science, 267 (1995) 1479. [ 121 M. Granstrijm and 0. Inganb, Synth. Met., 55-57 (1993) 460. [ 131 I.D. Parker, J. Appl. Phys., 75 (1994) 1656. [ 141 M. Berggren et al., Nature, 372 (1994) 444. [ 151 M. Granstriim and 0. Ingan&, Adv. Mater., submitted for publication. [16] D.M. de Leeuw, P.A. Kraakman, P.F.G. Bongaerts, C.M.J. Mutsaers and D.B.M. Klaassen, Synth. Met., 66 (1994) 263. [ 171 J.A. Walker, L.F. Warren and E.F. Witucki, J. Polym. Sci., Part A, Polym. Chem., 26 (1988) 1258. [ 181 N. Kuck, K. Liebennan, A. Lewis and A. Vecht, Appl. Phys. L.&t., 61 (1992) 139. [ 191 E. Betzig, J.K. Trautman, T.D. Harris, J.S. Weiner and R.L. Kostelak, Science, 2.51 (1991) 1468. [20] A. Lewis and K. Lieberman, Nature, 354 (1991) 214. [21] D. Braun, D. Moses, C. Zhang and A.J. Heeger, App/. Phys. Lett., 61 (1992)
3092.
Synthetic Metals 76 (1996) 141-143
Polymeric light-emitting diodes of submicron size - structures and developments M. Granstrijm, M. Berggren, 0. Inganiis Laboratory
of Applied
Physics, Department
of Physics (IFM),
Lintiping
University,
S-581 83 Linkiiping,
Sweden
Abstract Micron- andsubmicron-sized light-emittingdiodes(LEDs) madeusingconjugatedpolymersaselectroluminescent layersandcontact materialsarepresented. Two differentroutesto makearraysof suchsmalllight sources havebeendeveloped.Thebenefitsanddrawbacksof the useof the conjugatedpolymerpoly(2,3-ethylene-dioxythiophene) (PEDOT) asholeinjectorin polymerLEDsarealsodiscussed. Keywords:
Diodes; Electroluminescence
1. Introduction The field of organic materials as active materials in lightemitting diodes has gained an incredible amount of interest since the first discoveries with molecules [ 1,2] and conjugated polymers [ 31. The use of organic materials makes it possible to use different approaches when designing and processing the devices, resulting in a wide range of possible geometries and applications. In this context we present two different methods of producing sub-wavelength-sized polymeric light-emitting diodes (LEDs) , using the same conjugated polymer as emitting material in both cases,but radically different methods of obtaining the small size.
2. Experimental
stop the polymerization when the polymer fibres reach the upper surface of the membrane. The electrode size is also expected to define the size of the light sources since the upper limit of the exciton diffusion length in conjugated polymers has been estimated to be 5 nm [ 10,111. Polycarbonate membranes (Nuclepore@) with pore diameters of 100 nm (thickness 6 mm) were attached to Au contacts on top of glass substrates [ 121. PEDOT was polymerized from a water solution of 0.1 M monomer and 0.1 M NaPSS (sodium polystyrenesulfonate) using a Bioanalytical Systems BAS 1OOAelectrochemical analyser, A standard three-electrode setup was used with Ag/AgCl and Pt as reference and counter electrodes, respectively, and the polymerization potential was 1.3 V versus Ag/AgCl. The Llght
ia>
2.1. Type I
In both types of structures, poly(2,3-ethylene-dioxythiophene) (PEDOT) [ 4-61 was used as hole-injecting contact and poly [ 3- (4-octylphenyl) -2,2’-bithiophene] (PTOPT) [7] as light-emitting polymer. However, the ways of producing the small light sources are very different. In type I, we polymerized the doped and conducting polymer PEDOT electrochemically in the randomly distributed pores of commercially available micro filtration membranes [ 8,9] to define the size of the light sources. The pore sizes in such membranes span from 10 nm to 14 pm. Electrosynthesis of conjugated polymers inside these pores results in electrodes of the same size as the pores, if care is taken to 0379-6779/96/$15.00
0 1996 Elsevier Science S.A. All rights reserved
Light Fig. 1. Diode structures: (a) type I; (b) type II.
142
M. Granstrijrn
et al. /Synthetic
Metals
76 (1996)
141-143
*5,00
.2.50
H
.n
Fig. 2. SFM pictures of PTOPT/PMMA
IJn w blends with (A) 25% PTOPT and (B) 5% PTOPT. The bright spots are the PTOPT phases,
polymerization was interrupted when the conductive polymer fibres reached the upper surface of the membrane to achieve a membrane surface with electrical contacts defined by the doped polymer fibres that had grown in the pores. The electroluminescent polymer PTOPT was spin-coated in its undoped form from a warm xylene solution (5 mg/ml, 50 “C) on top of the membrane/contact structure. The thickness of this layer was estimated from optical spectra to be 400 A. As electron-injecting contact, a thin layer of the low work function metal calcium [ 13 ] was evaporated on the PTOPT surface. The calcium layer was then covered with aluminium to protect the calcium from the ambient atmosphere. The total thickness of the metal layer was 180-250 A. The resulting structure is shown in Fig. 1 (a). 2.2. Type II
The second type of nano-LED is shown in Fig. 1 (b). In this type, we have taken advantage of the fact that polymer blends normally phase-separate. Using this it was possible to find a stoichiometry between PTOPT and the insulating polymer poly (methyl methacrylate) (PMMA) that gives small ‘islands’ of PTOPT that have the appropriate size. To find this stoichiometry, scanning force microscopy was used to identify the phases and sizes [ 14,151 (see Fig. 2). PEDOT was used as hole-injecting contact in this type of structure as well, although a different polymerization method was used to obtain a transparent, sky-blue film through which the light could be emitted. For this purpose, chemically polymerized PEDOT (poly (3,4-ethylene-dioxythiophene) ) films were made on glass substrates using the method in [ 161. The monomer was kindly donated by Bayer AG and the iron( III) tris-p-toluenesulfonate salt was prepared as described in the literature [ 171. The films were thoroughly rinsed in ethanol after preparation. Solutions of PMMA and PTOPT were prepared in chloroform (5% PTOPT and 95% PMMA by weight) and the polymer mixture was spin-coated (5 mg./ ml, 1000 rpm) on the PEDOT contact to form a 600-700 A
thick film. On top of this (see Fig. 1 (b)) a thin layer of calcium was vacuum-evaporated (P < 10T6 Torr), and was subsequently covered by an aluminium layer, as in type I. For this type of diode there was no need for the metal layer to be transparent, so the total thickness was chosen to be about 1500 A. 2.3. Measurements Current and light emissions as functions of voltage were measured using a computerized setup with a photodiode (Hamamatsu lOlOBR), a programmable electrometer and a picoamperometer (Keithley 617 and 417). Scanning force microscopy (SFM) was performed with a NanoScope III (Digital Instruments Inc.) run in tapping mode and an Olympus BHSP equipped with a 35 mm camera was used for the optical microscopy. 3. Results
In Fig. 3 typical current-voltage and light output curves are shown for the two types of small polymer LEDs. As can
0
5
10
15
20
VaIlage(v) Fig. 3. Current-voltage (solid lines) and light-voltage (dotted lines) char-
acteristicsof the hvotypesof polymerLEDs:typeII ismarked withcircles.
M. GranstrBm
et al. /Synthetic
Metals
76 (1996)
141-143
143
that the polymeric devices are far from optimized for efficiency, for instance, the current densities could be increased and pulse techniques [21] could be used to increase the photon emission by two orders of magnitude. This would put the polymer LEDs in the low-mid range of available subwavelength light sources when comparing the light intensities. The polymer nano-LEDs will thereby rather find their use in applications where their unique properties are necessary or give advantages compared to other techkiques. Such properties are the possibility of making a large number of light sources simultaneously and of putting the diodes on a flexible substrate [ 151.
Acknowledgements The iron( III) tris-p-toluenesulfonate salt was prepared by Dr Xiwen Chen. This work was supported by the Micronics Program of the Swedish National Board for Industrial and Technical Development (NUTEK) .
References
Fig. 4. Photographs of emitting nano-LEDs: (A) type I; (B) type II. Magnification 1000 X (reduced in reproduction by 62%) ; exposure time 15 min.
be seen from the figure, the behaviour is very similar for both types, although with one clear difference: the light output from type II is higher. This is mainly explained by the fact that in type I diodes the light is viewed through a metal layer, whereas in type II it comes through a transparent contact. Compared to LEDs made with IT0 as hole-injecting contact, the efficiency is about two orders of magnitude lower, possibly due to a larger mismatch in the position of the band edges. From the values of measured light in Fig. 3, the number of photons emitted per second from individual LEDs can be estimated. Such a calculation, including corrections for the surface area being contacts (for type I) and PTOPT phase (type II) and number of working diodes (as determined from pictures taken in the optical microscope, see Fig. 4)) gives a number in the range of IO* photons per second for both types of diodes. This should then be compared to other sub-micronsized light emitters that range from lo3 to 10” photons per second [ 18-201. In such a comparison it must be remembered
[ 11 C.W. Tang and S.A. VanSlyke, Appl. Phys. I.&t., 51 (1987) 913. [2] C. Ada&i, T. Tsutsui and S. Saito, Appl. Phys. Lea., 55 (1988) 1489. [3] J.H. Burroughes et al., Nature, 347 (1990) 539. [4] G. Heywang and F. Jonas,Adv. Mater., 4 (1992) 116. [5] Q. Pei, G. Zuccarello, M. Ahlskogand 0. Inganb, Polymer, 35 (1994) 1347. [6] M. Dietrich, J. Heinze, G. Heywang and F. Jonas,J. Hectrochem. Sot., 369 (1994) 87. [7] M. Berggren, G. Gustafsson,0. Inganti, M.R. Andersson, T. Hjertberg and 0. Wennerstriim, J. Appl. Phys., 76 (1994) 7530. [8] Z. Cai and CR. Martin, J. Am. Chem. Sot., Ill (1989) 4138. [9] M. Granstriim and 0. Inganb, Polymer, (1995) in press. [lo] A.R. Brown et al., Chem. Phys. Lett., 200 (1992) 1. [ 111 M. GranstrGm, M. Berggren and 0. Inganb, Science, 267 (1995) 1479. [ 121 M. Granstrijm and 0. Inganb, Synth. Met., 55-57 (1993) 460. [ 131 I.D. Parker, J. Appl. Phys., 75 (1994) 1656. [ 141 M. Berggren et al., Nature, 372 (1994) 444. [ 151 M. Granstriim and 0. Ingan&, Adv. Mater., submitted for publication. [16] D.M. de Leeuw, P.A. Kraakman, P.F.G. Bongaerts, C.M.J. Mutsaers and D.B.M. Klaassen, Synth. Met., 66 (1994) 263. [ 171 J.A. Walker, L.F. Warren and E.F. Witucki, J. Polym. Sci., Part A, Polym. Chem., 26 (1988) 1258. [ 181 N. Kuck, K. Liebennan, A. Lewis and A. Vecht, Appl. Phys. L.&t., 61 (1992) 139. [ 191 E. Betzig, J.K. Trautman, T.D. Harris, J.S. Weiner and R.L. Kostelak, Science, 2.51 (1991) 1468. [20] A. Lewis and K. Lieberman, Nature, 354 (1991) 214. [21] D. Braun, D. Moses, C. Zhang and A.J. Heeger, App/. Phys. Lett., 61 (1992)
3092.