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Materials for polymer-light emitting diodes
Philips J. Res. 51 (1998)
527-533
MATERIALS FOR POLYMER-LIGHT EMITTING DIODES by H.F.M.
SCHOO
Philips Research Laboratories.
and R.J.C.E.
Prof. Hoktlaan
DEMANDT
4. 5656 AA Eindhoven.
The Netherland~y
Abstract Polymer light-emitting diodes have become feasible when suitable materials were available. The various relevant properties are interrelated parameters. The solubility can be improved in various ways but here the introduction of side-chains has been found most successful. Colour tuning is achieved by attaching electron withdrawing or donating side-chains. Some polymer defects in PPV are shown. Special functionalities can be built-in. As an example a self-doped PPV is given. Keywords:
electric field, intrinsic materials. colour-tuning. solubility.
polymer
LED,
side-chains,
1. Introduction The development of materials for polymer LEDs is one of the most challenging issues in the optimization of these devices. The operating mechanism of such devices where electrons and holes are injected into the active layer, are transported and have to recombine there emitting light, leads to a stringent set of demands on these materials. They should have good charge mobility, a high luminescence quantum efficiency and a bandgap corresponding to light emission in the visible wavelength regions. Furthermore, they should be extremely pure, easily processable, thermally stable and have good mechanical properties. Only an integral optimization of these variables will lead to a good material, since the optimization of one of these properties could very well affect the other characteristics. The first reports on electroluminescence of organic compounds date back to 1963, when Pope et al. [l] described electroluminescence in anthracene single crystals using liquid electrolytic electrodes for charge injection. It was not
Philips Journal of Research
Vol. 51
No. 4
1998
527
H.F.M. Schoo and R.J.C.E. Demandt
(2)
poly(p-phenylenevinylene) Fig.
I, The soluble material (polymer 1) is converted to poly(p-phenylenevinylene by thermal
[PPV] (PPV, polymer
2)
treatment.
until twenty-seven years later that an LED with an organic polymer as the active layer was reported for the first time [2], based on a single layer of polymer (type 2 in Fig. 1.) between two injecting electrodes. Several issues had to be solved, however, before this could lead to an industrial process. In this paper a number of important properties of the light emitting polymers used to make LEDs will be discussed. In the next Sec. the solubility will be addressed, Sec. 3 discusses colour tuning, in Sec. 4 the dependence of performance on structure is illustrated and in Sec. 5 it is shown that special functionalities can be achieved, for instance a self-doped emitting polymer.
2. Solubility One of the often mentioned advantages of polymers is that they are easily processable. Polymer (2) in Fig. 1, however, is completely insoluble and intractable. So, in order to apply spincoating for the deposition of the material, it was necessary to prepare films from a soluble precursor material (polymer 1) in Fig. 1, and convert it by thermal treatment. This process is not very reproducible, which makes it less attractive for industrial application. Several strategies have been employed in order to improve the solubility of PPV, among others lowering the molecular weight and the introduction of non-conjugated spacer units in the main chain. In most cases this also has a strong effect on other properties like photoluminescence efficiency, charge mobility and (mechanical) stability. The introduction of side-chains in these materials can lead to completely soluble polymers such as MEH-PPV (type 3) and OC,C,s-PPV (type 4), shown in Fig. 2, which can be processed by standard techniques like spincoating. The advantage of this approach is that these materials can be fully conjugated before being spincoated.
528
Philips Journal of Research
Vol. 51
No. 4
1998
Materials for polymer light-emitting diodes
‘1
k? \ 0
0
P (3)
n
(4)
MEH-PPV (3) Fig. 2. Two types of polymer,
-
oc
type 3 and 4, that are completely
c ,o-PPV
1
soluble due to the side-chains.
3. Colour Tuning An important advantage of using polymer materials in organic LEDs, is the ease with which their bandgap and redox potential can be tuned. This can be achieved by the introduction of electron-withdrawing or electron donating side-chains [3,4]. The alkoxy side-chains, used in the previous example, and cyano-groups lead to a decrease in the bandgap of the polymer, and thus to a more red emission (without these sidechains, polymer (2) in Fig. 1 emits yellowish-green light). If alkyl or aryl side-chains are used, however, more bluish emission is obtained. As an example the photoluminescence spectra are given in Fig. 3b for a range of polymers with general formula shown in Fig. 3a. This example demonstrates that it is possible to tune the colour of emission over the entire visible spectrum by choosing the appropriate side-chains.
4. Polymer structure and performance The (initial) performance and lifetime of polymer LED-devices are governed by many factors. One of them is the purity of the material. A large number of impurities and defects may be formed during synthesis of the materials. Impurities such as salts, monomer residues and low molecular weight reaction products can be removed thoroughly by several purification procedures. Defects in the polymer structure, however, are an integral part of the polymer, and are therefore much harder to remove, if it is possible at all. Some
PhilipsJournalof Research
Vol. 51
No. 4
1998
529
H.F.M. Schoo and R.J.C.E. Demandt
500
600
700
600
Wavelength (nm) Fig. 3. (a) General formula for a range of polymers with different photoluminescence spectra depending on the type of side-chain. (b) Normalized photoluminescence spectra of sidechainmodified PPV l- RI. Rl, Rj, R4 = alkyl; RS, Rs = CN, 2- R,, R2 = alkoxy; Rj. R4 = alkyl; R,, R6 = CN, 3- RI, R2 = alkoxy; Rx, R4 = alkoxy; R5, R6 = H, 4- R,, R2 = alkoxy; R3, R, = alkoxy; RS = R, = H annealed, 5- RI. RZ = alkoxy; R1, R4 = alkoxy; R5, R6 = CN.
of the (possible) defects that could be found in PPV are shown in Fig. 4. For clarity the sidechains have been omitted. Variation of the reaction conditions, such as monomer purity, type of solvent, reaction temperature, concentration, is the only way to improve the
Fig. 4. Some possible defects that could occur in PPV. The Greek capitals which the units appear in the polymers.
530
indicate
Philips Journal of Research
the fraction
Vol. 51
No. 4
in
1998
Materials for polymer
light-emitting
diodes
104 NE
n’ 103!
0, 3
lo*{
a 3 lo'<
,mmw
??
:
0 E
.
??
.rr,lO"i
lo-lo
H
0
1
!
!
2
.
??'
mm
‘Standard
??
?
!
4
’ !
6
f
!
8
!
10
Vbias (V) Fig. 5. Light output
versus applied
voltage
for standard
and improved
polymers.
purity of the polymer itself. Indeed, careful control of these parameters leads to far better materials and therefore improved performance of the devices, as is obvious from measurements of light output versus applied voltage, shown in Fig. 5. Similar observations can be made with respect to the lifetime of polymer LED devices. 5. Special functionalities
Polymers can contain many different functionalities. In fact, some of the defects shown above have a beneficial effect on the performance of the material. It is also possible to introduce special moieties into the polymer to improve the performance of the polymer. Charge injecting molecules, dyes, etc., can be attached via a covalent bond to the polymer backbone. In this way, the molecular dispersion of these molecules in the polymer matrix, which is often essential for its function, is guaranteed. As an example in Fig. 6 a selfdoped PPV is shown, made by introducing polymer-bound cationic species in the polymer [5].
Fig. 6. A self-doped
PPV, made by introducing
PhilipsJournalof Research Vol. 51
No. 4
l!WJ
polymer-bound
cationic
species in the polymer.
531
H.F.M.
Schoo and R.J.C.E.
Demandt
n-(self)doped PPV
Polycationic PPV
Fig. 7. The effect of an applied field in a LED device made with this polymer.
When an electric field is applied in an LED device made with this polymer, the anions in this material can diffuse to the anode while the polymer-bound cations cannot. The ionic groups then act as counterions for the injected charges. This is shown in Fig. 7. Effectively, this leads to a PIN-device in which the neutral (non-doped) emitting polymer is sandwhiched between an n-type and a p-type layer of the same material. With such a polymer it is possible to make an efficient single layer LED device, with any kind of metal as the injecting electrodes, since there will be no injection barrier for the charge carriers. 6. Conclusion There is still room for further improvement of the device performance through the design of new materials. This holds particularly if a combination of variables is considered when designing new materials, such as processability, stability, multilayer deposition, colour of emission, charge affinity. This will present a real challenge for synthetic-polymer chemists. Acknowledgements
The authors wish to thank Wolter ten Hoeve, Karin Spoelstra and Hans Wijnberg of Syncom B.V. in Groningen for their contribution to this work. REFERENCES M. Pope, J.P. Kallmann and P. Magnate, J. Chem. Phys. 38, 2042 (1963). J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Burns and A.B. Holmes. Nature (London) 347(6293), 539 (1990). N.C. Greenham, S.C. Moratti. D.D.C. Bradley. R.H. Friend and A.B. Holmes, Nature (London) 365(6447), 628 (1993). J. Cornil. D. Beljonne, D.A. dos Santos and J.L. Bredas, Synth. Met. 76(1-3), 101 (1996). H.F.M. Schoo, R.C.J.E. Demandt, J.J.M. Vleggaar and C.T.H. Liedenbaum, Macromol. Symp. 125 (Organic Light-Emitting Materials and Devices), 165 (1998).
532
PhilipsJournal of Research Vol. 51 No. 4
1998
Materials
for polwner
light-emitting
diodes
Authors Biographies
Herman F.M. Schoo: Drs. degree (chemistry), University of Groningen 1987; PhD, University of Groningen. 1991; Philips Research Laboratories, 1992%: work on polymer synthesis and improvement of material properties in several projects.
Rob Demandt: Ing. degree (chemical tories. 1992 1998; currently working
Philips Journal of Research
Vol. 51
No. 4
technology), HTS Heerlen. at Philips CCP, Eindhoven.
1!398
1992; Philips Research
Labord-
533
527-533
MATERIALS FOR POLYMER-LIGHT EMITTING DIODES by H.F.M.
SCHOO
Philips Research Laboratories.
and R.J.C.E.
Prof. Hoktlaan
DEMANDT
4. 5656 AA Eindhoven.
The Netherland~y
Abstract Polymer light-emitting diodes have become feasible when suitable materials were available. The various relevant properties are interrelated parameters. The solubility can be improved in various ways but here the introduction of side-chains has been found most successful. Colour tuning is achieved by attaching electron withdrawing or donating side-chains. Some polymer defects in PPV are shown. Special functionalities can be built-in. As an example a self-doped PPV is given. Keywords:
electric field, intrinsic materials. colour-tuning. solubility.
polymer
LED,
side-chains,
1. Introduction The development of materials for polymer LEDs is one of the most challenging issues in the optimization of these devices. The operating mechanism of such devices where electrons and holes are injected into the active layer, are transported and have to recombine there emitting light, leads to a stringent set of demands on these materials. They should have good charge mobility, a high luminescence quantum efficiency and a bandgap corresponding to light emission in the visible wavelength regions. Furthermore, they should be extremely pure, easily processable, thermally stable and have good mechanical properties. Only an integral optimization of these variables will lead to a good material, since the optimization of one of these properties could very well affect the other characteristics. The first reports on electroluminescence of organic compounds date back to 1963, when Pope et al. [l] described electroluminescence in anthracene single crystals using liquid electrolytic electrodes for charge injection. It was not
Philips Journal of Research
Vol. 51
No. 4
1998
527
H.F.M. Schoo and R.J.C.E. Demandt
(2)
poly(p-phenylenevinylene) Fig.
I, The soluble material (polymer 1) is converted to poly(p-phenylenevinylene by thermal
[PPV] (PPV, polymer
2)
treatment.
until twenty-seven years later that an LED with an organic polymer as the active layer was reported for the first time [2], based on a single layer of polymer (type 2 in Fig. 1.) between two injecting electrodes. Several issues had to be solved, however, before this could lead to an industrial process. In this paper a number of important properties of the light emitting polymers used to make LEDs will be discussed. In the next Sec. the solubility will be addressed, Sec. 3 discusses colour tuning, in Sec. 4 the dependence of performance on structure is illustrated and in Sec. 5 it is shown that special functionalities can be achieved, for instance a self-doped emitting polymer.
2. Solubility One of the often mentioned advantages of polymers is that they are easily processable. Polymer (2) in Fig. 1, however, is completely insoluble and intractable. So, in order to apply spincoating for the deposition of the material, it was necessary to prepare films from a soluble precursor material (polymer 1) in Fig. 1, and convert it by thermal treatment. This process is not very reproducible, which makes it less attractive for industrial application. Several strategies have been employed in order to improve the solubility of PPV, among others lowering the molecular weight and the introduction of non-conjugated spacer units in the main chain. In most cases this also has a strong effect on other properties like photoluminescence efficiency, charge mobility and (mechanical) stability. The introduction of side-chains in these materials can lead to completely soluble polymers such as MEH-PPV (type 3) and OC,C,s-PPV (type 4), shown in Fig. 2, which can be processed by standard techniques like spincoating. The advantage of this approach is that these materials can be fully conjugated before being spincoated.
528
Philips Journal of Research
Vol. 51
No. 4
1998
Materials for polymer light-emitting diodes
‘1
k? \ 0
0
P (3)
n
(4)
MEH-PPV (3) Fig. 2. Two types of polymer,
-
oc
type 3 and 4, that are completely
c ,o-PPV
1
soluble due to the side-chains.
3. Colour Tuning An important advantage of using polymer materials in organic LEDs, is the ease with which their bandgap and redox potential can be tuned. This can be achieved by the introduction of electron-withdrawing or electron donating side-chains [3,4]. The alkoxy side-chains, used in the previous example, and cyano-groups lead to a decrease in the bandgap of the polymer, and thus to a more red emission (without these sidechains, polymer (2) in Fig. 1 emits yellowish-green light). If alkyl or aryl side-chains are used, however, more bluish emission is obtained. As an example the photoluminescence spectra are given in Fig. 3b for a range of polymers with general formula shown in Fig. 3a. This example demonstrates that it is possible to tune the colour of emission over the entire visible spectrum by choosing the appropriate side-chains.
4. Polymer structure and performance The (initial) performance and lifetime of polymer LED-devices are governed by many factors. One of them is the purity of the material. A large number of impurities and defects may be formed during synthesis of the materials. Impurities such as salts, monomer residues and low molecular weight reaction products can be removed thoroughly by several purification procedures. Defects in the polymer structure, however, are an integral part of the polymer, and are therefore much harder to remove, if it is possible at all. Some
PhilipsJournalof Research
Vol. 51
No. 4
1998
529
H.F.M. Schoo and R.J.C.E. Demandt
500
600
700
600
Wavelength (nm) Fig. 3. (a) General formula for a range of polymers with different photoluminescence spectra depending on the type of side-chain. (b) Normalized photoluminescence spectra of sidechainmodified PPV l- RI. Rl, Rj, R4 = alkyl; RS, Rs = CN, 2- R,, R2 = alkoxy; Rj. R4 = alkyl; R,, R6 = CN, 3- RI, R2 = alkoxy; Rx, R4 = alkoxy; R5, R6 = H, 4- R,, R2 = alkoxy; R3, R, = alkoxy; RS = R, = H annealed, 5- RI. RZ = alkoxy; R1, R4 = alkoxy; R5, R6 = CN.
of the (possible) defects that could be found in PPV are shown in Fig. 4. For clarity the sidechains have been omitted. Variation of the reaction conditions, such as monomer purity, type of solvent, reaction temperature, concentration, is the only way to improve the
Fig. 4. Some possible defects that could occur in PPV. The Greek capitals which the units appear in the polymers.
530
indicate
Philips Journal of Research
the fraction
Vol. 51
No. 4
in
1998
Materials for polymer
light-emitting
diodes
104 NE
n’ 103!
0, 3
lo*{
a 3 lo'<
,mmw
??
:
0 E
.
??
.rr,lO"i
lo-lo
H
0
1
!
!
2
.
??'
mm
‘Standard
??
?
!
4
’ !
6
f
!
8
!
10
Vbias (V) Fig. 5. Light output
versus applied
voltage
for standard
and improved
polymers.
purity of the polymer itself. Indeed, careful control of these parameters leads to far better materials and therefore improved performance of the devices, as is obvious from measurements of light output versus applied voltage, shown in Fig. 5. Similar observations can be made with respect to the lifetime of polymer LED devices. 5. Special functionalities
Polymers can contain many different functionalities. In fact, some of the defects shown above have a beneficial effect on the performance of the material. It is also possible to introduce special moieties into the polymer to improve the performance of the polymer. Charge injecting molecules, dyes, etc., can be attached via a covalent bond to the polymer backbone. In this way, the molecular dispersion of these molecules in the polymer matrix, which is often essential for its function, is guaranteed. As an example in Fig. 6 a selfdoped PPV is shown, made by introducing polymer-bound cationic species in the polymer [5].
Fig. 6. A self-doped
PPV, made by introducing
PhilipsJournalof Research Vol. 51
No. 4
l!WJ
polymer-bound
cationic
species in the polymer.
531
H.F.M.
Schoo and R.J.C.E.
Demandt
n-(self)doped PPV
Polycationic PPV
Fig. 7. The effect of an applied field in a LED device made with this polymer.
When an electric field is applied in an LED device made with this polymer, the anions in this material can diffuse to the anode while the polymer-bound cations cannot. The ionic groups then act as counterions for the injected charges. This is shown in Fig. 7. Effectively, this leads to a PIN-device in which the neutral (non-doped) emitting polymer is sandwhiched between an n-type and a p-type layer of the same material. With such a polymer it is possible to make an efficient single layer LED device, with any kind of metal as the injecting electrodes, since there will be no injection barrier for the charge carriers. 6. Conclusion There is still room for further improvement of the device performance through the design of new materials. This holds particularly if a combination of variables is considered when designing new materials, such as processability, stability, multilayer deposition, colour of emission, charge affinity. This will present a real challenge for synthetic-polymer chemists. Acknowledgements
The authors wish to thank Wolter ten Hoeve, Karin Spoelstra and Hans Wijnberg of Syncom B.V. in Groningen for their contribution to this work. REFERENCES M. Pope, J.P. Kallmann and P. Magnate, J. Chem. Phys. 38, 2042 (1963). J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Burns and A.B. Holmes. Nature (London) 347(6293), 539 (1990). N.C. Greenham, S.C. Moratti. D.D.C. Bradley. R.H. Friend and A.B. Holmes, Nature (London) 365(6447), 628 (1993). J. Cornil. D. Beljonne, D.A. dos Santos and J.L. Bredas, Synth. Met. 76(1-3), 101 (1996). H.F.M. Schoo, R.C.J.E. Demandt, J.J.M. Vleggaar and C.T.H. Liedenbaum, Macromol. Symp. 125 (Organic Light-Emitting Materials and Devices), 165 (1998).
532
PhilipsJournal of Research Vol. 51 No. 4
1998
Materials
for polwner
light-emitting
diodes
Authors Biographies
Herman F.M. Schoo: Drs. degree (chemistry), University of Groningen 1987; PhD, University of Groningen. 1991; Philips Research Laboratories, 1992%: work on polymer synthesis and improvement of material properties in several projects.
Rob Demandt: Ing. degree (chemical tories. 1992 1998; currently working
Philips Journal of Research
Vol. 51
No. 4
technology), HTS Heerlen. at Philips CCP, Eindhoven.
1!398
1992; Philips Research
Labord-
533