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Transient electroluminescence spectroscopy of polyfluorene light-emitting diodes
Synthetic Metals 138 (2003) 233–236
Transient electroluminescence spectroscopy of polyfluorene light-emitting diodes J.M. Lupton*, J. Klein Max Planck Institute for Polymer Research, Ackermann-Weg 10, D-55128 Mainz, Germany
Abstract We present transient electroluminescence spectra of polyfluorene light-emitting diodes. The luminescence spectra recorded after switch-off of a 5 V driving pulse exhibit a dynamic change with time. A study of the temperature and bias offset dependence reveals that charge carriers are preferentially trapped on low energy oxidative sites present on the polyfluorene backbone. These observations of the charge carrier dynamics in light-emitting diodes (LEDs) compare to recent results on exciton trapping on emissive intra-chain defects in polyfluorenes. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Polymer light-emitting diodes; Polyfluorenes; Conjugated polymers; Transient electroluminescence; Charge transport
1. Introduction Polymer light-emitting diodes (LEDs) are an important application of conjugated polymers [1]. Besides the obvious technological interest in these systems, LEDs also offer a wealth of interesting physical properties relating to the dynamics of free charge carriers and excitons [2–8]. A particularly interesting sub-group of conjugated polymers are polyfluorenes, which have been used in a range of different device configurations [9–13]. These materials exhibit substantially different structure–property relations to phenylene vinylenes, on which most previous studies of polymer LEDs have been based. In particular, it was recently demonstrated that oxidation of polyfluorenes actually results in the formation of on-chain emissive defects [14,15], rather than non-emissive exciton traps as is the case in phenylene vinylenes [3]. These oxidative defects result in a broad emission spectrum red-shifted with respect to the main singlet emission band and have been linked to the oxidation of the fluorene unit which forms a fluorenone unit integrated on the polymer backbone [15]. Evidently, such chemical defects should not only modify the exciton dynamics in both one dimension on the chain and in three dimensions in the solid state, but also the charge transport properties. In order to investigate this we performed time-resolved electroluminescence (EL) spectroscopy of polymer LEDs, which takes into account both the spectral and temporal evolution of EL *
Corresponding author. Tel.: þ49-6131-379-484; fax: þ49-6131-379-100. E-mail address: [email protected] (J.M. Lupton).
after turn-off of a driving voltage [16]. Fig. 1 shows the experimental operating schematics. An electrical pulse is applied to the LED for a given time. The delayed EL spectrum is recorded at different delay times after turnoff of the driving voltage. Optionally, a constant bias offset V0 is applied to the modulated voltage.
2. Experiment Single layer LEDs were prepared on indium tin oxide (ITO) patterned by a solution etching process and subsequently cleaned under ultrasonication in isopropanol. A 50 nm layer of poly(3,4-ethylenedioxythiophene)/poly(styrene)-sulfonate, which is used as the hole injecting layer, was deposited on the ITO layer and dried on a hot plate at 80 8C. In order to gain a well-defined delayed luminescence signal, a small amount (approx. 10 ppm) of the phosphorescent dye 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin platinum(II) (PtOEP) was doped into the polyfluorene derivative poly(2,7-(9,9-bis(2-ethylhexyl)fluorene)) (PF2/6). This provides a single exponential luminescence decay due to longlived phosphorescence with a known lifetime. The structure of the material used is shown in the inset in Fig. 2. The blend was spin-coated to yield films typically 100 nm thick, which were then contacted with calcium electrodes typically 15 nm thick, covered subsequently by a 150 nm thick aluminium layer. The active sample area was approx. 4 mm2. The samples were mounted in a water-cooled cold finger cryostat under rotary pump vacuum. Delayed EL spectra were recorded by an EG&G intensified red-enhanced gated diode
0379-6779/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0379-6779(02)01293-6
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Fig. 1. Experimental set-up for time-resolved EL spectroscopy.
Fig. 2. Steady state polyfluorene EL spectrum. The inset shows the structure of PF2/6.
array coupled to a 0.3 m monochromator with a grating of 150 lines/mm. Note that in order to improve the signal to noise ratio gated spectra were recorded with a large entrance slit width corresponding to a spectral resolution of 10 nm.
Fig. 3. Time-resolved EL spectra recorded at 10 8C at 10, 80 and 140 ms delay normalised at 425 nm showing the green band increasing.
140 ms. The broad green band has recently been attributed to emissive on-chain chemical defects, which act as exciton traps and, for example, result in a reduction of exciton lifetime with increasing polymer chain length in dilute solution [14]. These defects are most likely associated with oxidatively generated fluorenone units [15]. The present results from transient EL suggest that these defects may also act as charge carrier traps for electrons. As the defect emission can also be excited directly [14] without requiring exciton transfer from the polymer backbone, it may be concluded that chemical defects have a smaller band gap than the polymer backbone. Charge carriers can therefore be trapped on these sites. As we are able to measure a delayed EL signal from both the defect and the polymer singlet it has to be concluded that charge carriers may also be localised on the polymer backbone, possibly by structural defects. However, the recombination rate clearly depends strongly on temperature, and the difference in time dependence of the two bands in the delayed spectra increases with temperature. This can be described in terms of the following. Comparatively shallow hole and electron traps exist on the polymer
3. Results and discussion Fig. 2 shows a typical PF2/6 cw EL spectrum recorded at a bias of 5 V. It is composed of the exciton band at 425 nm with a vibronic progression at 440 and 475 nm as well as a broad, featureless band centred at 525 nm. Fig. 3 shows time-resolved EL spectra measured at 10 8C at delay times of 10, 80 and 140 ms in time windows of 40 ms under excitation by a 5 V pulse of 40 ms duration and 10% duty cycle. Three features are observed in the delayed spectra corresponding to singlet emission (425 nm), defect emission (525 nm) and PtOEP phosphorescence (650 nm), which is totally masked in the cw spectrum. As the delay time is increased, the contribution of the broad green band increases with respect to the singlet exciton emission at 425 nm. The effect is much more pronounced at 50 8C, as seen in Fig. 4, where the spectra are shown for delays of 10, 60, 100 and
Fig. 4. Time-resolved EL spectra recorded at 50 8C at 10, 60, 100 and 140 ms delay at 425 nm showing the green band increasing.
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much more strongly with bias offset than the polymer singlet band indicates that more charge carriers are trapped on these lower energy sites than on shallower polymer backbone sites, which are possibly related to structural defects on the polymer backbone. As the bias offset is increased, more charge carriers are available for delayed recombination, which preferentially takes place on fluorenone defects.
4. Conclusions
Fig. 5. Time-resolved EL spectra recorded at 20 ms delay for different bias offsets of 2, 0, 1 and 2 V (green band increasing).
backbone, together with deep electron traps on the defect sites. As the temperature is raised, electrons trapped on the polymer backbone leave the device faster than electrons on the deeper traps. Likewise, holes trapped on the polymer backbone are more mobile at elevated temperatures, increasing the probability of recombination with electrons trapped on fluorenone sites with respect to electrons trapped on backbone sites. The same mechanism gives rise to an increase in fluorenone emission with increasing delay time, due to preferential detrapping of carriers localised on shallower backbone traps. Further evidence for the influence of charge trapping on the operation of these LEDs comes from a study of the delayed EL signal in dependence of a constant bias offset V0. Fig. 5 shows spectra recorded at 20 ms delay in 40 ms windows at bias offsets of 2, 0, 1 and 2 V. Note that the built-in field, which opposes the driving field, is typically larger than 2 V for the present diode geometry [17]. Also, the turn-on for EL is approx. 3 V. The bias offset does therefore not result in a steady state contribution to the EL. For negative bias offsets there is very little change to the delayed EL spectra. However, as the bias offset increases, there is a large increase in the delayed EL accompanied by a strong variation in the weighting between the different bands. Evidently the defect band at 525 nm is most sensitive to the bias offset and increases by almost an order of magnitude in intensity between 2 and 2 V. As the builtin field opposes the external bias offset for V > 0 it can be concluded that the built-in field is sufficient to remove all free charge carriers within the first 20 ms post turn-off, as increasing the reverse field does not alter the delayed EL intensity. In this case the delayed emission therefore has to arise purely from trapped charge carriers, rather than from mobile carriers, which would otherwise be perturbed by a negative bias offset. In contrast, as the bias offset is increased, the effective field post turn-off is reduced and the number of free carriers increases as a consequence. The fact that the fluorenone defect emission band increases
Time-resolved EL spectroscopy can provide important information on carrier dynamics and trapping in organic semiconductors. In combination with slight perturbations of the effective built-in field the influence of emissive chemical defects, which have been identified in timeresolved fluorescence measurements on dilute solutions of polyfluorene [14], on the charge transport can be studied. It is clear that such defects can act as strong trapping sites and should therefore be avoided by means of rigorous chemical purity. However, a control of transient carrier population as demonstrated in Fig. 5 may conceivably be of use for passive matrix display applications, which require pulsed addressing of LEDs. Our new experimental technique also provides an additional route to investigating structure–property relationships in conjugated polymers, which are presently intensively debated in the case of polyfluorenes [18].
Acknowledgements The authors are indebted to U. Scherf and H.-G. Nothofer for the kind provision of the polyfluorene polymer and for many helpful discussions.
References [1] R.H. Friend, R.W. Gymer, A.B. Holmes, J.H. Burroughes, R.N. Marks, C. Taliani, D.D.C. Bradley, D.A. DosSantos, J.L. Bredas, M. Logdlund, W.R. Salaneck, Nature 397 (1999) 121. [2] D.J. Pinner, R.H. Friend, N. Tessler, J. Appl. Phys. 86 (1999) 5116. [3] M. Yan, L.J. Rothberg, F. Papadimitrakopoulos, M.E. Galvin, T.M. Miller, Phys. Rev. Lett. 73 (1994) 744. [4] J.M. Lupton, I.D.W. Samuel, R. Beavington, M.J. Frampton, P.L. Burn, H. Ba¨ ssler, Phys. Rev. B 63 (2001) 5206. [5] J.M. Lupton, V.R. Nikitenko, I.D.W. Samuel, H. Ba¨ ssler, J. Appl. Phys. 89 (2001) 311. [6] V. Savvateev, A. Yakimov, D. Davidov, Adv. Mater. 11 (1999) 319. [7] P.W.M. Blom, M. Vissenberg, Phys. Rev. Lett. 80 (1998) 3819. [8] Y.V. Romanovskii, A. Gerhard, B. Schweitzer, U. Scherf, R.I. Personov, H. Ba¨ ssler, Phys. Rev. Lett. 84 (2000) 1027. [9] M. Gross, D.C. Mu¨ ller, H.G. Nothofer, U. Scherf, D. Neher, C. Bra¨ uchle, K. Meerholz, Nature 405 (2000) 661. [10] J.I. Lee, G. Klaerner, R.D. Miller, Chem. Mater. 11 (1999) 1083. [11] V.N. Bliznvuk, S.A. Carter, J.C. Scott, G. Klarner, R.D. Miller, D.C. Miller, Macromolecules 32 (1999) 361.
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[12] D. Sainova, T. Miteva, H.G. Nothofer, U. Scherf, I. Glowacki, J. Ulanski, H. Fujikawa, D. Neher, Appl. Phys. Lett. 76 (2000) 1810. [13] A.W. Grice, D.D.C. Bradley, M.T. Bernius, M. Inbasekaran, W.W. Wu, E.P. Woo, Appl. Phys. Lett. 73 (1998) 629. [14] J.M. Lupton, M.R. Craig, E.W. Meijer, Appl. Phys. Lett. 80 (2002) 4489.
[15] E.J.W. List, R. Guentner, P. Scanducci de Freitas, U. Scherf, Adv. Mater. 14 (2002) 374. [16] J.M. Lupton, J. Klein, Phys. Rev. B 65 (2002) 193202. [17] T.M. Brown, R.H. Friend, I.S. Millard, D.J. Lacey, J.H. Burroughes, F. Cacialli, Appl. Phys. Lett. 79 (2001) 174. [18] U. Scherf, E.J.W. List, Adv. Mater. 14 (2002) 477.
Transient electroluminescence spectroscopy of polyfluorene light-emitting diodes J.M. Lupton*, J. Klein Max Planck Institute for Polymer Research, Ackermann-Weg 10, D-55128 Mainz, Germany
Abstract We present transient electroluminescence spectra of polyfluorene light-emitting diodes. The luminescence spectra recorded after switch-off of a 5 V driving pulse exhibit a dynamic change with time. A study of the temperature and bias offset dependence reveals that charge carriers are preferentially trapped on low energy oxidative sites present on the polyfluorene backbone. These observations of the charge carrier dynamics in light-emitting diodes (LEDs) compare to recent results on exciton trapping on emissive intra-chain defects in polyfluorenes. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Polymer light-emitting diodes; Polyfluorenes; Conjugated polymers; Transient electroluminescence; Charge transport
1. Introduction Polymer light-emitting diodes (LEDs) are an important application of conjugated polymers [1]. Besides the obvious technological interest in these systems, LEDs also offer a wealth of interesting physical properties relating to the dynamics of free charge carriers and excitons [2–8]. A particularly interesting sub-group of conjugated polymers are polyfluorenes, which have been used in a range of different device configurations [9–13]. These materials exhibit substantially different structure–property relations to phenylene vinylenes, on which most previous studies of polymer LEDs have been based. In particular, it was recently demonstrated that oxidation of polyfluorenes actually results in the formation of on-chain emissive defects [14,15], rather than non-emissive exciton traps as is the case in phenylene vinylenes [3]. These oxidative defects result in a broad emission spectrum red-shifted with respect to the main singlet emission band and have been linked to the oxidation of the fluorene unit which forms a fluorenone unit integrated on the polymer backbone [15]. Evidently, such chemical defects should not only modify the exciton dynamics in both one dimension on the chain and in three dimensions in the solid state, but also the charge transport properties. In order to investigate this we performed time-resolved electroluminescence (EL) spectroscopy of polymer LEDs, which takes into account both the spectral and temporal evolution of EL *
Corresponding author. Tel.: þ49-6131-379-484; fax: þ49-6131-379-100. E-mail address: [email protected] (J.M. Lupton).
after turn-off of a driving voltage [16]. Fig. 1 shows the experimental operating schematics. An electrical pulse is applied to the LED for a given time. The delayed EL spectrum is recorded at different delay times after turnoff of the driving voltage. Optionally, a constant bias offset V0 is applied to the modulated voltage.
2. Experiment Single layer LEDs were prepared on indium tin oxide (ITO) patterned by a solution etching process and subsequently cleaned under ultrasonication in isopropanol. A 50 nm layer of poly(3,4-ethylenedioxythiophene)/poly(styrene)-sulfonate, which is used as the hole injecting layer, was deposited on the ITO layer and dried on a hot plate at 80 8C. In order to gain a well-defined delayed luminescence signal, a small amount (approx. 10 ppm) of the phosphorescent dye 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin platinum(II) (PtOEP) was doped into the polyfluorene derivative poly(2,7-(9,9-bis(2-ethylhexyl)fluorene)) (PF2/6). This provides a single exponential luminescence decay due to longlived phosphorescence with a known lifetime. The structure of the material used is shown in the inset in Fig. 2. The blend was spin-coated to yield films typically 100 nm thick, which were then contacted with calcium electrodes typically 15 nm thick, covered subsequently by a 150 nm thick aluminium layer. The active sample area was approx. 4 mm2. The samples were mounted in a water-cooled cold finger cryostat under rotary pump vacuum. Delayed EL spectra were recorded by an EG&G intensified red-enhanced gated diode
0379-6779/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0379-6779(02)01293-6
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Fig. 1. Experimental set-up for time-resolved EL spectroscopy.
Fig. 2. Steady state polyfluorene EL spectrum. The inset shows the structure of PF2/6.
array coupled to a 0.3 m monochromator with a grating of 150 lines/mm. Note that in order to improve the signal to noise ratio gated spectra were recorded with a large entrance slit width corresponding to a spectral resolution of 10 nm.
Fig. 3. Time-resolved EL spectra recorded at 10 8C at 10, 80 and 140 ms delay normalised at 425 nm showing the green band increasing.
140 ms. The broad green band has recently been attributed to emissive on-chain chemical defects, which act as exciton traps and, for example, result in a reduction of exciton lifetime with increasing polymer chain length in dilute solution [14]. These defects are most likely associated with oxidatively generated fluorenone units [15]. The present results from transient EL suggest that these defects may also act as charge carrier traps for electrons. As the defect emission can also be excited directly [14] without requiring exciton transfer from the polymer backbone, it may be concluded that chemical defects have a smaller band gap than the polymer backbone. Charge carriers can therefore be trapped on these sites. As we are able to measure a delayed EL signal from both the defect and the polymer singlet it has to be concluded that charge carriers may also be localised on the polymer backbone, possibly by structural defects. However, the recombination rate clearly depends strongly on temperature, and the difference in time dependence of the two bands in the delayed spectra increases with temperature. This can be described in terms of the following. Comparatively shallow hole and electron traps exist on the polymer
3. Results and discussion Fig. 2 shows a typical PF2/6 cw EL spectrum recorded at a bias of 5 V. It is composed of the exciton band at 425 nm with a vibronic progression at 440 and 475 nm as well as a broad, featureless band centred at 525 nm. Fig. 3 shows time-resolved EL spectra measured at 10 8C at delay times of 10, 80 and 140 ms in time windows of 40 ms under excitation by a 5 V pulse of 40 ms duration and 10% duty cycle. Three features are observed in the delayed spectra corresponding to singlet emission (425 nm), defect emission (525 nm) and PtOEP phosphorescence (650 nm), which is totally masked in the cw spectrum. As the delay time is increased, the contribution of the broad green band increases with respect to the singlet exciton emission at 425 nm. The effect is much more pronounced at 50 8C, as seen in Fig. 4, where the spectra are shown for delays of 10, 60, 100 and
Fig. 4. Time-resolved EL spectra recorded at 50 8C at 10, 60, 100 and 140 ms delay at 425 nm showing the green band increasing.
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much more strongly with bias offset than the polymer singlet band indicates that more charge carriers are trapped on these lower energy sites than on shallower polymer backbone sites, which are possibly related to structural defects on the polymer backbone. As the bias offset is increased, more charge carriers are available for delayed recombination, which preferentially takes place on fluorenone defects.
4. Conclusions
Fig. 5. Time-resolved EL spectra recorded at 20 ms delay for different bias offsets of 2, 0, 1 and 2 V (green band increasing).
backbone, together with deep electron traps on the defect sites. As the temperature is raised, electrons trapped on the polymer backbone leave the device faster than electrons on the deeper traps. Likewise, holes trapped on the polymer backbone are more mobile at elevated temperatures, increasing the probability of recombination with electrons trapped on fluorenone sites with respect to electrons trapped on backbone sites. The same mechanism gives rise to an increase in fluorenone emission with increasing delay time, due to preferential detrapping of carriers localised on shallower backbone traps. Further evidence for the influence of charge trapping on the operation of these LEDs comes from a study of the delayed EL signal in dependence of a constant bias offset V0. Fig. 5 shows spectra recorded at 20 ms delay in 40 ms windows at bias offsets of 2, 0, 1 and 2 V. Note that the built-in field, which opposes the driving field, is typically larger than 2 V for the present diode geometry [17]. Also, the turn-on for EL is approx. 3 V. The bias offset does therefore not result in a steady state contribution to the EL. For negative bias offsets there is very little change to the delayed EL spectra. However, as the bias offset increases, there is a large increase in the delayed EL accompanied by a strong variation in the weighting between the different bands. Evidently the defect band at 525 nm is most sensitive to the bias offset and increases by almost an order of magnitude in intensity between 2 and 2 V. As the builtin field opposes the external bias offset for V > 0 it can be concluded that the built-in field is sufficient to remove all free charge carriers within the first 20 ms post turn-off, as increasing the reverse field does not alter the delayed EL intensity. In this case the delayed emission therefore has to arise purely from trapped charge carriers, rather than from mobile carriers, which would otherwise be perturbed by a negative bias offset. In contrast, as the bias offset is increased, the effective field post turn-off is reduced and the number of free carriers increases as a consequence. The fact that the fluorenone defect emission band increases
Time-resolved EL spectroscopy can provide important information on carrier dynamics and trapping in organic semiconductors. In combination with slight perturbations of the effective built-in field the influence of emissive chemical defects, which have been identified in timeresolved fluorescence measurements on dilute solutions of polyfluorene [14], on the charge transport can be studied. It is clear that such defects can act as strong trapping sites and should therefore be avoided by means of rigorous chemical purity. However, a control of transient carrier population as demonstrated in Fig. 5 may conceivably be of use for passive matrix display applications, which require pulsed addressing of LEDs. Our new experimental technique also provides an additional route to investigating structure–property relationships in conjugated polymers, which are presently intensively debated in the case of polyfluorenes [18].
Acknowledgements The authors are indebted to U. Scherf and H.-G. Nothofer for the kind provision of the polyfluorene polymer and for many helpful discussions.
References [1] R.H. Friend, R.W. Gymer, A.B. Holmes, J.H. Burroughes, R.N. Marks, C. Taliani, D.D.C. Bradley, D.A. DosSantos, J.L. Bredas, M. Logdlund, W.R. Salaneck, Nature 397 (1999) 121. [2] D.J. Pinner, R.H. Friend, N. Tessler, J. Appl. Phys. 86 (1999) 5116. [3] M. Yan, L.J. Rothberg, F. Papadimitrakopoulos, M.E. Galvin, T.M. Miller, Phys. Rev. Lett. 73 (1994) 744. [4] J.M. Lupton, I.D.W. Samuel, R. Beavington, M.J. Frampton, P.L. Burn, H. Ba¨ ssler, Phys. Rev. B 63 (2001) 5206. [5] J.M. Lupton, V.R. Nikitenko, I.D.W. Samuel, H. Ba¨ ssler, J. Appl. Phys. 89 (2001) 311. [6] V. Savvateev, A. Yakimov, D. Davidov, Adv. Mater. 11 (1999) 319. [7] P.W.M. Blom, M. Vissenberg, Phys. Rev. Lett. 80 (1998) 3819. [8] Y.V. Romanovskii, A. Gerhard, B. Schweitzer, U. Scherf, R.I. Personov, H. Ba¨ ssler, Phys. Rev. Lett. 84 (2000) 1027. [9] M. Gross, D.C. Mu¨ ller, H.G. Nothofer, U. Scherf, D. Neher, C. Bra¨ uchle, K. Meerholz, Nature 405 (2000) 661. [10] J.I. Lee, G. Klaerner, R.D. Miller, Chem. Mater. 11 (1999) 1083. [11] V.N. Bliznvuk, S.A. Carter, J.C. Scott, G. Klarner, R.D. Miller, D.C. Miller, Macromolecules 32 (1999) 361.
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[12] D. Sainova, T. Miteva, H.G. Nothofer, U. Scherf, I. Glowacki, J. Ulanski, H. Fujikawa, D. Neher, Appl. Phys. Lett. 76 (2000) 1810. [13] A.W. Grice, D.D.C. Bradley, M.T. Bernius, M. Inbasekaran, W.W. Wu, E.P. Woo, Appl. Phys. Lett. 73 (1998) 629. [14] J.M. Lupton, M.R. Craig, E.W. Meijer, Appl. Phys. Lett. 80 (2002) 4489.
[15] E.J.W. List, R. Guentner, P. Scanducci de Freitas, U. Scherf, Adv. Mater. 14 (2002) 374. [16] J.M. Lupton, J. Klein, Phys. Rev. B 65 (2002) 193202. [17] T.M. Brown, R.H. Friend, I.S. Millard, D.J. Lacey, J.H. Burroughes, F. Cacialli, Appl. Phys. Lett. 79 (2001) 174. [18] U. Scherf, E.J.W. List, Adv. Mater. 14 (2002) 477.