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Light-emitting electrochemical cells based on poly(p-phenylene vinylene) copolymers with ion-transporting side groups
Synthetic Metals 122 (2001) 111±113
Light-emitting electrochemical cells based on poly(p-phenylene vinylene) copolymers with ion-transporting side groups J. Morgadoa,b,*, R.H. Friendb, F. Caciallib, B.S. Chuahc, H. Rostc, S.C. Morattic, A.B. Holmesc a
Departamento de Engenharia QuõÂmica, Instituto Superior TeÂcnico, Avenida Rovisco Pais, P-1049-001 Lisboa, Portugal b Cavendish Laboratory, Madingley Road, Cambridge CB3 0HE, UK c Melville Laboratory for Polymer Synthesis, Pembroke Street, Cambridge CB2 3RA, UK
Abstract We present the electro-optical characteristics of LECs based on two copolymers derived from poly(p-phenylene vinylene), DB-altBTEM-PPV and BDMOS-co-BTEM-PPV, having ion-transporting side groups. We ®nd typical LEC behaviour upon addition of lithium tri¯ate. Addition of poly(ethylene oxide), PEO, improves the EL ef®ciency, but the response time increases. We discuss these observations on the basis of PEO-induced phase separation. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Light-emitting electrochemical cells; Poly(p-phenylene vinylene); Copolymers
1. Introduction The ef®ciency of polymeric light-emitting devices is determined, among other factors [1], by the yield of formation of singlet-excitons from the injected electrons and holes. Pei et al. [2] showed that the presence of mobile ions within the emissive layer could lead to balanced charge injection, and therefore, to improved electroluminescence, EL, ef®ciency. Furthermore, these devices show little or no recti®cation, with light emission under both forward and reverse bias and starting at voltages close to the Eg/e, Eg being the optical energy gap. Pei et al. named these devices light-emitting electrochemical cells, LECs, after an electrochemical doping operating mechanism they proposed. Other mechanisms were proposed [3], but we will use the LEC designation with no direct reference to the operating mechanism. When the luminescent polymers do not possess ion-coordinating functionalities they are blended with a polymeric solvent, such as poly(ethylene oxide), PEO. In order to overcome the associated tendency for phase separation, Pei and Yan [4] proposed the grafting of ion-coordinating side groups to the luminescent polymer. In this communication we report on the electro-optical properties of LECs based on two copolymers derived from poly(p-phenylene vinylene), PPV, namely: poly [2,3-dibu*
Corresponding author. Tel.: 351-21-8418451; fax: 351-21-8417675. E-mail address: [email protected] (J. Morgado).
toxy-1,4-phenylene vinylene-alt-2,5-bis(triethoxymethoxy)1,4-phenylene vinylene], DB-alt-BTEM-PPV, an alternating copolymer, and poly[2,5-bis(dimethyloctylsilyl)-1,4-phenylene vinylene)]-co-[2,5-bis(triethoxymethoxy)-1,4-phenylene vinylene)], BDMOS-co-BTEM-PPV, a statistical copolymer (see Fig. 1). These copolymers were designed in order to combine the ion-coordinating ability of the side groups of BTEM-PPV, which has a PL ef®ciency of only 8.8% [5], with monomers of highly luminescent polymers: DB-PPV [6] and BDMOS-PPV [7], respectively. The ioncoordinating ability of the side groups of these copolymers, in spite of its ``dilution'' in relation to BTEM-PPV, was proven to be suf®cient to promote ion solvation and mobility as LECs were obtained upon mixing of the copolymers with lithium tri¯ate. Upon addition of PEO, the EL ef®ciency increases but the response time of the LECs increases also. We interpret these observations on the basis of PEO-induced phase separation. 2. Experimental The composition of the statistical copolymer (2) was established as 31:69 molar ratio of BDMOS:BTEM units [8]. The LECs were prepared by spin coating of oxygen plasma treated ITO/glass substrates with the polymer solution, followed by thermal evaporation of aluminium cathodes (4 mm2). The polymer solutions were prepared by addition of the required amount of lithium tri¯ate, LiTf or LiCF3SO3, in acetonitrile to the copolymer dissolved in
0379-6779/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 0 ) 0 1 3 6 6 - 7
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J. Morgado et al. / Synthetic Metals 122 (2001) 111±113
Fig. 1. Molecular structure of the copolymers.
chloroform, to which PEO, if required, had been added. The LECs were tested at 10ÿ2 mbar. 3. Results and discussion Fig. 2 shows the electro-optical characteristics of LECs based on the copolymers (1) and (2). Typical LEC behaviour is observed when LiTf is mixed in 8±10%, by weight, with the copolymers. This shows that a suf®cient amount of ions is solvated and that they are mobile under the applied electric ®eld. The addition of 22%, by weight, of PEO to the LEC solution of copolymer (1) increases both the EL ef®ciency and the response time (Fig. 3A), whereas the recti®cation ratio and turn-on voltages are not signi®cantly altered. The addition of PEO (15%, by weight) to the LEC solutions of copolymer (2) increases also the EL ef®ciency. However, the current± and luminance±voltage curves become much less symmetrical (see Fig. 2B). The maximum EL ef®ciency of the LECs based on copolymer (1) is about 0.9 cd/A for thinner devices (130 nm), increasing up to 1.5 cd/A upon addition of PEO. Thicker devices exhibit lower EL ef®ciency, being in the order of 0.05 cd/A for a 600 nm thick LEC and which increases by almost an order of magnitude upon addition of PEO. The LECs based on copolymer (2) shown in Fig. 2B have an EL ef®ciency of about 0.027 cd/A, which increases, in forward bias, to
0.49 cd/A upon addition of PEO. The lower ef®ciencies of the LECs based on copolymer (2) re¯ect its lower PL ef®ciency of about 17%, in relation to a maximum of 50% for copolymer (1). The in¯uence of PEO on the current of LECs based on copolymer (1) was investigated by dc transient studies. We have taken the current measured at ``time t 0'' and at a bias below 1 V as a measure of the ionic current [3]. Fig. 3B shows its dependence on the bias applied to LECs based on both copolymers. The non linear dependence for the LECs without PEO suggests the existence of ion association (ion aggregates) [3]. We explain the data in Fig. 3 on the basis of phase separation of PEO from the luminescent polymer, in spite of the glyme-like side groups of the copolymers. A fraction of the ionic salt will go into the PEO domains, leading to the formation of crystalline complexes, whose presence has been identi®ed in PEO-LiTf electrolytes [9]. As the ionic salt incorporated in these complexes is not able to move under the applied electric ®eld, less ions remain free to respond to the external electric ®eld. Within the ionic-space charge model [3], this implies that a longer time is required to build up the necessary ionic space charge at the electrode interfaces in order to reduce the width of the charge injection barriers to a minimum. This effect explains the longer time required for the light output and for the current of the devices containing PEO (see Fig. 3B) to reach maximum values. The presence of PEO reduces or even eliminates the presence of
Fig. 2. Current (I, thicker lines) and luminance (L, circles) as a function of the voltage applied to LECs, without (w/o) and with PEO, based on (A) DB-altBTEM-PPV, with thicknesses of 600 and 450 nm for the ``without'' and ``with PEO'' (w/o) devices, respectively; and based on (B) BDMOS-co-BTEM-PPV, with thicknesses in the range 205±235 nm.
J. Morgado et al. / Synthetic Metals 122 (2001) 111±113
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Fig. 3. (A) Time dependence of the current (I, thicker lines) and light intensity (L, circles) at 2.6 V bias for two different pixels of the same LECs based on copolymer (1) and whose characteristics are shown in Fig. 2A); (B) current at time t 0 for LECs based on both copolymers (cop(1) and cop(2)) and also for a LEC based on copolymer (1) with PEO.
ion aggregates, as concluded from the linear dependence on the bias voltage of the current at time t 0 (see Fig. 3B). AFM studies [10] on thin ®lms of the LECs based on copolymer (1) containing PEO provide evidence for the existence of phase separation. Further work is in progress to identify and characterise the crystalline complexes in these LEC ®lms. In conclusion, we have shown that these copolymers have improved PL ef®ciencies. The incorporation of highly ¯uorescent monomers in the BTEM-PPV chain is made at the expenses of a reduction of the concentration of the ioncoordinating groups. However, typical LEC behaviour could still be obtained upon mixing of these copolymers with LiTf. Therefore, enough ions (Li) are coordinated by the oxygen atoms of the ether units becoming mobile (presumably along with Tfÿ) under the applied bias. Furthermore, we have found that the addition of PEO leads to an increase of the EL ef®ciency, while increasing the response time. We attribute these effects to phase separation and salt trapping at PEO crystalline regions, due to the formation of crystalline complexes. Hence the improved luminescence ef®ciency can be attributed to lower ion-induced exciton quenching. The increase of the LECs response time upon addition of PEO is attributed to a reduction of the amount of free ions.
Acknowledgements Financial support from FundacËaÄo para a CieÃncia e a Tecnologia, The Royal Society, the European Comission, ORS and EPSRC is acknowledged. References [1] J. Morgado, F. Cacialli, J. GruÈner, N.C. Greenham, R.H. Friend, J. Appl. Phys. 85 (1999) 1784. [2] Q. Pei, G. Yu, C. Zhang, Y. Yang, A.J. Heeger, Science 269 (1995) 1086. [3] J.C. deMello, N. Tessler, S.C. Graham, R.H. Friend, Phys. Rev. B 57 (1998) 12951. [4] Q. Pei, Y. Yang, J. Am. Chem. Soc. 118 (1996) 7416. [5] D.-H. Hwang, B.S. Chuah, X.-C. Li, S.T. Kim, S.C. Moratti, A.B. Holmes, J.C. deMello, R.H. Friend, Macromol. Symp. 125 (1997) 111. [6] F. Cacialli, B.S. Chuah, J.S. Kim, D.A. Dossantos, R.H. Friend, S.C. Moratti, A.B. Holmes, J.-L. BreÂdas, Synth. Met. 102 (1999) 924. [7] H.Y. Chu, D.-H. Hwang, L.-M. Do, J.-H. Chang, H.-K. Shim, A.B. Holmes, T. Zyung, Synth. Met. 101 (1999) 216. [8] H. Rost, B.S. Chuah, D.-H. Hwang, S.C. Moratti, A.B. Holmes, J. Wilson, J. Morgado, J.J.M. Halls, J.C. De Mello, R.H. Friend, Synth. Met. 102 (1999) 937. [9] R. Frech, S. Chintapalli, P.G. Bruce, C.A. Vincent, Macromolecules 32 (1999) 808. [10] J. Morgado, R.H. Friend, F. Cacialli, B.S. Chuah, S.C. Moratti, A.B. Holmes, J. Appl. Phys. 86 (1999) 6392.
Light-emitting electrochemical cells based on poly(p-phenylene vinylene) copolymers with ion-transporting side groups J. Morgadoa,b,*, R.H. Friendb, F. Caciallib, B.S. Chuahc, H. Rostc, S.C. Morattic, A.B. Holmesc a
Departamento de Engenharia QuõÂmica, Instituto Superior TeÂcnico, Avenida Rovisco Pais, P-1049-001 Lisboa, Portugal b Cavendish Laboratory, Madingley Road, Cambridge CB3 0HE, UK c Melville Laboratory for Polymer Synthesis, Pembroke Street, Cambridge CB2 3RA, UK
Abstract We present the electro-optical characteristics of LECs based on two copolymers derived from poly(p-phenylene vinylene), DB-altBTEM-PPV and BDMOS-co-BTEM-PPV, having ion-transporting side groups. We ®nd typical LEC behaviour upon addition of lithium tri¯ate. Addition of poly(ethylene oxide), PEO, improves the EL ef®ciency, but the response time increases. We discuss these observations on the basis of PEO-induced phase separation. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Light-emitting electrochemical cells; Poly(p-phenylene vinylene); Copolymers
1. Introduction The ef®ciency of polymeric light-emitting devices is determined, among other factors [1], by the yield of formation of singlet-excitons from the injected electrons and holes. Pei et al. [2] showed that the presence of mobile ions within the emissive layer could lead to balanced charge injection, and therefore, to improved electroluminescence, EL, ef®ciency. Furthermore, these devices show little or no recti®cation, with light emission under both forward and reverse bias and starting at voltages close to the Eg/e, Eg being the optical energy gap. Pei et al. named these devices light-emitting electrochemical cells, LECs, after an electrochemical doping operating mechanism they proposed. Other mechanisms were proposed [3], but we will use the LEC designation with no direct reference to the operating mechanism. When the luminescent polymers do not possess ion-coordinating functionalities they are blended with a polymeric solvent, such as poly(ethylene oxide), PEO. In order to overcome the associated tendency for phase separation, Pei and Yan [4] proposed the grafting of ion-coordinating side groups to the luminescent polymer. In this communication we report on the electro-optical properties of LECs based on two copolymers derived from poly(p-phenylene vinylene), PPV, namely: poly [2,3-dibu*
Corresponding author. Tel.: 351-21-8418451; fax: 351-21-8417675. E-mail address: [email protected] (J. Morgado).
toxy-1,4-phenylene vinylene-alt-2,5-bis(triethoxymethoxy)1,4-phenylene vinylene], DB-alt-BTEM-PPV, an alternating copolymer, and poly[2,5-bis(dimethyloctylsilyl)-1,4-phenylene vinylene)]-co-[2,5-bis(triethoxymethoxy)-1,4-phenylene vinylene)], BDMOS-co-BTEM-PPV, a statistical copolymer (see Fig. 1). These copolymers were designed in order to combine the ion-coordinating ability of the side groups of BTEM-PPV, which has a PL ef®ciency of only 8.8% [5], with monomers of highly luminescent polymers: DB-PPV [6] and BDMOS-PPV [7], respectively. The ioncoordinating ability of the side groups of these copolymers, in spite of its ``dilution'' in relation to BTEM-PPV, was proven to be suf®cient to promote ion solvation and mobility as LECs were obtained upon mixing of the copolymers with lithium tri¯ate. Upon addition of PEO, the EL ef®ciency increases but the response time of the LECs increases also. We interpret these observations on the basis of PEO-induced phase separation. 2. Experimental The composition of the statistical copolymer (2) was established as 31:69 molar ratio of BDMOS:BTEM units [8]. The LECs were prepared by spin coating of oxygen plasma treated ITO/glass substrates with the polymer solution, followed by thermal evaporation of aluminium cathodes (4 mm2). The polymer solutions were prepared by addition of the required amount of lithium tri¯ate, LiTf or LiCF3SO3, in acetonitrile to the copolymer dissolved in
0379-6779/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 0 ) 0 1 3 6 6 - 7
112
J. Morgado et al. / Synthetic Metals 122 (2001) 111±113
Fig. 1. Molecular structure of the copolymers.
chloroform, to which PEO, if required, had been added. The LECs were tested at 10ÿ2 mbar. 3. Results and discussion Fig. 2 shows the electro-optical characteristics of LECs based on the copolymers (1) and (2). Typical LEC behaviour is observed when LiTf is mixed in 8±10%, by weight, with the copolymers. This shows that a suf®cient amount of ions is solvated and that they are mobile under the applied electric ®eld. The addition of 22%, by weight, of PEO to the LEC solution of copolymer (1) increases both the EL ef®ciency and the response time (Fig. 3A), whereas the recti®cation ratio and turn-on voltages are not signi®cantly altered. The addition of PEO (15%, by weight) to the LEC solutions of copolymer (2) increases also the EL ef®ciency. However, the current± and luminance±voltage curves become much less symmetrical (see Fig. 2B). The maximum EL ef®ciency of the LECs based on copolymer (1) is about 0.9 cd/A for thinner devices (130 nm), increasing up to 1.5 cd/A upon addition of PEO. Thicker devices exhibit lower EL ef®ciency, being in the order of 0.05 cd/A for a 600 nm thick LEC and which increases by almost an order of magnitude upon addition of PEO. The LECs based on copolymer (2) shown in Fig. 2B have an EL ef®ciency of about 0.027 cd/A, which increases, in forward bias, to
0.49 cd/A upon addition of PEO. The lower ef®ciencies of the LECs based on copolymer (2) re¯ect its lower PL ef®ciency of about 17%, in relation to a maximum of 50% for copolymer (1). The in¯uence of PEO on the current of LECs based on copolymer (1) was investigated by dc transient studies. We have taken the current measured at ``time t 0'' and at a bias below 1 V as a measure of the ionic current [3]. Fig. 3B shows its dependence on the bias applied to LECs based on both copolymers. The non linear dependence for the LECs without PEO suggests the existence of ion association (ion aggregates) [3]. We explain the data in Fig. 3 on the basis of phase separation of PEO from the luminescent polymer, in spite of the glyme-like side groups of the copolymers. A fraction of the ionic salt will go into the PEO domains, leading to the formation of crystalline complexes, whose presence has been identi®ed in PEO-LiTf electrolytes [9]. As the ionic salt incorporated in these complexes is not able to move under the applied electric ®eld, less ions remain free to respond to the external electric ®eld. Within the ionic-space charge model [3], this implies that a longer time is required to build up the necessary ionic space charge at the electrode interfaces in order to reduce the width of the charge injection barriers to a minimum. This effect explains the longer time required for the light output and for the current of the devices containing PEO (see Fig. 3B) to reach maximum values. The presence of PEO reduces or even eliminates the presence of
Fig. 2. Current (I, thicker lines) and luminance (L, circles) as a function of the voltage applied to LECs, without (w/o) and with PEO, based on (A) DB-altBTEM-PPV, with thicknesses of 600 and 450 nm for the ``without'' and ``with PEO'' (w/o) devices, respectively; and based on (B) BDMOS-co-BTEM-PPV, with thicknesses in the range 205±235 nm.
J. Morgado et al. / Synthetic Metals 122 (2001) 111±113
113
Fig. 3. (A) Time dependence of the current (I, thicker lines) and light intensity (L, circles) at 2.6 V bias for two different pixels of the same LECs based on copolymer (1) and whose characteristics are shown in Fig. 2A); (B) current at time t 0 for LECs based on both copolymers (cop(1) and cop(2)) and also for a LEC based on copolymer (1) with PEO.
ion aggregates, as concluded from the linear dependence on the bias voltage of the current at time t 0 (see Fig. 3B). AFM studies [10] on thin ®lms of the LECs based on copolymer (1) containing PEO provide evidence for the existence of phase separation. Further work is in progress to identify and characterise the crystalline complexes in these LEC ®lms. In conclusion, we have shown that these copolymers have improved PL ef®ciencies. The incorporation of highly ¯uorescent monomers in the BTEM-PPV chain is made at the expenses of a reduction of the concentration of the ioncoordinating groups. However, typical LEC behaviour could still be obtained upon mixing of these copolymers with LiTf. Therefore, enough ions (Li) are coordinated by the oxygen atoms of the ether units becoming mobile (presumably along with Tfÿ) under the applied bias. Furthermore, we have found that the addition of PEO leads to an increase of the EL ef®ciency, while increasing the response time. We attribute these effects to phase separation and salt trapping at PEO crystalline regions, due to the formation of crystalline complexes. Hence the improved luminescence ef®ciency can be attributed to lower ion-induced exciton quenching. The increase of the LECs response time upon addition of PEO is attributed to a reduction of the amount of free ions.
Acknowledgements Financial support from FundacËaÄo para a CieÃncia e a Tecnologia, The Royal Society, the European Comission, ORS and EPSRC is acknowledged. References [1] J. Morgado, F. Cacialli, J. GruÈner, N.C. Greenham, R.H. Friend, J. Appl. Phys. 85 (1999) 1784. [2] Q. Pei, G. Yu, C. Zhang, Y. Yang, A.J. Heeger, Science 269 (1995) 1086. [3] J.C. deMello, N. Tessler, S.C. Graham, R.H. Friend, Phys. Rev. B 57 (1998) 12951. [4] Q. Pei, Y. Yang, J. Am. Chem. Soc. 118 (1996) 7416. [5] D.-H. Hwang, B.S. Chuah, X.-C. Li, S.T. Kim, S.C. Moratti, A.B. Holmes, J.C. deMello, R.H. Friend, Macromol. Symp. 125 (1997) 111. [6] F. Cacialli, B.S. Chuah, J.S. Kim, D.A. Dossantos, R.H. Friend, S.C. Moratti, A.B. Holmes, J.-L. BreÂdas, Synth. Met. 102 (1999) 924. [7] H.Y. Chu, D.-H. Hwang, L.-M. Do, J.-H. Chang, H.-K. Shim, A.B. Holmes, T. Zyung, Synth. Met. 101 (1999) 216. [8] H. Rost, B.S. Chuah, D.-H. Hwang, S.C. Moratti, A.B. Holmes, J. Wilson, J. Morgado, J.J.M. Halls, J.C. De Mello, R.H. Friend, Synth. Met. 102 (1999) 937. [9] R. Frech, S. Chintapalli, P.G. Bruce, C.A. Vincent, Macromolecules 32 (1999) 808. [10] J. Morgado, R.H. Friend, F. Cacialli, B.S. Chuah, S.C. Moratti, A.B. Holmes, J. Appl. Phys. 86 (1999) 6392.