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Charge carrier injection and transport in polymer blend films
Solid State Communications 134 (2005) 291–294 www.elsevier.com/locate/ssc
Charge carrier injection and transport in polymer blend films Shan Yu Quana,b,*, Feng Tenga, DongDong Wanga, DeAng Liua, Zheng Xua,c,1, YongSheng Wanga, XuRong Xua a
Institute of Optoelectronics, Northern Jiaotong University, Beijing 100044, People’s Republic of China b ShenYang University of Technology, ShenYang 130021, People’s Republic of China c Institute of Material Physics, Tianjin Institute of Physics, Tianjin 300191, People’s Republic of China
Received 3 November 2004; received in revised form 8 November 2004; accepted 14 January 2005 by S. Das Sarma Available online 29 January 2005
Abstract The steady current–voltage characteristics of single layer organic devices based on MEH-PPV and N,N 0 -diphenyl-N,N 0 bis(4 0 -[N,N-bis(naphth-1-yl)-amino]-biphenyl-4-yl)-benzidine (TPTE) blend with different TPTE concentrations was investigated. The thickness dependence of the current–voltage relationship clearly demonstrates that the current at low voltage and at high voltage are all space charge limited. The current density–electric field characteristic proves the blend polymer LEDs to operate in the tunneling-controlled model. The effective hole mobility is directly determined by space charge limited current at high voltage and increases with increasing TPTE content in the blend. The EL efficiency shows concentration dependence, which is attributed to the change of the transport of holes in the blend film. q 2005 Elsevier Ltd. All rights reserved. PACS: 78.66.Qn; 85.60.Jb Keywords: D. Charge injection; D. Charge transport; D. Charge mobility; D. EL efficiency
1. Introduction Conjugated polymers have been extensively investigated due to the interesting physical properties of these materials as well as their large potential for electroluminescence applications [1]. Recently, polymer blends represent an alternative approach to new materials with improved performance for use in semiconducting devices. There have been many reports on blended organic- and polymerbased light-emitting diodes (LEDs) [2,3]. It has been * Corresponding author. Address: Institute of Optoelectronics, Physics, Northern Jiaotong University, B1005, no. ljaitong daxue road, Hai, Beijing 100044, People’s Republic of China. Tel.: C86 10 51684908; fax: C86 10 51688018. E-mail address: [email protected] (S.Y. Quan). 1 Now at 1Postdoctoral Working Station of Zhong-Huan San-Jin Ltd., Tianjin 300192, People’s Republic of China. 0038-1098/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2005.01.025
demonstrated that utilizing the blending techniques, it is easy to fabricate single-layer polymer LEDs with high performance. More importantly, we can improve both the carrier injection and the carrier transport [4–6] by carefully selecting the component polymer for blending and by adjusting their fractions in the blend polymer LEDs. However, the injection and transport properties of the charge carriers in the blend polymer LEDs are not yet understood well. It is well known that the interface barriers between the electrode and the polymer control the electron and hole injection. The smallest barrier height of the two barriers controls the I–V characteristics, while the larger barrier determines the device efficiency [7]. On the other hand, the charge carrier mobility controls the charge transport and charge recombination processes and also affects electrical and optical characteristic of polymer LEDs. In conjugated polymer films, it is generally considered that the charge
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S.Y. Quan et al. / Solid State Communications 134 (2005) 291–294
transport is a disordered hopping process [8]. Charge carrier mobilities in these disordered organic systems are governed by energetic and spatial disorder. The doping in the conjugated polymer should result in the change in energetic and spatial disorder. Therefore, it is possible to adjust the charge carrier mobility by blending. In this article, we present the investigation of the consequences of the concentration of N,N 0 -diphenyl-N,N 0 bis(4 0 -[N,N-bis-(naphth-1-yl)-amino]-biphenyl-4-yl)-benzidine (TPTE) on the charge carrier injection and transport in MEH-PPV:TPTE. The thickness of the current density– electric field characteristics indicates that the characteristics of light emitting diodes based on blend polymer are determined by tunneling of the hole through interface barriers. The dopant concentration dependence of the current–voltage relationship clearly indicates TPTE concentration significantly affects the hole mobility in the blend. The fact that electroluminescent efficiency shows concentration dependence further demonstrate that the hole mobilities are adjusted by changing the TPTE concentration in the blend so that transport balance is achieved.
2. Experiment Fig. 1 shows the organic materials N,N 0 -diphenyl-N,N 0 bis(4 0 -[N,N-bis(naphth-1-yl)-amino]-biphenyl-4-yl)-benzidine (TPTE) and MEH-PPV used in this study, TPTE is triaryl-amine tetramer with high hole mobility [9]. MEHPPV and hole transporter (TPTE) were dissolved respectively in THF by stirring the solution for 6 h. TPTE was blended in MEH-PPV at different concentrations (0.1:5, 0.3:5 and 0.5:5 weight ratio), and then sonicated for 2 h to give the appropriate weight percentage and then stirred for an additional 3 h. The devices used here are sandwiched structures of ITO/MEH-PPV:TPTE/Al for the study of transport properties and ITO/MEH-PPV:TPTE/Alq3/Al for the study of EL properties. The blend emitter was spin coated onto ITOcoated glass substrate. The doped polymer film thickness was measured using a surface profiler. The electron transport layer and cathode were evaporated at pressures less than 10K6 Torr. The thickness of each layer was 5 nm for Alq3, 1 nm for LiF, which was measured with a
Fig. 1. Molecular structures of MEH-PPV and TPTE.
calibrated oscillating crystal monitor. The annealing process of the emitting layer (60 8C for 1 h) was performed before Alq3 or cathode evaporation. The current–voltage (I–V) was measured using a source meter (Keithley 2400 source meter) and a luminometer (Minolta LS110 Luminometer) with a close-up lens (Minolta No. 110). All the measurements were carried out at room temperature in air.
3. Results and discussion Fig. 2 gives the I–V characteristics of ITO/MEHPPV:TPTE (5:0.1)/Al devices for different blend polymer thickness. The current dependence on electric field strength for ITO/MEH-PPV:TPTE (5:0.1) /Al devices is shown in Fig. 3. This figure shows demonstrate that the I–V characteristics depend, not on the voltage, but instead on the electric-field strength. This clearly indicates that the characteristics of light emitting diodes based on blend polymer are determined by tunneling of the hole through a barrier at the electrode/polymer interface. Increasing voltage of the I–V curve, three portions can be identified corresponding to space charge limited with a single discrete set of shallow traps (JfV2) (I) at low voltage, and a steeper increase which demonstrates a trap filled region (JfVm) (II) with a further increase in the voltage, and a final space charge limited (JfV2) (III) at high voltage. At low voltage, space charge limited current demonstrates that thermally generated free charge is negligible in comparison to the injected charge density ninj. The ninj dominate over the injected charge contribution. In this regime, a very low current is observed due to charge capture in traps. Current in this regime is determined by the bulk properties of the solid rather than contact effects. Increasing the forward bias results in an increase in injected charge, thereby filling the limited number of traps. The reduction in empty traps results in a rapid increase in the effective hole
Fig. 2. I–V characteristics of ITO/MEH-PPV:TPTE (5:0.1) /Al devices with different thickness.
S.Y. Quan et al. / Solid State Communications 134 (2005) 291–294
Fig. 3. Electric field vs. current density dependence for ITO/MEHPPV:TPTE (5:0.1)/Al devices.
mobility, and, therefore, a rapid power-law increases in current (JfVm). This will be observed experimentally as a rather sudden and large increase in the current, such as seen in region (II) shown in Fig. 2. At higher applied field, the charge mobility can be estimated by using the trap free space charge limited current (SCLC) model described as [10] 9 V2 JSCL Z 330 m 3 8 d
(1)
Where m is the effective charge carrier mobility, 30 is the permittivity of vacuum, 3 is the relative dielectric constant of the polymer, d is the thickness of the polymer, and V is the applied voltage. Assuming 3Z3, applying Eq. (1) to the data of segments (III) of Fig. 2, the carrier mobility of doped polymer can be determined. Considering that TPTE and MEH-PPV are all known as hole transporting material with low electrical affinity and ionization potential, the determined mobility values are considered to be the effective hole mobility in the blends. Table 1 presents the mobilities of MEH-PPV:TPTE blends for different TPTE concentrations. It can be seen the doping ratios really affect the effective hole mobility.
Within the range of selected concentration, ratio 10% by weight exhibited the best. The same space charge limited current characteristics are also observed in the other doping concentration. In luminescent conjugated polymer films, the main feature of the charge transport is its controlling by energetic and spatial disorder. The Gaussian disorder transport model [GDM] proposed by Ba¨ssler and co-workers [8,11] assumes that the charge transporting states are energetically localized. The energy of these states is subject to a random distribution introduced by disorder. The distribution of the density of states (DOS) can be described as a Gaussian function due to numerous independent contributions to the site energies coming from long-range electrostatic interactions with the surrounding disordered matrix [12,13], the width s of the DOS being a measure of the disorder of the transport states. It has been shown by both Monte-Carlo simulation [14] and analytically [15,16] that a carrier will most probably jump from a currently occupied state to a hopping site that belongs to effective transport level of the energy. In general, the jump probability exponentially decreases with increasing both the distance and energy difference between starting and target sites. In the MEHPPV:TPTE blend, we believe most holes prefer to jump through the highest occupied molecular orbital (HOMO) bands of TPTE due to its high mobility comparing to MEHPPV. Increasing TPTE concentration would result in the decreasing the jump distance, which lead to high jump probability. On the other hand, the overall DOS in the blend polymer will be a superposition of the Gaussian energy densities for the TPTE and MEH-PPV sites [17]. The concentration of TPTE in MEH-PPV would affect the Gaussian density of TPTE (DOS), which strongly affects the mobility charge carriers [18]. More TPTE would result in more superposition, which make the hole easy to jump from TPTE to the MEH-PPV. Thus the increasing doping concentration will strongly increase the charge carrier mobility. The energetic scheme of jump is summarized in Fig. 4. Fig. 5 shows the brightness-current characteristics of ITO/MEH-PPV:TPTE/Alq3/LiF/Al devices with different
Table 1 The mobilities at different MEH-PPV:TPTE Materials
Blend concentration (w/w)
m (cm2 VK1 sK1)
MEHPPV:TPTE MEHPPV:TPTE MEHPPV:TPTE MEHPPV:TPTE
0:5
(8.3G1.0)!10K6
0.1:5
(1.1G1.2)!10K5
0.3:5
(3.1G1.3)!10K5
0.5:5
(3.5G1.7)!10K5
293
Fig. 4. Energetic scheme of jump in the blend polymer.
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S.Y. Quan et al. / Solid State Communications 134 (2005) 291–294
achieved, which can be improve the performance of polymer LEDs.
Acknowledgements This work has been supported by National Natural Science Foundation of China (No. 90301004 and 10374001), and National Natural Science Foundation of Beijing city (No. 2032015), and RFDP (No. 20020004004).
References Fig. 5. The brightness-current characteristics of ITO/MEHPPV:TPTE/Alq3/Al devices with different concentrations of TPTE in MEH-PPV.
concentrations of TPTE in MEH-PPV. The EL efficiency as a function of current for these devices with different concentrations of TPTE in MEH-PPV is shown in the inset of Fig. 5. It is clearly seen that the EL efficiency shows concentration dependence, and the device with the concentration of MEH-PPV:TPTEZ5:0.3 exhibits the maximum EL efficiency, indicating that transport balance of holes and electrons in the blend can be achieved by adjusting the TPTE concentration in MEH-PPV. The decreased EL efficiency in the case of low and high TPTE concentration in MEH-PPV should be attributed to the imbalance electron and hole transport. In summary, we investigated the effect of the concentration of TPTE dopant molecule on the charge carrier injection and transport in MEH-PPV:TPTE by measuring the steady-state current–voltage characteristics. Our experimental results clearly demonstrate that the characteristics of light emitting diodes based on our blend polymer are determined by tunneling of the hole through interface barriers, and the current both at low voltage and high voltage are all space-charge limited. It has been found that the concentration of TPTE significantly affects the hole transport in MEH-PPV:TPTE, which is explained by increasing the hopping rate. Charge carrier mobility can be adjusted by varying the concentration of hole transporter so that a balance of electron and hole transport may be
[1] M. Seizo, Organic Electroluminescent Materials and Devices, OPA, Amsterdam, 1997. [2] S. Berleb, W. Bru¨tting, M. Schwoerer, J. Appl. Phys. 83 (1998) 4403. [3] M. Vchida, C. Adachi, T. Koyama, Y. Taniguchi, J. Appl. Phys. 86 (1999) 1680. [4] H. Vestweber, R. Sander, A. Greiner, W. Heitz, R.F. Mahrt, H. Bassler, Synth. Met. 64 (1994) 141. [5] C. Zhang, H. von Seggern, B. Kraabel, H.-W. Schmidt, A.J. Heeger, Synth. Met. 64 (1995) 141. [6] G. Yu, H. Nishino, A.J. Heeger, T.-A. Chen, R.D. Rieke, Synth. Met. 72 (1995) 249. [7] I.D. Parker, J. Appl. Phys. 75 (1994) 1656. [8] H. Baˆssler, Phys. Status Solidi B 175 (1993) 15. [9] S. Tokito, H. Tanaka, K. Noda, A. Okada, Y. Taga, Appl. Phys. Lett. 70 (1997) 1929. [10] K.C. Cao, W. Hwang, Electrical Transport in Solids, Pergamon, London, 1981. [11] P.M. Borsenberger, L.T. Pautmier, H. Baˆssler, Phys. Rev. B 46 (1992) 12145. [12] A. Dieckman, H. Bassler, M. Kenkre, J. Phys. Chem. 99 (1993) 8136. [13] S.V. Novikov, V. Vannikov, J. Phys. Chem. 99 (1995) 14573. [14] M. Gru¨newald, P. Thomas, Phys. Status Solidi B 94 (1979) 125. [15] D. Monroe, Phys. Rev. Lett. 54 (1985) 146. [16] V.I. Arkhipov, E.V. Emelianova, J. Adriaenssens, Phys. Rev. B 64 (2001) 125125. [17] M. Sugiuchi, H. Nishizawa, J. Imag. Sci. Technol. 37 (1993) 245. [18] V.I. Arkhipov, P. Heremons, E.V. Emelianova, G.J. Adriaenssens, H. Bassler, Appl. Phys. Lett. 82 (2003) 3245.
Charge carrier injection and transport in polymer blend films Shan Yu Quana,b,*, Feng Tenga, DongDong Wanga, DeAng Liua, Zheng Xua,c,1, YongSheng Wanga, XuRong Xua a
Institute of Optoelectronics, Northern Jiaotong University, Beijing 100044, People’s Republic of China b ShenYang University of Technology, ShenYang 130021, People’s Republic of China c Institute of Material Physics, Tianjin Institute of Physics, Tianjin 300191, People’s Republic of China
Received 3 November 2004; received in revised form 8 November 2004; accepted 14 January 2005 by S. Das Sarma Available online 29 January 2005
Abstract The steady current–voltage characteristics of single layer organic devices based on MEH-PPV and N,N 0 -diphenyl-N,N 0 bis(4 0 -[N,N-bis(naphth-1-yl)-amino]-biphenyl-4-yl)-benzidine (TPTE) blend with different TPTE concentrations was investigated. The thickness dependence of the current–voltage relationship clearly demonstrates that the current at low voltage and at high voltage are all space charge limited. The current density–electric field characteristic proves the blend polymer LEDs to operate in the tunneling-controlled model. The effective hole mobility is directly determined by space charge limited current at high voltage and increases with increasing TPTE content in the blend. The EL efficiency shows concentration dependence, which is attributed to the change of the transport of holes in the blend film. q 2005 Elsevier Ltd. All rights reserved. PACS: 78.66.Qn; 85.60.Jb Keywords: D. Charge injection; D. Charge transport; D. Charge mobility; D. EL efficiency
1. Introduction Conjugated polymers have been extensively investigated due to the interesting physical properties of these materials as well as their large potential for electroluminescence applications [1]. Recently, polymer blends represent an alternative approach to new materials with improved performance for use in semiconducting devices. There have been many reports on blended organic- and polymerbased light-emitting diodes (LEDs) [2,3]. It has been * Corresponding author. Address: Institute of Optoelectronics, Physics, Northern Jiaotong University, B1005, no. ljaitong daxue road, Hai, Beijing 100044, People’s Republic of China. Tel.: C86 10 51684908; fax: C86 10 51688018. E-mail address: [email protected] (S.Y. Quan). 1 Now at 1Postdoctoral Working Station of Zhong-Huan San-Jin Ltd., Tianjin 300192, People’s Republic of China. 0038-1098/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2005.01.025
demonstrated that utilizing the blending techniques, it is easy to fabricate single-layer polymer LEDs with high performance. More importantly, we can improve both the carrier injection and the carrier transport [4–6] by carefully selecting the component polymer for blending and by adjusting their fractions in the blend polymer LEDs. However, the injection and transport properties of the charge carriers in the blend polymer LEDs are not yet understood well. It is well known that the interface barriers between the electrode and the polymer control the electron and hole injection. The smallest barrier height of the two barriers controls the I–V characteristics, while the larger barrier determines the device efficiency [7]. On the other hand, the charge carrier mobility controls the charge transport and charge recombination processes and also affects electrical and optical characteristic of polymer LEDs. In conjugated polymer films, it is generally considered that the charge
292
S.Y. Quan et al. / Solid State Communications 134 (2005) 291–294
transport is a disordered hopping process [8]. Charge carrier mobilities in these disordered organic systems are governed by energetic and spatial disorder. The doping in the conjugated polymer should result in the change in energetic and spatial disorder. Therefore, it is possible to adjust the charge carrier mobility by blending. In this article, we present the investigation of the consequences of the concentration of N,N 0 -diphenyl-N,N 0 bis(4 0 -[N,N-bis-(naphth-1-yl)-amino]-biphenyl-4-yl)-benzidine (TPTE) on the charge carrier injection and transport in MEH-PPV:TPTE. The thickness of the current density– electric field characteristics indicates that the characteristics of light emitting diodes based on blend polymer are determined by tunneling of the hole through interface barriers. The dopant concentration dependence of the current–voltage relationship clearly indicates TPTE concentration significantly affects the hole mobility in the blend. The fact that electroluminescent efficiency shows concentration dependence further demonstrate that the hole mobilities are adjusted by changing the TPTE concentration in the blend so that transport balance is achieved.
2. Experiment Fig. 1 shows the organic materials N,N 0 -diphenyl-N,N 0 bis(4 0 -[N,N-bis(naphth-1-yl)-amino]-biphenyl-4-yl)-benzidine (TPTE) and MEH-PPV used in this study, TPTE is triaryl-amine tetramer with high hole mobility [9]. MEHPPV and hole transporter (TPTE) were dissolved respectively in THF by stirring the solution for 6 h. TPTE was blended in MEH-PPV at different concentrations (0.1:5, 0.3:5 and 0.5:5 weight ratio), and then sonicated for 2 h to give the appropriate weight percentage and then stirred for an additional 3 h. The devices used here are sandwiched structures of ITO/MEH-PPV:TPTE/Al for the study of transport properties and ITO/MEH-PPV:TPTE/Alq3/Al for the study of EL properties. The blend emitter was spin coated onto ITOcoated glass substrate. The doped polymer film thickness was measured using a surface profiler. The electron transport layer and cathode were evaporated at pressures less than 10K6 Torr. The thickness of each layer was 5 nm for Alq3, 1 nm for LiF, which was measured with a
Fig. 1. Molecular structures of MEH-PPV and TPTE.
calibrated oscillating crystal monitor. The annealing process of the emitting layer (60 8C for 1 h) was performed before Alq3 or cathode evaporation. The current–voltage (I–V) was measured using a source meter (Keithley 2400 source meter) and a luminometer (Minolta LS110 Luminometer) with a close-up lens (Minolta No. 110). All the measurements were carried out at room temperature in air.
3. Results and discussion Fig. 2 gives the I–V characteristics of ITO/MEHPPV:TPTE (5:0.1)/Al devices for different blend polymer thickness. The current dependence on electric field strength for ITO/MEH-PPV:TPTE (5:0.1) /Al devices is shown in Fig. 3. This figure shows demonstrate that the I–V characteristics depend, not on the voltage, but instead on the electric-field strength. This clearly indicates that the characteristics of light emitting diodes based on blend polymer are determined by tunneling of the hole through a barrier at the electrode/polymer interface. Increasing voltage of the I–V curve, three portions can be identified corresponding to space charge limited with a single discrete set of shallow traps (JfV2) (I) at low voltage, and a steeper increase which demonstrates a trap filled region (JfVm) (II) with a further increase in the voltage, and a final space charge limited (JfV2) (III) at high voltage. At low voltage, space charge limited current demonstrates that thermally generated free charge is negligible in comparison to the injected charge density ninj. The ninj dominate over the injected charge contribution. In this regime, a very low current is observed due to charge capture in traps. Current in this regime is determined by the bulk properties of the solid rather than contact effects. Increasing the forward bias results in an increase in injected charge, thereby filling the limited number of traps. The reduction in empty traps results in a rapid increase in the effective hole
Fig. 2. I–V characteristics of ITO/MEH-PPV:TPTE (5:0.1) /Al devices with different thickness.
S.Y. Quan et al. / Solid State Communications 134 (2005) 291–294
Fig. 3. Electric field vs. current density dependence for ITO/MEHPPV:TPTE (5:0.1)/Al devices.
mobility, and, therefore, a rapid power-law increases in current (JfVm). This will be observed experimentally as a rather sudden and large increase in the current, such as seen in region (II) shown in Fig. 2. At higher applied field, the charge mobility can be estimated by using the trap free space charge limited current (SCLC) model described as [10] 9 V2 JSCL Z 330 m 3 8 d
(1)
Where m is the effective charge carrier mobility, 30 is the permittivity of vacuum, 3 is the relative dielectric constant of the polymer, d is the thickness of the polymer, and V is the applied voltage. Assuming 3Z3, applying Eq. (1) to the data of segments (III) of Fig. 2, the carrier mobility of doped polymer can be determined. Considering that TPTE and MEH-PPV are all known as hole transporting material with low electrical affinity and ionization potential, the determined mobility values are considered to be the effective hole mobility in the blends. Table 1 presents the mobilities of MEH-PPV:TPTE blends for different TPTE concentrations. It can be seen the doping ratios really affect the effective hole mobility.
Within the range of selected concentration, ratio 10% by weight exhibited the best. The same space charge limited current characteristics are also observed in the other doping concentration. In luminescent conjugated polymer films, the main feature of the charge transport is its controlling by energetic and spatial disorder. The Gaussian disorder transport model [GDM] proposed by Ba¨ssler and co-workers [8,11] assumes that the charge transporting states are energetically localized. The energy of these states is subject to a random distribution introduced by disorder. The distribution of the density of states (DOS) can be described as a Gaussian function due to numerous independent contributions to the site energies coming from long-range electrostatic interactions with the surrounding disordered matrix [12,13], the width s of the DOS being a measure of the disorder of the transport states. It has been shown by both Monte-Carlo simulation [14] and analytically [15,16] that a carrier will most probably jump from a currently occupied state to a hopping site that belongs to effective transport level of the energy. In general, the jump probability exponentially decreases with increasing both the distance and energy difference between starting and target sites. In the MEHPPV:TPTE blend, we believe most holes prefer to jump through the highest occupied molecular orbital (HOMO) bands of TPTE due to its high mobility comparing to MEHPPV. Increasing TPTE concentration would result in the decreasing the jump distance, which lead to high jump probability. On the other hand, the overall DOS in the blend polymer will be a superposition of the Gaussian energy densities for the TPTE and MEH-PPV sites [17]. The concentration of TPTE in MEH-PPV would affect the Gaussian density of TPTE (DOS), which strongly affects the mobility charge carriers [18]. More TPTE would result in more superposition, which make the hole easy to jump from TPTE to the MEH-PPV. Thus the increasing doping concentration will strongly increase the charge carrier mobility. The energetic scheme of jump is summarized in Fig. 4. Fig. 5 shows the brightness-current characteristics of ITO/MEH-PPV:TPTE/Alq3/LiF/Al devices with different
Table 1 The mobilities at different MEH-PPV:TPTE Materials
Blend concentration (w/w)
m (cm2 VK1 sK1)
MEHPPV:TPTE MEHPPV:TPTE MEHPPV:TPTE MEHPPV:TPTE
0:5
(8.3G1.0)!10K6
0.1:5
(1.1G1.2)!10K5
0.3:5
(3.1G1.3)!10K5
0.5:5
(3.5G1.7)!10K5
293
Fig. 4. Energetic scheme of jump in the blend polymer.
294
S.Y. Quan et al. / Solid State Communications 134 (2005) 291–294
achieved, which can be improve the performance of polymer LEDs.
Acknowledgements This work has been supported by National Natural Science Foundation of China (No. 90301004 and 10374001), and National Natural Science Foundation of Beijing city (No. 2032015), and RFDP (No. 20020004004).
References Fig. 5. The brightness-current characteristics of ITO/MEHPPV:TPTE/Alq3/Al devices with different concentrations of TPTE in MEH-PPV.
concentrations of TPTE in MEH-PPV. The EL efficiency as a function of current for these devices with different concentrations of TPTE in MEH-PPV is shown in the inset of Fig. 5. It is clearly seen that the EL efficiency shows concentration dependence, and the device with the concentration of MEH-PPV:TPTEZ5:0.3 exhibits the maximum EL efficiency, indicating that transport balance of holes and electrons in the blend can be achieved by adjusting the TPTE concentration in MEH-PPV. The decreased EL efficiency in the case of low and high TPTE concentration in MEH-PPV should be attributed to the imbalance electron and hole transport. In summary, we investigated the effect of the concentration of TPTE dopant molecule on the charge carrier injection and transport in MEH-PPV:TPTE by measuring the steady-state current–voltage characteristics. Our experimental results clearly demonstrate that the characteristics of light emitting diodes based on our blend polymer are determined by tunneling of the hole through interface barriers, and the current both at low voltage and high voltage are all space-charge limited. It has been found that the concentration of TPTE significantly affects the hole transport in MEH-PPV:TPTE, which is explained by increasing the hopping rate. Charge carrier mobility can be adjusted by varying the concentration of hole transporter so that a balance of electron and hole transport may be
[1] M. Seizo, Organic Electroluminescent Materials and Devices, OPA, Amsterdam, 1997. [2] S. Berleb, W. Bru¨tting, M. Schwoerer, J. Appl. Phys. 83 (1998) 4403. [3] M. Vchida, C. Adachi, T. Koyama, Y. Taniguchi, J. Appl. Phys. 86 (1999) 1680. [4] H. Vestweber, R. Sander, A. Greiner, W. Heitz, R.F. Mahrt, H. Bassler, Synth. Met. 64 (1994) 141. [5] C. Zhang, H. von Seggern, B. Kraabel, H.-W. Schmidt, A.J. Heeger, Synth. Met. 64 (1995) 141. [6] G. Yu, H. Nishino, A.J. Heeger, T.-A. Chen, R.D. Rieke, Synth. Met. 72 (1995) 249. [7] I.D. Parker, J. Appl. Phys. 75 (1994) 1656. [8] H. Baˆssler, Phys. Status Solidi B 175 (1993) 15. [9] S. Tokito, H. Tanaka, K. Noda, A. Okada, Y. Taga, Appl. Phys. Lett. 70 (1997) 1929. [10] K.C. Cao, W. Hwang, Electrical Transport in Solids, Pergamon, London, 1981. [11] P.M. Borsenberger, L.T. Pautmier, H. Baˆssler, Phys. Rev. B 46 (1992) 12145. [12] A. Dieckman, H. Bassler, M. Kenkre, J. Phys. Chem. 99 (1993) 8136. [13] S.V. Novikov, V. Vannikov, J. Phys. Chem. 99 (1995) 14573. [14] M. Gru¨newald, P. Thomas, Phys. Status Solidi B 94 (1979) 125. [15] D. Monroe, Phys. Rev. Lett. 54 (1985) 146. [16] V.I. Arkhipov, E.V. Emelianova, J. Adriaenssens, Phys. Rev. B 64 (2001) 125125. [17] M. Sugiuchi, H. Nishizawa, J. Imag. Sci. Technol. 37 (1993) 245. [18] V.I. Arkhipov, P. Heremons, E.V. Emelianova, G.J. Adriaenssens, H. Bassler, Appl. Phys. Lett. 82 (2003) 3245.