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Structure and charge transport properties in MEH-PPV
Synthetic Metals 139 (2003) 581–584
Structure and charge transport properties in MEH-PPV Anto Regis Inigo a,b , Hsiang-Chih Chiu a,b , Wunshain Fann a,b,∗ , Ying-Sheng Huang c , U.S. Jeng d , C.H. Hsu d , Kang-Yung Peng e , Show-An Chen e a
c
Institute of Atomic and Molecular Sciences, Academia Sinica, P.O. Box 23-166, Taipei, Taiwan, ROC b Department of Physics, National Taiwan University, Taipei, Taiwan, ROC Department of Electronic Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan, ROC d National Synchrotron Radiation Research Center, Hsinchu, Taiwan, ROC e Department of Chemical Engineering, National Tsing-Hwa University, Hsinchu, Taiwan, ROC
Abstract The charge carrier transport in MEH-PPV is investigated with respect to different molecular weight distributions and with different tetrahedral defect densities. The defect density influences the charge transport behaviors significantly. With smaller defect density, MEH-PPV exhibits better charge transport which further depends upon the morphology. Position disorder parameter which is due to the morphology difference dominates the charge transport properties of low defect samples. © 2003 Elsevier Science B.V. All rights reserved. Keywords: MEH-PPV; Morphology; Tetrahedral defect density; TOF; Hole mobility; Charge transport
1. Introduction The demonstration of high performance electroluminescent devices of vacuum sublimed organic films and the subsequent discovery of electroluminescence from conjugated polymer films [1] opened up a new area in light emitting diodes. Although much progress has been made in studying charge generation and recombination, understanding the relationship between charge transport, morphology and chemical structure is still a big challenge. The polymer films are supposed to have different morphology with respect to processing conditions [2]. Hence it affects the charge carrier transport which in turn decides the injection of electrons and holes and their recombination to emit light. In this regard, we investigate the hole transport properties of MEH-PPV with respect to molecular weight, and defect density. Photoluminescence (PL) of polymeric compounds gives the direct information about the interchain and intrachain emissions. The morphology of the polymers is mainly characterized from the observation of emission from the interchain species and intrachain species. As the processing conditions varies, the morphology of the resulting polymer film may change. This will lead to the change in the photophysical properties of the ∗ Corresponding author. Tel.: +886-223-620-212; fax: +886-223-620-200. E-mail address: [email protected] (W. Fann).
resulting device structures [3,4]. The variation in molecular weight may also cause a change in the morphology of the films. The important issue behind the efficient light emission is balanced charge carrier injection and transport in the materials. Time of flight (TOF) is one of the methods to investigate the drift mobility of the charge carriers in the materials. Field and temperature dependent mobility can also provide more information about the basic transport properties [5]. In addition, the role defects can be significant [6]. The common defect in MEH-PPV is tetrahedral defect. These defects can act as charge traps and subsequently reduce the device lifetime [7]. We use time of flight to measure the mobility, photoluminescence to probe the effect of interchain interaction in the devices, gel permeation chromatography (GPC) for the analysis of molecular weight and nuclear magnetic resonance (NMR) to estimate the tetrahedral defect percentage.
2. Experimental procedure The devices were prepared by dissolving MEH-PPV in toluene (TL) at the concentration of 5 mg/cm3 from four batches of samples with different molecular weights from batches N1 to N4. The device preparation, TOF instrumentation and mobility measurements were described in our previous publication [5]. The molecular weight was measured
0379-6779/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0379-6779(03)00302-3
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by gel permeation chromatography. The photoluminescence spectra were recorded by spectrofluorometer (Spec Fluorolog-3).
3. Results and discussion Fig. 1. shows the representative figure of GPC for batch N1. The average molecular weight and poly dispersity index (PDI) are given in Table 1. Fig. 2 shows the representative spectrum of NMR of batch N2. The corresponding defect densities are also shown in Table 1. The densities of tetrahedral defects are 1.7, 2.46 and 3.0% for the devices from batches N1 to N3, respectively. We were unable to measure defect density in batch N4 due to solubility problem. Fig. 3 shows the PL spectrum of all the devices. The results show that the interchain emission decreases from batches N1 to N4, but there is no clear signature of molecular weight dependence. Fig. 4 shows the room temperature time of flight hole transients for all four batches. The samples from batches N1 and N2 show non-dispersive behavior and the transit time can be found in the linear scale. Samples from batches N3
and N4 show highly dispersive behavior. For the purpose of analysis the transit times were calculated by using log i–log t plot. It was even difficult to find out the transit times in log i–log t plots for N3 and N4 samples. It is much easy to obtain reliable transport parameters through non-dispersive transients. So, we further measured the field and temperature dependent mobilities for the devices made from batches N1 and N2. The data were analyzed using the Gaussian disorder model (GDM) proposed by Bässler [8]. Detailed analysis of that model can be found elsewhere [4,9]. The energy disorder and position disorder parameters were calculated and presented in the Table 1. Table 1 clearly shows that the defect density should be small to get a good charge transport in the materials. Batches N1 and N3 they have almost same molecular weight and PDI value but the hole transient in batch N1 is non-dispersive whereas in batch N3 the hole transient is highly dispersive. There is no significant correlation of transport properties with respect to the range of molecular weight investigated. But there seems to be a direct relationship between the defect density and the charge transport. The defect density is less for the samples from batches N1 and N2. Therefore, once the defect density is in control
Fig. 1. The representative figure for the GPC for batch 1. Table 1 The table shows the different physical parameters measured from four batches of MEH-PPV Sample
N1
N2
N3
N4
MW/PDI Thickness (m) Transport properties Mobility (cm2 /V s; E = 1.5 × 105 V/cm) Energy disorder (meV) Position disorder Defects (%)
249706/6.44 3.7 Non-dispersive 5.64 × 10−6 72.6 1.48 1.76
165371/5.53 4.0 Non-dispersive 4.01 × 10−6 81.9 4.57 2.46
253289/6.16 3.5 Dispersive – – – 3.00
407184/2.24 3.5 Dispersive – – – N/A
A.R. Inigo et al. / Synthetic Metals 139 (2003) 581–584
Fig. 2. The representative spectrum of NMR for batch 2.
Fig. 3. The PL spectrum for devices prepared from solution TL for all batches excited at 510 nm.
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Fig. 4. The room temperature hole transients for the devices of all batches.
then the effect of morphology can be visible in the charge transport. For example, the energy disorder and position disorder parameters are less for the batch N1 samples than that of batch N2 samples. Based on this analysis, we propose that the position disorder dominates the charge transport properties of the devices. Here, the morphology changes may be due the variation of polymer molecular weight and its PDI. 4. Conclusion The mobility and the type of charge transport, i.e. dispersive or non-dispersive may depend upon the defect density at the first instance. Once the defect density becomes less then the effect of morphology may be visible in the charge transport. In MEH-PPV, it seems that the position disorder dominates in devices depending upon the morphology induced by solvents. Acknowledgements This work is supported by National Research Council, Taiwan, ROC and MOE program for promoting academic
excellence of universities under the Grant no. 91-E-FA042-4A.
References [1] J.H. Buroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackey, R.H. Friend, R.L. Burn, A.B. Holmes, Nature (London) 347 (1990) 539. [2] C.Y. Yang, F. Hide, M.A.D. Garcia, A.J. Heeger, Y. Cao, Polymer 39 (1998) 2299. [3] C.H. Tan, A.R. Inigo, W.S. Fann, P.K. Wei, Y.S. Huang, G.Y. Perng, S.A. Chen, Org. Electron. 3 (2002) 85. [4] A.R. Inigo, C.H. Tan, W.S. Fann, Y.S. Huang, G.Y. Perng, S.A. Chen, Adv. Mater. 13 (2001) 504. [5] W.E. Spear, J. Non-Cryst. Solids 1 (1969) 197. [6] M. Yan, L.J. Rothberg, F. Papadimitrakopoulos, M.E. Galvin, T.M. Miller, Phys. Rev. Lett. 73 (1994) 744. [7] H. Becker, H. Sprizter, W. Kreuder, E. Kluge, H. Schenk, I. Parker, Y. Cao, Adv. Mater. 12 (2000) 42. [8] H. Bässler, Phys. Status Solidi B 175 (1993) 15. [9] C. Im, H. Bassler, H. Rost, H.H. Horhold, J. Chem. Phys. 113 (2000) 3802.
Structure and charge transport properties in MEH-PPV Anto Regis Inigo a,b , Hsiang-Chih Chiu a,b , Wunshain Fann a,b,∗ , Ying-Sheng Huang c , U.S. Jeng d , C.H. Hsu d , Kang-Yung Peng e , Show-An Chen e a
c
Institute of Atomic and Molecular Sciences, Academia Sinica, P.O. Box 23-166, Taipei, Taiwan, ROC b Department of Physics, National Taiwan University, Taipei, Taiwan, ROC Department of Electronic Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan, ROC d National Synchrotron Radiation Research Center, Hsinchu, Taiwan, ROC e Department of Chemical Engineering, National Tsing-Hwa University, Hsinchu, Taiwan, ROC
Abstract The charge carrier transport in MEH-PPV is investigated with respect to different molecular weight distributions and with different tetrahedral defect densities. The defect density influences the charge transport behaviors significantly. With smaller defect density, MEH-PPV exhibits better charge transport which further depends upon the morphology. Position disorder parameter which is due to the morphology difference dominates the charge transport properties of low defect samples. © 2003 Elsevier Science B.V. All rights reserved. Keywords: MEH-PPV; Morphology; Tetrahedral defect density; TOF; Hole mobility; Charge transport
1. Introduction The demonstration of high performance electroluminescent devices of vacuum sublimed organic films and the subsequent discovery of electroluminescence from conjugated polymer films [1] opened up a new area in light emitting diodes. Although much progress has been made in studying charge generation and recombination, understanding the relationship between charge transport, morphology and chemical structure is still a big challenge. The polymer films are supposed to have different morphology with respect to processing conditions [2]. Hence it affects the charge carrier transport which in turn decides the injection of electrons and holes and their recombination to emit light. In this regard, we investigate the hole transport properties of MEH-PPV with respect to molecular weight, and defect density. Photoluminescence (PL) of polymeric compounds gives the direct information about the interchain and intrachain emissions. The morphology of the polymers is mainly characterized from the observation of emission from the interchain species and intrachain species. As the processing conditions varies, the morphology of the resulting polymer film may change. This will lead to the change in the photophysical properties of the ∗ Corresponding author. Tel.: +886-223-620-212; fax: +886-223-620-200. E-mail address: [email protected] (W. Fann).
resulting device structures [3,4]. The variation in molecular weight may also cause a change in the morphology of the films. The important issue behind the efficient light emission is balanced charge carrier injection and transport in the materials. Time of flight (TOF) is one of the methods to investigate the drift mobility of the charge carriers in the materials. Field and temperature dependent mobility can also provide more information about the basic transport properties [5]. In addition, the role defects can be significant [6]. The common defect in MEH-PPV is tetrahedral defect. These defects can act as charge traps and subsequently reduce the device lifetime [7]. We use time of flight to measure the mobility, photoluminescence to probe the effect of interchain interaction in the devices, gel permeation chromatography (GPC) for the analysis of molecular weight and nuclear magnetic resonance (NMR) to estimate the tetrahedral defect percentage.
2. Experimental procedure The devices were prepared by dissolving MEH-PPV in toluene (TL) at the concentration of 5 mg/cm3 from four batches of samples with different molecular weights from batches N1 to N4. The device preparation, TOF instrumentation and mobility measurements were described in our previous publication [5]. The molecular weight was measured
0379-6779/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0379-6779(03)00302-3
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A.R. Inigo et al. / Synthetic Metals 139 (2003) 581–584
by gel permeation chromatography. The photoluminescence spectra were recorded by spectrofluorometer (Spec Fluorolog-3).
3. Results and discussion Fig. 1. shows the representative figure of GPC for batch N1. The average molecular weight and poly dispersity index (PDI) are given in Table 1. Fig. 2 shows the representative spectrum of NMR of batch N2. The corresponding defect densities are also shown in Table 1. The densities of tetrahedral defects are 1.7, 2.46 and 3.0% for the devices from batches N1 to N3, respectively. We were unable to measure defect density in batch N4 due to solubility problem. Fig. 3 shows the PL spectrum of all the devices. The results show that the interchain emission decreases from batches N1 to N4, but there is no clear signature of molecular weight dependence. Fig. 4 shows the room temperature time of flight hole transients for all four batches. The samples from batches N1 and N2 show non-dispersive behavior and the transit time can be found in the linear scale. Samples from batches N3
and N4 show highly dispersive behavior. For the purpose of analysis the transit times were calculated by using log i–log t plot. It was even difficult to find out the transit times in log i–log t plots for N3 and N4 samples. It is much easy to obtain reliable transport parameters through non-dispersive transients. So, we further measured the field and temperature dependent mobilities for the devices made from batches N1 and N2. The data were analyzed using the Gaussian disorder model (GDM) proposed by Bässler [8]. Detailed analysis of that model can be found elsewhere [4,9]. The energy disorder and position disorder parameters were calculated and presented in the Table 1. Table 1 clearly shows that the defect density should be small to get a good charge transport in the materials. Batches N1 and N3 they have almost same molecular weight and PDI value but the hole transient in batch N1 is non-dispersive whereas in batch N3 the hole transient is highly dispersive. There is no significant correlation of transport properties with respect to the range of molecular weight investigated. But there seems to be a direct relationship between the defect density and the charge transport. The defect density is less for the samples from batches N1 and N2. Therefore, once the defect density is in control
Fig. 1. The representative figure for the GPC for batch 1. Table 1 The table shows the different physical parameters measured from four batches of MEH-PPV Sample
N1
N2
N3
N4
MW/PDI Thickness (m) Transport properties Mobility (cm2 /V s; E = 1.5 × 105 V/cm) Energy disorder (meV) Position disorder Defects (%)
249706/6.44 3.7 Non-dispersive 5.64 × 10−6 72.6 1.48 1.76
165371/5.53 4.0 Non-dispersive 4.01 × 10−6 81.9 4.57 2.46
253289/6.16 3.5 Dispersive – – – 3.00
407184/2.24 3.5 Dispersive – – – N/A
A.R. Inigo et al. / Synthetic Metals 139 (2003) 581–584
Fig. 2. The representative spectrum of NMR for batch 2.
Fig. 3. The PL spectrum for devices prepared from solution TL for all batches excited at 510 nm.
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Fig. 4. The room temperature hole transients for the devices of all batches.
then the effect of morphology can be visible in the charge transport. For example, the energy disorder and position disorder parameters are less for the batch N1 samples than that of batch N2 samples. Based on this analysis, we propose that the position disorder dominates the charge transport properties of the devices. Here, the morphology changes may be due the variation of polymer molecular weight and its PDI. 4. Conclusion The mobility and the type of charge transport, i.e. dispersive or non-dispersive may depend upon the defect density at the first instance. Once the defect density becomes less then the effect of morphology may be visible in the charge transport. In MEH-PPV, it seems that the position disorder dominates in devices depending upon the morphology induced by solvents. Acknowledgements This work is supported by National Research Council, Taiwan, ROC and MOE program for promoting academic
excellence of universities under the Grant no. 91-E-FA042-4A.
References [1] J.H. Buroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackey, R.H. Friend, R.L. Burn, A.B. Holmes, Nature (London) 347 (1990) 539. [2] C.Y. Yang, F. Hide, M.A.D. Garcia, A.J. Heeger, Y. Cao, Polymer 39 (1998) 2299. [3] C.H. Tan, A.R. Inigo, W.S. Fann, P.K. Wei, Y.S. Huang, G.Y. Perng, S.A. Chen, Org. Electron. 3 (2002) 85. [4] A.R. Inigo, C.H. Tan, W.S. Fann, Y.S. Huang, G.Y. Perng, S.A. Chen, Adv. Mater. 13 (2001) 504. [5] W.E. Spear, J. Non-Cryst. Solids 1 (1969) 197. [6] M. Yan, L.J. Rothberg, F. Papadimitrakopoulos, M.E. Galvin, T.M. Miller, Phys. Rev. Lett. 73 (1994) 744. [7] H. Becker, H. Sprizter, W. Kreuder, E. Kluge, H. Schenk, I. Parker, Y. Cao, Adv. Mater. 12 (2000) 42. [8] H. Bässler, Phys. Status Solidi B 175 (1993) 15. [9] C. Im, H. Bassler, H. Rost, H.H. Horhold, J. Chem. Phys. 113 (2000) 3802.