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Possible transition from space-charge-limited to injection-limited conduction in poly(3-hexylthiophene) thin films
Applied Surface Science 252 (2006) 4023–4025 www.elsevier.com/locate/apsusc
Possible transition from space-charge-limited to injection-limited conduction in poly(3-hexylthiophene) thin films Yi Zheng a, Linda Kunardi b, Cedric Troadec b, Andrew T.S. Wee a, N. Chandrasekhar a,b,* a
Department of Physics, National University of Singapore (NUS), 10 Kent Ridge Crescent, Singapore 119260, Singapore b Institute of Material Research and Engineering (IMRE), 3 Research Link, Singapore 117602, Singapore Available online 21 October 2005
Abstract Two-terminal thin films of poly(3-hexylthiophene) (P3HT) with a wide electrode separation (150 mm) has been studied using current–voltage characteristics at different temperatures. Space-charge-limited conduction (SCLC) with high injection barriers (1.3 eV) has been observed at all temperatures in the low electric field regime. A possible transition from SCLC to injection-limited conduction (ILC) is reported. The experimental results have been compared with the disorder-controlled injection model proposed by Arkhipov et al. [V.I. Arkhipov, H. von Seggern, E.V. Emilianova, Appl. Phys. Lett. 83 (2003) 5074; V.I. Arkhipov, E.V. Emilianova, Y.-H. Tak, H. Ba¨ssler, J. Appl. Phys. 84 (1998) 848; V.I. Arkhipov, U. Wolf, H. Ba¨ssler, Phys. Rev. B 59 (1999) 7514]. # 2005 Elsevier B.V. All rights reserved. Keywords: P3HT thin films; Metal–organic interface; Charge injection; SCLC to ILC transition
1. Introduction Poly(3-hexylthiophene), P3HT, is one of the promising materials for organic thin-film transistors (OTFTs), with the advantages of solution processability and relatively high mobility (0.1 cm2 V1 s1) [4,5]. Compared to other vacuum-deposited organic thin films, solution-processed P3HT thin films show more structural complexity due to interchain stacking and chain-substrate interaction, which is hard to control and can determine the device performance [5,6]. In general, the microstructure of P3HT thin films can be described as ordered microcrystalline domains embedded in an amorphous matrix, which has been confirmed by multi-scale STM results [7]. Two models have been traditionally used to describe charge transport in a disordered organic material. The Gaussian disorder model of Ba¨ssler [8] ignores correlations between energies and positions of hopping sites, whereas the correlated hopping model of Dunlap and Novikov et al. [9] accounts for this interaction. Interactions between carriers can modify the nature of transport. There is usually an interplay between
* Corresponding author. Tel.: +65 6874 8586; fax: +65 6774 4657. E-mail address: [email protected] (N. Chandrasekhar). 0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.09.036
disorder and interaction in all electronic systems, i.e. in some cases the effects of disorder dominate, and in other cases the effect of interaction dominates the transport. Since the nature and extent of disorder is determined both by the organic material as well as its processing, it is certainly conceivable that the Gaussian disorder model applies to some organic systems, and the correlated hopping model applies to other systems. In this work, we study transport across an Au–P3HT interface, and we find that the properties can be well described by the Gaussian disorder model. For a disordered system such as Au–P3HT, charge transport can be controlled by two separate processes: charge injection at the metal–organic interface and transport in the bulk. For the latter, variable range hopping [8] has been widely adopted as the main mechanism. But it is still controversial in understanding interface charge injection. And if the interplay between interface injection and bulk conduction is considered, two different conduction regimes will be introduced: injectionlimited conduction (ILC) when injection is weaker than bulk transport, and space-charge-limited conduction (SCLC), corresponding to weaker bulk transport. I –V characteristics will therefore probe different regimes of transport across the metal–organic interfaces: in the SCLC regime, the measurement will probe the bulk properties; while in the ILC regime, only interface properties can be deduced from the same
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Y. Zheng et al. / Applied Surface Science 252 (2006) 4023–4025
experiments. Traditionally, such an interplay between the SCLC and ILC regimes is considered to be controlled only by one parameter, the contact barrier. It is believed that when the barrier height is lower than 0.25–0.30 eV, the observed I –V characteristics should be controlled by SCLC [10]. Recently, Arkhipov et al. proposed a two-step hopping model [1–3] to describe charge injection at metal–organic interfaces. In this model, carriers first jump from the Fermi level of a metal electrode into localized states in the organic, and diffuse under the influence of the applied electric field into the bulk. The latter step is well described by the onedimensional Onsager theory. Since this model predicts a both field and temperature dependent hopping injection current, the competition between ILC and SCLC becomes more complicated than the situation of barrier-height dominated transport due to the following reasons. First, for a fixed value of a high injection barrier (1 eV for instance), the charge transport can still be in the SCLC regime, which means linear I –V should be observed as if the barrier did not exist at all. Second, also for a fixed high barrier, there will be a transition from ILC to SCLC if I –V is measured as a function of temperature. Arkhipov et al. limited their calculation in the high electric field regime (105 to 108 V/m), based on a typical 100 nm device size. Therefore, experiments with lower electric field by using wide gap (150 mm) devices, should also be helpful in verifying this model. In this paper, we report I –V characteristics study of widegap (150 mm) P3HT thin films in the low and intermediate field regime (0–105 V/m) with high injection barriers (1.3 eV, from UPS results). We found that when the field is lower than several 103 V/m, I –V characteristics are linear and controlled by SCLC, despite the high barrier number. And as field goes up, there is a smooth transition from the linear SCLC regime to a different conduction regime, which could correspond to the ILC regime. Calculation based on the hopping injection model was performed and the results were compared with experiment.
3. Results and discussion Fig. 1 is the UPS spectra of P3HT thin film on gold substrate. The samples sent for UPS measurements are from the same batch of I –V experiments. Though some reports exist of ohmic contact between P3HT and Au [12], the UPS results clearly show a large injection barrier (about 1.3 eV) in our samples, which may be caused by surface dipole [13]. And the removal of oxygen (taken up by exposure to air) in P3HT thin films is a very slow process. After the sample has been left in vacuum for about 100 h, the I–V results showed no noticeable difference. Once the samples were stable, nitrogen gas was put into the chamber, and I –V was measured as a function of pressure as well as time. No change was observed in the measured I –V curves. These results are consistent with the literature and confirm the interaction between oxygen and P3HT. Fig. 2 is the I –V results of a P3HT thin film sample, measured at different temperatures. The main features are a linear part [14] at low electric field (0 to several 103 V/m), followed by a smooth transition into a different conduction regime at higher fields. The I –V’s in this high field regime are
2. Experimental
Fig. 1. UPS spectra of P3HT on Au, showing a 1.3 eV injection barrier at interface.
P3HT powder (98.5% regioregular) was purchased from Rieke Metals Inc., without further purification. P3HT solution was prepared in a nitrogen box with a concentration of 10 mg/ml using chloroform. The solution was then drop-cast onto the Au/SiO2/Si substrate with two predeposited Au electrodes in the nitrogen box. Finished samples were left in the box for half an hour before they were transferred to the cryostat for transport measurements. Au electrodes were deposited by thermal evaporation at a rate of 0.1 nm/s in 1 105 Pa vacuum. The electrodes were 50 nm thick, 3 mm wide and were separated by a 150 mm gap. Before measuring I –V characteristics, fresh samples were left in the 1 106 mbar vacuum for about 100 h to remove the oxygen doping [11]. After this process, I–V measurements were done using a Keithley 6430 source-measure unit as a function of temperature, which was controlled by a Lakeshore 340 temperature controller.
Fig. 2. The I–V characteristics of P3HT thin film at different temperatures. The maximum voltage is 5 V.
Y. Zheng et al. / Applied Surface Science 252 (2006) 4023–4025
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To understand the second part of the I –V results, we calculated the I –V characteristics at intermediate electric field using the hopping injection model [1–3]. The results of this calculation are shown in Fig. 4. In agreement with the experimental observations, the hopping injection model also yields a strong temperature-dependent I–V. The calculation also shows linear I –V’s at all temperatures, while there is a linear to non-linear transition for experimental data. Thus, the hopping injection model describes the high field data quite well. 4. Conclusion
Fig. 3. Resistivity at low electric field as a function of temperature reverse. Resistivity is obtained by linear fitting of the low-field I –V results.
In a conclusion, P3HT thin films with high injection barriers (1.3 eV) and wide electrode separation (150 mm) were studied using I –V technique in the low and intermediate field regime (0–105 V/m). Space-charge-limited conduction has been observed in the low electric field regime (0–103 V/m). A possible transition from the SCLC regime to the ILC regime has also been reported and compared with the theoretical model. Acknowledgements The authors are grateful to Dr. Gao Xinyu and Mr. Qi Dongchen for UPS experiments. And we thank Dr. A. Sellinger and Dr. S. Sudhakar for providing P3HT. References
Fig. 4. Calculation results using the hopping injection model. The red line is the experimental result at 300 K.
quite linear above 200 K but nonlinear at lower temperatures. The linear part of the I –V’s in the low field regime can be used to extract a resistance. And this extracted resistance has been plotted as a function of inverse temperature, as shown in Fig. 3. We can see that the dependence of resistivity on temperature can be described by a thermally activated process with a characteristic energy gap, R ¼ Aexp ðEg =2KTÞ. This characteristic semiconducting behaviour indicates that at low electric field, the conduction is controlled by the bulk, i.e. it is in the SCLC regime in spite of the existence of a high injection barrier.
[1] V.I. Arkhipov, H. von Seggern, E.V. Emilianova, Appl. Phys. Lett. 83 (2003) 5074. [2] V.I. Arkhipov, E.V. Emilianova, Y.-H. Tak, H. Ba¨ssler, J. Appl. Phys. 84 (1998) 848. [3] V.I. Arkhipov, U. Wolf, H. Ba¨ssler, Phys. Rev. B 59 (1999) 7514. [4] C.D. Dimitrakopoulos, P.R.L. Malenfant, Adv. Mater. (2002) 99. [5] H. Sirringhaus, Nature 401 (1999) 685. [6] S. Hugger, R. Thomann, T. Heinzel, T. Thurn-Albrecht, Colloid Polym. Sci. 282 (2004) 932. [7] B. Gre´vin, J. Chem. Phys. 118 (2003) 7097. [8] H. Ba¨ssler, Phys. Stat. Sol. B (1993) 15. [9] S.V. Novikov, Phys. Rev. Lett. 81 (1998) 4472. [10] M. Pope, C.E. Swenberg, Electronic Processes in Organic Crystals and Polymers, 2nd ed., Oxford University Press, Oxford, 1999. [11] M.S.A. Abdou, J. Am. Chem. Soc. 119 (1997) 4518. [12] C.H. Lei, Synth. Met. 145 (2004) 217. [13] A. Kahn, N. Koch, W.Y. Gao, J. Polym. Sci., Part B: Polym. Phys. 41 (2003) 2529. [14] Normally SCLC regime is associated with a power-law dependence of j / V m in a sandwich-structure device. This dependence could be different in devices with gap-cell setup, as Lei et al. [12] observed linear instead of parabolic I –V behaviour in P3HT thin films.
Possible transition from space-charge-limited to injection-limited conduction in poly(3-hexylthiophene) thin films Yi Zheng a, Linda Kunardi b, Cedric Troadec b, Andrew T.S. Wee a, N. Chandrasekhar a,b,* a
Department of Physics, National University of Singapore (NUS), 10 Kent Ridge Crescent, Singapore 119260, Singapore b Institute of Material Research and Engineering (IMRE), 3 Research Link, Singapore 117602, Singapore Available online 21 October 2005
Abstract Two-terminal thin films of poly(3-hexylthiophene) (P3HT) with a wide electrode separation (150 mm) has been studied using current–voltage characteristics at different temperatures. Space-charge-limited conduction (SCLC) with high injection barriers (1.3 eV) has been observed at all temperatures in the low electric field regime. A possible transition from SCLC to injection-limited conduction (ILC) is reported. The experimental results have been compared with the disorder-controlled injection model proposed by Arkhipov et al. [V.I. Arkhipov, H. von Seggern, E.V. Emilianova, Appl. Phys. Lett. 83 (2003) 5074; V.I. Arkhipov, E.V. Emilianova, Y.-H. Tak, H. Ba¨ssler, J. Appl. Phys. 84 (1998) 848; V.I. Arkhipov, U. Wolf, H. Ba¨ssler, Phys. Rev. B 59 (1999) 7514]. # 2005 Elsevier B.V. All rights reserved. Keywords: P3HT thin films; Metal–organic interface; Charge injection; SCLC to ILC transition
1. Introduction Poly(3-hexylthiophene), P3HT, is one of the promising materials for organic thin-film transistors (OTFTs), with the advantages of solution processability and relatively high mobility (0.1 cm2 V1 s1) [4,5]. Compared to other vacuum-deposited organic thin films, solution-processed P3HT thin films show more structural complexity due to interchain stacking and chain-substrate interaction, which is hard to control and can determine the device performance [5,6]. In general, the microstructure of P3HT thin films can be described as ordered microcrystalline domains embedded in an amorphous matrix, which has been confirmed by multi-scale STM results [7]. Two models have been traditionally used to describe charge transport in a disordered organic material. The Gaussian disorder model of Ba¨ssler [8] ignores correlations between energies and positions of hopping sites, whereas the correlated hopping model of Dunlap and Novikov et al. [9] accounts for this interaction. Interactions between carriers can modify the nature of transport. There is usually an interplay between
* Corresponding author. Tel.: +65 6874 8586; fax: +65 6774 4657. E-mail address: [email protected] (N. Chandrasekhar). 0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.09.036
disorder and interaction in all electronic systems, i.e. in some cases the effects of disorder dominate, and in other cases the effect of interaction dominates the transport. Since the nature and extent of disorder is determined both by the organic material as well as its processing, it is certainly conceivable that the Gaussian disorder model applies to some organic systems, and the correlated hopping model applies to other systems. In this work, we study transport across an Au–P3HT interface, and we find that the properties can be well described by the Gaussian disorder model. For a disordered system such as Au–P3HT, charge transport can be controlled by two separate processes: charge injection at the metal–organic interface and transport in the bulk. For the latter, variable range hopping [8] has been widely adopted as the main mechanism. But it is still controversial in understanding interface charge injection. And if the interplay between interface injection and bulk conduction is considered, two different conduction regimes will be introduced: injectionlimited conduction (ILC) when injection is weaker than bulk transport, and space-charge-limited conduction (SCLC), corresponding to weaker bulk transport. I –V characteristics will therefore probe different regimes of transport across the metal–organic interfaces: in the SCLC regime, the measurement will probe the bulk properties; while in the ILC regime, only interface properties can be deduced from the same
4024
Y. Zheng et al. / Applied Surface Science 252 (2006) 4023–4025
experiments. Traditionally, such an interplay between the SCLC and ILC regimes is considered to be controlled only by one parameter, the contact barrier. It is believed that when the barrier height is lower than 0.25–0.30 eV, the observed I –V characteristics should be controlled by SCLC [10]. Recently, Arkhipov et al. proposed a two-step hopping model [1–3] to describe charge injection at metal–organic interfaces. In this model, carriers first jump from the Fermi level of a metal electrode into localized states in the organic, and diffuse under the influence of the applied electric field into the bulk. The latter step is well described by the onedimensional Onsager theory. Since this model predicts a both field and temperature dependent hopping injection current, the competition between ILC and SCLC becomes more complicated than the situation of barrier-height dominated transport due to the following reasons. First, for a fixed value of a high injection barrier (1 eV for instance), the charge transport can still be in the SCLC regime, which means linear I –V should be observed as if the barrier did not exist at all. Second, also for a fixed high barrier, there will be a transition from ILC to SCLC if I –V is measured as a function of temperature. Arkhipov et al. limited their calculation in the high electric field regime (105 to 108 V/m), based on a typical 100 nm device size. Therefore, experiments with lower electric field by using wide gap (150 mm) devices, should also be helpful in verifying this model. In this paper, we report I –V characteristics study of widegap (150 mm) P3HT thin films in the low and intermediate field regime (0–105 V/m) with high injection barriers (1.3 eV, from UPS results). We found that when the field is lower than several 103 V/m, I –V characteristics are linear and controlled by SCLC, despite the high barrier number. And as field goes up, there is a smooth transition from the linear SCLC regime to a different conduction regime, which could correspond to the ILC regime. Calculation based on the hopping injection model was performed and the results were compared with experiment.
3. Results and discussion Fig. 1 is the UPS spectra of P3HT thin film on gold substrate. The samples sent for UPS measurements are from the same batch of I –V experiments. Though some reports exist of ohmic contact between P3HT and Au [12], the UPS results clearly show a large injection barrier (about 1.3 eV) in our samples, which may be caused by surface dipole [13]. And the removal of oxygen (taken up by exposure to air) in P3HT thin films is a very slow process. After the sample has been left in vacuum for about 100 h, the I–V results showed no noticeable difference. Once the samples were stable, nitrogen gas was put into the chamber, and I –V was measured as a function of pressure as well as time. No change was observed in the measured I –V curves. These results are consistent with the literature and confirm the interaction between oxygen and P3HT. Fig. 2 is the I –V results of a P3HT thin film sample, measured at different temperatures. The main features are a linear part [14] at low electric field (0 to several 103 V/m), followed by a smooth transition into a different conduction regime at higher fields. The I –V’s in this high field regime are
2. Experimental
Fig. 1. UPS spectra of P3HT on Au, showing a 1.3 eV injection barrier at interface.
P3HT powder (98.5% regioregular) was purchased from Rieke Metals Inc., without further purification. P3HT solution was prepared in a nitrogen box with a concentration of 10 mg/ml using chloroform. The solution was then drop-cast onto the Au/SiO2/Si substrate with two predeposited Au electrodes in the nitrogen box. Finished samples were left in the box for half an hour before they were transferred to the cryostat for transport measurements. Au electrodes were deposited by thermal evaporation at a rate of 0.1 nm/s in 1 105 Pa vacuum. The electrodes were 50 nm thick, 3 mm wide and were separated by a 150 mm gap. Before measuring I –V characteristics, fresh samples were left in the 1 106 mbar vacuum for about 100 h to remove the oxygen doping [11]. After this process, I–V measurements were done using a Keithley 6430 source-measure unit as a function of temperature, which was controlled by a Lakeshore 340 temperature controller.
Fig. 2. The I–V characteristics of P3HT thin film at different temperatures. The maximum voltage is 5 V.
Y. Zheng et al. / Applied Surface Science 252 (2006) 4023–4025
4025
To understand the second part of the I –V results, we calculated the I –V characteristics at intermediate electric field using the hopping injection model [1–3]. The results of this calculation are shown in Fig. 4. In agreement with the experimental observations, the hopping injection model also yields a strong temperature-dependent I–V. The calculation also shows linear I –V’s at all temperatures, while there is a linear to non-linear transition for experimental data. Thus, the hopping injection model describes the high field data quite well. 4. Conclusion
Fig. 3. Resistivity at low electric field as a function of temperature reverse. Resistivity is obtained by linear fitting of the low-field I –V results.
In a conclusion, P3HT thin films with high injection barriers (1.3 eV) and wide electrode separation (150 mm) were studied using I –V technique in the low and intermediate field regime (0–105 V/m). Space-charge-limited conduction has been observed in the low electric field regime (0–103 V/m). A possible transition from the SCLC regime to the ILC regime has also been reported and compared with the theoretical model. Acknowledgements The authors are grateful to Dr. Gao Xinyu and Mr. Qi Dongchen for UPS experiments. And we thank Dr. A. Sellinger and Dr. S. Sudhakar for providing P3HT. References
Fig. 4. Calculation results using the hopping injection model. The red line is the experimental result at 300 K.
quite linear above 200 K but nonlinear at lower temperatures. The linear part of the I –V’s in the low field regime can be used to extract a resistance. And this extracted resistance has been plotted as a function of inverse temperature, as shown in Fig. 3. We can see that the dependence of resistivity on temperature can be described by a thermally activated process with a characteristic energy gap, R ¼ Aexp ðEg =2KTÞ. This characteristic semiconducting behaviour indicates that at low electric field, the conduction is controlled by the bulk, i.e. it is in the SCLC regime in spite of the existence of a high injection barrier.
[1] V.I. Arkhipov, H. von Seggern, E.V. Emilianova, Appl. Phys. Lett. 83 (2003) 5074. [2] V.I. Arkhipov, E.V. Emilianova, Y.-H. Tak, H. Ba¨ssler, J. Appl. Phys. 84 (1998) 848. [3] V.I. Arkhipov, U. Wolf, H. Ba¨ssler, Phys. Rev. B 59 (1999) 7514. [4] C.D. Dimitrakopoulos, P.R.L. Malenfant, Adv. Mater. (2002) 99. [5] H. Sirringhaus, Nature 401 (1999) 685. [6] S. Hugger, R. Thomann, T. Heinzel, T. Thurn-Albrecht, Colloid Polym. Sci. 282 (2004) 932. [7] B. Gre´vin, J. Chem. Phys. 118 (2003) 7097. [8] H. Ba¨ssler, Phys. Stat. Sol. B (1993) 15. [9] S.V. Novikov, Phys. Rev. Lett. 81 (1998) 4472. [10] M. Pope, C.E. Swenberg, Electronic Processes in Organic Crystals and Polymers, 2nd ed., Oxford University Press, Oxford, 1999. [11] M.S.A. Abdou, J. Am. Chem. Soc. 119 (1997) 4518. [12] C.H. Lei, Synth. Met. 145 (2004) 217. [13] A. Kahn, N. Koch, W.Y. Gao, J. Polym. Sci., Part B: Polym. Phys. 41 (2003) 2529. [14] Normally SCLC regime is associated with a power-law dependence of j / V m in a sandwich-structure device. This dependence could be different in devices with gap-cell setup, as Lei et al. [12] observed linear instead of parabolic I –V behaviour in P3HT thin films.