- Email: [email protected]
Optical anisotropy of aligned pentacene molecules on a rubbed polymer corresponding to the electrical anisotropy
Current Applied Physics 10 (2010) 64–67
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Optical anisotropy of aligned pentacene molecules on a rubbed polymer corresponding to the electrical anisotropy Chang-Jae Yu a,*, Jin-Hyuk Bae b, Chang-Min Keum b, Sin-Doo Lee b a b
Department of Electronics and Computer Engineering, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, Republic of Korea School of Electrical Engineering #032, Seoul National University, Kwanak, P.O. Box 34, Seoul 151-600, Republic of Korea
a r t i c l e
i n f o
Article history: Received 21 October 2008 Received in revised form 7 April 2009 Accepted 22 April 2009 Available online 3 May 2009 PACS: 73.61.Ph 78.20.Fm 78.66.Qn
a b s t r a c t We report on the optical anisotropy of a pentacene film on a rubbed (poly)vinylalcohol (PVA) layer related to the electrical performances of the pentacene organic field effect transistors (OFETs) depending on the direction of a current flow. The optical anisotropies of the PVA films are negligible with respect to whether or not rubbing process. In the pentacene OFET on the rubbed PVA layer, however, the optical anisotropy is observed and the anisotropy of the electrical performances directly corresponds to the optical anisotropy of the pentacene thin-film on the rubbed PVA layer. Crown Copyright Ó 2009 Published by Elsevier B.V. All rights reserved.
Keywords: Organic field effect transistor Rubbing process Optical anisotropies Electrical performances
1. Introduction Organic field effect transistors (OFETs), performing large area coverage, low temperature processibility, and mechanical flexibility, have attracted much interest for electronic applications [1– 3]. To improve the electrical properties such as the field effect mobility in the OFETs, the interfacial interactions between an insulator and an organic semiconducting material have been extensively studied [4–8]. In previous works, the preferential alignment of the organic semiconducting molecules produced by an orientationally ordered insulator or an extra alignment layer on an insulator was found to enhance the mobility of the OFETs [9–13]. Especially, the pentacene molecule, which is one of the organic semiconductors with high performances, has a rod-shape like a liquid crystal (LC) molecules and thus it is expected that the structural ordering of the pentacene molecules can be induced on an alignment layer for the LC molecules through anisotropic surface anchoring [14]. In such reports, the anisotropy of the mobility, along two directions of the current flow parallel and perpendicular to the aligned direction of the surface-treated insulator by rubbing and/or photo-aligning process, was observed [11,12,15]. However, the optical anisotropy, indispensably pro* Corresponding author. E-mail address: [email protected] (C.-J. Yu).
duced in the ordered molecules with non-spherical shape, and the electrical performances of the ordered pentacene film have not been fully studied yet. In this paper, we report on the optical anisotropy of the aligned pentacene molecules on a rubbed (poly)vinylalcohol (PVA) layer corresponding to the anisotropy of the electrical performances in the pentacene-based OFETs. The optical anisotropies of the PVA layers and the pentacene films on them are measured with the photoelastic modulator (PEM) under crossed polarizers [16]. The optical anisotropy of the pentacene on the rubbed PVA layer is evidently observed while the pentacene on the untreated PVA layer is optically isotropic. It should be noted that the optical anisotropies of the PVA films are negligible with respect to whether or not rubbing process. In the pentacene-based OFETs on the rubbed PVA layer, the electrical performances parallel to the rubbing direction of the PVA layer are enhanced comparing to the performances perpendicular to the rubbing direction and thus the anisotropy of them is obviously measured even though the morphological effects such as the grain size and surface roughness are indistinguishable from the OFETs on the untreated PVA layer. 2. Experimental Fig. 1 shows a schematic diagram of the pentacene-based OFET in a top-contact and bottom gate structure. In such top-contact
1567-1739/$ - see front matter Crown Copyright Ó 2009 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2009.04.013
C.-J. Yu et al. / Current Applied Physics 10 (2010) 64–67
Fig. 1. The structure of our pentacene OFET with a PVA buffer layer in top-contact and bottom gate structure.
OFET configuration, source and drain electrodes are located on the top of the active layer below which a gate insulator is present on the gate electrode. The glass substrates coated with indiumtin-oxide (ITO) were patterned to form the gate electrode of 3 mm wide by photolithography. The patterned substrates were then cleaned sequentially with acetone, isopropyl alcohol, and methyl alcohol in an ultrasonicator for 10 min each. At each cleaning step, the substrates were rinsed with deionized water for 5 min in an ultrasonicator and purged with nitrogen gas. The cleaned substrates were dried at 90 °C in a vacuum oven for 10 min. Each patterned ITO substrate was coated with the polyvinylphenol (PVP) dissolved in cyclopentanone in 10 wt.% to prepare a gate insulator. The PVA, dissolved in DI water in 2 wt.%, was spin-coated on the top of the PVP layer as a commanding buffer layer. The rubbing process for PVA layer was conducted by a commercial rubbing machine (Namil Machinery Co., Korea) with a roller covered with a cotton velvet material under ambient environment. Here, rotation speed of the roller was 500 rpm. Note that the solvent of the PVP is chemically stable against the solution of the PVA. The thickness of the PVP layer and that of the PVA layer were measured as 380 and 20 nm, respectively. The total dielectric constant of the two layers was determined as about 4 at 1 kHz. The pentacene films and the gold (Au) electrodes for source and drain were deposited by thermal evaporation in a vacuum chamber at room temperature. The channel length and width of a shadow mask were 50 lm and 1 mm. Deposition of pentacene was carried out under a basal pressure of about 10 6 Torr at the deposition rate of 0.5 Å/s without further purification, and that of gold was under 10 5 Torr at 1.0 Å/s. The thickness of the pentacene layer and that of the Au layer were measured as 60 and 80 nm, respectively. The current–voltage characteristics were measured using a probe station (4155A, HP) under an ambient atmosphere at room temperature. An atomic force microscope (XE-150, PSIA) and a PEM (PEM90, Hinds) were used to determine the morphological properties and the optical anisotropy, respectively. 3. Results and discussion We first examine the surface morphologies of the PVA films and the pentacene films deposited on the PVA layers as shown in Fig. 2. Fig. 2a and b shows atomic force microscopic (AFM) images of the PVA film and the pentacene film on the non-rubbed PVA layer, and Fig. 2c and d shows the PVA film and the pentacene film on the rubbed PVA layer, respectively. In the surface morphologies of both the grain size and the roughness, no considerable differences between the both PVA layers were observed as shown in Fig. 2. Similarly, no remarkable differences between the pentacene layers on two different PVA layers were observed. To investigate more detail,
65
the Fourier transforms corresponding to the AFM images were carried out. In the pentacene film on the rubbed PVA layer, slight anisotropy was observed [12,17–19], but did not reflect the molecular orientation. Therefore, the alignment phenomena of the pentacene film on the rubbed PVA layer are indistinguishable from the pentacene on the untreated PVA layer through the AFM images. Next, the optical properties of the PVA layers treated by rubbing process and the resultant ordering of the pentacene molecules evaporated on them are measured. The directional alignment of the elongated molecules such as the pentacene and the LC is known to produce the optical anisotropy. The PEM method shown in Fig. 3a is commonly used to measure the small optical anisotropy of thin films such as an alignment layer [20–22]. Fig. 3b and c shows the phase retardations of the two different PVA layers, the rubbed and non-rubbed PVA layers, and the pentacene films on them as function of the rotational angle of the sample with respect to the x-axis. Here, the insets show the enlarged parts of the optical anisotropies in two different PVA layers (open circles) and in the pentacene film (open rectangles) on the untreated PVA layer. Both of the optical anisotropy of the non-rubbed PVA buffer layer and that of the pentacene film on it are negligible as shown in Fig. 3b. However, the optical anisotropy of the pentacene film on the rubbed PVA layer is obviously measured even though the rubbed PVA layer is optically isotropic as shown in Fig. 3c. As a result, it was found that the optical anisotropies of the PVA films are negligible with respect to whether or not rubbing process. On the other hand, the pentacene film deposited on the rubbed PVA layer produces the optical anisotropy, which is originated from the ordering of the pentacene film. It would be expected that the pentacene molecules might be substantially aligned on the rubbed PVA layer like the LC molecules on the alignment layer. We now measure the electrical performances of the pentacenebased OFETs depending on the rubbing direction of the PVA buffer layer. Fig. 4 shows the output characteristics and the transfer characteristics of the various OFET structures. In this work, the PVP without cross-linking agent was used, which strongly affected the gate leakage current and then on/off ratio was considerably reduced. Recently, in the PVP insulator mixed with cross-linking agent, it was reported that the leakage current of the OTFT was extremely reduced [23]. The cross-linking PVP layer as the insulator of the OTFT may provide the more stable device performances. However, this is not a main theme of this work. The drain current (IDS)–drain voltage (VDS) characteristics of the pentacene OFET with untreated PVA buffer layer are shown in Fig. 4a. Here, the IDS–VDS curves were obtained with varying the gate voltage (VGS) from 0 to 40 V in a step of 10 V. It should be noted that output characteristics of the pentacene OFET with untreated PVA buffer layer are equivalent irrespective of the direction of the current flow, that is, source-drain direction. In the pentacene OFETs with the rubbed PVA buffer layer, the output characteristics for two different directions of the current flow perpendicular to the rubbing direction and parallel to that of the PVA buffer layer were achieved as shown in Fig. 4b and c, respectively. The IDS–VDS curves of the OFET with the perpendicular-rubbed PVA layer are similar to those of the OFET with non-rubbed PVA layer. However, The IDS–VDS curves of the OFET with the parallel-rubbed PVA layer are enhanced compared to the output curves of the others. The transfer characteristics of the above three OFETs at a fixed VDS of 40 V are shown in Fig. 4d. In the saturation regime [24], the mobilities of the OFET with the non-rubbed PVA layer (open circles), the OFET with the perpendicular-rubbed one (open rectangles), and the OFET with the parallel-rubbed one (open reverse triangles) were determined as 0.13, 0.12, and 0.31 cm2/Vs, respectively. In our results, the anisotropic ratio of the mobility parallel to a rubbing direction to the mobility perpendicular to a
66
C.-J. Yu et al. / Current Applied Physics 10 (2010) 64–67
Fig. 2. The AFM images of (a) the non-rubbed PVA film and (b) the pentacene film deposited on the non-rubbed PVA layer and (c) the rubbed PVA film and (d) the pentacene film deposited on the non-rubbed PVA layer. The inset figures show the Fourier transforms of the corresponding AFM images of the pentacene films. The AFM images are scanned in the area of 5 lm 5 lm.
Fig. 4. The output characteristics of the pentacene OFET with (a) non-rubbed PVA layer, (b) perpendicular-rubbed PVA layer to the current flow, and (c) parallelrubbed PVA layer to the current flow with varying VGS from 0 to 40 V in a step of 10 V. The corresponding transfer curves at a fixed VDS of 40 V are shown in (d). Here open circles, rectangles, and reverse triangles represent the non-rubbed PVA layer, perpendicular-rubbed PVA layer, and parallel-rubbed PVA layer, respectively. Fig. 3. (a) The PEM experimental setup to measure the optical anisotropy of the PVA layers and pentacene films deposited on the PVA layers. (b) The optical anisotropy of the non-rubbed PVA layer and pentacene film deposited on it. (c) The optical anisotropy of the rubbed PVA layer and pentacene film deposited on it. The inset shows an enlarged part of the optical anisotropy in the PVA layer itself, in both cases.
rubbing direction was to be about a factor of 3. The similar factor was already reported for the pentacene OFETs with another rubbed polyimide [10]. As a result, the alignment of the pentacene molecules confirmed by the optical anisotropy enhances the electrical
C.-J. Yu et al. / Current Applied Physics 10 (2010) 64–67
performances of the pentacene OFETs with even similar morphologies observed from the AFM images. It would be expected that the enhancement of the electrical performances is originated from reducing a hopping barrier at grain boundaries because the macroscopically ordered state, observed through the optical anisotropy, decreases a difference of the crystalline directions between adjacent grains. 4. Conclusions We reported on the optical anisotropy of the pentacene molecules with the rubbed PVA buffer layer corresponding to the anisotropy of the electrical performances of the OFETs. The optical anisotropies of the PVA layers and the pentacene films on them were obtained using the PEM method. Both of the optical anisotropy of the non-rubbed PVA buffer layer and that of the pentacene film on it were negligible while the optical anisotropy of the pentacene film on the rubbed PVA layer was evidently observed even though the rubbed PVA layer is optically isotropic. In addition, the electrical performances of the OFET with the parallel-rubbed PVA layer were enhanced compared to the performances of the OFETs with the perpendicular-rubbed PVA layer or the non-rubbed PVA layer although the morphological effects such as the grain size and surface roughness are indistinguishable from the OFETs on the untreated PVA layer. In conclusion, the alignment of the pentacene molecules, confirmed by the optical anisotropy, enhanced the electrical performances of the pentacene OFETs. The anisotropy of the electrical performances of the aligned pentacene OFETs directly corresponded to the optical anisotropy of the pentacene film on the rubbed PVA layer. Acknowledgements This work was supported in part by the Ministry of Knowledge Economy of Korea through the 21st Century Frontier Research and
67
Development Program at the Information Display Center. C.-J. Yu would like to acknowledge financial support from the Korea Research Foundation Grant funded by the Korean Government (MOEHRD, Basic Research Promotion Fund) (KRF-2008-331D00385).
References [1] B. Crone, A. Dodabalapur, Y.Y. Lin, R.W. Filas, Z. Bao, A. Laduca, R. Sarpeshkar, H.E. Katz, W. Li, Nature 403 (2000) 521. [2] C.D. Dimitrakopoulos, P.R.L. Malenfant, Adv. Mater. 14 (2002) 99. [3] L. Zhou, A. Wanga, S.-C. Wu, J. Sun, S. Park, T.N. Jackson, Appl. Phys. Lett. 88 (2006) 083502. [4] X.T. Li, D.H. Pei, S. Kobayashi, Y. Imura, Jpn. J. Appl. Phys. 36 (1997) L432. [5] I. Yagi, K. Tsukagoshi, Y. Aoyagi, Appl. Phys. Lett. 86 (2005) 103502. [6] F. De Angelis, S. Cipolloni, L. Mariucci, G. Fortunato, Appl. Phys. Lett. 86 (2005) 203505. [7] J.-H. Bae, W.-H. Kim, H. Kim, C. Lee, S.-D. Lee, J. Appl. Phys. 102 (2007) 063508. [8] J.-H. Bae, J. Kim, W.-H. Kim, S.-D. Lee, Jpn. J. Appl. Phys. 46 (2007) 385. [9] P. Dyreklev, G. Gustafsson, O. Inganäs, H. Stubb, Synth. Met. 57 (1993) 4093. [10] X.L. Chen, A.J. Lovinger, Z. Bao, J. Sapjeta, Chem. Mater. 13 (2001) 1341. [11] H. Sirringhaus, R.J. Wilson, R.H. Friend, M. Inbasekara, W. Wu, E.P. Woo, M. Grell, D.D.C. Bradley, Appl. Phys. Lett. 77 (2000) 406. [12] W.-Y. Chou, H.-L. Cheng, Adv. Funct. Mater. 14 (2004) 811. [13] S.-Z. Weng, W.-S. Hu, C.-H. Cuo, Y.-T. Tao, L.-J. Fan, Y.-W. Yang, Appl. Phys. Lett. 89 (2006) 172103. [14] H. Fujikake, T. Suzuki, T. Murashige, F. Sato, Liq. Cryst. 33 (2006) 1051. [15] S.-J. Kang, Y.-Y. Noh, K.-J. Baeg, J. Ghim, J.-H. Park, D.-Y. Kim, J.S. Kim, J.H. Park, K. Cho, Appl. Phys. Lett. 92 (2008) 052107. [16] PEM-90 Application Note, Hinds Instruments. [17] D. Guo, K. Sakamoto, K. Miki, S. Ikeda, K. Saiki, Appl. Phys. Lett. 90 (2007) 102117. [18] W.-S. Hu, S.-Z. Weng, Y.-T. Tao, H.-J. Liu, H.-Y. Lee, L.-J. Fan, Y.-W. Yang, Langmuir 23 (2007) 12901. [19] W.-S. Hu, S.-Z. Weng, Y.-T. Tao, H.-J. Lie, H.-Y. Lee, Org. Electron. 9 (2008) 385. [20] A.J. Barlow, J. Lightwave Technol. LT-3 (1985) 135. [21] J.-H. Lee, C.-J. Yu, S.-D. Lee, Mol. Cryst. Liq. Cryst. 321 (1998) 317. [22] J.-H. Kim, S. Kumar, S.-D. Lee, Phys. Rev. E 57 (1998) 5644. [23] S.C. Lim, S.H. Kim, J.B. Koo, J.H. Lee, C.H. Ku, Y.S. Yang, T. Zyung, Appl. Phys. Lett. 90 (2007) 173512. [24] C.D. Dimitrakopoulos, D.J. Mascaro, IBM J. Res. Dev. 45 (2001) 11.
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Optical anisotropy of aligned pentacene molecules on a rubbed polymer corresponding to the electrical anisotropy Chang-Jae Yu a,*, Jin-Hyuk Bae b, Chang-Min Keum b, Sin-Doo Lee b a b
Department of Electronics and Computer Engineering, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, Republic of Korea School of Electrical Engineering #032, Seoul National University, Kwanak, P.O. Box 34, Seoul 151-600, Republic of Korea
a r t i c l e
i n f o
Article history: Received 21 October 2008 Received in revised form 7 April 2009 Accepted 22 April 2009 Available online 3 May 2009 PACS: 73.61.Ph 78.20.Fm 78.66.Qn
a b s t r a c t We report on the optical anisotropy of a pentacene film on a rubbed (poly)vinylalcohol (PVA) layer related to the electrical performances of the pentacene organic field effect transistors (OFETs) depending on the direction of a current flow. The optical anisotropies of the PVA films are negligible with respect to whether or not rubbing process. In the pentacene OFET on the rubbed PVA layer, however, the optical anisotropy is observed and the anisotropy of the electrical performances directly corresponds to the optical anisotropy of the pentacene thin-film on the rubbed PVA layer. Crown Copyright Ó 2009 Published by Elsevier B.V. All rights reserved.
Keywords: Organic field effect transistor Rubbing process Optical anisotropies Electrical performances
1. Introduction Organic field effect transistors (OFETs), performing large area coverage, low temperature processibility, and mechanical flexibility, have attracted much interest for electronic applications [1– 3]. To improve the electrical properties such as the field effect mobility in the OFETs, the interfacial interactions between an insulator and an organic semiconducting material have been extensively studied [4–8]. In previous works, the preferential alignment of the organic semiconducting molecules produced by an orientationally ordered insulator or an extra alignment layer on an insulator was found to enhance the mobility of the OFETs [9–13]. Especially, the pentacene molecule, which is one of the organic semiconductors with high performances, has a rod-shape like a liquid crystal (LC) molecules and thus it is expected that the structural ordering of the pentacene molecules can be induced on an alignment layer for the LC molecules through anisotropic surface anchoring [14]. In such reports, the anisotropy of the mobility, along two directions of the current flow parallel and perpendicular to the aligned direction of the surface-treated insulator by rubbing and/or photo-aligning process, was observed [11,12,15]. However, the optical anisotropy, indispensably pro* Corresponding author. E-mail address: [email protected] (C.-J. Yu).
duced in the ordered molecules with non-spherical shape, and the electrical performances of the ordered pentacene film have not been fully studied yet. In this paper, we report on the optical anisotropy of the aligned pentacene molecules on a rubbed (poly)vinylalcohol (PVA) layer corresponding to the anisotropy of the electrical performances in the pentacene-based OFETs. The optical anisotropies of the PVA layers and the pentacene films on them are measured with the photoelastic modulator (PEM) under crossed polarizers [16]. The optical anisotropy of the pentacene on the rubbed PVA layer is evidently observed while the pentacene on the untreated PVA layer is optically isotropic. It should be noted that the optical anisotropies of the PVA films are negligible with respect to whether or not rubbing process. In the pentacene-based OFETs on the rubbed PVA layer, the electrical performances parallel to the rubbing direction of the PVA layer are enhanced comparing to the performances perpendicular to the rubbing direction and thus the anisotropy of them is obviously measured even though the morphological effects such as the grain size and surface roughness are indistinguishable from the OFETs on the untreated PVA layer. 2. Experimental Fig. 1 shows a schematic diagram of the pentacene-based OFET in a top-contact and bottom gate structure. In such top-contact
1567-1739/$ - see front matter Crown Copyright Ó 2009 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2009.04.013
C.-J. Yu et al. / Current Applied Physics 10 (2010) 64–67
Fig. 1. The structure of our pentacene OFET with a PVA buffer layer in top-contact and bottom gate structure.
OFET configuration, source and drain electrodes are located on the top of the active layer below which a gate insulator is present on the gate electrode. The glass substrates coated with indiumtin-oxide (ITO) were patterned to form the gate electrode of 3 mm wide by photolithography. The patterned substrates were then cleaned sequentially with acetone, isopropyl alcohol, and methyl alcohol in an ultrasonicator for 10 min each. At each cleaning step, the substrates were rinsed with deionized water for 5 min in an ultrasonicator and purged with nitrogen gas. The cleaned substrates were dried at 90 °C in a vacuum oven for 10 min. Each patterned ITO substrate was coated with the polyvinylphenol (PVP) dissolved in cyclopentanone in 10 wt.% to prepare a gate insulator. The PVA, dissolved in DI water in 2 wt.%, was spin-coated on the top of the PVP layer as a commanding buffer layer. The rubbing process for PVA layer was conducted by a commercial rubbing machine (Namil Machinery Co., Korea) with a roller covered with a cotton velvet material under ambient environment. Here, rotation speed of the roller was 500 rpm. Note that the solvent of the PVP is chemically stable against the solution of the PVA. The thickness of the PVP layer and that of the PVA layer were measured as 380 and 20 nm, respectively. The total dielectric constant of the two layers was determined as about 4 at 1 kHz. The pentacene films and the gold (Au) electrodes for source and drain were deposited by thermal evaporation in a vacuum chamber at room temperature. The channel length and width of a shadow mask were 50 lm and 1 mm. Deposition of pentacene was carried out under a basal pressure of about 10 6 Torr at the deposition rate of 0.5 Å/s without further purification, and that of gold was under 10 5 Torr at 1.0 Å/s. The thickness of the pentacene layer and that of the Au layer were measured as 60 and 80 nm, respectively. The current–voltage characteristics were measured using a probe station (4155A, HP) under an ambient atmosphere at room temperature. An atomic force microscope (XE-150, PSIA) and a PEM (PEM90, Hinds) were used to determine the morphological properties and the optical anisotropy, respectively. 3. Results and discussion We first examine the surface morphologies of the PVA films and the pentacene films deposited on the PVA layers as shown in Fig. 2. Fig. 2a and b shows atomic force microscopic (AFM) images of the PVA film and the pentacene film on the non-rubbed PVA layer, and Fig. 2c and d shows the PVA film and the pentacene film on the rubbed PVA layer, respectively. In the surface morphologies of both the grain size and the roughness, no considerable differences between the both PVA layers were observed as shown in Fig. 2. Similarly, no remarkable differences between the pentacene layers on two different PVA layers were observed. To investigate more detail,
65
the Fourier transforms corresponding to the AFM images were carried out. In the pentacene film on the rubbed PVA layer, slight anisotropy was observed [12,17–19], but did not reflect the molecular orientation. Therefore, the alignment phenomena of the pentacene film on the rubbed PVA layer are indistinguishable from the pentacene on the untreated PVA layer through the AFM images. Next, the optical properties of the PVA layers treated by rubbing process and the resultant ordering of the pentacene molecules evaporated on them are measured. The directional alignment of the elongated molecules such as the pentacene and the LC is known to produce the optical anisotropy. The PEM method shown in Fig. 3a is commonly used to measure the small optical anisotropy of thin films such as an alignment layer [20–22]. Fig. 3b and c shows the phase retardations of the two different PVA layers, the rubbed and non-rubbed PVA layers, and the pentacene films on them as function of the rotational angle of the sample with respect to the x-axis. Here, the insets show the enlarged parts of the optical anisotropies in two different PVA layers (open circles) and in the pentacene film (open rectangles) on the untreated PVA layer. Both of the optical anisotropy of the non-rubbed PVA buffer layer and that of the pentacene film on it are negligible as shown in Fig. 3b. However, the optical anisotropy of the pentacene film on the rubbed PVA layer is obviously measured even though the rubbed PVA layer is optically isotropic as shown in Fig. 3c. As a result, it was found that the optical anisotropies of the PVA films are negligible with respect to whether or not rubbing process. On the other hand, the pentacene film deposited on the rubbed PVA layer produces the optical anisotropy, which is originated from the ordering of the pentacene film. It would be expected that the pentacene molecules might be substantially aligned on the rubbed PVA layer like the LC molecules on the alignment layer. We now measure the electrical performances of the pentacenebased OFETs depending on the rubbing direction of the PVA buffer layer. Fig. 4 shows the output characteristics and the transfer characteristics of the various OFET structures. In this work, the PVP without cross-linking agent was used, which strongly affected the gate leakage current and then on/off ratio was considerably reduced. Recently, in the PVP insulator mixed with cross-linking agent, it was reported that the leakage current of the OTFT was extremely reduced [23]. The cross-linking PVP layer as the insulator of the OTFT may provide the more stable device performances. However, this is not a main theme of this work. The drain current (IDS)–drain voltage (VDS) characteristics of the pentacene OFET with untreated PVA buffer layer are shown in Fig. 4a. Here, the IDS–VDS curves were obtained with varying the gate voltage (VGS) from 0 to 40 V in a step of 10 V. It should be noted that output characteristics of the pentacene OFET with untreated PVA buffer layer are equivalent irrespective of the direction of the current flow, that is, source-drain direction. In the pentacene OFETs with the rubbed PVA buffer layer, the output characteristics for two different directions of the current flow perpendicular to the rubbing direction and parallel to that of the PVA buffer layer were achieved as shown in Fig. 4b and c, respectively. The IDS–VDS curves of the OFET with the perpendicular-rubbed PVA layer are similar to those of the OFET with non-rubbed PVA layer. However, The IDS–VDS curves of the OFET with the parallel-rubbed PVA layer are enhanced compared to the output curves of the others. The transfer characteristics of the above three OFETs at a fixed VDS of 40 V are shown in Fig. 4d. In the saturation regime [24], the mobilities of the OFET with the non-rubbed PVA layer (open circles), the OFET with the perpendicular-rubbed one (open rectangles), and the OFET with the parallel-rubbed one (open reverse triangles) were determined as 0.13, 0.12, and 0.31 cm2/Vs, respectively. In our results, the anisotropic ratio of the mobility parallel to a rubbing direction to the mobility perpendicular to a
66
C.-J. Yu et al. / Current Applied Physics 10 (2010) 64–67
Fig. 2. The AFM images of (a) the non-rubbed PVA film and (b) the pentacene film deposited on the non-rubbed PVA layer and (c) the rubbed PVA film and (d) the pentacene film deposited on the non-rubbed PVA layer. The inset figures show the Fourier transforms of the corresponding AFM images of the pentacene films. The AFM images are scanned in the area of 5 lm 5 lm.
Fig. 4. The output characteristics of the pentacene OFET with (a) non-rubbed PVA layer, (b) perpendicular-rubbed PVA layer to the current flow, and (c) parallelrubbed PVA layer to the current flow with varying VGS from 0 to 40 V in a step of 10 V. The corresponding transfer curves at a fixed VDS of 40 V are shown in (d). Here open circles, rectangles, and reverse triangles represent the non-rubbed PVA layer, perpendicular-rubbed PVA layer, and parallel-rubbed PVA layer, respectively. Fig. 3. (a) The PEM experimental setup to measure the optical anisotropy of the PVA layers and pentacene films deposited on the PVA layers. (b) The optical anisotropy of the non-rubbed PVA layer and pentacene film deposited on it. (c) The optical anisotropy of the rubbed PVA layer and pentacene film deposited on it. The inset shows an enlarged part of the optical anisotropy in the PVA layer itself, in both cases.
rubbing direction was to be about a factor of 3. The similar factor was already reported for the pentacene OFETs with another rubbed polyimide [10]. As a result, the alignment of the pentacene molecules confirmed by the optical anisotropy enhances the electrical
C.-J. Yu et al. / Current Applied Physics 10 (2010) 64–67
performances of the pentacene OFETs with even similar morphologies observed from the AFM images. It would be expected that the enhancement of the electrical performances is originated from reducing a hopping barrier at grain boundaries because the macroscopically ordered state, observed through the optical anisotropy, decreases a difference of the crystalline directions between adjacent grains. 4. Conclusions We reported on the optical anisotropy of the pentacene molecules with the rubbed PVA buffer layer corresponding to the anisotropy of the electrical performances of the OFETs. The optical anisotropies of the PVA layers and the pentacene films on them were obtained using the PEM method. Both of the optical anisotropy of the non-rubbed PVA buffer layer and that of the pentacene film on it were negligible while the optical anisotropy of the pentacene film on the rubbed PVA layer was evidently observed even though the rubbed PVA layer is optically isotropic. In addition, the electrical performances of the OFET with the parallel-rubbed PVA layer were enhanced compared to the performances of the OFETs with the perpendicular-rubbed PVA layer or the non-rubbed PVA layer although the morphological effects such as the grain size and surface roughness are indistinguishable from the OFETs on the untreated PVA layer. In conclusion, the alignment of the pentacene molecules, confirmed by the optical anisotropy, enhanced the electrical performances of the pentacene OFETs. The anisotropy of the electrical performances of the aligned pentacene OFETs directly corresponded to the optical anisotropy of the pentacene film on the rubbed PVA layer. Acknowledgements This work was supported in part by the Ministry of Knowledge Economy of Korea through the 21st Century Frontier Research and
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Development Program at the Information Display Center. C.-J. Yu would like to acknowledge financial support from the Korea Research Foundation Grant funded by the Korean Government (MOEHRD, Basic Research Promotion Fund) (KRF-2008-331D00385).
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