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Photovoltaic properties of BiFeO3 thin film capacitors by using Al-doped zinc oxide as top electrode
Materials Letters 91 (2013) 359–361
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Photovoltaic properties of BiFeO3 thin film capacitors by using Al-doped zinc oxide as top electrode Wen Dong, Yiping Guo n, Bing Guo, Hongyan Liu, Hua Li, Hezhou Liu n State Key Laboratory of MMCs, School of Materials Science and Engineering, Shanghai Jiaotong University, Shanghai 200240, China
a r t i c l e i n f o
abstract
Article history: Received 28 September 2012 Accepted 10 October 2012 Available online 22 October 2012
We demonstrate using Al-doped zinc oxide films as electrode materials to form ferroelectric thin film capacitors. The photocurrent density and open circuit voltage for the capacitor with an Al-doped zinc oxide/ BiFeO3/Fluorine doped tin oxide structure are measured to be 0.13 mA/cm2 and 0.63 V, respectively, much higher than the reported values for traditional BFO based capacitors. To clarify the contribution of Al-doped zinc oxide to the photovoltaic effect, Au top electrode was used as a comparison group. The Al-doped zinc oxide based capacitors show a more than 3 times larger photovoltaic output than that of the Au counterpart, which indicates that the Al-doped zinc oxide makes a major contribution to the high photovoltaic effect. The comparison shows that the larger photovoltaic effect may be attributed to the larger build-in-electric field, depolarization electric field and higher transparency of top electrode. Our results suggest that the transparent Al-doped zinc oxide film can be a promising electrode material in ferroelectric film capacitors and could be a potential replacement of indium tin oxide materials. & 2012 Elsevier B.V. All rights reserved.
Keywords: Ferroelectric thin films BiFeO3 Oxide electrode Photovoltaic effect
1. Introduction Transparent conducting oxide (TCO) semiconductors that are used as transparent electrodes play an important role in most optoelectronic devices. Though indium tin oxide (ITO) thin films have been in practical use for transparent electrode applications, it may be faced with high cost and scarcity of indium. It is crucial to find out appropriate TCO materials to replace ITO. Some other TCO materials have been reported [1,2]. Doped zinc oxide thin films have been widely studied for their use as alternative to ITO [3–6]. Impurity-doped zinc oxide (ZnO) was proposed to be a potential alternative to ITO [3,4]. Among the doped zinc oxide films, aluminum-doped zinc oxide (AZO) films with a band gap of about 3.3 eV show the lowest electrical resistivity 2–4 10 4 O cm [5,6], which is similar to that of ITO films [7–9]. Unlike the more commonly used indium tin oxide (ITO), zinc oxide (ZnO) is a nontoxic and inexpensive material. It is chemically and thermally stable under hydrogen plasma processes commonly used for the production of solar cells [10] and light emitting diodes [11,12]. Thus, AZO is expected to be a promising material for fabricating transparent electrodes in optoelectronic devices. Recently, the photovoltaic effect in ferroelectric materials has gained much attention [13] due to the high output photovoltage and control of the photocurrent induced by mechanisms, such as electric field, magnetism and heat. Among the ferroelectric materials,
n
Corresponding authors. Tel.: þ 86 21 34202593; fax: þ 86 21 34202749. E-mail addresses: [email protected] (Y. Guo), [email protected] (H. Liu).
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.10.031
multiferroic BiFeO3 (BFO) with narrow band gap and high remnant polarization exhibit appreciable photovoltaic output in the visible light region. Chen et al. [14] used ITO and Au as top electrode to investigate the photovoltaic effect of BFO thin films and suggest that the electrode plays an important role in determining the photovoltaic effect of ferroelectric thin films. Zang et al. [13] obtained improved photovoltaic properties with a carbon nanotube/BFO/Pt as top electrode, with the open circuit voltage reaching as high as 0.47 V. However, most of the heterojunction structures are based on using metal and ITO electrodes, and the traditional process derived polycrystalline BFO with perovskite structure [13,14] still show limited photovoltaic effect. In this paper, we demonstrate the ferroelectric film based capacitors using AZO as the top electrode for the first time. The heterojunction structure consists of AZO/BFO/Fluorine tin oxide (FTO). BFO with rhombohedral structure belongs to the R3c space group was prepared as reported earlier [15]. An Au top electrode was used for comparison to further indicate the function of AZO in improving the photovoltaic effect of BFO based capacitors.
2. Experimental Polycrystalline BFO thin films were deposited on the commercial FTO glass substrates through a modified chemical solution deposition [15]. The optical properties were investigated using an UV–visible spectrophotometer in the wavelength range of 330–900 nm. AZO
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W. Dong et al. / Materials Letters 91 (2013) 359–361
electrodes of 0.5 mm 1 mm were deposited on the surface of BFO by the RF magnetron method [11] at room temperature in Ar gas with 2 wt% Al2O3-doped ZnO sintered sheet as the target. Au top electrodes of 0.5 mm 1 mm were prepared using a DC sputter coater. Photovoltaic properties were measured by a Keithley 2400 source meter. Illumination of 100 mW/cm2 (AM 1.5) generated by a solar simulator was used as the excitation light source. The external quantum efficiency (EQE) was measured by an EQE tester from Crowntech, and calibrated with a standard silicon solar cell.
3. Results and discussions The photovoltaic properties of the as-prepared polycrystalline BFO thin films are shown in Fig. 1(a). The open circuit voltage
(VOC) and the photocurrent density (JSC) of the capacitor can reach up to 0.63 V and 0.13 mA/cm2, respectively, much higher than the reported values for traditional polycrystalline BFO based capacitors. They are comparable with those of the ITO/BFO/FTO structure (VOC ¼0.65 V, JSC ¼0.13 mA/cm2) [15]. To clarify the effect of AZO electrode on improving the photovoltaic effect of AZO/BFO/ FTO structure, we provide a comparison group with an Au/BFO/ FTO structure. The VOC of the Au/BFO/FTO structure measured to be only 0.4 V, and the JSC of the Au/BFO/FTO structure is only onethird of the AZO/BFO/FTO structure. The time dependence of zero bias photocurrent density with light ON and OFF (Fig. 1(b)) displays repeatable and stable instantaneous responses of the photocurrent to the light illumination. The power conversion efficiency Z is calculated by the ratio of the output electric power to the input optical power [16]. It is found from Fig. 1(c) that the
Fig. 1. (a) J–V curves of the BFO film capacitors with Au and ITO top electrodes, (b) time dependence of zero bias photocurrent density with light ON and OFF, (c) power conversion efficiency calculated from J–V curves of (a) using spectral intensity of 100 mW/cm2, and (d) EQE of AZO/BFO/FTO structure within the wavelength from 200 to 900 nm.
Fig. 2. Energy band diagrams of the Au/BFO/FTO (a) and the AZO/BFO/FTO structures (b) upon illumination. (c) Sketch maps of Au/BFO/FTO and AZO/BFO/FTO structures. (d) Optical transmittance spectrum of BFO and AZO films.
W. Dong et al. / Materials Letters 91 (2013) 359–361
maximum Z of the AZO/BFO/FTO structure has an improvement of more than 3 fold (2.08 10 2%) in comparison with that of the Au/BFO/FTO structure (5.34 10 3%). To qualify the JSC as a function of the wavelength of incident, light external quantum efficiency (EQE) measurement was also carried out to give a better indication of the behavior of the devices when exposed to light adequate to promote charge excitation. It can be inferred from Fig. 1(d), a maximum conversion efficiency of about 7% is observed where the light energy is larger than the band gap of BFO (2.54 eV) [15] corresponding to absorption edge of 488 nm shown in Fig. 2(d). The drop-off at the shortest wavelengths ( o340 nm) is a result of light absorption in AZO top contacts. The above comparison and results indicate that AZO electrode can do a major contribution to the high photovoltaic output. The potential barrier voltage that equals to the maximum VOC is determined by work function difference of top and bottom electrode. According to the theoretical values of work function of top and bottom electrode, the maximum work function differences for the Au–FTO and AZO–FTO electrode pairs are calculated to be the same (0.8 eV) [17–19]. However the observed VOC in the AZO/BFO/FTO structure is more close to the potential barrier. The larger photovoltaic effect of the AZO based capacitors may be attributed to the larger build-in-electric field, depolarization electric field and higher transparency of top electrode. Firstly, it is well accepted that the photovoltaic effect of ferroelectric film capacitors is related to the carrier movement and carrier separation driven by the internal electric field that consists of build-in-electric field (EBi) and depolarization electric field (EDP). Fig. 2(a) and (b) shows schematic energy band diagrams of the capacitors. Generally, the top EBi (EBi-top) and bottom EBi (EBi-bottom) are back-to-back and then EBi may be defined as the difference between EBi-top and EBi-bottom. It is reported that the asymmetric electrode structure can provide a large EBi and makes contributions to the photovoltaic output [20]. Thus, the EBi can be generated both in AZO and Au based capacitors due to their asymmetric electrode structures (shown in Fig. 2(c)). However, the AZO based capacitor may show a much higher EBi than that of the Au based capacitors since the decrease of the schottky barrier induced by the interfacial defect states at the metal–ferroelectric interface is more severe than that of the oxide electrode–ferroelectric interface [16]. Secondly, it is reported that the oxide top electrodes combined with ferroelectric films could provide a larger EDP and induce a higher photovoltaic output than that of the metal electrode based capacitors [21]. Finally, it should be noted from Fig. 2(d) that the transmittance of the AZO film can be more than 80% in the visible region, thus more photo-hole pairs can be generated within the films with AZO electrode than that with a semitransparent Au electrode showing a transmittance of less than 20% as also given in Fig. 2(d).
361
4. Conclusions In summary, AZO was used to investigate the photovoltaic properties of ferroelectric film based capacitors for the first time. The open circuit voltage and photocurrent density for the AZO/BFO/ FTO structure are measured to be 0.13 mA/cm2 and 0.63 V, respectively, much higher than the reported values for traditional polycrystalline BFO based capacitors. They are comparable with those of the ITO/BFO/FTO structure (JSC ¼0.13 mA/cm2, VOC ¼0.65 V) previously reported [15]. A comparison with Au/BFO/FTO indicates that the higher photovoltaic effect of AZO based capacitors may be attributed to the larger build-in-electric field, depolarization electric field and higher transparency of top electrode. Our results suggest that AZO can be a promising electrode material in ferroelectric film capacitors and could be a potential replacement of ITO materials.
Acknowledgments This work is supported by the National Nature Science Foundation of China (no. 11074165).
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
Gordon RG. Mater Res Bull 2000;25:52–7. Coutts TJ, Young DL, Li X. Mater Res Bull 2000;25:58–65. Minami T. Mater Res Bull 2000;25:38–44. Minami T, Nanto H, Takata S. Appl Phys Lett 1982;41:958. Igasaki Y, Saito H. J Appl Phys 1991;70:3613–9. Kim H, Horwitz JS, Pique A, Murata H, Kushto GP, Schlaf R, et al. Appl Phys Lett 2000;76:259. Kim H, Pique A, Horwitz JS, Mattoussi H, Murata H, Kafafi ZH, et al. Appl Phys Lett 1999;74:3444. Kim H, Gilmore CM, Pique A, Horwitz JS, Mattoussi H, Murata H, et al. J Appl Phys 1999;86:6451–61. Kim H, Horwitz JS, Pique A, Gilmore CM, Chrisey DB. Appl Phys A 1999;69:S447–50. Hu J, Gordon RG. J Appl Phys 1992;71:880–90. Zhao JQ, Xie SJ, Han SH, Yang ZW, Ye L, Yang TL. Synth Met 2000;114:251–4. Park SH, Lee BH, Shin JM, Jeong SY, Song S, Suh HS, et al. Appl Phys Lett 2011;100:133306. Zang YY, Xie D, Chen Y, Wu X, Ren TL, Wei JQ, et al. Nanoscale 2012;4:2926–30. Chen B, Li M, Liu YW, Zuo ZH, Zhuge F, Zhan QF, et al. Nanotechnology 2011;22:195201. Dong W, Guo YP, Guo B, Liu HY, Li H, Liu HZ. Mater Lett 2012;88:140–2. Qin M, Yao K, Liang YC. J Appl Phys 2009;105:061624. Andersson A, Johansson N, Broms P, Yu N, Lupo D, Salaneck WR. Adv Mater 1998;10:859–63. ¨ Birgerson J, Keil M, Denier van der Gon AW, Crispin X, Logdlund M, Salaneck WR. Mater Res Soc Symp Proc 2000;660 JJ5.29.1–JJ5.29.6. Liu ZG, Zhao WM, Ji RB, Zhang ZL, Jiang XY, Xue MZ, et al. J Phys: Condens Matter 1996;8:3221–8. Zhang P, Cao DW, Wang CY, Shen MR, Su XD, Fang L, et al. Mater Chem Phys 2012;135:304–8. Qin M, Yao K, Liang YC. J Appl Phys 2009;95:22912.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Photovoltaic properties of BiFeO3 thin film capacitors by using Al-doped zinc oxide as top electrode Wen Dong, Yiping Guo n, Bing Guo, Hongyan Liu, Hua Li, Hezhou Liu n State Key Laboratory of MMCs, School of Materials Science and Engineering, Shanghai Jiaotong University, Shanghai 200240, China
a r t i c l e i n f o
abstract
Article history: Received 28 September 2012 Accepted 10 October 2012 Available online 22 October 2012
We demonstrate using Al-doped zinc oxide films as electrode materials to form ferroelectric thin film capacitors. The photocurrent density and open circuit voltage for the capacitor with an Al-doped zinc oxide/ BiFeO3/Fluorine doped tin oxide structure are measured to be 0.13 mA/cm2 and 0.63 V, respectively, much higher than the reported values for traditional BFO based capacitors. To clarify the contribution of Al-doped zinc oxide to the photovoltaic effect, Au top electrode was used as a comparison group. The Al-doped zinc oxide based capacitors show a more than 3 times larger photovoltaic output than that of the Au counterpart, which indicates that the Al-doped zinc oxide makes a major contribution to the high photovoltaic effect. The comparison shows that the larger photovoltaic effect may be attributed to the larger build-in-electric field, depolarization electric field and higher transparency of top electrode. Our results suggest that the transparent Al-doped zinc oxide film can be a promising electrode material in ferroelectric film capacitors and could be a potential replacement of indium tin oxide materials. & 2012 Elsevier B.V. All rights reserved.
Keywords: Ferroelectric thin films BiFeO3 Oxide electrode Photovoltaic effect
1. Introduction Transparent conducting oxide (TCO) semiconductors that are used as transparent electrodes play an important role in most optoelectronic devices. Though indium tin oxide (ITO) thin films have been in practical use for transparent electrode applications, it may be faced with high cost and scarcity of indium. It is crucial to find out appropriate TCO materials to replace ITO. Some other TCO materials have been reported [1,2]. Doped zinc oxide thin films have been widely studied for their use as alternative to ITO [3–6]. Impurity-doped zinc oxide (ZnO) was proposed to be a potential alternative to ITO [3,4]. Among the doped zinc oxide films, aluminum-doped zinc oxide (AZO) films with a band gap of about 3.3 eV show the lowest electrical resistivity 2–4 10 4 O cm [5,6], which is similar to that of ITO films [7–9]. Unlike the more commonly used indium tin oxide (ITO), zinc oxide (ZnO) is a nontoxic and inexpensive material. It is chemically and thermally stable under hydrogen plasma processes commonly used for the production of solar cells [10] and light emitting diodes [11,12]. Thus, AZO is expected to be a promising material for fabricating transparent electrodes in optoelectronic devices. Recently, the photovoltaic effect in ferroelectric materials has gained much attention [13] due to the high output photovoltage and control of the photocurrent induced by mechanisms, such as electric field, magnetism and heat. Among the ferroelectric materials,
n
Corresponding authors. Tel.: þ 86 21 34202593; fax: þ 86 21 34202749. E-mail addresses: [email protected] (Y. Guo), [email protected] (H. Liu).
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.10.031
multiferroic BiFeO3 (BFO) with narrow band gap and high remnant polarization exhibit appreciable photovoltaic output in the visible light region. Chen et al. [14] used ITO and Au as top electrode to investigate the photovoltaic effect of BFO thin films and suggest that the electrode plays an important role in determining the photovoltaic effect of ferroelectric thin films. Zang et al. [13] obtained improved photovoltaic properties with a carbon nanotube/BFO/Pt as top electrode, with the open circuit voltage reaching as high as 0.47 V. However, most of the heterojunction structures are based on using metal and ITO electrodes, and the traditional process derived polycrystalline BFO with perovskite structure [13,14] still show limited photovoltaic effect. In this paper, we demonstrate the ferroelectric film based capacitors using AZO as the top electrode for the first time. The heterojunction structure consists of AZO/BFO/Fluorine tin oxide (FTO). BFO with rhombohedral structure belongs to the R3c space group was prepared as reported earlier [15]. An Au top electrode was used for comparison to further indicate the function of AZO in improving the photovoltaic effect of BFO based capacitors.
2. Experimental Polycrystalline BFO thin films were deposited on the commercial FTO glass substrates through a modified chemical solution deposition [15]. The optical properties were investigated using an UV–visible spectrophotometer in the wavelength range of 330–900 nm. AZO
360
W. Dong et al. / Materials Letters 91 (2013) 359–361
electrodes of 0.5 mm 1 mm were deposited on the surface of BFO by the RF magnetron method [11] at room temperature in Ar gas with 2 wt% Al2O3-doped ZnO sintered sheet as the target. Au top electrodes of 0.5 mm 1 mm were prepared using a DC sputter coater. Photovoltaic properties were measured by a Keithley 2400 source meter. Illumination of 100 mW/cm2 (AM 1.5) generated by a solar simulator was used as the excitation light source. The external quantum efficiency (EQE) was measured by an EQE tester from Crowntech, and calibrated with a standard silicon solar cell.
3. Results and discussions The photovoltaic properties of the as-prepared polycrystalline BFO thin films are shown in Fig. 1(a). The open circuit voltage
(VOC) and the photocurrent density (JSC) of the capacitor can reach up to 0.63 V and 0.13 mA/cm2, respectively, much higher than the reported values for traditional polycrystalline BFO based capacitors. They are comparable with those of the ITO/BFO/FTO structure (VOC ¼0.65 V, JSC ¼0.13 mA/cm2) [15]. To clarify the effect of AZO electrode on improving the photovoltaic effect of AZO/BFO/ FTO structure, we provide a comparison group with an Au/BFO/ FTO structure. The VOC of the Au/BFO/FTO structure measured to be only 0.4 V, and the JSC of the Au/BFO/FTO structure is only onethird of the AZO/BFO/FTO structure. The time dependence of zero bias photocurrent density with light ON and OFF (Fig. 1(b)) displays repeatable and stable instantaneous responses of the photocurrent to the light illumination. The power conversion efficiency Z is calculated by the ratio of the output electric power to the input optical power [16]. It is found from Fig. 1(c) that the
Fig. 1. (a) J–V curves of the BFO film capacitors with Au and ITO top electrodes, (b) time dependence of zero bias photocurrent density with light ON and OFF, (c) power conversion efficiency calculated from J–V curves of (a) using spectral intensity of 100 mW/cm2, and (d) EQE of AZO/BFO/FTO structure within the wavelength from 200 to 900 nm.
Fig. 2. Energy band diagrams of the Au/BFO/FTO (a) and the AZO/BFO/FTO structures (b) upon illumination. (c) Sketch maps of Au/BFO/FTO and AZO/BFO/FTO structures. (d) Optical transmittance spectrum of BFO and AZO films.
W. Dong et al. / Materials Letters 91 (2013) 359–361
maximum Z of the AZO/BFO/FTO structure has an improvement of more than 3 fold (2.08 10 2%) in comparison with that of the Au/BFO/FTO structure (5.34 10 3%). To qualify the JSC as a function of the wavelength of incident, light external quantum efficiency (EQE) measurement was also carried out to give a better indication of the behavior of the devices when exposed to light adequate to promote charge excitation. It can be inferred from Fig. 1(d), a maximum conversion efficiency of about 7% is observed where the light energy is larger than the band gap of BFO (2.54 eV) [15] corresponding to absorption edge of 488 nm shown in Fig. 2(d). The drop-off at the shortest wavelengths ( o340 nm) is a result of light absorption in AZO top contacts. The above comparison and results indicate that AZO electrode can do a major contribution to the high photovoltaic output. The potential barrier voltage that equals to the maximum VOC is determined by work function difference of top and bottom electrode. According to the theoretical values of work function of top and bottom electrode, the maximum work function differences for the Au–FTO and AZO–FTO electrode pairs are calculated to be the same (0.8 eV) [17–19]. However the observed VOC in the AZO/BFO/FTO structure is more close to the potential barrier. The larger photovoltaic effect of the AZO based capacitors may be attributed to the larger build-in-electric field, depolarization electric field and higher transparency of top electrode. Firstly, it is well accepted that the photovoltaic effect of ferroelectric film capacitors is related to the carrier movement and carrier separation driven by the internal electric field that consists of build-in-electric field (EBi) and depolarization electric field (EDP). Fig. 2(a) and (b) shows schematic energy band diagrams of the capacitors. Generally, the top EBi (EBi-top) and bottom EBi (EBi-bottom) are back-to-back and then EBi may be defined as the difference between EBi-top and EBi-bottom. It is reported that the asymmetric electrode structure can provide a large EBi and makes contributions to the photovoltaic output [20]. Thus, the EBi can be generated both in AZO and Au based capacitors due to their asymmetric electrode structures (shown in Fig. 2(c)). However, the AZO based capacitor may show a much higher EBi than that of the Au based capacitors since the decrease of the schottky barrier induced by the interfacial defect states at the metal–ferroelectric interface is more severe than that of the oxide electrode–ferroelectric interface [16]. Secondly, it is reported that the oxide top electrodes combined with ferroelectric films could provide a larger EDP and induce a higher photovoltaic output than that of the metal electrode based capacitors [21]. Finally, it should be noted from Fig. 2(d) that the transmittance of the AZO film can be more than 80% in the visible region, thus more photo-hole pairs can be generated within the films with AZO electrode than that with a semitransparent Au electrode showing a transmittance of less than 20% as also given in Fig. 2(d).
361
4. Conclusions In summary, AZO was used to investigate the photovoltaic properties of ferroelectric film based capacitors for the first time. The open circuit voltage and photocurrent density for the AZO/BFO/ FTO structure are measured to be 0.13 mA/cm2 and 0.63 V, respectively, much higher than the reported values for traditional polycrystalline BFO based capacitors. They are comparable with those of the ITO/BFO/FTO structure (JSC ¼0.13 mA/cm2, VOC ¼0.65 V) previously reported [15]. A comparison with Au/BFO/FTO indicates that the higher photovoltaic effect of AZO based capacitors may be attributed to the larger build-in-electric field, depolarization electric field and higher transparency of top electrode. Our results suggest that AZO can be a promising electrode material in ferroelectric film capacitors and could be a potential replacement of ITO materials.
Acknowledgments This work is supported by the National Nature Science Foundation of China (no. 11074165).
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
Gordon RG. Mater Res Bull 2000;25:52–7. Coutts TJ, Young DL, Li X. Mater Res Bull 2000;25:58–65. Minami T. Mater Res Bull 2000;25:38–44. Minami T, Nanto H, Takata S. Appl Phys Lett 1982;41:958. Igasaki Y, Saito H. J Appl Phys 1991;70:3613–9. Kim H, Horwitz JS, Pique A, Murata H, Kushto GP, Schlaf R, et al. Appl Phys Lett 2000;76:259. Kim H, Pique A, Horwitz JS, Mattoussi H, Murata H, Kafafi ZH, et al. Appl Phys Lett 1999;74:3444. Kim H, Gilmore CM, Pique A, Horwitz JS, Mattoussi H, Murata H, et al. J Appl Phys 1999;86:6451–61. Kim H, Horwitz JS, Pique A, Gilmore CM, Chrisey DB. Appl Phys A 1999;69:S447–50. Hu J, Gordon RG. J Appl Phys 1992;71:880–90. Zhao JQ, Xie SJ, Han SH, Yang ZW, Ye L, Yang TL. Synth Met 2000;114:251–4. Park SH, Lee BH, Shin JM, Jeong SY, Song S, Suh HS, et al. Appl Phys Lett 2011;100:133306. Zang YY, Xie D, Chen Y, Wu X, Ren TL, Wei JQ, et al. Nanoscale 2012;4:2926–30. Chen B, Li M, Liu YW, Zuo ZH, Zhuge F, Zhan QF, et al. Nanotechnology 2011;22:195201. Dong W, Guo YP, Guo B, Liu HY, Li H, Liu HZ. Mater Lett 2012;88:140–2. Qin M, Yao K, Liang YC. J Appl Phys 2009;105:061624. Andersson A, Johansson N, Broms P, Yu N, Lupo D, Salaneck WR. Adv Mater 1998;10:859–63. ¨ Birgerson J, Keil M, Denier van der Gon AW, Crispin X, Logdlund M, Salaneck WR. Mater Res Soc Symp Proc 2000;660 JJ5.29.1–JJ5.29.6. Liu ZG, Zhao WM, Ji RB, Zhang ZL, Jiang XY, Xue MZ, et al. J Phys: Condens Matter 1996;8:3221–8. Zhang P, Cao DW, Wang CY, Shen MR, Su XD, Fang L, et al. Mater Chem Phys 2012;135:304–8. Qin M, Yao K, Liang YC. J Appl Phys 2009;95:22912.