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C60 composites
Colloids and Surfaces A: Physicochem. Eng. Aspects 284–285 (2006) 613–616
Photocurrent of thin-film cells of regioregular polyalkylthiophene/C60 composites Hiroshi Ito ∗ , Noritsugu Nomura, Takashi Suzuki, Sota Ukai, Kazuhiro Marumoto, Shin-ichi Kuroda Department of Applied Physics, Nagoya University, Nagoya 464-8603, Japan Received 25 June 2005; received in revised form 10 October 2005; accepted 12 October 2005 Available online 4 January 2006
Abstract We have studied steady-state photocurrents of poly(3-octylthiophene)/C60 composite thin-film cells formed with ITO/Al and ITO/Au electrodes. For the ITO/Al electrode, the photocurrent is dominated by photocarriers generated at or near the positively biased electrode, as demonstrated by clear antibatic or symbatic behaviors depending on the polarity of the bias. For the ITO/Au electrode, since the photocarrier generation near the Au electrode is not effective, the photocurrent-action spectra indicate the photo carrier generation in the bulk film. The efficiency of the bulk photogeneration enhances at the energy range of 1.4–1.9 eV, in agreement with the ESR measurements on the composite. © 2005 Elsevier B.V. All rights reserved. Keywords: Photocurrent; Poly(3-octylthiophene); C60 ; Antibatic; Symbatic
1. Introduction The regioregular polyalkylthiophene (PAT) attracts attention as a promising material for the growing area of molecular electronics, owing to the high solubility and the high carrier mobility. The mobility of the FET marks the highest value of 0.2 cm2 /V s among conducting polymers [1]. The self-organization of the alkyl-side chains in thin-films facilitates the formation of the lamella structure with – stacking between adjacent polymer chains [2]. Yoshino et al. have discovered that the photocurrent greatly enhances by addition of C60 into the conjugated polymers such as MEH-PPV and PAT [3], followed by the quenching of the photoluminescence [4,5]. This is because of the efficient photoinduced electron transfer from polymer to C60 with a quantum yield reaching unity [3]. The photoinduced holes relax into positive polarons on the conjugated polymer chain [6]. Since positive polarons travel with high mobility in PAT, the PAT/C60 system is promising as a prototype of the photocells taking advantage of the inherent bulk heterojunction between the polymer and C60 throughout the composites.
∗
Corresponding author. Tel.: +81 52 789 5164; fax: +81 52 789 3712. E-mail address: [email protected] (H. Ito).
0927-7757/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2005.10.009
Study on action spectra of the steady-state photocurrent of the photocell is important not only for application to solar cells but also for the physics of generation and transport of photocarriers [7–12]. However, the action spectra do not always reflect the energy dispersion relation of the polymer itself, since the photocurrent is often governed by the photocarrier generation at the surface or at the interface between the polymer and the electrode [7,8] In the sandwich-type photocell with transparent electrodes, the photocurrent is often governed by photocarriers generated at the interface of the efficient electrode [9]. Then the intensity of the light reaching the efficient electrode dominates the shape of the action spectra. If the light directly illuminates the efficient electrode, the action spectra generally follow the optical-absorption spectra of the polymer, resulting in the ‘symbatic’ behavior. On the other hand, the light illuminates from the reverse side of the efficient electrode, the intensity of the light attenuates before reaching the efficient electrode. Then the photocurrent is weak at the peak of the optical-absorption, resulting in the ‘antibatic’ behavior. In p-type semiconductors such as conjugated polymers, the efficient electrode is the positively biased electrode because the photogenerated electrons are easily absorbed into the positive electrode. When the workfunction of the positive electrode is lower, the Schottky barrier with large voltage gradient is formed between the polymer and the electrode and the photogeneration of carriers is more efficient.
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H. Ito et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 284–285 (2006) 613–616
The voltage gradient facilitates the separation of excitons. The symbatic and antibatic behaviors with respect to the opticalabsorption depending on the bias polarity and the illumination direction have been investigated in details for MEH-PPV [10,11] and PAT [12,13]. As for the composite of polymer/C60 , systematic studies on photocurrent in terms of bias polarity and illumination direction have not been reported yet. In the composite, since the bulk heterojunction plays role in photocarrier generation, the surface or the interface between the electrode and the composite may not be important for the photocarrier generation, and the antibatic or synbatic behavior may not be observed clearly. In order to clarify the generation and transport of photocarriers of the polymer/C60 system, we have investigated the photocurrentaction spectra of regioregular poly(3-octylthiophene)/C60 composite in sandwich-type photocells in view of the kind of electrodes and bias polarity. 2. Experimental Regioregular poly(3-octylthiophene) (RR-P3OT) (head-totail ratio 98.5%) [14] was purchased with Aldrich, Co. Ltd. High purity C60 (99.99%) was dissolved in toluene solution up to about 0.20 wt%. Then the RR-P3OT was added into the solution and the mixed solution was thoroughly stirred with use of the ultrasonic agitation. The molar ratio of C60 to RR-P3OT was 1:10. Then the solution was drop-cast under Ar gas atmosphere on a white-cut glass with a presputtered ITO electrode of 2 mm width. The thickness of the cast film was about 5 m. Then Au or Al electrode of 2 mm width was evaporated through a shadow mask. The thickness of the metal electrodes was 50 nm. The effective area of the sandwich cell was 2 mm × 2 mm. The schematic structure of the photocell is shown in Fig. 1. The photocurrent measurements were performed with ac technique at the frequency of 17.8 Hz. Two copper wires (100 m) were attached to the electrodes with silver paste and the device was set into a cryostat. The measurements were carried out under vacuum less than 1 Pa. A 500 W halogen lamp monochromated with a Nikon G250 grating was used as light source in the wavelength range from 400 to 1200 nm. Above 800 nm, an infrared filter was inserted to eliminate the second harmonics from the grating. The light intensity was calibrated with an Si photodiode. The intensity of the monochromated light
Fig. 1. Schematic structure of the photocells.
was approximately 20–30 W/cm2 . The photocurrent was normalized with the photon number. The light was chopped with an NF 5584 light chopper. The ac voltage appearing between the two ends of a series resistance (1 M) was measured with an NF 5610B lock-in amplifier. The positive bias was supplied by an Agilent 4339B high resistance meter, in which negative electrode was grounded. The optical-absorption spectra were measured with a Hitachi U-3200 automatic absorbance spectrometer. 3. Results In Fig. 2, we show the current–voltage characteristics of the ITO/Al and ITO/Au sandwich photocells obtained under dark condition. The bias polarity is defined as plus when the positive bias is applied to ITO. Clear rectifying behavior is observed for the ITO/Al cell. On the other hand, the current flows in both direction for the ITO/Au cell but a small asymmetry in current is observed. In the cell sandwiched with two Au electrodes, a linear current–voltage relation is observed (not shown). It is probable that the interface between the polymer and ITO has a small mismatch in energy, resulting that a small Schottky-like band bending remains at the interface. In Fig. 3, we show the photocurrent-action spectra of the ITO/Al photocell obtained under illumination through the ITO electrode in the condition that the positive bias is applied to Al (a) and the positive bias is applied to ITO (b). The optical-absorption spectra are shown by broken lines in the figures. Clear antibatic and symbatic behaviors are observed according to the polarity of the bias. When the Al electrode is positively biased, the action spectra show the antibatic behavior. When the ITO electrode is positively biased, the action spectra show the symbatic behavior. These results indicate that the photocarriers are generated at the Schottky barrier formed between Al and polymers. When the Al electrode is positively biased, the height of the barrier at the interface increases with a large band bending and the efficient charge separation occurs at or near the Al electrode. When the ITO electrode is positively biased, the barrier at the Al electrode
Fig. 2. Current–voltage characteristics of the ITO/Al and ITO/Au photocells obtained under dark condition. The bias polarity is defined as plus when the positive bias is applied to ITO.
H. Ito et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 284–285 (2006) 613–616
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Fig. 5. Comparison of the photocurrent-action spectra of the RR-P3OT/C60 and RR-P3OT-only photocells obtained under the illumination through ITO electrode with Al electrode positively (3 V) biased; these photocells are fabricated in the same condition. The optical-absorption spectra are shown by a broken line.
Fig. 3. The photocurrent-action spectra of the ITO/Al photocell obtained under the illumination through ITO electrode in the condition that the positive bias is applied to Al (a) and the positive bias is applied to ITO (b) at 1, 2, 3 and 4 V. The optical-absorption spectra are shown by broken lines.
disappears but a band bending at the side of the ITO electrode is enhanced, and then the charge separation occurs at or near the ITO electrode. These behaviors are basically the same for polymer-only photocells.
In Fig. 4, we show the photocurrent-action spectra of the ITO/Au photocell obtained under illumination through ITO electrode in the condition that the positive bias is applied to Au (a) and the positive bias is applied to ITO (b). When the positive bias is applied to ITO, the photocurrent-action spectra show the symbatic behavior, indicating that photocarriers are generated at or near the positively biased ITO electrode. On the other hand, when the positive bias is applied to Au, the photocurrent-action spectra appears neither symbatic nor antibatic to the opticalabsorption spectra. This is because the energy mismatch at the ITO electrode disappears and the band bending at the Au electrode is not formed. Hence, it is considered that the photocarriers generated by the charge separation between RR-P3OT and C60 in the bulk of the film contributes to the photocurrent. 4. Discussion
Fig. 4. Photocurrent-action spectra of the ITO/Au photocell obtained under the illumination through ITO electrode in the condition that the positive bias is applied to Au (a) and the positive bias is applied to ITO (b) at 1, 3 and 5 V. The optical-absorption spectra are shown by broken lines.
In the ITO/Al photocell, the photocurrent-action spectra are governed by the carrier generation at the interface of positively biased electrode because clear antibatic and symbatic behaviors are observed. This is because a Schottky barrier is formed not only at the interface of Al electrode but also at the ITO electrode, when they are positively biased. In Fig. 5, we show the comparison between the photocurrent-action spectra of the RR-P3OT/C60 composite and RR-P3OT measured at the same condition. The absolute value of the photocurrent of the composite enhances 80 times upon mixing C60 into the polymer, as in good agreement with the previous results [3]. The feature of the action spectra above 1.9 eV is common to the two action spectra, which are explained in terms of the antibatic behavior to the optical-absorption. In the RR-P3OT/C60 photocell, a photocurrent enhancement is observed as a shoulder from 1.4 to 1.9 eV. This shoulder is not understood by the antibatic model and corresponds to the photocurrent generated by the charge separation between RR-P3OT and C60 in the bulk. This photon-energy range of the charge separation agrees with the range that the
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H. Ito et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 284–285 (2006) 613–616
optically-forbidden transition of C60 occurs [3], which is consistent with the action spectra of the light-induced ESR [6]. In order to observe the photocurrent due to the charge separation between RR-P3OT and C60 in the bulk for the Schottky-type photocell, the photogenerated electrons must travel to the positive electrode in order to keep the charge balance, since the positive charges do not come into the polymer over the Schottky barrier. In the RRP3OT/C60 composites, the bipolar transport has been reported where the electronic path among C60 molecules is formed [15]. For the ITO/Au photocell, a symbatic behavior is observed when the ITO electrode is positively biased. A small energy mismatch remains at the interface of the ITO electrode when the positive bias is applied [9], as is observed as an asymmetric current–voltage relation shown in Fig. 2. When the Au electrode is positively biased, no Schottky barrier is formed at the interface of Au electrode. Then the photocurrent is considered to be caused by the bulk photogeneration of the carriers. In the absence of the Schottky barrier, the photoconduction only with holes is possible, since positive carriers are supplied from the positive electrode. The action spectra at the energy range of 1.4–1.9 eV are understood by the explanation that C60 is photoexcited and the positive charge is transferred to the polymer, resulting in positive polarons [6]. On the other hand, above 2.0 eV, the RRP3OT is photoexcited and electronic charge is transferred to C60 leaving positive polarons on the polymer chain. The shape of the action spectra is in qualitative agreement with those observed for the surface-type cells of the MEH-PPV/C60 and P3OT/C60 composites [16]. 5. Conclusions Steady-state photocurrents of thin-film cells with regioregular poly(3-octylthiophene)/C60 composite are studied using ITO/Al and ITO/Au electrodes. For the ITO/Al electrode, the photocurrent-action spectra are dominated by photocarriers generated at or near the Schottky barrier formed at the interface of the positive electrode, as demonstrated by the clear antibatic or symbatic behaviors depending on the polarity of the bias. For the ITO/Au electrode, since the photocarrier generation near the Au
electrode is not effective, the photocurrent-action spectra indicates the photocarrier generation in the bulk film, when the Au electrode is positively biased. The efficiency of the bulk photogeneration enhances at the energy range of 1.4–1.9 eV where the optically-forbidden transition of C60 occurs, in good agreement with the light-induced ESR measurements on the composite. Acknowledgements This research was partly supported by Grants-in-Aid for Scientific Research (17340094) and for Scientific Research in Priority Area “Super-Hierarchical Structures” (17067007) from the Ministry of Education, Culture, Sports, Science and Technology of Japan. References [1] G. Wang, J. Swensen, D. Moses, A. Heeger, J. Appl. Phys. 93 (2003) 6137. [2] H. Sirringhaus, P.J. Brown, R.H. Friend, Nature 401 (1999) 685. [3] K. Yoshino, X.H. Yin, S. Morita, T. Kawai, A.A. Zakhidov, Solid State Commun. 85 (1993) 85. [4] N.S. Saricifci, L. Smilowitz, A.J. Heeger, F. Wudl, Science 258 (1992) 1474. [5] S. Morita, A.A. Zakhidov, K. Yoshino, Solid State Commun. 82 (1992) 249. [6] K. Marumoto, N. Takeuchi, T. Ozaki, S. Kuroda, Synth. Met. 129 (2002) 239. [7] H.B. DeVore, Phys. Rev. 102 (1956) 86. [8] A.K. Ghosh, T. Feng, J. Appl. Phys. 49 (1978) 5982. [9] S. Barth, H. B¨assler, H. Rost, H.H. H¨orhold, Phys. Rev. B 56 (1997) 3844. [10] M.G. Harrison, J. Gruner, G.C.W. Spencer, Phys. Rev. B 55 (1997) 7831. [11] C.D. Stich, D.P. Halliday, A.P. Monkman, Mat. Res. Soc. Symp. Proc. 665 (2001) C8.17.1. [12] S.B. Lee, K. Yoshino, J.Y. Park, Y.W. Park, Phys. Rev. B 61 (2000) 2151. [13] K. Kaneto, K. Takayama, W. Takashima, T. Endo, M. Rikukawa, Jpn. J. Appl. Phys. 41 (2002) 675. [14] T.-A. Chen, X. Wu, R.D. Rieke, J. Am. Chem. Soc. 117 (1995) 233. [15] I. Balberg, R. Naidis, M.-K. Lee, J. Shinar, L.F. Fonseca, Appl. Phys. Lett. 79 (2001) 197. [16] C.H. Lee, G. Yu, D. Moses, K. Pakbaz, C. Zhang, N.S. Sariciftci, A.J. Heeger, F. Wudl, Phys. Rev. B 48 (1993) 15425.
Photocurrent of thin-film cells of regioregular polyalkylthiophene/C60 composites Hiroshi Ito ∗ , Noritsugu Nomura, Takashi Suzuki, Sota Ukai, Kazuhiro Marumoto, Shin-ichi Kuroda Department of Applied Physics, Nagoya University, Nagoya 464-8603, Japan Received 25 June 2005; received in revised form 10 October 2005; accepted 12 October 2005 Available online 4 January 2006
Abstract We have studied steady-state photocurrents of poly(3-octylthiophene)/C60 composite thin-film cells formed with ITO/Al and ITO/Au electrodes. For the ITO/Al electrode, the photocurrent is dominated by photocarriers generated at or near the positively biased electrode, as demonstrated by clear antibatic or symbatic behaviors depending on the polarity of the bias. For the ITO/Au electrode, since the photocarrier generation near the Au electrode is not effective, the photocurrent-action spectra indicate the photo carrier generation in the bulk film. The efficiency of the bulk photogeneration enhances at the energy range of 1.4–1.9 eV, in agreement with the ESR measurements on the composite. © 2005 Elsevier B.V. All rights reserved. Keywords: Photocurrent; Poly(3-octylthiophene); C60 ; Antibatic; Symbatic
1. Introduction The regioregular polyalkylthiophene (PAT) attracts attention as a promising material for the growing area of molecular electronics, owing to the high solubility and the high carrier mobility. The mobility of the FET marks the highest value of 0.2 cm2 /V s among conducting polymers [1]. The self-organization of the alkyl-side chains in thin-films facilitates the formation of the lamella structure with – stacking between adjacent polymer chains [2]. Yoshino et al. have discovered that the photocurrent greatly enhances by addition of C60 into the conjugated polymers such as MEH-PPV and PAT [3], followed by the quenching of the photoluminescence [4,5]. This is because of the efficient photoinduced electron transfer from polymer to C60 with a quantum yield reaching unity [3]. The photoinduced holes relax into positive polarons on the conjugated polymer chain [6]. Since positive polarons travel with high mobility in PAT, the PAT/C60 system is promising as a prototype of the photocells taking advantage of the inherent bulk heterojunction between the polymer and C60 throughout the composites.
∗
Corresponding author. Tel.: +81 52 789 5164; fax: +81 52 789 3712. E-mail address: [email protected] (H. Ito).
0927-7757/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2005.10.009
Study on action spectra of the steady-state photocurrent of the photocell is important not only for application to solar cells but also for the physics of generation and transport of photocarriers [7–12]. However, the action spectra do not always reflect the energy dispersion relation of the polymer itself, since the photocurrent is often governed by the photocarrier generation at the surface or at the interface between the polymer and the electrode [7,8] In the sandwich-type photocell with transparent electrodes, the photocurrent is often governed by photocarriers generated at the interface of the efficient electrode [9]. Then the intensity of the light reaching the efficient electrode dominates the shape of the action spectra. If the light directly illuminates the efficient electrode, the action spectra generally follow the optical-absorption spectra of the polymer, resulting in the ‘symbatic’ behavior. On the other hand, the light illuminates from the reverse side of the efficient electrode, the intensity of the light attenuates before reaching the efficient electrode. Then the photocurrent is weak at the peak of the optical-absorption, resulting in the ‘antibatic’ behavior. In p-type semiconductors such as conjugated polymers, the efficient electrode is the positively biased electrode because the photogenerated electrons are easily absorbed into the positive electrode. When the workfunction of the positive electrode is lower, the Schottky barrier with large voltage gradient is formed between the polymer and the electrode and the photogeneration of carriers is more efficient.
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H. Ito et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 284–285 (2006) 613–616
The voltage gradient facilitates the separation of excitons. The symbatic and antibatic behaviors with respect to the opticalabsorption depending on the bias polarity and the illumination direction have been investigated in details for MEH-PPV [10,11] and PAT [12,13]. As for the composite of polymer/C60 , systematic studies on photocurrent in terms of bias polarity and illumination direction have not been reported yet. In the composite, since the bulk heterojunction plays role in photocarrier generation, the surface or the interface between the electrode and the composite may not be important for the photocarrier generation, and the antibatic or synbatic behavior may not be observed clearly. In order to clarify the generation and transport of photocarriers of the polymer/C60 system, we have investigated the photocurrentaction spectra of regioregular poly(3-octylthiophene)/C60 composite in sandwich-type photocells in view of the kind of electrodes and bias polarity. 2. Experimental Regioregular poly(3-octylthiophene) (RR-P3OT) (head-totail ratio 98.5%) [14] was purchased with Aldrich, Co. Ltd. High purity C60 (99.99%) was dissolved in toluene solution up to about 0.20 wt%. Then the RR-P3OT was added into the solution and the mixed solution was thoroughly stirred with use of the ultrasonic agitation. The molar ratio of C60 to RR-P3OT was 1:10. Then the solution was drop-cast under Ar gas atmosphere on a white-cut glass with a presputtered ITO electrode of 2 mm width. The thickness of the cast film was about 5 m. Then Au or Al electrode of 2 mm width was evaporated through a shadow mask. The thickness of the metal electrodes was 50 nm. The effective area of the sandwich cell was 2 mm × 2 mm. The schematic structure of the photocell is shown in Fig. 1. The photocurrent measurements were performed with ac technique at the frequency of 17.8 Hz. Two copper wires (100 m) were attached to the electrodes with silver paste and the device was set into a cryostat. The measurements were carried out under vacuum less than 1 Pa. A 500 W halogen lamp monochromated with a Nikon G250 grating was used as light source in the wavelength range from 400 to 1200 nm. Above 800 nm, an infrared filter was inserted to eliminate the second harmonics from the grating. The light intensity was calibrated with an Si photodiode. The intensity of the monochromated light
Fig. 1. Schematic structure of the photocells.
was approximately 20–30 W/cm2 . The photocurrent was normalized with the photon number. The light was chopped with an NF 5584 light chopper. The ac voltage appearing between the two ends of a series resistance (1 M) was measured with an NF 5610B lock-in amplifier. The positive bias was supplied by an Agilent 4339B high resistance meter, in which negative electrode was grounded. The optical-absorption spectra were measured with a Hitachi U-3200 automatic absorbance spectrometer. 3. Results In Fig. 2, we show the current–voltage characteristics of the ITO/Al and ITO/Au sandwich photocells obtained under dark condition. The bias polarity is defined as plus when the positive bias is applied to ITO. Clear rectifying behavior is observed for the ITO/Al cell. On the other hand, the current flows in both direction for the ITO/Au cell but a small asymmetry in current is observed. In the cell sandwiched with two Au electrodes, a linear current–voltage relation is observed (not shown). It is probable that the interface between the polymer and ITO has a small mismatch in energy, resulting that a small Schottky-like band bending remains at the interface. In Fig. 3, we show the photocurrent-action spectra of the ITO/Al photocell obtained under illumination through the ITO electrode in the condition that the positive bias is applied to Al (a) and the positive bias is applied to ITO (b). The optical-absorption spectra are shown by broken lines in the figures. Clear antibatic and symbatic behaviors are observed according to the polarity of the bias. When the Al electrode is positively biased, the action spectra show the antibatic behavior. When the ITO electrode is positively biased, the action spectra show the symbatic behavior. These results indicate that the photocarriers are generated at the Schottky barrier formed between Al and polymers. When the Al electrode is positively biased, the height of the barrier at the interface increases with a large band bending and the efficient charge separation occurs at or near the Al electrode. When the ITO electrode is positively biased, the barrier at the Al electrode
Fig. 2. Current–voltage characteristics of the ITO/Al and ITO/Au photocells obtained under dark condition. The bias polarity is defined as plus when the positive bias is applied to ITO.
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Fig. 5. Comparison of the photocurrent-action spectra of the RR-P3OT/C60 and RR-P3OT-only photocells obtained under the illumination through ITO electrode with Al electrode positively (3 V) biased; these photocells are fabricated in the same condition. The optical-absorption spectra are shown by a broken line.
Fig. 3. The photocurrent-action spectra of the ITO/Al photocell obtained under the illumination through ITO electrode in the condition that the positive bias is applied to Al (a) and the positive bias is applied to ITO (b) at 1, 2, 3 and 4 V. The optical-absorption spectra are shown by broken lines.
disappears but a band bending at the side of the ITO electrode is enhanced, and then the charge separation occurs at or near the ITO electrode. These behaviors are basically the same for polymer-only photocells.
In Fig. 4, we show the photocurrent-action spectra of the ITO/Au photocell obtained under illumination through ITO electrode in the condition that the positive bias is applied to Au (a) and the positive bias is applied to ITO (b). When the positive bias is applied to ITO, the photocurrent-action spectra show the symbatic behavior, indicating that photocarriers are generated at or near the positively biased ITO electrode. On the other hand, when the positive bias is applied to Au, the photocurrent-action spectra appears neither symbatic nor antibatic to the opticalabsorption spectra. This is because the energy mismatch at the ITO electrode disappears and the band bending at the Au electrode is not formed. Hence, it is considered that the photocarriers generated by the charge separation between RR-P3OT and C60 in the bulk of the film contributes to the photocurrent. 4. Discussion
Fig. 4. Photocurrent-action spectra of the ITO/Au photocell obtained under the illumination through ITO electrode in the condition that the positive bias is applied to Au (a) and the positive bias is applied to ITO (b) at 1, 3 and 5 V. The optical-absorption spectra are shown by broken lines.
In the ITO/Al photocell, the photocurrent-action spectra are governed by the carrier generation at the interface of positively biased electrode because clear antibatic and symbatic behaviors are observed. This is because a Schottky barrier is formed not only at the interface of Al electrode but also at the ITO electrode, when they are positively biased. In Fig. 5, we show the comparison between the photocurrent-action spectra of the RR-P3OT/C60 composite and RR-P3OT measured at the same condition. The absolute value of the photocurrent of the composite enhances 80 times upon mixing C60 into the polymer, as in good agreement with the previous results [3]. The feature of the action spectra above 1.9 eV is common to the two action spectra, which are explained in terms of the antibatic behavior to the optical-absorption. In the RR-P3OT/C60 photocell, a photocurrent enhancement is observed as a shoulder from 1.4 to 1.9 eV. This shoulder is not understood by the antibatic model and corresponds to the photocurrent generated by the charge separation between RR-P3OT and C60 in the bulk. This photon-energy range of the charge separation agrees with the range that the
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H. Ito et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 284–285 (2006) 613–616
optically-forbidden transition of C60 occurs [3], which is consistent with the action spectra of the light-induced ESR [6]. In order to observe the photocurrent due to the charge separation between RR-P3OT and C60 in the bulk for the Schottky-type photocell, the photogenerated electrons must travel to the positive electrode in order to keep the charge balance, since the positive charges do not come into the polymer over the Schottky barrier. In the RRP3OT/C60 composites, the bipolar transport has been reported where the electronic path among C60 molecules is formed [15]. For the ITO/Au photocell, a symbatic behavior is observed when the ITO electrode is positively biased. A small energy mismatch remains at the interface of the ITO electrode when the positive bias is applied [9], as is observed as an asymmetric current–voltage relation shown in Fig. 2. When the Au electrode is positively biased, no Schottky barrier is formed at the interface of Au electrode. Then the photocurrent is considered to be caused by the bulk photogeneration of the carriers. In the absence of the Schottky barrier, the photoconduction only with holes is possible, since positive carriers are supplied from the positive electrode. The action spectra at the energy range of 1.4–1.9 eV are understood by the explanation that C60 is photoexcited and the positive charge is transferred to the polymer, resulting in positive polarons [6]. On the other hand, above 2.0 eV, the RRP3OT is photoexcited and electronic charge is transferred to C60 leaving positive polarons on the polymer chain. The shape of the action spectra is in qualitative agreement with those observed for the surface-type cells of the MEH-PPV/C60 and P3OT/C60 composites [16]. 5. Conclusions Steady-state photocurrents of thin-film cells with regioregular poly(3-octylthiophene)/C60 composite are studied using ITO/Al and ITO/Au electrodes. For the ITO/Al electrode, the photocurrent-action spectra are dominated by photocarriers generated at or near the Schottky barrier formed at the interface of the positive electrode, as demonstrated by the clear antibatic or symbatic behaviors depending on the polarity of the bias. For the ITO/Au electrode, since the photocarrier generation near the Au
electrode is not effective, the photocurrent-action spectra indicates the photocarrier generation in the bulk film, when the Au electrode is positively biased. The efficiency of the bulk photogeneration enhances at the energy range of 1.4–1.9 eV where the optically-forbidden transition of C60 occurs, in good agreement with the light-induced ESR measurements on the composite. Acknowledgements This research was partly supported by Grants-in-Aid for Scientific Research (17340094) and for Scientific Research in Priority Area “Super-Hierarchical Structures” (17067007) from the Ministry of Education, Culture, Sports, Science and Technology of Japan. References [1] G. Wang, J. Swensen, D. Moses, A. Heeger, J. Appl. Phys. 93 (2003) 6137. [2] H. Sirringhaus, P.J. Brown, R.H. Friend, Nature 401 (1999) 685. [3] K. Yoshino, X.H. Yin, S. Morita, T. Kawai, A.A. Zakhidov, Solid State Commun. 85 (1993) 85. [4] N.S. Saricifci, L. Smilowitz, A.J. Heeger, F. Wudl, Science 258 (1992) 1474. [5] S. Morita, A.A. Zakhidov, K. Yoshino, Solid State Commun. 82 (1992) 249. [6] K. Marumoto, N. Takeuchi, T. Ozaki, S. Kuroda, Synth. Met. 129 (2002) 239. [7] H.B. DeVore, Phys. Rev. 102 (1956) 86. [8] A.K. Ghosh, T. Feng, J. Appl. Phys. 49 (1978) 5982. [9] S. Barth, H. B¨assler, H. Rost, H.H. H¨orhold, Phys. Rev. B 56 (1997) 3844. [10] M.G. Harrison, J. Gruner, G.C.W. Spencer, Phys. Rev. B 55 (1997) 7831. [11] C.D. Stich, D.P. Halliday, A.P. Monkman, Mat. Res. Soc. Symp. Proc. 665 (2001) C8.17.1. [12] S.B. Lee, K. Yoshino, J.Y. Park, Y.W. Park, Phys. Rev. B 61 (2000) 2151. [13] K. Kaneto, K. Takayama, W. Takashima, T. Endo, M. Rikukawa, Jpn. J. Appl. Phys. 41 (2002) 675. [14] T.-A. Chen, X. Wu, R.D. Rieke, J. Am. Chem. Soc. 117 (1995) 233. [15] I. Balberg, R. Naidis, M.-K. Lee, J. Shinar, L.F. Fonseca, Appl. Phys. Lett. 79 (2001) 197. [16] C.H. Lee, G. Yu, D. Moses, K. Pakbaz, C. Zhang, N.S. Sariciftci, A.J. Heeger, F. Wudl, Phys. Rev. B 48 (1993) 15425.