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fullerene composites in surface-type photocells
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Thin Solid Films 516 (2008) 2743 – 2746 www.elsevier.com/locate/tsf
Photocurrent of regioregular poly(3-alkylthiophene)/fullerene composites in surface-type photocells H. Ito a,⁎, Y. Niimi a , A. Suzuki a , K. Marumoto b , S. Kuroda a a b
Department of Applied Physics, Nagoya University, Chikusa, Nagoya 464-8603, Japan Institute of Materials Science, University of Tsukuba, Tennodai, Tsukuba 305-8573, Japan Available online 29 April 2007
Abstract We have studied steady-state photocurrents of regioregular poly(3-alkylthiophene)/C60 composite on surface-type photocells formed with Au electrodes. The photocurrent is observed above 1.4 eV, in agreement with the light-induced ESR measurements on the composite. The photocurrent is small up to 1.8 eV and then increases abruptly at 1.9 eV and keeps increasing up to 3 eV. The action spectra is not symbatic nor antibatic with the optical absorption spectra. The exponent n of the light intensity I dependence of the photocurrent P, P ∝ In, is close to n = 0.5 above 1.9 eV, indicating the involvement of the bimolecular recombination. Below 1.9 eV, n is between 0.5 and 1, indicating the contribution of the monomolecular process. The change in the exponent is understood by a universal curve as a function of the photoexcitation density taking account of the fraction of the incident light absorbed and the optical penetration depth. © 2007 Elsevier B.V. All rights reserved. Keywords: Regioregular poly(3-alkylthiophene); Photocurrent; Surface-type cell; Bimolecular recombination
1. Introduction The regioregular poly(3-alkylthiophene) (RR-P3AT) attracts great attention as a promising material for the growing area of molecular electronics, owing to the high solubility and the high carrier mobility up to ∼ 0.5 cm2/V s among conducting polymers [1]. The self-organization of the alkyl-side chains in thin films facilitates the formation of a lamella structure with π–π stacking between adjacent polymer chains favorable for the high carrier mobility [2]. On the other hand, Yoshino et al. have shown that the photocurrent greatly enhances by the addition of C60 into P3AT [3], owing to the efficient photoinduced electron transfer from polymer to C60 as shown in Fig. 1, with a quantum yield reaching unity [4]. Then, due to the high carrier mobility of the regioregular polymer toward electrodes, the composite of RR-P3AT and C60 (or its derivative, PCBM) is a promising system as bulk-heterojunction-type solar cells in which nanoscale p–n junctions between the polymer and C60 are present throughout the composites [5]. The ⁎ Corresponding author. E-mail address: [email protected] (H. Ito). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.04.098
efficiency of the bulk-heterojunction-type solar cell is now reaching 5% under air mass 1.5 conditions [6]. Study on the spectral response of the photocurrent is important not only for the application to solar cells in order to improve the coverage of the solar spectral range but also for the understanding of the physics of generation and transport of photocarriers. We have studied the photocurrent action spectra of the RR-P3OT/C60 composite sandwiched with ITO and Al or Au electrodes [7]. For the ITO/Al electrode, the photocurrent is dominated by photocarriers generated at or near the positively biased electrode, similar to the action spectra of P3AT-only devices [8]. If the light directly illuminates the positive electrode, the action spectra generally follow the optical absorption spectra of the polymer, resulting in the ‘symbatic’ behavior. On the other hand, if the light illuminates the negative electrode, the intensity of the light attenuates before reaching the positive electrode. Then the photocurrent is weak at the peak of the optical absorption, resulting in the ‘antibatic’ behavior. Such ‘filter effect’ with respect to the direction of the illumination indicates that the holes on the polymer chain are the majority carrier of photoconduction. At the positive electrode, Schottky barrier with large voltage gradient is formed between
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Fig. 1. Photoinduced charge transfer between RR-P3AT and C60.
the polymer and the electrode at which the photogeneration of carriers is more efficient. Photogenerated electrons are easily absorbed into the positive electrode and holes as positive polarons on the polymer chain travels to the negative electrode with high carrier mobility [9]. On the other hand, for the ITO/Au electrode, efficient photogeneration does not occur at the positively biased Au electrode as evidenced by the absence of the filter effect when illuminated through the negatively biased ITO electrode. By the use of Au electrode, we can see more intrinsic nature of the action spectra of the photocurrent generated at the nanoscale heterojunctions between the polymer and C60. However, at the ITO electrode, small energy level mismatch between ITO and polymer may be present exhibiting a weak filter effect when the positive bias is applied on the ITO electrode. Even in the ITO/ Au cell, the effect of the electrode cannot be ruled out. In this paper we report the action spectra of the RR-P3AT/ C60 composite on surface-type photocells. The surface-type cell is free from the filter effect described above and is more appropriate to observe the intrinsic nature of the photogeneration in the bulk. Composite with polymer having hexyl side chain RR-P3HT/C60 exhibits larger photocurrent than the octyl one RR-P3OT/C60, in agreement with the difference in the hole mobility of the polymers [10]. We measured the light intensity dependence of the photocurrent. The intensity dependence is scaled with a unified curve at different photon energies taking account of the photoexcitation density, which is smaller for the low photon energy side but is larger for the high photon energy side. At lower intensity of light, carriers recombine with monomolecular process but at higher intensity of light, bimolecular recombination between photogenerated species becomes dominant.
from the electrode side. The photocurrent measurements were performed with ac technique at the frequency of 17.8 Hz with a monochromated 500 W halogen lamp as a light source. The ac voltage appearing on a series resistance (1 MΩ) was measured under applied dc bias of 30 V between two electrodes. Then the field gradient across the electrodes was 3000 V/cm. The light intensity was calibrated with a Si photodiode. The intensity of the monochromated light was 25 μW/cm2 at the peak photon energy of 1.9 eV. The photocurrent action spectra were normalized per constant photon flux. The light intensity dependence of the photocurrent was carried out by reducing the voltage supplied to the halogen lamp from 100 V down to 50 V [11]. The measurement was carried out under vacuum less than 1 Pa. The optical-absorption spectra were measured with a Shimadzu UV2450 spectrophotometer. 3. Results and discussion 3.1. Action spectra
2. Experimental
The photocurrent action spectra of (a) RR-P3HT/C60 and (b) RR-P3OT/C60 surface-type photocells are shown in Fig. 3(a) and (b), respectively. Solid lines represent the spectra when illuminated from the film side and the dash-dotted lines the spectra illuminated from the electrode side. The photocurrent is 1.5 to 2 times larger for RR-P3HT/C60 than for RR-P3OT/C60, in good agreement with the difference in the hole mobility of the two polymers [10]. The photocurrent is observed above 1.4 eV and remains nearly constant up to 1.8 eV. In this photon energy range, the enhancement of the light-induced ESR signal has been observed on the composite [9]. Here C60 is photoexcited across the forbidden transition owing to the slight lifting of the degeneracy by the dynamical Jahn–Teller effect. The exciton decomposes into an electron on C60 (C60 radical anion) and a hole on the polymer. The hole relaxes into a positive polaron and contributes to the conduction. Above 1.9 eV, the photocurrent abruptly increases and keeps increasing gradually up to 3 eV. Here, RRP3AT is photoexcited and holes as positive polarons contributes the conduction remaining electrons on C60. The overall shape of the photocurrent action spectra resembles the result on the ITO/ Au sandwich cell under positive bias applied on the Au electrode [7]. The spectra are also consistent with the earlier report for the steady-state photoconductivity measurement on the surface-type photocell of RR-P3OT/C60 composites [12].
Regioregular poly(3-alkylthiophene) (alkyl = hexyl, octyl, hereafter as RR-P3HT or RR-P3OT) (Aldrich, head-to-tail ratio 98.5%) was dissolved into toluene with addition of C60 (15 mol%, ∼60 wt.%). The mixed solution was thoroughly stirred with use of the ultrasonic agitation. Then, the solution was drop-cast onto a white-cut glass, on which two Au electrodes with a separation of 0.1 mm were pre-evaporated. The thickness of the cast film was about 3 μm. The thickness of the film was measured with an alpha-step surface profiler (KLATencor). The schematic structure of the photocell is shown in Fig. 2. The light was illuminated onto the film surface, or back
Fig. 2. Schematic diagram of the sample geometry and the direction of the light illumination.
H. Ito et al. / Thin Solid Films 516 (2008) 2743–2746
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3.2. Light intensity dependence The light intensity dependence of the photocurrent is crucial for the investigation of the generation and the kinetics of the photocarriers. If the photocarriers are trapped with monomolecular process, the rate equation of the hole carrier density p is written as, dp ¼ UI kp: dt
ð1Þ
where Φ denotes quantum yield, I the intensity of the incident light and k is a constant. At steady state, the carrier density p, i.e., the photocurrent P, is proportional to I. On the other hand, if the bimolecular recombination between photogenerated holes and electrons is the major process, the rate equation is dp ¼ UI bp2 dt
Fig. 3. Photocurrent action spectra of (a) RR-P3HT/C60 and (b) RR-P3OT/C60 surface-type photocells illuminated from the film side (solid lines) or the electrode side (dash-dotted lines). Dotted lines represent the optical absorption spectra.
The action spectra are not symbatic nor antibatic with the optical absorption indicated by dotted lines in Fig. 3(a) and (b). The spectra do not show filter effect with respect to the illumination direction from the film side or from the electrode side. Below 1.9 eV, the spectra are identical for two illumination directions, indicating that the whole film is photoexcited owing to the long optical penetration depth comparable to the film thickness (see Fig. 4). Above 2 eV, the photocurrent is slightly larger for the illumination from the film side. This is because the optical penetration depth is much shorter than the film thickness, as shown in Fig. 4. Then only the surface thin layer of the film is photoexcited. Since the available surface is smaller when illuminated from the electrode side, the smaller photocurrent is observed when illuminated from the electrode side.
Fig. 4. Optical penetration depth, Λ, for the RR-P3HT/C60 and RR-P3OT/C60 composites.
ð2Þ
where β is a constant. We assume that the same number of holes and electrons are photogenerated. Then, at steady state, the carrier density p, i.e., the photocurrent P, is proportional to the square root of I. In Figs. 5(a) to (d), we show the light intensity dependence of the photocurrent at photon energies of (a) 1.7 eV, (b) 2.0 eV, (c) 2.5 eV and (d) 3.0 eV for the RR-P3OT/C60 cell. Above 2.0 eV, the exponent n of the light intensity dependence of the photocurrent, P ∝ In, is 0.5∼0.6, indicating that the bimolecular recombination is relevant. Below 1.9 eV, n is ∼ 0.8, implying the involvement of the monomolecular process. No quadrimolecular behavior is observed as found for the light-induced ESR measurement [13]. Similar behavior is observed also for the RR-P3HT/C60 cell. We scale the light intensity dependence of the photocurrent as a function of the photoexcitation density [14]. Because of the
Fig. 5. The light intensity dependence of the photocurrent at (a) 1.7 eV, (b) 2.0 eV, (c) 2.5 eV, and (d) 3.0 eV for the RR-P3OT/C60 cell. The exponent of the light intensity dependence, n, is indicated for each measurement. Open symbols represent the photocurrent under illumination from the electrode side and closed symbols represent the photocurrent under illumination from the film side.
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surface-type cell is appropriate to observe the intrinsic nature of the generation and kinetics of the photocurrent. 4. Summary
Fig. 6. The light intensity dependence of the photocurrent of the RR-P3OT/C60 cell scaled with the photoexcitation density N = IFA/Λ at each photon energy. Open symbols represent the photocurrent under illumination from the electrode side and closed symbols represent the photocurrent under illumination from the film side.
difference in the optical absorption and hence the optical penetration depth, the density of the photoexcitation per unit volume of the film is quite different below or above 1.9 eV. Below 1.9 eV, the optical penetration depth Λ is comparable to the film thickness d and the fraction of the incident light absorbed by the film, FA = 1 − exp(− d / Λ), is small (∼ 0.6). On the other hand, above 1.9 eV, Λ is much shorter than d and FA is close to unity. Then, above 1.9 eV, almost all the incident light is absorbed by the thin layer of the film surface. The density of the photoexcitation per unit volume of the film is much larger above 1.9 eV than below 1.9 eV. In Fig. 6, we show the photocurrent as a function of the density of photoexcitation per unit volume of the film per second N = IFA / Λ for the RR-P3OT/C60 cell. The light intensity dependences of the photocurrent at four different photon energies seem to be scaled into a universal curve. The universality of the photocurrent dependence on the photoexcitation density indicates the constancy of the quantum yield on the photon energy throughout the measured photon energy range. The exponent of the photoexcitation density dependence of the photocurrent on the universal curve seems to change gradually from n ∼ 1 at low N of ∼ 1017/cm3 s to n ∼ 0.5 at high N of ∼ 1018/cm3 s. At the lower photoexcitation density, the probability to meet the separated species of electron on C60 and hole on polymer is low and each carrier is immobilized at trapped defect sites. But at the higher photoexcitation density, they meet together to recombine bimolecularly. At much higher photoexcitation density, the quadrimolecular process may be observed similar to that observed by ESR measurements [13]. This behavior is qualitatively the same as that found by the electrodeless microwave photoconductivity measurement on RR-P3HT [14]. This demonstrates that the present measurement is not affected by the presence of the electrode. The use of the
We have measured the action spectra of the RR-P3AT/C60 composite on surface-type photocells made with Au electrodes. The shape of the action spectra is not symbatic nor antibatic with respect to the optical absorption, indicating the intrinsic nature of the photogeneration in the bulk. A slight difference of the photocurrent on the illumination direction above 1.9 eV is understood by the difference in available surface area at which most of the photocarriers are generated. RR-P3HT/C60 device exhibits larger photocurrent than RR-P3OT/C60 device, indicating the difference in the hole mobility of the polymers. The light intensity dependence of the photocurrent is scaled with a unified curve at different photon energies taking account of the photoexcitation density. At lower intensity of 1017/cm3 s, carriers trapped with monomolecular process, but at higher intensity of 1018/cm3 s, bimolecular recombination between photogenerated species becomes dominant. Acknowledgements This research was 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] H. Sirringhaus, Adv. Mater. 17 (2005) 2411. [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] V.D. Mihailetchi, H. Xie, B. de Boer, L.J.A. Koster, P.W.M. Blom, Adv. Funct. Mater. 16 (2006) 699. [6] W. Ma, C. Yang, X. Gong, K. Lee, A.J. Heeger, Adv. Funct. Mater. 15 (2005) 1617. [7] H. Ito, N. Nomura, T. Suzuki, S. Ukai, K. Marumoto, S. Kuroda, Colloids Surf., A 284–285 (2006) 613. [8] K. Kaneto, K. Takayama, W. Takashima, T. Endo, M. Rikukawa, Jpn. J. Appl. Phys. 41 (2002) 675. [9] K. Marumoto, N. Takeuchi, T. Ozaki, S. Kuroda, Synth. Met. 129 (2002) 239. [10] K. Kaneto, W.Y. Lim, W. Takashima, T. Endo, M. Rikukawa, Jpn. J. Appl. Phys. 39 (2000) 872. [11] S. Matsuura, S.T. Ishiguro, K. Kikuchi, Y. Achiba, Phys. Rev., B 51 (1995) 10217. [12] 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. [13] K. Marumoto, Y. Muramatsu, S. Kuroda, Appl. Phys. Lett. 84 (2004) 1317. [14] G. Dicker, M.P. de Haas, L.D.A. Siebbeles, Phys. Rev., B 71 (2005) 155204.
Thin Solid Films 516 (2008) 2743 – 2746 www.elsevier.com/locate/tsf
Photocurrent of regioregular poly(3-alkylthiophene)/fullerene composites in surface-type photocells H. Ito a,⁎, Y. Niimi a , A. Suzuki a , K. Marumoto b , S. Kuroda a a b
Department of Applied Physics, Nagoya University, Chikusa, Nagoya 464-8603, Japan Institute of Materials Science, University of Tsukuba, Tennodai, Tsukuba 305-8573, Japan Available online 29 April 2007
Abstract We have studied steady-state photocurrents of regioregular poly(3-alkylthiophene)/C60 composite on surface-type photocells formed with Au electrodes. The photocurrent is observed above 1.4 eV, in agreement with the light-induced ESR measurements on the composite. The photocurrent is small up to 1.8 eV and then increases abruptly at 1.9 eV and keeps increasing up to 3 eV. The action spectra is not symbatic nor antibatic with the optical absorption spectra. The exponent n of the light intensity I dependence of the photocurrent P, P ∝ In, is close to n = 0.5 above 1.9 eV, indicating the involvement of the bimolecular recombination. Below 1.9 eV, n is between 0.5 and 1, indicating the contribution of the monomolecular process. The change in the exponent is understood by a universal curve as a function of the photoexcitation density taking account of the fraction of the incident light absorbed and the optical penetration depth. © 2007 Elsevier B.V. All rights reserved. Keywords: Regioregular poly(3-alkylthiophene); Photocurrent; Surface-type cell; Bimolecular recombination
1. Introduction The regioregular poly(3-alkylthiophene) (RR-P3AT) attracts great attention as a promising material for the growing area of molecular electronics, owing to the high solubility and the high carrier mobility up to ∼ 0.5 cm2/V s among conducting polymers [1]. The self-organization of the alkyl-side chains in thin films facilitates the formation of a lamella structure with π–π stacking between adjacent polymer chains favorable for the high carrier mobility [2]. On the other hand, Yoshino et al. have shown that the photocurrent greatly enhances by the addition of C60 into P3AT [3], owing to the efficient photoinduced electron transfer from polymer to C60 as shown in Fig. 1, with a quantum yield reaching unity [4]. Then, due to the high carrier mobility of the regioregular polymer toward electrodes, the composite of RR-P3AT and C60 (or its derivative, PCBM) is a promising system as bulk-heterojunction-type solar cells in which nanoscale p–n junctions between the polymer and C60 are present throughout the composites [5]. The ⁎ Corresponding author. E-mail address: [email protected] (H. Ito). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.04.098
efficiency of the bulk-heterojunction-type solar cell is now reaching 5% under air mass 1.5 conditions [6]. Study on the spectral response of the photocurrent is important not only for the application to solar cells in order to improve the coverage of the solar spectral range but also for the understanding of the physics of generation and transport of photocarriers. We have studied the photocurrent action spectra of the RR-P3OT/C60 composite sandwiched with ITO and Al or Au electrodes [7]. For the ITO/Al electrode, the photocurrent is dominated by photocarriers generated at or near the positively biased electrode, similar to the action spectra of P3AT-only devices [8]. If the light directly illuminates the positive electrode, the action spectra generally follow the optical absorption spectra of the polymer, resulting in the ‘symbatic’ behavior. On the other hand, if the light illuminates the negative electrode, the intensity of the light attenuates before reaching the positive electrode. Then the photocurrent is weak at the peak of the optical absorption, resulting in the ‘antibatic’ behavior. Such ‘filter effect’ with respect to the direction of the illumination indicates that the holes on the polymer chain are the majority carrier of photoconduction. At the positive electrode, Schottky barrier with large voltage gradient is formed between
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Fig. 1. Photoinduced charge transfer between RR-P3AT and C60.
the polymer and the electrode at which the photogeneration of carriers is more efficient. Photogenerated electrons are easily absorbed into the positive electrode and holes as positive polarons on the polymer chain travels to the negative electrode with high carrier mobility [9]. On the other hand, for the ITO/Au electrode, efficient photogeneration does not occur at the positively biased Au electrode as evidenced by the absence of the filter effect when illuminated through the negatively biased ITO electrode. By the use of Au electrode, we can see more intrinsic nature of the action spectra of the photocurrent generated at the nanoscale heterojunctions between the polymer and C60. However, at the ITO electrode, small energy level mismatch between ITO and polymer may be present exhibiting a weak filter effect when the positive bias is applied on the ITO electrode. Even in the ITO/ Au cell, the effect of the electrode cannot be ruled out. In this paper we report the action spectra of the RR-P3AT/ C60 composite on surface-type photocells. The surface-type cell is free from the filter effect described above and is more appropriate to observe the intrinsic nature of the photogeneration in the bulk. Composite with polymer having hexyl side chain RR-P3HT/C60 exhibits larger photocurrent than the octyl one RR-P3OT/C60, in agreement with the difference in the hole mobility of the polymers [10]. We measured the light intensity dependence of the photocurrent. The intensity dependence is scaled with a unified curve at different photon energies taking account of the photoexcitation density, which is smaller for the low photon energy side but is larger for the high photon energy side. At lower intensity of light, carriers recombine with monomolecular process but at higher intensity of light, bimolecular recombination between photogenerated species becomes dominant.
from the electrode side. The photocurrent measurements were performed with ac technique at the frequency of 17.8 Hz with a monochromated 500 W halogen lamp as a light source. The ac voltage appearing on a series resistance (1 MΩ) was measured under applied dc bias of 30 V between two electrodes. Then the field gradient across the electrodes was 3000 V/cm. The light intensity was calibrated with a Si photodiode. The intensity of the monochromated light was 25 μW/cm2 at the peak photon energy of 1.9 eV. The photocurrent action spectra were normalized per constant photon flux. The light intensity dependence of the photocurrent was carried out by reducing the voltage supplied to the halogen lamp from 100 V down to 50 V [11]. The measurement was carried out under vacuum less than 1 Pa. The optical-absorption spectra were measured with a Shimadzu UV2450 spectrophotometer. 3. Results and discussion 3.1. Action spectra
2. Experimental
The photocurrent action spectra of (a) RR-P3HT/C60 and (b) RR-P3OT/C60 surface-type photocells are shown in Fig. 3(a) and (b), respectively. Solid lines represent the spectra when illuminated from the film side and the dash-dotted lines the spectra illuminated from the electrode side. The photocurrent is 1.5 to 2 times larger for RR-P3HT/C60 than for RR-P3OT/C60, in good agreement with the difference in the hole mobility of the two polymers [10]. The photocurrent is observed above 1.4 eV and remains nearly constant up to 1.8 eV. In this photon energy range, the enhancement of the light-induced ESR signal has been observed on the composite [9]. Here C60 is photoexcited across the forbidden transition owing to the slight lifting of the degeneracy by the dynamical Jahn–Teller effect. The exciton decomposes into an electron on C60 (C60 radical anion) and a hole on the polymer. The hole relaxes into a positive polaron and contributes to the conduction. Above 1.9 eV, the photocurrent abruptly increases and keeps increasing gradually up to 3 eV. Here, RRP3AT is photoexcited and holes as positive polarons contributes the conduction remaining electrons on C60. The overall shape of the photocurrent action spectra resembles the result on the ITO/ Au sandwich cell under positive bias applied on the Au electrode [7]. The spectra are also consistent with the earlier report for the steady-state photoconductivity measurement on the surface-type photocell of RR-P3OT/C60 composites [12].
Regioregular poly(3-alkylthiophene) (alkyl = hexyl, octyl, hereafter as RR-P3HT or RR-P3OT) (Aldrich, head-to-tail ratio 98.5%) was dissolved into toluene with addition of C60 (15 mol%, ∼60 wt.%). The mixed solution was thoroughly stirred with use of the ultrasonic agitation. Then, the solution was drop-cast onto a white-cut glass, on which two Au electrodes with a separation of 0.1 mm were pre-evaporated. The thickness of the cast film was about 3 μm. The thickness of the film was measured with an alpha-step surface profiler (KLATencor). The schematic structure of the photocell is shown in Fig. 2. The light was illuminated onto the film surface, or back
Fig. 2. Schematic diagram of the sample geometry and the direction of the light illumination.
H. Ito et al. / Thin Solid Films 516 (2008) 2743–2746
2745
3.2. Light intensity dependence The light intensity dependence of the photocurrent is crucial for the investigation of the generation and the kinetics of the photocarriers. If the photocarriers are trapped with monomolecular process, the rate equation of the hole carrier density p is written as, dp ¼ UI kp: dt
ð1Þ
where Φ denotes quantum yield, I the intensity of the incident light and k is a constant. At steady state, the carrier density p, i.e., the photocurrent P, is proportional to I. On the other hand, if the bimolecular recombination between photogenerated holes and electrons is the major process, the rate equation is dp ¼ UI bp2 dt
Fig. 3. Photocurrent action spectra of (a) RR-P3HT/C60 and (b) RR-P3OT/C60 surface-type photocells illuminated from the film side (solid lines) or the electrode side (dash-dotted lines). Dotted lines represent the optical absorption spectra.
The action spectra are not symbatic nor antibatic with the optical absorption indicated by dotted lines in Fig. 3(a) and (b). The spectra do not show filter effect with respect to the illumination direction from the film side or from the electrode side. Below 1.9 eV, the spectra are identical for two illumination directions, indicating that the whole film is photoexcited owing to the long optical penetration depth comparable to the film thickness (see Fig. 4). Above 2 eV, the photocurrent is slightly larger for the illumination from the film side. This is because the optical penetration depth is much shorter than the film thickness, as shown in Fig. 4. Then only the surface thin layer of the film is photoexcited. Since the available surface is smaller when illuminated from the electrode side, the smaller photocurrent is observed when illuminated from the electrode side.
Fig. 4. Optical penetration depth, Λ, for the RR-P3HT/C60 and RR-P3OT/C60 composites.
ð2Þ
where β is a constant. We assume that the same number of holes and electrons are photogenerated. Then, at steady state, the carrier density p, i.e., the photocurrent P, is proportional to the square root of I. In Figs. 5(a) to (d), we show the light intensity dependence of the photocurrent at photon energies of (a) 1.7 eV, (b) 2.0 eV, (c) 2.5 eV and (d) 3.0 eV for the RR-P3OT/C60 cell. Above 2.0 eV, the exponent n of the light intensity dependence of the photocurrent, P ∝ In, is 0.5∼0.6, indicating that the bimolecular recombination is relevant. Below 1.9 eV, n is ∼ 0.8, implying the involvement of the monomolecular process. No quadrimolecular behavior is observed as found for the light-induced ESR measurement [13]. Similar behavior is observed also for the RR-P3HT/C60 cell. We scale the light intensity dependence of the photocurrent as a function of the photoexcitation density [14]. Because of the
Fig. 5. The light intensity dependence of the photocurrent at (a) 1.7 eV, (b) 2.0 eV, (c) 2.5 eV, and (d) 3.0 eV for the RR-P3OT/C60 cell. The exponent of the light intensity dependence, n, is indicated for each measurement. Open symbols represent the photocurrent under illumination from the electrode side and closed symbols represent the photocurrent under illumination from the film side.
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H. Ito et al. / Thin Solid Films 516 (2008) 2743–2746
surface-type cell is appropriate to observe the intrinsic nature of the generation and kinetics of the photocurrent. 4. Summary
Fig. 6. The light intensity dependence of the photocurrent of the RR-P3OT/C60 cell scaled with the photoexcitation density N = IFA/Λ at each photon energy. Open symbols represent the photocurrent under illumination from the electrode side and closed symbols represent the photocurrent under illumination from the film side.
difference in the optical absorption and hence the optical penetration depth, the density of the photoexcitation per unit volume of the film is quite different below or above 1.9 eV. Below 1.9 eV, the optical penetration depth Λ is comparable to the film thickness d and the fraction of the incident light absorbed by the film, FA = 1 − exp(− d / Λ), is small (∼ 0.6). On the other hand, above 1.9 eV, Λ is much shorter than d and FA is close to unity. Then, above 1.9 eV, almost all the incident light is absorbed by the thin layer of the film surface. The density of the photoexcitation per unit volume of the film is much larger above 1.9 eV than below 1.9 eV. In Fig. 6, we show the photocurrent as a function of the density of photoexcitation per unit volume of the film per second N = IFA / Λ for the RR-P3OT/C60 cell. The light intensity dependences of the photocurrent at four different photon energies seem to be scaled into a universal curve. The universality of the photocurrent dependence on the photoexcitation density indicates the constancy of the quantum yield on the photon energy throughout the measured photon energy range. The exponent of the photoexcitation density dependence of the photocurrent on the universal curve seems to change gradually from n ∼ 1 at low N of ∼ 1017/cm3 s to n ∼ 0.5 at high N of ∼ 1018/cm3 s. At the lower photoexcitation density, the probability to meet the separated species of electron on C60 and hole on polymer is low and each carrier is immobilized at trapped defect sites. But at the higher photoexcitation density, they meet together to recombine bimolecularly. At much higher photoexcitation density, the quadrimolecular process may be observed similar to that observed by ESR measurements [13]. This behavior is qualitatively the same as that found by the electrodeless microwave photoconductivity measurement on RR-P3HT [14]. This demonstrates that the present measurement is not affected by the presence of the electrode. The use of the
We have measured the action spectra of the RR-P3AT/C60 composite on surface-type photocells made with Au electrodes. The shape of the action spectra is not symbatic nor antibatic with respect to the optical absorption, indicating the intrinsic nature of the photogeneration in the bulk. A slight difference of the photocurrent on the illumination direction above 1.9 eV is understood by the difference in available surface area at which most of the photocarriers are generated. RR-P3HT/C60 device exhibits larger photocurrent than RR-P3OT/C60 device, indicating the difference in the hole mobility of the polymers. The light intensity dependence of the photocurrent is scaled with a unified curve at different photon energies taking account of the photoexcitation density. At lower intensity of 1017/cm3 s, carriers trapped with monomolecular process, but at higher intensity of 1018/cm3 s, bimolecular recombination between photogenerated species becomes dominant. Acknowledgements This research was 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] H. Sirringhaus, Adv. Mater. 17 (2005) 2411. [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] V.D. Mihailetchi, H. Xie, B. de Boer, L.J.A. Koster, P.W.M. Blom, Adv. Funct. Mater. 16 (2006) 699. [6] W. Ma, C. Yang, X. Gong, K. Lee, A.J. Heeger, Adv. Funct. Mater. 15 (2005) 1617. [7] H. Ito, N. Nomura, T. Suzuki, S. Ukai, K. Marumoto, S. Kuroda, Colloids Surf., A 284–285 (2006) 613. [8] K. Kaneto, K. Takayama, W. Takashima, T. Endo, M. Rikukawa, Jpn. J. Appl. Phys. 41 (2002) 675. [9] K. Marumoto, N. Takeuchi, T. Ozaki, S. Kuroda, Synth. Met. 129 (2002) 239. [10] K. Kaneto, W.Y. Lim, W. Takashima, T. Endo, M. Rikukawa, Jpn. J. Appl. Phys. 39 (2000) 872. [11] S. Matsuura, S.T. Ishiguro, K. Kikuchi, Y. Achiba, Phys. Rev., B 51 (1995) 10217. [12] 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. [13] K. Marumoto, Y. Muramatsu, S. Kuroda, Appl. Phys. Lett. 84 (2004) 1317. [14] G. Dicker, M.P. de Haas, L.D.A. Siebbeles, Phys. Rev., B 71 (2005) 155204.