- Email: [email protected]
C60 Organic Solar Cells by Optical Electric-Field-Induced Second-Harmonic Generation Measurement
Available online at www.sciencedirect.com
Physics Procedia 14 (2011) 167–171
9th International Conference on Nano-Molecular Electronics
Determination of Lifetime of Double-Layer CuPc/C60 Organic Solar Cells by Optical Electric-Field-Induced Second-Harmonic Generation Measurement Dai Taguchia, Tatsunori Shinoa, Xiangyu Chena, Le Zhanga, Jun Lia, Martin Weisb, Takaaki Manakaa, Mitsumasa Iwamotoa * a
Department of Physical Electronics, Tokyo Institute of Technology, 2-12-1 S3-33 O-okayama, Meguro-ku Tokyo 152-8552 Japan b Institute of Physics, Slovak Academy of Sciences, Dúbravská cesta 9, 845 11 Bratislava 45, Slovak Republic
Abstract By using optical electric-field-induced second-harmonic generation measurement (EFISHG), we directly probed carrier transport processes in double-layer organic solar cells (OSCs). Results showed that excessive negative charges accumulated at the CuPc/C60 interface under photoillumination, and the accumulated charges decayed in a two-step process in dark. The EFISHG measurement is available for catching carrier process at the donor/acceptor interface of multi-layer OSCs to evaluate carrier lifetime of the cell. © 2010 Published by Elsevier B.V. second-harmonic generation; organic solar cell; Maxwell-Wagner effect; space-charge field; interface
1. Introduction Organic solar cells (OSCs) are promising candidates for the next generation power device, where low-cost and printable methods are available in device fabrication [1]. Basically, OSCs are comprised of an organic layer with donor (D) and acceptor (A) molecules [2,3]. Excitons that are created in an OSC layer by photoillumination diffuse to the D-A interface to separate into electrons and holes. After the separation, electrons and holes, respectively, move to the opposite electrodes to generate a photo-voltage. In order to increase the efficiency of OSCs, novel ideas of the use of pn [4,5], pin [6] and bulk-hetero [7] structures have been proposed. As a result, the power conversion efficiency of OSCs has been improved [2,8]. However, these ideas are no longer sufficient. We need to focus on carrier lifetime in OSCs for further improvement. The electrical measurement such as photo-induced charge carrier extraction in a linearly increasing voltage (Photo-CELIV) measurement [9] would be available, but multi-carrier
* Corresponding author. Tel.: +81-3-5734-2191; fax: +81-3-5734-2191. E-mail address: [email protected].
1875-3892 © 2011 Published by Elsevier Ltd. doi:10.1016/j.phpro.2011.05.034
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(electrons and holes) transport in OSC layer complicates our understanding. On the other hand, the electric-fieldinduced optical second-harmonic generation (EFISHG) directly probes carrier motion and is very helpful to investigate carrier mechanism [10-14]. These results motivated us to use the EFISHG measurement for probing the carrier dynamics at the interface of OSCs. In this paper, using the EFISHG measurement, we study carrier dynamics, i.e., carrier generation and decay processes, in double-layer (copper-phthalocyanine (CuPc)/C60) OSCs. Analysis based on the Maxwell-Wagner model showed that excessive negative charges Qs<0 remained at the CuPc/C60 interface under photoillumination, and these charges decayed in a two step process with lifetimes W L1 =5.3u10-5 s and W L 2 =6.2u10-2 s. We conclude that the EFISHG measurement is very effective for evaluating the carrier lifetime of OSCs. 2. Experiment We prepared double-layer OSCs with an indium zinc oxide (IZO)/CuPc/C60/Al structure, as illustrated in Fig. 1.
Fig. 1. Experimental arrangement of the EFISHG measurement.
IZO-coated glass substrates were UV/ozone treated to remove organic residuals. The CuPc layer (thickness, 60 nm), C60 layer (40 nm), and Al electrode were successively deposited onto the UV/ozone treated IZO surface in vacuum. Prepared OSCs were sealed with a dry agent to avoid degradation during measurements. The working area of the OSCs was A=3.1 mm2. We also prepared single-layer IZO/CuPc/Al and IZO/C60/Al devices (thickness of CuPc and C60 layers, 100 nm) and used as a reference sample in the SHG measurement. Before the SHG measurements, the I-V characteristics of the OSCs were recorded under illumination. A red light from a lightemitting diode (wavelength 630 nm, full width at half maximum 30 nm, intensity 1 mW/cm2) was used as a light source. Note that CuPc and C60 layers absorb light at a wavelength of 630 nm, and excitons are created in the layers. By laser irradiation, EFISHG signal& is generated from organic layers due to the coupling & of electrons in molecules and electro-magnetic waves E (Z ) in the presence of electrostatic local electric field E (0) . The EFISHG intensity is given as [12]
& & & & I (2Z ) v| P (2Z ) |2 | H 0 F (3) E (0) E (Z ) E (Z ) |2 (1) & where P (2Z ) is the nonlinear polarization induced in the organic layers, H 0 is the vacuum permittivity, and F (3) & is the third order nonlinear susceptibility tensor. E (0) is the electrostatic local field in the organic layers (we define & & as that positive electric field E (0) points from the IZO electrode to the Al electrode), and E (Z ) is the electric field
of incident laser of the &SHG& intensity is& in proportion to the & beam. Equation (1) indicates that the square-root & electric field E (0) formed in the organic layers. Here, E (0) is given by E0& Es , where E0 ( Qm / H r H 0 ) is the electric field originated from charges rQm on top and bottom electrodes and Es is the electric field originated from charges in organic devices, e.g., charges Qs accumulated at the double-layer interface. Consequently, the EFISHG measurement probes carrier processes of the charges rQm on electrodes as well as charges Qs at the double-layer interface. Figure 1 portrays an experimental arrangement for the SHG measurement [12]. We used a pulsed laser as a probing light (repetition rate 10 Hz, average power 1 mW, duration 4 nsec), which was generated using an optical parametric oscillator pumped with the third-harmonic light of Q-switched Nd:YAG laser. p-polarized pulsed laser
Dai Taguchi et al. / Physics Procedia 14 (2011) 167–171
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beam focused onto the sample surface at an incident angle of 45q, where the spot size was smaller than the device area. The reflected fundamental laser beam was eliminated using optical filters and a monochromator. The SHG light generated from the sample was detected using the photomultiplier tube (PMT), and the intensity I (2Z ) was measured using a digital multimeter. The susceptibility tensor F (3) in Eq. (1) is a material dependent parameter and is a function of the optical frequency Z . The difference in F (3) among materials allows us the electric field in each layer of a double-layer OSCs to be selectively probed by the use of an appropriate laser beam wavelength. In the present study, we used the laser beam at a wavelength of OZ 1,000 nm and recorded the generated SHG at a wavelength of O2Z =500 nm to selectively measure the electric field in C60 layer ( E2 in Fig. 1). In the time-resolved SHG measurements, we illuminated the OSCs under short-circuit condition. Photocurrent was generated under a red illumination (50 msec illumination at the intensity 1 mW/cm2 followed by 50 msec break) from the LED synchronized with the incidence laser beam. Under the short-circuit condition, excess charges Qs would be accumulated at the CuPc/C60 interface due to the Maxwell-Wagner effect [15,16], while exciton separation might be provoked at the interface. The presence of charge Qs deforms the electric field distribution in OSCs. The EFISHG probes the change of electric field, hence, catches carrier behavior in OSCs. 3. Results and Discussion A red-light illumination of the OSC induced negative photocurrent (I<0) flowing through the external circuit. The solar cell parameters were determined from I-V characteristics, and were open-circuit voltage Voc=0.37 V, shortcircuit current density Jsc=-4.4u10-6 A/cm2, and the fill-factor 0.13. Under photoillumination, excitons created in the CuPc and C60 layers diffused to the CuPc/C60 interface to produce free holes and electrons. Then the holes (electrons) were conveyed across CuPc (C60) layer to the IZO (Al) electrode, and photocurrent flowed through the external circuit. These processes result in the negative photocurrent and well accounted for the experimental result. Figure 2 shows the square-root of the SHG intensity generated in the CuPc and C60 single-layer devices under voltage application. The SHG signal of the C60 single-layer device showed linear dependence on the applied voltage. Here, voltage V application to the single-layer device forms average electric field E V / d ( d : layer thickness) in the layer, and the linear dependence in Fig. 2 suggested that the observed SHG was induced through EFISHG process expressed in Eq. (1). The voltage at the SHG signal minima was non-zero, and suggested that internal electric field was formed in the C60 single-layer device. On the other hand, CuPc showed negligibly small SHG response to the voltage application. These results confirm that we can selectively probe the electric field in C60 layer of the double-layer CuPc/C60 OSC.
Fig. 2: The square-root of the SHG intensity of the C60 and CuPc single-layer devices induced by voltage application.
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Figure 3 shows the SHG response of the CuPc/C60 OSC to the external voltage. The SHG intensity decreased on the positive voltage application to the IZO electrode (Fig. 3a), while the SHG intensity increased on the negative voltage application (Fig. 3b). Thus the changing direction of the SHG intensity allows us to determine a direction of the electric field formed in the C60 layer.
a.
b.
Fig. 3: SHG response of the CuPc/C60 OSC to A.C. square-wave voltage. (a) Positive voltage application. (b) Negative voltage application. Figure 4 shows the SHG intensity of the OSC in response to the photoillumination. The SHG intensity increased under photoillumination in a way similar to the SHG response of the OSC under negative voltage application. These results suggested that the excessive negative charge additionally accumulated at the CuPc/C60 interface under photoillumination and established steady state with the presence of photocurrent. The negative excessive charges at the interface is a source of a space charge field and additionally formed negative electric field pointing from Al to IZO electrode in the C60 layer. Thus, it was probed in the SHG measurement. The analysis of the excess charge
Fig. 4: SHG response of the CuPc/C60 OSC to the photoillumination (50 msec illumination followed by 50 msec break). accumulation and its decay at the interface provide a way to evaluate carrier lifetime at the donor-acceptor interface of the OSC. For evaluating the lifetime, we used a filtering technique that can well reproduce relaxation process in solids [14,17,18]. The filtering analysis showed that the excess negative charges accumulated with a time-constant
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171
of W ch =1.4u10-4 sec under photoillumination, whereas charges decayed in a two-step process with lifetimes W L1 =5.3u10-5 sec and W L 2 =6.2u 10-2 sec. 4. Conclusion By using the EFISHG measurement, we directly probed carrier dynamics at the interface of the double-layer (CuPc/C60) OSC. The SHG induced in the C60 showed a large response to the electric field formed in the layer with the probing laser beam at the wavelength of 1,000 nm, and enabled us to selectively probe the electric field in the C60 layer of the CuPc/C60 cell. The Maxwell-Wagner model analysis of the SHG response suggested that excessive negative charges additionally accumulated at the CuPc/C60 interface under photoillumination and caused the change of electric field distribution in the OSCs. Analysis of the SHG response showed that the accumulated negative excess charges decayed in a two-step process. We conclude that the EFISHG measurement is available to investigate carrier dynamics in OSCs and provides a way to evaluate the carrier lifetime of the OSCs. 5. Acknowledgement A part of this work was financially supported by a Grant-in-Aid for Scientific Research (S) (No. 22226007) from Japan Society for the Promotion of Science and this development was supported by SENTAN from JST.
References [1] Organic Photovoltaics, eds. C. Brabec, V. Dyakonov, U. Scherf , Wiley-VCH, Weinheim, 2008. [2] B. P. Rand, J. Genoe, P. Heremans, and J. Poortmans, Prog. Photovolt: Res. Appl. 15 (2010) 659. [3] H. Hoppe and N. S. Sariciftci, J. Mater. Res. 19 (2004) 1924. [4] C. W. Tang, Appl. Phys. Lett. 48 (1986) 183. [5] N. S. Sariciftci, L. Smilowitz, A. J. Heeger, and F. Wudl, Science 258 (1992) 1474. [6] M. Hiramoto, H. Fujiwara, and M. Yokoyama, J. Appl. Phys. 72 (1992) 3781. [7] G. Yu, J. Gao, J. C. Hummelen, F. Wudl, and A. J. Heeger, Science 270 (1995) 1789. [8] M. A. Green, K. Emery, Y. Hishikawa, and W. Warta, Prog. Photovolt: Res. Appl. 17 (2009) 85. [9] A. J. Mozer, N. S. Sariciftci, L. Lutsen, D. Vanderzande, R. Österbacka, M. Westerling, and G. Juška, Appl . Phys . Lett. 86 (2005) 112104. [10] T. Manaka, E. Lim, R. Tamura, M. Iwamoto, Nat. Photonics 1 (2007) 581. [11] M. Iwamoto, T. Manaka, M. Weis, D. Taguchi, J. Vac. Sci. Technol. B 28 (2010) C5F12. [12] D. Taguchi, M. Weis, T. Manaka, M. Iwamoto, Appl. Phys. Lett. 95 (2009) 263310. [13] D. Taguchi, S. Inoue, L. Zhang, J. Li, M. Weis, T. Manaka, M. Iwamoto, J. Phys. Chem. Lett. 1 (2010) 803. [14] D. Taguchi, L. Zhang, J. Li, M. Weis, T. Manaka, M. Iwamoto, J. Phys. Chem. C 114 (2010) 15136. [15] R. Tamura, E. Lim, T. Manaka, M. Iwamoto, J. Appl. Phys. 100 (2006) 114515. [16] E. Lim, T. Manaka, R. Tamura, M. Iwamoto, Jpn. J. Appl. Phys. 45 (2006) 3712. [17] C. R. Crowel and S. Alipanahi, Solid-State Electron. 24 (1981) 25. [18] I. Thurzo, D. Pogány, and K. Gmucová, Solid-State Electron. 35 (1992) 1737.
Physics Procedia 14 (2011) 167–171
9th International Conference on Nano-Molecular Electronics
Determination of Lifetime of Double-Layer CuPc/C60 Organic Solar Cells by Optical Electric-Field-Induced Second-Harmonic Generation Measurement Dai Taguchia, Tatsunori Shinoa, Xiangyu Chena, Le Zhanga, Jun Lia, Martin Weisb, Takaaki Manakaa, Mitsumasa Iwamotoa * a
Department of Physical Electronics, Tokyo Institute of Technology, 2-12-1 S3-33 O-okayama, Meguro-ku Tokyo 152-8552 Japan b Institute of Physics, Slovak Academy of Sciences, Dúbravská cesta 9, 845 11 Bratislava 45, Slovak Republic
Abstract By using optical electric-field-induced second-harmonic generation measurement (EFISHG), we directly probed carrier transport processes in double-layer organic solar cells (OSCs). Results showed that excessive negative charges accumulated at the CuPc/C60 interface under photoillumination, and the accumulated charges decayed in a two-step process in dark. The EFISHG measurement is available for catching carrier process at the donor/acceptor interface of multi-layer OSCs to evaluate carrier lifetime of the cell. © 2010 Published by Elsevier B.V. second-harmonic generation; organic solar cell; Maxwell-Wagner effect; space-charge field; interface
1. Introduction Organic solar cells (OSCs) are promising candidates for the next generation power device, where low-cost and printable methods are available in device fabrication [1]. Basically, OSCs are comprised of an organic layer with donor (D) and acceptor (A) molecules [2,3]. Excitons that are created in an OSC layer by photoillumination diffuse to the D-A interface to separate into electrons and holes. After the separation, electrons and holes, respectively, move to the opposite electrodes to generate a photo-voltage. In order to increase the efficiency of OSCs, novel ideas of the use of pn [4,5], pin [6] and bulk-hetero [7] structures have been proposed. As a result, the power conversion efficiency of OSCs has been improved [2,8]. However, these ideas are no longer sufficient. We need to focus on carrier lifetime in OSCs for further improvement. The electrical measurement such as photo-induced charge carrier extraction in a linearly increasing voltage (Photo-CELIV) measurement [9] would be available, but multi-carrier
* Corresponding author. Tel.: +81-3-5734-2191; fax: +81-3-5734-2191. E-mail address: [email protected].
1875-3892 © 2011 Published by Elsevier Ltd. doi:10.1016/j.phpro.2011.05.034
168
Dai Taguchi et al. / Physics Procedia 14 (2011) 167–171
(electrons and holes) transport in OSC layer complicates our understanding. On the other hand, the electric-fieldinduced optical second-harmonic generation (EFISHG) directly probes carrier motion and is very helpful to investigate carrier mechanism [10-14]. These results motivated us to use the EFISHG measurement for probing the carrier dynamics at the interface of OSCs. In this paper, using the EFISHG measurement, we study carrier dynamics, i.e., carrier generation and decay processes, in double-layer (copper-phthalocyanine (CuPc)/C60) OSCs. Analysis based on the Maxwell-Wagner model showed that excessive negative charges Qs<0 remained at the CuPc/C60 interface under photoillumination, and these charges decayed in a two step process with lifetimes W L1 =5.3u10-5 s and W L 2 =6.2u10-2 s. We conclude that the EFISHG measurement is very effective for evaluating the carrier lifetime of OSCs. 2. Experiment We prepared double-layer OSCs with an indium zinc oxide (IZO)/CuPc/C60/Al structure, as illustrated in Fig. 1.
Fig. 1. Experimental arrangement of the EFISHG measurement.
IZO-coated glass substrates were UV/ozone treated to remove organic residuals. The CuPc layer (thickness, 60 nm), C60 layer (40 nm), and Al electrode were successively deposited onto the UV/ozone treated IZO surface in vacuum. Prepared OSCs were sealed with a dry agent to avoid degradation during measurements. The working area of the OSCs was A=3.1 mm2. We also prepared single-layer IZO/CuPc/Al and IZO/C60/Al devices (thickness of CuPc and C60 layers, 100 nm) and used as a reference sample in the SHG measurement. Before the SHG measurements, the I-V characteristics of the OSCs were recorded under illumination. A red light from a lightemitting diode (wavelength 630 nm, full width at half maximum 30 nm, intensity 1 mW/cm2) was used as a light source. Note that CuPc and C60 layers absorb light at a wavelength of 630 nm, and excitons are created in the layers. By laser irradiation, EFISHG signal& is generated from organic layers due to the coupling & of electrons in molecules and electro-magnetic waves E (Z ) in the presence of electrostatic local electric field E (0) . The EFISHG intensity is given as [12]
& & & & I (2Z ) v| P (2Z ) |2 | H 0 F (3) E (0) E (Z ) E (Z ) |2 (1) & where P (2Z ) is the nonlinear polarization induced in the organic layers, H 0 is the vacuum permittivity, and F (3) & is the third order nonlinear susceptibility tensor. E (0) is the electrostatic local field in the organic layers (we define & & as that positive electric field E (0) points from the IZO electrode to the Al electrode), and E (Z ) is the electric field
of incident laser of the &SHG& intensity is& in proportion to the & beam. Equation (1) indicates that the square-root & electric field E (0) formed in the organic layers. Here, E (0) is given by E0& Es , where E0 ( Qm / H r H 0 ) is the electric field originated from charges rQm on top and bottom electrodes and Es is the electric field originated from charges in organic devices, e.g., charges Qs accumulated at the double-layer interface. Consequently, the EFISHG measurement probes carrier processes of the charges rQm on electrodes as well as charges Qs at the double-layer interface. Figure 1 portrays an experimental arrangement for the SHG measurement [12]. We used a pulsed laser as a probing light (repetition rate 10 Hz, average power 1 mW, duration 4 nsec), which was generated using an optical parametric oscillator pumped with the third-harmonic light of Q-switched Nd:YAG laser. p-polarized pulsed laser
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beam focused onto the sample surface at an incident angle of 45q, where the spot size was smaller than the device area. The reflected fundamental laser beam was eliminated using optical filters and a monochromator. The SHG light generated from the sample was detected using the photomultiplier tube (PMT), and the intensity I (2Z ) was measured using a digital multimeter. The susceptibility tensor F (3) in Eq. (1) is a material dependent parameter and is a function of the optical frequency Z . The difference in F (3) among materials allows us the electric field in each layer of a double-layer OSCs to be selectively probed by the use of an appropriate laser beam wavelength. In the present study, we used the laser beam at a wavelength of OZ 1,000 nm and recorded the generated SHG at a wavelength of O2Z =500 nm to selectively measure the electric field in C60 layer ( E2 in Fig. 1). In the time-resolved SHG measurements, we illuminated the OSCs under short-circuit condition. Photocurrent was generated under a red illumination (50 msec illumination at the intensity 1 mW/cm2 followed by 50 msec break) from the LED synchronized with the incidence laser beam. Under the short-circuit condition, excess charges Qs would be accumulated at the CuPc/C60 interface due to the Maxwell-Wagner effect [15,16], while exciton separation might be provoked at the interface. The presence of charge Qs deforms the electric field distribution in OSCs. The EFISHG probes the change of electric field, hence, catches carrier behavior in OSCs. 3. Results and Discussion A red-light illumination of the OSC induced negative photocurrent (I<0) flowing through the external circuit. The solar cell parameters were determined from I-V characteristics, and were open-circuit voltage Voc=0.37 V, shortcircuit current density Jsc=-4.4u10-6 A/cm2, and the fill-factor 0.13. Under photoillumination, excitons created in the CuPc and C60 layers diffused to the CuPc/C60 interface to produce free holes and electrons. Then the holes (electrons) were conveyed across CuPc (C60) layer to the IZO (Al) electrode, and photocurrent flowed through the external circuit. These processes result in the negative photocurrent and well accounted for the experimental result. Figure 2 shows the square-root of the SHG intensity generated in the CuPc and C60 single-layer devices under voltage application. The SHG signal of the C60 single-layer device showed linear dependence on the applied voltage. Here, voltage V application to the single-layer device forms average electric field E V / d ( d : layer thickness) in the layer, and the linear dependence in Fig. 2 suggested that the observed SHG was induced through EFISHG process expressed in Eq. (1). The voltage at the SHG signal minima was non-zero, and suggested that internal electric field was formed in the C60 single-layer device. On the other hand, CuPc showed negligibly small SHG response to the voltage application. These results confirm that we can selectively probe the electric field in C60 layer of the double-layer CuPc/C60 OSC.
Fig. 2: The square-root of the SHG intensity of the C60 and CuPc single-layer devices induced by voltage application.
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Figure 3 shows the SHG response of the CuPc/C60 OSC to the external voltage. The SHG intensity decreased on the positive voltage application to the IZO electrode (Fig. 3a), while the SHG intensity increased on the negative voltage application (Fig. 3b). Thus the changing direction of the SHG intensity allows us to determine a direction of the electric field formed in the C60 layer.
a.
b.
Fig. 3: SHG response of the CuPc/C60 OSC to A.C. square-wave voltage. (a) Positive voltage application. (b) Negative voltage application. Figure 4 shows the SHG intensity of the OSC in response to the photoillumination. The SHG intensity increased under photoillumination in a way similar to the SHG response of the OSC under negative voltage application. These results suggested that the excessive negative charge additionally accumulated at the CuPc/C60 interface under photoillumination and established steady state with the presence of photocurrent. The negative excessive charges at the interface is a source of a space charge field and additionally formed negative electric field pointing from Al to IZO electrode in the C60 layer. Thus, it was probed in the SHG measurement. The analysis of the excess charge
Fig. 4: SHG response of the CuPc/C60 OSC to the photoillumination (50 msec illumination followed by 50 msec break). accumulation and its decay at the interface provide a way to evaluate carrier lifetime at the donor-acceptor interface of the OSC. For evaluating the lifetime, we used a filtering technique that can well reproduce relaxation process in solids [14,17,18]. The filtering analysis showed that the excess negative charges accumulated with a time-constant
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of W ch =1.4u10-4 sec under photoillumination, whereas charges decayed in a two-step process with lifetimes W L1 =5.3u10-5 sec and W L 2 =6.2u 10-2 sec. 4. Conclusion By using the EFISHG measurement, we directly probed carrier dynamics at the interface of the double-layer (CuPc/C60) OSC. The SHG induced in the C60 showed a large response to the electric field formed in the layer with the probing laser beam at the wavelength of 1,000 nm, and enabled us to selectively probe the electric field in the C60 layer of the CuPc/C60 cell. The Maxwell-Wagner model analysis of the SHG response suggested that excessive negative charges additionally accumulated at the CuPc/C60 interface under photoillumination and caused the change of electric field distribution in the OSCs. Analysis of the SHG response showed that the accumulated negative excess charges decayed in a two-step process. We conclude that the EFISHG measurement is available to investigate carrier dynamics in OSCs and provides a way to evaluate the carrier lifetime of the OSCs. 5. Acknowledgement A part of this work was financially supported by a Grant-in-Aid for Scientific Research (S) (No. 22226007) from Japan Society for the Promotion of Science and this development was supported by SENTAN from JST.
References [1] Organic Photovoltaics, eds. C. Brabec, V. Dyakonov, U. Scherf , Wiley-VCH, Weinheim, 2008. [2] B. P. Rand, J. Genoe, P. Heremans, and J. Poortmans, Prog. Photovolt: Res. Appl. 15 (2010) 659. [3] H. Hoppe and N. S. Sariciftci, J. Mater. Res. 19 (2004) 1924. [4] C. W. Tang, Appl. Phys. Lett. 48 (1986) 183. [5] N. S. Sariciftci, L. Smilowitz, A. J. Heeger, and F. Wudl, Science 258 (1992) 1474. [6] M. Hiramoto, H. Fujiwara, and M. Yokoyama, J. Appl. Phys. 72 (1992) 3781. [7] G. Yu, J. Gao, J. C. Hummelen, F. Wudl, and A. J. Heeger, Science 270 (1995) 1789. [8] M. A. Green, K. Emery, Y. Hishikawa, and W. Warta, Prog. Photovolt: Res. Appl. 17 (2009) 85. [9] A. J. Mozer, N. S. Sariciftci, L. Lutsen, D. Vanderzande, R. Österbacka, M. Westerling, and G. Juška, Appl . Phys . Lett. 86 (2005) 112104. [10] T. Manaka, E. Lim, R. Tamura, M. Iwamoto, Nat. Photonics 1 (2007) 581. [11] M. Iwamoto, T. Manaka, M. Weis, D. Taguchi, J. Vac. Sci. Technol. B 28 (2010) C5F12. [12] D. Taguchi, M. Weis, T. Manaka, M. Iwamoto, Appl. Phys. Lett. 95 (2009) 263310. [13] D. Taguchi, S. Inoue, L. Zhang, J. Li, M. Weis, T. Manaka, M. Iwamoto, J. Phys. Chem. Lett. 1 (2010) 803. [14] D. Taguchi, L. Zhang, J. Li, M. Weis, T. Manaka, M. Iwamoto, J. Phys. Chem. C 114 (2010) 15136. [15] R. Tamura, E. Lim, T. Manaka, M. Iwamoto, J. Appl. Phys. 100 (2006) 114515. [16] E. Lim, T. Manaka, R. Tamura, M. Iwamoto, Jpn. J. Appl. Phys. 45 (2006) 3712. [17] C. R. Crowel and S. Alipanahi, Solid-State Electron. 24 (1981) 25. [18] I. Thurzo, D. Pogány, and K. Gmucová, Solid-State Electron. 35 (1992) 1737.