Open Access
Add to BibSonomyAbstract
The aluminum and sliver multilayered nano-grating structure is fabricated by laser interference lithography and the intervals between nanoslits is filled with modified PEDOT:PSS. The grating structured transparent electrode functions as the anti-reflection layer which not only decreases the reflected light but also increases the absorption of the active layer. The performances of P3HT:PC61BM solar cells are studied experimentally and theoretically in detail. The field intensities of the transverse magnetic (TM) and transverse electrical (TE) waves distributed in the active layer are simulated by rigorous coupled wave analysis (RCWA). The power conversion efficiency of the plasmonic ITO-free polymer solar cell can reach 3.64% which is higher than ITO based polymer solar cell with efficiency of 3.45%.
© 2014 Optical Society of America
1. Introduction
Organic photovoltaic (OPV) devices have drawn a great deal of attention for energy- generation applications in recent years due to their advantages such as low cost, flexibility, light weight, large area etc. Many efforts have focused on designing new materials, device structures, and processing techniques to improve the solar cell efficiency. So far, polymer solar cells (PSCs) based on conjugated polymers blended with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) have achieved ~8% conversion efficiencies using a bulk heterojunction device structure [1–4]. However, current PSCs are typically fabricated on indium tin oxide (ITO) substrates and do not make full use of the processing advantages of organic materials. Therefore, there is an increasing demand for replacing ITO with alternative materials. To meet to this need, thin metals, metal grids, and conducting polymers are being developed. Although many studies [5–11] have successfully demonstrated the possibility of substituting the ITO with other transparent electrode layers in PSCs, the conversion efficiency of PSCs remained low. So far, the conversion efficiency of ITO-free P3HT/PCBM based solar cells on glass with large active area (>1 cm2) are around 1~2% and those of the devices with small active area (<0.25 cm2) are around ~3% [12–16]. Among the fabrication processes mentioned above, poly(3,4-ethylenedioxythiophene) (PEDOT) is widely used [12–16] and regarded as one of the most promising material for optoelectronic organic devices because of its high conductivity and transparency .
Another approach to enhance the conversion efficiency of PSCs is to use nano- particles or nano-structure to generate the plasmonic effects. To optimize the plasmonic effects in the devices, proper selection of nano-particle parameters such as size, shape and surface density are critical [17–19]. In general, nano-particle layer is deposited by spin coating. However, it is very hard to control the density of nano-particles on the surface [20,21]. As for the fabrication of nano-structure, one of the most common methods is to anneal the metal thin film [18]. Although the fabrication can be precisely controlled, the nano-structure is random.
In this study, PEDOT and nano-structure are combined together. As a result, a nano-grating composed of aluminum and sliver whose nanoslits were filled by PEDOT:PSS is used to replace the ITO layer. The nano-grating structure is fabricated by laser interference lithography [22] and the intervals between nanoslits are filled with modified PEDOT:PSS. The grating structure not only exhibits low sheet resistance (10 Ω/sq) comparable to ITO, but also has high transparency in the wavelength range of 400-700 nm. In addition, it can reduce the reflected light and form constructive interference in the photon-absorption active layer. To further understand the function of grating in solar cell, we use rigorous coupled wave analysis (RCWA) method to simulate the field intensities distributed in the active layer. The transverse magnetic (TM) wave results in a plasmonic wave on the surface of grating and electrode. On the other hand, the transverse electrical (TE) wave forms a strong constructive interference in the active layer under certain thickness (180 nm) of the devices. The simulated results agree well with previous reports [5,6].
2. Experiments
Figure 1(a) shows the fabrication process of the nano-grating structure. The photo-resist pattern was formed by laser interference lithography on an anti-reflective coating (ARC) layer [22]. The ARC (XHRiC-11, Brewer Science) layer was deposited to prevent the reflected light from damaging the photo-resist pattern. The laser beam at 325 nm wavelength was split into two pathways and eventually merged and interfered on the photo-resist and thereby generated the grating pattern (2.25 cm2). Then the photo-resist pattern was etched by the reactive ion etching (RIE) process. A metal layer composed of a single 20 nm thick aluminum layer (sample C) or adding an extra 2 nm thick buffer silver (sample A) layer were evaporated on and lifted off to form the nano-grating. To remove the ARC layer, the sample was put into the H2O2-NH4OH-H2O solution at 80°C for 5 minutes. Then the modified PEDOT:PSS (PH500 with 5% ethylene glycol, conductivity: 406 S cm−1) was spin coated on the sample and annealed at 120°C for 20 minutes to fill the slits and form the planarized transparent conductor. Finally, the metallic grating structure with a period of 500 nm and the line width of 200 nm on the glass substrate was completed. The sheet resistance of the grating structure was measured by the Hall system (HL 5500).
Fig. 1 (a) Schematic fabrication processes of nano-grating structure. (b) Schematic diagrams of the polymer solar cell structures on different transparent electrodes. Left is sample A, and right is sample B.
Figure 1(b) displays the solar cell structure on nano-grating (sample A) and ITO glass (sample B). A solar cell deposited directly on Al nano-grating without Ag buffer layer (sample C) was also fabricated for comparison. The device definition and performance are listed in Table 1.The layer structures of the devices consisted of glass substrate/ transparent electrode layer/ PEDOT:PSS / P3HT:PC61BM (~220 nm) /LiF (0.5 nm)/Al (150 nm). The ITO layer (70 nm) of sample B was deposited on the glass by sputtering. The PEDOT:PSS layer was then spin coated on the electrode and annealed at 120°C for 20 minutes. Subsequent deposition parameters were all the same for three samples. The active layer (P3HT:PC61BM (1:1 by weight)) was also spin coated and annealed at 120°C for another 20 minutes. The LiF layer and Al layer were deposited by evaporation. The active area (0.25 cm2) is defined by the area of Al electrode. The electrical characteristics of the solar cells were measured in atmosphere at room temperature. To measure the efficiency of solar cells, the devices were illuminated at 100 mW/cm2 from a 150 W Oriel solar simulator using an air mass 1.5 global (AM 1.5G) filter to obtain the current density–voltage (J–V) curve.
Table 1. The device performance of polymer solar cells with different transparent electrode layers.
Table 1 lists the measured short circuit current Jsc, open circuit voltage Voc, fill factor (FF), conversion efficiency η, average efficiencies PCEAVG, and standard deviation PCEσ over 16 devices. Although using the Al grating nanostructure as electrode can increase the Jsc of the device, sample C exhibits lower FF and lower Voc which lead to the lower conversion efficiency. The performance of device is improved by introducing the Ag buffer layer (i.e., sample A). Due to the improvement in the Rs and Rsh, the efficiency of sample A can reach 3.64% which is better than that (3.45%) of sample B using the ITO electrode.
To measure the period and the width of the nano-grating, scanning electron microscopy (SEM) images of the nano-grating were taken. Figures 2(a) and 2(b) show the SEM image of the nano-grating after the lift-off process and after the nanoslits were filled by PEDOT:PSS, respectively. To estimate the surface roughness and morphology of the sample, the nanostructure was further measured by atomic force microscopy (AFM) as shown in Fig. 2(c). The slight roughness at the interface can increase the interfacial area for carrier collection [23]. However, the large roughness may result in a shorted device. The root-mean-square (rms) roughness of the sample A is 4.39 nm. To make sure that the PEDOT:PSS is indeed filled into the slit and covered the well, SEM cross-sectional images were also taken as shown in Fig. 2(d).
Fig. 2 Scanning electron microscopy images of (a) nano-grating structure with a period of 500 nm and the line width of 200 nm on the glass substrate. (b) nano-grating structure whose slits are filled by PEDOT:PSS on the glass substrate. (c) atomic force microscopy (AFM) of (b). (d) SEM cross-sectional image of (b).
3. Results and discussion
To clarify the function of the grating structure, the reflection and absorption spectra of the transparent electrode on glass substrates were measured by the UV/Visible spectrophotometer (SHIMADZU UV-1650P) as shown in Figs. 3(a) and 3(b). The insets of Fig. 3(a) shows the device structures used for optical measurement. The reflectances of nano-gratings (samples A' and C') decreased to less than 5% from 400 to 600 nm as compared to ITO/PEDOT electrode (sample B') which exceeds 10% in the same range. It is because the periodic grating can improve the light scattering at the interface and guide more light into the active layer, which is similar to the increased light scattering in periodic structures used in light-emitting-diodes [6]. However, the absorption of sample A' from 400 to 600 nm is higher than that of sample B'. Consequently, the improvement in the external quantum efficiency (EQE) and the absorption spectra of sample from 400 to 600 nm are not obvious.
Fig. 3 (a) The reflection spectra of the glass substrates coated with different transparent electrode layers. The (b) absorption spectra.
The absorption of the P3HT:PC61BM active layer of samples A, B and C can also be measured and shown in Fig. 4(a).The insets of Fig. 4(a) shows the device structures used for optical measurement. Here, the absorption spectra can be obtained through the equation: A = 1-R-T [24], where A is the absorption, R is the reflection, and T is transmission of the samples. The absorption of samples A and C are enhanced in the wavelength range of 400~700 nm which also agrees with previous reports [5,6]. Moreover, the EQE of samples A, B and C were measured and shown in Fig. 4(b). Due to the excitation of waveguide mode from TE wave and the surface plasmon polariton (SPP) [5,6] from TM wave, both spectra of samples A and C are enhanced dramatically around 640 nm. To prove that the TM wave can excite the SPP mode, the absorption spectra with different periods are simulated and shown in Fig. 4(c). The simulated results show that the wavelength of SPP is linearly proportional to period, which can be regarded as a SPP mode [5,6].
Fig. 4 (a) The absorption spectra of the solar cells with different transparent electrode layers. The (b) EQE spectra, (c) The simulated results for the absorption spectra at different periods, and (d) J-V characteristics of samples A, B and C.
According to the EQE spectra, the Jsc of the device can be calculated by [5,6], where IAM1.5 is the reference solar spectral irradiance. The calculated Jsc of sample A, B and C are 9.23, 8.42 and 9.31 mA/cm2, respectively. The slight difference between the Jsc from EQE and PCE results are due to their different measurement light source. Even so, the Jsc from EQE measurement and PCE results still exhibit similar trends.
Figure 4(d) shows the J–V curves of samples A, B and C. The extracted device parameters are listed in Table 1. The Rs and the Rsh are calculated by the inverse of the slope of curve at V = 1 and V = 0, respectively [24].
To further understand the effects of TE and TM waves, the field intensities distribution in active layer of samples A, B and C were simulated by rigorous coupled wave analysis (RCWA) as shown in Figs. 5(a), 5(b), and 5(c), respectively. TE and TM waves with wavelength of 640 nm are incident onto these samples. The black dash line in Fig. 5(a) and Fig. 5(c) is the interface of the glass and transparent electrode. The active layers are located at the area between the red dash lines. It can be seen that the TM wave can generate the SPP mode near the metal surface which is mainly dominated by the period of the structure. On the other hand, the TE wave can excite the waveguide mode at the active layer which agrees with previous results [5,6]. The enhancement of the intensities can increase the absorption of the active layer which results in the improvement of the Jsc. Figure 5(b) displays the electrical and magnetic field intensity distribution for TE and TM wave in sample B. The black dash line is the interface between the glass and ITO layer. The active layers are located in the area between the two red dash lines, and the maximum intensities are lower than those of the devices with grating as shown in Figs. 5(a) and 5(c). The intensities in samples A and C are almost the same as the thickness of Ag buffer layer is only 2 nm. However, by introducing the buffer layer, the FF and Voc of the devices can be improved due to the matching of the work function between the active layer and the electrode. Figure 5(d) shows the SEM cross-sectional image of Si/PEDOT:PSS/P3HT:PCBM. The thickness of the PEDOT:PSS and P3HT:PCBM active layers are about 40 nm and 220 nm thick, respectively, which were used in the simulation.
Fig. 5 The field intensity distribution under TE and TM wave in samples (a) A, (b) B, and (c) C. (d) SEM cross- sectional image of Si/ PEDOT:PSS/P3HT:PCBM.
It is noticed that the maximum field in the devices with the grating is much more localized whereas the field in the devices without the grating is uniform. Therefore, the power absorption in the active layer was calculated. The absorbed power UA of active layer can be calculated by , where E is the electric field, ε is the spatially dependent permittivity, and V is the volume. If we only compare sample A with sample B, the ratio of the EQE spectra around the peak is 1.29, which is close to the ratio of the calculated absorbed power (1.37).
It is important to understand the relation between conductivity and FF. The grating structure exhibits low sheet resistance (10 Ω/sq), whereas the sheet resistance of ITO layer prepared by sputtering was measured to be 50 Ω/sq. The conductivity of the electrode directly impacts the Rs of the devices and the uniformity of the electrode layer [25–27] mainly impacts the Rsh, which is related to the leakage current. The Rs and Rsh will dominate the FF of the devices. Here, as the Rs and Rsh are known, we can calculate the theoretical FF of the solar cells [21]. The theoretical FF of the devices are 0.69, 0.68 and 0.61, respectively.
Due to the hydrophobicity of the metal thin film, PEDOT:PSS layer is hard to cover well on the metallic electrode in the past. Therefore, it is hard to predict the electrical characteristics of the device. Here, we only consider the band theory regardless of the sequence of device procedure. The ideal Voc is dominated by the difference in potential between LUMO of acceptor and the HOMO of donor [25–31] when the contact is perfect Ohmic contact. As a result, it is important to form an Ohmic contact on the electrode [24]. The anode of sample C is Al/PEDOT:PSS which is used for hole collection. However, the work function of Al (4.3 eV) is mismatch with the work function of PEDOT:PSS (5.1eV) and the HOMO of P3HT (5.2 eV). The mismatch of the work function will easily result in a Schottky contact, which leads to the decrease of the Voc. On the other hand, by introducing the Ag buffer layer (sample A), the mismatch of the work function can be solved and the Voc can be improved from 0.56 V to 0.6 V. It is noted that the work function of Ag is initially around 4.3 eV and will be closed to 5.0 eV due the oxidization [24–27] which is more effective for hole collection [27–29] and much close to those of PEDOT and P3HT. Because of the better ability for hole collection, and the well match in work function, the Rs is dramatically improved from 5.42 (Ω/sq) to 1.41 (Ω/sq). The Rsh of the device is also increased from 762 (Ω/sq) to 1212 (Ω/sq) due to the reduction of the leakage current at the interface of electrode. Hence, the FF can be enhanced from 54% to 66% and the efficiency of sample A can be improved from 2.91% to 3.64%.
4. Conclusions
In conclusion, the nano-grating structure filled by PEDOT:PSS layer as the transparent electrode is fabricated by laser interference lithography. The electrode can enhance the field intensities at the active layer and increase the absorption of the active layer. The TM wave can generate the SPP mode, and the TE wave can excite the wave guide mode at the active layer. The thin Ag buffer layer is used to reduce series resistance and match the work function between the electrode and active layer. Hence, the FF and Voc of the solar cell can be improved without sacrificing the transmission of the electrode layer. The power conversion efficiency of the plasmonic ITO-free polymer solar cell can reach as high as 3.64%.
Acknowledgments
The authors would like to thank the National Science Council of the Republic of China, the Center for Emerging Materials and Advanced Devices, and the Photonic Advanced Research Center of the National Taiwan University, for financial support under contracts of NSC 100-2120-M-002-014, 10R80908-4; 10R7b07-4, NSC-102-2221-E-002-205- MY3, 102R70607-4; 102R3401-1, NTU-CESRP-102R7607-2 and NTU-ICRP-102R7558, 100-2221-E-002-161-MY2.
References and links
1. L. Dou, J. You, J. Yang, C. C. Chen, Y. He, S. Murase, T. Moriarty, K. Emery, G. Li, and Y. Yang, “Tandem polymer solar cells featuring a spectrally matched low-bandgap polymer,” Nat. Photonics 6(3), 180–185 (2012). [CrossRef]
2. J. You, C. C. Chen, L. Dou, S. Murase, H. S. Duan, S. A. Hawks, T. Xu, H. J. Son, L. Yu, G. Li, and Y. Yang, “Metal oxide nanoparticles as an electron-transport layer in high-performance and stable inverted polymer solar cells,” Adv. Mater. 24(38), 5267–5272 (2012). [CrossRef] [PubMed]
3. L. Dou, J. Gao, E. Richard, J. You, C. C. Chen, K. C. Cha, Y. He, G. Li, and Y. Yang, “Systematic investigation of benzodithiophene- and diketopyrrolopyrrole-based low-bandgap polymers designed for single junction and tandem polymer solar cells,” J. Am. Chem. Soc. 134(24), 10071–10079 (2012). [CrossRef] [PubMed]
4. G. Li, R. Zhu, and Y. Yang, “Polymer solar cells,” Nat. Photonics 6(3), 153–161 (2012). [CrossRef]
5. C. Min, J. Li, G. Veronis, J. Y. Lee, S. Fan, and P. Peumans, “Enhancement of optical absorption in thin-film organic solar cells through the excitation of plasmonic modes in metallic gratings,” Appl. Phys. Lett. 96(13), 133302 (2010). [CrossRef]
6. Y. Zhan, J. Zhao, C. Zhou, X. Wang, Y. P. Li, and Y. Li, “Surface plasma coupled photovoltaic cell with double layered triangular grating,” IEEE Photonics J. 4(3), 1021–1026 (2012). [CrossRef]
7. K. Tvingstedt and O. Inganäs, “Electrode grids for ITO-free organic photovoltaic devices,” Adv. Mater. 19(19), 2893–2897 (2007). [CrossRef]
8. J. Meiss, M. K. Riede, and K. Leo, “Towards efficient tin-doped indium oxide ITO-free inverted organic solar cells using metal cathodes,” Appl. Phys. Lett. 94(1), 013303 (2009). [CrossRef]
9. Y. Galagan, J.-E. J. M. Rubingh, R. Andriessen, C.-C. Fan, P. W. M. Blom, S. C. Veenstra, and J. M. Kroon, “ITO free flexible organic solar cells with printed current collecting grids,” Sol. Energy Mater. Sol. Cells 94, 1339–1343 (2011). [CrossRef]
10. J. Meiss, N. Allinger, M. Riede, and K. Leo, “Improved light harvesting in tin doped indum oxide ITO free inverted bulk heterojunction organic solar cells using capping layers,” Appl. Phys. Lett. 93(10), 103311 (2008). [CrossRef]
11. S. I. Na, S. S. Kim, J. Jo, and D. Y. Kim, “Efficient and flexible ITO free organic solar cells using highly conductive polymer anodes,” Adv. Mater. 20(21), 4061–4067 (2008). [CrossRef]
12. T. R. Andersen, H. F. Dam, B. Andreasen, M. Hösel, M. V. Madsen, S. A. Gevorgyan, R. R. Søndergaard, M. Jørgensen, and F. C. Krebs, “A rational method for developing and testing stable flexible indium- and vacuum-free multilayer tandem polymer solar cells comprising up to twelve roll processed layers,” Sol. Energy Mater. Sol. Cells 120, 735–743 (2014). [CrossRef]
13. D. Angmo, S. A. Gevorgyan, T. T. Larsen-Olsen, R. R. Sondergaard, M. Hosel, M. Jorgensen, R. Gupta, G. U. Kulkarni, and F. C. Krebs, “Scalability and stability of very thin, roll-to-roll processed, large area, indium-tin-oxide free polymer solar cell modules,” Org. Electron. 14(3), 984–994 (2013). [CrossRef]
14. R. R. Søndergaard, M. Hösel, and F. C. Krebs, “Roll-to-roll fabrication of large area functional organic materials,” J. Polym. Sci. Pol. Phys. 51(1), 16–34 (2013). [CrossRef]
15. D. Angmo, I. Gonzalez-Valls, S. Veenstra, W. Verhees, S. Sapkota, S. Schiefer, B. Zimmermann, Y. Galagan, J. Sweelssen, M. Lira-Cantu, R. Andriessen, J. M. Kroon, and F. C. Krebs, “Low-cost upscaling compatibility of five different ITO-free architectures for polymer solar cells,” J. Appl. Polym. Sci. 130(2), 944–954 (2013). [CrossRef]
16. N. Espinosa, F. O. Lenzmann, S. Ryley, D. Angmo, M. Hosel, R. R. Søndergaard, D. Huss, S. Dafinger, S. Gritsch, J. M. Kroon, M. Jørgensen, and F. C. Krebs, “OPV for mobile applications: an evaluation of roll-to- roll processed indium and silver free polymer solar cells through analysis of life cycle, cost and layer quality using inline optical and functional inspection tools,” J. Mater. Chem. A 1(24), 7037–7049 (2013). [CrossRef]
17. B. Luk’yanchuk, N. I. Zheludev, S. A. Maier, N. J. Halas, P. Nordlander, H. Giessen, and C. T. Chong, “The Fano resonance in plasmonic nanostructures and metamaterials,” Nat. Mater. 9(9), 707–715 (2010). [CrossRef] [PubMed]
18. H. A. Atwater and A. Polman, “Plasmonics for improved photovoltaic devices,” Nat. Mater. 9(3), 205–213 (2010). [CrossRef] [PubMed]
19. Y. A. Akimov and W. S. Koh, “Resonant and nonresonant plasmonic nanoparticle enhancement for thin-film silicon solar cells,” Nanotechnology 21(23), 235201 (2010). [CrossRef] [PubMed]
20. C. I. Ho, D. J. Yeh, V. C. Su, C. H. Yang, P. C. Yang, M. Y. Pu, C. H. Kuan, I. C. Cheng, and S. C. Lee, “Plasmonic multilayer nanoparticles enhanced photocurrent in thin film hydrogenated amorphous silicon solar cells,” J. Appl. Phys. 112(2), 023113 (2012). [CrossRef]
21. C. E. Petoukhoff, D. K. Vijapurapu, and D. M. O’Carroll, “Computational comparison of conventional and inverted organic photovoltaic performance parameters with varying metal electrode surface workfunction,” Sol. Energy Mater. Sol. Cells 120, 572–583 (2014). [CrossRef]
22. M. Y. Lin, H. H. Chen, K. H. Hsu, Y. H. Huang, Y. J. Chen, H. Y. Lin, Y. K. Wu, L. A. Wang, C. C. Wu, and S. C. Lee, “White organic light emitting diode with linearly polarized emission,” IEEE Photonics Technol. Lett. 25, 1321–1323 (2013).
23. N. Sekine, C. H. Chou, W. L. Kwan, and Y. Yang, “ZnO nano-ridge structure and its application in inverted polymer solar cell,” Org. Electron. 10(8), 1473–1477 (2009). [CrossRef]
24. M. Y. Lin, C. Y. Lee, S. C. Shiu, I. J. Wang, J. Y. Sun, W. H. Wu, Y. H. Lin, J. S. Huang, and C. F. Lin, “Sol gel processed CuOx thin film as an anode interlayer for inverted polymer solar cells,” Org. Electron. 11(11), 1828–1834 (2010). [CrossRef]
25. M. Y. Liu, C. H. Chang, C. H. Chang, K. H. Tsai, J. S. Huang, C. Y. Chou, I. J. Wang, P. S. Wang, C. Y. Lee, C. H. Chao, C. L. Yeh, C. I. Wu, and C. F. Lin, “Morphological evolution of the poly(3- hexylthiophene)/[6,6]-phenyl-C61-butyric acid methyl ester, oxidation of the silver electrode, and their influences on the performance of inverted polymer solar cells with a sol–gel derived zinc oxide electron selective layer,” Thin Solid Films 518(17), 4964–4969 (2010). [CrossRef]
26. M. Zhang, T.-L. Chiu, C.-F. Lin, J.-H. Lee, J.-K. Wang, and Y. Wu, “Roughness characterization of silver oxide anodes for use in efficient top-illuminated organic solar cells,” Sol. Energy Mater. Sol. Cells 95(9), 2606–2609 (2011). [CrossRef]
27. H. L. Yip, S. K. Hau, N. S. Baek, H. Ma, and A. K. Y. Jen, “Polymer solar cells that use self-assembled- monolayer-modified ZnO/metals as cathodes,” Adv. Mater. 20(12), 2376–2382 (2008). [CrossRef]
28. T. Ameri, G. Dennler, C. Waldauf, H. Azimi, A. Seemann, K. Forberich, J. Hauch, M. Scharber, K. Hingerl, and C. J. Brabec, “Fabrication, optical modeling, and color characterization of semitransparent bulk- heterojunction organic solar cells in an inverted structure,” Adv. Funct. Mater. 20(10), 1592–1598 (2010). [CrossRef]
29. F. Zhang, X. Xu, W. Tang, J. Zhang, Z. Zhuo, J. Wang, J. Wang, Z. Xu, and Y. Wang, “Recent development of the inverted configuration organic solar cells,” Sol. Energy Mater. Sol. Cells 95(7), 1785–1799 (2011). [CrossRef]
30. R. Steim, F. R. Kogler, and C. J. Brabec, “Interface materials for organic solar cells,” J. Mater. Chem. 20(13), 2499–2512 (2010). [CrossRef]
31. V. D. Mihailetchi, P. W. M. Blom, J. C. Hummelen, and M. T. Rispens, “Cathode dependence of the open- circuit voltage of polymer:fullerene bulk heterojunction solar cells,” J. Appl. Phys. 94(10), 6849–6854 (2003). [CrossRef]
