pubs.acs.org/Langmuir © 2010 American Chemical Society
Photovoltaic Devices with an Active Layer from a Stamping Transfer Technique: Single Layer Versus Double Layer Dong Hwan Wang,† Dae-Geun Choi,‡ Ki-Joong Lee,‡ O Ok Park,*,† and Jong Hyeok Park*,§ †
Department of Chemical & Biomolecular Engineering (BK 21 Graduate Program), Korea Advanced Institute of Science and Technology(KAIST), 335 Gwahangno, Yuseong-gu, Daejeon, 305-701 Republic of Korea, ‡ Nano-Mechanical Systems Research Division, Korea Institute of Machinery & Materials(KIMM), 104, Jang-dong, Yuseong-gu, Daejeon 305-343, Republic of Korea, and §Department of Chemical Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea Received January 13, 2010. Revised Manuscript Received March 8, 2010
In this study, organic photovoltaic devices with single or double-layered active film were prepared from a stamping transfer technique. A P3HT/PCBM single-layered active layer and a ratio-controlled P3HT/PCBM double-layered active can be successfully fabricated with the help of ultraviolet curable polycarbonate films via a stamping transfer technique. The maximum conversion efficiency values 2.85 for a single active layer transferred device and 3.24% for an optimized double active layer transferred device. Even though transferred double layers should have a sharp interface boundary, an intermixed zone with a concentration gradient was generated by the interpenetration of a donor-rich layer and an acceptor-rich layer in a thermal annealing process. The generation of the intermixed zone is confirmed by Auger electron spectroscopy. The enhanced conversion efficiency levels are attributed to the increased efficiency of the carrier transporting process, which is due to the fact that the concentration gradient is combined with the efficient charge generation from the bulk heterojunction layers.
Introduction Photovoltaic (PV) devices based on polymer-fullerene thin films have received considerable attention from industrial companies and academic research groups because the devices have the capability of large area optoelectronics, flexibility, and ease of fabrication.1-8 The exciton diffusion length of organic semiconductors has an extremely short limit in the 1 to 10 nm level.9,10 Hence, most of the created electron-hole charge carriers in double-layer systems can be easily recombined in an active layer before generating power from the charges that reach outside circuits. A bulk-heterojunction (BHJ) structure has therefore been considered as a promising device structure. Recent reports have suggested various means of improving solar conversion efficiency. Some of the various kinds of novel processes featured in efficient active layer design include nanopattern structures,11,12 *Corresponding authors. E-mail: (O.O.P.)
[email protected]; (J.H.P.)
[email protected]. (1) Riede, M. K.; Mueller, T.; Maennig, B.; Leo, K.; Hvid, K. O. S.; Zimmermann, B.; Niggemann, M.; Gombert, A. Appl. Phys. Lett. 2008, 92, 076101. (2) Heeger, A. J. Angew. Chem., Int. Ed. 2001, 40, 2591. (3) Thompson, B. C.; Frechet, J. M. J. Angew. Chem. Rev. 2008, 47, 58. (4) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995, 270, 1789. (5) Dennler, G.; Scharber, M. C.; Brabec, C. J. Adv. Mater. 2009, 21, 1323. (6) Blom, P. W. M.; Mihailetchi, V. D.; Koster, L. J. A.; Markov, D. E. Adv. Mater. 2007, 19, 1551. (7) Krebs, F. C. Sol. Energy Mater. Sol. Cells 2009, 93, 394. (8) Helgesen, M.; Søndergaard, R.; Krebs, F. C. J. Mater. Chem. 2010, 20, 36. (9) Choong, V.; Park, Y.; Gao, Y.; Wehrmeister, T.; Mullen, K.; Hsieh, B. R.; Tang, C. W. Appl. Phys. Lett. 1996, 69, 1492. (10) Markov, D. E.; Amsterdam, E.; Blom, P. W. M.; Sieval, A. B.; Hummelen, J. C. J. Phys. Chem. A 2005, 109, 5266. (11) Emah, J. B.; Curry, R. J.; Silva, S. R. P. Appl. Phys. Lett. 2008, 93, 103301. (12) Kim, M. S.; Kim, J. S.; Cho, J. C.; Shtein, M.; Guo, L. J.; Kim, J. Appl. Phys. Lett. 2007, 90, 123113. (13) Wang, D. H.; Im, S. H.; Lee, H. K.; Park, J. H.; Park, O. O. J. Phys. Chem. C 2009, 113, 17286. (14) Hayakawa, A.; Yoshikawa, O.; Fujieda, T.; Uehara, K.; Yoshikawa, S. Appl. Phys. Lett. 2007, 90, 163517.
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the insertion of effective interlayers,13-15 donor-acceptor ratio control,16,17 and printing techniques.18,19 Generally, the fabrication of a PV device with bilayer structures of an active layer is constrained by the common solubility of many polymeric materials in most organic solvents.20 To overcome this drawback, Huck and Kim recently reported the use of a stamping transfer technique to fabricate organic PVs. However, the efficiency of that technique is very low. More recently, Chu and Ho reported a new method of using a stamping technique to fabricate organic PVs; in case of an active layer is spin-coated on polydimethylsiloxane (PDMS) as a stamp material.17 Generally, however, PDMS is easily swelled by an organic solvent. Even though they used a nondestructive solvent treatment to spin-coat the BHJ film on top of the PDMS surface due to its instability by an organic solvent, this technique is a time-consuming process. It also has a maximum cell efficiency of around 2.7%. Another issue of bilayer devices is their low efficiency, which is due to the flat interfacial contact between the donor-acceptor layer. The low efficiency leads to an effective charge separation but only at the small interfacial regions of the two layers. In this study, we aim to solve the low efficiency problem of transfer-printed bilayer organic PVs, first by determining the optimum blend ratio for each layer and then by preparing each layer with a different P3HT/PCBM blend ratio (which is either P3HT-rich or PCBM-rich). In addition, to remove the sharp (15) Yoon, S. J.; Park, J. H.; Lee, H. K.; Park, O. O. Appl. Phys. Lett. 2008, 92, 143504. (16) Baumann, A.; Lorrmann, J.; Deibel, C.; Dyakonov, V. Appl. Phys. Lett. 2008, 93, 252104. (17) Chirvase, D.; Parisi, J.; Hummelen, J. C.; Dyakonov, V. Nanotechnology 2004, 15, 1317. (18) Huang, J. H.; Ho, Z. Y.; Kuo, T. H.; Kekuda, D.; Chu, C. W.; Ho, K. C. J. Mater. Chem. 2009, 19, 4077. (19) Yim, K. H.; Zheng, Z.; Liang, Z.; Friend, R. H.; Huck, W. T. S.; Kim, J. S. Adv. Func. Mater. 2008, 18, 1012. (20) Kumar, A.; Li, G.; Hong, Z.; Yang, Y. Nanotechnology 2009, 20, 165202.
Published on Web 03/19/2010
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interface of the top layer and bottom layers in the bilayer active film, which can deteriorate the interfacial contact, we observed the formation of an intermixed middle zone between the P3HTrich bottom part of the active layer and the PCBM-rich top part of the active layer as we controlled various kinds of annealing conditions. Furthermore, by using a novel ultraviolet- (UV-) curable polycarbonate (PC) film instead of PDMS material in the printing process, we successfully transferred the active layers of the UV-PC film to the top of poly(3,4-ethylene dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS, Baytron P) coated indium tin oxide (ITO) glass without any pretreatment (such as intermediate solvent treatment or plasma treatment). The transfer was successful because the UV-PC film has a good organic solvent wettability with smaller contact angle than PDMS stamp from the contact angle analyzer of Phoenix 450 as shown in Figure S3 (Supporting Information). Generally, conjugated polymers are an attractive alternative to the traditional silicon solar cells because they can be deposited onto flexible substrates over large areas using wet-processing techniques such as roll-to-roll coating or printing. We belive that the proposed method could prepare master films coated with respectively P3HT and PCBM rich mixture and then transfer them to flexible substrate by using roll-to-roll process.21,22 We also discuss how a TiOx interlayer between the transferred active layer and an Al cathode on the device enhances the performance of the device.
Experimental Section Preparation of UV-Curable Resin Coated PC-Film. A cleaned silicon wafer was treated by trichloro-(1H,1H,2H,2Hperfluorooctyl)silane (FOTS) to reduce the surface energy, and then the self-assembled monolayers (SAMs) were created in wafer surface to progress the stamping process. UV-curable resin of Norland Optical Adhesive was dropped between the flexible PC-film and silicon wafer, then flattened with a roller. Then the PC film was cured by UV light of 365 nm and the silicon wafer was detached. The UV-PC film of uniform morphology was realized due to smooth surface of wafer and the film size was also dependent on the area of used silicon wafer. Synthesis of Titanium Oxide Interlayer. Acetic acid exothermically reacts with titanium isopropoxide (TIP) and can form metal alkoxoacetates. While the isopropoxide groups in TIP are hydrolyzed and then condensated, the bridged acetate ligands remain bonded to titanium throughout the condensation. The typical synthesis is that the 80 g of TIP was dropped into 80 g of methanol in a flask and 16 g of glacial acetic acid was then added to the solution at room temperature. After 30 min, 5 g of deionized water (18 MΩ) was added to the previous solution drop by drop, and the reaction was allowed to stir with magnetic bar for 24 h.13
Fabrication of the Ratio Controlled Photovoltaic Devices via Stamping Transfer Technique. After cleaning ITO glass with chloroform, acetone, and 2-propanol solvents to remove residual organic impurities and materials, we subjected the glass to oxygen plasma treatment to reform the ITO surface. We then used diluted methanol to spin-coat to a thickness of ∼35 nm a hole transporting buffer layer made of a conducting polymer, namely poly(3,4-ethylene dioxythiophene):poly(styrenesulfonate) (PEDOT: PSS, Baytron P). The PEDOT:PSS-coated ITO was prebaked with a digital controllable hot plate at 200 °C for 5 min to evaporate the water in the PEDOT:PSS layer. After that, we transferred the P3HT-rich bottom layer and the PCBM-rich top layer from the UV-PC film to the PEDOT:PSS-coated ITO substrate. The transfer was conducted in accordance with the usual procedure of continual heating on a hot plate at 90 °C in air. The total thickness of 220 nm the active layer was composed from (21) Krebs, F. C.; Gevorgyan, S. A.; Alstrup, J. J. Mater. Chem. 2009, 19, 5442. (22) Krebs, F. C. Org. Electron. 2009, 10, 761.
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approximately 110 nm of P3HT-rich and 110 nm of PCBM-rich layer found from focused ion beam image (see Figure S4 (Supporting Information)), then we spin-coated a ∼3 nm thick TiOx interlayer on the printed active bilayer at 2500 rpm for 30 s.13 An aluminum (Al) metal electrode was then uniformly deposited by means of a thermal evaporator with a thickness of 150 nm under a pressure of ∼10-6 Torr. A thermal annealing process was executed at different temperatures of 70, 110, 150, and 190 °C with direct placement on a hot plate for different periods. X-ray diffraction patterns show that the stamp-transferred active layer has almost the same crystalline ordering and lattice spacing as before the stamping of the active layer and after the annealing process (see Figure S2 (Supporting Information)). The stamping transferred BHJ area on the ITO/PEDOT:PSS was confirmed the value of 1.44 cm2 while the transferred area can be realized more large area depending on the substrate of wafer size with defect free morphology (see Figure S5 (Supporting Information)). The active area with the thermally deposited Al electrode was also confined 3-6 mm2 with a video microscope (SV-35). Furthermore, the J-V (photocurrent-potential) curves were measured with an Oriel 91193, 1000 W lamp, which served as the light source for the device. To calibrate the intensity of the light, we used Keithley 2400 source measure unit with a silicon reference cell (Fraunhofer ISE, Certificate No. C- ISE269), which had an illumination rating of AM 1.5. The surface morphology and the roughness of the transferred double-layered active layer were observed with an atomic force microscope (Veeco, USA; noncontact tapping mode, D3100).
Results and Discussion To prepare PV devices with a ratio-controlled bilayer active film, we used a stamping transfer technique with a UV-curable resincoated PC-film. The film was prepared in a two-step method. The steps are shown in Figure 1, parts a and 1b. Figure 1a shows the processing steps of the UV-PC film. First, we treated a cleaned silicon wafer with trichloro-(1H,1H,2H,2H-perfluorooctyl) silane to reform the surface energy of the wafer. We then adapted the self-assembled monolayers (SAMs) to perform the stamping transfer process.23 UV-curable resin (Norland Optical Adhesive 63) was dropped between flexible PC film and a wafer substrate. The film was then flattened with a common roller to enhance the adhesive strength and remove any inner air bubbles. The resincoated PC film was cured in 365 nm UV light for 3 min, and the silicon wafer was continually detached so that it remained clear of the surface of the UV-PC film. Each active layer was either P3HTrich (6:4, 7:3, 8:2, 9:1) (110 nm) or P3HT-rich (6:4, 7:3, 8:2, 9:1) (110 nm, as shown in Figure S4 (Supporting Information)), and spin-coated from a 4 wt % solution on the UV-PC film. To confirm the active layer thickness, we observe the surface profile before and after the stamping. The active layer on the UV-PC film had almost the same film thickness as the transferred active layer (as shown in Figure S1 (Supporting Information)). Figure 1b shows the fabrication procedures for the PV device with the printed P3HT-rich/ PCBM-rich ratio-controlled bilayer film. Figure 2a shows the J-V (photocurrent-potential) curves of the stamping-transferred P3HT-rich/PCBM-rich bilayer devices and their dependence on the P3HT/PCBM ratio of the bottom or top layer. In general, the performance of a single-layered BHJ PV device performance is critically influenced by the donor/acceptor ratio because of the morphological changes of the active layer. The morphology of the active layer has a major influence on the cell performance of a BHJ device because the charge separation and charge transportation strongly and simultaneously depend (23) Choi, D. G.; Jeong, J. H.; Shim, Y. S.; Lee, E. S.; Kim, W. S.; Bea, B. S. Langmuir 2005, 21, 9390.
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Figure 2. (a) J-V curves of the stamping-transferred P3HT-rich/ PCBM-rich bilayer devices with and without a TiOx interlayer between the active layer and the Al electrode for various P3HT/ PCBM ratios. (b) Efficiency trend curve of the stamping-transferred P3HT-rich/PCBM-rich bilayer devices with a TiOx interlayer between the active layer and the Al electrode.
Figure 1. Schematic diagram of the proposed device structure via a stamping technique of the ratio-controlled P3HT-rich/PCBMrich bilayer with a UV-curable resin-coated PC film: (a) the processing steps for making the UV-curable resin-coated PC-film; (b) device fabrication with the stamping technique and the P3HTrich/PCBM-rich bilayer.
on morphology. Hence, the optimization of the weight ratio of the donor (P3HT) and the acceptor (PCBM) reveal the important steps for single-layer BHJ-based solar cells.16 The stampingtransferred P3HT-rich/PCBM-rich bilayer devices were also influenced strongly by the blend ratio of the top and bottom layers. Generally, a p-n junction bilayer junction cell has low conversion efficiency due to the bilayer; there is only a small interfacial contact area between the donor and acceptor. However, Figure 2 clearly shows that stamping-transferred bilayer devices can perform better than a BHJ device when the ratiocontrolled bilayer is optimized. In terms of charge separation, a single-layer BHJ device has a greater possibility of generating more charge carriers. However, in terms of charge transportation, the P3HT-rich layer is printed on the PEDOT-coated ITO side, which means that generated holes can be easily extracted or transported through the P-type P3HT-rich region pathways. 9586 DOI: 10.1021/la100164k
In the same manner, a PCBM-rich layer that is transferred onto a P3HT-rich layer near an Al metal electrode can easily extract and transport generated electrons through the n-type PCBM-rich pathways. A bilayer device in which the top layer has a P3HT/ PCBM ratio of 3:7 and the bottom layer has a P3HT/PCBM ratio of 7:3 shows an optimized performance of 2.50% with a short circuit current (Jsc) of 8.64 mA/cm2 and a fill factor (FF) of 0.47. This result confirms deduce that printed bilayer devices with an optimized weight ratio have increased efficiency due to the large charge generation regions of the two different BHJ structures. They also have high carrier mobility from the efficient charge transport through each donor-rich and acceptor-rich pathway. As a result, the Jsc values are enhanced. The efficiency of the printed bilayer device is also greatly increased when the TiOx interlayer is inserted between the printed active bilayer and the Al metal electrode. The bilayer device shown in Figure 2b has a top layer with a P3HT/PCBM ratio of 2:8 and a bottom layer with a P3HT/ PCBM ratio of 8:2 and these layers are combined with a 3 nmthick TiOx interlayer. This bilayer device which provided the best efficiency consistently and reproducibly has the performance of 3.24% with a short circuit current (Jsc) of 10.12 mA/cm2 and a fill factor (FF) of 0.52 (see Figure S6 (Supporting Information)). Because TiOx has the lowest unoccupied molecular orbital energy level of 4.4 eV and the highest occupied molecular orbital energy level of 8.1 eV, it is expected to perform as an exciton-blocking Langmuir 2010, 26(12), 9584–9588
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Figure 3. Three-dimensional atomic force microscope images of the morphology of the active layer of (a) a stamping-transferred P3HT-rich (6:4)/PCBM-rich (4:6) bilayer with a TiOx interlayer;, (b) a stamping-transferred P3HT-rich (7:3)/PCBM-rich (3:7) bilayer with a TiOx interlayer, (c) a stamping-transferred P3HT-rich (8:2)/PCBM-rich (2:8) bilayer with a TiOx interlayer, and (d) a stamping-transferred P3HTrich (9:1)/PCBM-rich (1:9) bilayer with a TiOx interlayer.
layer and as a hole-blocking layer because TiOx has its lowest unoccupied molecular orbital level near the Fermi level of Al and possesses a large bandgap.13 Therefore, the capacity of the TiOx interlayer to serve as a hole-blocking property with increasing the device efficiency. The enhanced efficiency can also be explained with the help of atomic force microscope images of a printed bilayer with a fairly flat surface (in terms of the root-mean-square of the surface roughness). The smooth morphology of printed bilayer devices is expected because UV-PC film is molded on a Si wafer and the next coated layer consequently has a similar surface roughness as the Si wafer, which has a very smooth surface. In addition, the regular force of 36.8 N/cm2 from the rubbing process during the transfer printing also helps ensure that the active layer has a uniform flat surface. As shown in parts a-d of Figure 3, the printed bilayer surfaces with the TiOx interlayer have a fairly flat surface with a root-mean-square of 1.5, 0.62, 1.1, and 0.84 nm, depending on the different P3HT/PCBM ratio of the top and bottom layers. The coverage of the inserted TiOx might be enhanced by these morphologies due to the small undulation of surface. Figure 2a and Table 1 confirm that devices with a TiOx interlayer in ratio-controlled P3HT-rich (6:4. 7:3, 8:2, 9:1)/ PCBM-rich (4:6, 3:7, 2:8, 1:9) printed bilayers are more efficient than devices without a TiOx interlayer. All the results show that the device with a TiOx interlayer has an enhanced Jsc value and an enhanced fill factor (FF). This outcome is attributed to the special role of TiOx improved charge carrier harvesting and compensation for the poor contact between the P3HT-rich/PCBM-rich bilayer and the Al electrode. Thus, the uniform surface morphology of the printed bilayer prepared from the stamping transfer of the Langmuir 2010, 26(12), 9584–9588
stamping technique can have more positive effects when the printed bilayer devices are combined with a uniformly coatedTiOx interlayer system. In addition to the effect of the ratio-controlled bilayer device with TiOx, the creation of an intermixed zone between different ratio-controlled P3HT-rich and PCBM-rich layers appears to play an important role in enhancing the properties of printed bilayer devices.24 Because a chlorobenzene solvent is used equally to dissolve the P3HT-rich/PCBM-rich bottom and top layers, the residual solvent in the upper PCBM-rich layer can partially swell the bottom P3HT-rich layer during the printing process. Additionally, the upper layer can be slightly penetrated and diffused to the bottom layer because of the polymer chains are softened during thermal annealing at temperatures above the glass transition temperature. Thus, an intermixed zone with a P3HT-rich and PCBM-rich concentration gradient can be definitely created between two different ratio-controlled top and bottom printed layers. This concentration graded bilayer structure induces an increase in charge generation and a separation of regions that are similar to a spin-coated single BHJ; however, it also effectively connects the P3HT-rich layer and the PCBM-rich layer. To confirm whether an intermixed zone can exist in the active layer, we used an Auger electron spectroscope for our analysis. As shown in the inset of Figure 4, P3HT has some sulfur elements in the polymer chains; these elements can be detected by means of a sulfur signal when we use the Auger electron spectroscopy. On the (24) Wang, D. H.; Lee, H. K.; Choi, D. G.; Park, J. H.; Park, O. O. Appl. Phys. Lett. 2009, 95, 043505.
DOI: 10.1021/la100164k
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Wang et al. Table 1. Efficiency Levels of Devices with a Single Active Layer Transferred or with a Ratio-Controlled P3HT-Rich (6:4. 7:3, 8:2, 9:1)/PCBM-Rich (4:6, 3:7, 2:8, 1:9) Double Active Layer Transferred by a Stamping Technique single or double active layer transferred device (vacuum: ∼10-6 Torr)
Voc (V)
Jsc (mA/cm2)
FF
eff (%)
0.57
9.21
0.54
2.85
0.60
7.26
0.52
2.29
P3HT-rich(7:3)/ PCBM-rich(3:7) with TiOx
0.60
9.66
0.48
2.80
P3HT-rich(8:2)/ PCBM-rich(2:8) with TiOx P3HT-rich(9:1)/ PCBM-rich(1:9) with TiOx
0.61
10.12
0.52
3.24
0.61
7.11
0.46
2.00
single active layer device with TiO P3HT-rich(6:4)/ PCBM-rich(4:6) with TiOx
Figure 4. An Auger electron spectroscope depth profile of an optimized double active layer transferred device with a P3HT-rich/ PCBM-rich (8:2/2:8) ratio controlled with a TiOx interlayer.
annealing temperatures. As the annealing temperature is increased from 70 to 150 °C beyond the glass temperature of P3HT,25 all of the cell values, namely Voc, Jsc, FF, and η (%) (see Table 1), are improved due to the reorganization of the polymer chains. Closed packing structures then provide act provide efficient exciton dissociation. This result confirms that the power conversion efficiency (PCE) is increased from 1.44% to 3.24% in optimized ratio-controlled printed bilayer devices after the annealing process.
Conclusions
Figure 5. J-V curves of a stamping-transferred P3HT-rich(8:2)/ PCBM-rich(2:8) bilayer device with a TiOx interlayer between the active layer and the Al electrode for different annealing temperatures.
other hand, PCBM has no sulfur content, which means it cannot be detected. We therefore analyzed the depth profiles of the ratiocontrolled bilayer active films by recording the sulfur (S) concentration of the PCBM-rich top layer and the P3HT-rich bottom layer in combination with ion-beam milling in a vertical direction. Figure 4 shows the peak-to-peak Auger signal of the optimized ratio-controlled bilayer with P3HT-rich (8:2)/PCBM-rich (2:8) active films as a function of the depth (in nanometers). The sulfur signal was slightly detected at the surface of the PCBM-rich top layer for a distance of 90 nm in a vertical inner direction. In contrast, an increased sulfur signal was observed for a distance of up to 90 nm from the top point. This result means that the optimized ratio-controlled bilayer successfully created an intermixed zone with a sharply increased gradient slope in the range 90-150 nm between the donor-rich region and the acceptor-rich region during the printing process and the thermal annealing treatment. Figure 5 shows the J-V curves of the optimized P3HT-rich (8:2)/PCBM-rich (2:8) bilayer device with a TiOx interlayer between the active layer and the Al electrode for different (25) Youngkyoo, K.; Choulis, S. A.; Nelson, J.; Bradley, D. D. C. Appl. Phys. Lett. 2005, 86, 063502.
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In conclusion, we have demonstrated that PV devices with a P3HT-rich/PCBM-rich bilayer structure and a concentration gradient can be successfully fabricated, without destruction of the underlayer, by using UV-PC films in a stamping technique. In spite of its low efficiency, we can enhance the printed bilayer device by designing the active layer with optimized ratio control of the top and bottom layers. This type of ratio control improves the charge generation and transport through each electrode. Moreover, an intermixed zone is created in a bilayer film during the printing or annealing process. Auger electron microscopy reveals that polymer PVs with an optimized P3HT-rich (8:2)/ PCBM-rich (2:8) ratio and TiOx have enhanced Jsc, FF, and PCE (%) values due to the concentration gradient in the intermixed zone. This novel method of printing enables us to fabricate a multilayer device and to initiate more in-depth research on simple printing of tandem polymer-based PVs. Acknowledgment. This work was supported by a grant from the ERC program of National Research Foundation (NRF) funded by the Korea Ministry of Education, Science and Technology (MEST) (No. R11-2007-045-01002-0(2009)). This research was also partially supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2009-0083540). D.G.C. and K.J.L. acknowledge the support from a grant (M102 KN010001-08K1401-00210) from CNMM. Supporting Information Available: Figures showing plots of surface profile data and X-ray diffraction data, images of contact angle measurement and a UV-cured resin coated polycarbonate film, and plots of average device efficiencies. This material is available free of charge via the Internet at http://pubs.acs.org. Langmuir 2010, 26(12), 9584–9588
