Article pubs.acs.org/Langmuir
Layer-by-Layer All-Transfer-Based Organic Solar Cells Jung Kyu Kim,† Wanjung Kim,† Dong Hwan Wang,‡ Haksoo Lee,† Sung M. Cho,† Dae-Geun Choi,§ and Jong Hyeok Park*,† †
SKKU Advanced Institute of Nanotechnology (SAINT) and School of Chemical Engineering, Sungkyunkwan University (SKKU), Suwon 440-746, Republic of Korea ‡ Center for Polymers and Organic Solids, University of California at Santa Barbara, Santa Barbara, California 93106-5090, United States § Nano-Mechanical Systems Research Division, Korea Institute of Machinery and Materials (KIMM), 171 Jang-dong, Yuseong-gu, Daejeon 305-343, Republic of Korea S Supporting Information *
ABSTRACT: For the first time, we describe a novel cost- and time-effective vacuum-free process to fabricate bulk-heterojunction (BHJ) organic photovoltaics (OPVs) via layer-bylayer selective stamping transfer of all layers. By controlling the surface properties of polyurethane acrylate (PUA) stamping molds with ultraviolet (UV)−ozone (UVO) exposure, poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS), BHJ layer, and metal cathode were uniformly transferred layer by layer onto each of the bottom layers. Among several interfaces between each layer, we found that the interface between the active layer and metal cathode is a critical factor in obtaining conventional device-like efficiency. To enhance the interfacial connectivity between the BHJ layer and metal cathode and increase electron extraction from the BHJ layer, a titanium oxide (TiOx) interlayer was introduced. Cell performance was optimized by controlling the concentration of TiOx solution. The poly(3-hexylthiophene-2,5-diyl)/[6,6]phenyl-C61-butyric acid methyl ester (P3HT/PC60BM) BHJ device fabricated by transferring PEDOT/PSS, TiOx/active layer, and Al cathode showed 2.01% power conversion efficiency. This efficiency is not comparable to those of conventional OPVs, but our approach shows the possibility of fabricating OPVs via the layer-by-layer transfer method for the first time.
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INTRODUCTION Organic semiconductor-based photovoltaic devices with a donor−acceptor bulk-heterojunction (BHJ) structure have alluring advantages. Large-area devices via simple and lowcost processes based on solution processing, such as roll to roll or inkjet, have driven organic photovoltaic devices (OPVs) close to commercialization.1−5 Extensive studies have been performed on the improvement of power conversion efficiencies and have demonstrated that OPVs have a great deal of possibilities as the next generation solar cell. Because of these vibrant studies, the device performance has gradually enhanced and around 10% of cell efficiency was achieved.6−10 Even though cell performances of OPVs almost fulfill the industrial requirements, the methods to realize low-cost and vacuum-free fabrication have not been fully investigated to solidify their position in the market. Fabrication of OPVs composed of a hole extracting conductive polymer, active BHJ layer, and metal cathode can be carried out by a solution process, such as spin-cast, bladecoating, screen-printing, and roll-to-roll processes, combined with metal deposition under high vacuum conditions.11−14 These techniques, however, waste large amounts of solutions or © XXXX American Chemical Society
require high-cost facilities. Deposition of the top metal electrode through a time-consuming vacuum evaporation process is also one of the impediments toward manufacturing large area devices. Therefore, studies on the fabrication of the vacuum-free process using highly conductive polymers15,16 or metal-transfer and cold-welding methods17−20 have been intensively studied. Our group and several research groups previously reported that polymer BHJ films can be easily transferred from a mother substrate to the target substrate and showed comparable cell performances to those from the spincoating process.21−24 In particular, investigation of the I−V characteristics revealed that multilayer transfer was also possible without any significant interfacial problems between poly(3,4ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/ PSS) and the active layer and between the bottom active layer and the top active layer.25 In the case of a general vacuum-free process for the preparation of top electrodes, the p-type conducting polymers Received: November 4, 2012 Revised: March 29, 2013
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Figure 1. (Top) Schematic diagram of the introduced solar cell device structure fabricated by stamping and transferring of all layers and stamping and (bottom) transferring of the Al cathode on the TiOx-coated BHJ layer with the surface-tuned regiflex mold.
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RESULTS AND DISCUSSION The stamping-transfer imprinting technique was adapted to fabricate all-layer-transferred BHJ OPVs. To realize the formation of each layer with a uniform surface, a smooth layer with low roughness is required on the stamping mold. Therefore, an ultraviolet (UV)-curable resin-coated PC rigiflex mold was cured on a very flat silicon wafer to induce a smooth surface. Because of the smooth surface of the wafer, the film had a uniform morphology and the surface of the film was coterminous with that of the wafer.21 By tuning the surface energy of the PUA/PC films with UV−ozone (UVO) treatment, the films were exploited by the stamping−transferring process. A schematic diagram for the preparation of OPVs from the all-layer transfer method is presented in Figure 1. As seen in Figure 1, PEDOT/PSS (Clevios AI4083), poly(3hexylthiophene-2,5-diyl)/[6,6]-phenyl-C61-butyric acid methyl ester (P3HT/PC60BM) BHJ layer, and metal cathode were sequentially transferred onto the indium tin oxide (ITO) substrate. The rigiflex film was hydrophobic because of the inherent property of UV-curable resin (PUA) and used for transferring the active layer. On the other hand, transferring two layers, PEDOT/PSS on ITO and the metal layer on the active layer, was achieved by UVO-treated PUA/PC films. In Figure 2, the all-layer transfer process was demonstrated. The surface energy change of PUA/PC film from hydrophobicity to hydrophilicity was dependent upon the UVO exposure time. When we consider inherent properties of the PEDOT/PSS layer, the PEDOT/PSS layer could be coated on the mold having hydrophilicity. However, if the adhesive interaction from the hydrophilicity of the mold between them was much stronger than the interaction between the ITO substrate and the surface of the PEDOT/PSS layer on the mold, it will be difficult to transfer the layer on the ITO target substrate. For this reason, we need to use the UVO-treated mold with proper hydrophilicity. Because it is very difficult to predict those interactions, we need a trial-and-error approach. After UVO
with high conductivity have been adapted on the top of the organic layer. Because of the high work function characteristics of the conducting polymers, however, there was no choice but to fabricate an inverted structural device.26−28 On the other hand, the metal-transfer and cold-welding processes can be adapted to both the inverted and normal structures.17,29 To imprint the metal layer on the organic layer with uniform morphology, polydimethyl siloxane, Teflon, polyurethane acrylate, and quartz were each used as a transferring mold. The molds were required to be very carefully prepared because the performance of metal transfer is highly dependent upon their properties.27−34 To fabricate an organic device by the vacuum-free process, the metal-imprinting process has already been exploited for organic light-emitting diodes. However, academic results regarding organic solar cells have not yet been reported because device performance is extremely sensitive to the interface properties between the cathode and the active layer, which strongly influence the carrier extraction from the organics to the electrode.35−40 Moreover, several recent studies have reported the fabrication of OPVs using active layer transfer, but significant challenges still remain in the fabrication of OPVs using dry transfer of all layers, including PEDOT/PSS, BHJ layer, interlayer, and metal cathode. In this study, vacuum-free and cost-effective fabrication was achieved using all layers of transfer for OPVs for the first time. PEDOT/PSS, active layer, and metal cathode with an interlayer were deposited layer-by-layer by the stamping-transfer technique using surface-energy-modified rigid-flexible (rigiflex) molds composed of polyurethane acrylate (PUA)-coated polycarbonate (PC) films. Furthermore, we succeed in solving the interfacial problems between the BHJ layer and metal cathode using titanium oxide (TiOx) as an interlayer. The conversion efficiency of metal-transferred device with the modified interface was increased to double the value of the only metal-transferred device. B
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were not coated on the pure PUA/PC film and the 2 min UVO-treated PUA/PC film. Via atomic force microscopy (AFM) measurements, as shown in Figure 2, it is ascertained that the PEDOT/PSS layer was transferred onto ITO and the transferred layer had lower roughness than that of the spin-cast layer because of the smooth surface of the PUA/PC film. Because no residue is left after transfer, so that the PEDOT/ PSS layer can be transferred very uniformly, and hydroxyl groups on the surface are very stable, the PUA/PC stamp can be reused without any additional treatment. It should be noted that the stamp is reusable after single UVO treatment. In the same manner, 200 nm of BHJ layers was stamptransferred on the as-coated PEDOT/PSS layer under various temperature conditions from 15 to 90 °C using the pure PUA/ PC film (UVO treatment free) having a contact angle of 90.2° (Figure 3D). BHJ layers were uniformly transferred over 75 °C, as shown in Figure 3. It was demonstrated in panels B−D of Figure 2 that we can selectively transfer polymers and metal layers onto anywhere with various shapes. To apply the TiOx interlayer between the active layer and metal cathode, as shown in Figure 1, TiO x interlayers with different solution concentrations were deposited on the PUA/PC film, followed by application of the next active layer. After this, these two layers were transferred on the top of the PEDOT/PSS layer simultaneously. In this case, the 2 min UVO-treated film (Figure 3C, with a contact angle of 83.6°) was the best stamping mold because the solution for the TiOx interlayer was alcohol-solvent-based. As the final layer for manufacturing of all-layer-transferred OPV devices, Al was thermally deposited on various surface-modified PUA/PC films. For the metal layer transfer, the 12 min UVO-treated PUA/PC film with a contact angle of 50.1° was chosen to provide the best condition. Interestingly, the Al layer on the PUA/PC film can be easily transferred in a uniform and selective fashion on the active or TiOx layer without any residues, as shown in panels C and D of Figure 2 and inset image. To investigate the morphology and the presence/absence of cracks of the transferred layers, the following two experiments were conducted. First, field emission scanning electron microscopy (FE-SEM) images from each 5 points of the transferred layers were measured. As shown in Figures S-1 and S-2 of the Supporting Information, despite the edges on the transferred layers being slightly deformed, the PEDOT/PSS and BHJ layers were uniformly transferred overall. Figure S-3 of the Supporting Information shows 5 × 5 μm topographic images of the surface of BHJ layers and TiOx-coated BHJ layers using AFM. Both of the images of transferred layers without (B) or with (D) TiOx show that the layers were uniformly transferred on the target substrate (35 nm thick PEDOT/PSS-coated ITO glass) and there was little change in the morphology between spin-coated and transferred layers. Because of the flat surface of the PUA/PC mold, the transferred layers had lower surface roughness than the spincoated layers. In this paper, the spin-coating method was adapted to prepare PEDOT/PSS and the active layer on the PUA/PC mother substrate as a model case, but the polymer-coating process could be replaced by other processes, such as dropcasting, spray, blade-printing, dip-coating, or screen-printing process, which are suitable for mass production because of their solution-based nature and flexible substrate films. Besides, even though the thermal evaporation process under vacuum was used to deposit Al on the PUA/PC film as a model case, this process can also be substituted by sputter and colloidal
Figure 2. (A) Stamping-transferred PEDOT/PSS layer on the ITO glass and (B) BHJ layer (200 nm thick) on the PEDOT/PSS-coated ITO glass (15 × 15 mm). (C) Al metal (100 nm thick) on the PUA/ PC mold film. (D) All-transfer OPV device by stamping−transferring Al cathode on the pre-transferred ITO/[PEDOT/PSS]/[BHJ and TiOx] substrate with the Al-deposited mold in panel C. (E, F, and G) AFM images of the surface of ITO glass, spin-cast PEDOT/PSS, and transferred PEDOT/PSS, respectively. The image size is 5 × 5 μm.
exposure to the film for 2, 8, and 12 min, water contact angles could be controlled, as shown in Figure 3. First, 35 nm of
Figure 3. (A−D) Water droplets (11 μL) on the PUA/PC films and their contact angles after UVO exposure and (E−J) stampingtransferred BHJ layers on the PEDOT/PSS-coated ITO glass (25 × 25 mm) under various temparatures from 15 to 90 °C.
PEDOT/PSS was coated on the 8 min treated PUA/PC film, of which the contact angle was 54.5° (Figure 3B). After this, the PEDOT/PSS layer was transferred immediately onto the precleaned ITO glass (Figure 2A) at 90 °C, followed by drying for 15 min at 150 °C. On the other hand, when the PUA/PC film was exposed for 12 min to gain more hydrophilicity (Figure 3A, with a contact angle of 50.1°), PEDOT/PSS deposited on the PUA/PC film was not transferred successfully to the ITO substrate. This might be due to the stronger interaction between the PEDOT/PSS layer and the PUA/PC film with strong hydrophilicity rather than between the PEDOT/PSS layer and the ITO substrate. PEDOT/PSS layers C
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solutions with metal nanoparticles.33,34,37 All processes, involving both transfer and spin-coating, were carried out under the conditions of Ar-filled globe box, and all of the stamping-transfer processes were conducted under 0.1 kgf of press (all of the layers were peeled off with about 2 mm/s of rate). Furthermore, PEDOT/PSS, BHJ, and Al layers had 35, 200, and 100 nm thicknesses, respectively. The each spin-cast or evaporated metal layer and transferred layer had the same thickness, which was confirmed by an α step measurement. Figure 4A and Table S-3 of the Supporting Information provide the J−V characteristics of the BHJ devices fabricated by various methods: (a) PEDOT/PSS (spin-coating)/BHJ layer (spin-coating)/Al (thermal evaporation), (b) PEDOT/PSS (transfer)/BHJ layer (transfer)/Al (thermal evaporation), and (c) PEDOT/PSS (transfer)/BHJ layer (transfer)/Al (transfer). As seen in Figure S-4 and Table S-1 of the Supporting Information, when only the PEDOT/PSS layer was transferred and combined with spin casting of the active layer and thermal evaporation of the metal cathode to fabricate OPVs, a slight lowering of efficiency was observed in comparison to conventional OPVs (device a). However, when PEDOT/PSS and the BHJ active layers were prepared by transferring and when the metal cathode was evaporated for OPVs (device b), 17% higher power conversion efficiency (PCE) was obtained in comparison to conventional OPVs (device a) made by spin casting of PEDOT/PSS and the BHJ layer with metal cathode evaporation. Because of the enhanced interfacial characteristics between PEDOT/PSS and the active layers during the pressing procedure,21,25 the polymer-transferred device showed higher cell performance: 9.24 mA/cm2 of Jsc and 53.09% of fill factor versus 7.964 mA/cm2 of Jsc and 51.43% of fill factor for the spin-cast device. However, the cell performance of the all-layertransferred device (device c) was decreased 3 times as low as the polymer-transferred device with metal evaporation (device b), which presumably demonstrated poor interfacial connection between the transferred metal and the active layer.17,34−40 To improve the connection between the active layer and cathode in the metal-transferred device, a TiOx interlayer was inserted between the two layers (Figure 1). As shown in panels B and C of Figure 4, the concentration of TiOx solution was varied to optimize the cell performances of OPVs. The effectiveness of the TiOx interlayer was much stronger in the all-layertransferred OPVs: 9.1% PCE improvement in the spin-cast device, 14.4% in the PEDOT/PSS and BHJ layers transferred device, and 97.1% in the all-layer-transfer device. Using 5 mM TiOx solution, the PCE of the all-layer-transferred device was tremendously increased by about 2 times higher than that of the TiOx-free all-layer-transferred device (from 1.02 to 2.01%). The Jsc and fill factor were also increased from 5.781 to 9.628 mA/cm2 and from 29.73 to 35.37%, respectively, because of the enhanced electron extraction and interfacial connection between the active layer and cathode. However, a thicker TiOx layer than its optimum point prohibited the contact between the active layer and metal cathode. This efficiency is the first record of OPV prepared using the all-layer-transfer method. By adapting the optimized condition of the TiOx layer, 3.76% of PCE was achieved from the layer-by-layer transfer process using poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]]/[6,6]-phenyl-C71-butric acid methyl ester (PTB7/PC70BM), as shown in Figure S-5 of the Supporting Information. The proposed method took a
Figure 4. Photocurrent potential (J−V) curves of the BHJ devices in relation to various kinds of layer-coating processes (A) without and (B) with the TiOx layer between the active and cathode layers in relation to (C) layer-coating process produced by stamping− transferring all of the layers in relation to various concentrations of the TiOx solution. Here, HEL means hole extraction layer, PEDOT/ PSS.
significant leap for the fabrication of vacuum-free organic electronics.
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CONCLUSION BHJ OPVs fabricated via layer-by-layer all transfer were achieved with surface-modified PUA/PC rigiflex films for the first time. UVO exposure time to the stamp mold was determined as a critical factor for succeeding in transferring of the PEDOT/PSS and BHJ layers, TiOx layer, and Al metal cathode. When the transferred PEDOT/PSS and BHJ layers on the ITO substrate were combined with the thermal-evaporated D
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with TiOx (Figure S-5), device performance metrics of the BHJ devices fabricated by controlling the cathode deposition technique (Table S-2), and device performance metrics of the BHJ devices (P3HT/PCBM) fabricated by controlling the HEL (PEDOT/PSS), BHJ, and metal cathode coating technique and the thickness of the TiOx layer between BHJ and cathode (Table S-3). This material is available free of charge via the Internet at http://pubs.acs.org.
metal cathode, the device showed more enhanced power conversion efficiency compared to spin-casted devices. However, to realize all-layer-transfer-based OPVs, a controlled insertion of a TiOx interlayer was an important factor. These all-layer-transfer-based OPVs provide an important viable route to future electronic devices requiring low-cost fabrication and simple patterning.
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EXPERIMENTAL SECTION
Preparation of Stamping-Transfer Molds. Pre-cleaned silicon wafer was treated by trichloro(1H,1H,2H,2H-perfluorooctyl)silane (FOTS) to diminish its surface energy, and then self-assembled monolayers (SAMs) were created on the surface. Following this, a drop of UV-curable resin of Norland Optical Adhesive (PUA) was placed on the pretreated silicon wafer surface and spread to the whole surface area by covering it with a flexible PC film and flattening with a roller. The sandwiched UV-curable resin between the wafer and the PC film was cured by UV light of 365 nm. The UV-curable resincoated PC (PUA/PC) film was detached from the wafer. Fabrication of Solar Cells. The substrate of ITO glass was prepared through the cleanging process using detergent, acetone, and isopropyl alcohol with ultrasonication. Then, ITO was exposed to UVO (20 min) to reform the surface before transferring to the PEDOT/PSS layer. An active layer was prepared by mixing P3HT (Rieke Metals, Inc.) and PC60BM (Nano-C) (1:0.6 in weight ratio). To prepare the PTB7-based active layer, the active solution was prepared by the blend of PTB7 (1-material Chemscitech, Inc.) and PC70BM (Nano-C). The blend ratio of the donor and acceptor was 10:15 (mg/mg) in mixed solution of 0.97 mL of chlorobenzene and 0.03 mL of 1,8-diiodoctane. In a typical TiOx synthesis, 8 g of titanium isopropoxide was mixed with 8 g of methanol and then 1.6 g of glacial acetic acid and 0.5 g of deionized water (18 MΩ) were added to the solution. After 24 h, the TiOx solution was subsequently diluted with methanol.35 Before Al deposition, the samples was thermally treated at 90 °C for 10 min for pre-annealing and then baked at 150 °C for 30 min after Al deposition for post-annealing. The Al cathode (thickness of 100 nm) was deposited on a PUA/PC substrate by a thermal evaporator under a vacuum pressure of 5 × 10−7 Torr. Characterization and Measurements. PCE values were obtained from photocurrent−potential measurements with Oriel 91193 and Keithley 2400 source measure units. The light source was a 1000 W lamp, and its light intensity was modified for 1 sun (100 mW/cm2) using a National Renewable Energy Laboratory (NREL)calibrated Si solar cell having an illumination rating of AM 1.5. AFM images of the surface morphology were obtained from Dimension 3100, Veeco, Plainview, NY. To measure water contact angles, a water droplet of 11 μL was placed on the surface of the stamping-transfer mold using a microsyringe at room temperature and the contact angle was estimated by a photographic method.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by a grant from the National Research Foundation (NRF) funded by the Korea Ministry of Education, Science and Technology (MEST) (20110023215, 20110027677, 2010-0029321, NCRC program (20110006268)).
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ASSOCIATED CONTENT
S Supporting Information *
SEM images of the transferred PEDOT/PSS layer on ITO glass (Figure S-1), SEM images of ITO/[PEDOT/PSS]/BHJ samples prepared by the transfer process (Figure S-2), AFM images of spin-coated BHJ layers (A) without TiOx/(C) with TiOx and transferred BHJ layers (B) without TiOx/(D) with TiOx (Figure S-3), J−V charicteristics of the BHJ OPV device fabricated by spin casting or transferring the PEDOT/PSS layer on the ITO substrate, followed by spin casting the active and TiOx layers and thermally evaporating the Al cathode (Figure S-4), device performance metrics of the BHJ devices fabricated by controlling the HEL (PEDOT/PSS) coating technique (Table S-1), J−V charicteristics of OPV devices composed of PTB7 (1-material) and PC70BM (Nano-C) BHJ fabricated by (a) thermally evaporating the Al cathode on the TiOx layer or stamping−transferring the Al cathode (b) without TiOx and (c) E
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dx.doi.org/10.1021/la400137g | Langmuir XXXX, XXX, XXX−XXX
