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Interfacial Engineering of Ultrathin Metal Film Transparent Electrode for Flexible Organic Photovoltaic Cells Jingyu Zou, Chang-Zhi Li, Chih-Yu Chang, Hin-Lap Yip,* and Alex K.-Y. Jen*
Polymer solar cells (PSC) are considered to be a promising technology for alternative renewable energy.[1–4] It has been rapidly developed with new advancements in novel light-harvesting and charge-transporting materials[5,6] together with advanced device architectures.[7–9] Currently, indium tin oxide (ITO) is the most commonly used commercially available transparent electrode for PSCs, since it possesses both high optical transparency (>80% in the visible spectrum) and low resistivity (∼10–20 Ω/ⵧ on glass). However, ITO has become the main limiting factor in making large-area devices for new generation of flexible optoelectronic applications, due to its low conductivity on flexible substrates (sheet resistance ∼60 Ω/ⵧ) and poor mechanical property. In addition, the price of ITO has been escalating in recent years due to the limited reserve of indium.[10,11] Therefore, vigorous efforts have been invested in developing alternative materials for transparent electrode, including new transparent conducting oxides,[12,13] carbon nanotubes,[14] graphene,[15] conducting polymers,[16,17] metal nanowires or meshes,[18] patterned metal grids,[19–22] and ultrathin metal films (UTMF).[23–26] However, it is quite challenging to produce devices with similar performance to those made from ITO. UTMF-based electrodes combining high electrical conductivity and good mechanical flexibility is considered as a promising candidate to replace ITO. The UTMF is also scalable for large area without the need of using complicated patterning processes like those needed for patterning metal grids. Among all metals tried, silver is considered as the best choice for UTMF due to its very low resistivity (1.62 µΩ cm), good ambient stability, and high ductility (only surpassed by Au). For application as transparent electrode, the optical transparency and conductivity of UTMFs are highly dependent on the film thickness.[23] The transparency of Ag thin film will decrease exponentially when its thickness is increased, therefore, it is critical to maintain a thickness < 10 nm in order to achieve reasonable transparency for visible light. However, Ag favors 3D island growth and the percolation threshold thickness for thermally evaporated Ag film is typically around 10–25 nm.[27] Since sheet resistance is critically dependent on the continuity and smoothness of the metal film, it is important to simultaneously achieve low percolation threshold, continuous film formation, J. Zou, Dr. C.-Z. Li, Dr. C.-Y. Chang, Dr. H.-L. Yip, Prof. A. K.-Y. Jen Department of Materials Science and Engineering University of Washington BOX 352120 Seattle, Washington 98195, USA E-mail: [email protected]; [email protected]
DOI: 10.1002/adma.201306212
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and good film quality in order to get optimal transparency and low sheet resistance. The continuity of ultrathin Ag film is governed by its nucleation and growth kinetics on substrate, which can be affected by the surface energy of the seed layer and the deposition conditions. The poor wettability of Ag on electrically insulating substrates (i.e., glass, polyethylene naphthalate (PEN)) often leads to poor film continuity due to unfavorable disparity in surface energy and poor adhesion to the substrate.[26] Therefore, it is critical to have an adequate seed layer to improve the quality of Ag film. Transparent metal oxide is commonly used as seed layer to achieve high quality UTMF film through optimized wettability and chemical interactions between metal and metal oxide.[23,28] Once the UTMF is formed, another metal oxide layer can be deposited on top of the UTMF to serve as a charge selecting layer and an optical spacer to improve the electrical and optical properties of the transparent electrode. Such metal oxide/thin metal/metal oxide tri-layer structure (MO/M/MO) has been shown to be promising as transparent electrode for organic solar cells.[25,29–31] Since most of the metal oxides are prepared under high vacuum to ensure good quality films, it is much more challenging to produce suitable oxide films for tri-layer transparent electrode due to the complexity involved in coating. Therefore, it requires precise engineering of the interfaces to achieve adequate wetting properties to produce uniform films with electrically coherent interface for efficient charge collection. Here, we report a novel protocol for achieving high quality ultrathin Ag films in the MO/M/MO tri-layer structure. Each of the interfaces was individually optimized with rationally designed functional self-assembled monolayer (SAM) to ensure the resulting electrode has very smooth surface, high conductivity, and good optical transparency. The MO/M interface was modified with a SAM that can interact with the incident metal atoms to minimize surface diffusion and enhance metal nucleation. As a result, it promotes the growing of continuous and smooth ultrathin metal films.[26] At the M/MO interface, a dipolar SAM with desired dipole can also applied to realign the interfacial energy levels for achieving Ohmic contact to improve charge collection. Finally, a fullerene-based SAM (C60-SAM) was applied on top of the MO layer to passivate the inorganic surface traps and improve interfacial exciton dissociation between MO and active layer.[32,33] With these improved interfacial properties, the resulting PSC with UTMF transparent electrode showed higher performance than those fabricated with ITO substrates. This approach can also be applied to the plastic substrates to result in highly efficient flexible PSCs with superior mechanical properties.
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COMMUNICATION Figure 1. AFM images of 10 nm ultrathin Ag films on top of (a) glass (surface roughness RMS = 6.07 nm, sheet resistance = N/A) (b) glass/ZnO (surface roughness RMS = 2.68 nm, sheet resistance = 13.59Ω/ⵧ (c) glass/ZnO/MUA (surface roughness RMS = 0.95 nm, sheet resistance = 8.61 Ω/ⵧ (d) glass/ZnO/Lauric acid (surface roughness RMS = 9.38 nm, sheet resistance = 10.87 Ω/ⵧ).
ZnO was chosen as the seed layer and also as the chargeselecting layer to fabricate the tri-layer structured UTMF due to its matched surface energy with Ag and good transparency to visible light. It can be prepared on glass and plastic substrates easily through solution processes to enable its usage for roll-toroll processing. In the case without the ZnO seed layer, discontinuous Ag islands were formed on glass with extremely low electrical conductivity (Figure 1a). By applying a sol-gel ZnO film as seed layer, a continuous thin Ag film (10 nm) could be obtained through thermal evaporation to achieve a relatively low sheet resistance (13.59 Ω/ⵧ) and a root-mean-square (RMS) roughness (2.68 nm) (Figure 1b). The quality and robustness of the thin Ag film can be further improved by applying a double-end functionalized 11-mercapto-undecanoic acid (MUA) SAM on top of ZnO as a molecular binder to covalently attach Ag and ZnO together. The carboxylic acid on the MUA will react with the hydroxyl group on ZnO surface to form an ester linkage while the thiol group will bond with Ag.[34] This SAM layer can modify the Ag growth kinetics to form a very smooth thin film with lower RMS roughness (0.95 nm), and improved sheet resistance (8.61 Ω/ⵧ) (Figure 1c).
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The resulting UTMF also showed exceptional robustness and electrical property even after being ultrasonic agitated with cleaning solvents and heat treated (the details are described in supporting information). This enables its future use for various large-area optoelectronic applications. For comparison, the lauric acid SAM which has an inert -CH3 terminal group was also applied on top of the ZnO seed layer. Although the sheet resistance of the resultant Ag films didn’t change significantly, it has a very rough surface (with a RMS roughness of 9.38 nm) (Figure 1d), which is almost one order of magnitude higher than that modified by MUA. Such a rough metal film will exacerbate light scattering to reduce transparency and increase parasitic current shunting in PSCs.[23] To avoid potential oxidation and morphological change in thin Ag films using high temperature annealing, a low temperature processed ZnO layer[35] was spin-coated on top of the UTMF to form the MO/M/MO tri-layer structure. The transparency of the resultant UTMFs was tested alongside with the ITO on glass and plastic substrates (Figure 2a). Based on the unified 10 nm targeted thickness using the same deposition condition, the Ag films on glass with isolated islands showed a dip at ∼450 nm due to the excitation of surface plasmons. The ZnO/Ag/ZnO
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Wavelength (nm) Figure 2. Transmission spectra of (a) glass, ITO-coated glass and UTMF electrodes on glass with different tri-layer structures and (b) tri-layer UTMF electrodes on PEN, together with PEN and ITO-coated PEN.
transparent electrode shows a maximum transparency of 78% at 400 nm but it gradually decrease as the wavelength is increased. The lauric acid-modified transparent electrode shows an even lower transmittance due to stronger light scattering from very rough surface. By inserting a MUA adhesion layer, the transparency of the tri-layer transparent electrode is increased more than 10% compared to that of the unmodified one. This transparency is comparable to that of ITO in the spectral range between 400 nm and 600 nm. This improved optical transparency is due to significantly reduced light scattering since its surface is very smooth. However, its transmittance is lower than ITO at the wavelengths above 600 nm due to stronger absorption and reflection nature of metal at higher wavelengths. Nevertheless, the lower transmittance of the UTMF transparent electrode in this spectral range did not really affect the light absorption of the active layer because the actual absorption is determined by the electrical field distribution within a completed PSC device structure. As it will be shown later, the increased reflection of UTMF indeed helps create a resonant cavity effect between the bottom and top Ag electrodes, which results in enhanced light harvesting in longer wavelength region.
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Figure 3. (a) Schematic drawing of the PSC devices and the molecular structure of MUA and C60-SAM employed for interfacial modifications; (b) J–V characteristics and (c) EQE of devices with ITO electrode and tri-layer UTMF electrode on both glass and PEN.
To demonstrate that the UTMF fabrication can be applied to flexible substrates, the tri-layer structure was fabricated on polyethylene naphthalate (PEN) substrate using the same condition as it was fabricated on glass. The same low sheet resistance could be achieved for the MUA-modified transparent electrode on PEN (9.00 Ω/ⵧ), which is significantly lower than that of ITO on PEN (60 Ω/ⵧ). Due to lower optical transparency of the PEN substrate (80–85%) compared to glass (90%), the corresponding tri-layer electrode has lower overall optical transparency (Figure 2b). To demonstrate the application of UTMF transparent electrodes in PSCs, they were incorporated into devices with the following configuration: glass or PEN/ZnO (40 nm)/Ag (10 nm)/ ZnO (20 nm)/PIDT-PhanQ:PC71BM (70 nm)/MoO3(5 nm)/Ag (120 nm) (Figure 3a). The active layer was based on the BHJ film of PIDT-PhanQ and PC71BM, which is a well characterized photoactive material system that constantly shows PCE over 6% in devices.[36–38] The reference devices were made with the
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Electrode
ITO on glass (15 Ω/ⵧ)
UTMF on glass (8.6 Ω/ⵧ)
ITO on plastic (60 Ω/ⵧ)
UTMF on plastic (9 Ω/ⵧ)
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Voc
Jsc
FF
PCE
[V]
[mA cm−2]
0.87
11.6
0.64
6.38
(0.87)
(11.5)
(0.64)
(6.34)
0.87
12.0
0.63
6.59
(0.87)
(11.9)
(0.63)
(6.58)
0.82
11.9
0.57
5.58
(0.82)
(11.9)
(0.56)
(5.49)
0.87
11.0
0.64
6.04
(0.86)
(11.1)
(0.63)
(6.00)
(a)
(b)
(c) Absorbed Light Intensity Fraction
Table 1. Performance of devices with ITO electrodes and tri-layer structured UTMF electrodes on both glass and PEN substrates. The Average values obtained from at least eight cells are given in brackets.
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commercially available ITO-coated (120 nm) glass or PEN substrates as the transparent electrodes. This inverted device structure has been shown to have better ambient stability[39] and is compatible with roll-to-roll coating process.[38] In addition to modify the ZnO/Ag interface, the Ag/ZnO and the ZnO/BHJ interfaces were also modified with SAMs that are tailored to improve device performance. Our previous study has shown that the dipolar MUA SAM can be used to provide desired interfacial dipole and adhesion to minimize contact resistance between the interface of Ag cathode and ZnO electron-transporting layer in the conventional structure PSCs.[34] The same strategy was also applied here to improve the contact resistance between ultrathin Ag and upper ZnO layer. In addition, a fullerene-based SAM was also used to modify the interface between top ZnO layer and BHJ to enhance electronic coupling and charge selectivity.[32] The performance of UTMF-based devices with or without interfacial modifications are summarized in supporting information (SI Table 1 and Figure SI 2). In general, devices with dual modifications of the Ag/ZnO and ZnO/BHJ interfaces, showed the best PCEs with simultaneously improved short circuit current (Jsc), open circuit voltage (Voc), and fill factor (FF). The photovoltaic parameters of PSCs fabricated from UTMF and ITO transparent electrodes on glass and plastic substrates are summarized in Table 1 and Figure 3b. It is evident that the devices with SAM modified tri-layer transparent electrode exhibit higher PCE than those employ the ITO-coated glass electrode. Since these devices have similar Voc and FF, the improved performance should come from the enhanced Jsc of UTMF-based device. The external quantum efficiency (EQE) measured from these devices show dramatic spectral differences (Figure 3c). Although the EQE of the UTMF-based device is lower than that of the ITO-based device in the spectral range between 350 and 480 nm, it is higher in the range between 480 and 700 nm. Since light harvesting in PSC is closely related to the electric field distribution and intensity within the device, field intensity profiles versus position and wavelength were simulated using transfer matrix formalism calculations to provide some insights about the difference in EQE spectra (Figure 4a and 4b). For the UTMF-based device, a strong resonant cavity effect is observed in the spectral range between 600 nm and 720 nm owing to
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the reflective nature of two Ag electrodes. It also coincides well with the absorption peak of PIDT-PhanQ at ∼650 nm, therefore, it significantly improves the absorbance of the BHJ layer
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high transparency (>80% between 400 and 600 nm) and low surface resistance (8.61 Ω/ⵧ). The inverted PSC devices based on this modified UTMF electrode show superior performance compared to those using ITO electrode. The UTMF-based flexible devices also show superior bending resilience compared to those fabricated on ITO electrodes. The EQE measurements of these UTMF electrode based devices reveal that the improved device performance is due to coherent light trapping in devices. This study offers a new and innovative approach for improving light harvesting and compatibility with roll-to-roll process for fabricating high-performance flexible PSCs.
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at longer wavelengths and leads to enhanced EQE. By combining the electric field distribution and absorption of the active layer, the profile of the light absorbance in the active layer of devices is calculated and shown in Figure 4c. The simulated result clearly shows coherent light trapping in the UTMF-based devices, which results in enhanced absorbance in the longer wavelength region, which correlates very well with the measured EQE spectra. Although the transmittance of the UTMF on PEN substrate is lower than that of the PEN/ITO electrode (∼10–20%) over the visible range (Figure 2b), more efficient light harvesting can be achieved in the completed device due to microcavity-induced light trapping. This is evident by the red-shifted EQE spectrum (Figure 3c) observed for the UTMF-coated PEN based devices since the Jsc of the resultant devices (11.0 mA/cm2) is only slightly lower than that of the ITO-based devices (11.9 mA/cm2). In spite of the lower Jsc in UTMF-based devices, higher Voc and FF can be obtained due to significantly improved electrical conductivity in UTMF-coated PEN electrode (sheet resistance ∼ 9 Ω/ⵧ) compared to that of ITO/PEN electrode (sheet resistance ∼ 60 Ω/ⵧ). As a result, higher PCE can be achieved in UTMF-based device (6.04%) than that of ITO/PEN-based device (5.64%). (Table 1 and Figure 3b). To evaluate the mechanical properties of ITO and UTMF electrode based flexible PSCs, bending test was performed on devices using a roller with a bending radius of 0.55 cm. The plot of PCE against the number of bending cycles is shown in Figure 5. The ITO-based devices exhibit dramatically decreased performance starting from the first bending cycle and show continuous decrease during the test. The PCE degrades from its original 5.58% to only 0.3% after 200 bending cycles due to ITO brittleness. On the contrary, the device with UTMF electrode is much more resilient with only a slight decrease of its original PCE (from 6.04% to 5.58%) after 200 cycles due to excellent ductility of thin metal. In conclusion, we have developed a generally applicable method for fabricating highly transparent ultrathin Ag films on both glass and plastic substrates using self-assembled monolayer modified ZnO/Ag/ZnO tri-layer structure. The resulting electrodes show remarkably low surface roughness with very
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Experimental Section Electrode Fabrication and Characterization: The substrates (glass and PEN: Teonex Q65FA) were sequentially cleaned with detergent, de-ionized water, acetone, and isopropyl alcohol. After drying, the glass substrates were treated with air-plasma for 20 s to remove any residual organic materials. ZnO precursor prepared using the method described by Sun et al.[39] was spin-coated onto the pre-cleaned glass or PEN substrates, and the films were annealed at 200 °C (for glass) or 150 °C (for PEN) for 1 h in air. Self-assembled monolayer of MUA or lauric acid was deposited by spin coating from the solution of the corresponding molecules in ethanol (1 mM), followed by rinsing with ethanol. 10 nm thick Ag films were deposited by thermal evaporation at a rate of 5 Å s−1 under vacuum pressure < 10−6 Torr. After thermal evaporation, another layer of MUA was spin-coated with same procedure described above. The transmission spectra of the electrodes were measured in air by using a Perkin-Elmer Lambda-9 UV/VIS spectrometer, while an Alessi four-point probe was used to measure the sheet resistance. Device Fabrication: ZnO precursor from zinc acetylacetonate hydrate (purchased from Aldrich and used as received) dissolved in ethanol (20 mg mL−1) was spin-coated onto the pre-cleaned ITO surface or the unmodified and SAM-modified ultrathin Ag films, and subsequently baked at 130 °C for 10 min in air.[35] The substrates were then transferred into a nitrogen-filled glovebox. The C60-SAM was deposited on the ZnO surface by spin coating from a 1 mM solution of the molecules in tetrahydrofuran (THF)/chlorobenzene (CB) (1:1 v/v) followed by rinsing with pure THF solvent. Subsequently, the PIDT-PhanQ:PC71BM active layer (ca. 70 nm) was spin-coated on top of the ZnO/C60-SAM layer. The solution was prepared by dissolving the polymer and fullerene at a weight ratio of 1:3 in o-dichlorobenzene overnight and filtered through a PTFE filter (0.2 µm). The substrates were annealed at 110 °C for 10 min prior to electrode deposition. Finally, molybdenum trioxide (5 nm) and silver (120 nm) was thermally evaporated onto the active layer sequentially under vacuum < 1 × 10−6 Torr to complete the device fabrication. Device characterization: The current-voltage (I–V) characteristics of un-encapsulated photovoltaic devices were measured under ambient conditions using a Keithley 2400 source-measurement unit. An Oriel xenon lamp (450 Watt) with an AM1.5 G filter was used as the solar simulator. The light intensity was set to be one Sun (100 mW cm−2) using a calibrated Hamamatsu silicon diode with a KG5 color filter, which can be traced to the National Renewable Energy Laboratory (NREL). The EQE system uses a lock-in amplifier (Stanford Research Systems SR830) to record the short circuit currents under chopped monochromatic light.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
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The authors thank the support from the Air Force of Scientific Research (FA9550–09–1–0426), the Office of Naval Research (N00014–11–1– 0300), and the Asian Office of Aerospace R&D (FA2386–11–1–4072). We acknowledge the use of variable angle ellipsometry provided by the UW Nanotechnology User Facility (NTUF), which is a member of the NSF National Nanotechnology Infrastructure (NNIN). A. K.-Y. J. thanks the Boeing Foundation for support. Received: December 19, 2013 Revised: February 8, 2014 Published online: March 13, 2014
[1] G. Dennler, M. C. Scharber, C. J. Brabec, Adv. Mater. 2009, 21, 1323. [2] B. C. Thompson, J. M. J. Frechet, Angew. Chem. Int. Ed 2008, 47, 58. [3] C. J. Brabec, S. Gowrisanker, J. J. M. Halls, D. Laird, S. Jia, S. P. WIlliams, Adv. Mater. 2010, 22, 3839. [4] S. B. Darling, F. You, RSC Adv. 2013, 3, 17633. [5] Y. Liang, Z. Xu, J. Xia, S.-T. Tsai, Y. Wu, G. Li, C. Ray, L. Yu, Adv. Mater. 2010, 22, E135. [6] Y.-X. Xu, C.-C. Chueh, H.-L. Yip, F.-Z. Ding, Y.-X. Li, C.-Z. Li, X. Li, W.-C. Chen, A. K.-Y. Jen, Adv. Mater. 2012, 24, 5356. [7] Z. He, C. Zhong, S. Su, M. Xu, H. Wu, Y. Cao, Nat. Photonics 2012, 6, 591. [8] L. Dou, J. You, J. Yang, C.-C. Chen, Y. He, S. Murase, T. Moriarty, K. Emery, G. Li, Y. Yang, Nat. Photonics 2012, 6, 180. [9] T. Ameri, G. Dennler, C. Lungenschmied, C. J. Brabec, Ener. Envir. Sci. 2009, 2, 347. [10] O. Inganäs, Nat. Photonics 2011, 5, 201. [11] C. G. Granqvist, Sol. Energy Mater. Sol. Cells 2007, 91, 1529. [12] E. Fortunato, D. Ginley, H. Hosono, D. C. Paine, MRS Bulletin 2007, 32, 242. [13] H. Kim, J. S. Horwitz, W. H. Kim, A. J. Mäkinen, Z. H. Kafafi, D. B. Chrisey, Thin Solid Films 2002, 420, 539. [14] R. C. Tenent, T. M. Barnes, J. D. Bergeson, A. J. Ferguson, B. To, L. M. Gedvilas, M. J. Heben, J. L. Blackburn, Adv. Mater. 2009, 21, 3210. [15] J. Wu, H. A. Becerril, Z. Bao, Z. Liu, Y. Chen, P. Peumans, Appl. Phys. Lett. 2008, 92, 263302.
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[16] Z. Tang, L. M. Andersson, Z. George, K. Vandewal, K. Tvingstedt, P. Heriksson, R. Kroon, M. R. Andersson, O. Inganäs, Adv. Mater. 2012, 24, 554. [17] Y. Xia, K. Sun, J. Ouyang, Adv. Mater. 2012. 24, 2436. [18] J.-Y. Lee, S. T. Connor, Y. Cui, P. Peumans, Nano Lett. 2008, 8, 689. [19] K. Tvingstedt, O. Inganäs, Adv. Mater. 2007, 19, 2893. [20] J. Zou, H.-L. Yip, S. K. Hau, A. K.-Y. Jen, Appl. Phys. Lett. 2010, 96, 203301. [21] D. S. Ghosh, T. L. Chen, V. Pruneri, Appl. Phys. Lett. 2010, 96, 041109. [22] M.-G. Kang, M.-S. Kim, J. Kim, L. J. Guo, Adv. Mater. 2008, 20, 4408. [23] B. O’Connor, C. Haughn, K.-H. An, K. P. Pipe, M. Shtein, Appl. Phys. Lett. 2008, 93, 223304. [24] J.-F. Salinas, H.-L. Yip, C.-C. Chueh, C.-Z. Li, J.-L. Maldonado, A. K.-Y. Jen, Adv. Mater. 2012, 24, 6362. [25] N. P. Sergeant, A. Hadipour, B. Niesen, D. Cheyns, P. Heremans, P. Peumans, B. P. Rand, Adv. Mater. 2012, 24, 728. [26] H. M. Stec, R. J. Williams, T. S. Jones, R. A. Hatton, Adv. Funct. Mater. 2011, 21, 1709. [27] R. S. Sennett, G. D. Scott, J. Opt. Soc. Am. 1950, 40, 203. [28] D. Jeannot, J. Pinard, P. Ramoni, E. M. Jost, IEEE Trans. Comp. Hybrids Manuf. Technol. 1994, 17, 17. [29] J. C. C. Fan, Appl. Phys. Lett. 1974, 25, 693. [30] H. Cho, C. Yun, J.-W. Park, S. Yoo, Org. Elec. 2009, 10, 1163. [31] D. R. Sahu, J.-L. Huang, Thin Solid Films 2006, 515, 876. [32] S. K. Hau, H.-L. Yip, H. Ma, A. K.-Y. Jen, Appl. Phys. Lett. 2008, 93, 233304. [33] C.-H. Hsieh, Y.-J. Cheng, P.-J. Li, C.-H. Chen, M. Dubosc, R.-M. Liang, C.-S. Hsu, J. Am. Chem. Soc. 2010, 132, 4887. [34] H.-L. Yip, S. K. Hau, N. S. Baek, A. K.-Y. Jen, Appl. Phys. Lett. 2008, 92, 193313. [35] C.-Y. Chang, Y.-J. Cheng, S.-H. Hung, J.-S. Wu, W.-S. Kao, C.-H. Lee, C.-S. Hsu, Adv. Mater. 2012, 24, 549. [36] Y. Zhang, J. Zou, H. L. Yip, K. S. Chen, D. F. Zeigler, Y. Sun, A. K.-Y. Jen, J. Mater. Chem. 2011, 23, 2289. [37] J. Zou, H.-L. Yip, Y. Zhang, Y. Gao, S.-C. Chien, K. O’Malley, C.-C. Chueh, H. Chen, A. K.-Y. Jen, Adv. Func. Mater. 2012, 22, 2804. [38] K.-S. Chen, H.-L. Yip, C. W. Schlenker, D. S. Ginger, A. K.-Y. Jen, Org. Electron. 2012, 13, 2870. [39] S. K. Hau, H.-L. Yip, N. S. Baek, J. Zou, K. O'Malley, A. K.-Y. Jen, Appl. Phys. Lett. 2008, 92, 253301.
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Interfacial Engineering of Ultrathin Metal Film Transparent Electrode for Flexible Organic Photovoltaic Cells Jingyu Zou, Chang-Zhi Li, Chih-Yu Chang, Hin-Lap Yip,* and Alex K.-Y. Jen*
Polymer solar cells (PSC) are considered to be a promising technology for alternative renewable energy.[1–4] It has been rapidly developed with new advancements in novel light-harvesting and charge-transporting materials[5,6] together with advanced device architectures.[7–9] Currently, indium tin oxide (ITO) is the most commonly used commercially available transparent electrode for PSCs, since it possesses both high optical transparency (>80% in the visible spectrum) and low resistivity (∼10–20 Ω/ⵧ on glass). However, ITO has become the main limiting factor in making large-area devices for new generation of flexible optoelectronic applications, due to its low conductivity on flexible substrates (sheet resistance ∼60 Ω/ⵧ) and poor mechanical property. In addition, the price of ITO has been escalating in recent years due to the limited reserve of indium.[10,11] Therefore, vigorous efforts have been invested in developing alternative materials for transparent electrode, including new transparent conducting oxides,[12,13] carbon nanotubes,[14] graphene,[15] conducting polymers,[16,17] metal nanowires or meshes,[18] patterned metal grids,[19–22] and ultrathin metal films (UTMF).[23–26] However, it is quite challenging to produce devices with similar performance to those made from ITO. UTMF-based electrodes combining high electrical conductivity and good mechanical flexibility is considered as a promising candidate to replace ITO. The UTMF is also scalable for large area without the need of using complicated patterning processes like those needed for patterning metal grids. Among all metals tried, silver is considered as the best choice for UTMF due to its very low resistivity (1.62 µΩ cm), good ambient stability, and high ductility (only surpassed by Au). For application as transparent electrode, the optical transparency and conductivity of UTMFs are highly dependent on the film thickness.[23] The transparency of Ag thin film will decrease exponentially when its thickness is increased, therefore, it is critical to maintain a thickness < 10 nm in order to achieve reasonable transparency for visible light. However, Ag favors 3D island growth and the percolation threshold thickness for thermally evaporated Ag film is typically around 10–25 nm.[27] Since sheet resistance is critically dependent on the continuity and smoothness of the metal film, it is important to simultaneously achieve low percolation threshold, continuous film formation, J. Zou, Dr. C.-Z. Li, Dr. C.-Y. Chang, Dr. H.-L. Yip, Prof. A. K.-Y. Jen Department of Materials Science and Engineering University of Washington BOX 352120 Seattle, Washington 98195, USA E-mail: [email protected]; [email protected]
DOI: 10.1002/adma.201306212
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and good film quality in order to get optimal transparency and low sheet resistance. The continuity of ultrathin Ag film is governed by its nucleation and growth kinetics on substrate, which can be affected by the surface energy of the seed layer and the deposition conditions. The poor wettability of Ag on electrically insulating substrates (i.e., glass, polyethylene naphthalate (PEN)) often leads to poor film continuity due to unfavorable disparity in surface energy and poor adhesion to the substrate.[26] Therefore, it is critical to have an adequate seed layer to improve the quality of Ag film. Transparent metal oxide is commonly used as seed layer to achieve high quality UTMF film through optimized wettability and chemical interactions between metal and metal oxide.[23,28] Once the UTMF is formed, another metal oxide layer can be deposited on top of the UTMF to serve as a charge selecting layer and an optical spacer to improve the electrical and optical properties of the transparent electrode. Such metal oxide/thin metal/metal oxide tri-layer structure (MO/M/MO) has been shown to be promising as transparent electrode for organic solar cells.[25,29–31] Since most of the metal oxides are prepared under high vacuum to ensure good quality films, it is much more challenging to produce suitable oxide films for tri-layer transparent electrode due to the complexity involved in coating. Therefore, it requires precise engineering of the interfaces to achieve adequate wetting properties to produce uniform films with electrically coherent interface for efficient charge collection. Here, we report a novel protocol for achieving high quality ultrathin Ag films in the MO/M/MO tri-layer structure. Each of the interfaces was individually optimized with rationally designed functional self-assembled monolayer (SAM) to ensure the resulting electrode has very smooth surface, high conductivity, and good optical transparency. The MO/M interface was modified with a SAM that can interact with the incident metal atoms to minimize surface diffusion and enhance metal nucleation. As a result, it promotes the growing of continuous and smooth ultrathin metal films.[26] At the M/MO interface, a dipolar SAM with desired dipole can also applied to realign the interfacial energy levels for achieving Ohmic contact to improve charge collection. Finally, a fullerene-based SAM (C60-SAM) was applied on top of the MO layer to passivate the inorganic surface traps and improve interfacial exciton dissociation between MO and active layer.[32,33] With these improved interfacial properties, the resulting PSC with UTMF transparent electrode showed higher performance than those fabricated with ITO substrates. This approach can also be applied to the plastic substrates to result in highly efficient flexible PSCs with superior mechanical properties.
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COMMUNICATION Figure 1. AFM images of 10 nm ultrathin Ag films on top of (a) glass (surface roughness RMS = 6.07 nm, sheet resistance = N/A) (b) glass/ZnO (surface roughness RMS = 2.68 nm, sheet resistance = 13.59Ω/ⵧ (c) glass/ZnO/MUA (surface roughness RMS = 0.95 nm, sheet resistance = 8.61 Ω/ⵧ (d) glass/ZnO/Lauric acid (surface roughness RMS = 9.38 nm, sheet resistance = 10.87 Ω/ⵧ).
ZnO was chosen as the seed layer and also as the chargeselecting layer to fabricate the tri-layer structured UTMF due to its matched surface energy with Ag and good transparency to visible light. It can be prepared on glass and plastic substrates easily through solution processes to enable its usage for roll-toroll processing. In the case without the ZnO seed layer, discontinuous Ag islands were formed on glass with extremely low electrical conductivity (Figure 1a). By applying a sol-gel ZnO film as seed layer, a continuous thin Ag film (10 nm) could be obtained through thermal evaporation to achieve a relatively low sheet resistance (13.59 Ω/ⵧ) and a root-mean-square (RMS) roughness (2.68 nm) (Figure 1b). The quality and robustness of the thin Ag film can be further improved by applying a double-end functionalized 11-mercapto-undecanoic acid (MUA) SAM on top of ZnO as a molecular binder to covalently attach Ag and ZnO together. The carboxylic acid on the MUA will react with the hydroxyl group on ZnO surface to form an ester linkage while the thiol group will bond with Ag.[34] This SAM layer can modify the Ag growth kinetics to form a very smooth thin film with lower RMS roughness (0.95 nm), and improved sheet resistance (8.61 Ω/ⵧ) (Figure 1c).
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The resulting UTMF also showed exceptional robustness and electrical property even after being ultrasonic agitated with cleaning solvents and heat treated (the details are described in supporting information). This enables its future use for various large-area optoelectronic applications. For comparison, the lauric acid SAM which has an inert -CH3 terminal group was also applied on top of the ZnO seed layer. Although the sheet resistance of the resultant Ag films didn’t change significantly, it has a very rough surface (with a RMS roughness of 9.38 nm) (Figure 1d), which is almost one order of magnitude higher than that modified by MUA. Such a rough metal film will exacerbate light scattering to reduce transparency and increase parasitic current shunting in PSCs.[23] To avoid potential oxidation and morphological change in thin Ag films using high temperature annealing, a low temperature processed ZnO layer[35] was spin-coated on top of the UTMF to form the MO/M/MO tri-layer structure. The transparency of the resultant UTMFs was tested alongside with the ITO on glass and plastic substrates (Figure 2a). Based on the unified 10 nm targeted thickness using the same deposition condition, the Ag films on glass with isolated islands showed a dip at ∼450 nm due to the excitation of surface plasmons. The ZnO/Ag/ZnO
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Wavelength (nm) Figure 2. Transmission spectra of (a) glass, ITO-coated glass and UTMF electrodes on glass with different tri-layer structures and (b) tri-layer UTMF electrodes on PEN, together with PEN and ITO-coated PEN.
transparent electrode shows a maximum transparency of 78% at 400 nm but it gradually decrease as the wavelength is increased. The lauric acid-modified transparent electrode shows an even lower transmittance due to stronger light scattering from very rough surface. By inserting a MUA adhesion layer, the transparency of the tri-layer transparent electrode is increased more than 10% compared to that of the unmodified one. This transparency is comparable to that of ITO in the spectral range between 400 nm and 600 nm. This improved optical transparency is due to significantly reduced light scattering since its surface is very smooth. However, its transmittance is lower than ITO at the wavelengths above 600 nm due to stronger absorption and reflection nature of metal at higher wavelengths. Nevertheless, the lower transmittance of the UTMF transparent electrode in this spectral range did not really affect the light absorption of the active layer because the actual absorption is determined by the electrical field distribution within a completed PSC device structure. As it will be shown later, the increased reflection of UTMF indeed helps create a resonant cavity effect between the bottom and top Ag electrodes, which results in enhanced light harvesting in longer wavelength region.
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Figure 3. (a) Schematic drawing of the PSC devices and the molecular structure of MUA and C60-SAM employed for interfacial modifications; (b) J–V characteristics and (c) EQE of devices with ITO electrode and tri-layer UTMF electrode on both glass and PEN.
To demonstrate that the UTMF fabrication can be applied to flexible substrates, the tri-layer structure was fabricated on polyethylene naphthalate (PEN) substrate using the same condition as it was fabricated on glass. The same low sheet resistance could be achieved for the MUA-modified transparent electrode on PEN (9.00 Ω/ⵧ), which is significantly lower than that of ITO on PEN (60 Ω/ⵧ). Due to lower optical transparency of the PEN substrate (80–85%) compared to glass (90%), the corresponding tri-layer electrode has lower overall optical transparency (Figure 2b). To demonstrate the application of UTMF transparent electrodes in PSCs, they were incorporated into devices with the following configuration: glass or PEN/ZnO (40 nm)/Ag (10 nm)/ ZnO (20 nm)/PIDT-PhanQ:PC71BM (70 nm)/MoO3(5 nm)/Ag (120 nm) (Figure 3a). The active layer was based on the BHJ film of PIDT-PhanQ and PC71BM, which is a well characterized photoactive material system that constantly shows PCE over 6% in devices.[36–38] The reference devices were made with the
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Electrode
ITO on glass (15 Ω/ⵧ)
UTMF on glass (8.6 Ω/ⵧ)
ITO on plastic (60 Ω/ⵧ)
UTMF on plastic (9 Ω/ⵧ)
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Voc
Jsc
FF
PCE
[V]
[mA cm−2]
0.87
11.6
0.64
6.38
(0.87)
(11.5)
(0.64)
(6.34)
0.87
12.0
0.63
6.59
(0.87)
(11.9)
(0.63)
(6.58)
0.82
11.9
0.57
5.58
(0.82)
(11.9)
(0.56)
(5.49)
0.87
11.0
0.64
6.04
(0.86)
(11.1)
(0.63)
(6.00)
(a)
(b)
(c) Absorbed Light Intensity Fraction
Table 1. Performance of devices with ITO electrodes and tri-layer structured UTMF electrodes on both glass and PEN substrates. The Average values obtained from at least eight cells are given in brackets.
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commercially available ITO-coated (120 nm) glass or PEN substrates as the transparent electrodes. This inverted device structure has been shown to have better ambient stability[39] and is compatible with roll-to-roll coating process.[38] In addition to modify the ZnO/Ag interface, the Ag/ZnO and the ZnO/BHJ interfaces were also modified with SAMs that are tailored to improve device performance. Our previous study has shown that the dipolar MUA SAM can be used to provide desired interfacial dipole and adhesion to minimize contact resistance between the interface of Ag cathode and ZnO electron-transporting layer in the conventional structure PSCs.[34] The same strategy was also applied here to improve the contact resistance between ultrathin Ag and upper ZnO layer. In addition, a fullerene-based SAM was also used to modify the interface between top ZnO layer and BHJ to enhance electronic coupling and charge selectivity.[32] The performance of UTMF-based devices with or without interfacial modifications are summarized in supporting information (SI Table 1 and Figure SI 2). In general, devices with dual modifications of the Ag/ZnO and ZnO/BHJ interfaces, showed the best PCEs with simultaneously improved short circuit current (Jsc), open circuit voltage (Voc), and fill factor (FF). The photovoltaic parameters of PSCs fabricated from UTMF and ITO transparent electrodes on glass and plastic substrates are summarized in Table 1 and Figure 3b. It is evident that the devices with SAM modified tri-layer transparent electrode exhibit higher PCE than those employ the ITO-coated glass electrode. Since these devices have similar Voc and FF, the improved performance should come from the enhanced Jsc of UTMF-based device. The external quantum efficiency (EQE) measured from these devices show dramatic spectral differences (Figure 3c). Although the EQE of the UTMF-based device is lower than that of the ITO-based device in the spectral range between 350 and 480 nm, it is higher in the range between 480 and 700 nm. Since light harvesting in PSC is closely related to the electric field distribution and intensity within the device, field intensity profiles versus position and wavelength were simulated using transfer matrix formalism calculations to provide some insights about the difference in EQE spectra (Figure 4a and 4b). For the UTMF-based device, a strong resonant cavity effect is observed in the spectral range between 600 nm and 720 nm owing to
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the reflective nature of two Ag electrodes. It also coincides well with the absorption peak of PIDT-PhanQ at ∼650 nm, therefore, it significantly improves the absorbance of the BHJ layer
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high transparency (>80% between 400 and 600 nm) and low surface resistance (8.61 Ω/ⵧ). The inverted PSC devices based on this modified UTMF electrode show superior performance compared to those using ITO electrode. The UTMF-based flexible devices also show superior bending resilience compared to those fabricated on ITO electrodes. The EQE measurements of these UTMF electrode based devices reveal that the improved device performance is due to coherent light trapping in devices. This study offers a new and innovative approach for improving light harvesting and compatibility with roll-to-roll process for fabricating high-performance flexible PSCs.
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at longer wavelengths and leads to enhanced EQE. By combining the electric field distribution and absorption of the active layer, the profile of the light absorbance in the active layer of devices is calculated and shown in Figure 4c. The simulated result clearly shows coherent light trapping in the UTMF-based devices, which results in enhanced absorbance in the longer wavelength region, which correlates very well with the measured EQE spectra. Although the transmittance of the UTMF on PEN substrate is lower than that of the PEN/ITO electrode (∼10–20%) over the visible range (Figure 2b), more efficient light harvesting can be achieved in the completed device due to microcavity-induced light trapping. This is evident by the red-shifted EQE spectrum (Figure 3c) observed for the UTMF-coated PEN based devices since the Jsc of the resultant devices (11.0 mA/cm2) is only slightly lower than that of the ITO-based devices (11.9 mA/cm2). In spite of the lower Jsc in UTMF-based devices, higher Voc and FF can be obtained due to significantly improved electrical conductivity in UTMF-coated PEN electrode (sheet resistance ∼ 9 Ω/ⵧ) compared to that of ITO/PEN electrode (sheet resistance ∼ 60 Ω/ⵧ). As a result, higher PCE can be achieved in UTMF-based device (6.04%) than that of ITO/PEN-based device (5.64%). (Table 1 and Figure 3b). To evaluate the mechanical properties of ITO and UTMF electrode based flexible PSCs, bending test was performed on devices using a roller with a bending radius of 0.55 cm. The plot of PCE against the number of bending cycles is shown in Figure 5. The ITO-based devices exhibit dramatically decreased performance starting from the first bending cycle and show continuous decrease during the test. The PCE degrades from its original 5.58% to only 0.3% after 200 bending cycles due to ITO brittleness. On the contrary, the device with UTMF electrode is much more resilient with only a slight decrease of its original PCE (from 6.04% to 5.58%) after 200 cycles due to excellent ductility of thin metal. In conclusion, we have developed a generally applicable method for fabricating highly transparent ultrathin Ag films on both glass and plastic substrates using self-assembled monolayer modified ZnO/Ag/ZnO tri-layer structure. The resulting electrodes show remarkably low surface roughness with very
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Experimental Section Electrode Fabrication and Characterization: The substrates (glass and PEN: Teonex Q65FA) were sequentially cleaned with detergent, de-ionized water, acetone, and isopropyl alcohol. After drying, the glass substrates were treated with air-plasma for 20 s to remove any residual organic materials. ZnO precursor prepared using the method described by Sun et al.[39] was spin-coated onto the pre-cleaned glass or PEN substrates, and the films were annealed at 200 °C (for glass) or 150 °C (for PEN) for 1 h in air. Self-assembled monolayer of MUA or lauric acid was deposited by spin coating from the solution of the corresponding molecules in ethanol (1 mM), followed by rinsing with ethanol. 10 nm thick Ag films were deposited by thermal evaporation at a rate of 5 Å s−1 under vacuum pressure < 10−6 Torr. After thermal evaporation, another layer of MUA was spin-coated with same procedure described above. The transmission spectra of the electrodes were measured in air by using a Perkin-Elmer Lambda-9 UV/VIS spectrometer, while an Alessi four-point probe was used to measure the sheet resistance. Device Fabrication: ZnO precursor from zinc acetylacetonate hydrate (purchased from Aldrich and used as received) dissolved in ethanol (20 mg mL−1) was spin-coated onto the pre-cleaned ITO surface or the unmodified and SAM-modified ultrathin Ag films, and subsequently baked at 130 °C for 10 min in air.[35] The substrates were then transferred into a nitrogen-filled glovebox. The C60-SAM was deposited on the ZnO surface by spin coating from a 1 mM solution of the molecules in tetrahydrofuran (THF)/chlorobenzene (CB) (1:1 v/v) followed by rinsing with pure THF solvent. Subsequently, the PIDT-PhanQ:PC71BM active layer (ca. 70 nm) was spin-coated on top of the ZnO/C60-SAM layer. The solution was prepared by dissolving the polymer and fullerene at a weight ratio of 1:3 in o-dichlorobenzene overnight and filtered through a PTFE filter (0.2 µm). The substrates were annealed at 110 °C for 10 min prior to electrode deposition. Finally, molybdenum trioxide (5 nm) and silver (120 nm) was thermally evaporated onto the active layer sequentially under vacuum < 1 × 10−6 Torr to complete the device fabrication. Device characterization: The current-voltage (I–V) characteristics of un-encapsulated photovoltaic devices were measured under ambient conditions using a Keithley 2400 source-measurement unit. An Oriel xenon lamp (450 Watt) with an AM1.5 G filter was used as the solar simulator. The light intensity was set to be one Sun (100 mW cm−2) using a calibrated Hamamatsu silicon diode with a KG5 color filter, which can be traced to the National Renewable Energy Laboratory (NREL). The EQE system uses a lock-in amplifier (Stanford Research Systems SR830) to record the short circuit currents under chopped monochromatic light.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
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The authors thank the support from the Air Force of Scientific Research (FA9550–09–1–0426), the Office of Naval Research (N00014–11–1– 0300), and the Asian Office of Aerospace R&D (FA2386–11–1–4072). We acknowledge the use of variable angle ellipsometry provided by the UW Nanotechnology User Facility (NTUF), which is a member of the NSF National Nanotechnology Infrastructure (NNIN). A. K.-Y. J. thanks the Boeing Foundation for support. Received: December 19, 2013 Revised: February 8, 2014 Published online: March 13, 2014
[1] G. Dennler, M. C. Scharber, C. J. Brabec, Adv. Mater. 2009, 21, 1323. [2] B. C. Thompson, J. M. J. Frechet, Angew. Chem. Int. Ed 2008, 47, 58. [3] C. J. Brabec, S. Gowrisanker, J. J. M. Halls, D. Laird, S. Jia, S. P. WIlliams, Adv. Mater. 2010, 22, 3839. [4] S. B. Darling, F. You, RSC Adv. 2013, 3, 17633. [5] Y. Liang, Z. Xu, J. Xia, S.-T. Tsai, Y. Wu, G. Li, C. Ray, L. Yu, Adv. Mater. 2010, 22, E135. [6] Y.-X. Xu, C.-C. Chueh, H.-L. Yip, F.-Z. Ding, Y.-X. Li, C.-Z. Li, X. Li, W.-C. Chen, A. K.-Y. Jen, Adv. Mater. 2012, 24, 5356. [7] Z. He, C. Zhong, S. Su, M. Xu, H. Wu, Y. Cao, Nat. Photonics 2012, 6, 591. [8] L. Dou, J. You, J. Yang, C.-C. Chen, Y. He, S. Murase, T. Moriarty, K. Emery, G. Li, Y. Yang, Nat. Photonics 2012, 6, 180. [9] T. Ameri, G. Dennler, C. Lungenschmied, C. J. Brabec, Ener. Envir. Sci. 2009, 2, 347. [10] O. Inganäs, Nat. Photonics 2011, 5, 201. [11] C. G. Granqvist, Sol. Energy Mater. Sol. Cells 2007, 91, 1529. [12] E. Fortunato, D. Ginley, H. Hosono, D. C. Paine, MRS Bulletin 2007, 32, 242. [13] H. Kim, J. S. Horwitz, W. H. Kim, A. J. Mäkinen, Z. H. Kafafi, D. B. Chrisey, Thin Solid Films 2002, 420, 539. [14] R. C. Tenent, T. M. Barnes, J. D. Bergeson, A. J. Ferguson, B. To, L. M. Gedvilas, M. J. Heben, J. L. Blackburn, Adv. Mater. 2009, 21, 3210. [15] J. Wu, H. A. Becerril, Z. Bao, Z. Liu, Y. Chen, P. Peumans, Appl. Phys. Lett. 2008, 92, 263302.
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[16] Z. Tang, L. M. Andersson, Z. George, K. Vandewal, K. Tvingstedt, P. Heriksson, R. Kroon, M. R. Andersson, O. Inganäs, Adv. Mater. 2012, 24, 554. [17] Y. Xia, K. Sun, J. Ouyang, Adv. Mater. 2012. 24, 2436. [18] J.-Y. Lee, S. T. Connor, Y. Cui, P. Peumans, Nano Lett. 2008, 8, 689. [19] K. Tvingstedt, O. Inganäs, Adv. Mater. 2007, 19, 2893. [20] J. Zou, H.-L. Yip, S. K. Hau, A. K.-Y. Jen, Appl. Phys. Lett. 2010, 96, 203301. [21] D. S. Ghosh, T. L. Chen, V. Pruneri, Appl. Phys. Lett. 2010, 96, 041109. [22] M.-G. Kang, M.-S. Kim, J. Kim, L. J. Guo, Adv. Mater. 2008, 20, 4408. [23] B. O’Connor, C. Haughn, K.-H. An, K. P. Pipe, M. Shtein, Appl. Phys. Lett. 2008, 93, 223304. [24] J.-F. Salinas, H.-L. Yip, C.-C. Chueh, C.-Z. Li, J.-L. Maldonado, A. K.-Y. Jen, Adv. Mater. 2012, 24, 6362. [25] N. P. Sergeant, A. Hadipour, B. Niesen, D. Cheyns, P. Heremans, P. Peumans, B. P. Rand, Adv. Mater. 2012, 24, 728. [26] H. M. Stec, R. J. Williams, T. S. Jones, R. A. Hatton, Adv. Funct. Mater. 2011, 21, 1709. [27] R. S. Sennett, G. D. Scott, J. Opt. Soc. Am. 1950, 40, 203. [28] D. Jeannot, J. Pinard, P. Ramoni, E. M. Jost, IEEE Trans. Comp. Hybrids Manuf. Technol. 1994, 17, 17. [29] J. C. C. Fan, Appl. Phys. Lett. 1974, 25, 693. [30] H. Cho, C. Yun, J.-W. Park, S. Yoo, Org. Elec. 2009, 10, 1163. [31] D. R. Sahu, J.-L. Huang, Thin Solid Films 2006, 515, 876. [32] S. K. Hau, H.-L. Yip, H. Ma, A. K.-Y. Jen, Appl. Phys. Lett. 2008, 93, 233304. [33] C.-H. Hsieh, Y.-J. Cheng, P.-J. Li, C.-H. Chen, M. Dubosc, R.-M. Liang, C.-S. Hsu, J. Am. Chem. Soc. 2010, 132, 4887. [34] H.-L. Yip, S. K. Hau, N. S. Baek, A. K.-Y. Jen, Appl. Phys. Lett. 2008, 92, 193313. [35] C.-Y. Chang, Y.-J. Cheng, S.-H. Hung, J.-S. Wu, W.-S. Kao, C.-H. Lee, C.-S. Hsu, Adv. Mater. 2012, 24, 549. [36] Y. Zhang, J. Zou, H. L. Yip, K. S. Chen, D. F. Zeigler, Y. Sun, A. K.-Y. Jen, J. Mater. Chem. 2011, 23, 2289. [37] J. Zou, H.-L. Yip, Y. Zhang, Y. Gao, S.-C. Chien, K. O’Malley, C.-C. Chueh, H. Chen, A. K.-Y. Jen, Adv. Func. Mater. 2012, 22, 2804. [38] K.-S. Chen, H.-L. Yip, C. W. Schlenker, D. S. Ginger, A. K.-Y. Jen, Org. Electron. 2012, 13, 2870. [39] S. K. Hau, H.-L. Yip, N. S. Baek, J. Zou, K. O'Malley, A. K.-Y. Jen, Appl. Phys. Lett. 2008, 92, 253301.
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