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Christoph Sachse,* Nelli Weiß, Nikolai Gaponik, Lars Müller-Meskamp, Alexander Eychmüller, and Karl Leo* combination with high flexibility.[4,5] Due to advantages such as performance, comThe great potential of solution-processed metal nanowire networks utilized as mercial availability and feasible synthesis a transparent electrode has attracted much attention in the last years. Typiroutes, most of the reported nanowirecally, silver nanowires are applied, although their replacement by more abunbased conducting networks consist of dant and cheaper materials is of interest. Here, a hydrazine-free synthesis silver. Furthermore, silver is a noble metal route for high aspect ratio copper nanowires is used to prepare conductive and therefore shows a rather weak oxidanetworks showing an enhanced electrode performance. The network deposition behavior which is of importance for the network stability. Hence, recently many tion is done with a scalable spray-coating process on glass and on polymer papers were published on post-processing foils. By a pressing or an annealing step, highly conductive transparent (e.g., by pressing,[6] matrix integration[7,8] electrodes are obtained, and they reveal transmittance-resistance values or plasmonic welding[9]) and improving similar to indium tin oxide (ITO) and networks made of silver nanowires. The the electrode performance[10,11] of silver application potential of the copper nanowire electrodes is demonstrated by nanowire (AgNW) networks. On the downside, silver is an expensive and scarce integrating them into an evaporated small-molecule organic solar cell with material itself and therefore might limit 3% efficiency. ultimate production scaling, which is also the case for the use of gold nanowires.[12,13] Thus, although just a small amount of −2 about 50–100 mg m is necessary for a highly conductive 1. Introduction AgNW electrode,[5] low-cost alternatives are needed. Organic solar cells are a promising technology for future low The replacement of silver by copper offers several supecost energy supply. Recently, cells with efficiencies above 10% rior key parameters such as price (ca. 100 times cheaper) and were certified and their operation lifetimes exceed industrially availability (ca. 1000 times higher abundance[14]). Very recently interesting levels.[1] Among the great advantages of thin film Rathmell et al. reported on the potential of copper nanowire solar cells is the opportunity to employ flexible substrates, ena(CuNW) networks as transparent contact layers.[15,16] bling new application fields and production technologies such Herein, we extended this approach by the following improveas roll-to-roll manufacturing. Although flexibility is an often ments: i) synthesizing CuNWs without the hazardous matementioned key issue, the proper encapsulation[2] and suitable rial hydrazine; ii) obtaining higher aspect ratio CuNWs enatransparent electrodes[3] are still challenges to be solved. The bling superior electrode performance, which is comparable to current state-of-the-art electrode material indium tin oxide AgNW based networks; iii) applying a scalable spray-coating (ITO) is rather expensive due to limited resources. Moreover, it system for large area network deposition, which works ink-free is not flexible since its highly conductive polycrystalline microwith low-boiling alcohols; and iv) investigating post preparastructure is brittle and fails when the layer is bent repeatedly. tive treatment steps, such as pressing and reduction at elevated In the last years promising results were published using metal temperatures to improve electrode performance. Finally, with nanowire networks as an alternative. As shown in the literature, annealing in hydrogen, the best performance could be achieved these electrodes offer a high conductivity and transmittance in while the networks show reasonable stability under ambient conditions. Subsequently, the CuNW electrode is integrated into a vacuum-deposited small-molecule organic solar cell with C. Sachse, Dr. L. Müller-Meskamp, Prof. K. Leo about 3% efficiency. Institut für Angewandte Photophysik Technische Universität Dresden 01069, Dresden, Germany E-mail:
[email protected];
[email protected] Dr. N. Weiß, Dr. N. Gaponik, Prof. A. Eychmüller Physikalische Chemie Technische Universität Dresden 01069, Dresden, Germany
DOI: 10.1002/aenm.201300737
Adv. Energy Mater. 2014, 4, 1300737
2. Results and Discussion 2.1. Nanowire Synthesis The copper nanowires were synthesized based on the hydrothermal method developed by Mohl et al.[17] The nanowires (NWs), which were prepared with 0.39 g glucose possessed an
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ITO-Free, Small-Molecule Organic Solar Cells on SprayCoated Copper-Nanowire-Based Transparent Electrodes
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Figure 1. Transmission electron microscopy (TEM) image of CuNWs prepared with 0.39 g glucose drop-coated onto a TEM-grid with the corresponding diameter distribution as an inset.
average diameter of 44.6 nm calculated from 200 arbitrarily chosen nanowires on transmission electron microscopy images (see example in Figure 1). These wires had been used throughout the rest of this study, unless otherwise noted. The X-ray diffraction (XRD) data of the as-prepared crystalline nanowires can be found in the Supporting Information (Figure S1). If the amount of glucose in the synthesis was doubled to 0.78 g, thinner nanowires with an average diameter of 33 nm were obtained. Lowering the temperature of the autoclave from 120 °C to 100 °C caused a diameter reduction to 24 nm for the 0.39 g glucose approach, which is consistent with results published by Jin et al.[18] All nanowire dispersions synthesized by this method showed a similar length distribution, which was extremely wide containing wires from five to some tens and even hundreds of micrometers. A small amount of nanoparticles of various shapes were also present as by-products in all cases. After synthesis, the NWs were washed with water and n-hexane to remove the excess of non-reacted species since they might deteriorate the wire-wire-contact in the network films later on. Subsequently, the solvent was exchanged and nanowires formed low concentration dispersions in isopropanol without larger aggregates. Using these nanowire dispersions, homogeneous nanowire films could be spray-coated on glass and polymer substrates (see Figure 2). In comparison with the results presented by Rathmell et al.[16] no additional dispersants or film-coating matrix materials were necessary to prepare uniform copper NW films, which largely simplifies the post-processing. 2.2. Post-Processing After film deposition, the network was not conductive, most likely due to the high junction resistance between adjacent wires in the network. Similar observations were shown for AgNW networks identifying the surfactant as probable cause.[4] In case of the CuNWs presented here and in accordance with literature, the presence of hexadecylamine (HDA) molecules should result in a gap between neighboring CuNWs of less than twice the length
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Figure 2. Copper nanowire dispersion ready for A) coating and B) a spraycoated glass substrate with C) the corresponding SEM image of the film.
of HDA, which was reported to be 2.26 nm.[19] The polyvinylpyrrolidone (PVP) shell on the surface of AgNWs produced by the most common synthesis route is assumed to result in a similar distance between adjacent wires.[4] Therefore, we expected that a fast and simple mechanical pressing step, similar to the technique successfully applied by Gaynor et al. for AgNWs should enhance the contacts and lead to better network conductivity.[6] The CuNW networks for this experiment were spray-coated on a polyethylene terephthalate (PET) foil and pressed together with a covering glass slit to avoid damage. As evidenced from the scanning electron microscopy (SEM) image in Figure 3, after pressing, the wires remained on the plastic foil due to strong adhesion. Thus, the method is sufficient to achieve a well-connected network without any delamination from the plastic foil. Pressures up to 4 GPa were applied, which affected both the conductivity and the transmittance. In our examinations the minimum pressure at which a reasonable conductivity could be achieved was 0.4 GPa. At similar wire densities, higher pressures resulted in better conductivities. Unfortunately, the total optical transmittance was decreased by the pressing process and dropped even more at higher pressures. This high transmittance loss counteracts the performance improvement (see Supporting Information, Figure S2,S3) and spoils the performance of the electrode. Although the networks were conductive after pressing, the performance values (see examples in Supporting Information Figure S2) were far below the values previously achieved for silver nanowires and thus are not sufficient for a competitive transparent electrode. As the conductivity values for silver and copper are very similar, a better performance was expected for the long, crystalline copper wires. Therefore, the conductivity and especially the inter-wire contact had to be improved. We supposed that a copper oxide layer might be responsible for the poor contact between the wires.[17,18,20] Although our X-ray diffraction (XRD)
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Figure 3. Tilted SEM image (angle of 60°) of a dense, spray-coated CuNW film after being pressed for 30 s at 3.9 GPa at room temperature.
study did not show any copper oxide related peaks (see Supporting Information, Figure S1), it should be noted that amorphous or thin nano-crystalline oxide layers are difficult to detect with XRD. An indication for a slightly oxidized Cu surface can be observed in fluorescence measurements at CuNW films on glass. The emission spectrum (excitation wavelength of 530 nm) presented as Supporting Information Figure S4 shows a clear peak around 820 nm which is attributed to oxygen vacancies in a copper(I) oxide lattice.[21] In the literature, oxidation was intensively studied on copper nanoparticles utilized for inkjet printing.[22,23] The formation of an oxide layer in air was also reported for CuNWs[17,24–26] and investigated for thin copper films.[27] Since the copper oxide layer is assumed to be the major limit for the conductivity, a reduction approach was used to convert the oxide into elementary Cu. In the reduction process the samples were heated to 175 °C under a hydrogen flow. Subsequently, the films showed a strong decrease in sheet resistance to values as low as 24 Ω/sq at 82% total transmittance (corresponding to about 88% when subtracting the glass substrate). The temperature applied during the hydrogen treatment is lower than that in other reduction experiments reported in the literature, e.g., 300 °C[17,20,28] and 400 °C,[29] but is sufficient to improve the conductivity significantly. Recently, Kholmanov et al. also reported lower temperatures of 180 °C to enhance the conductivity of copper nanowires,[30] however the annealing process and its resulting effects are not further discussed. The required temperature is crucial for the application of polymer foils. Although 175 °C is still a quite high temperature and there is certainly room for improvement towards more gentle post-processing, we were able to achieve functioning electrodes on temperature stabilized PET. The complete transmittance-resistance-data for reduced CuNW electrodes of different coverage on glass substrates is displayed in Figure 4. For comparison, the data of our standard ITO electrode on the same substrate and recent results obtained for AgNW electrodes[31] were added to the figure showing similar performance. To the best of our knowledge, this is the best CuNW performance published so far. This illustrates the advantage
Adv. Energy Mater. 2014, 4, 1300737
Figure 4. Performance graph of transparent NW electrodes obtained with our copper wires (black squares). For comparison recently published results for silver wires with 60 nm (black triangles) and 90 nm (gray circles) are shown.[31] Assuming percolation behavior the complete data sets were fitted with a figure of merit model presented by De et al. revealing a Π of around 60 for the CuNWs.[35] For comparison the ITO reference is marked with a cross symbol.
that can be gained from the very long nanowires achieved by the Mohl et al. synthesis route.[16,30,32,33] Additional experiments using nitrogen and a 10% H2-N2 gas mixture did not cause any detectable decrease in sheet resistance. This observation proves that the application of heat alone is not sufficient and a highly reducing environment is necessary to attain acceptable conductivity levels. In addition, it further backs our hypothesis, that the increase in conductivity is indeed caused by the reduction of oxide and not by the thermal desorption or rearrangement of the ligand shell. Furthermore, annealing has an influence on the transmittance spectra. As shown in Figure 5, annealing in the presence of hydrogen causes a transmittance increase in the short wavelength regime. An explanation of the origin of this effect probably needs complex calculations involving the scattering of the nanowires. In addition to the nanowire material, the substrate, the wire dimensions, and the shell environment can affect the result. Nevertheless, from our observation, we can deduce that it is probably caused by the expected reduction of copper(I) oxide (Cu2O) to elementary copper. The Cu2O in the oxide shell has a band gap of about 2.0 eV and starts absorbing in the region
