High-Conductivity Solution-Processed Carbon Nanotube Networks as Transparent Electrodes in Organic Solar Cells
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High-Conductivity Solution-Processed Carbon Nanotube Networks as Transparent Electrodes in Organic Solar Cells Aminy E. Ostfeld1, Siân Fogden2, Amélie Catheline1,2, Kee-Chan Kim2, Kathleen Ligsay2, Graham A. McFarlane2, and Ana Claudia Arias1 1 Dept. of Electrical Engineering and Computer Sciences, University of California, Berkeley, CA 94720, U.S.A. 2 Linde Nanomaterials, Linde LLC, 1970 Diamond Street, San Marcos, CA 92078, U.S.A.
ABSTRACT Solutions of individual, unbroken single-walled carbon nanotubes in organic solvent were fabricated in a reductive dissolution process. Transparent conductive films deposited from these organic inks gave a significantly higher conductivity to absorptivity ratio than those cast from an aqueous dispersion of carbon nanotubes. For example, films from the organic ink have achieved a sheet resistance of 250 Ω/□ with transmittance of 92% at 550 nm wavelength, compared to 76% transmittance for a 250 Ω/□ film from the aqueous dispersion. The promise of these films as transparent electrodes has been demonstrated by their incorporation into organic solar cells with power conversion efficiency of 2.3%, comparable to that of solar cells produced using indium tin oxide transparent electrodes. INTRODUCTION Networks of single-walled carbon nanotubes (CNTs) are a highly promising alternative to indium tin oxide (ITO), the leading transparent electrode material in organic optoelectronics. While ITO electrodes are brittle [1] and expensive to produce [2], films of CNTs are flexible and can be deposited from inks by a number of scalable and low-temperature processes. To date, there have been several successful demonstrations of organic solar cells using single-walled CNT transparent electrodes [3-6]. The conductivity of a transparent CNT network follows percolation theory and is further limited by the contact resistance at tube-tube junctions. Therefore, the highest-performing CNT networks are those composed of individual tubes or small bundles (which have low contact resistance and a large number of percolation pathways) of maximum length (minimizing the number of junctions) [7-9]. Reducing the amount of defects in the CNTs themselves also enhances conductivity [8]. However, the typical method of CNT ink fabrication, dispersing CNTs in water with the aid of sonication, is known to damage and shorten the nanotubes [10]. In this work, an alternative method is used to produce solutions of individual, unbroken nanotubes. First, the nanotubes are chemically reduced in the presence of alkali metal and liquid ammonia. The liquid ammonia is then removed, leaving a nanotubide salt that will dissolve spontaneously in a polar organic solvent, without the need for sonication [11-12]. Solution processed transparent conductive films were fabricated from this organic ink as well as from an aqueous dispersion of CNTs, and we report an improved conductivity-absorptivity ratio for these films relative to those made from aqueous dispersion. CNT films were successfully incorporated
into poly(3-hexylthiophene): [6
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