Flexible, Light‐Weight, Ultrastrong, and Semiconductive Carbon Nanotube Fibers for a Highly Efficient Solar Cell

Abstract

Carbon nanotubes have been widely introduced to fabricate high-efficiency organic solar cells because of their extremely high surface area (e.g., ca. 1600 mg 1 for single-walled nanotubes) and superior electrical properties. In one direction, nanotubes are used in electrode materials. For example, the incorporation of nanotubes onto titania nanoparticle films has been shown to increase the roughness factor and decrease the charge recombination of electron/hole pairs, and the replacement of platinum with nanotubes as counter electrode catalyzed the reduction of triiodide to improve the cell performance. In another direction, the distribution of nanotubes within the photoactive layer improved the short circuit current density and fill factor owing to rapid charge separation at the nanotube/electron donor interface and efficient electron transport through nanotubes. However, the degrees of improvement are far from what is expected for nanotubes, mainly because of random aggregation of nanotubes in the cells. For a random nanotube network, the electrons have to cross many more boundaries. Therefore, alignment of nanotubes will further greatly improve cell performance as charge transport is more efficient. Solar cells have typically been fabricated from rigid plates, which are unfavorable for many applications, especially in the fields of portable and highly integrated equipment. As a result, flexible devices have recently become the subject of active research as a good solution. In particular, weavable fiber solar cells are very promising and have attracted increasing attention in recent years. Fiber solar cells based on metal wires, glass fibers, or polymer fibers have been investigated. Herein, we first made a family of novel organic solar cells with excellent performance from the highly aligned nanotube fiber. Compared with traditional solar cells fabricated from rigid plates or recently explored flexible films/fibers, nanotube fiber solar cells demonstrate some unique and promising advantages. Firstly, as the building nanotubes are highly aligned, the fiber shows excellent electrical properties, which provide the novel solar cell with higher short-circuit photocurrent, better maximum incident monochromatic photon-to-electron conversion efficiency, and higher power conversion efficiency. Secondly, nanotube fibers show excellent mechanical properties, much stronger than Kevlar and comparable to the strongest commercial fibers of zylon and dyneema in tensile strength. Thirdly, these fibers are flexible, light-weight, and weavable and have tunable diameters ranging from micrometers to millimeters. The above properties provide nanotube fiber solar cells with a broad spectrum of applications, including power regeneration for space aircraft and clothing-integrated photovoltaics. To produce desired nanotube fibers, high-quality nanotube arrays were first synthesized by a typical chemical vapor deposition method. The synthetic details are reported elsewhere. To summarize, Fe/Al2O3 was used as the catalyst, ethylene served as the carbon source, and Ar with 6%H2 was used to carry the precursor to a tube furnace, where the growth took place. The reaction temperature was controlled at 750 8C and the reaction time was typically between 10 and 20 min. Fibers were directly spun from the nanotube array (see Figure S1 in the Supporting Information), and the fiber diameter was controlled from 6 to 20 mm by varying the initial ribbon, that is, a bunch of nanotubes pulled out of the array at the beginning of the spinning. The nanotube fiber can be spun with lengths of tens of meters or even longer, and the fiber is uniform in diameter. The density of the nanotube fiber was calculated to be on the order of 1 gcm , and its linear density was on the order of 10 mgm , relative to 10 mgm 1 and 20– 100 mgm 1 for cotton and wool yarns, respectively. As shown in Figure 1a, the nanotube fiber is flexible and will not break after being bent, folded, or even tied many times. Highresolution transmission electron microscopy (see Figure 1b) [*] T. Chen, S. Wang, Z. Yang, Q. Feng, X. Sun, Dr. L. Li, Prof. Z.-S. Wang, Prof. H. Peng Laboratory of Advanced Materials, Fudan University Shanghai 200438 (China) E-mail: zs.wang@fudan.edu.cn penghs@fudan.edu.cn T. Chen, Z. Yang, X. Sun, Dr. L. Li, Prof. H. Peng Key Laboratory of Molecular Engineering of Polymers of Ministry of Education, Fudan University (China) T. Chen, Z. Yang, X. Sun, Dr. L. Li, Prof. H. Peng Department of Macromolecular Science, Fudan University (China) S. Wang, Prof. Z.-S. Wang Department of Chemistry, Fudan University (China) [] These authors contributed equally to this work.