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We propose a self-organized micro-honeycomb network  structure of single-walled carbon nanotubes (SWNTs)  obtained by water vapor treatment of as-synthesized  vertically aligned SWNTs (VA-SWNTs) [1] for SWNT/nSi heterojunction solar cell [2-6]. Dependence of photovoltaic conversion efficiency (PCE) on the nan otube network structure was examined. The VA-SWNTs was synthesized by the standard alcohol-catalytic CVD method with Co/Mo di pcoated on Si/SiO2 substrate [1]. The fabrication process of the micro-honeycomb structured film involved only t wo steps. First, an as-synthesized VA-SWNTs film was exposed to water vapor by hanging over heated water . Then, the film was dried by simply turning over the film upwards. By carefully controlling the temperature a nd pressure of water vaporization, different morpholog ies of SWNT film as shown in Fig. 1 were obtained. Fig. 1 (a) shows a quasi-regular honeycomb structure made from 5 μm thick VA-SWNT film. By the vapor treatment, collapsed spaghetti-like SWNTs make contact to the substrate in the middle of each honeycomb cell. Cel l wal s consist of vertically aligned SWNTs with heavily bu ndled top part as shown in Fig. 1 (d). The cell walls are less pronounced for morphologies in Fig. 1 (b) and (c) m ade from shorter VA-SWNTs; 3 μm and 2 μm thick VASWNTs, respectively. The schematic of SWNT/n-Si heterojunction solar cell built with the SWNT thin film is shown iFig. 2. The micro-honeycomb SWNTs network film was placed on top of the substrate which has a 3 mm × 3 mm bare n-type silicon contact window in the center. T he contact window is surrounded by SiO 2 as insulating layer and Pd as electrode. By using the hot water techniq ue [7], the removal of micro-honeycomb SWNTs network was easier than as-grown VA-SWNTs. Figure 3 shows current density and voltage (J-V)  characteristic of the SWNT-Si heterojunction solar cell using SWNT films shown in Fig. 1. The optimal photovoltaic conversion efficiency (PCE) under AM1. 5 is 5.1% (from the network structure shown in Fig. 1(b) ), with the fill factor of 46%. The open-circuit volta ge and short-circuit current are 0.54V and 20.72 mA/cm , respectively. This showed a substantial improvement  compared with previous reported result (PCE = 8.4% [3], 2.4% [4], 1.7% [5], 1.53% [6]). The improvement sho uld be attributed to the self-organizing micro-honeycom b network, in which the spaghetti-like SWNTs in good contact with the Si serve as the charge carrier sep aration interface, and the artery-like condensed SWNTs serv e as the holes collector and transport high-way to the e lectrode. Part of this work was financially supported by Grant-in-Aid for Scientific Research (22226006, 19054003), JSPS Core-to-Core Program, and Global CO E Program 'Global Center for Excellence for Mechanica l Systems Innovation'. References: [1] Y. Murakami, S. Chiashi, Y. Miyauchi, M. Hu, M.  Ogura, T. Okubo, S. Maruyama, Chem. Phys. Lett., 385 (2004) 298. [2] Y. Jia, J. Wei, K. Wang, A. Cao, Q. Shu, X. Gui , Y. Zhu, D. Zhuang, G. Zhang, B. Ma, L. Wang, W. Liu, Z . Wang, J. Luo, D. Wu, Adv. Mat., 20 (2008) 4594. [3] P. Wadhwa, B. Liu, M. A. McCarthy, Z. Wu, A. G.  Rinzler, Nano Lett., 10 (2010) 5001. [4] D. Kozawa, K. Hiraoka, Y. Miyauchi, S. Mouri, K . Matsuda, Appl. Phys. Express, 5 (2012) 042304. [5] P. Ong, W. Euler, I. A. Levitsky, Nanotechnology, 21 (2010) 105203. [6] Y. Jia, P. Li, J. Wei, A. Cao, K. Wang, C. Li, D. Zhuang, H. Zhu, D. Wu, Mater. Res. Bull., 45 (2010) 1401. [7] Y. Murakami, S. Maruyama, Chem. Phys. Lett., 422 (2006) 575. Fig. 2 Schematic of SWNT/n-Si heterojunction solar cell. Fig. 1 (a, b, c) SEM images of different self-organ izing micro-honeycomb network of SWNTs; (d) enlarged image of condensed SWNTs bundles from (c).

Micro-Honeycomb Network Structure of Single-Walled Carbon Nanotubes for Heterojunction Solar Cell