Self-organized Micro-honeycomb Network Structure of Single-walled Carbon Nanotubes for Photovoltaic Devices

Single-walled carbon nanotubes (SWNTs) are expected to be a promising nanomaterial because of their outstanding electronic, mechanical, and thermal properties. For macroscopic device applications, an assembly of SWNTs is a critical issue. We propose a self-organized micro-honeycomb network structure of SWNTs obtained by water vapor treatment of as-synthesized vertically-aligned SWNTs (VA-SWNT) for solar cell devices with higher performance. The microhoneycomb structure was realized by simply exposing VASWNT to water vapor and drying in ambient condition. Honeycomb cell walls consist of capillary-aggregated vertically aligned SWNTs with heavily bundled top part. Within each cell, collapsed spaghetti-like SWNTs make contact to the substrate. The SWNT/n-Si heterojunction solar cell was built by placing the micro-honeycomb SWNTs network film on top of the substrate which has a 3 mm  3 mm bare n-type silicon contact window in the center. The contact window is surrounded by SiO2 as insulating layer and Pt as electrode. Our preliminary tests showed that optimal photovoltaic conversion efficiency (PCE) under AM1.5 was 5.91%, with the fill factor of 72%. The open-circuit voltage and short-circuit current are 0.53V and 15.5 mA/cm, respectively. This showed a substantial improvement compared with heterojunction solar cells using spaghetti-like SWNTs. Furthermore, the superior performance of dye-sensitized solar cells with the microhoneycomb SWNTs was demonstrated. NOMENCLATURE J Current density (mA/cm) L Thickness of vertically aligned SWNT film V Bias voltage (V) INTRODUCTION Single-walled carbon nanotubes (SWNTs) are featured with outstanding electronic, optical, mechanical and thermal properties, hence considered as one of most promising material for next-generation electronic and optical devices such as field effect transistors (FET), photovoltaic devices and pulsed optical fiber laser. To bridge these nanoscale properties with corresponding performance of macroscale applications, the assembly of SWNTs is a critical issue. For photovoltaic devices, so far, spaghetti-like thin film of SWNTs known as bucky paper or vertically aligned SWNT (VA-SWNTs) have been examined for SWNT/Si heterojunction solar cells or counter electrode of dye-sensitized solar cells. Here, we propose a self-organized micro-honeycomb network structure of single-walled carbon nanotubes (SWNTs) obtained by water vapor treatment of as-synthesized VA-SWNTs for solar cell devices with higher performance. The pursuit of high-efficiency carbon nanotube-based solar cells has long been an intriguing research topic since the discovery by Iijima in 1991 [1], yet still without any fundamental breakthrough and practical applications. In 2008, Cao group deposited double-walled carbon nanotube (DWNT) films onto n-type silicon substrate and showed the maximum PCE of 7% [2]. Since then, this simple but efficient structure ignited great research interest worldwide. Wadhwa et al. [3] used SWNTs “bucky-paper” with SOCl2 ionic liquid and obtained the efficiency of 8%. SWNTs showed good performance but the fabrication process of SWNTs film included surfactant, sonication, vacuum filtration and liquid state doping chemicals which made the device life-time extremely short. The photo-conversion mechanism has been argued whether it is p-n junction or Schottky junction. In order to explore this mechanism of SWNT/n-silicon, Kozawa et al. used (6,5) enriched CoMoCAT on top and obtained the PCE of 2.4% [4]. Ong et al. used SWNTs with (7,6) major chirality, DRAFT