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 x 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/cm², 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.