Single-walled carbon nanotubes for heterojunction solar cells

Single-walled carbon nanotubes (SWNTs) with outstanding electronic, optical, mechanical and thermal properties are expected to be the most promising materials for next-generation energy as well as optical and electronic devices. However, the structure-controlled assembling of SWNTs to macroscale for various devices is still challenging. This study focused on the SWNT assemblies for improving the performance of SWNT-Si heterojunction solar cells. A water vapor breath figure method to build up SWNTs into a self-assembled microhoneycomb network for the applications of solar cells has been developed. The micro-honeycomb network consists of vertical aggregated SWNT walls and a buckypaper bottom (quasi two-dimensional random network). The pristine micro-honeycomb structured SWNT-Si heterojunction solar cell shows a record-high fill factor of 72% as well as a stable power conversion efficiency (PCE) of 6.04%. The PCE increased to 10.02% in the dry state after the dilute nitric acid treatment. For comparison, the state-of-the-art randomly oriented SWNT films with high transparency and with low sheet resistance were also used for fabricating solar cells. The PCE and fill factor of the randomly oriented SWNT-Si solar cell are 10.12% and 61%, respectively. In addition, the relationship between the equivalent series resistance and the fill factor was investigated through the diode equation modeling. This study demonstrates that the micro-honeycomb structure formed by the condensation and evaporation of the water vapor is beneficial for improving the performance of solar cell, especially in terms of the fill factor.