Scalable and Solid‐State Redox Functionalization of Transparent Single‐Walled Carbon Nanotube Films for Highly Efficient and Stable Solar Cells

This study reports a scalable and room-temperature solid-state redox functionalization process for single-walled carbon nanotubes (SWNTs) with instant efficacy and high stability. By drop-casting/spin-coating CuCl2/Cu(OH)2 colloidal ethanol solution onto SWNT films, the sheet resistance of the SWNT films achieves 69.4 Ω sq−1 at 90% transparency without noticeable increase for more than 12 months. The charge transfer mechanism between the redox and the SWNTs is revealed by Raman and X-ray photoelectron spectroscopies. The SWNT/silicon solar cells are utilized as a benchmark to evaluate the effectiveness of the redox functionalization process and its compatibility for device integration. The power conversion efficiency of the SWNT/Si solar cell increases by 115% after redox functionalization, reaching the value of 14.09% without degradation in the ambient for over 12 months. Temperature-dependent operation characteristics of the redox functionalized SWNT/Si solar cells demonstrate that the Fermi level unpinning and enhanced tunneling of the charge carriers contribute to the significant improvement of the photovoltage and fill factor. The CuCl2/Cu(OH)2 redox also serves as an antireflection layer, resulting in a 20% increase of the photocurrent. The proposed redox functionalized SWNTs are promising as multifunctional transparent conductive films for wide-range solar cell applications.