A small-bundle single-wall carbon nanotube electrothermal film for smart windows
The integration of electrothermal films into smart windows demands simultaneous high optical transparency and exceptional heating performance, a trade-off that has constrained flexible transparent heater development. This work reports a transparent conductive single-wall carbon nanotube (SWCNT) film composed of highly crystalline, long SWCNTs in small bundles, synthesized by floating catalyst chemical vapor deposition (FCCVD). The small-bundle SWCNT film, with an average bundle diameter of 7.1 nm, achieves a sheet resistance of 26 Ω/□ at 82% transmittance and reaches a stable temperature of 102 °C under a low applied voltage of 20 V. The superior electrothermal performance relative to large-bundle counterparts originates from a higher areal nanotube density and more efficient conductive pathways at equivalent transmittance. Integrating this transparent heating film with a paraffin wax/polydimethylsiloxane (PW/PDMS) thermochromic functional layer yields a large-area flexible smart window. The device exhibits a reversible visible light transmittance range from 0.17% to 78% and exceptional cycling stability. This study overcomes the transparency–conductivity trade-off in transparent electrothermal films, providing a viable route for flexible smart windows and related thermal management devices.