Characterization of flexible transparent conductive films fabricated using CVD graphene on heat-resistant transparent polyimide films

Polymethylmethacrylate (PMMA) is commonly used as a support material in graphene transfer. Heat treatment exceeding 250 °C is required to remove the PMMA after the transfer process. However, the transparent flexible substrates conventionally used for graphene transfer generally exhibit low heat resistance. This hinders the complete removal of the PMMA, resulting in low electrical conductivity of graphene. Therefore, we focused on developing heat-resistant transparent polyimide (TPI) films as substrates for graphene transfer. The interactions between the TPI substrates and chemical vapor deposition graphene were systematically investigated. The effects of the TPI surface roughness, chemical composition, and chemical structure of the TPI, and doping effects of the substrate were examined, and a silane coupling agent (SCA) was coated to bring the TPI surface properties closer to those of a quartz glass substrate. Three-layer stacked graphene (3LG) on TPI, in which the -CF₃group in TPI is replaced with -CH₃, exhibited the highest carrier mobility of 3610 cm²Vs^-1at a constant carrier density after annealing. The sheet resistance of the 3LG on TPI annealed after vapor phase deposition of the SCA was lower than that of the standard TPI and decreased to 82 Ω/sq. after doping with bis(trifluoromethanesulphonyl)amide. This is comparable to the electrical properties of graphene on a quartz glass substrate. Furthermore, after doping, the 3LG/TPI maintained a high optical transmittance of 88.8% at a wavelength of 550 nm. The knowledge obtained from this study will allow graphene flexible transparent conductive films to be transferred onto TPI for use in transparent heaters and solar cells as well as to control the electrical properties for various device applications by modifying the roughness, chemical structure, and coating layers on the surface of the TPI substrate.