Carbon Nanotube Composite Mesh Film with Tunable Optoelectronic Performance

Abstract

In this study we fabricated novel transparent composite electrodes based on multi-walled carbon nanotubes (MWCNTs) and sodium dodecyl sulfate (SDS). The target optoelectronic properties of the flexible composite films are ± 100 /sq at 77% transmission of 550 nm light based on the properties of indium tin oxide (ITO) films on poly-ethylene terephthalate (PET). The optical properties of the MWCNTs composite thin films were measured in the UV-Vis and IR regions. The corresponding band-gap of composite films was 3.82 eV and complementary to the band-gaps' of dye-sensitized and organic solar cell components. The band-gaps of films deposited between 30 minutes and 60 minutes were tunable. Composite films over PET had tunable transparency of 550 nm light from 85% to 83% to 80%. The respective resistivities were 1141 /sq, 378 /sq and 110 /sq with an inverse exponential relationship between transparency and resistivity. Preferential deposition of MWCNT composites was demonstrated by low coverage on glass substrates and dense coverage on PET substrates. The roughness (Ra) of the MWCNT composites over PET was 20.40 nm as compared to 0.70 nm for indium tin oxide (ITO) depositions over glass.Materials with high transparency, flexibility, and conductivity are important for progressing optoelectronic devices. 1,7,9 Ideally transparent electrodes would be highly transparent, easily fabricated, and serve as efficient electron acceptors with low resistivity. 1,7 Current optoelectronics use metal oxides as conductive electrodes, the most common material being indium tin oxide (ITO). ITO electrodes deposited on flexible PET substrates (Sigma-Aldrich), have exceptional optoelectronic properties: 100 /sq at 77% transmittance of 550 nm light. ITO has been shown to have tunable band-gap of 3.27 to 3.75 eV and minimal absorption peaks in the UV-NIR regions; at 450 nm, 550 nm, 725 nm and 1100 nm. 2,6 These properties make ITO an optimal electrode currently for solar cells and displays. 1,23-26 Limitations of ITO electrodes such as their brittleness and scarcity of indium metal have prevented their applications in flexible devices. 5 Flexing of ITO electrodes alters the homogenous coverage over the flexible substrate resulting in cracking and reduced performance over time. 7,9 With progressive flexible designs of optoelectronics, there is a need for flexible electrode materials on flexible substrates with similar performance to ITO. [5][6][7]10,14,15 Carbon nanotubes (CNTs) have been used in many applications because of their mechanical, thermal, and electronic properties. In particular, CNTs have been incorporated into the solar cell active layers and as composite electrodes. 4-18 The conductivity of arm-chair type single walled carbon nanotubes (SWCNTs) allows for hole mobility along the conjugated 1-dimensional (1 D) structure. 4-18 The mechanical properties of CNTs also allow for film flexibility without significant damage. [8][9][10][11] Multi-walled carbon nanotubes demonstrate optical properties similar to that of arm-chair SWCNTs ...

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