Optoelectronic and mechanical properties of Sol-Gel derived Multi-Layer ITO thin films improved by elemental doping, Carbon Nanotubes and Nanoparticles

Hatem Taha

Transparent conductors (TCs) are an essential ingredient in numerous new applications which are emerging in the 21st century including high efficiency solar cells, rigid and tactile displays, light emitting diodes, photonics for communications and computing, energy efficient and smart windows and gas sensors, since they allow efficient light transmission while electric signals are applied or collected. So far, indium tin oxide (ITO) reflects the best trade-off between low electrical resistivity and high optical transparency, making it the first candidate as transparent conductor for most optoelectronics technologies despite its drawbacks such as expensiveness and poor mechanical characteristics. However, due to the intricacy of ITO, the coating characteristics strongly depend on the deposition conditions. Despite many developments in ITO-based transparent conductive coatings; these coating are yet to be commercialized for optoelectronic applications. Many challenges still exist in terms of the fabrication of high quality ITO-based transparent conductive coatings, in order to meet the criteria of better, cost-effectiveness and environmentally-friendly characteristics, especially in the context of transparent conductive electrodes. In this study, the deposition conditions along with incorporating thin ITO films with transition metals (Ti and Ag), carbon nanotubes (e.g., SWCNTs) and metals nanoparticles (Ag and Au nanoparticles) were optimized to synthesize high quality ITO based TCs via a facile, environmentally friendly and cost-effective sol-gel spin-coating method. ITO thin films were fabricated with different Ti and Ag doping concentration, different annealing temperature and different geometry such as single layer, bi-layer and multi-layer. The structural, surface morphology and composition, electrical, optical and mechanical properties were characterized using a wide range of complementary techniques, namely, X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), electron dispersive X-ray (EDX), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), four point probes and Hall Effect, UV-Vis, nanoindentation and FEM modeling. All the fabricated ITO-based TCs showed a nano-sized grain-like morphology forming a homogenous surface structure. XRD results demonstrated a relatively good crystallinity of ITO-based thin film coatings after applying a suitable heat treatment. XPS and EDX analysis corroborated the existence of the main elements for each case of thin ITO coating. In the case of pure ITO and (Ti- and Ag-) doped ITO thin films with a thickness of 350 ±5 nm and 500 C annealing temperature, the highest optical transparency was determined to be 92% for pure ITO, 4 at.% Ti-doped ITO and 2 at.% Ag-doped ITO thin films, while the lowest electrical resistivity of 1.6×10-4 Ωcm was achieved for the ITO film prepared with 4 at.% Ti content. However, these thin films exhibit mechanical characteristics namely hardness and Young’s modulus in the range of (5.3 – 6.8) GPa and (128 – 148) GPa, respectively. In order to enhance their mechanical characteristics while maintaining their optoelectronic properties, SWCNTs were incorporated with ITO with different films thicknesses (i.e. 150, 210, 250 and 320) ± 5 nm, and the characterizations were carried out with respect to the film thickness. The hardness and Young’s modulus for SWCNTs/ITO thin films were in the range of (22 – 28) GPa and (254 – 306) GPa, respectively. The lowest electrical resistivity of 4.6×10-4 (Ω cm) was achieved for the thicker film, while the highest transmittance of 91.5 % was obtained from the thinner film. Obtained results show a significant improvement in the mechanical properties of SWCNT/ITO thin films compared with their counterparts of pure and doped ITO thin films, along with distinctively good optoelectronic properties. To minimize the consumption of indium, ITO thin films were combined with (very thin metal-, metal nanoparticles- and metal oxide-) layers in bi-layered and tri-layered geometries of (AuI), ((Au)nI), ((Ag)nI) and ((AgO)I), and (IAuI), (I(Au)nI), (I(Ag)nI) and (I(AgO)I), respectively, with films thicknesses (~ 130 ± 5 nm). For these coating systems, the lowest electrical resistivity 1.2×10-4 Ωcm was for ITO/Au/ITO thin film, while the highest optical transparency ~ 91.5% was for ITO/AgO/ITO thin film. Two thin films with the configurations of ITO\AgO\ITO and AgO\ITO for tri-layer and bi-layer coatings, respectively with the best optoelectronic performance were nominated as transparent conductive electrodes in designing inverted organic solar cells, and compared with pure ITO thin films. Power conversion efficiencies of 4.9%, 4.2% and 3.8% were found for ITO/AgO/ITO, AgO/ITO and ITO thin film coatings, respectively. To conclude, sol-gel spin-coating derived ITO based transparent conductive coatings present high quality crystal structure, distinctively good optoelectronic properties as well as reasonably mechanical characteristics, and comparable with those achieved from other sophisticated fabrication techniques such as sputtering, pulsed laser deposition, electron beam evaporation etc. All these attributes render the ITO-based coatings promising as transparent conductors in many industrial and technological applications.

Optoelectronic and mechanical properties of Sol-Gel derived Multi-Layer ITO thin films improved by elemental doping, Carbon Nanotubes and Nanoparticles

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