Carve out a new future: Practical use of flexible transparent conductive film

Transparent conductive oxides (TCOs) have been at the forefront of research for flexible device electrodes due to their excellent electrical conductivity and high transparency. Indium tin oxide (ITO) has historically been the material of choice for TCOs, despite its susceptibility to bending and its association with toxic indium. However, in recent years, alternative materials such as Al-doped ZnO (AZO) have gained prominence due to their abundance, non-toxic nature, and excellent electrical properties when prepared at high temperatures.

In a recent study published in the Journal of Vacuum Science & Technology B, Professor Atsushi Nitta and colleagues from the Department of Electronic Control Engineering at the National Institute of Technology, Kagoshima College, have made significant advancements in the development of transparent conductive films. These films have gained immense importance in various applications, including transparent antennas, particularly in addressing the growing demands of 5G communication networks.

The research team focused on the development of flexible multilayer transparent conductive films on a polyethylene naphthalate (PEN) substrate. These substrates, though unsuitable for high-temperature deposition, were used to investigate the potential of AZO/Ag/Cu/AZO structures as transparent conductive films. The inclusion of a conductive metal layer (Ag) sandwiched between AZO layers was explored as a means to enhance electrical properties at low temperatures. The authors showed that the deposition rate of the Cu layer had a significant impact on the electrical and optical properties of the films. As the Cu deposition rate increased from 0.02 to 0.08 nm/s, the resistivity improved while transmittance remained constant. However, at a deposition rate of 0.10 nm/s, both resistivity and transmittance were negatively affected. This highlights the delicate balance between achieving low resistivity and high transmittance in transparent conductive films. When the authors conducted X-ray diffraction (XRD) analysis the results showed that the crystallinity of the AZO layer improved with increasing Cu deposition rate up to 0.08 nm/s. Beyond this rate, the crystallinity deteriorated due to increased energy input during deposition, which damaged the AZO layer. The presence of a Cu layer did not alter the crystallinity of Ag, but it had a significant impact on the AZO layer’s structural properties.

Moreover, the researchers performed Auger electron spectrometer (AES) which provided important data into the distribution and diffusion of elements within the films. Interestingly, the Cu layer effectively inhibited the oxidation of Ag and its diffusion into the AZO layer, particularly when the Cu deposition rate was optimal. This observation is crucial for improving the resistivity of transparent conductive films. Furthermore, varying the thickness of the Cu layer revealed that thicker Cu layers improved resistivity while decreasing transmittance. The study found that a 5 nm Cu layer yielded a resistivity comparable to commercially available transparent conducting films. Importantly, this resistivity improvement was attributed to the prevention of Ag oxidation and diffusion into the AZO layer by the Cu layer.

In conclusion, Professor Nitta and his team have made significant progress in the development of transparent conductive films. Their research highlights the importance of an intermediate Cu layer in enhancing the electrical properties of AZO/Ag/AZO multilayer structures. By carefully controlling the deposition rate and thickness of the Cu layer, they achieved a balance between resistivity and transmittance, ultimately obtaining films with practical resistivity. These findings are not only relevant for transparent conductive films but also hold promise for various applications of TCOs, including flexible devices and antennas for 5G communications. The study opens up avenues for further research into highly transparent metal conductors as intermediate layers, which could advance the field of transparent electronics.