Surface Tuning of Transparent Electrodes with Atmospheric Pressure Plasma for Third Generation Photovoltaics

Transparent semiconductor thin films such as indium tin oxide (ITO) and fluorine-doped tin oxide (FTO) are commonly used in optoelectronic devices due to their favorable properties such as high conductivity and transparency and suitable work function 1. Their use ranges from light-emitting diodes through flat panel displays up to solar cells. In this context, surface stoichiometry plays a crucial role in influencing the final properties of the final device. Therefore, efficient surface treatment has been a topic in research for years, covering a wide range of approaches from wet and dry treatments. Among these possibilities, plasma treatment remains today the most effective. In several studies 2, 3, atmospheric pressure plasma shown an increase of surface energy, decrease the contamination while enhancing the work function of both ITO and FTO. Low temperature, fast processing times, and no special requirements for working gas are the main advantages of this approach. In this study, the authors compare the effects of cold (<100°C) atmospheric pressure plasma on ITO and FTO after air and nitrogen treatment to provide deeper insight into the interaction of discharge with semiconductive materials during treatment. A coplanar dielectric barrier discharge has been used for the surface modification (more information available in 4). For this configuration both electrodes lay in one plane, and the discharge occurs on the top of the barrier. Chemical changes in the surface were also evaluated as a function of plasma parameters without any pre-treatment of the substrates. For both studied materials, similar trends were observed at fast treatment times (<60s of plasma exposure). The plasma treatment resulted in a decrease of water contact angle from the initial 90° for ITO to 10° after 30 seconds of exposure. For FTO, the initial angle 60° was lowered to 10° after just 5 seconds of exposure, and complete hydrophilicity was obtained after 30 seconds of plasma treatment. An interesting trend in oxygen concentration measured by XPS was observed for ITO but not FTO samples. From a physical point of view, the interaction with the conductive sample affects the orientation of filaments during the surface modification. Additionally, the results suggest that for small working distances, the physical regime can vary from coplanar to volume discharge as shown in Figure 1. All changes were observed with ICCD imaging (Pi-Max 4, Princeton Instruments). Regarding the physical changes over the surface, the plasma treatment resulted in decreased roughness with no significant effect on conductivity. In conclusion, the results obtained in this study can be utilized in the manufacturing process of microelectronic devices and provide a deeper insight into how the treatment process influences discharge parameters. 1 Armstrong et al. https://doi.org/10.1016/j.tsf.2003.08.067 2 Šimor et al. https://doi.org/10.1063/1.1513185 3 Donley et al. https://doi.org/10.1021/la011101t 3 Chaney et al. https://doi.org/10.1016/S0169-4332(03)00617-2 Figure 1