| Description |
Over the past two decades, many different cold atmospheric pressure plasmas (CAPP) sources have been developed in research labs and brought to industrial use. Due to their versatility, CAPPs are now employed in a wide range of applications, including water or exhaust gas purification [1], sterilization of medical equipment [2], or the surface treatment of sensitive materials [3]. Plasma-surface interaction can induce various types of modifications, such as etching, deposition, cross-linking, or functionalization. All these different treatments have proven to be suitable to modify and tune the surface properties of materials used in organic photovoltaic (OPV). The possibility of operating those plasma devices at atmospheric pressure makes them particularly attractive for further industrial scale-up, allowing, for example, treatments of significant amounts of materials in roll-to-roll systems compared to the batch approach used at low pressure. Indium tin oxide (ITO) and fluorine-doped tin oxide (FTO) are the most common transparent electrodes used in OPV cells. The substrate on which the electrodes are usually deposited is glass or polyethylene terephthalate (PET). To optimize the interface between the electrode and the electron transport layer, the charge transfer needs to be maximized. Therefore, the work functions of these two materials need to be matched, and charge traps at the interface must be passivated [4]. Preliminary results suggest that these objectives can be reached by using plasma treatment at atmospheric pressure [5]. Still, the exact mechanisms leading to the increase of the work function induced by the plasma remain unclear. Possible explanations can be related to the removal of organic contaminants, change in the electronic state of the surface and the band bending or the modification of surface charge traps affecting the work function. In this work, the effect of different plasma treatments, such as volume, surface or coplanar dielectric barrier discharge, on ITO and FTO is analyzed. More specifically, the influence of the substrate, as well as its thickness is correlated to the surface charges and the related modification of the work function. The different modified samples are analyzed by isothermal surface potential decay (ISPD), and the work function is determined by conductive atomic force spectroscopy. Such a comprehensive study is of interest to tune the materials used in OPV and obtain the desired properties to optimize the final cells.
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