Abstract
Atmospheric solution based processes are being developed for the fabrication of thin film photovoltaic devices. Deposition techniques such as electrodeposition, spin coating, spraying or printing are promising techniques to increase the throughput and reduce the cost per Watt of Copper-Indium-Gallium-Selenide (CIGS), Copper-Zinc-Tin-Sulphide (CZTS) and perovskite thin film solar technologies. All these technologies require pre-treatment of the substrate prior to the deposition of the thin film and ideally this pre-treatment should also be performed at atmospheric pressure. Results presented in this paper show that use of an atmospheric-pressure plasma is highly effective in activating the surface of substrates commonly used in thin film photovoltaic (PV) device fabrication. Surface activation improves the adhesion of thin films. The use of an atmospheric activation process is compatible with a continuous vacuum-free PV fabrication process. Soda lime glass (SDL) and fluorine doped tin oxide (FTO) coated glass are substrates commonly used in the fabrication of photovoltaic modules. These substrates have been surface treated using a He/O2 atmospheric-pressure plasma, resulting in increased surface energy as evidenced by Water Contact Angle (WCA) measurements. The pre-treatment reduces adventitious surface contamination on the substrates as shown using X-ray Photoelectron Spectroscopy (XPS) measurements. The advantages of using the atmospheric plasma surface pre-treatment has been demonstrated by using it prior to atmospheric deposition of Cadmium Sulphide (CdS) thin films using a sonochemical process. The CdS thin films show pinhole-free coverage, faster growth rates and better optical quality than those deposited on substrates pre-treated by conventional wet and dry processes.
Highlights
Reducing the cost of energy is a major driving force in solar energy research as it leads directly to increased deployment of photovoltaic (PV) modules
The low wettability of the substrates before the plasma treatment ($44° and $86° for the soda lime (SDL) and TEC10 substrates respectively) was significantly improved by the atmospheric plasma treatment, which reduced the water contact angle (WCA) to values less than 10°
Even 300 min after the plasma treatment, the WCA remained below 10° for both surfaces
Summary
Reducing the cost of energy is a major driving force in solar energy research as it leads directly to increased deployment of photovoltaic (PV) modules. Four approaches are typically combined in these atmospheric-pressure plasma systems to maintain the non-thermal character of the plasma despite the high collisionality encountered at atmospheric pressure (Iza et al, 2008): (1) large flows, (2) dilution of molecular gases in noble gases (e.g. He, Ar), (3) small scale discharges to take advantage of large surface to volume ratios and (4) pulsed operation to prevent thermalisation of the discharge The latter can be achieved either by pulsing the input voltage to the plasma source or by introducing dielectric barriers that quench the plasma at each cycle. We report on the use of a He/O2 atmosphericpressure dielectric barrier discharge as a dry plasma cleaning and surface activating pre-treatment process for atmosphericdeposition of CdS thin films. 14.30%, K2O 1.20%, CaO 6.40%, MgO 4.30%, Al2O3 1.20%, Fe2O3 0.03% and SO3 0.30% (‘‘Menzel-Gläser_Microscope Slides,” n.d.) Both the TEC 10 and SDL substrates were treated with the atmospheric-pressure plasma and their surface chemical composition was studied before and after the atmospheric plasma treatment. SDL glass is used for CIGS and CZTS devices in the substrate configuration
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