Rapid Plasma-Assisted, Ambient-Pressure Deposition of Conformal Nanocrystalline Zinc Oxide Thin Films for Solar Cell App
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Rapid Plasma-Assisted, Ambient-Pressure Deposition of Conformal Nanocrystalline Zinc Oxide Thin Films for Solar Cell Applications Joachim D. Pedersen and Kwok Siong Teh School of Engineering, San Francisco State University 1600 Holloway Avenue, San Francisco, CA 94132 ABSTRACT This paper reports a plasma-assisted, rapid, ambient-pressure, low-temperature one-step process for depositing conformal, non-porous nanocrystalline ZnO thin film on various substrates ranging from Si (100), fused quartz, glass, muscovite, c- and a-plane sapphire (Al2O3), to the common polymer polyimide (KaptonTM). The as-synthesized polycrystalline films range in thickness from 20nm to 200nm, deposited at a growth rate ranging from 2 nm/min to 50 nm/min. The lowest deposition temperature achieved with this method is 180°C and progress is being made in further lowering this temperature. The as-deposited films are highly oriented in the caxis, with (002) being the dominant planes. INTRODUCTION Current global climate change and unsustainable reliance on fossil fuels have created a compelling imperative to develop photovoltaic energy on a large scale at an affordable cost. Among competing photovoltaic technologies, dye-sensitized solar cell (DSSC) is viewed as a strong complement to silicon solar cell due to its temperature-independent performance and the ease of manufacturing. Zinc oxide (ZnO) thin film—a key material used in DSSC both as a dyesupport layer and a transparent electrode—has conventionally been synthesized by solutionphase method such as hydrothermal deposition and by vapor-phase methods such as sputtering, spray pyrolysis, thermal evaporation, and low-pressure chemical vapor deposition. Despite continued progresses made in our understanding of the mechanical, optical and electronic properties of ZnO in both undoped and doped states, major shortcomings in fabrication techniques—lengthy deposition time, need for high vacuum and pre-seeding of substrate— continue to constrain the widespread use of ZnO in the manufacturing of DSSC. As an effort to overcome such challenges, this paper reports a plasma-assisted, rapid, ambient-pressure, low-temperature one-step inductive heating process for depositing conformal, non-porous nanocrystalline ZnO thin film on various substrates ranging from silicon to polyamide and from crystalline to amorphous substrates. Previous research has shown that inductive heating can provide a useful and efficient means to introduce large amount of heat for nanomaterial synthesis [1-3]. This is attributed to inductively coupled plasma being capable of rapid-on and rapid-off switching capabilities, making it well suited for applications where hightemperature and high-heating rate heat treatments are needed. In particular, inductively coupled plasma systems have shown industry-scale utility for synthesis of high-quality nanoparticles at ambient pressures [4]. In inductively coupled plasma nanoparticle synthesis methods, concurrent introduction of complex liquid, gas, or powder precursors enables a one-step, cost-eff
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