Characterization of Ag Nanocrystals for use in Solar Cell Applications
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CHARACTERIZATION OF AG NANOCRYSTALS FOR USE IN SOLAR CELL APPLICATIONS Annett Thøgersen1, Jack Bonsak1, Jeyanthinath Mayandi1, Erik S. Marstein1 and Mahalingam Umadevi2 1
Department of Solar Energy, Institute for Energy Technology, Norway.
2
Department of Physics, Mother Teresa Women’s University, India.
ABSTRACT Ag nanocrystals made by chemical synthesis have been used in solar cell applications as a part of light trapping. The shape, crystal structure, defects and composition of these nanocrystals have been studied in detail. Samples with different ratios of silver solution (AgNO3) and reductant (NaBH4) were made, and a difference in nanocrystal size was observed. HRTEM and diffraction patterns showed that the samples contained mostly Ag nanocrystals, and some of them contained Ag2O nanocrystals as well. Some nanocrystals contained large defects, mostly twinning, which induced facets on the nanocrystal surface.
INTRODUCTION Metal nanocrystals like Ag and Au exhibit a strong absorption band that is not present in the spectrum of their corresponding bulk metal. This is due to localized surface plasmon resonance, which is a collective oscillation of the conduction electrons [1]. This oscillation results in a local electromagnetic field at the nanocrystal surface and wavelength selective photon absorption and scattering [1]. By dispersing metal nanocrystals on top of an optically thin solar cell, the surface plasmon resonance will scatter the light further into the solar cell, thereby increasing the absorption and efficiency. Ag nanocrystals have attracted much attention due to the flexible electrical, optical and chemical properties they exhibit compared to the fixed properties of their corresponding bulk materials [2]. Ag nanocrystals dispersed onto an optically thin solar cell has shown to scatter the light into the solar cell and increase absorption. Pillai et al. [3] showed that Ag nanocrystals with an average nanocrystal diameter of 16 nm contributed up to a 16-fold enhancement in the photocurrent at certain wavelengths and a 33% increase of the total current of the device. The surface plasmon resonance is dependent on the shape of the nanocrystal surface, size, spatial arrangement and configuration of the nanocrystals [4]. It is therefore very important to investigate these features in detail. There are different methods for making Ag nanocrystals, such as chemical and electrochemical synthesis [5], γ-radiation [6] photochemical [7] and laser
ablation [8], and physical vapour deposition [2]. Using a chemical reduction of silver salts by sodium borohydride has shown to be a simple and popular method [2]. EXPERIMENTAL The Ag nanocrystals studied in this work were made by a wet chemical reduction synthesis. The silver nanoparticles were obtained by the drop wise addition of aqueous silver nitrate (AgNO3) to a highly reducing solution of sodium borohydride (NaBH4). The mole concentration of the sodium borohydride solution was twice that of the silver nitrate solution to prevent agglomeration onc
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