Increasing the optical absorption in a-Si thin films by embedding gold nanoparticles
Introduction
The incorporation of plasmonic nanostructures in the design of solar cells has shown potential for the development of high-efficiency, low-cost thin-film solar cells [1], [2]. Light trapping techniques using plasmonic nanostructures have been the subject of considerable interest and, specifically, in solar cell architectures employing metallic nanoparticles as scattering elements [3], [4], [5] or in textured plasmonic-back reflectors [6], [7], [8], [9]. The metallic nanoparticles enhance the optical absorption by increasing the optical path of the incident light inside the thin semiconductor absorber layer with the theoretical possibility of exceeding the Yablonovitch -thermodynamic limit on the light absorption [10].
Another possible approach to enhance the optical absorption consists in embedding the metal nanoparticles inside the semiconductor absorbing layer making use of their plasmonic near field properties. The metal nanoparticles act as antennas, capturing the energy of the incident light into localized surface plasmon resonance (LSPR) modes. The local electric field near the metallic surface can become orders of magnitude higher than that of the incident field. The plasmonic near field enhancement has been exploited both in organic and dye-sensitized solar cells [11], [12], [13], [14], [15] and in inorganic solar cells, such as CdSe/Si [16] and amorphous Silicon (a-Si) thin-film hetero-structures [6], [17], [18], [19].
In silicon based solar cells, the inclusion of metallic nanoparticles inside the semiconductor photoactive layer theoretically favors the direct absorption of the radiation through scattering and the near-field enhancement effect [20]. However, it may also be responsible for an efficiency drop of the device performance since the introduction of additional metallic structures into the semiconductor photoactive layer causes further ohmic losses and may introduce significant structure defects which are quite detrimental to the photocurrent by increasing the recombination rate [21], [22]. It is therefore of paramount importance to tailor the size, density, shape and location of the nanoparticles in the design of an appropriate solar cell architecture in order to provide both an efficient light trapping scheme and a reduction of the energy losses allowing an enhancement over the entire spectrum of the photovoltaic performance [21], [23]. Although Ohmic losses in the metallic nanoparticles are considered an obstacle for thick solar cells [24], for very thin film solar cells the benefits may outweigh these parasitic losses since their optical absorption can benefit from the smallest boost [25]. In particular in thin a-Si photoactive layers, the light trapping scheme should be able to achieve a large absorption cross section near the band-edge regions (600–800 nm) of the absorption spectrum where the absorption coefficients typically become small and the efficiency is low. Therefore a better understanding of the optical properties of a-Si nanocomposite containing metallic nanoparticles is important and may lead the further optimization of light-trapping schemes being effective across the whole useful solar spectrum. This is particularly of importance to thin film solar cells which have a considerable market share and offer interesting prospects [26], [27].
In the present work, we report the possibility to tune the light absorption inside thin a-Si films deposited on glass substrates and incorporating a random array of Gold nanoparticles from the visible toward the NIR as a function of the film-thickness. The nanoparticles were fabricated by a gas phase aggregation clusters source which produces ultra clean nanoparticles of any composition at a fast rate [28]. Recently such nanocluster sources have been used to explore light management for photovoltaics [29], [30], [31], providing a novel and economically interesting approach. UV–Vis spectroscopy measurements showed thickness-dependent absorption properties in the Gold/a-Si nanocomposite material. The effective dielectric environment causes a redshift of the Gold nanoparticles plasmonic resonance inside a-Si with increasing thickness. These experimental findings are explained with finite-difference time domain (FDTD) simulations, by which also the role played by the near field coupling between Gold nanoparticles is elucidated. The intensity and the broadening of the plasmonic resonance as a function of the a-Si layer thickness and Gold nanoparticles deposition time is discussed as well.
Section snippets
Experimental procedure
Gold nanoparticles were deposited with a gas aggregation nanocluster source based on magnetron sputtering (NC200U-B Oxford Applied Research Ltd.) [32], [33] on glass substrates. For the deposition of nanoparticles, argon gas was used as both sputter and carrier gas inside the gas aggregation chamber. The Au target had a purity of 99.99%, the argon flow rate in the cluster source was 15 sccm, the magnetron DC power 30 Watts and the aggregation length 60 mm for all samples. The deposited
Morphology
The optical properties of a nanocomposite depend both on the material and morphological properties of its structural units (cluster or nanoparticles) and of its surfaces [37].
The geometrical structure as well as the mean size of the deposited Gold nanoparticles are determined primarily by the parameters chosen during the deposition. Different studies on the fabrication of nanoparticles by the gas aggregation process have shown that the cluster size distribution can be accurately described by a
Conclusions
The control of the red-shift of the plasmon resonance by embedding a random array of glass supported Gold nanoparticles in different thicknesses of a-Si thin films forms a route to precisely tune the optical absorption properties. This could be exploited in the design of efficient light-trapping techniques in a-Si thin-film solar cells as well as in the design of NIR-tunable optical coatings. Although the experimental red-shift of the plasmonic resonance is qualitatively reproduced by FDTD
Acknowledgement
The Dept. Of Medical Biotechnology and Translational Medicine (BIOMETRA), Universita’ degli Studi di Milano is thanked for using the TEM and Maura Francolini for assistance.
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