The nanostructure of silicon thin films for solar cells
published: Jan. 18, 2008, recorded: October 2007, views: 1723
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A typical thin-film silicon solar cell is deposited on a glass substrate covered with a conductive transparent metal oxide. The active part of a solar cell consists of three to six silicon layers, each with a thickness of ten to several hundred nanometers, deposited in a layer-by-layer fashion. In these structures layers with different individual optical gaps is stacked together, in order to cover as much of the solar spectrum as possible. By changing the structure of the material, going from pure anorphous to monocrystalline, it is possible to obtain the variation in optical gap using the same material. Silicon in the form of nanocrystals drags in that sense particular attention in last decade. For any practical use, it is important to know size and size distribution of nano particles in this kind of structure. A series of multilayered silicon thin films was prepared by the decomposition of silane gas, diluted with hydrogen, in a radio-frequency glow discharge. Films with nanocrystallites (nc-Si) of different sizes were processed by varying the silane-to-hydrogen ratio. The nanostructures of the silicon thin films were studied by Raman spectroscopy (RS) and high-resolution transmission electron microscopy (HRTEM). Raman spectrum of the microcrystalline Si shows one intensive sharp band at 521 cm-1. For crystallites smaller than 30 nm this band (transversal optical - TO mode) is broadened and its position is shifted to lower frequencies. The shift is dependent on the average crystallite sizes. The size of the nanocrystallites in the investigated samples was estimated from the shift of the TO mode in the Raman spectra of the nc-Si after the deconvolution of the spectra. The volume fraction of the crystalline phase can be estimated from the ratio of the integrated intensities of the crystalline TO and the amorphous TO modes after the deconvolution of the Raman spectra. Since the deconvolution procedure influences the accuracy of obtained result, various methods of deconvolution were applied. Therefore, for the calculation of the Raman shifts and the integrated intensities, the spectra are frequently fitted as the sum of amorphous (Gaussian) contributions and crystalline contribution (Voight). We further deconvoluted the spectra, but using a somewhat different procedure than that one usually described in the literature. Since the spectra of the multilayered nc-Si thin films can be deconvoluted to the modes belonging to the a-Si and the TO mode of the nc-Si crystalline fraction, we first removed the amorphous contribution directly by subtracting the experimental spectra of completely a-Si from the spectra of our multilayered nc-Si samples. The TO band in the nc-Si appears as asymmetric and broad, which suggests the coexistence of smaller and larger crystals, so we deconvoluted the TO mode of the nc-Si into two components. The two components can be assigned as one belonging to the small crystallites, and the other to the larger crystallites.
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