Si Nanocrystal Networks for Photovoltaic Applications


Si Nanocrystal Networks for Photovoltaic Applications

Ozen, E.; Gundogdu, S.; Keles, U.; Bulutay, C.; Aydinli, A.; Heinig, K.-H.; Rigato, V.

Self-assembling during spinodal decomposition of Si nanocrystals in a dielectric matrix is a very promising synthesis process of novel nanocrystaline Si structures for 3rd generation thin-film solar cells. Thanks to quantum confinement in nanocrystals, this approach can be utilized to improve the single band gap silicon solar cells efficiency by spectrum management through the incorporation of larger band gap nanocrystaline silicon into the solar cell structure allowing a better use of the solar spectrum.
Conventional techniques use high-temperature processing to activate the spinodal decomposition process. However, these methods are incompatible with glass substrates or thin-film stacked structures usually employed in mass production techniques (e.g in pilot lines for deployment of solar cells). An alternative approach reducing the thermal budget and allowing localised processing is the laser irradiation of substochiometric silicon oxides.
We present cw laser annealing of Si-rich oxide thin films with varying Si content to obtain Si nanocrystals embedded in silica. SiOx thin films with x<2 were obtained by plasma enhanced chemical vapor deposition (PECVD). Hydrogen or nitrogen diluted silane (SiH4) gas was used as the Si source and two different precursor gasses, N2O and CO2, were used for oxygen incorporation. We have achieved the control of the Si ratio in the films by adjusting the relative gas flow ratios. Fine tuning the Si excess in SiOx and optimizing the annealing conditions is pursued to control the inter-nanocrystal distance to generate a network of Si nanocrystals with controlled Si/SiO2 phase separation. Our computational studies of silicon nanowire networks based on realistic pseudopotential techniques have unraveled the systematics of the band gap variation under topological and structural variations. With the aid of these atomistic modeling tools, nanocrystal networks are optimized for solar cell applications.
We have investigated the nanocrystal network formation for different composition samples. In the case of PECVD grown Si-rich oxides, different elements such as nitrogen, carbon and especially hydrogen can be present in the films. A detailed elemental study has been performed to determine the precise composition of the films using ion beam techniques such as elastic recoil detection analysis (ERDA) and Rutherford back scattering (RBS), as well as X-Ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). The Si/SiO2 phase separation has been studied by energy filtered Transmission Electron Microscopy (EFTEM) and the nanowires' diameters have been identified to be in the order of a few nanometers suggesting the possibility of quantum confinement.
This research is supported by TÜBITAK-BMDF grant no 109R037

Keywords: Nanocomposite; Si; SiO2; Sponge; photovoltaic; EFTEM; theory

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