engleski

Amorphous-nano-crystalline silicon thin films in next generation of solar cells

The optical properties of amorphous-nano-crystalline thin films deposited by Plasma Enhanced Chemical Vapor Deposition were studied in correlation with the size distribution of the individual crystal sizes and the crystal to amorphous fraction. A possible application of this material as active part in solar cells was tested by integrating the actual Si layers in a typical p-i-n solar cell structure. The nano-structural properties were examined by Raman spectroscopy, Grazing Incidence Small-Angle X-ray Scattering and High-Resolution Electron Microscopy. X-ray scattering measurements were done at the Austrian Small Angle X-ray Scattering beam-line (Synchrotron Elettra, Trieste). The in-depth size distribution of the nano-crystals was found either uniform across the sample, or the crystals were slightly larger when located closer to the surface. Typical sizes were between 2 and 10 nm while the Raman crystal fraction varied from 8 to 35 vol. %. Measurements of the optical properties showed that the spectral distribution of the absorption coefficient in the whole range of crystal to amorphous fractions remained similar to pure amorphous silicon in the visible part of the spectrum and showed a square dependence on the photon energy (Tauc gap). The average optical gap was larger for smaller nano-crystals and a higher crystal fraction just confirming the quantum size effects that correspond to quantum dots. The spectral response of solar cells with the examined thin films as active elements showed a narrower spectral distribution and a blue shift compared to pure amorphous solar cells. The effect was larger for samples with a higher nano-crystal fraction and smaller crystals suggesting a possible application in multi-layer solar cells

nanocrystalline silicon; GISAXS; Raman; optical spectroscopy

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