Amorphous-Nano-Crystalline Silicon Thin Films in Next Generation of Solar Cells

Abstract

The optical properties of amorphous-nano-crystalline thin films deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD) were studied in correlation with the size distribution of individual crystal sizes and the crystalline to amorphous fraction. A possible application 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 Grazing Incidence Small-Angle X-ray Scattering (GISAXS), Grazing Incidence Wide Angle X-ray scattering (GIWAXS) and High-Resolution Electron Microscopy (HRTEM). X-ray scattering measurements were done at the Austrian SAXS 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 5 nm while the Raman crystal fraction varied from 15 to 30 vol. %. Measurements of the optical properties showed that the spectral distribution of the absorption coefficient in a whole range of crystal to amorphous fractions remained similar to pure amorphous silicon in the visible part of the spectrum and showed 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 comparing to pure amorphous solar cells. The effect was larger for samples with a higher nanocrystal fraction and smaller crystals suggesting a possible application in multi-layer solar cells.