ON THE BANDGAP OF HYDROGENATED NANOCRYSTALLINE SILICON THIN FILMS

Hydrogenated nanocrystalline silicon (nc-Si:H) has attracted a great deal of attention in solar cell applications. However, the material properties have not been very well understood because of the complexity of the structure. The objective of this paper is to find the optical bandgap by measuring the absorption coefficients as a function of wavelength. We found that no good linearity was observed on the Tauc plot for nc-Si: H films. It means that one cannot obtain the optical bandgap of nc-Si: H from the Tauc plot. Instead, a plot of (alpha h nu)(1/5) versus h nu shows a straight line for a wide range of photon energies. The intersection on the h nu-axis is around 1.1 eV, which is coincidentally the same as the bandgap of c-Si. The simplest explanation of the 1/5 power could be a superposition of absorptions from nanocrystallites and amorphous tissues. We also measured the dark current versus voltage characteristics as a function of temperature for nc-Si: H solar cells. The pre-factor of the diode characteristics shows a thermal activation energy of 0.55-0.65 eV. If the Fermi level is at the middle of the bandgap, the mobility bandgap of nc-Si: H is around 1.1-1.3 eV.