Piezoelectric photothermal study of the optical properties of microcrystalline silicon near the bandgap

The optical absorption spectra of hydrogenated microcrystalline silicon (μc-Si:H) films deposited on glass and transparent conductive oxide (TCO) covered glass substrates were measured by using the piezoelectric photothermal (PPT) technique. The effects of the deposition rate on the optical absorption of μc-Si:H thin films were investigated from the nonradiative transition point of view. It was found that increasing the deposition rate resulted in a decrease of optical absorption and a shift of effective energy gap to the higher photon energy side. These changes in the optical properties of μc-Si:H cause the decrease of the number of carriers optically generated by absorbing sunlight, and results in a reduction in the photovoltaic conversion efficiency of the solar cells for high deposition rate samples. The usefulness of the PPT method for investigating the optical properties of thin and transparent μc-Si:H films was also demonstrated.