Improving structural, optical, and electrical properties of μc-Si:H using non-uniform hydrogen dilution treatment on RF-PECVD prepared layers

Abstract

Hydrogenated microcrystalline silicon (μc-Si:H) has been used in solar cell technology to improve solar conversion efficiency. In this work, the effects of some of the main preparation parameters of μc-Si:H have been studied. The μc-Si:H films were deposited in a standard RF-PECVD at the frequency of 13.56 MHz on glass and silicon substrates without any extra heat treatment. By changing the combination of silane (SiH4) and hydrogen (H2) in the deposition chamber, the proportion of crystalline silicon in the resulting film can be altered. The effects of hydrogen dilution ratio, defined as R=H2SiH4, on the structural, optical, and electrical properties of hydrogenated silicon, have been investigated. Two hydrogen dilution strategies have been used: uniform and non-uniform. The hydrogen dilution ratio was changed from 20 to 33, while other parameters such as RF power, deposition pressure, temperature, and time were remained constant at 30 W (power density of 300 mW/cm2), 0.3 Torr, 300 °C, and 60 min, respectively. Since increasing the number of hydrogen atoms during the deposition process can cause surface damage and consequently, the reduction of crystal growth, the effect of non-uniform dilution ratio has also been considered. The properties of the resulting hydrogenated microcrystalline silicon films were analyzed using X-ray diffraction (XRD), atomic force microscope (AFM), Raman spectroscopy, UV–Visible–near-infrared spectroscopy, and Fourier transform infrared (FTIR) spectroscopy etc. The maximum crystalline ratio of 61%, the crystallite size of 83.3 nm, the energy gap of 1.6 eV, the hydrogen content of 6.7%, and structural coefficient of 0.53, and the highest conductivity have been obtained for the hydrogenated microcrystalline silicon films prepared using the non-uniform dilution ratio of hydrogen. The lower energy gap of hydrogenated microcrystalline silicon with respect to that of the hydrogenated amorphous silicon, results in the absorption of a wider band of the solar spectrum and, therefore, higher efficiency of the related solar cells. In addition, the higher conductivity of hydrogenated microcrystalline silicon compared to that of the hydrogenated amorphous silicon, is of interest for application in photovoltaic and opto-electronic devices.