Characterisation of intrinsic silicon oxide absorber layers for use in silicon thin film solar cells

Abstract

The use of a wide bandgap absorber layer in the top cell of a multi-junction silicon thin film solar cell is necessary to achieve a high-conversion efficiency. A higher bandgap of the absorber results in a higher open-circuit voltage (V oc) of the cell. In this work, intrinsic hydrogenated amorphous silicon oxide (i)a-SiO:H films have been prepared by using 13.56 MHz radio frequency plasma enhanced chemical vapour deposition (RF-PECVD) at a substrate temperature of 195 C. The carbon dioxide (CO 2) to silane (SiH 4) ratio r c was varied and the influence of the ratio r c on the optoelectronic film properties was investigated. These thin films have been studied in detail in terms of their dark (σ d) and photo (σ ph) conductivity and photoresponse PR (σ ph /σ d). The defect density N d and Urbach energy E U were determined by constant photocurrent method (CPM). The optical bandgap E g,Tauc was derived from Tauc plots. Optical constants were determined by spectroscopic ellipsometry measurements in the range between 300 and 1000 nm. It was found that the increase in the CO 2 to SiH 4 ratio r c not only leads to a higher optical bandgap, but also higher defect density N d and Urbach energy E U and on the other hand to a lower photoresponse PR. A suitable photoresponse PR = 8.64 × 10 5 at a high bandgap of E g,Tauc = 1.91 eV and a defect density of N d = 1.7 × 10 16 cm -3 was achieved. The refraction index and extinction coefficient were lowered with increasing r c. The analysis of light-induced degradation in the (i)a-SiO:H layers showed a smaller increase of deep defects N d and a higher increase of the Urbach energy E U in terms of the light soaking time with respect to the (i)a-Si:H reference layer.