Processing and Characterization of Novel low bandgap Ge Alloys for PV Applications

Abstract

Low-cost multijunction photovoltaic devices are the next step in the solar energy revolution. Adding a bottom junction with a low bandgap energy material through plasma-enhanced chemical vapour deposition (PECVD) processing could provide a low-cost boost in conversion efficiency. A logical candidate for this low-bandgap material is germanium. A showerhead configuration instead of a direct PECVD reactor is investigated to better control reaction mechanics. In this work, hydrogenated amorphous germanium (a-Ge:H) films processed with a wide range of deposition pressures, powers, temperatures and GeH4 dilution in hydrogen are characterized using elemental analysis, vibrational analysis and analysis of the optoelectrical properties. We have identified a small processing window in which intrinsic a-Ge:H films are processed reproducibly. Two types of degradation were also studied: light-induced and oxidation in air. The lowest E04 achieved for intrinsic films was 0.8 eV with an activation energy of 0.39 eV which lies at the centre of the bandgap. The degradation experiments showed no sign of oxygen signature from EDX measurements performed after two months and FTIR measurements performed after two weeks. Light-induced degradation (LID) was performed by exposing the samples to over 80 hours; photoconductivity did not degrade below the initial value for the duration of the exposure. We believe that the showerhead configuration has increased the surface mobility of Ge-radicals during deposition, allowing them to reach sites in the layer that would otherwise be void.