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.

To make silicon thin film solar cells attractive to the market, their efficiencies should reach the efficiencies of the dominant crystalline silicon solar cell. Therefore the search for ways to improve the efficiency of silicon thin film solar cells continues. TCO front contact layers and back reflector layers play a significant role in the increase of thin film solar cell efficiencies. For the optical characterization of these TCO layers, commonly used methods are found to differ significantly in results regarding the extinction coefficient which makes their accuracy questionable.

This research consists of two main parts. For the first part, the commonly used optical characterization methods spectroscopic ellipsometry and spectrophotometry + data analysis in SCOUT are analyzed in accuracy and compared to newly introduced methods using spectrophotometry + data analysis in GenPro4 and photothermal deflection spectroscopy + data analysis in GenPro4 to build a guide on optical characterization of TCO materials. For the second part, the optical response determined using the software GenPro4 of a double junction silicon thin film solar cell for a novel bi-layer front contact design consisting of IOH and i-ZnO will be compared to the optical response for standard used AZO, and ITO single layers to find the best front contact design. Furthermore, the optical response of a double junction silicon thin film solar cell for a back reflector containing an i-ZnO layer on top of the silver back contact will be compared to the optical response for a back reflector containing an AZO layer on top of the silver back contact to find the best back reflector design.

From the results of the optical characterization methods, it is concluded that photothermal deflection spectroscopy + data analysis in GenPro4 is the most accurate method for determining the extinction coefficient of a TCO material. The results of the optical simulations for front contact TCO and back reflector TCO designs showed that the bi-layer can enhance the optical response of a double junction silicon thin film solar cell significantly compared to the AZO and ITO single layers. The i-ZnO TCO back reflector layer was found to induce less parasitic absorption and therefore a better optical response of the double junction silicon thin film solar cell.