M.Sc. / Thin film solar cell devices based on amorphous silicon absorber films are promising candidates for the efficient conversion of sunlight into useable, cheap electrical energy. However, typical device structures are rather complex and consist of semiconductor/metal contacts as well as a complicated p-i-n junction. Against this background, the present study focussed on the optimization of certain key components of the device, including the transparent conductive oxide, amorphous silicon absorber layers and substrate/metal structures. These thin films were deposited by direct current (DC) magnetron sputtering and radio frequency (RF) capacitativecoupled gas discharge. In each case, a systematic study was conducted in which all the relevant processing parameters were varied over a broad range. The material quality of the respective films was subsequently correlated against the growth parameters. In the case of DC magnetron sputtered ZnO, w hich is generally used as a transparent conductive oxide in the device structure, the material quality were critically influenced by geometric orientation of the sample with respect to the target, the substrate-target distance, deposition power, working pressure and substrate temperature. Optimum structural, optical and electrical properties were obtained in the case of samples deposited at an angle of 80° with respect to the surface of the target. Bombardment damage was to a large extent prevented when the samples were placed at a vertical substrate-to-target distance of 70 mm, 75 mm away from the center zone of the plasma. The optimum substrate temperature, deposition power and working pressure was experimentally found to be 100°C; 600 mW/cm2 and 5.25 ´ 10-3 Torr, respectively. The structural features of the substrates influenced the morphology and optical properties of the DC sputtered metallic films. In general, the surface roughness increased when the glass substrates were replaced by kapton. The glass/silver structures were characterized by relatively smooth surface morphologies, while glass/aluminium films exhibited spike-like growth features. The material properties of intrinsic amorphous silicon were influenced by the RF power, substrate temperature and deposition pressure. A systematic study revealed optimum structural, optical and electrical properties at depositions powers around 43 mW/cm2, substrate temperatures close to 200°C and deposition pressures in the order of 500 mT. / Professor V. Alberts
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:uj/uj:13542 |
| Date | 28 October 2008 |
| Creators | Sheppard, Charles Johannes |
| Source Sets | South African National ETD Portal |
| Detected Language | English |
| Type | Thesis |
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