Abstract
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