Herstellung und Charakterisierung von a-Si:H / c-Si Heterostruktur-Solarzellen

This thesis is concerned with the analysis, performance, and optimization of solar cells comprised of a commercially available crystalline silicon wafer base, with low temperature plasma assisted deposited silicon emitter layers. In particular heterojunction solar cells comprised of a n-type amorphous silicon-based layer deposited onto p-type crystalline silicon materials is studied. First relatively simple solar cells are presented in which an uniform layer of n-type amorphous silicon is used in order to establish reproducible preparation conditions and to set a baseline by which more complicated structures could be evaluated. These simple structures achieved 11%solar conversion efficiencies. Analysis of the dark diode characteristics including the current-voltage behavior reveal that the diode quality factor is independent of temperature; the pnjunction dominates the transport in forward direction. The capacitance-voltage profiles (C$^{-2}$ vs. V) are linear; this holds at varied temperature and under illumination. The capacitance varies very slowly with frequency. This indicates a low interface defect state density. As a consequence the performance of these simple solar cells is reasonably good, with a high open circuit voltage (V$_{oc}$ > 600 mV). In contrast the fill factor (FF) is low ($\sim$ 60 %). The underlying causes found was probed through careful analysis of the dark and illuminated current-voltage characteristics: They show a cross-point for different illumination at fixed temperature. The photo current case can not at all be modeled by a simple superposition of the dark diode characteristic and the photo current as is often the case for solar cells in which photo carrier drift is the dominate carrier collection mechanism. The jV-curve and the photo current show an S-shaped characteristic for low temperature and high illumination level. Junction transport modeling indicate that the photo and dark diode characteristics are consistent with a conduction band offset ($\Delta E_{c}$) between the deposited amorphous silicon emitter and the crystalline silicon base of approximately 0.2 eV. This barrier tends to shift the potential drop from the crystalline base to the amorphous emitter. Also the conduction band offset is relatively small it hinders the photogenerated electrons in passing the interface region when the potential drop in the crystalline base is lowered. Samples with p-type amorphous silicon layers on n-type doped wafers are analysed. Qualitatively the influence of the band offset on solar cell characteristics (in this case the valence band due to photogenerated holes) are found, namely cross-point of dark andilluminated current voltage curves and S-shape. In order to further characterize the effects of conduction band offsets and to explore means by which to remove them alloyed n-type amorphous silicon based emitter layers are studied. A larger deviation from superposition is found when larger band gap n-type amorphous silicon-carbon alloy emitters are used. Narrower band gap amorphous silicon-germanium alloy and micro-crystalline silicon emitter layers display characteristics consistent with reduced band offsets. However, in these cases the solar cell performance is limited by a larger interface defect state density: The frequency dependence of the capacity is significantly enlarged, the dark current voltage characteristic is poorer and V$_{oc}$ is lower. In this case intrinsic interface layers improve the interface propertiesand consequently V$_{oc}$ relative, but the solar cell performance of the narrower band gap emitter lavers failed to show improvement relative to the simple case