Hydrogenated amorphous silicon: impact of process conditions on material properties and solar cell efficiency

Stuckelberger, Michael (Stückelberger, Michael Elias)
Advisors: Ballif, Christophe; Haug, Franz-Josef

Thin-film silicon solar cells are one possible answer to the increasing energy demand of today. Hydrogenated amorphous silicon (a-Si:H) plays a crucial role therein - as absorber layers, but also as doped layers to build p-i -n junctions. This thesis is devoted to a-Si:H, with the main focus on thin-film silicon solar cells, but also with applications for opto-electronic devices, detectors, and other types of solar cells such as heterojunction solar cells. We discuss models of a-Si:H and develop further the representation of defects by amphoteric states. Using a simple model, we show - in agreement with layer-by-layer simulations and experimental results - that trapped electrons tend to dominate the electric field deformation in the initial state, whereas positively charged defects dominate in the degraded state. Experimentally, we define the deposition parameter space accessible by plasma-enhanced chemical vapor deposition (PECVD) and explore that space by varying the deposition temperature, pressure, excitation frequency, power, and H2/SiH4 ratio for intrinsic absorber layers. This leads to a catalog of a-Si:H absorber layers with tunable properties and we incorporate these materials into solar cells. For every pressure, we find an optimum hydrogen dilution where the light-induced degradation of solar cells is minimal and comparable for all pressures. Using narrow-bandgap absorbers, we demonstrate short-circuit current densities of Jsc=18.2 mA/cm2 with a 300-nm-thick absorber layer and extract more than 20 mA/cm2 from a cell with a 1000-nm-thick absorber layer. Using wide-bandgap absorbers, we achieve open-circuit voltages (Voc) of 1.04 V and Voc-fill factor products of 739mV. For such materials, we find an increased Voc dependence on substrate roughness. This is investigated by transmission electron microscopy and is attributed to porous a-Si:H material grown above peaks on the textured substrates. Depositing absorber layers in a triode reactor, we achieve efficiencies of 10.0% after light soaking. Further, we describe observations of a reversible, light-induced Voc increase of solar cells with thin p-type layers, and decrease with thick p-type layers, with a magnified effect on rough substrates. Based on layer measurements and simulations, we attribute the Voc increase to the degradation of the p-layer and the Voc decrease to the degradation of the absorber layer.

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