Low power high growth rate deposition of microcrystalline silicon
Microcrystalline growth regimes and solar cells obtained in different pressure and silane depletion conditions are studied in a large area KAI-S plasma reactor. The microcrystalline material quality is systematically investigated by Fourier Transform Photocurrent Spectroscopy (FTPS) to evaluate the defect density. It is shown that higher pressure and silane depletion positively affect the material quality. A clear correlation between FTPS measurements and cell efficiency is established, showing that the limiting factor is the material quality of the intrinsic microcrystalline layers. The highest efficiency achieved so far is 8.2% for a 1.2 μm thick microcrystalline solar cell deposited at 3 Å/s on glass coated with ZnO deposited by low pressure chemical vapor deposition. The efficiency drops to 6.4% for solar cells deposited at 9 Å/s in conditions where silane powder is present in the plasma. Upon modifying the reactor geometry in order to suppress powder formation, solar cells achieve 7.0% efficiency at 12 Å/s. The results can be described by pressure and silane depletion dependent ion bombardment. Transition curves from amorphous to microcrystalline are studied, revealing ion bombardment induced amorphization of the layer at low pressure and low silane depletion. Modeling of the ion energy bombarding the substrate supports this interpretation.