Plasma Deposition of Microcrystalline Silicon Solar Cells: Looking Beyond the Glass

Microcrystalline silicon emerged in the past decade as highly interesting material for application in efficient and stable thin film silicon solar cells. It consists of nanometer-sized crystallites embedded in a micrometer-sized columnar structure, which gradually evolves during the SiH4 based deposition process starting from an amorphous incubation layer. Understanding of and control over this transient and multi-scale growth process is essential in the route towards low-cost microcrystalline silicon solar cells. This thesis presents an experimental study on the technologically relevant high rate (5-10 $\mathring{A}$ s$^{−1}$) parallel plate plasma deposition process of state-of-the-art microcrystalline silicon solar cells. The objective of the work was to explore and understand the physical limits of the plasma deposition process as well as to develop diagnostics suitable for process control in eventual solar cell production. Among the developed non-invasive process diagnostics were a pyrometer, an optical spectrometer, a mass spectrometer and a voltage probe. Complete thin film silicon solar cells and modules were deposited and characterized. It was established that under state-of-the-art high rate deposition conditions new challenges arise regarding temperature control since the high RF power dissipated in the plasma causes the substrate to heat up significantly during film growth. On the basis of experimental results a semi-empirical engineering model was developed that describes the magnitude of this plasma induced substrate heating for arbitrary reactor geometry and process settings. The experimental study revealed that plasma induced substrate heating leads to sub-optimal material quality and solar cell performance and it should be prevented by designing and incorporating a fast active substrate temperature control in deposition reactors. Another treated aspect of high rate deposition is the required high dilution of the SiH$_{4}$ gas in H$_{2}$, which is of importance to the on-going cost price reductions. It was established that under conditions of low H2 dilution transient depletion of the SiH$_{4}$ source gas evolves through diffusion of SiH4 from the surrounding reactor volume back into the plasma and prevents successful nucleation of crystallites. A self-consistent analytical engineering model was developed for the general description of this transient depletion of source gases as the [...