Production of polycrystalline silicon thin films on foreign substrates using electron cyclotron resonance plasma enhanced chemical vapour deposition
The wide spread adoption of solar photovoltaic cells is impeded by a number of factors,
the primary one of which is the cost. The technology behind the most used cells today is
based on bulk single crystalline silicon wafers. These wafers subsequently undergo
numerous processesto produce a finished module capableo f delivering usable direct
current electricity. Even with all these processes, the single biggest contributor to
production costs is the starting wafer - estimated to account for some 50% of
manufacturing costs. Removing these costs by replacing the wafer is the leading topic in
solar cell research today.
Glass is the most convenient starting point for replacing silicon wafers - it is benign, both
from an environmental and manufacturing viewpoint, and is considerably less expensive
than silicon wafers for a given quantity. As an amorphous material, glass is well suited to
acting as a substrate for amorphous silicon layers used in low cost cells. Amorphous
silicon cells suffer from stability issues and can degrade in performance substantially over
the operational lifetime of the solar cell. To overcomethese problems the amorphous
silicon can be replaced with crystalline silicon material. Generally, the deposition of suitable
crystalline material occurs at a temperature in excess of the softening point of glass. So
however useful glass is as a substrate it is incompatible with simple, low temperature
formation of crystalline silicon using most techniques.
There are two outstanding issues relating to the manufacture of thin film silicon solar cells
that have been researched for this thesis. One is the deposition of silicon layers at a
growth rate high enough to allow for a reasonable throughput of material. The second is
the production of material suited to the task i.e. structurally and electrically.
In this thesis the direct deposition of high quality polycrystalline silicon( near-single orientation
with suitable electrical characteristics) using electron cyclotron resonance plasma enhanced
chemical vapour deposition(E CR PECVD) onto glass is demonstrated.
A new visualisation of the magnetic field used in E R PECVD has given an insight into
the optimisation of the deposition process using this technique. By adjusting the magnetic
field appropriately, an increase in growth rate for deposition of polycrystalline silicon of 2-
25 times that reported in the literature was found. In addition to the characterisation of the deposited material, the process parameters have been fully investigated by analysing the
process plasma characteristics using a Langmuir probe.
An amorphous incubation layer 1 micron thick is seen when the polycrystalline material is
deposited directly on glass, however this layer can be substantially reduced by depositing
on a thin layer of silicon (on the glass) which has been crystallised by excimer laser
irradiation. This indicatesa possible direction in combining these two approaches in future
manufacturing processes for the growth of low-temperature polycrystalline silicon layers
on glass to form photovoltaic devices.