ALICE, a Novel Poly-Si on Glass PV Technology
In the ALICIA solar cell,
the absorber material is directly grown as crystalline silicon,
using Si epitaxy at about 600°C by ion-assisted deposition.
One difficulty in the ALICIA approach is to maintain a contamination-free
initial growth surface prior to the start of epitaxy. We use hydrogen
termination of the initial growth surface, whereby a well-controlled
sample heating procedure is required to ensure that the hydrogen
does not desorb from the initial growth surface before Si epitaxy
starts.
In 2003, we developed and patented an alternative
approach that avoids the problem of a contaminated initial growth
surface [1,2]. The idea is to deposit the solar cell absorber material
onto the hydrogen-terminated seed layer at low temperature (~200°C)
as amorphous material and then to crystallise the amorphous material
in a subsequent thermal anneal at elevated temperature (about 600°C).
This method is related to solid-phase crystallisation (SPC), however,
because of the presence of a crystalline seed layer, it is actually
a solid-phase epitaxy (SPE) process. The resulting devices are
thus termed ALICE solar cells (aluminium-induced crystallisation,
solid-phase epitaxy). The key feature in the ALICE process is a
crystallographic transferral of information during a thermal annealing
step from a crystalline layer (e.g., an AIC poly-Si seed layer
on glass) into another, non-crystalline layer (e.g., an a-Si layer).
The non-crystalline layer crystallizes epitaxially on the crystalline
seed layer during the anneal step.
Cross-sectional Schematic representation of
the precursor structure for the ALICE process
The a-Si is deposited by a beam evaporation onto
an oxide-free, hydrogen-terminated, smooth AIC poly-Si film on
glass. The precursor formation is followed by an in-situ vacuum
anneal, whereby the a-Si crystallises via solid phase epitaxy (SPE)
on the AIC poly-Si layer.
A cross-sectional TEM image of an ALICE solar cell
is shown below. This image confirms that a crystallographic transferral
of information has taken place from the AIC seed layer into the
SPE film. Defects are visible, especially along the interface,
however, regions that are virtually free of defects are also visible.
Bright-field, cross-sectional TEM image of
an ALICE poly-Si solar cell on glass (0.5µm scale).
ALICE is a promising novel poly-Si on glass thin-film
PV technology. Current work on ALICE solar cells focuses on the
improvement of the open-circuit voltage (currently 350 mV) and
the fabrication of functioning Mesa-type solar cells.
[1] P.I. Widenborg, A.G. Aberle, A. Straub, N.-P.
Harder, D.H. Neuhaus and O. Nast-Hartley, Fabrication method for
crystalline semiconductor films on foreign substrates, International
PCT patent application PCT/AU03/01313 (7 Oct 2003).
[2] P.I. Widenborg, A. Straub, Y. Huang, and A.G. Aberle, “Solid phase
epitaxy of a-Si on AIC poly-Si seed layers on glass substrates”, 19th
European Photovoltaic Solar Energy Conference, Paris, June, 2004. |
