Thin Film Group
ALICIA Poly-Si on Glass PV Technology
In 1999, a research project was initiated at UNSW aiming at the realisation of a novel type of polycrystalline silicon thin-film solar cell on glass. The idea was to first create a thin large-grained polycrystalline seed layer on glass using aluminium-induced crystallisation (AIC) of amorphous silicon and then to epitaxially thicken the seed layer with ion-assisted deposition (IAD). AIC was chosen in 1999 because of promising initial results at UNSW by Nast et alia [1]. IAD was chosen because it was known to enable high-rate Si growth at low (i.e., glass-compatible) temperatures [2]. The resulting PV devices are thus referred to as ALICIA solar cells (aluminium-induced crystallisation, ion-assisted deposition; [3]).
The first phase (1999-2002) of the ALICIA project focussed on materials aspects of the grown films [3-5]. In 2003 the focus was shifted towards the open-circuit voltage (Voc) of the fabricated devices, since Voc is an easily measurable indicator of the electronic quality of the fabricated material.
Schematic representation of an ALICIA poly-Si solar cell on glass.
Our earlier work, using an IAD machine at ANTEC Solar in Germany, proved the fundamental feasibility of making ALICIA poly-Si solar cells on glass [3,4].Specifically, the combination of the AIC and IAD techniques enables the formation of large-grained poly-Si material (average grainsize about 10 µm) on glass at high deposition rates (compared to other low-temperature techniques) without having toxic gases involved.
The first major achievement in 2003 using the UNSW IAD machine was the realisation of high-quality epitaxial growth on (100)-oriented Si wafers in a non-ultra-high vacuum (non-UHV) environment. Evidence for the high quality of the grown diodes is a Voc of 550 mV, which is comparable to the Voc’s achieved in other IAD systems operating under UHV conditions. This is an important result since lower vacuum requirements immediately translate into lower operating and maintenance costs and hence make the IAD method potentially suitable for the PV industry.
The second major achievement in 2003 using this machine was the realisation of epitaxial Si growth on AIC-seeded glass substrates [6]. The cross-sectional TEM image of a current ALICIA solar cell clearly shows that epitaxial growth has occurred on the AIC seed layer.
Cross-sectional bright-field TEM image of a current ALICIA solar cell. The picture shows, from bottom to top: glass, 80 nm SiN, 200 nm AIC seed layer and 1800 nm IAD-grown poly-Si material.
The IAD-grown film displays, overall, good crystalline quality and a large-grained character. However, as can also be seen in the TEM image, there are grains in the IAD layer in which the epitaxial growth quality has deteriorated during the Si deposition process (the colour changes from bright to dark within one grain). This could indicate a grain-dependent epitaxial growth quality, as predicted for low-temperature epitaxy by Wagner et alia [7]. Furthermore, our TEM studies showed that some of the defects in the IAD layer originate in the underlying AIC seed layer. Note that, in contrast to previously reported ALICIA solar cells, the entire AIC seed layer is now free of silicon islands.
[1] O. Nast, S. Brehme, S. Pritchard, T. Puzzer,
A.G. Aberle and S.R. Wenham, “Aluminium induced crystallisation
of silicon on glass for thin-film solar cells”, Technical
Digest, 11th Inter¬national Photovoltaic Science and Engineering
Conference, Sapporo, 1999, p.727
[2] S. Oelting, D. Martini and D. Bonnet, “Crystalline thin film silicon
solar cells by ion-assisted deposition”, Proc., 12th European Photovoltaic
Solar Energy Conference, Amsterdam, 1994, p. 1815.
[3] A.G. Aberle, P.I. Widenborg, A. Straub, and N.-P. Harder, “Polycrystalline
silicon on glass thin-film solar cell research at UNSW using the seed layer
concept”, Proc., 3rd World Conference on Photo¬voltaic Energy Conversion,
Osaka, 2003 (in print).
[4] A.G. Aberle, N.-P. Harder and S. Oelting, “Formation of large-grained
uniform poly-Si films on glass at low temperature”, Journal of Crystal
Growth 226, 209 (2001).
[5] P.I. Widenborg and A.G. Aberle, “Surface morphology of poly-Si films
made by aluminium-induced crystallisation on glass substrates”, Journal
of Crystal Growth 242, 270 (2002).
[6] A. Straub, P.I. Widenborg, N.-P. Harder, A.B. Sproul, Y. Huang, and A.G.
Aberle, “Present status of ALICIA solar cells on glass”, Tech.
Digest, 14th International Photovoltaic Science and Engineering Conference,
Bangkok, January, 2004, p. 29.
[7] T.A. Wagner, L. Oberbeck and R.B. Bergmann, Materials Science and Engineering
B89, 319 (2002).