Evaporated Silicon-based SPC Poly-Si on
Glass PV Technology
In 2003, we developed a poly-Si on glass solar cell
technology based on solid-phase crystallisation (SPC) of evaporated
amorphous silicon (a-Si). Compared to the established SPC poly-Si
thin-film PV technology (which uses PECVD for a-Si deposition;
[1]), evaporation has a number of advantages such as high deposition
rate, excellent Si source material usage, and avoidance of toxic
gases. Our process starts with the deposition of an approximately
2µm thick layer of a-Si on bare (or SiN-coated) glass by
electron beam evaporation at a high deposition rate of about 200
nm/min. The resulting diode structure is glass/SiN (optional)/n+/n-/p+.
Doping of the evaporated a-Si is realised in-situ using phosphorus
and gallium effusion cells. Then, the a-Si is crystallised by SPC
in a tube furnace (N2 ambient) at 600°C during 48
hours.
Our work in 2003 has shown that good-quality poly-Si
diodes can be made on glass by this technology [2]. This can be
seen in the cross-sectional transmission electron microscope (TEM)
image of one of our films shown below. The poly-Si grains are up
to 1.5 µm wide and are preferentially (111)-oriented.
Cross-sectional TEM image of a SPC poly-Si
film made on planar SiN-coated glass using evaporated a-Si as
the precursor material.
After SPC, the thin-film diodes received two follow-up
treatments (first a rapid thermal anneal (RTA), followed by a hydrogenation
process in a PECVD ammonia plasma)to improve their structural and
electronic quality. It was found that the two follow-up treatments
more than double the 1- Sun Voc of samples.
Furthermore, first solar cells were made from the
hydrogenated material. The cells are circular (Ø = 4mm),
Mesa-type structures with a thickness of about 2.5µm, as
shown in the figure below. A transparent conductive oxide (Al-doped
ZnO, 80 nm) is used as the top electrode and antireflection coating,
whereas the bottom n++ layer is contacted with an evaporated Al
film.
Cross-sectional view of the Mesa-type SPC poly-Si
solar cells. Note that the glass is planar and that the device
has no light trapping feature, limiting the short-circuit current
potential.
One of the fabricated solar cells demonstrated a
Voc of 345 mV, a short-circuit current density of 6.03 mA/cm2,
a fill factor of 64 %, and a conversion efficiency of 1.35 %.
[1] T. Matsuyama, N. Terada, T. Baba, T. Sawada,
S. Tsuge, K. Wakisaka and S. Tsuda, “High-quality poly¬crystalline
silicon thin film prepared by a solid phase crystallisation method”,
Journal of Non-Crystalline Solids 198-200, 940 (1996).
[2] D. Song, P. Widenborg, A. Straub, Y. Huang and A.G. Aberle, “Polycrystalline
silicon films on glass by solid phase crystallization of evaporated a-Si”,
Tech. Digest, 14th International Photo¬voltaic Science and Engineering
Conference, Bangkok, January,. 2004, p. 327. |
