Complete Report - University of New South Wales

Secondary ion mass spectroscopy (SIMS) measurements were performed externally to
investigate the dopant and impurity profi les in a fi nished EVA cell, see Fig. 4.4.11. The dopant
profi les for both boron and phosphorus are satisfying; however, despite the good voltage
of the cell, the impurity levels of carbon (C) and oxygen (O) are surprisingly high. Tails of
contaminants close to the air-cell interface indicate high carrier recombination in this region,
and thus a small diffusion length. This will most severely affect the minority carrier collection
and hence the short-circuit current of the cell. More work is required to fi nd and eliminate the
source of these contaminants, and to thereby further improve these solar cells.
1E21
Cell-SiN interface
Fig. 4.4.11: SIMS analysis of the
dopants (B, P) and the contaminants
O, C, and N in a recent EVA solar
cell.
concentration (atoms/cm -3 )
1E+2
1E+1
1E+1
1E+1
N
C
O
B. ALICIA Solar Cells
B
P
1E16
0 0.2. 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
ALICIA stands for “ALuminium-Induced Crystallisation Ion-Assisted deposition” [4.4.15]. The
idea behind the patented ALICIA solar cell technology is to directly (i.e., epitaxially) grow the
crystalline absorber layer on a hydrogen-terminated seed layer made by Aluminium-Induced
Crystallisation (AIC) on glass. The AIC precursor structure is glass / SiN (100 nm) / Al
( ~ 200 nm, evaporated) / a-Si ( ~ 300 nm, sputtered). This structure is annealed for 12 hours
at about 450°C in a tube furnace at atmospheric pressure and then the Al and the excess Si
are removed. The resulting average grain size is about 10 to 20 µm. For low-temperature Si
epitaxy we use IAD (Ion-Assisted Deposition) because this method is capable of high-rate Si
growth at low (i.e., borosilicate glass compatible) temperatures of about 600°C. Figure 4.4.12
schematically shows a Mesa-type ALICIA solar cell. The two IAD-grown poly-Si layers have
a combined thickness of about 2 µm and are deposited in less than 30 minutes (this time
includes sample heating, Si deposition, cooling and unloading of the sample). In recent years
we have developed an IAD process that is capable of achieving good-quality epitaxial Si in a non
-UHV environment [4.4.16]. This makes IAD Si epitaxy potentially suitable for the PV industry.
p+ layer
Rear metal
n+ layer
Absorber
layer
Seed layer
Glass
Sunlight
V, I
Front metal
SiN
One limitation of the ALICIA cell to date has been poor
light absorption. Due to the fl at surfaces of the planar
glass superstrates used so far, a great deal of light
refl ects from the front surface of the cell, and this is
a severe limitation for the cell’s effi ciency. In addition,
a large fraction of low-energy light which does enter
the cell reaches the back side, and can be transmitted
through. An aluminium fi lm has been used to date to
refl ect this light back into the cell. This should allow light
which is usually transmitted to refl ect back through
the cell before being lost, giving an increase in light
absorption and cell current.
Figure 4.4.12: Schematic of a Mesa-type ALICIA solar
cell realised by shadow-mask epitaxy (not to scale).
61