Complete Report - University of New South Wales

Current Density (mA/cm 2 )
16
14
12
10
8
6
4
2
Jsc = 14.8mA/cm 2
Voc = 403mV
FF = 0.51
Eff. = 3.0%
4
Power density (mW/cm 2 )
3
2
1
Figure 4.4.15: Current-voltage
and power-voltage curves of
the best ALICIA solar cell made
as yet. The glass substrate is
planar.
0
0
0 100 200 300 400 500
Voltage (mV)
4.4.9 Optimisation of PECVD Alice Solar Cells
ALICE stands for “ALuminium-Induced Crystallisation solid-phase Epitaxy”. The idea behind the
patented ALICE solar cell technology is to deposit the absorber onto an H-terminated AIC
seed layer at very low temperature ( ~ 200°C) as amorphous material and then to crystallise
the amorphous material in a thermal annealing step at elevated temperature (570-600°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 key feature
in the ALICE process is a crystallographic transferral of information during a thermal anneal
from the seed layer into the crystallising a-Si overlayer [4.4.17]. The SPE process is a twostage
process where the fi rst step is an a-Si deposition at around 200-300°C substrate
temperature onto a hydrogen-terminated AIC poly-Si seed layer, followed by the second step
which is a thermal anneal at about 570°C substrate temperature during which the a-Si
crystallises via solid phase epitaxy. It seems possible that, compared to SPC solar cells (which
do not feature a seed layer), the epitaxial growth feature in SPE solar cells improves the
crystal quality and hence the effi ciency of the resulting solar cells.
During 2005, we investigated the structural quality as well as the diode properties of poly-Si
thin-fi lm solar cell fabricated by the SPC and SPE methods, utilizing PECVD a-Si:H precursor
material. Two post-deposition treatments, rapid thermal annealing (RTA) and hydrogenation,
were used to increase the voltages of the cells. The poly-Si fi lms were characterized with
Raman, UV refl ectance, and Suns-Voc measurements. All samples were deposited onto 3
mm thick Borofl oat33 glass substrates (Schott AG). A conventional 13.56 MHz parallelplate
PECVD machine was used for the SiN and the doped a-Si:H layers. Phosphine and
diborane-doped silane gas was used as the doping gas for the deposition of n-type and p-
type a-Si:H fi lms, respectively. SPC and SPE crystallisation anneals were performed ex-situ
in a conventional nitrogen-purged atmospheric-pressure tube furnace. The SPC anneal was
performed iso-thermally at about 600°C for 15 hours, whereas the SPE anneal had a 2-step
anneal (15 hours at 570°C, followed by 6 hours at 600°C). RTA was done at 900°C for 4
minutes. Hydrogenation was performed in a parallel-plate reactor at 13.56 MHz, using a
hydrogen plasma for 1 hour at a glass temperature of about 470°C. Table 4.4.1 summarizes
the different sample structures used in this study.
Table 4.4.1: Structures of the investigated samples.
Sample Final structure
SPE-A Glass / ~ 70 nm SiN / 200 nm p++ AIC poly-Si / 1200 nm n- SPE poly-Si /100 nm n+ SPE poly-Si
SPE-B Glass / ~ 70 nm SiN / 200 nm p++ AIC poly-Si / 1200 nm p- SPE poly-Si / 100 nm n++ SPE poly-Si
SPC Glass / ~70 nm SiN / 200 nm n++ SPC poly-Si / 1200 nm p- SPC poly-Si / 100 nm p+ SPC poly-Si
63