Complete Report - University of New South Wales

Silicon Nitride Passivation of Bare, n-type Silicon Surfaces
Figure 4.3.2.12 shows the effective surface recombination velocity (SRV) versus injection level
curves for three different refractive index fi lms on FZ wafers. For comparison purpose, the
effective SRV of a sample that had a 100 ohm/sq phosphorus diffusion and oxidation are
also plotted on the same graph. The two dotted lines on the graphs that cross the effective
SRV curves represent the locus of the 1-sun open-circuit voltage and approximate maximumpower
voltage operating points for a high-effi ciency, 200-micron thick solar cell.
As illustrated in the graph, the surface passivation quality of as-deposited SiN is exceptional,
achieving better recombination than the baseline phosphorus diffused and oxidized surface.
In our work, silicon-rich fi lms give the best surface passivation at as-deposited conditions.
Table 4.3.2.2 summarizes the extracted SRV and implied open
circuit voltages for various as-deposited silicon nitride fi lms.
Figure 4.3.2.12: The effective surface recombination velocity vs.
minority carrier density plots of various SiN on 1 ohm.cm n-type FZ
wafer.
Table 4.3.2.2: The best effective SRV, effective SRV at 1-sun Voc, and
implied voltage at 1-Sun Voc operating points of various SiN fi lms on
planar n-type surface of a 200-micron thick wafer.
Refractive index Wafer type Best SRVeff (cm/s) SRVeff at Voc (cm/s) Imp Voc (mV)
2.00 n-FZ 7.01 13.34 723
2.35 n-FZ 2.29 10.74 732
2.75 n-FZ 1.96 9.53 739
Phos and oxide n-FZ 5.23 15.48 709
A voltage test structure device was prepared to further demonstrate the exceptional
passivation quality provided by our SiN fi lms on n-type wafers. The device has two very small
diffused contact regions, one was phosphorus diffused and the other boron diffused. A lowindex
(nr = 2.00) SiN fi lm was deposited on both sides of the wafer in the as-deposited condition.
Small openings made photolithographically in the contact regions allowed the measurement
of the open-circuit voltage at approximately 1-sun, 25ºC of 719 mV. Further improvement
is expected during 2006, with open-circuit voltages approaching 725 mV expected for test
structures and 700 mV in fully fabricated silicon nitride passivated solar cells.
Silicon Nitride Passivation of Boron Diffused Emitters
42
The ability of silicon nitride to passivate a boron-diffused emitter is important for making higheffi
ciency n-type silicon solar cells. Our results show that for some diffusion sheet resistances,
boron diffusion process, and subsequent thermal treatment, SiN is particularly well-suited for
passivating boron-diffused surface on n-type silicon. Figure 4.3.2.13a shows the implied open
circuit voltages (extracted from photoconductance measurements) of various passivation
fi lms on boron-diffused emitters with sheet resistances ranging from roughly 45 ohm/sq to
250 ohm/sq, including silicon dioxide and various silicon nitride fi lms. The results illustrate
that at the as-deposited condition for the sheet resistance range studied in this work, SiN
provides poorer passivation compared to silicon dioxide. Interestingly, it appears that, at the
as-deposited condition, the three different SiN fi lms provide roughly the equivalent passivation
quality and have similar trend across the range of sheet resistances studied.