Complete Report - University of New South Wales

The fact that the SPE diodes are n=1 limited strongly suggests that the voltage problem is
not associated with grain boundaries or the p-n junction depletion region. Investigation of
the active doping levels in the base and emitter regions of the SPE cells is in progress, but it
should be noted that the 393-mV SPE diode had no intentionally different doping level from
that of the 463-mV SPC diode.
From the results obtained in this study it is clear that SPE poly-Si material grown on AICseeded
glass has a signifi cantly better crystal quality compared to SPC poly-Si material grown
on bare (i.e., non-seeded) glass. So far, this advantage has not been translated into superior
photovoltaic performance. Analysis of Suns-Voc curves indicates that the problem lies in the
absorber region and/or the emitter region of the SPE diodes.
10..0
2-diode model
Light Intensity (suns)
1.0
0.1
n=1
Exp. data
n=2
Figure 4.4.18: Suns-Voc data
(symbols) and 2-diode model analysis
(solid lines) of sample SPE-A.
0.0
0.36
0.38 0.40 0.42 0.44 0.46 0.46
Voltage (V)
Table 4.4.2: Suns-Voc data of samples SPE-A, SPE-B and SPC.
Sample p-n junction location 1-Sun Voc n=1 Voc n= 2 Voc
SPE-A glass side 423 424 561
SPE-B air side 393 401 499
SPC glass side 463 474 523
4.4.10 Optimisation of the Rapid Thermal Annealing and Hydrogenation
Processes
Due to the fabrication process and structure of poly-Si thin-fi lm solar cells, defects are
present in the material. These defects can drastically reduce the open-circuit voltage (Voc)
and the short-circuit current (Jsc) of the device. As such, the removal of these defects is
essential and is typically done via thermal annealing and subsequent hydrogen passivation. An
additional concern is achieving complete activation of dopants within the device.
Thermal annealing is well known to dramatically reduce defects and also to activate dopants.
Traditionally this annealing has been done in a tube furnace. Rapid thermal annealing (RTA)
has replaced the tube furnace in many applications. RTA processes are ideally suited for
poly-Si thin-fi lm solar cells on glass substrates [4.4.18 - 4.4.21] as precise control of the
thermal profi le the device receives can be achieved. It is well known that hydrogen passivation
(hydrogenation) is crucial in passivating defects. In thin-fi lm devices this hydrogenation is
typically done using an RF or microwave plasma. The Voc is typically more than doubled and
the Jsc is tripled by the hydrogenation step [4.4.18 - 4.4.21]. The combination of these two
processes is critical in drastically improving poly-Si thin-fi lm solar cells.
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