Complete Report - University of New South Wales

to avoid excessive shading losses. Such cells therefore typically have emitters with sheet
resistivities in the range of 40-50 ohms per square, which inevitably give signifi cantly degraded
response to short wavelength light. To raise this sheet resistivity to above 100 ohms per
square as required for near unity internal quantum effi ciencies for short wavelength light,
serious resistive losses are introduced, both in the emitter and the contact resistance at the
metal to n-type silicon interface.
Furthermore, the conventional design of Figure 4.7.1 has quite poor surface passivation in
both the metallised and non-metallised regions. Even if good ohmic contacts could be made
to more lightly doped emitters, the large metal/silicon interface area would signifi cantly limit
the voltages achievable due to the high levels of recombination in these regions and hence
contribution to the device dark saturation current. These voltage limitations are not of major
signifi cance at the moment due to the limitations imposed by the substrates. However, In the
future as wafer thicknesses are reduced to improve the device economics, the cells will have
the potential for improved open circuit voltages, but only provided the surfaces, including
under the metal, are well passivated.
150-200
p+
n++
p-type
3mm
patterned metal contact
phosphorus
bulk of wafer
rear metal contact
Figure 4.7.1: Conventional screenprinted
solar cell with heavily
diffused emitter and large metal/
silicon interface area for the front
surface metallisation.
metal
Innovative Emitter Design
The new emitter design is shown in Figure 4.7.2. The top surface is diffused to 100 ohms per
square, while the heavily diffused grooves act as semiconductor fi ngers to carry the current
to the screen printed silver fi ngers that run perpendicular to the grooves as shown in Figure
4.7.3. Also not shown in Fig 4.7.2 is the surface passivating dielectric that not only passivates
the lightly diffused surface so as to give near unity internal quantum effi ciencies for short
wavelength light as shown in Figure 4.7.4, but it also isolates the metal from these same
regions to minimise the device dark saturation current. The metal only contacts the silicon
within the heavily diffused grooves, therefore giving a low area contact while still achieving low
contact resistance. Fill factors of 79-80% have been demonstrated with this structure on
large area devices of approximately 150cm 2 , area verifying the effectiveness of this contacting
scheme. Effi ciencies as high as 18.3% (inhouse
measurements) have been achieved,
30 microns
contributed to signifi cantly by the high short
circuit currents gained through the excellent
100 / n-type
spectral response of Figure 4.7.4 and the low
metal and top surface shading losses.
45 microns
Figure 4.7.2: Cross-section of the
investigated innovative emitter design using
semiconductor fi ngers, developed to address
the fundamental limitations of screen-printed
metal contacts with their inability to produce
fi ne lines and make ohmic contact to lightly
diffused emitters.
5 / n++
p-type silicon
n++
rear surface of solar cell
121