Complete Report - University of New South Wales

Figure 4.7.3 : Screen-printed fi ngers running
perpendicular to the heavily diffused grooves
where electrical contact is made. A dielectric/
AR coating passivates the top surface and
isolates the metal from the lightly diffused top
surface.
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90
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50
40
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S6A-33452
EQE
Reflection
IQE
0
300 400 500 600 700 800 900 1000 1100 1200
Wavelength (nm)
Figure 4.7.4: Excellent IQE achieved through
the use of the innovative emitter design
and low refl ection losses due to AR coated
textured surface and low metal shading
The grooves are typically spaced less
than a millimetre apart so as to minimise
resistive losses within the lightly diffused
emitter, while the screen-printed metal
lines can be spaced signifi cantly further
apart than in normal screen-printed cells of Figure 4.7.1 due to the comparatively excellent
lateral conductivity of the emitter achieved by the very heavy doping within the grooves. This
concept of semiconductor fi ngers does not appear to have ever been used in commercial
solar cells, and has considerable appeal as it facilitates good conductivity within the emitter.
This is acheived without the normal trade-off found in screen printed cells where such regions
of good emitter conduction are located at the top surface and therefore degrade the cell
spectral response and current generating capability, due to the corresponding extremely
short minority carrier diffusion lengths in such regions.
Fig. 4.7.5 shows an SEM of a screen-printed metal line crossing one of the heavily phosphorus
diffused laser grooves. The silicon is only exposed within the grooves, with the screen-printed
metal having been shown to make excellent ohmic contact to the heavily phosphorus
diffused silicon in these regions. Both thick oxides and silicon nitride layers, when used with
appropriate pastes, appear to provide adequate protection to the lightly diffused surface
regions, preventing the screen-printed metal from contacting the silicon.
A simplifi cation of the proposed emitter design is to apply a phosphorus doped passivating
dielectric after lightly diffusing the top surface. The laser scribing conditions for groove
formation are then modifi ed so as to melt the silicon rather than ablate it, thereby allowing
large amounts of phosphorus to penetrate into the molten silicon, producing heavily doped
channels rather then grooves. This avoids the need for etching the grooves and subsequently
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