High-Efficiency N-Type Commercial Silicon
Solar Cell
One of the biggest advantages of the DSBC Buried
Contact technology is that the processing technology is ambipolar
and can thus easily be applied to p-type or n-type substrates without
the introduction of new processing steps. The final metallisation
sequence, for example, is identical irrespective of substrate polarity.
Presently, two solar cell designs are being developed
for commercial n-type crystalline silicon wafers. They are the
DSBC design and the Interdigitated Back Buried Contact
(IBBC) design. The IBBC design locates all metal electrodes
on the rear surface of the wafer (shown below) and enjoys reduced
optical shading losses, but requires exceptionally high bulk lifetime
and low front-surface recombination velocity. Laboratory solar
cells fabricated on float-zoned wafers demonstrate these and other
important benefits of the IBBC design.
Photograph of an Interdigitated Backside Buried
Contact (IBBC) solar cell being tested for localised shunting
using a temperature-sensitive liquid crystal plastic sheet. The
dashed lines are included to indicate the location of the gridlines.
The circles are centred on the location of the shunt. The culprit
shunt is the base contact gridlines.
Compared to the DSBC solar cells, the IBBC solar
cells are superior in terms of both VOC and JSC.
The VOC of the IBBC design is about 10 mV higher and
the front illumination JSC is about 0.6 mA/cm2 higher
than the DSBC design. It has been determined that the 10 mV of
VOC difference comes from differences in surface recombination
velocities in the two designs, while the difference in short-circuit
current are due to changes in shading losses.
Shunt Visualisation in N-Type IBBC Solar
Cells
One of the key technical problems associated with IBBC solar cells
is controlling electrical properties of the diffusion overlap region
between the rear-side p-diffused emitter and the n-diffused base
groove contacts. Recent investigation shows that shunting from
the phosphorus groove contacting the boron emitter is the cause
of a significant and intermittent shunt resistance problem in n-type
IBBC solar cells.
In collaboration with Dr. Otwin Breitenstein, we have implemented
a simple method of viewing the shunts in IBBC solar cells. Covering
the solar cell, which is under reverese bias, with a thermally
sensitive liquid-crystal (LC) sheet reveals the shunting points.
As shown in the figure, the dash lines indicate the location of
the phosphorus-doped grooves, while circles locate the origins
of the heat source, where shunting exists.
The IBBC/DSBC devices and processes are presently being optimised
with the aim of achieving 20.5% confirmed efficiency. Key problems
under investigation are series resistance analysis and metallization
improvement, boron emitter optimization to remove shunts in the
diffusion overlap region, and fabrication on textured wafers. Currently
textured wafers have unconfirmed efficiencies of about 18%. |
