Complete Report - University of New South Wales

4.1 INTRODUCTION TO RESEARCH
Photovoltaics, the direct conversion of sunlight to electricity using solar cells, is recognised
as one of the most promising options for a sustainable energy future. The ARC Photovoltaics
Centre of Excellence commenced in mid-2003, drawing together previous disparate strands
of work, supported under a variety of programs, into a coherent whole addressing the key
challenges facing photovoltaics, as well as “spin-off” applications in microelectronics and
optoelectronics.
The Centre’s photovoltaics research is divided into three interlinked strands addressing nearterm,
medium-term and long-term needs, respectively. The present photovoltaic market is
dominated by “fi rst-generation” product based on silicon wafers, either single-crystalline as in
microelectronics, or a lower-grade multicrystalline wafer (Fig. 4.1.1). This market dominance
is likely to continue for at least the next decade. First-generation production volume is growing
rapidly, with the technological emphasis upon streamlining manufacturing to reduce costs
while, at the same time, improving the energy conversion effi ciency of the product. Other key
issues involve reducing the manufacturing spread on multicrystalline wafer lines caused by
variability in wafer quality (typically 20% spread in cell output) and elimination of the effects of
boron-oxygen defects in both types of wafers. These defects become active under illumination
and reduce the performance of most commercial modules by about 3%. They also constrain
the specifi cation of the starting silicon wafer, restricting cell design possibilities. Also important
is the reduction of the thickness of the starting silicon wafer without losing performance, to
save on material use, and the development of low-cost techniques for reducing refl ection
from multicrystalline cells.
Figure 4.1.1:
“First-generation” waferbased
technology (BP
Solar Saturn Module,
the photovoltaic product
manufactured in the highest
volume by BP in Europe,
using UNSW buried-contact
technology).
The Centre’s fi rst-generation research is focussed on these key issues. Major emphasis is
upon the “buried-contact” solar cell, originally developed by Centre researchers, the fi rst of
the modern high-effi ciency cell technologies to be successfully commercialised (Fig. 4.1.1).
Centre research seeks improvements to these devices to increase effi ciency, particularly
for devices fabricated on thin wafers. Of key interest is the development of buried-contact
sequences for substrates doped with phosphorus, rather than boron, to avoid the boronoxygen
defect problem previously noted.
Wafers are expensive and need quite elaborate and expensive encapsulation, since they
are brittle and also thermally mismatched to the glass coversheet, making fi rst-generation
technology inherently material-intensive. To avoid the associated cost penalties, several
companies worldwide are commercialising “second-generation” thin-fi lm cell technology
based on depositing thin layers of the photoactive material onto supporting substrates or
superstrates, usually sheets of glass (Fig. 4.1.2). Although materials other than silicon are
of interest for these fi lms, silicon avoids problems that can arise with these more complex
compounds due to stability, manufacturability, moisture sensitivity, toxicity and resource
availability issues. CSG Solar, a partner in the Centre, has commercialised an approach
pioneered by Centre researchers that is unique in that it is based on the use of the same high
quality silicon used for fi rst-generation production, but deposited as a thin layer onto glass.
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