TEST & MEASUREMENT
Improvement by understanding
The manufacturing of solar cells is driven by the need for constant improvement in
efficiency and cost reduction. A key tool for these drivers is accurate measurements to
provide the correct data to implement improvements. Igor Lyuboshenko, CEO of
PhaseView provides an overview of major photovoltaic technologies and the positive
Issue I 2009
impact 3D solar cells surface measurement has on the outcomes.
-pv-management.com
T
oday, the photovoltaic industry is driven alternative technologies, which use little or no
by two technologies: crystalline silicon silicon. Thin-film cell technology has efficiencies in
.solar
and thin film. Crystalline silicon (c-Si) wafer-based the range between 8% and 18%, uses only 2% of
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solar cells remain dominant with over 85% of the the silicon required for conventional c-Si or µ-Si
solar cell production, and will remain at 77% of the solar cells, generates less waste and can be
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solar cell production by 2010. They are made from produced through fewer production steps by
either mono- or multi-crystalline silicon wafers and already existing facilities in e.g. flat panel display
have efficiencies typically in the 13% to 22% range. industry.
In the wafer-based sector, cylindrical 5 or 6 inch
wafers are processed into 220 µm thick solar cells, Patterning and manufacturing solar cells
and then connected in series of 60 to 70 cells to In the Czochralski process for the mono-crystalline
form a typical solar panel with a rated output silicon solar cell, solid cylindrical ingots, 400 mm in
power at the 200 W level. As the dominant cost diameter and 1 to 2 m in length, are grown, while
within the c-Si solar cell manufacture is the silicon n-type or p-type dopants are added during the
raw material, reducing wafer thickness to 200 µm process. In the c-Si approach, after the p-type
and increasing surface area by moving to 8 inch phosphorous doping, edges of the cell have to be
wafers by 2015 will produce significant cost etched to avoid contact between the front (n-type
savings. silicon) and back (p-type silicon). This operation,
known as edge isolation, reduces the
Today solar applications account for more than recombination rate of elementary charges and
55% of the silicon wafers consumed worldwide. influences the cell efficiency. The technologies
The rapid ramp in demand for wafers has created currently used for edge isolation are plasma etch,
Figure 1. The laser a shortage in the feedstock supply and has wet chemical etch, screen printing and laser
scribing pattern temporarily pushed the price of silicon cells higher. scribe, the latter having 50% share in this
geometry This has motivated the industry to look for application.
In multi-crystalline silicon solar cell, molten silicon
is poured into a large molding container and
carefully cooled and solidified. Trends in the multi-
crystalline technologies have focused on
increasing the efficiency of modules. One way to
achieve this is by adding more usable surface
area, e.g. by roughening the surface of a cell by
wet chemical process. By creating peaks and
valleys, the cell – in addition to having lateral
surface area – has also a vertical depth, increasing
its usable size.
In thin-film solar cell technology, starting from a
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