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|Title: ||Development of non-vacuum and low-cost techniques for Cu(In, Ga)(Se, S)2 thin film solar cell processing|
|Authors: ||Hibberd, Christopher J.|
|Keywords: ||Solar cells|
|Issue Date: ||2009|
|Publisher: ||© Christopher John Hibberd|
|Abstract: ||Solar photovoltaic modules provide clean electricity from sunlight but will not be able to
compete on an open market until the cost of the electricity they produce is comparable to that
produced by traditional methods. At present, modules based on crystalline silicon wafer solar
cells account for nearly 90% of photovoltaic production capacity. However, it is anticipated
that the ultimate cost reduction achievable for crystalline silicon solar cell production will be
somewhat limited and that thin film solar cells may offer a cheaper alternative in the long
term. The highest energy conversion efficiencies reported for thin film solar cells have been
for devices based around chalcopyrite Cu(In, Ga)(Se, S)2 photovoltaic absorbers.
The most efficient Cu(In, Ga)(Se, S)2 solar cells contain absorber layers deposited by vacuum
co-evaporation of the elements. However, the cost of ownership of large area vacuum
evaporation technology is high and may be a limiting factor in the cost reductions achievable
for Cu(In, Ga)(Se, S)2 based solar cells. Therefore, many alternative deposition methods are
under investigation. Despite almost thirty companies being in the process of commercialising
these technologies there is no consensus as to which deposition method will lead to the most
cost effective product.
Non-vacuum deposition techniques involving powders and chemical solutions potentially
offer significant reductions in the cost of Cu(In, Ga)(Se, S)2 absorber layer deposition as
compared to their vacuum counterparts. A wide range of such approaches has been
investigated for thirty years and the gap between the world record Cu(In, Ga)(Se, S)2 solar
cell and the best devices containing non-vacuum deposited absorber layers has closed
significantly in recent years. Nevertheless, no one technique has demonstrated its superiority
and the best results are still achieved with some of the most complex approaches.
The work presented here involved the development and investigation of a new process for
performing one of the stages of non-vacuum deposition of Cu(In, Ga)(Se, S)2 absorber layers.
The new process incorporates copper into an initial Group III-VI precursor layer, e.g. indium
gallium selenide, through an ion exchange reaction performed in solution. The ion exchange
reaction requires only very simple, low-cost equipment and proceeds at temperatures over
1000°C lower than required for the evaporation of Cu under vacuum.
In the new process, indium (gallium) selenide initial precursor layers are immersed in
solutions containing Cu ions. During immersion an exchange reaction occurs and Cu ions
from the solution exchange places with Group III ions in the layer. This leads to the
formation of an intimately bonded, laterally homogeneous copper selenide – indium (gallium)
selenide modified precursor layer with the same morphology as the initial precursor.
These modified precursor layers were converted to single phase chalcopyrite CuInSe2 and
Cu(In, Ga)Se2 by annealing with Se in a tube furnace system. Investigation of the annealing
treatment revealed that a series of phase transformations, beginning at low temperature, lead
to chalcopyrite formation. Control of the timing of the Se supply was demonstrated to
prevent reactions that were deemed detrimental to the morphology of the resulting
chalcopyrite layers. When vacuum evaporated indium (gallium) selenide layers were used as
initial precursors, solar cells produced from the absorber layers exhibited energy conversion
efficiencies of up to 4%. While these results are considered promising, the devices were
characterised by very low open circuit voltages and parallel resistances.
Rapid thermal proce|