Diffusion of sodium in copper indium gallium diselenide based materials
Date
2015
Authors
Journal Title
Journal ISSN
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Publisher
University of Delaware
Abstract
Cu(In,Ga)Se2 (CIGS) thin film photovoltaic technology is in the early stage
of commercialization with an annual manufacturing capacity over 1 GW and has
demonstrated the highest module efficiency of any of the thin film technologies.
However there still is a lack of fundamental understanding of the relationship
between the materials properties and solar cell device operation. It is well known
that the incorporation of a small amount of Na into the CIGS film during processing
is essential for high efficiency devices. However, there are conflicting explanations
for how Na behaves at the atomic scale. This dissertation investigates how Na is
incorporated into the CIGS device structure and evaluates the diffusion of Na into
CIGS grain boundaries and bulk crystallites.
Most commercially available CIGS modules are fabricated on soda-lime glass
coated with Mo as the back electric contact, and Na in the glass diffuses through
the Mo layer into the CIGS during film growth. The transport of Na through Mo
was evaluated using a combination of X-ray photoelectron surface accumulation
measurements along with diffusion modeling to obtain diffusion coefficients at
several temperatures. It was determined that Na diffusion in Mo only occurs
along grain boundaries and that oxygen provides an additional driving force to
enhance Na transport. Device data revealed that older Mo substrates with a greater
amount of surface oxide resulted in slightly higher efficiencies due to enhanced Na
incorporation caused by the oxide. This finding shows that Mo substrates could
potentially undergo an oxidation treatment prior to CIGS deposition to optimize
the incorporation of Na.
While it is known that Na segregates at CIGS grain boundaries, debate
remains whether Na diffusion into grain interiors is significant enough to affect device
performance. Single crystal CuInSe2 was used as a model system to represent the
grain interiors of CIGS, and crystals of different composition and dislocation density
were evaluated. Diffusion coefficients and solubility were obtained for each crystal
at two temperatures using concentration depth profiles measured after Na diffusion
with secondary ion mass spectrometry. Characterization of extended defects with
transmission electron microscopy confirmed that the dislocation density was too low
to significantly impact the effective diffusion coefficient. The Cu-poor crystal had a
higher solubility suggesting that Na diffusion is mediated by Cu-vacancies, but this
was not accompanied by an expected increase in diffusion coefficient. The activation
energy for diffusion was similar to values expected for interstitial diffusion, but the
large size of Na+ ions should result in a solubility that is much lower than what
was experimentally measured. To resolve this contradiction, a hybrid interstitialsubstitutional
mechanism is proposed that combines the fast diffusion of interstitial
atoms with the high solubility common for substitutional impurities. Lattice
diffusion of Na proceeds fast enough that CIGS grain interiors should have Na
concentrations near the solubility of 1018 cm−3 limit when manufactured under
standard conditions.
To determine if Na in grain interiors affects device performance, Na was
selectively removed from only grain boundaries using a series of heat treatments in
air at 200 !C to drive Na out of grain boundaries onto the CIGS surface followed
by rinsing in water to dissolve accumulated Na. Due to the low temperature of
this heat treatment, Na at grain boundaries remained mobile while diffusion within
bulk grains was too slow for significant removal. Changes in electrical properties
were evaluated by measuring conductivity and Seebeck coefficient and both were
found to decrease as Na was removed reaching a value similar to Na-free films. This
simultaneous decrease in both properties can be explained by the compensation of
donor defects causing an increase in the free carrier concentration. Devices also
showed a decrease in efficiency and open-circuit voltage after Na removal confirming
that the beneficial effects of Na must be due to its presence at grain boundaries
and not associated with Na within the grains. The findings of this dissertation
potentially could provide guidance for rational optimization of Na incorporation
procedures in the manufacturing of CIGS solar cells.