**Date:** January 18, 2005

**Subject:** Quarterly Technical Status Report, May through July 2004

THIN-FILM PHOTOVOLTAICS PARTNERSHIP PROGRAM

Process R&D for CIS-Based Thin-Film PV

NREL Letter Subcontract \# ZDJ-2-30630-16

**To:** Harin Ullal, NREL Contract Monitor

**From:** Dale E. Tarrant

**cc:** Loretta Schmidt, Subcontract Administrator

Ken Zweibel, Manager of TFPPP

Robert R. Gay, SSI

David Jeffrey, SSI

Robert Wieting, SSI

The primary objectives of this subcontract are to:

-   Address key near-term technical R&D issues for continued improvement
    in thin-film PV products

-   Continue process development for increased production capacity

-   Develop processes capable of significantly contributing to the DOE
    goals for PV shipments in the year 2020

-   Advance mid- and longer-term thin-film R&D needed by industry for
    future product competitiveness including improving module
    performance, decreasing production process costs per watt produced,
    and improving reliability of CIS thin-film technology

-   Perform aggressive module lifetime R&D directed at developing
    packages that address the DOE goal for modules that should last up
    to 30 years while retaining 80% of initial power

Process and capacity development during this subcontract period
emphasized scale-up of minimodule production capacity. Process changes
to increase capacity were explored for depositions, precursor reaction
and patterning. The impact of new encapsulation methods on transient
effects was explored. In turn, measurement and production protocols were
developed considering the impact of transient effects and the need for
100% testing of a much higher volume of smaller circuits. New patterning
approaches based on depositing a line of material that inhibits ZnO
growth are under development. Syringe based and inkjet printing
development addressed writing multiple lines, automation, pattern width,
thickness and alignment.

As discussed in more detail in the last quarterly report, SSI is also
emphasizing development of "glass/glass" package designs that eliminate
the TPAT backsheet primarily to decrease packaging costs. Simplification
of the package and decreased operating temperature are additional
potential advantages. Two fully operational ST80 prototypes were
fabricated during this subcontract period using prototype components.
Remaining tasks include additional environmental and stress testing,
final process definition for covering the back and finalization of
component designs for the frame, junction box, PC board and front glass.

SSI independently and in conjunction with TFPPP team activities has
pursued improved understanding of transient effects. Transient effects
are difficult to quantify since the magnitude and time constant for
transient effects depend on processing, handling, light exposure history
and measurement protocols. Significant progress has been made through
NREL TFPPP teaming activities. Investigation of the impact on transient
effects for several processes steps and processing conditions have
continued during this subcontract period:

-   Environmental exposure between process steps - Reducing the exposure
    to ambient humidity before and after the CdS deposition
    significantly reduces lamination transients.

-   Buffer layer variations - Thicker CdS decreases transients while
    partial electrolyte soaks apparently have no effect.

-   Reactor process parameters -- Studies for limited variations in
    process parameters, such as the degree of sulfidation, indicate a
    potential dependence on this absorber formation parameter.

-   Positions within reactors -- Initial results indicate that
    transients can be, but are not necessarily, dependent on position
    within a reactor.

-   Differences between reactors

-   Plate orientations and separations within the reactor

Both minimodules and circuit plates have subjective aesthetic
requirements making features that are sometimes seen after precursor
reaction, "dark spots", a potential yield issue. CIS has an
aesthetically pleasing uniform black appearance. This uniformity may be
disrupted by dark spots if the product is viewed close up; the dark
spots typically aren't visible from a few feet away. Optically, dark
spots appear as differences in surface texture often around a "dark
core" (Status Report, Feb. through Apr. 2004). Present understanding is
that dark spots do not have a significant impact on performance although
the probability of generating dark spots may correlate with process
parameters that do have an impact on performance or adhesion. This is
consistent with initial results from NREL indicating that the elemental
profiles are nearly identical for dark spots and for typical areas.

Process R&D is being pursued because of the aesthetics issue and to gain
understanding of the limits of process options. Changes in both
precursor and absorber formation processes have been shown to decrease
the number or visibility of dark spots; however, at the expense of
adding additional processing or compromising throughput. Planarization
by buffing precursors practically eliminated dark spots but at the
expense of adding a process step that would require significant
development. Modifying the thermal profile for absorber formation
dramatically decreased the occurrence of dark spots with potential
negative impact on adhesion, process variation and yield. Additional
process explorations were pursued to learn about the source of dark
spots. The formation of dark spots does not seem to correlate with the
thickness of the SiO~2~ or pinholes, debris from Mo patterning or the
order of precursor deposition.

Sputter deposition of the In precursor may at times lead to "Spits" that
are roughly spherical with diameters of about 12 microns but ranging
from 6 to 25 microns. Although the density has not been extensively
studied, when present, a density of about 20/cm^2^ has been observed.
Spits appear to be on top of the film with minimal disruption of the
surrounding film implying that the spits are from the target; however,
it is possible that a spit landing on the surface early in the
deposition or a spit generated on the surface increase in size during
the deposition.

Dark spots seen after CIGSS formation at times seem to be associated
with spits that are assumed to become dark "cores" (Status Report, Feb.
through Apr. 2004); however, there are other conditions that lead to the
presence of dark spots without the presence of spits. Dark cores are
often observed at the center of a dark spot; however, dark cores are
also observed with no surrounding dark spot. After CIGSS formation,
pattern lines in the Mo may be decorated with a few to a continuum of
dark spots and these dark spots are almost never coincident with a dark
core. Optical micrographs indicate that there are differences in the
structure of the precursor at the Mo patterning presumably due to
modification of precursor nucleation and growth associated with the
topography of the patterning. The density of dark spots with or without
dark cores may be much higher than the density of spits and dependent on
reaction conditions and position within the reactor. Similarly, the
presence or absence of a dark core and decoration of patterning lines
with dark spots are dependent on reaction conditions and position within
the reactor. These density and position dependence observations indicate
that dark spots, with or without dark cores, can be generated by spits
and also by other mechanisms that are related to precursor topology and
the impact of topology on reaction pathways.

NREL (coordinated by Sally Asher) provided SSI with SEM micrographs of
absorber structures selected to explore the impact of process R&D on
dark spot formation, transient effects and the impact of Na content on
adhesion. The study emphasized crossections to explore structural
differences in the "fine grain" region near the Mo/CIS interface that is
characterized by fine grains and voids. Although there are few images
for each sample type, imaged areas were selected as being "typical".
Images of areas for the following conditions were supplied:

-   Relatively low transients

-   Relatively high transients

-   Lower Na content

-   Modified precursor formation processes for decreased dark spots

The two baseline absorber images, Figure 1 and Figure 2, seem to be
typical; a fine grain or gravelly appearance at the Mo/CIS interface,
voids are common near the Mo/CIS interface and the main absorber has
fairly large (on the order of the film thickness) "grains" or fracture
features. The grains are not dramatically different for the two samples;
however, with the exception of very small grains, grain size is not
necessarily a predictor of performance. TEM analysis of SSI absorbers
indicates that grain boundaries are highly ordered, possibly even more
highly ordered than twins, rather than a highly disrupted extended
grouping of dislocations. Current density is high for CIS indicating
that grain boundaries are not strong recombination centers. No
distinction between the two samples is possible for this limited set of
images; there appears to be no dramatic difference in absorber structure
that correlates with transient effects.
![](media/image1.wmf){width="3.457638888888889in"
height="3.004166666666667in"}![](media/image2.png){width="3.5in"
height="3.0166666666666666in"}

The void region is extensive for absorbers with decreased Na content
(Figure 3). It isn't clear if the fine grain, gravelly appearance, at
the Mo/CIS interface extends further into the absorber or if grain size
distribution is different than is typical.
![](media/image3.png){width="3.4895833333333335in"
height="3.0076388888888888in"}

Subject to interpretation, the appearance (Figure 4 and Figure 5) of
absorbers (in this case also with ZnO) formed from buffed precursors
seems to support the observation of better adhesion:

-   A more continuous fine grain or gravelly appearance at the Mo/CIS
    interface

-   Smaller or much smaller voids near the Mo/CIS interface

![](media/image4.jpeg){width="3.5in"
height="3.0166666666666666in"}![](media/image5.jpeg){width="3.5in"
height="3.0166666666666666in"}

The absorber thickness is variable however differences in the appearance
of the main grains are within the variation seen for baseline absorbers.
The CIS (and ZnO) appears to be thicker at one location in Figure 5
while maintaining a good CIS/Mo interface.

SIMS analysis was performed at NREL (Sally Asher, Matthew Young ) on SSI
absorbers from the standard production process and other samples for
comparison. The contrast between baseline absorbers and absorbers with
lower than normal Na concentration will be compared in this report.
Additional samples exploring the dependence of thermal transient on
location within a reactor, absorbers with and without CdS deposition,
absorbers with and without dark spots and Na variations have also been
analyzed but are not reported here since similar results have previously
been reported, analysis is not complete or sample selection did not
adequately consider the dependence on position within the reactor.
Additional sample sets will address some of these issues in the future.

SSI substrates include a SiO~2~ barrier to Na diffusion between the
glass and Mo electrode to avoid poor adhesion and dead areas at pinholes
and patterning lines. Most of the Na incorporated in SSI absorbers is
from adjacent glass substrates; the Na is transported as sodium selenide
in the reactor (CIS Thin Film Manufacturing At Shell Solar: Practical
Techniques In Volume Manufacturing, R. Wieting et al., 31^st^ IEEE PV
Specialist Conference, 2005). Low Na concentration in the absorber was
achieved during absorber formation by facing the precursors toward a
SiO~2~ coated glass plate rather than the glass side of an adjacent
substrate. Figure 6 is a chart showing Na concentration versus depth for
multiple absorbers with baseline processing and two absorbers with low
Na concentrations. Na concentrations throughout all regions, except the
front surface, of low Na absorbers are lower than baseline by about an
order of magnitude. Na concentrations are lowest through the front half
of the absorbers; away from the front surface and surfaces in the fine
grain region. Excluding the surface, Na concentrations are variable
particularly through the front, about 0.5 μ, of the absorbers.
Characterizing this variability will be emphasized in future
studies.![](media/image6.wmf){width="2.922222222222222in"
height="3.7784722222222222in"}

**Additional Highlights:**

-   Dale Tarrant attended a two-day module team meeting held at NREL on
    June 23^rd^ and 24^th^ , 2004 and presented an overview of SSI
    status and plans, "Packaging and Reliability Experience, Questions
    and Plans." (attached).

-   Robert Wieting presented a paper titled "The Shell Solar 245 KW
    Grid-Connected CIS Thin Film PV Rooftop, Array: System Design and
    First Year Performance" at the 19th European Photovoltaic Solar
    Energy Conference, Paris, 2004 (attached).

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-   SSI received a test report from Jill Adelstein with a quarterly
    update on performance of the array of SSI modules deployed at the
    OTF.

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-   SSI supplied a module and module component data to Vasilis Fthenakis
    for fire-simulations at Brookhaven National Laboratories.

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-   An abstract submitted titled "Product Line Expansion and Production
    Scaleup at SSI" was submitted for the DOE Solar Energy Technologies
    Program Review Meeting - 2004

-   In conjunction with TFPPP National Team activities, SSI deposited
    ZnO on multiple sets of Unisun absorber layers and supplied Unisun
    with \~4 sq. ft. of patterned Mo coated glass

-   Eighteen \~10x10 cm samples from SSI minimodule production were sent
    to Kannan Ramanathan NREL for studies related to TFPPP activities.

-   SSI deposited ZnO SSI absorbers with NREL buffers for Kannan
    Ramanathan NREL for studies related to TFPPP activities.

-   SSI sent samples to Angus Rockett, University of Illinois, for TEM
    studies related to TFPPP activities.