**Date:** January 24, 2005

**Subject:** Quarterly Technical Status Report, August through October
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

During recent subcontract periods, process and capacity development
emphasized scale-up of minimodule production capacity. Long-term plans
included returning to power module production in parallel with
minimodule production after synergistic process development. During this
subcontract period, minimodule production was abandoned and plans were
made to return to power module production. Power module production has
started at a lower production rate than before minimodule production and
the scaleup rate will be determined by:

-   Market demand

-   Development of plans for reduced production costs

-   Development of production plans including new products

-   Training of production personnel for multiple shifts

-   Lead time for power module materials

As discussed in previous quarterly reports, 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. Humidity penetration during accelerated tests was observed
for the first glass/glass package designs. Very promising results have
been demonstrated for a package with an edge seal ("Swiggle" from
TruSeal Technologies) developed in collaboration with the NREL sponsored
National Thin-Film PV Module Reliability Team. A test plan was developed
with UL and testing to qualify Swiggle for use in modules is now
complete. As a result, UL categorizes Swiggle as a "Group 1" insulating
material, which is the top insulating group. Testing primarily with
Swiggle as a component rather than in a laminate included:

-   Dielectric breakdown strength before and after environmental
    conditioning

-   High voltage arc track rate

-   High voltage arc track rate in the presence of aqueous contaminates

-   Volume resistance

-   Damp heat exposure

-   Dielectric breakdown strength for a laminate

-   Damp heat exposure for a laminate

A hermetically sealed package for present module configurations is
created using EVA to laminate circuit plates to a tempered cover glass
and to apply a Tedlar/polyester/Al/Tedlar (TPAT) backsheet. The cover
glass and backsheet are purposely larger than the circuit plate thereby
surrounding the circuit plate with a region where the cover glass and
backsheet are laminated directly together. This region provides a
hermetic seal and the offset between the circuit plate and the edge of
the cover glass provides electrical isolation from the frame. This
configuration fully encapsulates the circuit and any contaminates on the
back of the circuit plate. Elimination of the TPAT backsheet for the
glass/glass package was predicated on the assumption that contaminates
on the back of the circuit plate would be polished off. However, a back
cleaning system purchased for this task has proven inadequate for the
ST80 where two circuit plates are laminated to one cover glass and the
backs of the circuit plates may not be parallel. The approach may also
be inadequate for single circuit plates. Short-term alternative plans
are to use a lower cost backsheet, probably Tedlar supplied with a lower
cost adhesive. Long-term options include sealing the back of the circuit
plate with a low cost coating that is applied during or after
lamination.

The previous quarterly report discussed process development for control
of Na and changes in both precursor and absorber formation processes
that have decreased the number or visibility of dark spots. Modifying
the thermal profile for absorber formation dramatically decreased the
occurrence of dark spots but with a potential negative impact on
adhesion, process variation and yield. Planarization of precursors by
buffing practically eliminated dark spots. However, this process
requires development to demonstrate a practical process. During this
subcontract period, another precursor modification process was explored
with the goal of minimizing dark spots, improving performance and
minimizes transients. Process development emphasized preparing for and
executing initial experiments to implement a sodium sputter dosing
process developed by our Munich R&D colleagues (J. Palm et al., "CIS
module pilot processing applying concurrent rapid selenization and
sulfurization of large area thin film precursors", Thin Solid Films
431-432 (2003), pp. 514-522). As implemented at SSI, a sodium compound
is sputtered on Mo base electrodes prior to deposition of nominally
standard precursors and the precursors are processed through nominally
the baseline SSI process. Preparation efforts included preparing for
safe processing with a new material, evaluating sputtering parameters,
demonstrating XRF process feedback and defining process procedures.

The sputter dosing experiments were implemented with minimal Na
incorporation in the precursors during the reaction process. As
discussed in the previous quarterly report, low Na concentration in
absorbers can be achieved by minimizing Na incorporated from adjacent
glass substrates. Sputter dosed precursors were processed using
nominally baseline conditions with the exception of loading the reactor
with precursors facing other precursors thereby minimizing conventional
sodium dosing (CIS Thin Film Manufacturing At Shell Solar: Practical
Techniques In Volume Manufacturing, R. Wieting et al., 31st IEEE PV
Specialist Conference, 2005). The actual need to minimize the
conventional sodium dosing will be explored later.

Initial results of the sputter dosing development indicate good
performance that is possibly better than baseline performance, good
adhesion, no or minimal dark spots and improved overall visual
uniformity. Circuit plates without sputter dosing are not always
visually uniform, even after ZnO deposition that dramatically decreases
the visibility of nonuniformities. Circuit plates with sputter dosing
nearly always have a uniform appearance. Figure 1 and Figure 2
illustrates differences in visual appearance for a circuit plates with
sputter dosing and a circuit plate that exhibits nonuniformities that
may be seen without sputter dosing.

SSI presently deposits a Mo base electrode and precursors in the same
sputtering system. Purchasing an additional sputtering system and
performing these processes in separate systems is a scaleup option that
could more than double capacity. However, purchasing substrates with the
barrier and base electrode layers is another scaleup option if quality
and cost criteria can be met. The possibility of using purchased base
electrodes with a barrier coating was explored during this subcontract
period. The ability to pattern this base electrode (Mo) and produce
quality plates was explored.

Prior to this subcontract work, shunting along laser P1 scribes
sporadically caused poor performance. Studies of the interdependence of
laser processing conditions and Mo properties, including water or O~2~
incorporation during deposition, were conducted to address this issue
(\"TFPPP - Commercialization of CIS-Based Thin-Film PV. Final Technical
Report, NREL/SR-520-32772). An affected Mo region along the edge of the
patterned Mo was found to be dependent on both base electrode deposition
conditions and laser parameters but primarily on Mo deposition
conditions. The affected region is typically seen as lifting and
cracking along the edge of the patterned Mo and leads to shunting.
Within the range of laser parameters explored, which did not include
pulse durations shorter than 10 ns, modifying laser parameters could
only minimize the effected region. Only modifying the Mo deposition
conditions could eliminate the affected region.

An affected region was found for the purchased Mo and module performance
was poor due to shunting. Figure 3 and Figure 4 are optical micrographs
of laser-produced scribes in SSI baseline Mo and the purchased Mo.
Although features in the center of the scribe in baseline Mo may appear
suspect, there is minimal lifting at the edge of the scribe. Lifting at
the edge of the scribe and cracking is observed for the purchased Mo.

The Dektak scans in Figure 5 quantify differences in lifting at the
affected region for SSI baseline Mo and the purchased Mo. Damage at the
center of the scribes is also seen in the Dektak scribes. This damage
has been further studied by etching off the Mo and characterizing the
damage using optical micrographs and DekTak scans. The damage does not
appear to have a first order impact on performance and further study of
this damage is only likely in conjunction with study of the role of Na
for circuit plate performance.
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height="3.5284722222222222in"}

Additional process development during this subcontract period:

-   Run to run consistency of the introduction of reactant gasses was
    improved by changing from control based on mass flow controllers to
    control based on using an unregulated but fixed flow to a specified
    pressure.

-   Studies were conducted to minimize the production of dark spots and
    improve performance by decreasing the number of parts loaded in a
    reactor and by improving thermal uniformity by decreasing heat loss
    to the door section of the reactor.

-   Thermal uniformity improvements in one reactor were demonstrated by
    modifying the heater controls to emphasize heating the bottom of
    circuit plates.

-   Initial experiments were conducted to improve efficiency by
    increasing absorber thickness while avoiding poor adhesion by
    applying the sputter dosing process. Improved adhesion relative to
    thicker absorbers without sputter dosing was achieved; however, the
    adhesion was not excellent and experimental results may have been
    tainted by glass washing problems.

**Additional Highlights:**

> Dale Tarrant attended the DOE Solar Energy Technologies Program Review
> Meeting -- 2004, October 25-28 in Denver, Colorado and presented a
> paper titled "CIS Product Line Expansion and Production Scaleup at
> SSI" (paper and presentation slides attached).
>
> NREL has performed SIMS depth profile analysis on baseline absorbers
> and two types of low Na samples. Low Na was achieved during reaction
> by facing the absorbers on adjacent plates and by facing an absorber
> to a SiO~2~ coated glass plate.
>
> Robert Gay supplied information on the SSI CdS dip process regarding
> environmental concerns and recycling to Vasilis Fthenakis, Brookhaven
> National Laboratories.
>
> Robert Wieting (SSI) submitted an abstract for 31^st^ IEEE PV
> Specialists Conference, January 3-7, 2005.
>
> In conjunction with TFPPP National Team activities, SSI deposited CVD
> ZnO on Unisun absorber layers.
>
> SSI supplied about 70 minimodules to Larry Olsen's group at PNNL for
> testing multi-layer barrier coatings
>
> SSI supplied minimodules and small gridded devices to David Albin for
> NREL "baselining" activities - developing tools or accelerated tests
> to predict long-term performance. These devices were from previous
> TFPPP studies by Larry Olsen's group at PNNL and Kannan Ramanathan at
> NREL.
>
> SSI received preliminary data from Angus Rockett, University of
> Illinois. SSI samples were supplied for TEM studies related to
> adhesion and exploration of the Mo/CIS interface at dark spots.
> Initial results indicate changes in ZnO microstructure and Cl
> contamination around dark spots.