**Date:** August 8, 2004

**Subject:** Quarterly Technical Status Report, February through April
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:** Vicki Riddell, 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
for 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.

SSI is developing "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. Figure 1 is a sketch comparing the present production
package and a glass/glass package. In addition to decreased cost,
simplification of the package may increase yield. Decreasing the
operating temperature will lead to higher efficiency for modules in the
field.

![](media/image1.wmf){width="5.626388888888889in"
height="2.6118055555555557in"}

Figure 1. "TPAT" and "glass/glass" package designs.

Progress developing and testing glass/glass packages using a single 40W
circuit plates is being extended to a new higher power, ST80, product.
Figure 2 is a sketch of the proposed ST80 glass/glass package which is
designed for cost reduction and marketability. Two nominally 40W circuit
plates are laminated to a common tempered glass front sheet. Laser edge
deletion, removal of all films forming an approximately 1 cm boarder,
provides electrical isolation from the frame. An edge seal developed in
collaboration with the NREL sponsored National Thin-Film PV Module
Reliability Team (TFMRT) is used at the perimeter of both plates. A
screen in front of the ribbons is included to achieve an aesthetically
pleasing uniform black appearance.

![](media/image2.wmf){width="5.36875in" height="2.1930555555555555in"}

Figure 2. Proposed ST80 glass/glass package.

Humidity penetration during accelerated tests was observed during
testing of initial glass/glass package designs. Very promising results
have now been demonstrated for prototype packages with the edge seal
developed in collaboration with the TFMRT. In addition to a barrier to
humidity ingress, this edge seal solved problems during lamination -
breakage of the circuit plate due to compression of the laminate edges.
Various package design options were explored including variation in edge
seal type, edge seal width, EVA thickness and lead routing. Prototype
40W circuit plates packages incorporating an edge seal have passed
accelerated tests, including the damp heat test, after outdoor recovery
from thermally induced transients.

PVB as an alternative to EVA as a pottant was tested. Package failures
occurred during damp heat accelerated testing of a package with EVA as
the pottant and without an edge seal. Previous testing indicated that
water vapor ingress for small EVA packages was primarily due to
diffusion along the EVA/glass interfaces rather than through the EVA.
PVB was tested because it may have lower interface diffusion than EVA.
Initial damp heat tests using 10W modules indicated potentially lower
losses and less humidity ingress during damp heat testing. However, at
least part of the lower losses was due to lower initial efficiencies
after lamination with PVB. Including an edge seal with EVA as the
pottant addresses the water vapor ingress issue thereby making
exploration of the PVB option less important. Also, there are several
potential issues with the use of PVB:

-   PVB is hygroscopic making it particularly sensitive to handling
    during manufacturing.

-   PVB is stiff and tends to shrink nonuniformly which may make
    lamination lay-up particularly difficult for ST80 configurations.

-   The lamination temperature is high and the cycle time potentially
    long thereby exacerbating transient effects and potentially limiting
    the use of low temperature solders.

-   Softening may complicates accelerated testing by making laminates
    stick to the fixturing within the environmental chambers.

-   High conductivity may make packages more susceptible to corrosion or
    high voltage failure.

Accelerated testing beyond the normal damp heat test, 1000 hours of
exposure to 85ºC and 85% relative humidity, has been pursued to identify
potential weaknesses in the glass/glass package design, particularly the
impact of humidity. Accelerated environmental testing in the dark
introduces thermally induced transients that recover with light
exposure. Control modules were exposed for the same duration at 85ºC but
without high humidity thereby allowing distinction between humidity
induced effects and solely transient thermal effects. All measurements
for control and test modules were made after a two hour outdoor
exposure.

In previous testing of glass/glass packages without edge seals (Annual
Technical Report, 2002 -- 2003), electrical performance differences
between controls and modules exposed to damp heat occurred at the onset
of visible corrosion - darkening and broadening of the interconnects at
the edge of the circuit plates. In the current testing using an edge
seal, there was no significant difference in electrical performance
between controls and modules exposed to high humidity and no visible
corrosion after 1000 hours of exposure. Corrosion was not observed for
2300 hours of exposure to high humidity. However, the electrical
performance seems to be slightly lower than for the controls. This
observation is supported by trends in the electrical performance data
but the number of modules tested is inadequate to claim statistical
significance at a 95% confidence level. Corrosion is visible on most
modules after 3000 hours of damp heat exposure. Lower electrical
performance with no visible corrosion is observed for one module.
Typically, corrosion of the interconnects is observed along the short
edge of the 1x4 ft. module from the edge into the package for up to 3 cm
(Figure 3, short edge to the right). Corrosion along the long edge of a
module may be as far inboard as the first interconnect and varies from
unobservable through sporadic to continuous. One module failed
catastrophically after 3500 hours due to failure of solder connections
to a printed circuit board that extends the circuit plate buss ribbons
and routes power to the back of the module. The majority of modules
failed after 4000 hours due to this failure mechanism and breakage.
These results indicate that the glass/glass package design with an edge
seal provides protection from humidity ingress well beyond the standard
1000 hours at 85ºC and 85% relative humidity.
![](media/image3.png){width="4.139583333333333in" height="3.1125in"}

Both minimodules and circuit plates have subjective aesthetic
requirements making features that are sometimes seen after selenization,
"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 typically
around a "dark core" (Figure 4). Surface texture differences range from
barely noticeable to dramatic and there may be concentric rings with
differences in surface texture. When present, the size of the dark core
varies from barley noticeable at the magnification used for Figure 4 to
about 0.04 mm. Dark cores are practically invisible because of their
small size and the visibility of dark spots varies from virtually
undetectable to a variation in shades of very dark gray. With few
exceptions, dark cores are observed at the center of a dark spot;
however, dark cores are often observed with no surrounding dark spot.
Dark spots at times appear centered on the Mo patterning typically with
no dark core.

![](media/image4.wmf){width="3.2666666666666666in"
height="2.4569444444444444in"}

Figure 4. Optically micrograph of a dark spots

Sally Asher, NREL, coordinated analysis of samples with dark spots in
parallel with analysis of the absorber structures for standard
production processes and for process variations; these particular
process variations were not defined to explore dark spot generation but
samples with dark spots were included in the sample set. NREL has
identified the dark cores, or minimally some dark cores, as In rich
regions reacted with S and Se. Figure 5 is a SEM image of a dark core
and Figure 6 is an AES In map of the same area. These results are
consistent with nonuniform In sputter deposition, "spits", as the source
for some but not all dark cores and dark spots. Specifically, spits are
at times seen on precursors with a density of about 20/cm^2^, whereas
the density of dark cores and dark spots in some locations is much
higher.

![](media/image5.wmf){width="3.5319444444444446in" height="3.0125in"}

Figure 5. SEM image of a dark core (NREL).

![](media/image6.wmf){width="3.5277777777777777in"
height="3.0541666666666667in"}

Figure 6. AES In map of a dark core (NREL).

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 the
area of a dark spots and typical areas. Although dark spots do not have
a significant impact on performance, 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 potentially at the expense of adding additional processing or
compromises in other production criteria. Exploration of process options
will continue. The appearance of Mo after removing the CIG (for example
using tape) is different in the immediate area a spots and for regions
that tend to generate high or low spot densities. Also, improved
adhesion may be indicated for the same changes in the precursor
formation processes that decreased the number or visibility of dark
spots. Therefore, cross-sectional SEM and analysis of the CIS/Mo
interface will be explored at NREL.

**Additional Highlights:**

-   SSI deposited CVD ZnO on glass substrates and multiple sets of
    experimental absorbers for Unisun.

-   SSI supplied about 70 minimodules to Larry Olsen's group at PNNL for
    testing multi layer barrier coatings.

-   SSI supplied twenty six minimodules with wider than standard
    boarders (\~5 mm) to Larry Olsen's group at PNNL for testing multi
    layer barrier coatings.

-   Bill Shafarman, IEC, visited and toured the facilities.

-   About 60 Mo coated 10x10 cm glass substrates were sent to Ryan
    Kaczynski, a graduate student working with Tim Anderson at the
    University of Florida, for CIS studies in conjunction with TFPPP
    team activities.

-   Harin Ullal requested modules for the "Hot and Humid" test program
    that will go to Texas A&M University.

-   SSI sent a CIS module and a list of module components to Vasilis
    Fthenakis for fire-simulating experiments; Vasilis Fthenakis,
    Brookhaven National Laboratory, National Photovoltaic EH&S
    Assistance Center.

-   Process development has included the analysis and upgrade of mass
    flow controllers that tend to corrode when used with H~2~Se.

-   Vivian Alberts, Physics Department, of the Rand Afrikaans
    University, has made single phase absorbers using SSI precursors
    with sulfur content, S/(S+Se), as high as 0.7. Additional work is
    required to produce lower bandgap absorbers and efficient devices.