**Barrier Coatings and Stability of Thin Film Solar Cells**

**3rd Quarterly Report - Phase I:**

**March 1, 2005 \-- May 31, 2005**

**NREL Subcontract: 48027**

**Subcontractor: Pacific Northwest National Laboratory**

**Principal Investigator: Larry C. Olsen**

**1. OBJECTIVES/APPROACH**

The key objectives of the program are to develop low cost barrier
coatings for CIS and CdTe solar cells and to develop an improved
understanding of the effects of water on the stability of these types of
cells. The scope of this work entails investigations of multilayer,
barrier coatings for CIS and CdTe thin film solar cells, and studies of
stability issues, particularly those related to moisture ingress.
Investigation of barrier coatings on SSI and CSU devices will continue
in an effort to establish effective approaches to encapsulate CIS and
CdTe modules. Studies will also be directed towards issues concerning
cost of the coating process. The program will be structured into three
major tasks: (1) Barrier coatings and stability studies for CIS Solar
Cells; (2) Barrier coatings and stability studies for CdTe solar cells;
(3) Low cost coating process development.

**2. PROGRESS FOR THIS REPORTING PERIOD**

Work concentrated on studies concerning encapsulation of CdTe cells.
Three topics are discussed: further consideration of results for a CSU
cell encapsulated with a single dyad/Al-film combination; results for
studies of stressed bare cell cells from CSU and University of Toledo;
and studies of encapsulated calcium samples subjected to heat and
humidity conditions.

**2.1 Results for CSU Cell Encapsulated with One Dyad and Al Film**

Results for stress tests for a CSU cell encapsulated with a single dyad
and an aluminum film were discussed in the last quarterly report. Very
positive results were obtained for the cell stressed for 1000 hours
under 60ºC/90%RH conditions. After being stressed for 1000 hours, we
decided to change the stress conditions to 85C/85%RH to further
characterize the ability of this relatively simple approach to
encapsulation. Results for I-V parameters are shown in Figure 1. The
main effect of the 60/90 stress

![](media/image1.wmf){width="3.3751968503937007in"
height="2.372047244094488in"}![](media/image2.wmf){width="3.3751968503937007in"
height="2.3751968503937007in"}

**Figure 1.** I-V parameters for CSU cell encapsulated by a single dyad
combined with aluminum film subjected to 60ºC/90%RH followed by
85ºC/85%RH.

was an increase in R~s~ which caused a decrease in FF. The effects of
85/85 stress are more extensive. In particular, all parameters degraded
as a result of the 85/85 stress. Part of the cell degradation may simply
be due to thermal effects. Dry heat studies at 85ºC will be carried out
next quarter so that we can identify thermal effects separately. Another
key point to be determined is the effect of lateral migration of
moisture. All encapsulation procedures used to date have not
incorporated an edge seal. Studies discussed below show that lack of an
edge seal could be affecting our results.

**2.2 Studies with Bare CSU Cells**

Efficiency results for bare CSU cells subjected to 60C/90%RH are given
in Figure 2. The cell areas are 0.3 cm^2^. Cells were enclosed in an
environmental chamber under dark and open circuit conditions. These
results represent a very significant improvement relative to efficiency
data reported for bare CSU cells in 2004. CSU indicated that the
carbon/nickel material used to form a back contact is greatly improved
over that used in 2004. These results compare very favorably to those
reported below for bare Toledo cells.

![](media/image3.wmf){width="4.274409448818898in"
height="3.0811023622047244in"}

**Figure 2.** Efficiency vs time for bare CSU cells in environment of
60ºC/90%RH.

**2.3 Studies with Bare UT Cells**

Encapsulation studies of University of Toledo cells were initiated last
quarter. The effort began with examining the effects of moisture on bare
devices. The UT cells utilize a superstrate configuration and are
approximately one cm^2^ in area. Data were taken at three conditions,
dry heat at 60ºC, 60ºC/90%RH and 85ºC/85%RH. These results suggest that
moisture moves through the front contact structure rather slowly.
However, encapsulation is clearly required to achieve a stable cell.
Future studies will involve application of transparent multilayer
encapsulation.

![](media/image4.wmf){width="4.231102362204725in"
height="3.1720472440944882in"}

**Figure 3.** Efficiency vs time for bare UT cells in three different
environments, dry conditions at 60ºC, 60ºC/90%RH and 85ºC/85%RH.

**2.4** Investigation of Moisture Ingress with Encapsulated Calcium
Layers

Studies were carried out with encapsulated calcium films in an effort to
understand the pathways taken by moisture in the encapsulated cell
structures. Samples described by Figure 4 were used for the study. A 500
Å thick calcium film was deposited onto glass and then transferred to
the coating system where it was stored in nitrogen prior to
encapsulation. The encapsulated samples were then subjected to various
environmental conditions involving stress due to temperature and
moisture. Moisture interaction with the calcium films results in the
formation of Ca(OH)~2,~ which is transparent. The multi-layers of
polymer and Al~2~O~3~ utilized for encapsulation are transparent to the
visible

**Figure 4.** Sample structure for testing dynamics moisture ingress.

spectrum. Thus, whereas the coated calcium films are not transparent at
the beginning of the various tests, moisture ingress results in
formation of transparent regions of Ca(OH)~2~ in the sample allowing one
to observe where moisture ingress occurs.

Figures 5 and 6 show results for samples with one dyad and five dyad
coatings after being subjected to 60ºC/90%RH, 85ºC /50%RH and 85ºC
/85%RH for 120 hours. For each type of coating three samples were
subjected to 60ºC/90%RH, two samples to 85ºC /50%RH and one sample to
85ºC /85%RH. In case of the single dyad coating, moisture penetrates
through the interior of the encapsulation as well as from the edges. In
fact for an environment of 85ºC /85%RH, the calcium film has been
converted completely to Ca(OH)~2~ in the case the sample with only one
dyad coating. The results are much different for the calcium films
encapsulated with five dyad coatings. After 120 hours, there is no
indication of moisture ingress for any of the stress conditions.

Figures 7 and 8 give results for the encapsulated calcium samples after
550 hours. The calcium samples with only one dyad coating are completely
converted to Ca(OH)~2~ in the case of two of the three samples subjected
to 60ºC/90%RH and for the sample subjected to 85ºC/85%RH. The samples
stressed at 85/50 are nearly completely converted also. It is
interesting that the 60/90 condition appears to have a larger effect
than 85/50. It should also be noted that results for 85/50 stress
indicate that although moisture penetrates the interior region,
diffusion of water from the edges is a very significant cause of water
ingress. The results for the samples with five dyad coatings are very
instructive. They clearly establish that with five dyads, the dominant
mechanism for moisture ingress is due to diffusion from the edges. It
seems clear that edge seals will be a subject we must address in future
studies. The results for 60/90 stress described in Figure 8 are quite
impressive, and, in fact, consistent with results of life tests carried
out with encapsulated CSU CdTe cells and SSI CIGSS mini-modules.

![](media/image6.jpeg){width="0.9633858267716535in"
height="0.9826771653543307in"}![](media/image7.jpeg){width="0.9633858267716535in"
height="0.9826771653543307in"}![](media/image8.jpeg){width="0.9633858267716535in"
height="0.9826771653543307in"}![](media/image10.jpeg){width="0.9633858267716535in"
height="0.9811023622047244in"}![](media/image11.jpeg){width="0.9633858267716535in"
height="0.9811023622047244in"}![](media/image12.jpeg){width="0.9633858267716535in"
height="0.9811023622047244in"}

**Figure 5.** Samples with one dyad encapsulation (See Figure 4) that
were subjected to 60ºC/90%RH, 85ºC/50%RH and 85ºC/85%RH for 120 hours.

![](media/image13.png){width="1.0492125984251968in"
height="0.8755905511811024in"}![](media/image14.png){width="1.0492125984251968in"
height="0.8755905511811024in"}![](media/image15.png){width="1.0480314960629922in"
height="0.8755905511811024in"}![](media/image16.png){width="1.4173228346456692in"
height="1.4661417322834647in"}![](media/image17.png){width="1.0480314960629922in"
height="0.8755905511811024in"}![](media/image18.png){width="1.0480314960629922in"
height="0.8755905511811024in"}![](media/image19.png){width="1.0492125984251968in"
height="0.8755905511811024in"}![](media/image13.png){width="1.0492125984251968in"
height="0.8755905511811024in"}![](media/image14.png){width="1.0492125984251968in"
height="0.8755905511811024in"}![](media/image15.png){width="1.0480314960629922in"
height="0.8755905511811024in"}![](media/image16.png){width="1.4173228346456692in"
height="1.4661417322834647in"}![](media/image17.png){width="1.0480314960629922in"
height="0.8755905511811024in"}![](media/image18.png){width="1.0480314960629922in"
height="0.8755905511811024in"}![](media/image19.png){width="1.0492125984251968in"
height="0.8755905511811024in"}

**Figure 6.** Samples with five dyad encapsulation (See Figure 4) that
were subjected to 60ºC/90%RH, 85ºC/50%RH and 85ºC/85%RH for 120 hours.

![](media/image20.jpeg){width="1.2007874015748032in"
height="0.9019685039370079in"}![](media/image21.jpeg){width="1.2007874015748032in"
height="0.9019685039370079in"}![](media/image22.jpeg){width="1.2007874015748032in"
height="0.9019685039370079in"}

**Figure 7.** Samples with one dyad encapsulation (See Figure 4) that
were subjected to 60ºC/90%RH, 85ºC/50%RH and 85ºC/85%RH for 550 hours.

![](media/image23.jpeg){width="1.1346456692913385in"
height="1.002755905511811in"}![](media/image24.jpeg){width="1.1366141732283463in"
height="1.002755905511811in"}![](media/image25.jpeg){width="1.1346456692913385in"
height="1.002755905511811in"}![](media/image26.jpeg){width="1.1334645669291339in"
height="1.002755905511811in"}![](media/image27.jpeg){width="1.1334645669291339in"
height="1.002755905511811in"}![](media/image28.jpeg){width="1.1346456692913385in"
height="1.002755905511811in"}

**Figure 8.** Samples with five dyad encapsulation (See Figure 4) that
were subjected to 60ºC/90%RH, 85ºC/50%RH and 85ºC/85%RH for 550 hours.