**Low Toxic Processing of Thin and Ultra-thin CIGSS Absorber Solar
Cells**

Neelkanth G. Dhere, Ankur A. Kadam, Sachin S. Kulkarni, Jyoti S.
Shirolikar and Shirish A. Pethe

Florida Solar Energy Center, University of Central Florida,
dhere@fsec.ucf.edu

#### ABSTRACT

CuIn~1-x~Ga~x~Se~2-y~S~y~ (CIGSS)/CdS thin-film solar cells were
prepared on molybdenum coated glass substrates. A low toxic approach
using diethylselenide (DESe) as a selenium source was used for
selenization of metallic precursors. Magnetron sputtering technique
which is easy to scale up was used for deposition of back contact,
metallic precursors and transparent and conducting oxide. A compact and
large-grain absorber layer was produced by heating the elemental stack
at 475-500^o^C in the diluted DESe gas. The threshold limit value (TLV)
for DESe is 5 times less stringent than that of H~2~Se. After
stabilizing deposition and selenization/sulfurization parameters on a
2.75 μm thick absorber layer, preliminary experiments were carried out
on an ultra-thin (\<1μm) absorber to reduce indium consumption. Rapid
Thermal Processing (RTP) technique was developed as an alternative to
conventional selenization and sulfurization to reduce the process time
and thermal budget, thus enhancing throughput.

**1. Objectives**

The objective of the proposed project is to develop a high throughput,
low toxic, very thin, low-indium content and highly efficient CIGSS
solar cell. The work is oriented to fulfill the DOE goal of developing
technology capable of reducing the cost of solar energy to a point where
it becomes competitive in the energy market and enabling a sustainable
solar industry.

**2. Technical Approach**

Mo back contact and the stack of metallic precursors CuGa and In were
deposited by DC magnetron sputtering over a substrate area of 15 cm x 10
cm. The thickness of layers was adjusted by varying the linear speed of
the substrate moving over the target. Selenization and sulfurization was
carried out by heating the elemental stack in diluted DESe and dilute
H~2~S gases respectively. The absorber layer was characterized by
scanning

![](media/image1.jpeg){width="1.4986111111111111in"
height="1.24375in"}![](media/image2.jpeg){width="1.625in"
height="1.25in"}

Fig. 1. SEM micrograph of sample selenized at 400^o^C for 20 minutes.
(a) unetched, (b) etched

electron microscopy (SEM), x-ray diffraction (XRD), electron probe
microanalysis (EPMA), Auger electron spectroscopy (AES), secondary ion
mass spectroscopy (SIMS) and transmission electron microscopy (TEM).
Cells were completed by depositing CdS by chemical bath deposition
(CBD), i:ZnO/ZnO:Al by RF magnetron sputtering and Cr/Ag contact fingers
by thermal evaporation.

**3. Results and Accomplishments**

*3.1 Parameter optimization*

The approach was to optimize the parameters for selenization and then
carry out sulfurization. Diluted DESe was used as a selenium source and
based on previous experience, selenization was carried out at 400^o^C.
The copper-rich phase i.e. Cu~2-x~Se was removed by 10% KCN etch, making
the grains clearly visible. From the SEM microstructure (Fig. 1a, 1b) it
was inferred that the grains are compact. However, they are not faceted
to a great extent. Complete dissociation of DESe takes place at
temperatures greater than 450^o^C^1^. Therefore, further experiments
were carried out at temperatures of 475^o^C and 500^o^C.

The metallic precursors were deposited at near-stoichiometric
composition and selenized at 475^o^C for 30 minutes. Absorber thickness
was measured to be 2.75 μm. Figure 2a indicates grain size to be in the

range of 0.5 μm to 1.5 μm. An efficiency of 6.12% was obtained on the
I-V setup built in-house. A major improvement was observed by selenizing
the near-stoichiometric composition at 500^o^C for 30 minutes with
addition of minute quantity of NaF (Fig. 2b). In selenide-free
chalcopyrite, world record V~oc~ of 830 mV was obtained on
CuIn~1-x~Ga~x~S2 (Fig 5). The efficiency (11.99 %) is the highest ever
demonstrated using sputtering technique.

*3.2 Ultra-thin CIGS*

CuGa-In metallic precursors were selenized in DESe at 400ºC for 10
minutes and further sulfurized using dilute H~2~S as a sulfur source at
475ºC for 20 minutes. Figure 3a shows the SEM image showing compactly

![](media/image3.jpeg){width="1.4986111111111111in"
height="1.395138888888889in"}
![](media/image4.jpeg){width="1.4986111111111111in" height="1.40625in"}

Fig. 2.(a) Selenized at 475^o^C/30 min, (b) 500^o^C/30 min

![](media/image5.png){width="1.5569444444444445in"
height="1.4in"}![](media/image6.wmf){width="1.7034722222222223in"
height="1.4215277777777777in"}

Fig. 3. SEM and XRD of ultra-thin CIGSS

![](media/image7.png){width="2.1222222222222222in"
height="1.5416666666666667in"}![](media/image8.wmf){width="0.7993055555555556in"
height="0.1986111111111111in"}

Fig. 4. X-sectional view of CIGS grains.

![](media/image9.png){width="1.71875in" height="1.9166666666666667in"}

Fig. 5. I-V characteristics of CIGS2 solar cell

packed, small grains (\~0.5µm) with negligible porosity. Figure 3b shows
the XRD pattern with (112), (220/204), and (312) chalcopyrite
reflections from the CuIn~0.99~Ga~0.01~Se~1.08~S~0.92~ phase with a =
5.7294 Å and c = 11.4587 Å. Later a thin precursor layer was only
sulfurized at 475^o^C/20 minutes to observe the grain growth. The SEM
micrograph (Fig. 4) shows the formation of grains of size \< 1 μm
extending from the back contact to surface.

*3.2 Rapid Thermal Processing*

Glass substrates were coated with Mo back contact and CuGa-In precursor
layers by DC magnetron sputtering. Selenium and NaF were deposited by
thermal evaporation. RTP was carried out in a mixture of H~2~S and
hydrogen at a temperature below 550°C. Preliminary SEM results with only
selenized RTP thin films are shown in Fig. 6a and b. Grain size of \~1-2
microns was obtained on an etched film (Fig. 6b). Working CIGSS
thin-film solar cells with dark and light I-V characteristics were
obtained on cells completed by CdS deposition using CBD,

![](media/image10.png){width="1.6236111111111111in"
height="1.2645833333333334in"}
![](media/image11.png){width="1.5888888888888888in" height="1.25in"}

Fig. 6. SEM images at x5500 of (a) Unetched and (b) Etched RTP CIGS thin
films.

ZnO(i)/ZnO:Al bilayer window by RF magnetron sputtering and Cr/Ag front
contact fingers by thermal evaporation through metal masks. Experiments
are being carried out to improve the properties of these cells.

**4. Conclusion**

The work presents the capability of DESe to be used as a low toxic
alternative source for preparing a high quality absorber. Two approaches
for reducing the cost of photovoltaic were demonstrated. First, costly
indium can be reduced by preparing ultra-thin CIGSS layer and still
maintaining high quality and second, high throughput can be gained by
RTP. Future work would focus on further improving the absorber quality
by material analysis and characterization as well as fine tuning of
remaining process steps.

## ACKNOWLEDGMENT

This work was performed under NREL contract XXL-5-44205-08. Authors are
thankful to Bobby To and Helio Moutinho of NREL for characterization
support.

## REFERENCES

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### **MAJOR FY 2005 PUBLICATIONS**

1\. A. Kadam, A. Jahagirdar and N. Dhere

"Performance Enhancement of CIGSS Absorber Layer on Glass Substrate" ,
SWC 2005 proceedings

2\. N. Dhere, S. Kulkarni, J. Shirolikar and M. Nugent

"Review of Rapid Thermal Process and Setup for Fabrication of Absorber
Layer for CIGSS Solar Cells by RTP", SWC 2005 proceedings