**High Throughput, Low Toxic Processing of Very Thin, High Efficiency
CIGSS Solar Cells**

Year 1, Second Quarter Report

Report no. FSEC-CR 1559-05

December 19, 2005

Prepared for

National Renewable Energy Laboratory

######## 1617 Cole Boulevard

## Golden, CO 80401

## Submitted by

Neelkanth G. Dhere

Florida Solar Energy Center^®^

## 1679 Clearlake Road 

## Cocoa, FL 32922-5703

# Cocoa, FL 32922-5703 Cocoa, FL 32922-57CC

![](media/image1.wmf){width="1.2in" height="1.1027777777777779in"}

High Throughput, Low Toxic Processing of Very Thin, High Efficiency
CIGSS Solar Cells. **NREL contract no. XXL-5-44205-08, UCF/FSEC Account
no. 2012 8098**

Year 1, Second Quarter Report no. FSEC- CR-1559-05

**[Introduction]{.underline}**

In the Year 1, First Quarter report, initial experiments towards
optimizing the selenization parameters were discussed in detail. During
this quarter the experiments were continued using both conventional and
rapid thermal processing approaches. In the following discussion, the
preparation and the characterization of CuIn~1-x~Ga~x~Se~2~ and
CuIn~1-x~Ga~x~S~2~ thin films and solar cells are discussed in detail.

**[Optimization of Selenization Parameters]{.underline}**

Experiments were carried out to determine the adequate selenization
temperature and soaking time at that temperature. The metallic
precursors CuGa and indium were deposited using DC magnetron sputtering
technique. Prior to the deposition of metallic precursors, a small
amount (\~8.7 nm) of NaF was added to facilitate the crystalline growth
of the chalcopyrite crystal \[1\]. A thin layer of NaF was deposited by
thermal evaporation on Mo coated glass prior to the deposition of
metallic precursors. The thickness of the NaF layer was monitored by
quartz crystal rate and thickness monitor and control unit. Selenization
and sulfurization parameters were varied with each experiment. The
following micrograph shows the morphology of the film selenized at
475^o^C for 30 minutes and sulfurized at 475^o^C for 20 minutes. The
sample was etched in 10% KCN to remove the excess Cu layer.

![](media/image2.jpeg){width="2.7083333333333335in" height="2.125in"}

Figure 1. SEM of film selenized at 475 ^o^C for 30 minutes followed by
sulfurization at 475 ^o^C for 20 minutes

Table 1. XEDS result of KCN etched sample

  --------------- ------------- ------------- ------------- -------------
  Elem.           Cu            In            Se            S

  At%             28.68         22.90         41.23         7.20
  --------------- ------------- ------------- ------------- -------------

Since the sulfurization time and temperature were reduced, the amount of
sulfur incorporation in the film was limited to 7.2 at.%. Larger grains
were also observed at subsurface level. Having achieved some promising
parameters a series of experiments were carried out on 2 mil (50 μm)
polished stainless steel foil. The back of the foil was deposited with a
700 nm layer of Mo to prevent it from reacting with selenium or sulfur
vapor. On the front side, a Mo layer of approximately 1 μm was deposited
as a back contact followed by metallic precursors CuGa and Indium. The
films were selenized in a gas mixture of DESe, H~2~ and N~2~.

Figure 2 shows SEM micrographs of samples after each heat treatment. The
inset image in each figure is the same micrograph at a lower
magnification. It is observed that heat treatment carried out at 475
^o^C gave rise to a secondary phase that was rich in indium, while the
selenization carried out at 500^o^C led to a homogeneous phase. The
second phase appeared as small globules clearly observed at lower
magnification. The grain size for all four samples was approximately 1
μm. The grain size on stainless steel substrate was comparatively
smaller than that on glass substrate. The precursor deposition
parameters were those corresponding to slightly copper-poor chemistry.
Therefore, it can also be seen that the sulfurized film shows the
smallest grain size. These observations were helpful in understanding
that higher temperatures or even longer soaking times are required to
obtain larger grains.

![](media/image3.wmf){width="5.999305555555556in"
height="4.996527777777778in"}

Figure 2. (a) Sulfurized at 475^o^C/20 min, (b) Selenized at 475^o^C/30
min, (c) Selenized at 500^o^C/30 min, (d) Selenized at 475^o^C/30 min
and sulfurized at 475^o^C/20 min

Table II. XEDS: Chemical composition of samples a-d in Figure 2

  ------------------- -------- --------- --------------- ------------- ---------
  Sample ID           Cu (At%) In (At%)  Ga (At%)        Se (At%)      S (At%)

  a (unetched) - S    29.52    20.62     1.26            48.60         \-\--

  a (etched) - S      26.67    22.6      \-\-\-\-\-\--   50.73         \-\-\--

  b (unetched) - Se   27.09    18.63     5.48            48.81         \-\--

  b (etched) - Se     23.43    20.08     6.68            49.81         \-\--

  c (unetched) - Se   24.54    19.77     5.26            50.43         \-\--

  c (etched) - Se     23.73    19.92     7.3             49.05         \-\--

  d (unetched) --     27.21    18.96     5.5             36.94         11.40
  Se/S                                                                 

  d (etched) -- Se/S  23.14    19.16     7.27            38.12         12.31
  ------------------- -------- --------- --------------- ------------- ---------

Further the DESe setup was modified and experiments were carried out on
glass substrates. Figure 3 represents the micrograph of the film that
was selenized at 500 ºC for 30 minutes in dilute DESe atmosphere. The
precursor layer was modified to a sandwiched pattern with indium layer
sandwiched between two CuGa layers. In this experiment DESe and hydrogen
gas were filled separately to establish different quantities of each
gas. The grains were compact and approximately 1 μm in size. Further
experiments will be carried out to optimize the quantity of DESe to
achieve a high-quality absorber layer.

![](media/image4.wmf){width="5.999305555555556in"
height="4.613194444444445in"}

Figure 3. (a) SEM micrograph at 2500X, (b) at 5500X, (c) cross-sectional
view at 16000X.

Table III. Chemical composition of the selenized and sulfurized film
using XEDS

  ------------ ----------- ----------- ----------- ----------- -----------
  Elem.        Cu          In          Ga          Se          S

  At%          23.14       19.16       7.27        38.12       12.31
  ------------ ----------- ----------- ----------- ----------- -----------

**[Rapid Thermal Processing]{.underline}**

In continuation of the work carried out in Quarter 1 \[2,3\], Auger
electron spectroscopy (AES), Secondary Ion Mass Spectroscopy (SIMS),
X-ray diffraction (XRD) and electron probe microanalysis (EPMA) analyses
were carried out. The results are reported in ure 4,5,6.

![](media/image5.wmf){width="4.247222222222222in"
height="3.196527777777778in"}

Figure 4: AES depth profile

![](media/image6.wmf){width="3.290277777777778in"
height="3.9541666666666666in"}

Figure 5: Secondary Ion Mass Spectroscopy analysis

![](media/image7.wmf){width="5.116666666666666in"
height="3.5284722222222222in"}

Figure 6: X-ray diffraction pattern

In earlier experiments, the sheet resistance of the sample was observed
to be very low. It was suspected that there was an extreme loss of Se in
the form of In~2~Se~3~. In order to ensure proper intermixing of the
metallic precursors i.e. CuGa and In, they were deposited in the form of
a few alternating layers, which were designed to obtain a Cu/In+ Ga
ratio of around 0.95. EPMA analysis data of selenized sample is given in
Table IV.

Table IV- EPMA analysis data for selenized sample.

Cu/In+Ga = 0.93

  ---------------- -------------------- --------------- -----------------
  Cu               In                   Ga              Se

  24.52            20.55                6.01            24.44
  ---------------- -------------------- --------------- -----------------

Table V- EPMA analysis data for selenized and sulfurized sample.

Cu/In+Ga = 0.92

  -------------- -------------- -------------- -------------- --------------
  Cu             In             Ga             Se             S

  24.24          19.28          6.92           27.05          22.48
  -------------- -------------- -------------- -------------- --------------

EPMA data (from NREL) for this selenized and sulfurized sample (Table V)
showed more sulfur (\~20 %) than the required amount. In the subsequent
experiment the metallic precursors were deposited with different
thicknesses so as to obtain a Cu/In+ Ga ratio of around 0.95. Further
experiments were designed to reduce the amount of sulfur to \~10% (at.
wt) \[4\]. The amount of Se evaporated was 1.6 times the value required
for stoichiometric CIGS. The amount of NaF was reduced to 30-40 Å. The
precursor layers were pre-annealed at 80-120 °C for 15-30 minutes in the
RTP setup. Selenization/sulfurization was carried out at 500-550 °C for
6 minutes. The sheet resistances achieved after rapid thermal processing
were around 10KΩ/. CIGSS thin-film solar cells were completed by
chemical bath deposition of CdS hetrojunction partner layer followed by
RF magnetron sputter deposition of ZnO(i)/ZnO:Al transparent and
conducting oxide window layer and then Cr/Ag contact fingers by thermal
evaporation. Current-voltage (I-V) characteristics (figure 7) were
plotted for these cells using the I-V setup that was built in-house.

![](media/image8.wmf){width="3.167361111111111in"
height="2.2180555555555554in"}

Figure 7: I-V characteristics for CIGSS solar cell processed by RTP

  ------------------------------------- ---------------------------------
  ***Parameter***                       ***Value***

  Rsh                                   279.22 Ω

  Rs                                    31.65 Ω

  Area                                  0.441 cm^2^

  I~sc~                                 -7.01 mA

  V~oc~                                 589.99 mV

  I~pk~                                 -4.60 mA

  V~pk~                                 370.00 mV

  Fill Factor                           41.24 %

  Efficiency                            3.87 %
  ------------------------------------- ---------------------------------

Table VI. Device parameters for the cell processed by RTP

In the following experiment the amount of thermally evaporated Se on the
CuGa and In layers was doubled. No NaF was deposited in this experiment.
RTP was carried out by pre-annealing the substrate at 80-120 °C for
15-30 minutes. Selenization/sulfurization was carried out at 450-550 °C
for 6 minutes. The sheet resistance achieved was in the range of 10kΩ/
to 15kΩ/.

In another experiment, 30-40 Å of NaF was deposited by thermal
evaporation on Mo back contact followed by multi-layer sputter
deposition of CuGa and In similar to above experiments designed to
obtain a Cu/In+Ga ratio of around 0.95. The amount of Se deposited by
thermal evaporation was maintained at a level twice that required for a
stoichiometric film. The absorber layers are now being analyzed.

**[Sulfurization approach for low toxic absorber]{.underline}**

CuIn~1-x~Ga~x~S~2~ (CIGS2) thin films were prepared in two steps. Step
one consisted of the deposition of CuGa-In metallic precursors using DC
magnetron sputtering. The metallic precursors were deposited in the
copper excess composition of Cu/(In+Ga) = 1.4. Step two consisted of
sulfurization of metallic precursors in dilute H~2~S ambient (8% H~2~S).
Since the CIGS2 growth was carried out in the copper excess composition,
excess copper segregated on top of CIGS2 in the form of Cu~2-x~S was
etched away using 10% KCN solution.

XRD pattern of etched CIGS2 thin film shows (101), (112), (103), (200),
(220), (312), (400), (316) and (424) reflections of highly crystalline
chalcopyrite CIGS2 and also reflections from molybdenum (Figure 8). The
strongest reflection was from the (112) plane at 2θ = 27.92°. The
lattice parameters calculated were a = 5.52 Å and c = 11.04 Å.
Molybdenum reflection was observed at 2θ = 40.4°. The intensity ratio of
I(112)/I(220/204), measured for these films was 2.74, showing a
preferred {112} orientation.

![](media/image9.wmf){width="4.659027777777778in"
height="3.0409722222222224in"}

Figure 8. XRD pattern of series cell prepared in Cu/(In+Ga)=1.4 ratio

Figure 9 shows the I-V characteristic of the best cell measured at NREL.
Device parameters were V~oc~, 830.2 mV, J~sc~, 20.88 mA/cm^2^, FF 69.13%
and efficiency of 11.99%. This is the world record efficiency to date
for selenide-free CIGS2/CdS thin film solar cells prepared by
sulfurization of metallic precursors. The highest efficiency so far on
CIGS2/CdS thin film solar cells is 12.3% developed using two stage
co-evaporation techniques \[5\]. The open circuit voltage reported in
this research (830.2 mV) is the highest ever obtained on CIGS2/CdS
thin-film solar cells. The cells have been awarded the "Voc champions"
title. The same cell was sent to NASA Glenn Research Center (NASA GRC)
for I-V measurements in AM0 conditions. AM0 simulates the conditions in
space. The best values were V~oc~, 827.52 mV, J~sc~, 25.0 mA/cm^2^, FF,
67.7% and efficiency of 10.25%. Table VII and VIII shows the device
parameters obtained from I-V measurements in AM 1.5 and AM 0 conditions.

![](media/image10.png){width="3.3756944444444446in"
height="3.4368055555555554in"}

Figure 9. I-V characteristic of the highest reported Voc cell, measured
at NREL

Table VII. Device parameters of I-V measurements carried out at AM 1.5
conditions

  ------------- ---------- --------------- ----------- --------------------
  Sample ID     Voc (mV)   Jsc (mA/cm^2^)  FF (%)      Efficiency (AM 1.5)

  SS1020-1      810        18.69           63.77       9.65

  SS1020-2      810        18.23           63.81       9.42

  SS1020-3      810        18.79           64.49       9.81

  SS1020-4      810        18.55           64.86       9.75

  SS1020-5      810        19.64           63.07       10.03

  SS1020-6      820        19.09           60.89       9.53

  SS1020-7      810        19.36           63.55       9.96

  SS1020-8      800        18.46           63.53       9.38

  SS1020-9      800        19.29           62.42       9.63
  ------------- ---------- --------------- ----------- --------------------

Table VIII. Device parameters of I-V measurements carried out at AM 0
conditions

  -------------- ------------- ------------- ------------ ----------------
  Cell           Voc (mV)      Jsc           Fill Factor  Efficiency (AM0)
                               (mA/cm^2^)    (%)          

  SS1020-1       827.08        24.13         68           9.93

  SS1020-2       824.14        23.76         68           9.75

  SS1020-3       825.13        24.40         68.6         10.1

  SS1020-4       825.69        24.00         69.1         10.01

  SS1020-5       827.52        25.00         67.7         10.25

  SS1020-6       825.37        24.56         63.5         9.41

  SS1020-7       822.37        24.57         67.1         9.92

  SS1020-8       819.96        24.31         67.5         9.84

  SS1020-9       814.83        25.23         64.7         9.73
  -------------- ------------- ------------- ------------ ----------------

**TEM analysis**

The CIGS2 absorber layer is fairly smooth throughout with occasional
waviness. Large-sized CIGS2 grains of about 1 µm size are seen on the
top side towards the CdS layer whereas small grains of about 150 nm size
are observed close to the Mo back contact (Figure 10). Adhesion between
the layers looks good especially that between CIGS2 and Mo. Some
porosity is seen in the small-sized CIGS2 grains near the Mo layer.

![](media/image11.jpeg){width="3.0104166666666665in"
height="3.0104166666666665in"}

Figure 10. Bright field transmission electron micrograph giving an
overview of the cross-section of the CIGS2 --based thin film solar cell

![](media/image12.jpeg){width="2.75in" height="2.0104166666666665in"}

![](media/image13.jpeg){width="2.6354166666666665in"
height="2.3333333333333335in"}![](media/image14.jpeg){width="2.46875in"
height="2.375in"}

![](media/image15.jpeg){width="2.5104166666666665in"
height="2.34375in"}![](media/image16.jpeg){width="2.5104166666666665in"
height="2.40625in"}

Figure 11. Scanning transmission electron microscopy (STEM) high-angle
annular dark-field (HAADF) image (top) and energy dispersive x-ray (EDX)
elemental maps (bottom) of an area at the ZnO-Ag/Cr interface.

The sample has Cr-Ag front contacts with Cr very close to the ZnO layer.
In the regions where CIGS2 is wavy, a rough ZnO layer is observed and
there is a break in the continuity of the Cr-Ag contacts as revealed by
the elemental area maps (Figure 11). As we scan the CIGS2 layer from top
to bottom (Figure 12), there is an exchange of Ga to In ratios. The
distribution of Cu and S is constant throughout the CIGS2 layer. A line
scan over the pores (Figure 12) close to the Mo layer reveals an
increase in the Si and O signal very close to the pores. There is also
segregation of indium closer to the pores but just beyond the Si and the
O signal.

![](media/image17.jpeg){width="4.395833333333333in" height="3.75in"}

Figure 11. HAADF image (left) and energy dispersive x-ray (EDX) line
scan (right) of the cross-section of the thin-film solar cell from top
to bottom.

![](media/image18.jpeg){width="4.552083333333333in"
height="3.6458333333333335in"}

Figure 12. HAADF image (left) and EDX line scan (right) across some
pores close to the Mo layer showing segregation of In and an increase in
the Si and O signals at the pore surfaces.

Even though pores are found in the first micron above the back contact,
efficiencies of the best cells were measured to be 11.99%. This gives
hope that a complete elimination of pores leads to improved performance.
The CIGS2 grain size above the back contact may be increased by further
heat treatments. This would also possibly reduce the fraction of pores.
However, the beneficial Ga/In gradient across the absorber layer should
be maintained.

**Acknowledgements**

Authors are thankful to Helio Moutinho and Bobby To (from NREL) for XRD
and EPMA analysis respectively and to Mr. Mikhail Kilmov of AMPAC's MCF
at UCF for SIMS analysis.

**References**

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2.  NREL Proposal on High Throughput, Low Toxic Processing of Very Thin,
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