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

Year 1, First Quarter Report

Report no. FSEC-CR 1528-05

August 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, First Quarter Report no. FSEC-CR-1528-05

######### Introduction

Copper indium diselenide, CuInSe2 (CIS), is an excellent absorber
material for the development of highly efficient and low-cost thin film
solar cell devices due to its high absorption coefficient and near
optimal bandgap value. The addition of gallium (CuIn~1-x~Ga~x~Se~2~,
CIGS) and sulfur (CuIn~1-x~Ga~x~Se~2-y~S~y~, CIGSS) has been used to
produce graded bandgap structures in order to further increase the
efficiency of devices by obtaining a higher charge carrier collection.
The present highest cell efficiencies of CIGS cells and CIGSS module are
19.5% and 13.1% respectively \[1,2\].

Many recent presentations and publications from Linda L. Horton Oak,
Ridge National Laboratory, Richard Smalley, Rice University and Nathan
Lewis, California Institute of Technology, have emphasized that among
the renewables, only solar has a the resource to make a substantial
contribution to non-CO~2~ energy in the 21st century. The rest of the
renewable options (wind, biomass, geothermal, hydro) do not have
adequate global resources to do more than fill in for solar when solar
is unavailable -- although such "filling in" may be about a Terawatt
each (still a very significant contribution). This means that solar
(with about 125,000 TW of global incident energy) has both a huge
opportunity and a huge responsibility \[3-6\].

The idea of thin film solar cells is simple: use mostly low cost
material (glass, metal, plastic) and very little high-cost
semiconductor. Since the mass per unit area of 1 micron thick
semiconductor is approximately 3-6 g/m^2^, even ultra-expensive material
costs only pennies per watt at this level. Among all thin films, CIS
based thin films have the highest cell and module efficiencies and to
date, stability has also been considered a strength of CIS. However,
with the increase in the cost of indium, the need for thin CIS, CIGS or
CIGSS thin film solar cells has increased.

Indium is a rare metal that commonly occurs as an impurity in zinc and
to a lesser extent in tin, lead and copper. The abundance of indium in
the earth's crust is believed to be equivalent to the occurrence of
silver, however, the similarities end there. Over the last two years
cost of indium per kilogram has increased ten times (\$60/kg in 2002 to
\$600/kg in 2004) \[7\].

The availability of indium could become a limiting factor if CIGSS PV
modules were to satisfy a sizable fraction of global energy demand.
Indium consumption could be lowered by higher materials utilization, use
of thinner films, substitution by gallium, and higher module
efficiencies. Based on the high absorption coefficient of 105 cm-1 for
photon energy above the bandgap, Zweibel \[8\] estimates that the CIGS
thickness can be reduced to 0.5 µm without light-trapping and to 0.2 µm
with sophisticated light-trapping. For thinner cells, the collection
efficiency would improve because the distance between the space-charge
edge and light-generated electron-hole pairs would become smaller
compared to the minority-carrier diffusion length. Spitzer et al \[9\]
estimated that efficiencies exceeding 25% should be achievable for
single-junction, chalcopyrite-based devices using a 1-µm absorber layer
with bandgaps of 1 - 1.5 eV, assuming negligible surface recombination
velocity and no absorption in the window material.

Making cells thinner could reduce the time required for deposition and
thereby increase the throughput. However, there is a higher probability
of photogenerated minority carriers being lost at the back contact in a
thinner CIGSS layer. This can be avoided by adding an adequate amount of
gallium that will diffuse to the back contact and create a back-surface
field repelling the electron toward the p-n junction. Lundberg et al
\[10\] obtained efficiencies as high as 15% on a 1-µm-thick CIGS
absorber layer with back-surface field provided by Ga near the back
contact. Below a thickness of 0.8 µm, recombination increased due to
absorbance-related losses and poor material quality while short circuit
current decreased due to absorbance related losses and reduced carrier
collection. Negami et al \[11\] reported a decrease in open-circuit
voltage and fill factor for 0.47-µm-thick CIGS because of shunting paths
through CdS formed due to excessive surface roughness. Absorbance as
well as shunting-related losses could be reduced by improving the
material quality. Thus there is a formidable but achievable goal of
meeting terawatt challenge at a reduced cost and keeping efficiencies up
to the mark.

Over the last few years, excellent facilities have been developed at
FSEC for the development of p-type CIGSS thin films on large, 10 cm x 15
cm substrates \[12-14\]. CIGSS thin films were prepared on glass in two
steps. Step one involved the deposition of CuGa-In metallic precursors
on molybdenum coated glass substrate using DC magnetron sputtering and
step two involved the selenization/sulfurization of these metallic
precursors using diluted diethylselenide (DESe) as a selenium source and
diluted H~2~S as a sulfur source respectively.

Samples for transmission electron microscopy (TEM) were prepared using
the focused ion beam system FIB200TEM for cross-sectional cuts of the
layers. The TEM samples were taken from the surface using the ex-situ
lift out method with quartz needles \[15\] and deposited on a holey
carbon film on a Cu-TEM grid. The TEM measurements were performed on a
Tecnai F30 transmission electron microscope equipped with a field
emission source, a GATAN imaging filter and an energy dispersive x-ray
detector for elemental analysis. The focused ion beam system and the
Tecnai F30 were both at the Materials Characterization Facility (MCF)
within the Advanced Materials Processing and Analysis Center (AMPAC) of
University of Central Florida. The TEM sample preparation and the TEM
measurements were done by the Co-PI Dr. H. Heinrich and his collegue. A
similar study using the MCF equipment by the Co-PI on Cu-In-Ga-Se
samples from ETH Zurich, Switzerland has recently been published \[16\].

**[Optimization of Deposition and Selenization/Sulfurization
Parameters]{.underline}**

Experiment 1

The metallic precursors were deposited with excess CuGa. The total
thickness of CuGa was divided into two layers. The first layer was
comparatively thick and was obtained by linearly moving the substrate
over CuGa target at a speed of 0.02 cm/sec while tha other layer was
deposited at substrate speed of 0.07 cm/sec. A thick indium layer was
sandwiched between two CuGa layer by moving the substrate over indium
target at speed of 0.021 cm/sec. the elemental stack was selenized at
400 ^o^C for 10 minutes. Figure 1a shows a micrograph of unetched
absorber while figure 1b shows etched film. Copper rich phase Cu~2~Se
was removed by 10% KCN etch making the grains clearly visible. From the
microstructure it was inferred that the grains are compact however they
are not faceted to a great extent.

![](media/image2.jpeg){width="1.8645833333333333in"
height="1.4583333333333333in"}
![](media/image3.jpeg){width="1.8645833333333333in"
height="1.4583333333333333in"}

Figure 1a: Sample 1a unetched Figure 1b: Sample 1a etched

XEDS: sample 1a, unetched XEDS: sample 1a, etched

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

  At%             22.49          21.19          5.38          49.13
  --------------- -------------- -------------- ------------- --------------

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

  At%             21.25          22.71          4.24          50.54
  --------------- -------------- -------------- ------------- --------------

Sample 1b

This film had similar deposition parameters as film 1a. The film was
selenized at 450 ^o^C for 60 minutes. The temperature and soaking time
were increased to achieve faceted grains. Figure 2a shows micrograph of
unetched absorber layer. Whiskers of Cu~2~Se were clearly visible. The
appearance of the sample was brown as compared to gray after
selenization. As seen from the SEM micrograph the grains were not
well-formed.

![](media/image4.jpeg){width="1.8541666666666667in"
height="1.4583333333333333in"} XEDS: sample 1b, unetched

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

  At%             22.49          21.19          5.38          49.13
  --------------- -------------- -------------- ------------- --------------

Figure 2: SEM of sample 1b

Experiment 2

Sample 2a

The deposition parameters were maintained same as those used for
sample 1. This experiment was an effort to observe the behaviour of the
film under different temperature and time. The sample was selenized at
425 ^o^C for 30 minutes. Diluted DeSe was filled in the reaction tube of
the furnace to the pressure of 60 torr (2%). On the surface of the film
withered patches were observed as seen in figure 3a. On carrying out
X-ray energy dispersive spectroscopy (XEDS) analysis they were found to
be indium rich indicating segregation of indium.

![](media/image5.jpeg){width="1.8645833333333333in"
height="1.4583333333333333in"}

XEDS: sample 2a, unetched

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

  At%             11.66          52.07                        36.27
  --------------- -------------- -------------- ------------- --------------

Figure 3a: SEM of sample 2 a

Sample 2b

This sample had similar precursor deposition parameters and diluted DeSe
amount as 2a. The figure 3b shows a selenized sample at 400 ^o^C for 30
minutes in. On the surface of the film spherical grains were observed.
XEDS was carried out on these grains and were found to be indium rich.
At higher magnification small grains less than micron were seen beneath
the indium rich grains. It was also observed from the XEDS analysis of
the film that the amount of Se was low

![](media/image6.jpeg){width="1.8645833333333333in"
height="1.4583333333333333in"}

XEDS: sample 2b, unetched

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

  At%             9.37           62.73                        27.9
  --------------- -------------- -------------- ------------- --------------

Figure 3b: SEM of sample 3c

Experiment 3

![](media/image7.jpeg){width="2.2083333333333335in"
height="1.4583333333333333in"}

Figure 4: Selenized and KCN etched sample

Precursor deposition parameters and amount of DeSe was kept same as
experiment 2. The film was selenized at 425 ^o^C for 60 minutes. The
film peeled partly after taking out of the furnace while it fell apart
completely during KCN etching as seen in the figure 4. The peeling off
of the sample is already indicating that either the selenium amount was
less, or the temperature used was not adequate to form well adherent
film.

Experiment 4

![](media/image8.jpeg){width="2.2083333333333335in"
height="1.4583333333333333in"}

Figure 5: Selenized sample 4

The deposition parameters and amount of DeSe was kept same as previous
experiments. The film was selenized at 500^o^C for 30 minutes. After
taking out of the furnace the film appears uniformly gray with some dark
patches on one side (Fig. 5) During etching in 10% KCN the film
completely peeled off. The Mo surface came out clean indicating no
reaction with Mo. It was believed that the excess copper in the film
formed Cu~2~Se layer at Mo/CIGS interface that dissolved during etching
resulting in peeling of the film. The other reason would have been the
insufficient amount of Se due to which adherent CIGS layer was not
formed.

Experiment 5

In-order to eliminate possible effect of Cu~2~Se the deposition
parameters were changed. Amount of CuGa was reduced by increasing the
speed of substrate moving over the CuGa target while indium was kept
same. One layer of CuGa was deposited by linearly moving the substrate
over the target at a speed of 0.04 cm/sec. The sample was cut in half
and one part of it was selenized at 450^o^C for 30 minutes. However, the
problem continued as the film peeled completely during etching. It was
confirmed that the peeling problem was due to lack of selenium in the
film. DeSe solution was refilled and further experiments were carried
out.

Sample 5b

![](media/image9.jpeg){width="2.2083333333333335in"
height="1.4583333333333333in"}

Figure 6a: Selenized sample 5b Figure 6b: ZnO coated absorber film

![](media/image11.jpeg){width="1.8645833333333333in"
height="1.4583333333333333in"}

Figure 6c: SEM of unetched absorber Figure 6d: SEM of etched absorber

EPMA: sample 5b, unetched EPMA: sample 5b, etched

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

  At%             23.3           25.32          2.93          48.44
  --------------- -------------- -------------- ------------- --------------

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

  At%             23.06          25.53          2.99          48.40
  --------------- -------------- -------------- ------------- --------------

The remaining part of the sample 5 was selenized at 475 ^o^C for 30
minutes. A minute amount of oxygen was deliberately added as it prevents
the segregation of indium during alloy formation. After the selenization
treatment the sample appeared uniform gray with no patches at all. As
selenized sample had a sheet resistance of 37- 47 KΩ/□. After etching in
10 % KCN the change in sheet resistance was negligible, this was
expected as the precursor deposition parameters were adjusted to obtain
near-stoichiometric composition. The SEM micrograph in figures 6c and 6d
shows the absorber layer before and after etching. It was also noted
that the grain size was in the range of 0.5 μm to 1.5 μm. The thickness
of the film was 3.5 to 3.6 μm. This sample was further cut in half to
produce two pieces of 5 cm x 5 cm. one of the pieces was etched while
the other was carried as selenized for CdS deposition. i:ZnO/ZnO:Al and
Ni/Al contact fingers were deposited with the earlier stabilized
parameters. 6.12 % efficiency was measured on the etched and CdS sample
while the unetched and CdS deposited provided lower values.

Experiment 6

![](media/image13.jpeg){width="1.8645833333333333in"
height="1.4583333333333333in"}

Figure 7a: SEM of selenized Figure 7b: SEM of NaF deposited, selenized
and and etched sample etched sample

EPMA: sample 6, No NaF and etched EPMA: sample 6, NaF and etched

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

  At%             20.77          29.28          1.18          48.76
  --------------- -------------- -------------- ------------- --------------

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

  At%             21.67          26.75          3.07          48.50
  --------------- -------------- -------------- ------------- --------------

The amount of precursors was reduced in-order to obtain absorber
thickness of approximately 2.75 μm. The speed of the substrate moving
over the respective targets was increased. Substrate speed over CuGa
target was 0.05 cm/sec while that over indium target was 0.027 cm/sec. A
very thin layer of NaF was deposited on half of the sample while the
other half was selenized as deposited. The difference in the
microstructures between both the films is shown in figures 7a and 7b.
Addition of sodium improved the grain size as well as the grains were
faceted. The resulting films had a thickness of approximately 2.8 μm.
The amount of precursors was reduced proportionally during deposition;
however from EPMA data it was found that the films became indium rich.
The sheet resistance of the film with NaF was in the range of 109 to 190
KΩ/□ while that of film with no NaF layer was 500 KΩ/□ to 2 MΩ/□. Thus
addition of NaF improved conductivity due to doping.

Experiment 7

![](media/image15.jpeg){width="1.8645833333333333in"
height="1.4583333333333333in"}

Figure 8a: SEM of unetched sample Figure 8b: SEM of etched sample

XEDS: sample 7, NaF and unetched XEDS: sample 7, NaF and etched

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

  At%             21.98          25.96          3.04          48.99
  --------------- -------------- -------------- ------------- --------------

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

  At%             21.87          26.02          3.13          48.96
  --------------- -------------- -------------- ------------- --------------

In this experiment amount of CuGa was deposited at substrate speed of
0.052 cm/sec. Amount of indium was decreased by increasing the substrate
speed to 0.03 cm/sec. NaF was deposited on the precursor layer by
thermal evaporation. NaF layer was deposited on all the experiments
discussed henceforth. Selenization was carried out at 500^o^C for 30
minutes. SEM micrograph shown in figure 8a and 8b depicts the etched and
unetched absorber layer. The grains were well developed and faceted with
average size of 1.5 μm. The resulting film had a thickness of 2.7 to 2.8
μm.

Experiment 8

![](media/image17.jpeg){width="1.875in" height="1.46875in"}
![](media/image18.jpeg){width="1.8645833333333333in"
height="1.4583333333333333in"}

Figure 9a: Sample 8a unetched Figure 9a: Sample 9a etched

EPMA: sample 8a, NaF and unetched EPMA: sample 8a, NaF and etched

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

  At%             22.4           25.77          2.49          49.32
  --------------- -------------- -------------- ------------- --------------

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

  At%             21.46          26             3.11          49.42
  --------------- -------------- -------------- ------------- --------------

Sample 8 was deposited with the same parameters as sample 7. A thin
layer of NaF was deposited on top of the precursor layer. The 10 cm x 15
cm piece was cut in two equal half. One of the pieces was selenized at
500 ^o^C for 60 minutes. The grains were highly faced and could not be
identified from one another with clear boundaries.

![](media/image19.jpeg){width="2.0104166666666665in"
height="2.0104166666666665in"}

Figure 9c: TEM of X-section view of sample 8a

The TEM image shows porous structure near the interface with Mo
backcontact layer. The grains near the interface are comparatively
smaller that those in the top part of the film. The grain size ranged
from 1 to 2 μm. The porosity was believed to be an artifact of sample
preparation procedure. It also indicated that the phase formed near the
interface is volatile or poorly adherent.

Sample 8b

![](media/image20.jpeg){width="1.9791666666666667in" height="1.5625in"}
![](media/image21.jpeg){width="1.9895833333333333in" height="1.5625in"}

Figure 10a: Unetched sample 8b Figure 10b: Etched sample 8b

XEDS: sample 8b, unetched XEDS: sample 8b, etched

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

  At%           21.45        25.26        5.19        3.02       45.07
  ------------- ------------ ------------ ----------- ---------- ------------

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

  At%          22.9        24.27       5.35        2.72        44.77
  ------------ ----------- ----------- ----------- ----------- -----------

Sample 8 b was the remaining half of the deposition carried out in
experiment 8. The piece was selenized at 500 ^o^C for 30 minutes
followed by sulfurization at 500 ^o^C for 20 minutes. Approximately 10
minutes were required to evacuate the Se gas and refill the H~2~S gas.
Therefore the sample was at 500 ^o^C for total time of 60 minutes. As
seen from the SEM micrograph the grains were highly faceted. XEDS
analysis showed that the amount of sulfur in the film was 2.72 at %
which can be further increased by sulfurization for longer time.

TEM analysis of sample 8bE:

The Cu-In-Ga-Se-S absorber layer contains two different parts. The top
part near the CdS-ZnO layer has low S and Ga contents. However, an
increase in the S content accompanied by a decrease in Se is observed
near the top 300 nm of this layer close to the CdS layer. In the lower
part of the absorber layer near the Mo back contact, the Ga content
increases gradually at the expense of In. Additionally, strong
variations in the S and Se content occur, with S locally replacing Se. A
high fraction of pores and small grains are observed in this lower 1000
nm of the absorber layer.

![](media/image22.wmf){width="5.2868055555555555in"
height="3.8625in"}![](media/image23.wmf){width="3.06875in"
height="0.5576388888888889in"}

Figure 10c: Scanning transmission electron microscopy (STEM) high-angle
annular dark-field (HAADF) image (left) of a cross section of the
Cu-In-Ga-Se-S thin-film solar cell. The line scan on the right shows the
energy dispersive x-ray (EDX) intensities from the top of the cell to
the bottom with the Mo layer.

Experiment 9

Processing of sample 9 was carried out with similar parameters of
experiment 8a. After selenization step the sample was found to be
partially selenized as part of it had sheet resistance of 20 to 30 Ω/□
while the selenized region had sheet resistance of 20 to 40 K Ω/□. DeSe
solution was refilled for further experiments. On calculating it was
found that 20 gm of DeSe is sufficient for approximately 7 experiments
corresponding to diluted DeSe pressure of 280 Torr.

Experiment 10

![](media/image24.jpeg){width="1.4583333333333333in" height="1.65625in"}
![](media/image25.jpeg){width="1.8541666666666667in"
height="1.4583333333333333in"}

Figure 11a: Cross-section of sample 10 Figure 11b: topographic view of
sample 10

From the EPMA data of sample 8a it was found that the Cu/In+Ga ratio of
the films was 0.74. In-order to increase this ration to near 0.92 amount
of CuGa was increased by 24%. The substrate was now moved over the CuGa
target at the speed of 0.041 cm/sec. Indium amount was kept same. The
elemental stack was selenized at 500^o^C for 30 minutes. Sulfurization
was not carried out on this film as it was decided to check the
repeatability of selenization process parameters. Figure 11a shows the
cross-section of sample 10a, small grains are seen at the interface with
Mo while large (approx. 2μm) grains were observed towards the surface.
The surface of the film appeared rough due to sputtering technique used
in preparation of the film. The grain size was confirmed from figure 11b
and also it can be seen that the grains are faceted.

Experiment 11

![](media/image26.jpeg){width="1.21875in" height="1.5520833333333333in"}
![](media/image27.jpeg){width="1.9895833333333333in" height="1.5625in"}

Figure 12a: X-section of sample 11 Figure12b: SEM of sample 11

The deposition parameters were kept same in this experiment.
Selenization was carried out at 500 ^o^C for 60 minutes. Cross-section
view showed large grains along the thickness of the film.

TEM analysis of sample 11

The micrographs reveal a homogeneous Cu-In-Ga-Se-S absorber layer with
fewer pores than in the previously reported samples. The differences
between top and bottom layer are less pronounced. Still, grain sizes in
the top part of the absorber layer reach sizes up to 1000 nm in
diameter, while grains in the lower part are typically 100 to 200 nm in
diameter. The EDX analysis reveals the formation of a Mo and Se-rich
layer (probably MoSe~2~) of 50 nm thickness above Mo layer. This layer
appears gray in the figure 12c. Increase in the amount of CuGa has
substantially reduced the porosity observed in sample 8. It is believed
that increasing the amount of CuGa prevented that formation of any
volatile phase causing porosity. Adhesion of the film was also better
due to probable formation of defect free Cu-Ga-Se-S phase at interface
with Mo.

![](media/image28.jpeg){width="3.0104166666666665in"
height="3.0104166666666665in"}

Figure 12c: Bright-field transmission electron micrograph of the
interface between Mo and CIGSS. A Mo and Se rich layer on top of Mo and
a low fraction of pores is found. The adhesion between the Mo and the
CIGSS layer appears to be good.

![](media/image29.wmf){width="5.377083333333333in"
height="3.9006944444444445in"}

Figure 12d: HAADF-STEM of the Mo-CIGSS interface (left) and EDX line
scan (right). A clear increase in the Se signal as well as Mo is found
at the interface indication MoSe~2~

**[Very Thin (1 µm) CIGSS thin film solar cells]{.underline}**

Detailed study was carried out to understand the necessity of thinner
CIGSS absorber layers through the "Terawatt Challenge" and the indium
cost. Initial experiments were carried out to achieve 1 µm thick CIGSS
absorber layer. The absorber layer was studied through materials
characterization techniques such as scanning electron microscopy (SEM)
for morphology, X-ray energy dispersive spectroscopy (XEDS) for
composition, secondary ion mass spectroscopy (SIMS) and Auger electron
spectroscopy (AES) for depth profile and X-ray diffraction (XRD) for the
structure.

CuGa-In metallic precursors were selenized using dilute diethylselenide
(DESe) as selenium source at 400ºC for 10 minutes and further sulfurized
using dilute H~2~S as sulfur source at 475ºC for 20 minutes. Sheet
resistance of the films was 9-12 KΩ/. Figure 13a shows the XEDS graph
carried out at 20 kV. Atomic concentrations measured by XEDS were
Cu:In:Ga:Se:S =18.52:22.99:4.25:27.91:26.34 showing very low gallium
concentration. Figure 13b shows the XRD pattern with (112), (220/204),
and (312) reflections from CuIn~0.99~Ga~0.01~Se~1.08~S~0.92~ phase with
a = 5.7294 Å and c = 11.4587 Å as well as reflections from molybdenum.
Figure 13c shows the SEM image showing compactly packed, small grains
(\~ 0.5 µm) with negligible porosity.

![](media/image30.wmf){width="2.7in" height="2.0520833333333335in"}
Figures 13d and 13e give SIMS and AES depth profiles respectively. Both
SIMS and AES profiles showed increase in gallium concentration and
decrease in indium concentration towards the molybdenum back contact.
Copper signal was steady through the bulk of CIGSS.

![](media/image31.png){width="2.3006944444444444in"
height="2.025in"}Figure 13a. XEDS image of thin CIGSS. Figure 13b. XRD
pattern of thin CIGSS.

Further, completion of these thin CIGSS films into thin film solar cells
will be carried out by the deposition of n-type CdS layer using CBD
technique, ZnO/ZnO:Al bilayer window by RF magnetron sputtering, and
Ni-Al front contacts by e-beam evaporation through metal masks. I-V and
quantum efficiency characteristics will subsequently be presented.

![](media/image32.png){width="1.8875in" height="1.5in"}

Figure 13c. SEM image of thin CIGSS at x5000 magnification.

![](media/image33.wmf){width="2.640972222222222in"
height="1.6909722222222223in"}

Figure 13d. SIMS depth profile of thin CIGSS.

![](media/image34.wmf){width="2.501388888888889in"
height="1.6458333333333333in"}

Figure 13e. AES depth profile of thin CIGSS.

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

After completion of the RTP setup several experiments were carried out
to prepare CIGS thin films following evaporation of NaF and Se on the
CIG precursor layers. Some of the most significant experiments are as
follows

A precursor film of CuGa and In was deposited at 350 W and Ar pressure
of 1.5 x 10^-3^ Torr and 230W at Ar pressure of 7 x 10^-4^ Torr
respectively on a 10 cm x 10 cm Mo coated glass substrate. The speed of
the substrate over CuGa target was 0.052 cm/sec and over indium target
was 0.03 cm/sec. NaF (60-120 Å) and Se (0.8 gm = 2.3 μm) were deposited
by thermal evaporation. The following RTP experiments were carried out
for this sample

Experiment 1

Before switching lamp power ON, N~2~ was filled into the quartz tube at
2.4 Torr. A 2.5 cm x 10 cm piece was kept in the graphite tray. The
power was kept at 100% for 2.5 minutes, 50% for 3 minutes and 40% for 5
minutes. Temperature could not be measured as contact to the
thermocouple was lost. The sample peeled off. It was concluded that the
sample was overheated, unevenly heated or N2 pressure was too low.

Experiment 2

N~2~ pressure was kept at 22 Torr. A 2.5 cm x 10 cm piece was kept in
the graphite tray. The bottom heating array was switched ON to 100% 1
minute before the top heating array was switched ON. Total process time
was same as Experiment 1. The sample did not peel. Its sheet resistance
was 100-200 KΩ/□. SEM and X-ray energy dispersive spectroscopy (XEDS)
results for the same are shown in figure 14. SEM image shows uniform and
compact grains with grain size of \~ 1 to 2 μm. Gallium was not detected
as it was below detection limit.

![](media/image35.jpeg){width="1.90625in" height="1.4895833333333333in"}

XEDS analysis

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

  At%                25.86             22.92             51.21
  ------------------ ----------------- ----------------- -----------------

Figure 14: SEM micrograph showing small and compact grains

Experiment 3

N~2~ was filled upto 20 Torr. Both lamp arrays were kept at 100% for 2.5
minutes and 50% for 8 minutes. A sample appeared uniform with sheet
resistance of 140 KΩ/□. Figure 15 shows compact and large grains of size
2 μm and more.

![](media/image36.jpeg){width="1.9270833333333333in"
height="1.5104166666666667in"}

XEDS analysis

  ------------ ----------- ----------- ----------- ----------- -----------
  Elem.        Cu          In          Ga          Se          Mo

  At%          21.67       21.06       8.2         46.3        2.76
  ------------ ----------- ----------- ----------- ----------- -----------

Figure 15: SEM micrograph showing large and non faceted grains

Experiment 4

Later on some additional experiments were carried out with no samples
for calibration purpose to find out the extent of achievable ramp rates.

a\) In vacuum, it was observed that a temperature of 500 °C could be
achieved in 25 seconds (20 °C/sec).

b\) At 0.8 Torr of N~2~ pressure, it took 2 minutes to reach 500 °C (4
°C/sec).

c\) At 0.43 Torr of N~2~ pressure, it took 2.5 minutes to reach 506 °C
(3.37 °C/sec).

d\) An experiment was carried out to attain 550 °C and to adjust dwell
at that temperature for 6 minutes. At 0.31 Torr, this was possible with
a power setting of 100% for 115 seconds. The SCR control was manually
adjusted thereafter to dwell at the same temperature. Thus, it was
concluded that ramp rates decrease with increase in chamber pressure.

Following experiments were carried out on copper rich sample. The speed
of the substrate over the CuGa target was reduced to 0.046 cm/sec.
Amount of indium was kept same. It was realized that the amount of Se
used in the previous runs was excess so the amount was reduced to 0.63
gm corresponding to 1.8 μm thickness.

Experiment 5

RTP was carried out for this sample with same parameters as in
experiment 4d. The resulting sample was uniform and did not peel off
after etching. Sheet resistance value of 20-100 Ω/□ was obtained. Both
the etched and unetched samples were sent to NREL for electron probe
microanalysis (EPMA) and X-ray diffraction (XRD) analysis.

Experiment 6

RTP was carried out in vacuum with continuous pumping with a mechanical
pump throughout the experiment. A temperature of 550 °C was reached in 2
min 23 sec and maintained at 550°C for the next 6 minutes. A sheet
resistance of \~ 20-100 Ω/□ was observed. This was attributed to loss of
Se during RTP. It was decided that the next sample should be soaked for
just 4 minutes at 550 °C to avoid excessive loss of Se.

Experiment 7

The sample was heated to 550 °C in 180 seconds in N~2~ pressure of 22
Torr with a dwell of 4 minutes at that temperature. The SEM show uniform
and compact grains with grain size of \~ 1 μm (Fig. 16).

![](media/image37.jpeg){width="1.7916666666666667in" height="1.40625in"}
XEDS analysis

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

  At%                20.68             23.58             55.74
  ------------------ ----------------- ----------------- -----------------

Figure 16: SEM micrograph showing compact grains

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