CIGSS Thin Film Solar Cells

Year 2, Quarter 3 Report

Report no. FSEC CR1416-03

July 21, 2003

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"}

**CIGSS Thin Film Solar Cells**

###### NREL contract no. NDJ-2-30630-03, UCF/FSEC Account no. 26-56-511

Year 2, Quarter 3, Report no. FSEC-CR-1416-2003

######### Introduction

Deposition of CuIn~1-x~Ga~x~S~2~ (CIGS2) thin film solar cells was
carried out on a routinely basis both by DC and RF magnetron sputtering
systems on Mo coated glass substrates and devices were sent for
characterizations and efficiency measurements. This report presents the
process development towards improving Mo back contact layer along with
working of the evaporation system by Joule heating as well as status of
the electron beam evaporation system. Experiments were also carried out
for improving cadmium sulphide heterojunction partner layer and
recycling of the CdS waste, thus reducing the problem of toxic waste
disposal. Satisfactory functioning of Selenization/Sulfurization furnace
unit donated by Shell (formerly Siemens) and Scrubber for furnace
connected with a CM4 analyzer for preparation and development of
CuIn~1-x~Ga~x~Se~2-y~S~y~ (CIGSS) thin-film solar cells on large (6"x4")
substrates is also discussed.

**[Mo Back Contact]{.underline}**

## 

Depending on the working gas pressure, residual stresses are developed
in refractory metal films prepared by magnetron sputtering. Films
deposited below transition pressure develop compressive stresses,
whereas those deposited above the transition pressure develop tensile
stresses. Gross stress may be determined by visual inspection in that
highly compressed films tend to buckle up, frequently in zigzag
patterns, whereas films under extreme tensile stress develop a system of
stress lines that appear like scratches.

Earlier during 1992-93, PV Materials Laboratory at FSEC had carried out
extensive research on back contact molybdenum layer for CIGS thin film
solar cells to study the effect of deposition parameters on the
morphology and electrical properties of Mo layer. Films deposited under
high power and low pressure (HW/LP) showed compressive stresses while
those deposited under low power and high pressure (LW/HP) showed tensile
stresses. As the thickness and residual stress build, films deposited at
high power and low pressure or low power and high-pressure both show
stress lines that appeared like scratches and even leading to cracks.
Residual stress measurements were carried out by using XRD technique at
Oak Ridge National Laboratories (ORNL). Based on these results five
alternate cycles of HW/LP and LW/HP starting with HW/LP and ending with
the same were used for deposition of back contact molybdenum to obtain
the desired thickness of approximately 5500 Ǻ. The films deposited under
these conditions were found to be relatively stress free. The resultant
1"x1.25" CIGS solar cells did not peel off during selenization or
sulfurization and showed efficiency over 10% AM1.5. It was observed that
after sulfurization in 4 % H~2~S the exposed molybdenum layer turned
purple. Therefore, effect of deposition and sulfurization parameters on
the morphology and electrical properties of Mo layer was reinvestigated
especially in relation to enhancing the size to larger areas 4"x4".

Thin flat strips of clean titanium foil of 1cm x 15cm were attached at
their ends to a glass substrate prior to deposition (Figure 1) using
vacuum compatible tape. It was observed that films deposited at high
power of 300W and low argon pressure of 3 x 10^-4^ Torr resulted in
compressive stresses. The thin foil deposited under these parameters
showed convex bending (Figure 2). On the other hand films deposited at
power of 200W and argon pressure of 5 x 10^-3^ Torr resulted in tensile
stresses showing concave bending (Figure 3).

![](media/image2.png){width="4.0in" height="1.5583333333333333in"}

Fig. 1: Thin strips of flat foil attached to glass substrate prior to
deposition.

# ![](media/image3.jpeg){width="4.545138888888889in" height="1.6881944444444446in"}

# 

Fig. 2: Films deposited at high power of 300W and low argon pressure of
3 x 10^-4^ Torr resulting in compressive stresses observed as lifting of
the flat foil.

![](media/image4.jpeg){width="3.2465277777777777in" height="1.5875in"}

Fig. 3: Films deposited at low power of 200W and high argon pressure of
5 x 10^-3^ Torr resulting in tensile stresses observed as lifting of the
inverted deposited foil.

![](media/image5.jpeg){width="2.7270833333333333in" height="1.625in"}

Fig 4: Scratches observed due to tensile stresses on films deposited
200W/ 5 x 10^-3^ Torr.

Films deposited at low power and high-pressure showed stress lines on
the film (Figure 4) that appeared like scratches. These samples were
sulfurized in 4% H~2~S in argon mixture at 475°C for 20 minutes. The
reactivity of the molybdenum in sulfur/ selenium atmosphere has been
reported to occur at temperatures over 600°C. However, it was observed
that the sample under compressive stress peeled off from several regions
and turned reddish pink indicating a chemical reaction occurring even at
475°C. On the other hand, the sample under tensile stress remained as it
was with slight change in color from brownish to brownish red. Thus, the
possible cause for the reaction of molybdenum in sulfur atmosphere might
be due to increase of chemical reactivity induced by residual stresses
especially compressive formed during deposition. Morphology of these
samples was analyzed by SEM. Also, if the total thickness of the
molybdenum layer is very low, either deposited by multiple layer
sequence or deposited at once, the reactivity of H~2~S with the
molybdenum can cause an excessive increase in the sheet resistance of
the molybdenum after sulfurization. However, if molybdenum thickness is
adequate it would not result in excessive increase in the sheet
resistance in spite of the very thin reacted layer H~2~S at the surface.

### [Adhesion Measurement]{.underline}

Adhesion of molybdenum films to glass substrates is one of the very
important parameter in the fabrication of CIGSS thin film solar cells.
Good adhesion is essential to avoid peeling off. It also affects the
device performance. Films exhibited various degrees of adhesion to the
soda-lime glass substrates. This was immediately evident when
as-deposited films were ultrasonically cleaned for less than 1 min in
isopropyl alcohol. After cleaning, the degree of adhesion was
qualitatively assessed using the adhesive-tape test. It was observed
that films sputtered at high power and low-pressure had poor adhesion to
substrate. Also pieces of the molybdenum films flaked off during this
cleaning process, while the films sputtered at low power and
higher-pressure exhibited good adhesion.

[Stress Measurement]{.underline}

Various techniques such as X-rays diffraction pattern, interferometry,
bending substrate curvature method, glancing-angle X-ray diffraction
etc. are available for

Table I: Loads applied to make foil flat.

measurement of stress in thin films. In the present case, a simple
bending foil technique was used to estimate the magnitude of stress
present in the foils. Loads were applied to the concave and convex
region of the foil and weight required to make the foil flat similar to
its position prior to deposition was measured. The load required and the
curvature in the cycles mentioned above has been tabulated below.

The applied load as well as the curvature of the foil was used as a
measure for estimation of stress. Foils were deposited with a number of
combinations of the cycles of high power/low pressure and low power/high
pressure to obtain a very flat foil after deposition. It is believed
that such stress reversal is dependent on energetic bombardment by
reflected neutral and/or sputtered atoms. At relatively low pressure,
the mean free path is high, as a result of which the arriving atoms have
higher kinetic energy and the resulting film has dense microstructure
thereby experiencing compressive stresses. At higher working pressure
the mean free path is reduced resulting in moderate flux of atoms
reaching the substrate leaving the film in tensile stress. However, for
producing highly efficient 4"x 4" size solar cell, the back contact
molybdenum has to be completely free of residual stresses. Therefore,
combination of tensile and compressive layers of molybdenum was
deposited. The deposition cycles and time was altered to obtain the
desired thickness.

Two different cycles (Table I) were designed and studied for this
purpose. The desired thickness for molybdenum for CIGS2 thin film solar
cells is approximately 5500 Å. Therefore, the deposition cycles were
designed in such a way that the desired thickness was achieved at the
same time the films obtained were free of residual stresses.

In the first experiment two tensile stress cycles (low-power and
high-pressure) were sandwiched between three cycles of compressive
stress (high-power and low-pressure). In the second experiment two high
power and low-pressure cycles were sandwiched between three low power
and high-pressure cycle. Films obtained from both the

Table II: Deposition sequences used for reducing the residual stress.

experiments were sulfurized and tested for adhesion using simple scotch
tape test. Both the films showed no peeling. It can be inferred that
when alternate layers with compressive and tensile stress are present
then the reaction does not proceed through the entire thickness of
molybdenum during sulfurization and thereby maintaining good adhesion
and morphological and electrical properties.

### [Morphological Analysis]{.underline}

Morphology of deposited Mo films was analyzed by scanning electron
microscopy (SEM). Current study highlights the importance of film
microstructure in determining the level of intrinsic stress present in
polycrystalline thin-films. When sputtering at high pressures, multiple
gas-phase collisions reduce the energy of sputtered atoms and
neutralized gas ions. In addition, the angle at which these incident
species impact the substrate becomes more oblique. The resulting
microstructure is characterized by porous columnar (fish-like) grain
growth and significant intergranular voids. The observed increase in
resistivity for the films at higher sputtering pressure is a direct
result of this sputtering induced porosity. Attractive forces (i.e.
tensile force between these grains) are inversely proportional in
strength to the intergranular spacing. With decreasing pressure and less
scattering of sputtered species, the films become less porous and more
tightly packed. This results in both an increase in the in-plane tensile
stress and a decrease in film resistivity (Table II). When the
intergranular spacing decreases to a point where grains begin to touch
(low pressure). Compressive forces associated with atomic peening

![](media/image6.png){width="2.9868055555555557in"
height="2.5215277777777776in"}

Fig. 5: SEM micrograph of sample deposited at 300W / 0.3x10^-3^ Torr.

begin to exceed the above-mentioned tensile force. As this compressive
force starts to increase, films exhibit a maximum in tensile stress and
finally go into compression at very low sputtering pressures.

The microstructure of the compressive stressed films (Figure 5) consists
of tightly packed columns, whereas in the tensile stressed films (Figure
6) the development of a void network structure surrounding the columnar
grains is observed. Fish-like morphology has been shown to be desirable
by earlier studies at NREL.

![](media/image7.png){width="2.9868055555555557in"
height="2.4881944444444444in"}

Fig. 6: SEM micrograph of sample deposited at 200W / 5x10^-3^ Torr.

![](media/image8.png){width="2.467361111111111in"
height="2.217361111111111in"}

Fig. 7: SEM micrograph of sample deposited at 300W / 0.3x10^-3^ Torr and
sulfurized at 475°C for 20 minutes.

However the SEM micrograph of sulfurized molybdenum film did show
different structure as compared to the as deposited samples. This
clearly showed the occurrence of chemical reaction between molybdenum
and hydrogen sulfide even at 475°C. Figures 7 and 8 show the SEM
micrographs of sample deposited at 300W / 0.3x10^-3^ Torr and 200W
/5x10^-3^ Torr and sulfurized for 20 minutes in H~2~S atmosphere. It is
clearly observed that the structure has become more dense also the
grains are having different shape as compared to as deposited samples.

![](media/image9.png){width="2.5972222222222223in" height="2.0875in"}

Fig. 8: SEM micrograph of sample deposited at 200W / 5x10^-3^ Torr and
sulfurized at 475°C for 20 minutes.

### [Resistivity measurements]{.underline}

A four-point probe technique was used to measure sheet resistance of
molybdenum films. 1cm x 1cm samples were cut from the large samples.
These samples were covered with aluminum foil in such a way that the
four corners remained exposed. These samples were then sputter deposited
by thin layer of Au/Pt for approximately 12 minutes. The deposited
regions were then soldered to the four probes of the Hall effect
measuring equipment. The sheet resistance of the films are tabulated in
the table III below. Thickness was measured using DEKTAK profilometer.

It was observed that films deposited under high power and low-pressure
showed lower sheet resistance as compared to the films deposited at low
power and high pressure. This can be the microstructure formed after
deposition due deposition parameters. However the sulfurized films
showed considerable rise in the values of sheet resistance. If there is
excessive reaction between molybdenum and hydrogen sulfide during
sulfurization and sheet resistance increases considerably, it can damage
the device performance by increasing the series resistance.

### [Optical Profilometry]{.underline}

Profilometry measurements on the thin Mo layers allowed determining the
surface average roughness. The average surface roughness was found to be
increasing with film deposition rate (Table III). Films deposited at
high power and low pressure

Table III: Thickness, sheet resistance and surface roughness
measurements for different cycles of deposition.

  ----------------------------- ----------- -------------- ----------------
  Deposition Parameters         Thickness Å Surface        Sheet Resistance
                                            Roughness nm   (Ω / )

  300W / 0.3x10^-3^ Torr (44    2870        1.88, 1.24,    1.88
  sec)                                      2.13           

  300W / 0.3x10^-3^ Torr (66    3800        \-             1.08
  sec)                                                     

  200W / 5x10^-3^ Torr (132     5852        0.83, 0.74     3.56
  sec)                                                     

  200W / 5x10^-3^ Torr (121     5430        \-             4.57
  sec)                                                     

  300W / 0.3x10^-3^ Torr        2870        \-             \-
  (sulphurized)                                            

  200W / 5x10^-3^ Torr          5852        \-             2420
  (sulphurized)                                            
  ----------------------------- ----------- -------------- ----------------

![](media/image10.jpeg){width="3.0in" height="2.3180555555555555in"}

Fig. 9: Optical profilometry micrograph of sample deposited at 300W /
0.3x10^-3^ Torr.

having higher deposition rate showed higher roughness values (Figure 9),
while those deposited at low power and high pressure showed lower
roughness values (Figure 10).

[X-ray Diffraction]{.underline}

The films deposited with 5 alternate cycles of compressive and tensile
were sulfurized and further characterized by XRD. Only molybdenum peaks
were observed and no other compound of sulfur or any other compound was
detected. Thus even though

![](media/image11.jpeg){width="3.0034722222222223in"
height="2.123611111111111in"}

Fig. 10: Optical profilometry micrograph of sample deposited at 200W /
5x10^-3^ Torr.

the films had discolored during the sulfurization, the bulk of the Mo
layer remained intact and that the reacted Mo layer was not thick enough
to provide detectable XRD reflections. These films will be further
characterized by SEM to observe the change in morphology and four-point
probe technique will be used for measuring the sheet resistance.

It is planned to augment the analysis. The as-deposited samples will be
further characterized by X-ray diffraction technique to quantify the
stress obtained for each deposition cycle and resistivity measurement.
Sulphurized samples will be characterized by Auger electron spectroscopy
for spectroscopic analysis as well as by X-ray diffraction for
structural, compositional as well stress analysis, and resistivity
measurements for estimating series resistance losses.

Presently the 5-cycle deposition of alternate cycles of high power low
pressure and low power high pressure has been changed. The new process
has 3-cycles starting with low power high pressure, high power low
pressure in the middle and ending with low power high pressure. Though
the cycles have been reduced the total deposition time has remained the
same as in the previous 5-cycle process because the time for the
deposition of each layer has been increased proportionately. This cycle
may be further modified depending on the sheet resistance measurements
obtained on sulfurized molybdenum layers.

**[Heterojunction Partner CBD CdS Layer]{.underline}**

Experiments were carried out to recycle chemicals and to reduce chemical
waste during chemical bath deposition (CBD) of heterojunction partner
CdS layers on 4"x4" CIGS samples. Earlier, experiments concentrated on
achieving uniform CdS films on 4" x 4" substrates. At present,
considerable emphasis is being laid on recycling of the waste that is
produced during the chemical bath deposition. Disadvantages of the CBD
method are the low materials yield and production of toxic cadmium
containing waste. Experiments were carried out to reduce waste by
reusing the ammonia and the non-reacted thiourea for the subsequent CdS
depositions.

Table VI: Amount of chemical and time during continuous recycling

To improve yield and reduce waste, the CdS precipitate was separated
from the waste after deposition by ultra-filtration. The permeate that
contains ammonia and thiourea was re-used for the next CBD process after
addition of cadmium salt. The amount of thiourea and ammonium hydroxide
for obtaining satisfactory CdS deposition under approximately the same
conditions were determined. Table VI provides the amount of chemical
added to obtain a thickness of approximately 500 Å and the corresponding
time required. It can be seen that due to formation of reaction products
the reaction proceeds at a slower rate and thus the quantity of
chemicals added at the end of sixth recycling is same as the original
amount. Figure 11 a and b show the microstructure of

![](media/image12.png){width="2.5700787401574803in"
height="2.0070866141732284in"}![](media/image13.png){width="2.55in"
height="2.013779527559055in"}

Figure 11: SEM images a) Image for CdS layer with fresh solution and b)
Image for CdS layer with recycled CdS solution.

CdS layer on Tin Oxide coated glass substrate. The recycled solution
shows similar morphology to that obtained from CdS layer derived from
fresh solution.

**[Transparent Conducting ZnO/ZnO: Al Bilayer]{.underline}**

Experiments were carried out for optimizing parameters for simultaneous
and post regeneration of targets for reducing waste of valuable targets
and time of regeneration. As explained in the earlier report,
simultaneous regeneration was carried out using Ar+O~2~ mixture
containing 1% O~2~. 5500 Å thick, \~85% transparent ZnO:Al thin films
with sheet resistance in the range 27-45 could be deposited at oxygen
partial pressure of 5 x10^-6^ Torr, total Ar+O~2~ pressure of 1.5x10^-3^
Torr, RF power of 300 watts, deposition times of 4 minutes at each
substrate movement step of ½".

Depositions were also carried out to optimize the down time and minimize
waste of ZnO:Al target by post regeneration. Regeneration time was
varied over a wide range while maintaining the same RF power levels as
used during depositions i.e. 200 and 300 Watts for ZnO and ZnO:Al
targets respectively. For this, O~2~ pressure was adjusted at 7.5x10^-4^
Torr with mass flow controller and the total pressure was increased to
1.5x10^-3^ Torr by adding argon gas. The films of ZnO/ZnO:Al bilayer,
thus obtained were analyzed by photospectrometer and Hall measurements.
The transparency, resistivity, sheet resistance, carrier density and
Hall mobility of these films were compared.

After series of depositions and analysis of ZnO/ZnO:Al thin films, the
time of regeneration was adjusted to be 2 minutes and 13 minutes at RF
power levels of 200 watt and 300 watt for ZnO intrinsic and ZnO:Al
respectively. Oxygen and argon partial pressures were adjusted at
7.5x10^-4^ Torr each.

**[Vacuum System Refurbishment for Evaporation]{.underline}**

The old vacuum system with a large deposition chamber (\~18\" diameter x
24\" height) for evaporation pumped with a 1500 l/s diffusion pump,
liquid nitrogen trap, a two-stage rotary pump and a gate valve was being
refurbished since the last quarter. Completed tasks in refurbishment
process involved power connection cables testing and replacing old
connections with new ones, water connections along with polishing and
assembly of individual electrodes assembly and evaporation boats
mounting. Preliminary testing was carried out satisfactorily.

[Vacuum Evaporation of Cr/Ag contact Fingers]{.underline}

It was decided to deposit Cr/Ag contact fingers by thermal evaporation
in the old vacuum system. For this, gold plated copper electrodes were
fitted in the chamber used earlier for selenization. Water connections
were provided to the electrodes. The hoisting mechanism for lifting bell
jar of the vacuum chamber failed during operation. The problem was
solved by replacing bearings of hoist bolt and by replacing the worn out
belt of motor of hoist. Experiments were carried out for uniform
depositions of Cr/Ag contact fingers on solar cells. It was found that
\~600Å thick chromium could be deposited at 3.6 volts and 110 amperes
for 6 minutes from tungsten evaporation boat. Silver was deposited from
molybdenum boat for 9 minutes at 2.1 volts and 50 amperes. Thickness of
silver deposited was \~5000Å.

**[Electron Beam Evaporation Source System]{.underline}**

The design for grounding was mentioned in the earlier report. In this
quarter the grounding work for eliminating split beam problem in the
electron beam evaporation system was completed. The electron beam
evaporation system also arrived at FSEC in this quarter. Installation of
the system is being carried out and preliminary trails shall be carried
out for contact finger deposition.

**[Shell (Siemens) Selenization/Sulfurization Unit]{.underline}**

Sulfurization of 4" x 4" substrate with CuGa/In metallic precursor
layers is being satisfactorily carried out at FSEC. CIGS2 samples have
shown characteristic colors after the treatment.

# [Scrubber for Selenization/Sulfurization Furnace]{.underline} 

Both the scrubbers with all the connections including KOH impregnated
carbon drums; exhaust fans and four point CM4 analyzer were completed.
Using a mixture of 4% H~2~S in nitrogen and further nitrogen dilution of
60 times, the toxicity level of H~2~S has been continuously monitored at
the following four points.

[Point 1]{.underline}: The furnace enclosure that will immediately show
rise in the toxic concentration in case there is any leak. This point is
very important from operating personnel point of view.

[Point 2]{.underline}: This point is located at the exit of the first
packed bed scrubber. The effectiveness of the venturi scrubber and first
packed bed scrubber and the amount of toxic gas entering the second wet
scrubber as well as the second packed bed scrubber can be determined
using the toxic gas concentrations obtained from this point.

[Point 3]{.underline}: This point is located at the exit of the second
packed bed scrubber. The effectiveness of the second wet scrubber and
second packed bed scrubber and also the

amount of toxic gas entering the activated carbon drums can be
determined by using toxic gas concentrations obtained from this point.

[Point 4]{.underline}: This point is at the exit of the activated carbon
drums. The effectiveness of the overall scrubbing system can be
determined by using the toxic gas concentrations obtained from this
point. Results of gas concentrations as detected by CM4 continuous gas
monitor at the above points are provided in following Table V.

Table V. H~2~S gas concentration at four points in ppb scale

  --------------- --------------------------- ------------------------------
  Point of        Location of the point in    H~2~S gas concentration in ppb
  Detection for   the scrubbing system        scale
  toxic gas       (Figure 38)                 
  concentration                               

  1               Near the furnace            0

  2               First packed bed scrubber   18
                  top                         

  3               Second packed bed scrubber  17
                  top                         

  4               After activated carbon      0
                  drums                       
  --------------- --------------------------- ------------------------------

From these results it can be clearly seen that the exhaust gases form
the furnace are getting detoxified to the level that is much lower than
the threshold limit value for H~2~S. The results also show that the
system is leak tight. Thus scrubbing system is functioning
satisfactorily and is reducing the toxic gas concentrations to level
that are below the Occupational Safety and Health Administration (OSHA)
threshold limit values (TLV), and it is safe to work in the FSEC PV
materials laboratory. In case of H~2~S, the threshold limit values (TLV)
as per Occupational Safety and Health Administration (OSHA) is 10 ppm,
whereas the concentration of gas leaving the first packed bed scrubber
is 0 in ppm scale and 18 in ppb scale. Already this is considerably
below the required amount of threshold limit values (TLV) for H~2~S.With
the second scrubber certainly much lower values will be obtained.

The scrubbing system is currently being used only to detoxify H~2~S type
of gas. Thus no data is available for the H~2~Se type of gas. For H~2~Se
the threshold limit values (TLV) as per Occupational Safety and Health
Administration (OSHA) is 50 ppb. After using the scrubbing system for
H~2~Se type of gas the concentration levels at various points for H~2~Se
will be known. The reactions for H~2~Se are expected to be analogous.
The molecules of H~2~Se and H~2~S have similar chemical structures,
however the former are slightly larger. H~2~Se is expected to be more
reactive with NaOH than is H~2~S. Thus H~2~Se will be effectively
scrubbed and will be reduced to below 50 ppb, the threshold limit values
(TLV) for H~2~Se on a continuous basis.The CM4 analyzer is being
connected to the present building alarm system.

**[CIGS2 Thin Film Solar Cell device Characterization and
Testing]{.underline}**

CuIn~1-x~Ga~x~S~2~ (CIGS2) films were prepared on 4"x4" Mo-coated glass
samples by sulfurization at 475 ^0^C. Reasonably good crystalline
quality, copper-rich CIGS2 thin film were prepared by sulfurization of
DC magnetron sputtered stacked CuGa/In metallic precursor layers in a
mixture of argon and hydrogen sulfide. Near stoichiometric, copper-poor,
CIGS2 thin film were obtained by etching Cu-rich CIGS2 layers in dilute
KCN. The absorber layers were characterized by Scanning electron
microscopy, Auger Electron Spectroscopy and X-ray diffraction. Further
deposition of CdS, ZnO/ZnO:Al, was carried out. Final deposition of
Ni/Al/Ni contact fingers was carried out at NREL and efficiency
measured. The efficiency measured at AM1.5 for thin films on glass
substrates was 7.89% while for the samples with antireflection coating
(MgF2) the efficiency was 8.65%.

**[Graduate Teaching and Thesis Research]{.underline}**

The PI taught one Graduate course on Photovoltaic Solar Energy
Materials. Three students of the PI Mr. Harshad P. Patil, Mr. Sachin S.
Kulkarni and Mr. Sachin M. Bet successfully defended their Masters
theses. Mr. Harshad P. Patil shall be continuing as a Ph.D. student.