CIGSS Thin Film Solar Cells

Year 2, Annual Report

Report no. FSEC-CR1429-03

October 20, 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 32922-57

![](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, Annual report, Report no. FSEC-CR-1429-2003**

# Introduction

Efforts of during the Year one consisted of scaling-up from small 1"x 1"
to 4"x 4" substrate area. Activities in second year were mainly focused
on stabilizing the parameters for large areas so as to obtain highly
efficient CIGSS thin film solar cells. As a part of this, thickness
optimization over large area (6"x4") for both DC and RF magnetron
sputtering was taken up during initial period of second year. Routine
depositions of CIGS2 absorber layer were carried out after stabilizing
the thickness-uniformity, deposition parameters for molybdenum back
contact, metallic precursors, and sulfurization heat treatment cycle.
Experiments were carried out for improving cadmium sulfide
heterojunction partner layer and recycling of the CdS waste, thus
reducing the problem of toxic waste disposal. Selenization/Sulfurization
furnace unit donated by Shell (formerly Siemens Solar) was refurbished
and made operational. Two scrubbers were commissioned and connected to
the furnace. CM4 analyzer for detecting H~2~S/H~2~Se gas leaks was
coupled with scrubber and central alarm unit of the building. Electron
beam assembly was received and tested. Malfunctioning of the unit
continues and is being presently addressed.

**[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.

Thin flat strips of clean titanium foil of 0.4" x 6" 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

![](media/image2.png){width="2.823611111111111in"
height="1.3020833333333333in"}

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

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 5 x 10^-3^ Torr argon pressure resulted in tensile
stresses showing concave bending and stress lines (Figures 3-4)

![](media/image3.png)

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

![](media/image4.png)

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

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

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

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 curvatures in the cycles mentioned above are provided
in Table I.

Table I: Loads applied to make foil flat.

  ------------------------ --------------------- ------------------------
  **Power / Argon          **Height of           **Loads applied**
  Pressure**               curvature**           

  300W / 3 x 10^-4^        0.88 mm               450 mg

  200W / 5 x 10^-3^        0.48 mm               130 mg
  ------------------------ --------------------- ------------------------

The applied loads as well as the curvature of the foil were used for
estimating the stress. These samples were sulfurized in 4% H~2~S in
argon mixture at 475°C for 20 minutes in selenization/sulfurization
furnace. 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, mostly as it was with slight change in color from
brownish to brownish red. 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. 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 fairly free of residual stresses.
Therefore, the deposition started with low power and high pressure to
reduce chemical reactivity. Two compressive stress cycles sandwiched
between three tensile cycles were deposited in order to lower the stress
and to achieve required thickness.

## Characterization of Molybdenum films

### **1. Morphological Analysis**

Morphology and nature of stress in the films deposited at different
combinations of operating conditions (power and pressure) were studied
by scanning electron microscopy (SEM). High pressure-low power and low
pressure-high power films were independently deposited on glass
substrate. The film deposited at high pressure-low power was
characterized by porous columnar (fish-like) grain growth and
significant intergranular voids (Figure 5). This caused the film to be
in compressive stress and increase in resistivity was a direct result of
this sputtering induced porosity. The film deposited at low
pressure-high power was tightly packed and showed less porosity (Figure
6). This resulted in both an increase in the in-plane tensile stress and
a decrease in film resistivity.

![](media/image6.png){width="3.2465277777777777in"
height="2.0284722222222222in"}

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

![](media/image7.png){width="3.2465277777777777in"
height="2.0284722222222222in"}

Figure 6: SEM micrograph of sample deposited at 300W / 0.3x10^-3^ Torr.

Following are the SEM micrograph showing the sulfurization effect on
these films (Figure 7 and 8). The difference in the microstructure
before and after sulfurization clearly demonstrated the chemical attack
of sulfur on molybdenum.

![](media/image8.png){width="2.9868055555555557in"
height="2.0284722222222222in"}

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

![](media/image9.png){width="2.9868055555555557in" height="2.0875in"}

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

### 

### **2. Resistivity measurements**

A four-point probe technique was used to measure sheet resistance of
molybdenum films. 0.4" x 0.4" size 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 sputter
deposited with thin layer of Au/Pt for approximately 12 minutes. The
deposited regions were soldered to the four probes of the Hall effect
measuring equipment. Films deposited under high power-low-pressure
showed lower sheet resistance as compared to the films deposited at low
power-high pressure. However the sulfurized films showed considerable
rise in the values of sheet resistance. Excessive reaction between
molybdenum and hydrogen sulfide during sulfurization increased sheet
resistance considerably.

### **3. Optical Profilometry**

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

![](media/image11.jpeg){width="3.0034722222222223in"
height="2.123611111111111in"}Figure 9: Optical profilometry micrograph
of sample deposited at 300W / 0.3x10^-3^ Torr.

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

Optical profilometer was used to measure the surface roughness of the
films. Average surface roughness increased with film deposition rate.
Films deposited at high power-low pressure had higher deposition rate
and were therefore observed to be rougher than the high pressure-low
power films (Figures 9 and 10).

**4. X-ray Diffraction**

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

## CIGS2 Thin Films

Problem in maintaining stable plasma was resolved during last year by
changing the shield arrangement. This year, problem of peeling-off of
films after sulfurization was attended. The problem originated because
insufficient amount of gallium reached the back contact as well as
inadequate Cu-excess. As described earlier in the Year 1, Annual Report,
a new CuGa (24%) target with adequate and uniform gallium concentration
was installed in place of the old one having lower and non-uniform
gallium content. Moreover, the thickness of CIGS2 films was lower than
that utilized earlier (2.75 μm). The thickness of metallic precursor was
increased by increasing the thickness of indium by 20% and
copper-gallium by 44% instead of 20%. Comparatively higher amount of
CuGa was added as the excess copper is consumed for CuS layer formation
during sulfurization and is etched away in subsequent KCN treatment.
This totally eliminated the peeling-off problem. Thickness obtained
after making these changes was found to be more than 2.75 μm. Metallic
precursors were individually deposited on glass substrate for thickness
measurement using DEKTAK profilometer. Deposition parameters were
relatively varied to achieve desired thickness. The elemental stack was
sulfurized in old Lindberg furnace at Ar:H~2~S 4% flow throughout the
cycle with the maximum temperature of 475 ^o^C for 60 minutes. Later
part of the year the furnace donated by Shell Solar was made
operational. In this furnace gas ratio was maintained same while the
maximum processing temperature and time were of 475 ^o^C and 20 minutes.
Periodic maintenance of the both DC and RF sputtering vacuum systems was
carried out by changing the conveyor foils, and accessories. Well
adherent large area (6"x 4") thin film absorber layers were routinely
fabricated.

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

![](media/image12.jpeg){width="4.0256944444444445in"
height="2.4402777777777778in"}

Figure 11. CBD Set-up used for CdS deposition

Heterojunction partner CdS layers were satisfactorily deposited by
chemical bath depositions on 1"x 4" CIGS2 samples. Design and
construction of the corresponding setup was explained in previous annual
report. Successful fabrication of 4"x 4" substrate size till
sulfurization stage was driving force for developing new CdS setup to
accommodate the new size. Figure 11 shows the new setup made using
Teflon sheets and rods. Water bath shown in the same figure was not
sufficient enough to achieve uniform temperature within the reacting
solution. The setup was modified as shown in Figure 12. The outer bath
was changed from smaller glass container to bigger aluminum pot. The pot
could hold more volume of outer bath water and therefore could heat the
inner chemical bath to the desired temperature in comparatively less
time.

![](media/image13.jpeg){width="2.8569444444444443in"
height="2.998611111111111in"}

Figure 12. Modified CBD set up used for CdS deposition

Quantities of chemicals were optimized by carrying out systematic
experiments. Considerable emphasis was laid on recycling of the waste
that was 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 subsequent CdS
depositions.

Table II: Amount of chemical and time during continuous recycling

To improve yield and reduce waste, the CdS precipitates were 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 II 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 13 (a) and (b) show the microstructure of CdS layer on
Tin Oxide coated glass

![](media/image14.png){width="2.5700787401574803in"
height="2.0070866141732284in"}![](media/image15.png){width="2.55in"
height="2.013779527559055in"}

Figure 13: SEM images a) Image for CdS layer with fresh solution and b)
Image for CdS layer with recycled CdS solution.

substrate. The recycled solution shows similar morphology to that
obtained from CdS layer derived from fresh solution.

## Transparent Conducting ZnO/ZnO:Al Bilayer

Experiments were carried out for the deposition of ZnO:Al and ZnO thin
films. Regeneration process carried out for targets replenishes the
depleted oxygen species during deposition. Initially, simultaneous
regeneration of the ZnO:Al sputtering target was carried out so as to
minimize the down time and also to avoid waste of expensive target
material. This was carried out using by 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 was 1.5x10^-3^ Torr,
RF power was 300 watts for ZnO:Al and deposition time was 4 minutes at
each substrate movement step of ½". However, it was observed that
simultaneous regeneration of ZnO:Al target might have an adverse effect
on the efficiency of CIGS2 cells. Hence, regeneration of ZnO:Al target
was carried out after the completion of deposition cycle. After
completion of each deposition cycle, samples were taken out of the
vacuum chamber and a piece of glass was mounted for sputtering on glass
during regeneration process. ­O2 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.

­Regeneration time was varied over a wide range while maintaining the
same RF power levels as used during depositions i.e. 200 watts for ZnO
and 300 watts for and ZnO:Al targets. Depositions of ZnO/ZnO:Al bilayer
on glass sample were carried out after every regeneration and the films
thus obtained were analyzed by photospectrometer and Hall effect
measurements, to compare transparency, resistivity, sheet resistance,
carrier density and Hall mobility. Best performance was obtained for the
replenishment time of 14 minutes for ZnO:Al and 2 minutes for intrinsic
ZnO target. The method of post-deposition replenishment of targets
resulted in overuse of ZnO/ZnO:Al target materials but by optimizing the
time of post-regeneration, down time and waste of the ZnO:Al target
material was reduced. Bilayer window depositions were carried out at 200
watts and 40 seconds per ½" step for intrinsic ZnO to achieve 500 Å
thickness and at 300 watts with 4 minutes per ½" step for ZnO:Al to
achieve 5000 Å thickness. Efficiencies of the cells fabricated with
these thicknesses were not found promising. Experiments were carried out
for reduced thickness of intrinsic ZnO. Thickness values of 30 Å, 250 Å
and 450 Å were used for fabricating CIGS2 thin film solar cells.
Complete analysis of these cells is awaited.

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

The old vacuum system equipped with 1500 l/s diffusion pump, liquid
nitrogen trap, a two-stage rotary pump and a gate valve was refurbished
for evaporation of Cr/Ag contact fingers. Electrical and water
connections were re-established. Individual electrodes were polished and
assembled together with evaporation boats. 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
observed 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 \~5000 Å.

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

Last year, electron beam evaporation source (e-gun) was purchased from
Thermionics Laboratory Inc for deposition of Ni/Al contact fingers to
complete the fabrication of CIGSS thin film solar cells. The e-beam
source was installed in a high-vacuum system with the requisite
electrical and water connections as per instructions by the
manufacturer. In order to hold the substrate and to facilitate their
movement a rotation mechanism was designed and developed in-house.
During trial run the electron beam appeared to split in two components
one of which hit the evaporant in the crucible and other was incident on
the body screw thus endangering the assembly. After discussing with the
manufacturer the entire assembly was shipped to manufacturer to rectify
the faulty operation of the e-gun system. The e-gun system was reported
to be operating normally without any split beam problem at the
manufacturer's facility. After the receipt of the e-beam system at FSEC,
it was mounted in vacuum system that had a cryopump and mechanical pump
combination for attaining vacuum in the low 10^-6^ Torr range. The
problem was found to persist with no improvement. With repeated trials
the power supply became unstable and fluctuated over a range of values;
and the power supply switched itself OFF as soon as the emission current
was switched ON. It was not possible to carry out any further
experimentation under these circumstances. Therefore, the e-gun system
was again shipped back to the manufacturer for repair work. The
manufacturer reported that a capacitor in the power supply had failed
and the malfunctioning of the e-gun system was because of improper RF
grounding. In order to provide independent RF grounding eight copper
clad grounding rods of dimension 10 ft. x 0.75 in. diameter were driven
in the ground in a form of a network that provided effective resistance
of 0.6-0.7 Ω. On receiving the e-gun assembly it was reinstalled in
vacuum chamber and hooked up to the grounding network. The e-gun was
tested to verify if the problem of split beam and malfunctioning of
power supply still persist. During the first run it was seen that the
power supply had problems reaching 9.82 KV owing to arc faults. In
subsequent experiments sufficient dwell was given at each KV reading for
the power supply to stabilize. This helped in reaching 9.82 KV and to
remain stable. Aluminum was successfully evaporated to achieve
approximately 1-micron thickness. While trying to operate the system
again the power supply could not go beyond 6 KV and shut off owing to
arc fault. The experiment was performed again after resetting. However,
the voltage threshold of shutting off reduced in each subsequent
attempt. It was not possible to go even beyond 4 KV. Local
representative of Thermionics visited the facility at FSEC. In his
presence the gun was again tested. In this attempt the arc fault
occurred at 3.5 KV and the system switched off. It was mutually decided
to plan the visit of Thermionics engineer from the manufacturing
division to solve the problem onsite. If the problem cannot be solved
the assembly will be returned to Thermionics for repair work.

[Selenization/Sulfurization Unit]{.underline}

Selenization/Sulfurization furnace was donated by Shell Solar (formerly
Siemens Solar) for sulfurization of large substrates. Refurbishment work
was taken up in 4^th^ quarter of last year and it was completed by 2^nd^
quarter of this year. Before actual experimentation the unit was tested
for leaks using Residual Gas Analyzer (RGA) at joints and fittings.
Preliminary tests were carried out using 4" x 4" Mo coated glass
substrates with CuGa/In metallic precursor layers. Systematic
experimentation was carried out to stabilize the sulfurization treatment
parameters in order to achieve uniform and conformal CIGS2 absorber
layer. Sulfurization was carried out in 4% H~2~S in H~2~S/N gas mixture
where nitrogen was used as carrier gas. Initial experimentation was
carried out at maximum temperature of 475 ^O^C for 60 minutes. The films
obtained from this experiments were non conformal, patchy and
peeled-off. Experiments were carried out at different dwell times of 10,
15 and 20 minutes. 475 ^O^C for 20 minutes was observed to be the best
combination for achieving uniform sulfurized layer. Sulfurization cycle
of H~2~S: N of 4% and 475 ^O^C for 20 minutes are being routinely used
for fabricating CIGS2 absorber layer.

![](media/image16.png){width="3.7868055555555555in"
height="2.689583333333333in"}

Figure 14. Selenization/Sulfurization furnace

**[Scrubber for Selenization/Sulfurization furnace]{.underline}**

Design part of the scrubber was explained in last year's annual report.
This year marked the completion of commissioning and successful
operation of the scrubber. The packed-bed column was split in two. First
column of 10.25 ft. was installed beside the selenization/sulfurization
furnace inside the FSEC PV Mat Lab. A new 12 feet x12 feet room was
built in the FSEC high bay lab. The second 9.25 feet packed-bed column
was installed in this room. The new room will also house a Fume hood for
NaOH neutralization. The double containment pipe connection from first
column to the second was completed. Both the columns had sprinklers on
top for fire safety. The Florida State Fire Marshal had approved the
sprinkler set-up. Installing KOH impregnated carbon drums, exhaust fans
and four point CM4 analyzer completed the commissioning. CM4 analyzer
was also connected to building alarm system for safety measures in case
of accidental leaks. Following table shows the CM4 results for H~2~S
gas. These values are well within the specification of Occupational
Safety and Health Association (OSHA) i.e. 10 ppm. H~2~Se gas will be
used after assuring complete leak proof operation of the furnace and
scrubber.

Table III. CM4 results

+-----------+---------------------------+------------------------------+
| Point of  | Location of the point in  | H~2~S gas concentration in   |
| Detection | the scrubbing system      | ppb scale                    |
| for toxic |                           |                              |
| gas       |                           | (OSHA requirement is 10 ppm) |
| conc      |                           |                              |
| entration |                           |                              |
+-----------+---------------------------+------------------------------+
| 1         | Near the furnace          | 0                            |
+-----------+---------------------------+------------------------------+
| 2         | First packed bed scrubber | 18                           |
|           | top                       |                              |
+-----------+---------------------------+------------------------------+
| 3         | Second packed bed         | 17                           |
|           | scrubber top              |                              |
+-----------+---------------------------+------------------------------+
| 4         | After activated carbon    | 0                            |
|           | drums                     |                              |
+-----------+---------------------------+------------------------------+

![](media/image17.png){width="3.192361111111111in"
height="3.6354166666666665in"}

Figure 15: Scrubber

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

CIGS2/CdS thin film solar cells were prepared on 4"x4" Mo-coated glass
samples and stainless steel foil substrates. Elemental stack sequence
deposited by DC magnetron sputtering was sulfurization at 475 ^0^C for
20 minutes to obtain CIGS2 absorber layer. Excess CuS layer developed
during sulfurization was etched away by KCN treatment. CdS n-type
heterojunction partner was deposited by chemical bath deposition. Window
bilayer of ZnO/ZnO:Al was deposited by RF magnetron sputtering. Ni/Al
contact fingers were deposited at NREL, as e-gun source was not
operational. The absorber layers were characterized by Scanning electron
microscopy (SEM), Auger Electron Spectroscopy and X-ray diffraction. CdS
layer was analyzed by SEM. Transparency and conductivity of window
layers was studied by photospectrometer while sheet resistance and
mobility were measured with Hall effect measurements.

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

During this year, five students: Mr. Ankur A. Kadam, Mr. Vivek S. Gade,
Mr. Sachin S. Kulkarni, Mr. Sachin M. Bet and Mr. Harshad P. Patil,
successfully defended their Masters theses. Mr. Ankur A. Kadam and Mr.
Anant H. Jahagirdar enrolled for PhD Program at UCF starting spring
2003. PI taught two Graduate level courses: Photovoltaic Solar Energy
Materials and Physics of Failure and Reliability of Microelectronic
Packages.