Fabrication and Characterization of Various

Layers of Thin Film CIGS Solar Cells

Students: Robert Vittoe1 and Cansu Sener2

Mentors: Mangilal Agarwal2, Sudhir Shrestha2, and Kody Varahramyan2

Integrated Nanosystems Development Institute (INDI)Indiana University-Purdue University Indianapolis (IUPUI)

1Department of Physics, Purdue School of Science, Indiana University-Purdue University Indianapolis (IUPUI), Indianapolis, IN 462022Department of Electrical and Computer Engineering, Purdue School of Engineering and Technology, Indiana University-Purdue University Indianapolis (IUPUI), Indianapolis, IN 46202

Table of Content

1. Abstract……………………………………………………………………………………………

………………..3

2. Introduction………………………………………………………………………………………

……………...4

3. Materials and

Methods……………………………………………………………………………….……..7

4. Results……………………………………………………………………………………………

…………….….13

5. Discussion and

Conclusions…………………………………………………………………………….17

6. Acknowledgement………………………………………………………………………………

……………19

7. References………………………………………………………………………………………

……………….20

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1. Abstract

Flexible thin film solar cells are considered the new generation in solar

cell technology. Copper Indium Gallium Selenide (CIGS) solar cells have been

highly regarded for cost-competitive energy production. Fabrication of CIGS

solar cells was accomplished by using cost-effective methods, such as

spraying and chemical bath deposition. Atomic Force Microscopy (AFM) was

used to study the surface characteristics of the different of films.

Semiconductor Characterization Instrument and Micromanipulator Probing

Station were used to measure the performance and characteristics of the

solar cells. To determine the thickness of the films, a scratch was made in

each film using the micromanipulator needles and then imaged with the

AFM. The CIGS layer was deposited with spray deposition. The average

thickness of the Cadmium Sulfide (CdS) film deposited with chemical bath

deposition was determined to be between 300 nm and 500 nm. The surface

was observed to be rough which requires further improvement. The initial

solar cell devices measured with the Keithley instrument showed a short

circuit current (Isc) of 3x10-8 A and an open circuit voltage (Voc) of 0.2 V.

Improvement of the solar cell devices is in progress and the results will be

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presented in the future. CIGS solar cells have the potentials to be cost

efficient alternative to silicon-based solar cells. The ability to fabricate the

CIGS solar cells on a flexible substrate will also broaden the range of

practical applications possible for solar cell devices.

2. Introduction

The purpose of this study is to work toward creating a viable renewable

and sustainable energy source. Flexible thin film solar cells are considered

the new generation in solar cell technology. Copper Indium Gallium Selenide

(CIGS) have the potential to reduce solar cell fabrication costs per kilowatt

thereby allowing solar cells to compete with current power production

technology. A 2005 NREL study suggests, “High efficiency CIGS solar cells

can be fabricated up to a band gap of about 1.2 eV. The advantage of using

a higher band gap is the higher open-circuit voltage and a lower temperature

coefficient.” The paper also states they “achieved Voc of 650-660 mV, current

density of 32-33 mA cm-2, fill factor of 77-78% and the best conversion

efficiency of 16.5%.” (K. Ramanathan, 2005) There have been significant

improvements in CIGS solar cell efficiencies over the last few years. A 2011

National Renewable Energy Laboratory (NREL) study states “CuIn1-xGaxSe2

(CIGS)-based solar cells have achieved the highest energy conversion

efficiency (>20%) among photovoltaic (PV) thin-film materials. Those solar

cells are typically made from CIGS alloys with low Ga content (x~0.3)

resulting in absorber energy band gap values ~1.1-1.2 eV.” (Miguel

Contreras, 2011) Also some researches indicates how to attain the maximum

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solar cell performance “Optimum composition can be found to have an

atomic ratio of 0.88<Cu/(In+Ga)<0.95 and x~0.3. Because composition has

a direct effect on the optical (and other properties) of the absorber we

review data for our best solar cell performers in terms of their bandgap Eg

and other device parameters.” (Contreras, 2005)

Like traditional silicon solar cells, CIGS solar cells produce current through

a p-n junction. The p-type layer has a positive charge due to “holes” in its

structure where electrons are “missing” while the n-type layer has a

negative charge due to an excess of electrons. In a CIGS solar cell the CIGS

material is the p-type layer. The n-type layer is composed of Cadmium

Sulfide (CdS) covered by layers of Transparent Conducting Oxides (TCO) like

Intrinsic Zinc Oxide (i-ZnO) and Aluminum doped Zinc Oxide (ZnO:Al). A 2009

NREL paper states “CIGS, as the absorber and the depleted semiconductor,

is the primary layer in determining junction properties and device

performance. Its formation tends to be the chief challenge facing

manufacturers, as the highest performing CIGS requires the difficult

combination of elevated temperatures, extended deposition times, controlled

composition and reaction of four elements, Na doping, and large grain size.”

(I. Repins, 2009) Also according to a 2000 paper from The Japan Society of

Applied Physics “It was found that the improvement of cell performance was

achieved by Zn doping after CIGS formation. Furthermore, it was concluded

from the JEQIC, SlMS and QE measurements that Zn was doped on the CIGS

layer by Zn evaporation and that the conversion from p-type to n-type

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occurred at the surface. The conversion efficiency of 1 1.5% has been

achieved using the Zn-doped CIGS layer without a buffer layer and by the

formation of the pn homojunction in the CIGS absorber.” (Takeshi SUGIYAMA,

2000)

Consistency in the performance from one CIGS solar cell device to the

next can be an issue. The 2009 NREL paper also states “properties of the

CIGS that may change from one film to the next and are expected to affect

device performance are band gap (Eg), lifetime (τ), carrier density (p),

mobility (μ), and front and rear surface recombination velocities (SF and SR).

Profiles (gradients) in these properties may occur through the film.” (I.

Repins, 2009) Figure 2.1 shows the band gap diagram of a typical CIGS

device.

Figure 2.1 Band gap diagram of a typical CIGS solar cell device.

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Silicon solar cell devices are heavy, rigid and brittle which limits the range

of locations and applications for which they can be used. “Non-crystalline

form of silicon is applied as a thin film to various surfaces. Amorphous silicon

cells have been commercially available since the 1970s, and they are

relatively inefficient at converting sunlight to electricity (reaching a

maximum of about 12 percent). However, because they are a thousand

times thinner than c-Si cells, they use less silicon and are much cheaper to

produce. They can also be made more efficient by stacking with other thin-

film semiconductors.” (Dustin Mulvaney, 2009) The use of CIGS nanoparticles

allows for construction of thinner device with a much greater surface area

than with bulk materials. The increased surface area increases the number of

photons that can be absorbed by the material and increase the amount of

current produced by the solar cell device. This also reduces the weight of the

solar cell device. Along with this significant reduction in weight, fabrication of

CIGS solar cells on flexible substrates will greatly expand both the range of

application for solar cell devices and increase the number locations where

they can be installed.

3. Materials and Methods

Fabrication of CIGS solar cells was accomplished by using a number of

cost-effective methods, such as non-vacuum spray deposition, vacuum

thermal deposition, sputtering, and chemical bath deposition.

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As it is shown in Figure 3.1 our CIGS solar cell devices consist of

multiple layers. A glass substrate is used as the base of the device. On a few

of the devices Aluminum was deposited to the substrate using the Vacuum

Thermal deposition device at the Indiana University, Bloomington campus.

This device uses an electric current to heat and vaporize the desired material

inside a vacuum chamber. The material is then deposited on the substrate.

The majority of the CIGS solar cell substrates were shipped to an outside

vendor to sputter a Molydbenum layer onto the substrate. Sputtering is a

method of deposition that uses high energy particles, usually electrons, to

overcome the surface binding energy and remove atoms from a solid

material. The atoms are then deposited on the target object. The layer of

Molybdenum acts as the anode. A layer of CIGS is added on top of the Mo

using non-vacuum spray deposition of the CIGS nanoparticles suspended in

an ethanol solution. CIGS is the p-type layer of the solar cell device. This is

the layer of the device with the “holes”.

Figure 3.1 Structure of a typical CIGS solar cell

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Next the n-type layer of Cadmium Sulfide (CdS) is applied using

chemical bath deposition. This is the layer of the device with the surplus of

electrons. Chemical Bath deposition is performed in a solution of water,

Ammonium Hydroxide and Cadmium Sulfate. These two layers form the

desired p-n junction.

The i-ZnO and n-ZnO TCO layers are deposited next using the same

non-vacuum spray deposition method as was used to deposit the CIGS layer.

A paper presented at Chang Gung University, Taiwan stated that “The highly

efficient solar cell devices were normally using a thin buffer layer of CdS

deposited onto a Cu(In,Ga)Se2 absorber layer with a highly resistive i-ZnO

Figure 3.2 Spray deposition CIGS sample under the hood

Figure 3.3 In the process of Chemical Bath Deposition

Heater

CIGS layered samplesSolution 1: CH4N2S +H20

Solution 2: CdS +NH40H +H2O

N2

Spray device

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layer introduced between the CdS and ZnO:Al to prevent leakage current.”

(Ming-Yang Hsieh) The intrinsic Zinc Oxide (i-ZnO) is first TCO layer to be

applied. The i-ZnO is suspended in an Ethanol solution to be sprayed onto

the device. The final TCO layer of the device is the layer of Aluminum doped

Zinc Oxide (ZnO:Al) which is also applied with spray deposition while

suspended in an Ethanol solution. Finally, small amount of Silver or

Aluminum was then place on the top of the device to act as the cathode.

Analyses of CIGS Solar Cells were performed using Atomic Force

Microscopy (AFM), Keithley 4200 Semiconductor Characterization, and

Micromanipulator Probing Station. AFM was used to study the surface

characteristics of the different of films. The AFM operates in one of three

modes; contact, tapping or non-contact. The tapping mode gave the clearest

images of our samples. To determine the thickness of the films a scratch was

made in each film using the micromanipulator needles. The sample was then

imaged with the AFM device. The image of the scratches allowed us to see

the difference between the RMS height of the sample and the depth of the

scratch as shown for the CdS sample in Figures 4.2 and 4.3.

The Keithley 4200 Semiconductor Characterization Instrument and

Micromanipulator Probing Station were used to measure the performance

and I/V characteristics of the CIGS solar cells devices.

Micromanipulator Probing Needle

Solar Cell Device

Microscope lens

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The short circuit current (Isc), which is measured in Amperes (A), and the

open circuit voltage (Voc) which is measured in Volts (V), are used to

determine the efficiency and power of the solar cell device. The short circuit

current is the measured current when the voltage is zero. The open circuit

voltage is the measured voltage when the current is zero. The website

PVEducation.org provides a concise explanation of how these two factors are

determined. “The efficiency of a solar cell is determined as the fraction of

incident power which is converted to electricity and is defined as:

where Voc is the open-circuit voltage;

where Isc is the short-circuit current; and

where FF is the fill factor

where η is the efficiency.

In a 10 x 10 cm2 cell the input power is 100 mW/cm2 x 100 cm2 = 10 W.”

(Christiana Honsberg) The website also describes how the power is

determined by using Isc and Voc along with the Fill Factor (FF) “The short-

circuit current and the open-circuit voltage are the maximum current and

voltage respectively from a solar cell. However, at both of these operating

Figure 3.4 Timed measurements with silver electrodes on Probing Station

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points, the power from the solar cell is zero. The "fill factor", more commonly

known by its abbreviation "FF", is a parameter which, in conjunction with Voc

and Isc, determines the maximum power from a solar cell. The FF is defined

as the ratio of the maximum power from the solar cell to the product of V oc

and Isc. Graphically, the FF is a measure of the "squareness" of the solar cell

and is also the area of the largest rectangle which will fit in the IV curve.

”(Christiana Honsberg) Diagram 3.5 depicts a FF graph.

4. Results

Multiple CIGS solar cell devices were successfully fabricated in our lab and

tested by our team using the described methods.

The CdS layer was successfully deposited and imaged (Figure 4.1)

showing thicknesses ranging from approximately 300 nm (Figure 4.2) to over

500 nm (Figure 4.3). The CdS layer was also observed to be somewhat

rougher than was desired. The paper from Taiwan makes the claim that 40

nm is the optimum thickness for the CdS buffer layer. “The thickness of CdS

buffer layer at 40 nm has been used as the base parameter in this study to

Figure 3.5 A graph of the Fill Factor from PVEducation.org

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be found optimum from the results. The values are decreasing with the

increase of CdS thickness after 40nm. The reason cause that is Jsc

decreasing result in efficiency increasing.” (Ming-Yang Hsieh)

Figure 4.1 Used Atomic Force Microscopy to image the scratch in a layer of CdS at 50 µm

Figure 4.2 AFM result of a CdS sample layer

0 2 4 6 8 10 12 14 16 18 20-900

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Figure 4.4 show the surface of a CIGS sample imaged at 4 μm and

figure 4.5 shows a scratch in the same sample imaged at 50 μm.

Figure 4.4 AFM images the surface of a CIGS layer at 4 µm

0 2 4 6 8 10 12 14 16 18 20-900

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Figure 4.3 An Excel graph for the AFM result of a CdS sample layer

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Figure 4.5 AFM images the surface of a CIGS layer at 50 µm

The ZnO layer was also imaged with the AFM device and shown to have a

thickness just over 1 µm which is also a little thicker than what is believed to

be the optimum thickness which is about 500 nm for the combined TCO

layers. These results are shown in Figure 4.6.

Figure 4.6 An Excel graph for the AFM results of a ZnO sample layer.

0 0.5 1 1.5 2 2.5 3 3.5 4-1500

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July 12 #2

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Initial results showed the solar cell device measurements of short

circuit current (Isc) to be 3x10-8 A and open circuit voltage (Voc) to be 0.2 V.

These results along with an example of the curve seen from a solar cell

device on an I/V graph are shown in Figure 4.7.

5. Discussion and Conclusions

The roughness and the excess thickness of the CdS layer both require

further improvement. Reducing the time the CIGS sample is left in the

current formulation of the chemical bath to deposit the CdS should be a

relatively easy way to improve the issue with the thickness. By reducing the

chemical bath time from the current two minutes to something along the

order of 20 or 30 seconds the thickness should decrease from the 300 nm to

500 nm range to the desired, optimum range of 30 nm to 50 nm. This is

assuming that the CdS is deposited at something close to a linear rate.

Another possibility is to increase the Ammonium Hydroxide concentration to

alter the pH and inhibit the rate of the CdS deposition as shown in an article

from the Brazilian Journal of Physics. “Experiments 5 and 6 give interesting

results for more basic values of pH. When a high ammonium concentration

Figure 4.7 I/V Characteristics of CIGS device

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was used in experiment 5, an inhibition of the deposition was found since the

first minutes, just after a thin film was deposited with an unchanged

morphology and similar band gap. However, for experiment 6, in which a low

ammonium concentration was used, the deposition of the film was totally

inhibited. It is important to say that in these two experiments the color of the

solution changed to yellow immediately after the addition of Thiourea,

although the deposition was inhibited since the first minutes of reaction.”

(Oliva, 2006)

Other methods to optimize the solar cell devices bandgap are currently

being researched. Some of the methods under consideration include;

Selenization and/or Sulferization of the CIGS layer. According to an article

from the Institute of Energy Conversion at the University of Delaware

Selenization was able to optimize the bandgap from the top to the bottom in

the CIGS layer itself, “These devices were made from the precursors with the

90 minute selenization treatments. The shifts of the long wavelength edge of

the EQE curve toward shorter wavelengths imply wider bandgap for the

devices made from the ED Cu-Se/Ga/In and ED Cu2-xSe/Ga/In precursors.

The wider bandgaps are consistent with the increased Ga incorporation near

the front of the CIGS for the films made from the two precursors.” (Rui

Kamada) From the preliminary literature research on the topic, it appears

that using Selenization and Suferization in tandem is preferable to just using

one technique or the other.

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Another method for optimizing the bandgap of the solar cell device being

considered is to switch from a CdS n-type layer (2.4 eV) to a ZnS n-type layer

(3.7 eV). A third method for optimizing the band gap of the solar cell device

is to switch the TCO layers from the i-ZnO and ZnO:Al to a TCO layer of

Indium Tin Oxide (ITO). ZnO has a band gap of 3.3 eV while ITO has a larger

band gap of 4.0 eV. The ITO combined with the ZnS would seem to have the

potential to substantially improve the performance of the CIGS solar cell

devices. The wider band gaps in both the n-type layer and the transparent

oxide layer would allow more photons at a wider range of energies to pass

through those layers of the solar cell device and strike the p-type layer. This

will in turn give the solar cell device the ability to produce a larger current.

The results of these methods and other possible modifications to the

CIGS solar cell devices are to be presented at a future date.

6. Acknowledgement

This study was sponsored by the Indiana University‐Purdue University

Indianapolis (IUPUI) Multidisciplinary Undergraduate Research Institute

(MURI) and supported by Integrated Nanosystems Development Institute

(INDI). Thank you to Dr. Mangilal Agarwal, Dr. Sudhir Shrestha, and Dr. Kody

Varahramyan for all their guidance and support on this project. And special

thanks to Parvin Ghane for all her guidance throughout this research project,

Dr. Ricardo Decca for the use of the Atomic Force Microscopy device and to

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Dan Minner for his time spent training all of us on the usage of the Atomic

Force Microscopy device.

7. References

Christiana Honsberg, a. S. B. Fill Factor, from http://www.pveducation.org/pvcdrom/solar-cell-operation/fill-factor

Contreras, M. A. R., K. AbuShama, J. Hasoon, F. Young, D. L. Egaas, B. Noufi, R. (2005). SHORT COMMUNICATION: ACCELERATED PUBLICATION: Diode characteristics in state-of-the-art ZnO/CdS/Cu(In1?xGax)Se2 solar cells. Progress in Photovoltaics: Research and Applications, 13(3), 209-216. doi: 10.1002/pip.626

Dustin Mulvaney, V. B., Monica Cendejas, Sheila Davis, Lauren Ornelas, Simon Kim, Serena Mau, William Rowan, Esperanza Sanz, Peter Satre, Ananth Sridhar, Dean Young. (2009). Toward a Just and Sustainable Solar Energy Industry: A Silicon Valley Toxics Coalition White Paper.

I. Repins, S. G., J. Duenow, T.J. Coutts, W. Metzger, and M.A. Contreras. (2009). Required Materials Properties for High-Efficiency CIGS Modules. To be presented at the Society of Photographic Instrumentation Engineers (SPIE) 2009 Solar Energy + Technology Conference San Diego, California

K. Ramanathan, J. K., and R. Noufi. (2005). Properties of High-Efficiency CIGS Thin-Film Solar Cells. Prepared for the 31st IEEE Photovoltaics Specialists Conference and Exhibition Lake Buena Vista, Florida.

Miguel Contreras, L. M., Brian Egaas, Jian Li, Manuel Romero, and Rommel Noufi. (2011). Improved Energy Conversion Efficiency in Wide-Bandgap Cu(In,Ga)Se2 Solar Cells. Presented at the 37th IEEE Photovoltaic Specialists Conference (PVSC 37) Seattle, Washington.

Ming-Yang Hsieh, S.-Y. K., Fang-I Lai, Ming-Hsuan Kao, Pei-Hsuan Huang, Hsun Wen Wang, Min-An Tsai, Hao-Chung Kuo. Optimization of CdS buffer layer on the performance of copper indium gallium selenide solar cells. http://research.cgu.edu.tw/ezfiles/14/1014/img/651/1138P.pdf.

Oliva, S. H. C. M. R. R. P. J. L. P. W. C. A. I. (2006). Analysis of the chemical bath and its effect on the physical properties of CdS/ITO thin films. Braz. J. Phys., vol.36(no.3b). doi: 10.1590/S0103-97332006000600068

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Rui Kamada, W. N. S., and Robert W. Birkmire. Cu(In,Ga)Se2 FILM FORMATION FROM SELENIZATION OF MIXED METAL/METAL-SELENIDE PRECURSORS. Institute of Energy Conversion.

Takeshi SUGIYAMA, S. C., Akira YAMADA, Makoto KONAGAI, Yuriy KIJDRI AVTSEV , Antonio GODINES, Antonio VILLEGAS and Rene ASOMOZA. (2000). Formation of pn Homojunction in Cu(InGa)Se2 Thin Film Solar Cells by Zn Doping. Jpn. J. Appl. Phys., Vol. 39(Part 1, No. 8,), pp. 48164819.

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