I

THE PENNSYLVANIA STATE UNIVERSITY

DEPARTMENT OF ENGINEERING SCIENCE AND MECHANICS

DETECTING DEFECTS IN CdTe SOLAR CELLS

USING EPR SPECTROSCOPY

LIAM A. YOUNG

Spring 2013

A thesis

submitted in partial fulfillment

of the requirements

for a baccalaureate degree

in Engineering Science

Reviewed and approved* by the following:

Patrick M. Lenahan

Distinguished Professor of Engineering Science

Thesis Supervisor

Bernhard R. Tittmann

Schell Professor of Engineering Science

Honors Adviser

Judith A. Todd

P. B. Breneman Deparment Head Chair

Professor, Department of Engineering Science and

Mechanics

* Signatures are on file in the Engineering Science and Mechanics office.

II

We approve the thesis of Liam A. Young:

Date of Signature

Patrick M. Lenahan

Distinguished Professor of Engineering Science

Thesis Supervisor

Bernhard R. Tittmann

Schell Professor of Engineering Science

Honors Adviser

Judith A. Todd

P. B. Breneman Department Head Chair

Professor, Department of Engineering Science

and Mechanics

Student ID# 911695783

III

Abstract

Cadmium telluride is a II-VI direct bandgap semiconductor with properties

conducive to becoming a photovoltaic cell. Recently, these compounds have been

researched as possible replacements for silicon based solar cells. With current

efficiencies much lower than anticipated, we assume that a defect in the compound is

causing electron mobility to decrease. In this study we will examine the CdTe samples

with Electron Paramagnetic Resonance spectroscopy at 300, 80, and 5 K.

IV

Table of Contents

List of Figures………………………………............................................................. V

Literature Review…………………………………………………………………... 1

Photovoltaics……………………………………………………………….... 1

EPR Spectroscopy…………………………………………………………... 4

Lab Configuration…………………………………………………………………... 7

Results………………………………………………………………………………... 11

Conclusions………………………………………………………………………….. 16

Future Work…………………………………………………………………………. 17

Appendix I…………………………………………………………………………… 19

Acknowledgments…………………………………………………………………… 20

Work Cited…………………………………………………………………………... 21

V

List of Figures

Figure 1: Shockley-Quisser Curve…………………………………………………..... 1

Figure 2: Direct and Indirect Band Gaps……………………………………………... 2

Figure 3: Superstrate CdTe Slar Cell………………………………………………..... 2

Figure 4: As-Deposited CdTe Device………………………………………………… 3

Figure 5: Post Pyrrole Treatment CdTe Device……………………………………..... 3

Figure 6: Magnetic Field Vs Energy………………………………………………….. 4

Figure 7: Simple EPR Signal………………………………………………………….. 5

Figure 8: Basic EPR Spectrometer……………………………………………………. 7

Figure 9: Insulated Dewars……………………………………………………………. 8

Figure 10: Liquid Helium System…………………………………………………...... 9

Figure 11: Cryostat Schematic………………………………………………………… 10

Figure 12: First CdTe Signal. 50K…………………………………………………..... 12

Figure 13: Spectral Comparison. 5K………………………………………………….. 13

Figure 14: CdTe + Cu. Calculating g………………………………………………….. 14

1

Literature Review

Photovoltaics

Photovoltaic (PV) cells are slowly becoming a large percentage of our nation’s

power supply. With the increasing needs of electricity, it is a high priority to find cleaner

sources of energy without creating too much cost. For solar cells to be more cost

effective, we must increase their efficiencies and lower their production expenses.

A solar panel is essentially little more than a p-n junction that absorbs light. Light

of a specific wavelength enters the cell and excites an electron. This electron is moved to

the conduction band where it escapes into a biased n-doped region. These escaped

electrons then become a current [18].

Photon energy is of the utmost

importance in this case. If the photon does not

have the energy to lift the electron into the

conduction band, then it is reflected and will

not create any current. If we compare the

energy of the photons coming from the sun, we

can see that there is an optimum energy level

that would allow for the greatest amount of photons

to be absorbed [5]. Band gaps ranging from 1.3 to 1.5 eV have the highest theoretical

potential for solar cell efficiency. These energy levels have been determined by the

Shockley-Queisser curve.

The energy required to allow the electron to escape is called the band gap. There

are two types of band gaps; direct and indirect. The direct band gap allows electrons to

Figure 1: Shockley-Quisser Curve

2

shift from the valance to conduction band smoothly by absorbing a photon. The indirect

band gap, however, requires an increase in energy and a change in momentum. This

makes indirect band gap semiconductors less conducive to photovoltaic research. CdTe

has a band gap of 1.45 eV which makes it a wonderful candidate for replacing silicon

based PV.

For years, Solar Cells have been built from

single crystal Si which is bulky and expensive.

Because the efficiency of Si PV is based mainly on

the quality of the crystal structure, it is difficult to

increase it without prices skyrocketing. With thin

film CdTe PV, it is possible to have smaller, better,

and more cost effective solar cells.

CdTe solar cells have several different

configurations. Figure 3 contains the most

Figure 2: Direct and Indirect Band Gaps

Figure 3: Superstrate CdTe Slar Cell

3

common form in which the cell is constructed on top of a glass substrate and then flipped

for actual function [2]. The fist layer is built from a transparent conducting oxide (TCO)

which allows light through to the junction and acts as the front contact. The CdS and

CdTe act as the n and p doped junction and are loaded with electrons and holes

respectively. Pure CdTe single crystals have a carrier concentration of about

[8]. These middle sections are built with chemical bath deposition (CBD)

which has creates efficiencies upwards of 16% [9]. The final layer is the back contact

that allows for current to flow.

The cell also goes through a serious of

treatments that is meant to reduce the better

known and commonly occurring defects.

Introducing the sample to CdCl₂ has shown to

reduce impurities along the outer surface as

well as remove many Cd vacancies. If left for

too long however, the Cl will begin to etch the

material [3]. In other cases the sample is

placed in a Telluride rich environment for

hours in order to get rid of Te vacancies [8]. In

later phases of the building process, it has been

observed that Cu ions can contaminate the

sample. To counteract this, the samples go

through a Pyrrole and light treatment which

also bolsters crystal growth [3].

Figure 4: As-Deposited CdTe Device

Figure 5: Post Pyrrole Treatment CdTe

Device

4

EPR Spectroscopy

Electron Paramagnetic Resonance (EPR) Spectroscopy is a sensitive and accurate

tool for viewing structural defects in semiconductors and other materials. This technique

functions by creating a large magnetic field through the sample. This overpowering field

lines up electrons which will spin either with the field (low energy) or against the field

(high energy). By creating this new energy difference, we can examine patterns through

the electrons’ behavior. While inducing the magnetic field, we exposed the sample to

high energy microwaves. The photons will react with electrons which will then flip their

spins. By measuring the number of flips from low to high energy levels, we get what is

called an absorption spectrum.

In each EPR experiment, the field

strength is slowly increased. This

proportionally increases the energy

required to flip the spinning electrons.

Figure 4 explains that the total energy

required to flip the spins is the difference

between the two energy levels. The

phenomenon known as magnetic resonance happens whenever the photons the sample is

exposed to and the field induced energy gap are equal. The equation used to determine

resonance conditions is as follows:

By understanding material properties, we can tell exactly what a defect is by its

spectrum. Many different kinds of defects exist with their own interactions and

Figure 6: Magnetic Field Vs Energy

5

idiosyncrasies. Due to EPR’s relatively extensive use, hundreds of these unique spectra

have already been analyzed and cataloged. The trouble lies mostly in being able to

identify the acquired spectrum and matching it to known samples.

Each element has naturally occurring

spins that appear under EPR. The number of

absorption lines seen in a spectrum

corresponds to these spins. The simplest spin

being 0, this contains only 1 line. Figure 5

shows the absorption region of a spin 0

spectrum. The first derivative is the most

commonly displayed form for analysis and

allows for other helpful techniques such as amplification modifications to be used.

For this study, we will take advantage of several defining equations. The signal

strength of each spectrum is actually inversely proportional to the temperature of the

sample. We therefore assume that by lowering the temperature, it will become easier to

view defining features of the spectrum. The most basic form of this equation is equal to

M₀ which is the Magnetization.

After decreasing the energy of the system, we must begin to worry about the

Fermi Dirac distributions. At lower temperatures, it will be much more likely to find

electrons at a lower energy state. With more electrons at the lower state, we are able to

excite a greater number into the conduction band and therefore increase the signal

strength.

Figure 7: Simple EPR Signal

6

The tests will be run at 300 K, 80 K, 50 K, and 5 K. By decreasing the

temperature from 300 K to 5 K, we can theoretically increase the signal strength by a

factor of 60. 300K is the approximate room temperature of the lab in which the tests are

run. 80 K is the approximate temperature of the sample after being inserted into a liquid

nitrogen Dewar. 50 K and 5 K will be achieved through the use of a Janis Cryostat

system.

The spectrometer consists mainly of two large electromagnets, a microwave

bridge, and a microwave cavity. The magnets control the large field that forces the

electrons into order. These experiments are run at a magnetic field strength of 3400

Gauss (340 mT). The microwave bridge contains a Gunn oscillator which emits

microwaves between 9 and 10 GHz. These travel down to the cavity which is where the

sample is located. The cavity contains a standing wave and must be tuned by the

operator before every use. Improper tuning can lead to damage as seen during later

experimentation. Any photons not absorbed by the sample are reflected back and

counted.

7

Lab Configuration

For the first set of room temperature experiments, we used a Bruker EXM

Spectrometer with a TE 102 single cavity. This spectrometer used a Bruker designed

program for all controls and detection. This was the simplest setup of the study and

produced no results. At room temperature, the signal strength was not strong enough and

the collected spectra showed only noise and cavity contaminations.

During the second round of experiments, we tried reducing the sample

temperature with liquid nitrogen. We also moved to different spectrometer with a control

console, not a computer program, and a TE 104 double cavity. To accomplish the lower

temperatures we used a special insulated glass Dewar. The Dewar has a long hollow

protrusion that the sample is lowered into. This protrusion then goes into the cavity and

allows for testing. We added a stopper around the test tube in order to keep liquid

nitrogen from entering into the cavity’s “sweet spot.” The quickly evaporating liquid

nitrogen also caused small vibrations that would have ruined the spectrum if we had not

Figure 8: Basic EPR Spectrometer

8

stabilized the sample with the stopper. We took the following steps to ensure that the

sample was and stayed cold for the entire experiment:

Fill the Dewar with liquid nitrogen and wait for the boiling to slow

Fill a cup with liquid nitrogen and place the sample inside to chill for 30 seconds

Empty out the Dewar making sure all the nitrogen has evaporated

Place the sample down into the Dewar and secure the stopper

Refill the Dewar with liquid nitrogen around the sample

The nitrogen with evaporate off as the

experiment is running and the size of the Dewar will

determine the number of runs possible. The small glass

Dewar we used allowed for 45 minutes of uninterrupted

run time. On several occasions we attempted to refill the

Dewar mid-experiment. We hoped that by pausing the

machines and pouring gently into the Dewar, we could

run the experiment for a longer period. This turned out to be incorrect however, because

the sample would move every time and ruin the already fragile tuning

Tuning at this temperature became very difficult and it soon became evident that

the cavity was being “loaded.” This means that there is a magnetic field within the cavity

causing a disruption to the already existing fields. Loading takes place when the sample

is conductive or if there is water in the cavity. In this case, the CdTe has become

conductive. The movement of electrons in the sample causes the internal disrupting field.

Figure 9: Insulated Dewars

9

In an attempt to properly tune the cavity, we turned the attenuation to 15 db. The

unstable loaded tuning at such a high power strained the microwave bridge. This

eventually became too much for the bridge and it broke down. After realizing our

mistake, we replaced the bridge and moved to liquid helium temperatures.

For liquid helium, the set up was

slightly more involved. We used a Janis

turbomolecular pump to vacuum insulate

the sample. A low temp apparatus is

mounted on the spectrometer and attaches

to the cavity. This system holds the

sample in place and forces the helium

around it. We were originally concerned

about the seals which hold the helium in and keep it from escaping into the cavity but the

seals held and caused no problems.

The helium tank is placed in front of the spectrometer and feeds into the system

via a vacuum insulated tube. The temperature of the sample is controlled by a valve on

the tube that regulates the flow of helium. Pressure in the tank must be monitored and

kept just below 3 psi. This allows for steady flow and testing temperatures.

We ran the samples at 50 K first as a test to ensure the apparatus was functioning

properly. Once at 50 K, the cavity tuned perfectly indicating that the loading problem

had been solved. The signal achieved was the first meaningful spectrum of the study.

All samples were run at 50 K in order to replicate the signal and make certain we would

have no more issues with loading.

Figure 10: Liquid Helium System

10

By increasing

the flow rate of helium

greatly, we achieved

temperatures

fluctuating about 5 K.

The signal size and

clarity of the spectra

increased in

accordance with the

Bloche equations. The

only issue encountered during this stage of the study was warming the samples back to

room temperature. Cold air inside the test tube was condensing at 5 K and when the

sample reached the boiling point of nitrogen, it was expand rapidly. This posed a

possible threat at it could throw powdered cadmium throughout the lab. We solved the

issue by covering the top of the test tubes with small balloons. A small incision was then

made in each balloon to allow for the slow escape of gasses instead of the explosive

alternative.

Figure 11: Cryostat Schematic

11

Results

The first round of texting took place at room temperature. Due to the relative size

of the defect, however, the spectra collected were meaningless. The background signal

was so large compared to the defect that even after several hundred runs and hours of

averaging there was no discernible data. Spectra for each sample varied due to noise

which caused random fluctuations in the signal.

Liquid nitrogen temperatures caused several unexpected problems. The reduction

in temperature should have produced a proportional increase in signal strength, but it

made the sample temporarily conductive. At 80 K, all the samples showed signs of

cavity loading caused by magnetic fields within the cavity. This in turn made tuning the

sample properly impossible. In our attempts to tune the cavity, the microwave bridge

was strained and began to malfunction. The AFC Lock-in had broken and the bridge was

unable to attain a signal even for weak pitch. Any and all data collected at this

temperature is meaningless due to the lack of control and damage done to the system.

Using liquid helium would increase the signal strength further by decreasing

temperature. The samples were chilled to 50 K as a test. It became immediately apparent

that the conductivity issue had been solved. The samples were able to tune perfectly

within seconds. The first usable spectrum was collected from the CdTe sample at 50 K.

12

We can see right away that the spectrum is dominated by a strong six line signal.

We interpret the six lines an element with a spin 5/2. This means that each line

corresponds to a directionality of spin for an electron on the defect. The other samples

showed similar signals with varying strengths. These variations are likely caused by

volume differences of each sample instead of a reduction in spins. By reducing the

temperature further, we can increase the defect signal again which will also raise the

single-to-noise ratio.

-0.80

-0.60

-0.40

-0.20

0.00

0.20

0.40

0.60

0.80

1.00

CdTe 50K

Figure 12: First CdTe Signal. 50K

13

The spectrums collected at 5 K share the six line signal found at 50 K. By using a

table of spin constants, we can narrow down the element responsible for this defect. Due

to the strength of the six lines, we assume that the element is approximately 100% spin

5/2, meaning all of the natural isotopes match that spin. There are only four elements that

correspond to this spin ratio; aluminum, manganese, iodine, and praseodymium

We can rule out praseodymium immediately being that it is a lanthanide. To

narrow the possibilities down further, we examine the g values. The free electron g is

2.0023 and large atoms have larger g values. In this case, we have a center field of 3400

Gauss and the middle of the six lines is very well aligned with the center field. Al and

2650.00 2850.00 3050.00 3250.00 3450.00 3650.00 3850.00 4050.00

EPR

Am

plit

ud

e (

AU

)

Magnetic Field (Gauss)

CdTe

CdTe+CdCl2

CdTe+CdCl2

+Cu

CdTe+Cu

Figure 13: Spectral Comparison. 5K

14

Mn are small enough to leave the spectrum close to the central field. We can calculate

the experimental g from the following graph.

To find g, we use the following equation that employs the range of the signal. By

taking B1 and B2, we calculate the center of the signal. We then find the percentage of

change from the original center field.

The g calculated comes out to be 2.011. According to another paper, Mn in CdTe

has a g value of 2.005. This difference however does not immediately exclude Mn as a

possibility. There is very little information on Al in CdTe and it is therefore hard to use g

to concretely determine if the impurity is Al or Mn. Mn however, is still much more

-0.20

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

0.20

0.25

2,600.00 2,800.00 3,000.00 3,200.00 3,400.00 3,600.00 3,800.00 4,000.00 4,200.00

CdTe + Cu

B1

B2

Figure 14: CdTe + Cu. Calculating g

15

likely due to its use in other research areas it being seen in CdTe as an impurity before.

Al isn’t used in any treatments nor does it come in contact with the CdTe at any point

during construction of the Solar Cell. Because the CdTe of these samples was created

sterilely for this experiment, we can assume that it should not come in contact with any

Al. We deduce from this that Mn is the most likely impurity

16

Conclusions

After three round of experimentation, two of which were wildly unsuccessful, we

can see that the defect creates a six line signal with a g close to that of the free electron.

The functioning experiment being helium cooled samples at 50 K and below. At this

temperature we have increased the signal size enough to be detected and gotten rid of the

conductivity that caused loading.

By examining contemporary articles, we have determined that an interstitial

defect of Mn is the most likely culprit. There are several papers on Mn as an impurity of

CdTe and almost no information about Al. Our sponsor, GE, was very surprised by our

findings and was unsure of the defect’s origin. Other companies we work with, however,

have considered Mn as a possibility which only further supports our initial conclusion.

17

Future Work

To continue the work done during this study, we will examine the samples under

different conditions and using different techniques. To truly gauge the defect’s

interactions, we need to study a complete solar cell. This would allow for several tests

that could explain other aspects of the defect.

Cavity loading was an immense issue for this study and caused damage to one of

the systems. We will test the samples starting at room temperature and slowly lowering

them to 80 K. At some point the sample will become conductive and no longer function

in its intended manner. This will determine if CdTe based solar cells can be used in some

of the colder parts of the world.

Between 80 and 50 K, the sample became nonconductive and the charge carriers

would no longer move. Although this has no current use to us, this temperature and

phenomenon may apply to similar devices and may therefore be useful in the future. For

both the cavity loading and carrier freezeout experiments, we would use the Janis

temperature control system. This would allow for a steady drop in temperature while still

being capable of reaching carrier freezeout.

If the sample is a complete photovoltaic cell then we can test its functionality

while under observation. By applying light to a cell that is already undergoing EPR

spectroscopy, we might see how the hanging bond changes and interacts with other

electrons. We would run the sample at varying temperatures again however it is unlikely

to work at 50 K or below due to carrier freezout.

Under the six line signal is a very sizable amount of background and other

signals. They are much more difficult to see being that there is such a strong other signal.

18

If we could subtract the six lines away and leave everything else, we could see everything

in the sample. We have attempted to create an artificial six line signal from the original

signal but other factors came into play such as hyperfine interactions. These slightly

alter the signal and made the initial subtraction attempts useless. The program EasySpin,

which works through MATLAB, can simulate a much better signal. We should be able

to use EasySpin to build better spectral subtractions.

19

Appendix 1

Tools

Experiment 1:

EXM Burker, EPR Spectrometer

EXM Control and Collection Program

Experiment 2:

EXM Burker, EPR Spectrometer

Labview (data collection)

Control Console

Insulated Glass Dewar

Liquid Nitrogen

Experiment 3:

EXM Burker, EPR Spectrometer

Labview (data collection)

Control Consol

Liquid Helium

Janis Turbomolecular Vacuum Pump

Cryostat System and Mount

Vacuum Insulated Transfer Tube

20

Acknowledgements

I would first and foremost like to thank Dr. Lenahan who provided me with this

opportunity and showed the utmost patience during this project. Michael Pigott was the

best lab partner I could have asked for and I’m well aware that this project would not

have gone as smoothly without him. Corey Cochran, Tom Pomorski, Mike Mutch, Mark

Anders, and Jake Fulman who worked alongside us in the lab and were never too busy to

give a helping hand.

I would also like to thank my brothers who listened to hours of brain storming and

never once complained.

21

Work Cited

[1] Galazka, R. R. and Wojtowicz, T. (2010). CdTe and Related Compounds; Physics,

Defects, Hetero- and Nano-structures, Crystal Growth, Surfaces and Applications

[2] Compaan, A. and Bohn, R. (1992). Thin Film Cadmium Telluride Photovoltaic Cells,

Annual Subcontract Report. 23 July 1990 – 31 October 1991

[3] Koll, D. K., Taha, A. H., Giolando, D. M. (2011). Photochemical “Self-Healing”

Pyrrole Based Treatment of CdS/CdTe Photovoltaics. Solar Energy Materials and

Solar Cells, 95(7).

[4] Gessert, T. A. (2012). Cadmium Telluride Photovoltaic Thin Film: CdTe.

Comprehensive Renewable Energy, 423-438.

[5] Al-Dhafiri, A. M. (1998). Photovoltaic Properties of CdTe – Cu2Te. Renewable

Energy, 14(1-4).

[6] Chin, K. K. (2012). U.S. Cl. 136/260; 438/95; 257/E31.015. Washington, DC: U.S.

[7] Smith, L. C., Bingham, S. J., Davies, J. J., and Wolverson, D. (2005). Electron

Paramagnetic Resonance of Manganese Ions in CdTe Detected by Coherent

Raman Spectroscopy. Department of Physics, University of Bath, U.K.

[8] Emanuelsson, P., Omling, P., Meyer, B. K., Wienecke. M., and Schenk. M. (1993).

Identification of Cadmium Vacancy in CdTe by Electron Paramagnetic

Resonance. Physical Review, 47(13).

[9] Morales-Acevedo, A. (2006). Thin Film CdS/CdTe Solar Cells: Research

Perspectives. Solar Energy, 80(6).

[10] Lane, D. W., Rogers, K. D., Painter, J. D., Wood, D. A., and Ozsan, M. E. (2000).

Structural Dynamics in CdS-CdTe Thin Films. Thin Solid Films, 361-362.

22

[11] Schwartz, R. N., Wang, C., Trivedi, S., Jagannathan, G. V., Davidson, F. M., Boyd,

P. R., and Lee, U. (1997). Spectroscopic and Photorefractive Characteristics of

Cadmium Telluride Crystals Codoped with Vanadium and Manganese. Physical

Review, 55(23).

[12] Stefaniuk, I., Bester, M., Virt, I. S., and Kuzma, M. (2005). EPR Spectra of Cr in

CdTe Crystals. ACTA Physica Polonica A, 108(2).

[13] Pires, R. G., Dickstein, R. M., Titcomb, L. S., and Anderson, R. L. (1990). Carrier

Freezeout in Silicon. Cryogenics, 30(12).

[14] First Solar. (2011). First Solar Sets World Record for CdTe Solar PV Efficiency.

Available: http://investor.firstsolar.com/releasedetail.cfm?ReleaseID=593994.

[15] Eaton, G. R., Eaton, S. S., Barr, D. P., and Weber, R. T. (2010). Quantitative EPR.

Germany: Springer-Verlag/Wien.

[16] Dhere, R. G., Joel, N. D., DeHart, C. M., Li, J. V., Kuciauskas, D., and Gessert, T.

A. (2012). Development of Substrate Structure CdTe Photovoltaic Devices with

Performance Exceeding 10%. NREL. Golden CO, U.S.

[17] Lenahan, P. M. and Cochrane, C. J., (2013). A Brief Introduction to Electron Spin

Resonance. Unpublished Manuscript.

[18] Gessert, T. A. (2010). Junction Evolution During Fabrication of CdS/CdTe Thin-

film PV Solar Cells. NREL. Presented: 9-2010. China New Energy International

Forum and Fair.