solar cells based on cdte thin film and composite of organic and inorganic nano-scale materials

8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials

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Solar Cells Based on CdTe Thin Film

and Composite of Organic and Inorganic Nano-Scale Materials

BY

Hyeson JungB. S., Dongguk University, S. Korea, 1999M.S., Dongguk University, S. Korea, 2001

THESIS

Submitted as partial fulfillment of the requirementsfor the degree of Doctor of Philosophy in Electrical and Computer Engineering

in the Graduate College of theUniversity of Illinois at Chicago, 2010

Chicago, Illinois


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This thesis is dedicated to my loving mother, Okmyung ChOi(Sl^1U), and to the

memory of my father, Goonsoo Jung^Z^r, 1945-2003), whose great love and

everlasting support accompanies me from my childhood to now and beyond.

m


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ACKNOWLEDGMENTS

First of all, I would like to express my gratitude to my advisor Prof. Mitra

Dutta for the continuous support of my Ph. D study and research, for her

knowledge, encouragement, and patience. Without her advice not only for research

but also for life, I could not have completed my Ph. D study.

Beside my advisor, I would like to thank the rest of my thesis committee:

Prof. Michael A. Stroscio, Prof. Michael Trenary, Prof. Vitali Metlushko, and Prof.

Su Gupta, for their informative and valuable comments.

I thank my fellow members of Nano Research Laboratory: Ayan Kar, Yang

Li, Jinyong Yang, Clare Sun, Takayuki Yamanaka, Milana Vasudev, Jun Qian,Sushmita Biswas, Sicheng Liao, Donna Wu, Rade Kuljic, Banani Sen, Prof.

Michael A. Stroscio, and Prof. Mitra Dutta for their support and friendship. My

special thanks also go to Dr. Seyong An and Mr. Bob Lajos at Nano Core Facility

(UIC), Dr. Ke-Bin Low and Dr. Alan Nicholls at Research Resource Center (UIC),

and Dr. David J. Gosztola and Gary P. Wiederrecht at Center for Nano

Materials(Argonne National Laboratory) for their insightful advice for my research,

iv


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ACKNOWLEDGMENTS (continued)

and to Prof. P. T. Snee in the Department of Chemistry for providing the CdSe

NQDs and allowing me to use a characterizing system.

My sincere thanks also goes to Dr. Sivaligam Sivananthan and Dr. Sung-

shik Yoo, who are alumni of the UIC, for encouraging me to start Ph. D program,

and for giving me career guidance.

Last but not the least, I would like to thank my family: my mother Okmyung

Choi, little sister Hehsoon Jung, brother-in-law Yongsoo Hong, and little brother

Yongkyu Jung, for supporting me spiritually throughout my life.

I offer my regards and blessings to all of those who supported me in any

respect during the completion of my study.

?

HS


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TABLE OF CONTENTS

CHAPTER PAGE

1. INTRODUCTION 1

1.1. Motivation 11.2. Objectives 4

2. SOLAR CELL FUNDAMENTALS 5

2.1. History of photovoltaic 52.2. Type of solar cells 72.3. Physics of photovoltaic cells 10

3. EXPERIMENTS 16

3.1. Material deposition 163.2. Diagnostic techniques 18

4. POLYCRYSTALLINE CdTe SOLAR CELL 23

4.1. Background 234.2. Prior work/Literature research 264.3. CdTe/CdS structure fabrication 284.4. CdCI2 vapor process 394.5. Characterization of solar cells 41

4.6. Demonstration 454.7. Tandem solar cells 47

Vl


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TABLE OF CONTENTS (continued)

CHAPTER PAGE

5. Quantum confinement in PbSe nanowire 50

5.1. Background 505.2. Quantum confinement in nanowires 515.3. Growth of the PbSe nanowire 53

5.4. Measurement of effective energy levels 605.5. Calculation of effective energy levels 63

6. ENERGYTRANSFER IN THE COMPOSITE 68

6.1 Background 686.2. Colloid quantum dots 696.3. Photosystem 1 716.4. Energy transfer mechanisms 73

6.5. Energy transfer measurement in the composite system 766.6. Current-Voltage measurement of the composite 92

7. Conclusion 97

APPENDIX 99

CITED LITERATURE 105

VITA 112

vii


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LIST OF TABLES

PAGE

ADVANTAGEAND disadvantage of photovoltageOF PHOTOVOLTAIC SOLAR CELL 4

development and COMMERCIALIZATION OF PV cell . 7

SUMMARY OF CdS/CdTe PROCESSES AT MAJORITYRESEARCH PALCES 28

SUMMARY OF PROCESSES AND MATERIALS 30

PERFORMANCE OF CdS/CdTe SOLAR CELLS 44

PROCESS IMMPROVEMENTS OF CdS/CdTe SOLAR CELLS 44

FITTING RESULTS FROM FIGURE 36 AND 37 85

Vili


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LIST OF FIGURES

FIGURE PAGE

1 . The energy flow chart in the United State in 2008. Source:Lawrence Livermore National Laboratory and the Departmentof Energy 3

2. Progress in record efficiency of PV solar cell. Source: TheUnited State Department of Energy 9

3. Solar cell fabrication cost vs. energy conversion efficiency 11

4. Schematic diagram of ideal photovoltaic cell and itsequivalent circuit 13

5. Ideal J-V characteristics and figures of merit of a p-n junctionsolar cell 16

6. Non-ideal J-V characteristics of a p-n junction solar cell withseries resistance and shunt resistance; and its equivalentcircuit 16

7. Solar spectral irradiance and ¡deal solar cell efficiency (ASTMStandard Extraterrestrial Spectrum Reference E-490-00 for AM 0 and ASTM G-173forAM 1.5:www.rredc.nrel.gov, Idealsolar cell efficiency reproduced from Sze. T=300K) 25

8. Energy band diagram of CdTe/CdS solar cell 29

9. XPS results on CdS thin film (top) survey scan (bottom) detailscans of Cd and S 33

IX


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LIST OF FIGURES (continued)

FIGURE EAGE

1 0. AFM image of CdTe before/after CdCI2 treatment 36

1 1 . AFM ¡mage of CdTe after post annealing at 425 0C for 20minutes 36

12. Cross-sectional view of CdTe solar cell after post annealing at

425 0C for 20 minutes 36

13. XPS results on as-grown CdTe thin film (top) survey scan(bottom) detail scans of Cd and Te 38

14. XPS results on CdTe solar cell (top) survey scan (bottom)detail scans of Cl and Cu 39

41

1 5. I-V measurement on samples without/with CdCI2 vapor

process

16. Schematic diagram of CdCI2 vapor process system built in-house 4^

1 7. Energy conversion efficiency measurement results 43

18. Demonstration of CdTe/CdS solarceli 47

19. Calculated band gaps of PbSe nanowires and ideal solar cellefficiency reproduced from Sze. T=300K 49

20. Illustration of the density of states (DOS) of electronsreproduced from "Quantum Heterostructure 53

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PAGE

A picture of 3" PbSe film 55

Schematic illustration of the sputtering process 55

Surface view of SEM ¡mage of grown PbSe nanowiresrevealed the form of pyramids 57

Sectional view of SEM image of the perpendicularly grownnanowires is shown. The diameter of the wires is about

200nm or smaller (-100 nm), approximately. Some wires aremerged together 58

The wires were grown in the direction of < 1 1 1 > orientation of rock-salt cubic structure according to x-ray diffraction

spectrum 59

Atomic position map, rock-salt cubic structure inside small box 59

(a)FTIR spectrum and (b) PL spectrum of the PbSenanowires are presented. Cut-off wavelength (where 50 % of maximum transmittance) at 2.5 µ?? and PL peak at 2.45 µG?are found, despite energy band gap of bulk PbSe crystal is4.46 µG? at 300K 62

XPS spectrum of (a)Pb 4f region and (b)Se 3d region areobtained. Peak fitting indicated strong presence of oxygen atthe PbSe nanowire surface 64

Xl


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FIGURE PAGE

29. Schematic illustration of the Fermi-level pinning at thenanowire surface due to surface oxidization. Because of the

existence of depletion space charge layer(W) at the surface,the effective diameter(d) of PbSe nanowires are decreased,and leads to quantum confinement. The first electron (En-Oand hole (Em) quantum confinement levels above the

conduction energy band (Ec) and below the valence energy band (Ev) are marked. Triangular wires are simplified intocylinders with diameter, D. They are not on scale 66

30. Bright field ¡mage and fluorescence ¡mage of CdTe NQDs .... 70

31. Absorption spectra of PSI 72

32. Photo emission spectra of PSI with excitation 442 nm 72

33. Energy diagram of energy transfer in the CdSe NQDs+PSIcomposite system 75

34. Fluorescence microscope ¡mages of CdSe NQDs and CdSe NQDs/PSI system 75

35. (a)Photoluminescence of CdSe NQDs(575 nm, square),PSI(682 nm, circle), and NQD-PSI(solid line) composite usingexcitation at 442 nm. (b)Overlap between PSI

absorption(dashed line) and NQDs(solid line) 79

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FIGURE PAGE

36. Fluorescence decay of (a)NQDs sample and (b)NQDs in thecomposite sample in short time scale. A negative absorptionchange (??) means transient bleaching of NQDs. In thecomposite, bleaching (negative ??) is switched to absorption(positive ??) after 0.12 ps 81

37. Absorption decay of (a) NQDs only sample with excitation at477 nm (b) NQDs of the composite (c) PSI of the compositewith excitation at 610 nm (d) PSI of the composite withexcitation at 477 nm. The square symbol lines represent theexperimental measurements, and the solid lines represent 84exponential decay fits

38. (a) Transient absorption spectra, at different times, of thecomposite of NQDs and PSI are shown. When excited at 477

nm, both NQDs and PSI are excited, (b) Each spectrum in thefigure 4(a) is represented at different times separately in 4(b).. 87

39. Transient absorption spectra, at different times, of (a) NQDsonly with excitation at 477 nm (b) PSI of the composite withexcitation at 610 nm, where only PSI is excited 90

40. I-V measurements on the composite CdSe NQDs-PSI. Biasapplied was from -2V to 2V, and then back to -2V. Currentincreased with light 91

41. Energy diagram of a CdSeQD-PSI composite system 92

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PAGE

Photoluminescence of (A) 520 nm QDs, (B) 605 nm QDs, (C) 94PS1, (D) 520 nm QD-PSI composite, and (E) 605 nm QD-PSIcomposite, using excitation at 442 nm

(a) l-V measurements on the pure PSI under dark and lightcondition. The data clearly showed a response to light. Bias

was applied from -2V to 2V, and back to -2V. (b) l-Vmeasurement plot on the composite of CdSe QDs/PS1 under light condition and plot with pure PSI plot are presented 96

FTIR results on PbSe nanowires before/after annealing 15sec at 400 °C 99

Hall coefficient and carrier concentration of (top) as-grownPbSe and (bottom) oxidized PbSe film 101

XlV


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LIST OF ABBREVIATIONS

AFM Atomic Force Microscope

CBD Chemical Bath Deposition

CSS Closed Spaced Sublimation

DOS Density of States

FWHM Full Width at Half Maximum

FRET Fluorescence Resonance Energy Transfer

FTIR Fourier Transmittance Infrared Spectroscopy

HOMO Highest Occupied Molecular Orbit

HRT High Resistance TCO

l-V Current-Voltage Measurement

LUMO Lowest Unoccupied Molecular Orbit

NQDs Nanocrystalline Quantum Dots

MOCVD Metal Organic Chemical Vapor Deposition

OPA Optical Parametric Amplifier

PL Photoluminescence

xv


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LIST OF ABBREVIATIONS (continued)

PS I Photosystem I

PS Il Photosytem Il

PV Photovoltaic

VTD Vapor Transport Deposition

QDs Quantum dots

RF Radio Frequency

SEM Scanning Electron Microscopy

TCO Transparent Conducting Oxide

XPS X-ray Photoelectron Spectroscopy

XRD X-ray Diffraction

XVl


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SUMMARY

Solar cells based on polycrystalline cadmium telluride (CdTe) thin film and

composite of organic and inorganic nano-scale materials has been investigated in

this research. CdTe solar cells were fabricated, and 12 % energy conversion

efficiency achieved with first efforts from scratch. As possible promising materials

for tandem solar cells lead selenide (PbSe) nanowires were studied. In addition,

the integration of photosystem I and cadmium selenide (CdSe) NQDs

(nanocrystalline quantum dots) was explored for enhancement of light harvesting in

photosynthesis for solar cells.

The CdTe thin film solar cell structures were grown by means of vacuum

deposition systems followed by annealing treatments. A new cadmium chloride

(CdCI2) vapor process was developed in-house. The structures were characterized

using atomic force microscope (AFM), scanning electron microscopy (SEM), x-ray

photoelectron spectroscopy (XPS), and l-V measurement. Using four 3 cm ? 3 cm

CdTe/CdS solar cell, an alarm clock and a toy dragon fly were operated. For very

high efficiency (>20 %) solar cells, tandem solar cell concept is

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SUMMARY (continued)

promising. The uses of multiple solar cells of different band gaps give rise to higher

efficiency. One with the narrower band gap, such as PbSe nanowires, combined

with CdTe solar cell is suitable example.

PbSe nanowires were grown by magnetron sputtering on silicon with silicon

dioxide (Si02/Si) substrates, and characterized by SEM, x-ray diffraction (XRD),

Fourier Transform Infrared spectroscopy (FTIR), photoluminescence (PL) and XPS.

Closely packed PbSe nanowires of approximately 100 nm diameter grew in the

rock-salt cubic structure orientation. These large wires showed a large blue

shift in the luminescence and absorption compared to the bulk crystal,

demonstrating quantum confinement. This is attributed to a strong built-in field

due to surface states, band bending and a depletion layer which confines the

carrier states.

In natural photosynthesis process, light harvesting complexes (LHCs) harvest

light and pass excitation energy to photosystem I (PSI) and photosystem Il (PSII).

In this study, we have used NQDs as an artificial LHC by integrating them with PSI

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SUMMARY (continued)

in order to extend their spectral range. We have performed PL and ultrafast time-

resolved absorption measurements to investigate this process. Our PL experiments

showed that emission from the NQDs is quenched, and the fluorescence from PSI

is enhanced. Transient absorption and bleaching results can be explained by FRET

(fluorescence resonance transfer) from the NQDs to the PSI. This non-radiative

energy transfer occurs in ~ 6.5 ps. Current-voltage (l-V) measurements on the

composite NQD-PSI samples demonstrate a clear photoresponse. This exciting

breakthrough provides a basis for design of novel energy harvesting devices such

as solar cells based on photosynthesis.

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1. INTRODUCTION

1.1. Motivation

In recent years there has been much interest in the solar cells as a source of

renewable energy. The motivations include high oil prices, problems in energy

production and distribution, but most of to find alternative sources of energy rather

than fossil fuels. Figure 1 is the energy flow chart in the United States in 2008.

Overall, most of the energy is made from burning fossil fuels such as natural gas,

coal, and petroleum. The problems of burning fossil fuels are well known. The fuel

source is limited, and cause pollution. Besides, in the current energy distribution

system, half of the energy is lost.

About 40% of total fuel source is used to generate electricity, and it is the most

critical energy form these days Especially, most of petroleum is used to

transportation. For alternatives, many nuclear fission reactors built in the past, but

radioactive residue disposal and accidents are big concerns. Hydro, wind,

geothermal, and solar technologies are also developed and contributed to generate

electricity. Among them, solar energy is a promising technology for the clean

energy, yet the percentage (0.09 %) is still low as shown in the figure 1 . As shown

1


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2

in the table I, there are many advantages of solar cells compared to other fuel

sources.1"2 Using solar cell technology, electrical power can be generated in sun

light. It is clean with no emission or radioactivity. This fuel source is infinite. It can

be installed quickly, and at nearly any point of use, which reduces loss during

transmission. It does not require large amounts of cooling water. On the other hand,

this technology depends on weather. It is still more expensive than fossil fuels

because of several technical and non technical issues. The purpose of this thesis

is to explore alternative, novel solar cells.

This research is composed of three parts. The first part represents solar cell

fabrication of polycrystalline CdTe. CdTe photo-voltaic cells were grown, and solar

cells were fabricated. The second part of this research studies nanowires for

possible promising materials for tandem solar cells. PbSe nanowires were grown,

and its quantum confinement effect was investigated. In the third part of the

research, enhancement of light harvesting in photosynthesis was investigated. PSII

has been replaced by NQDs which are integrated with PSI. This study provides a

basis for the design of novel energy harvesting and other electronic devices based

on photosynthesis.


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?


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4

TABLE I

ADVANTAGEAND disadvantage of photovoltage

_____________ OF PHOTOVOLTAIC SOLAR CELL

Advantage Disadvantage

• Abundant and inexpensive resource · Low density

energy• High public acceptance (no one wants a coal burning

power plant in their neighborhood) · Weather

dependence• Carbon free energy (no fossil fuels releasing CO2, SO2,

NO2) · Lack of storage

• High reliability in modules ( >20years)

• Quick installation

• Can be installed at nearly any point-of-use (Stand alone

system, less transmission and distribution loss, space

shuttle etc.)

• Daily output peak may match local demand


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5

1.2. Objectives

The objectives of this study were:

¦ to demonstrate material growth, device fabrication, and energy

conversion efficiency testing of polycrystalline CdTe/CdS thin film

solar cells,

¦ to study quantum confinement effects in the nanowires(PbSe) for

possible promising materials for tandem solar cells,

¦ to explore an integration of useful organic (CdSe QDs) and inorganic

(PSI) characteristics for light harvesting enhancement of solar cells.


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2. SOLAR CELL FUNDAMENTALS

2.1. History of photovoltaic

Since 1839, large variety of solar cells were introduced.1,3 The photovoltaic effect

was first discovered by Edmund Becquerel. He observed that metal plates

immersed into an electrolyte produced a small voltage and current. The first solid

state material that showed photovoltaic (PV) behavior was selenium with platinum

wire by Adams and Day in 1877. The first functional and intentionally made solar

cell was by Fritts in 1883. He melted selenium or copper oxide or thallium sulfide

into a thin sheet on a metal plate, and pressed a gold leaf film as the top contact. In

1954, researchers at Bell lab accidentally discovered that the semiconductor pn

junction diodes generate a voltage when the room lights were on. From mid 1950

to mid 1970, R&D was focused toward space application such as satellite power.

Then, there was oil crisis in 1973, which initiated the R&D on solar cells for civilian

applications. Silicon was the first to be used for commercial solar cells. In the early

1980s, the solar-powered calculators and wristwatches were sold by the Japanese

companies. The first solar cells power plant was built in 1982 in the state of

California. It took about 15 years to cumulate electrical power- 100MW by the solar

6


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TABLE II

DEVELOPMENTAND commercialization OF PV cell

1839 Becqurel (FR) demonstrated the PV effect.

1 877 Adams and Day (UK) observed the first PV in solid.

1883 Fritts (US) showed large area solar cell with rectifying metal contact.

1 955 Hoffman electronics offers 2% efficient Si PV cell at $1 500/W

1973 Worldwide oil crisis

1 974 Tyco (USA) grows 2.5 cm wide Si ribbon for PV

1982 First 1 MW PV power plant using c-Si, CA, USA

1986 First a-Si thin film solar power module

1994 GalnP/GaAs concentrator multijunction > 30% (NREL, USA)

1996 dye-sensitized solid/liquid cell achieved 11% (Switzerland)1997 Cumulative world wide PV production reached 100 MW

1998 Cu(lnGa)Se2 thin film solar cell reached 19 % (NREL, USA)

1 999 Cumulative world wide PV production reached 1 000 MW

2008 Cumulative world wide PV production reached 8775 MW*

Source: *EERE News, U. S. Department of Energy, March 25, 2009


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cells, but It took only 2 years to increase it to 10 times, 1 GW. In 2008, the

cumulative world wide PV production reached closed to 10 GW. The history is

summarized in table II.

2.2. Type of solar cells

In figure 2, the line graph shows the increase of the energy conversion efficiency

of the solar cells since 1975. The left hand vertical axis is energy conversion

efficiency, while the horizontal axis shows the time in years. The solid lines (square

markers) represent the solar cells made of single crystal silicon. The solid lines

(circles) represent the so called thin film solar cells which are made of CdTe or

CIGS or amorphous silicon. The two lines from the lowest efficiency represent the

so called organic cells. The lines (triangles) represent the solar cells made of GaAs

related materials, and they are used for the space applications. The NREL,

National Renewable Energy Laboratory has the highest efficiency record made of

MBE grown GaAs related materials.


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£3) m


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10

In the reference 4, Green summarize the solar cells into three groups as shown in

figure 3.4 The first-generation solar cell was a single crystal solar cell, which has

been commercialized the most. Recently with high demand, the price silicon wafers

has increased more than three times. Thus, the second-generation less expensive

solar cells are desirable. These are solar cells made of thin film of amorphous

silicon, CdTe, and CIGS (CuInGaSe). Since these are thin films, which require less

material than bulk wafers, they are less expensive than solar cells made of silicon

wafers. However, energy conversion efficiency of polycrystalline cells is lower than

single crystal ones. The third-generation solar cells are tandem cells, organic cells

and other cells including cells made of nano-sized materials. The solar cells under

this group are still under research to achieve high efficiency and low cost. In this

study, one type of the second generation solar cell, CdTe PV cells were fabricated

and tested. Following that PbSe nano-rods were grown for future nanomaterial

based solar cells, and finally the study of energy transfer from inorganic CdSe

quantum dots to organic photosystem I was carried out and investigated for the

third and future generations of PV cells.


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11

USSOJWW US$().20/W

s 40

USS0.50/W

Thermodynamiclimit

USS1.00/W

Present limit

US$3 .50/W

0 U)Q 200 300 400 500

Cost. US$/m2

Figure 3. Solar cell fabrication cost vs. energy conversion efficiency

2.3. Physics of photovoltaic cells

There are many different types of solar cells, but the basic operation is explained

by a p-n junction operation under solar radiation.5 When the cell is exposed to the

solar spectrum, a photon that has energy less than the band gap makes no

contribution to the cell output. A photon that has energy greater than band gap is


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¦M-

13

'\J\J¥ + +

+ +

+ +

If

+

-o V o-

Ev

I = I(e1V/kT-ï)-IL

Rl

Figure 4. Schematic diagram of ideal photovoltaic cell and its equivalent circuit


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14

A solar cell is thus a p-n junction diode without any external voltage applied.

Because of built-in electrical field in the depletion region, lL, a photocurrent flows

even without external bias, (i.e. at zero bias), in the direction of the electric field.

The l-V behavior for an ¡deal solar cell is shown in figure 5. With illumination, the

excited electrons contribute to the current density, which is Jsc, the short circuit

current. Jsc is equal to the photocurrent density JL. This short circuit current is the

amount of current generated by photons. Another important parameter is the Voc,

open circuit voltage. This parameter is depends on the built-in potential, which is

depends on the material properties. The current and voltage which deliver the

maximum power are referred as the maximum power current density Jm and

maximum power voltage Vm. A fill factor, FF, can be defined by

FF =J VJ SCy OC

(3)

and the energy conversion efficiency is given by

J V

?~~?~ (4)in

where Pin is the incident solar power on the PV cell. To increase the energy

conversion efficiency, we need larger short circuit current and larger open circuit


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15

voltage. To increase short circuit current, basically, we need more excitedelectrons/holes by choosing appropriate materials with direct band gap than

indirect, designing multi-junction cells and so on. To increase open circuit voltage,

fabrication process needs to be improved to provide strong abrupt junctions with

minimum leakages.

In the case of non-ideal solar cell, the equivalent circuit for the ideal diode l-V

characteristics needs to be modified by adding the series resistance (Rs) and shunt

resistance (Rsh). This non-ideal l-V characteristic allows us to determine properties

of the PV cell. As shown in figure 6, these series and shunt resistance reduce the

fill factor, and result in poor energy conversion efficiency. Series resistance is

related to the non-ohmic component of metal contact. Shunt resistance is from

leakage in the diodes. In other words, observing series resistance implies that the

metal contact is non-ohmic, and shunt resistance reveals the poor condition of the

p-n junction of the PV cells. To achieve higher energy conversion efficiency,

selection of materials and processes should be considered carefully to obtain

larger open circuit voltage, larger short circuit current, smaller series resistance,

and smaller shunting resistance.


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16

Figure 5. Ideal J-V characteristics and figures of merit of a p-n junction solar cell

Light.

sh? WV-

L· MR.

\— ???t-

s *> sh

TV

Jl

R,

Figure 6. Non-ideal J-V characteristics of a p-n junction solar cell with series

resistance and shunt resistance; and its equivalent circuit


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3. EXPERIMENTS

3.1. Material deposition

Three thin film deposition techniques are selected and utilized in this work based

on accessibility of source materials and systems.

Thermal evaporation

The thermal evaporation technique is one of vacuum thin film deposition

techniques. It consists with a heating unit (resistance heating), a source boat, and

sample holder. The source material is vaporized by applied heat, and condensed

on the sample surface and vacuum chamber walls. Relatively low vacuum

pressures (~10"5 Torr) are used to avoid reaction between source material and air.

In addition, in poor vacuum pressure, the deposition is not carried out because the

mean free path of vapor is not enough. In this deposition technique, the deposited

films are often non-crystalline since vaporized atoms reaching the sample surface

are low. Heating and vacuum pressure are deposition rate parameters. In this work,

CdS thin films were deposited by thermal evaporation. CdS evaporation source

(99.99%) was purchased from Cerac Inc. (Milwaukee, Wl).

?-beam evaporation (e-beam evaporation)

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The e- beam evaporation is another vacuum thin film deposition technique,similar to thermal evaporation. Systemically only difference is a heating unit.

Instead of resistance heating, electron beam gun is used. Usually several KeV high

energy electron beam bombard the source in a crucible. Generally, the source

materials are available with high purity quality, so higher vacuum pressure (~10"7

Torr) is used to obtain high purity films. In this work, CdTe thin films and goldcontacts were deposited by e-beam evaporation. CdTe e-beam evaporation

sources (99.999%) were purchased from Cerac Inc. and Piasmaterials (Livemore,

CA)

RF magnetron sputtering

The sputtering evaporation system is, again, one of vacuum thin film deposition

system, but its operation is more complicated. Instead of heating unit, RF power

generate a plasma (argon in this work), and it bombards the source target. In some

cases, reactive gases used, therefore the plasma reacts with the source too. By

adding magnet in the target side, it makes plasma more concentrated betweentarget and sample surface. PbSe nanowires were grown using this technique.

PbSe sputtering target (99.999%) was purchased Cerac Inc. (Milwaukee, Wl).


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3.2. Diagnostic techniques

Morphology, crystal structures, identification of elements and compounds, optical

band gap and photoconductivity of materials were analyzed by means of several

techniques.

X-ray photoelectron spectroscopy (XPS)

Since favorite Albert Einstein discovered photoelectric effect, it has influenced

numerous areas. XPS is one of them. Kei Siegbahn demonstrated photoemission

as analytical tool in 1980s. The basic operation is that a detector collects

photoemission from a sample surface after its exposure to x-rays. By measuring

emitted electron's kinetic energy, its binding energy is calculated, and elements at

the sample surface can be identified. Usually, x-ray can penetrate only top -10 nm

surface. To remove surface contaminations or oxide layers, sputtering by argon

gas is carried out. This technique is non-sample destructive, and very effective

elemental, chemical analysis of any sample, which is compatible with ultra high

vacuum. Kratos Axis-165 XPS system, which equipped with monochromatic x-ray

source(AI Ka) and concentric hemispherical analyzer, was used to analyze CdS,

CdTe materials, and the PbSe nanowires.


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Atomic force microscope (AFM)

AFM is one kind of scanning probe microscopes. It ¡mages atomic resolution

sample surface by measuring the vertical and horizontal deflection of laser beam

which reflected from a cantilever. Since it is measuring atomic interaction between

the cantilever and sample surface, unlike measuring tunneling current in case of

scanning tunneling microscope operated in ultra high vacuum, it can be operated in

ambient air or even in liquids. It is also a non-sample destructive techniques, but

the samples have to cut into small pieces to mount in AFM sample holder. A

PicoScan 2500 system was used to measure CdTe grain size.

Scanning electron microscope (SEM)

The Hitachi S-3000N SEM system equipped with a tungsten electron source

(accelerating voltage 0.3 - 30 kV), was used to image cross-sectional view of CdTe.

The sample was coated with 10 nm Pt film prior to imaging. We also obtained SEM

images of PbSe nanowires.

Current-voltage measurement (l-V measurement)

Photovoltaic behavior of CdS/CdTe junction was characterized using a current-

voltage (l-V) measurement system. The system consists of KEITHLEY 2400


41/135

21

source meter and two probes. To measure solar cell's energy conversion efficiency,

a light source was added, and the irradiance at the sample surface was calibrated

using a reference cell (p-Si solar cell, VLSI Standards, Inc., San Jose, CA). Photo-

response of PSI and composite of NQDs and PSI was also measured.

X-rav diffraction (XRD)

The crystallinity of PbSe nanowires was analyzed by Siemens Diffracktometer D-

5000 x-ray diffraction system (Cu radiation, graphite monochromator).

Fourier transmittance infrared spectroscopy (FTIR)

Thermo Nicolet Nexus 870 FTIR system (spectral resolution: 0.125 cm"1,

frequency range: 400-12000 cm"1, detector: DTGS, MBTB) was used to measure

transmittance of PbSe nanowires. The measurements were carried out at room

temperature.

Spectrofluorometer

Photoluminescence of PbSe nanowires was measured using Horiba Jobin Yvon

FluoroLog system. What is unique about this system is that it allows us measure in

infrared range to 3/zm. The probe range of the system is between 800 nm to 3/zm.

Hall measurement


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22

Hall measurement with the van der Paw technique is the most common

measurement technique to determine the sheet carrier density by measuring Hall

voltage with HP 4156B Semiconductor Parameter analyzer, where four indium

contacts were made on the samples to be measured.

Fluorescence image

Both bright field images and fluorescence ¡mages of PSI and NQDs were

obtained using Nikon Ellipse TE 2000-S microscope. Three exciter (405 nm+45,

460 nm±45, 545 nm±15), two beamspliter (470DCXR and 475DCXRU), and six

emitter (525 nm±10, 585 nm±10, 605 nm + 10, 610 nm+32, 655 nm+ 10 705 nm

±10) filters were available.

Absorption measurement

The absorption measurements were carried out by a Cary 300 UV-Vis

spectrophotometer system at room temperature.

Photoluminescence (PL)

A single-stage ACTON SpectraPro 2500 spectrometer with a 1200 g mm-1

grating with 442 nm line of a Kimmon Helium-Cadmium laser at an initial laser

power of 80 mW was used.


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23

Transient absorption measurement

Transient absorption measurements were carried out at the Center for

Nanoscale Materials, Argonne National Laboratory. The system consists of a

femtosecond Ti:sapphire oscillator regeneratively amplified at 1.7 kHz. A small

amount of the output is used to generate the white light continuum probe, and the

remaining 95% pumps an optical parametric amplifier (OPA) to produce theexcitation pulses. The OPA produces a tunable fs output from the UV through to

the infrared. For these experiments, the OPA was set to 477 nm. The two outputs

then enter a transient absorption spectrometer (Helios, Ultrafast Systems), where

the probe is variably delayed relative to the pump on a mechanical delay line. The

pump beam is chopped at half the repetition rate of the laser so that an absorption

change (??) can be measured as a function of delay: AA=-[log(lp/lnp)]. Here lp is

the intensity of the transmitted probe with the pump on, while lnp is the intensity of

the transmitted probe with no pump. A spectrograph is used to collect the spectral

content of the probe from 440 nm to 760 nm as a function of delay. The data is

chirp corrected to within 100 fs over this spectral range. The excitation pulse with

610 nm was also used.


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25

Solar spectral irradiance

E 2000 U

e

5

.S¦?

a.m

AMO, 1353 W/m'relevant one

for satellite and space-vehicle

AN1 1.5, 844W/nr average for terrestrial application

1 2 3Wavelength [µp?]

Solar spectral irradiance & Ideal solar cell efficiency

?cf

?

£UJ

CuInGaSe

CdTe CdS

a-Si:H

KlLiLi

1500^

C

1000 1B

a

tfí

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3 .5 4.0Bandgap energy [eV]

Figure 7. Solar spectral irradiance and ideal solar cell efficiency (ASTM Standard

Extraterrestrial Spectrum Reference E-490-00 for AM 0 and ASTM G-173 for AM

1.5:www.rredc.nrel.gov, Ideal solar cell efficiency reproduced from Sze. T=300K)


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26

van

ety of deposition techniques such as chemical bath deposition, chemical vapordeposition, closed-space sublimation, e-beam deposition, and so on .1 The record

laboratory efficiency for CdTe solar cell has reached 16.7% at National Renewable

Energy Laboratory, and the module efficiency reached in solar cells of this material

is 10.9% by BP Solarex.6 Single crystal CdTe has been well studied and used for

gamma ray detectors, but the polycrystalline material technology is not developedas much as single crystal technologies because polycrystalline CdTe technology

has not many application so far. The goal of this work was to demonstrate CdTe

thin film solar cell fabrication and to try to understand the various factors in the

fabrication process to identify and improve what contributes to the efficiency of

energy conversion process.


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27

4.2. Prior work/Literature research

Thin film polycrystalline CdTe solar cell has been developed world wide,

especially in University of South Florida (Tampa, Florida), the University of Toledo

(Toledo, Ohio), Colorado State University (Fort Collins, Colorado), University of

Delaware (Newark, Delaware), and National Renewable Energy Laboratory

(Golden, Colorado) in U. S. Although they used different processes to fabricate

CdTe solar cells, the basic structure is CdS/CdTe p-n junction on TCO coated

glasses, the so called a superstrate configuration. The processes are summarized

in the table III.

We have demonstrated this superstrate configuration CdS/CdTe solar cell, from

material deposition, processing such as annealing, doping, cell fabrication and performed its energy conversion efficiency test.


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28

Table

SUMMARY OF CdS/CdTe PROCESSES AT MAJORITY RESEARCH PALCES

Superstrate

Transparentcontact

HRT layer

CdS

CdTe

CdCI2

University of SouthFlorida78

Borosilicate glass

(Corning 7059)

Sn02:F by MOCVD

ITO by sputtering

SnO2 by MOCVD

SnO2 by sputtering

CBD(90nm-100nm)

Back contact

CSS (5-6 w),

Tsub=550 1C

thermal evaporation

followed by heat

treatment @390 "C

Bromine/methanol

etch for 7-10 sec

Graphite doped with

HgTe:Cu by heat

treatment @250 °CGraphite

Mo

University of

Toledo9, First Solar,BP solar

Pilkington Tec-7, Tec-

15

ZnO:AI on Corning

1737 aluminosilicate

glass

Yes, no details

Sputtering

Sputtering (2-4 ,um)

Colorado School

of Mines,

Colorado State

University,

NREL1 10

Pilkington Tec-7,

Tec-15,

Sn02/coming

7059

Cd2SnO2 or

Zn2Sn04 coating

N/A

CBD

Vapor process

@375-400 °C

Yes and no, no detail

Cu/Au followed by

diffusion annealing at

150-170 °C

ZnTe:N/Au or

ZnTe:Cu/Au study

CSS @ Tsub=570-

625 °C

University of Delaware

11-12

TEC-15

N/A

CBD(


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29

4.3. CdTe/CdS structure fabrication

Figure 8 shows the structure of a CdS/CdTe solar cell and its corresponding

energy band diagram. Processes and purchased materials are summarized in

table IV. We first explored the TCO coated glasses. CdS was deposited by thermal

deposition technique. CdTe was deposited by e-beam, and the film was annealed

at high temperature under CdCI2 vapor. Cu was diffused into CdTe film by low

temperature annealing. Finally, Au back contact was applied.

-"W* Glass Sn02 CdS

3.7 eV2.4 eV

CdTe

1 .45 eV

Au

"Ë7" _Ef_

Ev

Figure 8. Energy band diagram of CdTe/CdS solar cell


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31

The first challenge was preparation of the TCO coated glasses. It should havevery high optical transmittance and low resistivity. Four different types of glasses

were tested. First, indium tin oxide (ITO) coated glasses, which were purchased

from Sigma-Aldrich were used, but the CdTe film peeled off after high temperature

annealing. Indium in ITO coating seems deteriorated at elevated temperature. Next,

three different Sn02:F coated glasses were used. The glasses purchased from

Pilkington was more transparent compared it purchased from Woo Yang. Thinner

Pilkington glasses,TEC15, showed best results.

A high resistance TCO (HRT) layer is necessary to produce high efficiency cells.

Additional Sn02:F or ZnO:AI films, with a higher resistance than the TCO, have two

benefits. One is that these ?-type oxide layers allows for thinner CdS, therefore it

increases the light absorption. Another is that HRT appears to improve non-

uniformity of the solar cells by blocking electrically short channels in the films. The

properties of HRT layers mainly depend on its thickness and annealing

temperature, which decides the resistivity. Although we have tried 0.04 µG? thickZnO:Al films followed by 500 0C annealing, but have not seen significant

improvement such as increasing open circuit voltage and/or short circuit current.


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32

The CdS thin films (0.2 µ??) were thermally deposited at room temperature.

Rather thick layers were deposited to avoid pin holes in this deposition. If CdS film

is thinner (


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33

200000

150000

5*?m

55?—*

e

g 100000

£ 50000

-I — ? — ? — ? — ? — ¦ — I — ? — I — ¦ — I — ' — I — ¦ — I — ' — I — "~

CdS: Survey

t — ¦ — t

Cd

3d Cd

5/2 3d,

Cd 2d c

4??? 1S4s

Cd

3p3Cd3P1

O1s

Cd

3s

Cd (A)

' ¦ ¦ I I L. _l ? I ?-

? 100 200 300 400 500 600 700 800 900 100011001200Binding Energy(eV)

Cd at CdS thin film surface

3d 5/2

404.9

3d 3/2

404 406 408 410 412 4 14 416Binding Energy(eV)

S at CdS thin film surface

61.4

162.5

160 162 164Binding Energy(eV)

Figure 9. XPS results on CdS thin film (top) survey scan (bottom) detail scans of

Cd and S


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34

in the as-grown CdTe. The CdCI2 treatment increase grain size and improve itselectrical properties. In addition, it influences p-type doping. AFM and SEM images

of the CdTe film are presented in the figure 10, 11, and 12.

As-grown CdTe film has grain size about 0.1 µt? as shown in figure 10. Its

chemical analysis was carried out by XPS, and the results are presented in figure

13. Strong Cd and Te peaks were found, and no other impurity was observed. TheCdTe film was placed inside an oven while 0.1 M CdCI2 was vaporized, using a

ultrasonic nubilizer, and delivered to its surface. The CdCI2 treatment temperature

has great role on increasing the grain size. The AFM surface images of CdTe

annealed at different temperature are shown in figure 10 and 11. By exposing the

sample to the CdCI2 treatment at 425 0C for 20 minutes, a grain size of 5 µ?? was

achieved as shown in the figure 12.

Finally, the back contact deposition is the last step to complete the CdTe/CdS

solar cell. This step consists of etching off CdCI2 treated CdTe surface, Cu

deposition and diffusion by low

temperature annealing, and Au contact deposition.

The biggest concern in this step is that it is impossible to make ohmic contact with

p-type CdTe and any metal materials. All metal contacts result in formation of


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35


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Figure 11 AFM ¡mage of CdTe after post annealing at 425 0C for 20 minutes

^rararofi^^^M

HRRH

Figure 12. Cross-sectional view of CdTe solar cell after post annealing at 425 °

for 20 minutes


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37

Schottky junctions. In order to minimize this effect, a highly doped area is

desired between CdTe and metal. One method is etching CdTe surface in

bromine/methanol solution. By using this etching, the Te-rich surface is created,

which increase Cd vacancies at the surface. Another method is adding Cu to the

CdTe surface. As a result, the width of the Schottky barrier is decreased. In this

experiment, we tried both etching in bromine/methanol solution and Cu doping.25A of Cu was deposited using e-beam deposition system, and annealed at 180 0C

for 20 minutes.

Figure 13 and 14 show XPS analysis on as-grown CdTe surface and CdTe

surface after CdCI2 treatment followed by Cu doping. In addition to Cd and Te

peaks, Cl and Cu peaks were observed after the treatment and the doping. No

bromine residue was found. Lastly, Au was deposited as a back contact using e-

beam deposition system.

In summary, CdS/CdTe solar cell was fabricated based on best known published

information. The most difficult challenge was CdCI2 vapor process, which is

explained in the following section.


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38

300000

250000

l'I1 — t-

? CdTe: Survey

200000

?U)

E 1500003O

c¿&¦¡j 100000COl

*-»

_ç

50000

Cd

3d„

-|— ? — ? — ?— | — ¦ GTe

3cU. Te3d,„

Ih CdCd r,4sTe4dlCd 4s C

|4p 1s

_!__! I L-

Cd

3d,,.

O

1s

Cd

3p Te Te Cd -

3Cd Sp3Te 3s (A)3P1 3P1Cd

3s

0 100 200 300 400^ ¿00^ 800 900 1000 1100 1200

45000

40000

35000

30000

25000

20000

15000

I 10000

5000

Cd at CdTe thin film surface

3d 5/2405.3

6.7

400 402 4 04 406 408 410 412Binding Energy(eV)

414 416

Te at CdTe thin film surface

3d 5/2572.6

576 580

Binding Energy(eV)

Figure 13. XPS results on as-grown CdTe thin film (top) survey scan (bottom)

detail scans of Cd and Te


59/135

39

350000

300000

250000 -

-?2 200000(?

·*— '

C3

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C 100000?

50000

CdTe after CdCI2 treatment and Cu dopingTe

3d Te6^ 3d,,

Cd 3d

Cd

5ß3d,

G Cd

Te

Te Cd4s4d 4s

Cl

4d lCd 2d C

J ?-

?

1s

Cd

3P3Cd

l3Pl\\\

Te

3P3Te

Cd \ 3PiCu3s

Te

3s

kk2p3

Cd

(A)

Jl! \j

J ? — L J ? I — ?-

? 100 200 300 400 500 600 700 800 900 100011001200Binding Energy(eV)

Cl at CdTe surface

after CdCI2 treatment and Cu doping

2P,

2P1i 200.4

2P3932

kààAm

?' ?

2P,952

I I I

1IW Ì«1Cu at CdTe surface ' ' I ] T|!|'i'after CdCI2 treatment and Cu doping

185 190 195 200 205 210 215Binding Energy(eV)

915 920 925 930 935 940 945 950 955 960 965Binding Energy(eV)

Figure 14. XPS results on CdTe solar cell (top) survey scan (bottom) detail scans

of Cl and Cu


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40

4.4. CdCI? vapor process

CdCI2 post treatment is the most critical process in CdTe thin film solar cell

fabrication. Spin coating of CdCI2, thermal evaporation of CdCI2 and so called

vapor process were used by other groups. 7"12 Firstly, spin coating of CdCI2

followed by annealing was tested. However, CdTe films were peeled off after

annealing. In addition, the applied CdCI2 crystallized and it was not possible to

clean it away from its surface. Next, coated CdCI2 was briefly rinsed, and samples

were annealed. Less than 5% energy conversion efficiency was measured on

these samples as shown in figure 15.

To achieve higher efficiency, a process using CdCI2 vapor was needed. Other

groups reported solar cells with higher efficiency using a so called vapor process,which is annealing samples under continuous CdCI2 vapor flow generated by

thermally sublimated from solid form CdCI2 source.9"12 Our own vapor process was

built in house as shown in figure 16. In our case, we used a nubilizer to generate

CdCI2 vapor, and the vapor was transferred into a oven where CdTe/CdS samples

are annealed. As a results, we found increase of both Voc and Jsc, which leads to

higher efficiency as shown in figure 15.


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41

CM

eo

<E

U)Ca>Q

ca>L-?-

3

O

5

0

-5

-10

-15

-20

-25

Annealing temperature: 38O0C, 30min

Solid lines

: spin coatedfollowed by rinsed

Dashed lines: spin coated followed by rinsedand then applied using sonic during annealing

J ? L·

-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Voltage (V)

Figure 15. 1-V measurement on samples without/with CdCI2 vapor process

Exhaust

Oven

CdCI2vapor Carrier

Sample

Nubilizer

Figure 16. Schematic diagram of CdCI2 vapor process system built in-house


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42

4.5. Characterization of solar cellsThe CdTe/CdS solar cells were tested by current-voltage measurement (l-V) with

a light source at 20 0C. To allow meaningful comparison between different solar

cells under different conditions, a standard is generally used. The standard is the

total power density of the exposed light at the solar cell surface is 1000 W/square

meters. Solar energy conversion efficiency is defined as the maximum power over

the total power. In our best solar cell we achieved open circuit voltage 0.79 V, and

short circuit current density 22.96 mA, and FF 67 %. The efficiency achieved was

12.16 %, and device size was 0.5 mm ? 0.5 mm. The l-V system was calibrated

using a reference cell (p-Si solar cell, VLSI Standards, Inc., San Jose, CA).

//(%)Pu,

J^VFF ? 100 (5)Pu,

0.02296 ? 0.79 ? 67.06 ? 100 ^ ] on~ HXK)= 12.16%

l-V results from different CdTe/CdS solar cell samples with different efficiency were

shown in figure 17. As explained in chapter 2.3., using values of open circuit

voltage, short circuit current, shunt resistance and series resistance obtained from


63/135

43

l-V curves, we were able to analyze the performance of CdTe/CdS solar cells. Theequivalent circuit of the solar cell, shown in figure 6, has two shunt resistor

components (one for the p/n junction, another for the back contact junction) and

two series resistor components (one for the back contact, another due to the front

contact and electrical wires), the values are summarized in table IV.

^ 10.2% -Iv 12.2%

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0Voltage (V)

Figure 17. Energy conversion efficiency measurement results


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45

To achieve the short circuit current, Jsc, first of all, a high density generation of

electron-hole pairs are needed without recombination. Optimized thicknesses of

CdTe layer and CdCI2 treatment are critical. Other sources of JSc losses are

electrical contacts and reflection at the glass surface. The key determining open

circuit voltage, Voc, is the recombination in the depletion-region. Lower

recombination rate give higher Voc. In other words, less trapping levels-high crystal

quality at the p/n junction- all yield higher V0c-

Fill factor is another important figure of merit for the solar cells, and it can be

calculated as shown in chapter 2.3. Solar cells always have a parasitic resistance

which is a combination series resistance and shunt resistance. The series

resistance consists of bulk resistance of the semiconductor, contact resistance

between semiconductor and metal contact, and electrical contacts connected to

outside. The shunt resistance is mainly caused by leakage across p/n junction. By

optimizing the solar cell fabrication processes, as described in previous section,

we've improved the performance of CdTe/CdS solar cells as shown in table Vl.


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46

4.6. Demonstration

Using four 3 cm ? 3 cm CdTe/CdS solar cell, an alarm clock and a toy dragon fly

were operated as shown in figure 18. Top three pictures are the CdTe solar cells

connected to the alarm clock. As shown in the middle picture, the battery was

removed, and the power terminals were connected to the CdTe solar cells. In the

case of the toy dragon fly, the turn-on voltage was about 0.6V, and about 40 mA of current was required to start the small electrical motor, which attached to the

dragon flies. By connecting the solar cells in series, the solar cells provide enough

voltage and power to the alarm clock and toy dragon fly. These CdTe solar cell

used for the demonstration had an efficiency of about 5%.


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47

Figur® 18. Demonstration of CdTe/CdS solarceli.


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48

4.7. Tandem solar cells

Tandem solar cells, which consist of multiple solar cells with each solar cell

absorbing the solar radiation closed to each solar cell's band gap. Tandem solar

cells made of single crystalline GaAs and Si have been achieved high energy

conversion efficiency, over 40%. 2"6 In our study, we suggest PbSe nanowires with

band gaps smaller than the band gap of CdTe (1.5 eV). Band gaps of nanowires

can be engineered by varying size of nanowires. It is known as quantum

confinement effect, which will be explained in section 5.2. Calculated band gaps of

PbSe nanowires with different sizes were superimposed on ¡deal solar cell

efficiency marked with band gap of different solar cell materials in figure 19. The

band gaps of nanowires are calculated using 1-D infinite well case as shown inequation (6).

^Vi2 ?2p2\2",? ImJa2+ 2mh*a2+ 8 (6)

As shown in the figure 19, band gap of PbSe nanowires can be engineered, so it

can join together with other solar cells such as CdTe thin film for tandem solar cell

configuration.


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49

There have been proposals of solar cells made of nanowires.13"14 Despite of the

high energy conversion efficiency potential, realization of nanowire solar cell

require better understanding and control over materials and device structure. As a

initial step, we have shown the quantum confinement effect in our PbSe nanowires

in this study.

50

40

30

P 20

10h

?c

?

EUJ

CuInGaSe.

PbSe a-Si:Hnanorod

20

15

Ec

"so

o

10 If

5 eQ

0

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0Bandgap energy [eV]

Figure 19. Calculated band gaps of PbSe nanowires and ideal solar cell efficiency

reproduced from Sze. T=300K


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5. Quantum confinement in PbSe nanowire

5.1. Background

To explore materials for tandem solar cells, PbSe nonowires were studied.

Although PbSe has direct and small band gap, bulk PbSe is not very efficient to

make solar cell because the energy band gap is not in visible range. The solar

irradiance outside visible range is very weak. However, with the magic of quantum

confinement, PbSe nanowires could be another good candidate material for solar

cell. We can engineer the energy band by making the PbSe material smaller;

therefore move the absorption energy gap toward visible range.

Material growth of semiconductor nanowires has attracted much interest for

conceptual devices such as single electron transistors, field effect transistors,

sensors, emitters, and solar cells.15"20 Bulk PbSe has a narrow direct band gap

(0.28 eV at 300K) at the L point of the Brillouin zone, large dielectric constant (e„~

23) and high optical sensitivity (absorption coefficient > 104 cm"1) near room

temperature in the technologically important regions of near infrared (0.75-3/™) and

mid-wave infrared (3-5/™). 21"25 Nanowires of PbSe are of enhanced interest due to

their special properties where the relatively large Bohr excitonic radius (46 nm) and

50


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51

small effective masses lead to strong electron and hole confinement in PbSe

nanowires. PbSe nanowires have been grown by solution-phase synthesis21 and

direct current electrodeposition22, vapor-liquid-solid growth.23 In this study, we

demonstrate strong confinement in wires grown by RF magnetron sputtering on

non-single-crystal Si02/Si substrates, without an intermediate layer, where the

wires are much larger than the Bohr radius of the electrons.

5.2. Quantum confinement in nanowires

In a bulk semiconductor, density state of electrons, N(E) is as a function of

energy band level and effective mass, written as follows.

NiE) = An^-Y4Ëh2 (7)It is the number of allowed energy states per unit energy per unit volume (eV1-cm"3)

for a three dimensional semiconductor. In the figure 20, the parabolic curve shows

the density state of bulk. For a two dimensional semiconductor such as quantum

well, density of states become steplike. Nanowires, in which electrons are confined

along a line, are one dimentional quantum structure with spikelike density of states.


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52

To calculate confinement energy calculation in the nanowires, Schrodinger

equation for free electrons in two dimensions using polar coordinates .27

h2 ( d2 id Id 2 ?

2m· + + ¦

2 a/i2dr r dr r d? ?(?,T) = ??^,T)

(8)

?-2^2 · 2? p j(n

The solution for a cylindrical well with infinitely high walls is e?1- — 2ma

I. „? 1, where angular momentum quantum number is Je,n ~ (n + 9 Kl ?)p

The state of lowest energy has zero angular momentum( £=Q).

Thé lowest confined energy level of nanowires is ?2p\-p)2

&1,02ma¿


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53

?

B"co?

?—

O

>>^-'

"toCf

Q

tEc bulk C dot

C wire

C wei

Energy

Figure 20. Illustration of the density of states (DOS) of electrons reproduced from

"Quantum Heterostructure" 26

5.3. Growth of the PbSe nanowire

The sputtering deposition technique has been used to grow other semiconductor

nanowires such as ZnO.28 The advantages of the sputtering method include growth

of quality PbSe nanowires which are contamination free, compatible with device

processing, less expensive and simple.

PbSe thin film was deposited on a 3" Si02/Si substrate, as shown in figure 21 ,


74/135

54

using RF magnetron sputtering system (CVC SC4000) at the Nanotechnology

Core Facility at University of Illinois at Chicago using 2" lead selenide (99.9%)

sputtering target from GERAC, USA. The PbSe was deposited for 5 minutes under

a pressure of 4.3 mTorr with constant flow of Ar gas (40 seem), with the substrate

at room temperature. The applied power on target was at 200W, and the sample

was rotated. Schematic illustration of the sputtering process is shown in figure 22.

The substrate was cleaned with acetone followed by a methanol and deionized

water rinse. Surface morphology and cross-sectional ¡mages of the film were

obtained using SEM. The crystallinity of the film was checked by XRD.


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55

Figure 21 . A picture of 3" PbSe film

Rotation

MColumnar ?« .·PbSe

Substrate

Growth

direction

©<

o

G

©

Q

o Plasma

PbSe target

Figure 22. Schematic illustration of the sputtering process


76/135

56

Material deposition using a sputtering technique usually leads to a randomly

oriented polycrystalline thin film. However, our PbSe nanowires were obtained by

RF magnetron sputtering deposition as shown in the figure 23 and 24. In the plane

view of SEM image, randomly distributed, close-packed triangular structures were

seen with a density of 9x1 09 cm"2. The cross-sectional view of SEM images

revealed perpendicular growth of 2.8 µ ?? long wires. The average side length of the

triangles has been found to be in the range of 140 nm to 200 nm. The thicker wires

appeared to be a merging of thinner wires. The outer wires are shorter than inner

columns due to damage occurring during sample cleaving. The XRD pattem(figure

25) of the wires showed only two strong peaks - (111) and (222) - corresponding to

rock-salt cubic structure of PbSe crystal, implying that the wires were grown only in

the direction. Other PbSe nanowires have been reported with multiple

diffraction peaks.19"20 In these reports, PbSe formed islands in the rock-salt cubic

structure grown in direction on SiO2 substrates as illustrated in figure 26.

The islands were randomly distributed single crystals, and agglomerated into a

polycrystalline thin film with large grains where the grains were related to the

no

substrate orientation.


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57

MTi

1 ¦*/ ,..¦ Í -;

Figure 23. Surface view of SEM image of grown PbSe nanowires revealed the form

of pyramids.


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58

.Sî?S'

Figure 24. Sectional view of SEM image of the perpendicularly grown nanowires is

shown. The diameter of the wires is about 200nm or smaller (-100 nm),

approximately. Some wires are merged together.


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59

10000PbSe film (111)

10 20

300

SiO, Substrate

M'**«|M^yl'"*wwW'M*'t' '*

10 20 30 40 50 602T

Figure 25. The wires were grown in the direction of orientation of rock-salt

cubic structure according to x-ray diffraction spectrum.

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Figure 26. Atomic position map, rock-salt cubic structure inside small box


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normalized with respect to the Si02/Si substrate which was used in this study.

The high absorption of the thick layer of PbSe wires is consistent with previous

results of films at these wavelengths.22 The cut-off absorption wavelength of about

2.5,™ (0.496 eV) was observed instead of the known cut-off wavelength of 4.46/¿m

(0.278 eV) for of single- or poly-crystalline PbSe.23

The pronounced interference fringes are indicative of Franz Keldysh effect seen

below the bandgap in semiconductors with built in or applied field. 30 Franz Keldysh

effect is a shift of wavelength toward longer wavelength - smaller energy gap,

showing oscillations, when a strong electric field is applied. It has been shown in

GaAs/AIGaAs quantum well stru

solar cells based on cdte thin film and composite of organic and inorganic nano-scale materials

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