Post on 06-Jul-2018
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
1/135
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
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
2/135
UMI Number: 3431234
All rights reserved
INFORMATION TO ALL USERSThe quality of this reproduction is dependent upon the quality of the copy submitted.
In the unlikely event that the author did not send a complete manuscriptand there are missing pages, these will be noted. Also, if material had to be removed,
a note will indicate the deletion.
UMTDissertation Publishing
UMI 3431234
Copyright 2010 by ProQuest LLC.All rights reserved. This edition of the work is protected against
unauthorized copying under Title 17, United States Code.
ProQuest®
ProQuest LLC
789 East Eisenhower ParkwayP.O. Box 1346
Ann Arbor, Ml 48106-1346
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
3/135
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
4/135
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
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
5/135
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
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
6/135
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
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
7/135
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
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
8/135
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
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
9/135
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
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
10/135
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
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
11/135
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
?
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
12/135
LIST OF FIGURES (continued)
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
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
13/135
LIST OF FIGURES (continued)
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
XIl
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
14/135
LIST OF FIGURES (continued)
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
XlIl
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
15/135
LIST OF FIGURES (continued)
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
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
16/135
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
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
17/135
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
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
18/135
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
xvii
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
19/135
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
xviii
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
20/135
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.
XlX
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
21/135
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
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
22/135
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.
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
23/135
?
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
24/135
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
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
25/135
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.
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
26/135
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
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
27/135
7
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
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
28/135
8
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.
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
29/135
9
£3) m
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
30/135
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.
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
31/135
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
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
32/135
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
33/135
¦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
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
34/135
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
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
35/135
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.
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
36/135
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
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
37/135
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)
17
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
38/135
18
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).
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
39/135
19
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.
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
40/135
20
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
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
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
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
42/135
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.
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
43/135
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.
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
44/135
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
45/135
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)
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
46/135
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.
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
47/135
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.
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
48/135
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(
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
49/135
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
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
50/135
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
51/135
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.
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
52/135
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 (
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
53/135
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
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
54/135
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
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
55/135
35
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
56/135
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
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
57/135
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.
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
58/135
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
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
59/135
39
350000
300000
250000 -
-?2 200000(?
·*— '
C3
o
O 150000
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
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
60/135
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.
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
61/135
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
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
62/135
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
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
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
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
64/135
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
65/135
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.
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
66/135
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%.
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
67/135
47
Figur® 18. Demonstration of CdTe/CdS solarceli.
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
68/135
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.
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
69/135
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
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
70/135
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
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
71/135
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.
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
72/135
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¿
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
73/135
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 ,
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
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.
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
75/135
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
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
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.
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
77/135
57
MTi
1 ¦*/ ,..¦ Í -;
Figure 23. Surface view of SEM image of grown PbSe nanowires revealed the form
of pyramids.
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
78/135
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.
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
79/135
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.
o ©O ©
°*®?.??·©?? ô
OT o
O ? «
O0O0O0O ?»???°©
?°?? © ? O O
Figure 26. Atomic position map, rock-salt cubic structure inside small box
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
80/135
8/17/2019 Solar Cells Based on CdTe Thin Film and Composite of Organic and Inorganic Nano-Scale Materials
81/135
61
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