Advanced electron beam techniques for solar cell characterization

ADVANCED ELECTRON-BEAM BASED TECHNIQUES
FOR SOLAR CELL CHARACTERIZATION







MENG LEI
(B. Eng. (Hons.), NUS)




A THESIS SUBMITTED
FOR THE DEGREE OF DOCTOR OF PHILOSOPHY


DEPARTMENT OF ELECTRICAL AND
COMPUTER ENGINEERING
NATIONAL UNIVERSITY OF SINGPAORE
2014





i
DECLARATION
DECLARATION

I hereby declare that this thesis is my original work and it
has been written by me in its entirety.
I have duly acknowledged all the sources of information
which have been used in the thesis.

This thesis has also not been submitted for any degree in
any university previously.



MENG LEI
04 May 2014


ii
Advanced Electron-Beam Based Techniques for Solar Cell Characterization
Acknowledgements
My first and also my most sincere gratitude goes to my Ph.D. supervisors, Professors
Charanjit Singh Bhatia and Jacob Phang from Department of Electrical and Computer
Engineering, National University of Singapore (ECE, NUS), for their continuous
guidance and support throughout my doctoral studies. Professor Bhatia is someone you
will instantly love and never forget once you meet him. His mentorship has always been
paramount in providing a well-rounded experience consistent with my long-term career
goals. He has given me the freedom to pursue various areas that I am interested in and
has been very supportive in all my Ph.D. projects. Professor Phang had always been
motivating and inspiring me to take up new challenges and had made one of the biggest
difference in my life. His attitude of living every moment to its fullest and his strong
determination has helped me come a long way and will always guide me in future.
My special thanks also go to my Ph.D. mentor, Alan Street, for always being so kind,

helpful and motivating. I have always enjoyed the personal discussion with him and the
time I spent with him during dry runs of my presentations. His technical inputs and
friendly nature has always made me feel at ease with him.
I would like to express my deep gratitude to Professor Armin Aberle, Dr. Bram Hoex and
Dr. Johnson Wong from Solar Energy Research Institute of Singapore (SERIS); and
Professors Aaron Danner and Yang Hyunsoo from Spin Energy Lab (SEL). The
discussion and suggestions from them are always valuable to me. My special appreciation
goes to Johnson for his kind help in reviewing my thesis chapters on short notices.



iii
Acknowledgements
I am very much thankful to Dr. Steven Steen, Dr. Satyavolu S. Papa Rao, Dr. Ron Nunes
and Dr. Harold Hovel from IBM Thomas J. Watson Research Centre for their valuable
support and collaboration with Professor Bhatia (ECE, NUS) during the period of NUS-
IBM Joint Study Agreement # W0853529. It provided me with the unique opportunity to
gain a wider breadth of research experience while I was still a graduate student.
I would like to thank the ECE and SERIS for offering me the NUS Research Scholarship
as well as equipment support during my Ph.D. candidature.
My acknowledgement will never be complete without the special mention of my lab
seniors at the Centre of Integrated Circuits Failure Analysis and Reliability (CICFAR):
Dr. Xie Rongguo, Dr. Hao Yufeng, Dr. Huang Jinquan, Dr. Wong Chee Leong, Dr. Jason
Teo, Dr. Zhang Huijuan, Dr. Pi Can, Dr. Wang Ziqian, Dr. Wang Rui and Dr. Ren Yi for
all their personal and professional help during the initial days of my stay in the lab. I
would also like to extend my sincere thanks to Mrs. Ho, Mr. Koo and Linn Linn for
keeping a friendly and healthy lab atmosphere and bearing with me all these days.
I am grateful to my fellow lab mates and friends: Liu Dan, Yihong, Jiayi, Wei Sun, Bai
Xue, Yuya, Yunshan, Dr. York Lin, Dr. Ma Fusheng, Baochen, Mridul, Fajun, Cangming,
Yang Yue for always being there and bearing with me for the good and bad times during

the wonderful days of my Ph.D. life. I find myself lucky to have friends like them.
Finally, I would like to acknowledge my parents, grandparents and all elders to me in my
family for their constant support and strong faith in me. I cannot imagine a life without
their love and care.

iv
Advanced Electron-Beam Based Techniques for Solar Cell Characterization
Table of Contents
DECLARATION i
Acknowledgements ii
Table of Contents iv
Abstract vii
List of Figures viii
List of Tables xiv
List of Symbols xv

Chapter 1 Introduction and Motivation 1
1.1 Photovoltaic Technology and Challenges 1
1.2 Current Characterization Techniques for Solar Cells 3
1.3 Strengths of Electron-Beam Based Techniques 4
1.4 Organization of thesis 5

Chapter 2 Theory and Literature Review 7
2.1 Introduction 7
2.2 Electron Beam and Sample Interaction 7
2.3 Secondary Electron Imaging in SEM 9
2.4 Scanning Electron Acoustic Microscopy (SEAM) 10
2.4.1 Physical Principles 10
2.4.2 Applications of SEAM 14
2.5 Conventional Electron Beam Induced Current (EBIC) 19

2.5.1 Physical Principles 19
2.5.2 Applications of EBIC Imaging 20
2.5.3 Quantitative EBIC Measurements 24
2.6 Single Contact Electron Beam Induced Current (SCEBIC) 41
2.6.1 Physical Principles 41
2.6.2 Applications of SCEBIC 44



v
Table of Contents
2.6.3 Limitations and Challenges of SCEBIC 45
2.7 Strength and Challenges for Solar Cell Characterization 46

Chapter 3 Experimental Setup 48
3.1 Introduction 48
3.2 Experimental Setup (SEAM, EBIC and SCEBIC) 49
3.3 Summary 52

Chapter 4 SEAM Imaging on SDE Multicrystalline Silicon Wafers 53
4.1 Introduction 53
4.2 SEAM Signal Detection 55
4.3 Sample Procedures of Saw Damage Etch (SDE) 56
4.4 Defect Characterization of Saw-Damage-Etched Wafers 56
4.5 Optimization of SDE Duration 60
4.6 Summary 64

Chapter 5 Defect Characterization of Solar Cells 65
5.1 Morphological and Electrical Defects in Multicrystalline Silicon Solar Cells 65
5.1.1 Principle of Signal Detection 65

5.1.2 Defect Characterization in Isolation Trenches 68
5.1.3 Distinguishing Morphological and Electrical Defects 70
5.2 Defect Characterization of Amorphous Silicon (a-Si:H) Thin Film Solar Cells 76
5.2.1 Device Fabrication and Performance 77
5.2.2 Defect Characterization Using LBIC Imaging and FIB Cross-Sectioning . 79
5.3 Studies of Photon Emission at Defects in Multicrystalline Silicon Solar Cells . 90
5.4 Summary 93

Chapter 6 SCEBIC Imaging on Solar Cells 95
6.1 Introduction 95
6.2 SPICE Model of SCEBIC 96
6.2.1 SCEBIC Transient Phenomenon 97

vi
Advanced Electron-Beam Based Techniques for Solar Cell Characterization
6.2.2 Factors of SCEBIC Transient Signals 99
6.3 Experimental Verification of SCEBIC Model 101
6.4 SCEBIC Imaging on Multicrystalline Silicon Solar Cells 104
6.5 SCEBIC Imaging on Partially-Processed Solar Cells 106
6.6 Summary 107

Chapter 7 Extraction of Surface Recombination Velocity 108
7.1 Introduction 108
7.2 One-dimensional Numerical Approach for SRV 110
7.3 Three-dimensional Simulative Approach for SRV 115
7.4 Sample Preparation and Experiment Setup 118
7.5 Results and Discussion 120
7.6 Summary 127

Chapter 8 Conclusions 129

8.1 Summary 129
8.2 Future Work 131

References 134
Appendix A: List of Publications 148





vii
Abstract
Abstract
This dissertation presents a detailed comparative study of advanced electron-beam based
techniques for solar cell characterization. Firstly, the advantage of the subsurface imaging
of scanning electron acoustic microscopy (SEAM) was utilized to characterize the
structural properties of saw-damage-induced defects and the non-destructive nature of
SEAM could enable accurate optimization of saw-damage etch process duration. SEAM
was also employed together with electron beam induced current (EBIC) to investigate
defects in photovoltaic devices. It was found that combination of these two techniques
could provide complementary information that clearly distinguishes the morphological
and electrical nature of the defects. The first demonstration of single contact EBIC
(SCEBIC) on solar cells is then reported and the experimental results were supported
with an analytical model and clearly explained using SPICE simulations. The
requirement on only one contact enables SCEBIC to be performed on partially processed
solar cells, thus allowing a high degree of flexibility of SCEBIC and its potential
applications in photovoltaic industry. Lastly, highly localized quantitative EBIC were
demonstrated to measure surface recombination velocity (SRV) for solar cells with
different surface passivation conditions. A three-dimensional Monte Carlo simulation for
electron-beam sample interaction was first employed to create a three-dimensional carrier

generation profile for accurate modelling of EBIC using Sentaurus TCAD. These
simulation results were then verified using experimental data that were almost perfectly
matching, clearly demonstrating the capability and benefit of the high resolution and
accuracy of quantitative EBIC for the extraction of SRV for solar cells.

viii
Advanced Electron-Beam Based Techniques for Solar Cell Characterization
List of Figures
Figure 2-1. Electron scattering in silicon using CASINO Monte Carlo simulation at an
electron beam energy of 10 keV 9
Figure 2-2. Schematic of SEAM thermo-elastic mode 11
Figure 2-3. Schematic comparison of (a) SEAM (< 1 MHz), whose acoustic wavelength
is longer than the sample thickness, and (b) conventional SAM (~ few GHz), whose
acoustic wavelength is much smaller than the sample thickness. 13
Figure 2-4. (a) Secondary electron (SE) and (b) SEAM images (at 165 kHz) of the
domain structure in Polycrystalline Mn
50
Ni
28
Ga
22
alloy. 15
Figure 2-5. SE images of a multi-level IC (a) before and (b) after removing the top metal
layer; and corresponding SEAM amplitude images prior to the top-down de-processing at
electron beam energy of 30 keV and electron beam modulation frequency of (b) 25 kHz,
(c) 60 kHz, (d) 173.8 kHz and (e) 200 kHz. 16
Figure 2-6. (a) SE image of an IC; and SEAM phase images at modulation frequency of
173.2 kHz and different phases respect with the reference signals when b(1) θ = 40
o
, b(2)

θ = 80
o
, b(3) θ = 100
o
, b(4) θ = 120
o
, b(5) θ = 160
o
17
Figure 2-7. (a) SEAM image taken at 71.9 kHz of a multi-level IC, (b) SE image of the
cross-section of the sample after focus ion beam (FIB) milling at the highlighted location
indicated at the SEAM image. 18
Figure 2-8. EBIC images of (a) a continuous junction; and (b) a discontinuous junction
regions created by different laser diode currents. 21
Figure 2-9. Temperature dependence of EBIC contrasts of dislocations for different
concentrations of contaminating impurities 22
Figure 2-10. Comparison of EBIC (30 keV) and band-to-band luminescence or SiPHER
(532 nm) on block-cast mc-Si. 23



ix
List of Figures
Figure 2-11. Schematic of excess-charge collection geometries of a p-n junction. (a)
normal; and (b) planar geometries 25
Figure 2-
12. Schematic of an ideal point source at z = ξ within a semi
-infinite
semiconductor that has a planar surface at z = 0 29
Figure 2-

13. Schematic of an ideal point source at z = ξ within a semi
-infinite
semiconductor that has a planar surface at z = 0. A p-n junction is inserted parallel to the
semiconductor surface at z = z
j
. 31
Figure 2-14. Δp(z) as a function of S for steady-state uniform electron-beam excitation
when (a) ξ/L
p
= 1, i.e. ξ ≈ L
p
, (b) ξ/L
p
= 3, i.e. ξ >> L
p
, and (c) ξ/L
p
= 0.3 35
Figure 2-15. Schematic of EBIC measurement on a p-n junction solar cell 37
Figure 2-16. Experimental results on the effective diffusion length, L
eff
versus the
penetration depth, ξ of the electron beam in GaAs p-n junction structure. 38
Figure 2-17. Schematic of electron-hole pair excitation in a p-n junction structure 40
Figure 2-18. A plot of EBIC current versus ξ for a planar p-n junction silicon device 40
Figure 2-19. Schematic of (a) conventional EBIC and (b) SCEBIC configurations 42
Figure 2-20. A typical SCEBIC transient of IC when electron beam is turned on at a
modulation frequency of 1 kHz 44
Figure 2-21. (a) EBIC image of a CMOS transistor array with connections to power pins
V

dd
and V
ss
; and (b) SCEBIC image of the same device with substrate as single contact.
45
Figure 3-1. Overview of the electron-beam based characterization techniques: (a)
Conventional EBIC, (b) single contact EBIC (SCEBIC), and (c) SEAM. 50
Figure 3-2. Block diagram of the experimental setup of EBIC and SEAM 51
Figure 3-3. Modification of the setup for single-contact EBIC (SCEBIC) 52

x
Advanced Electron-Beam Based Techniques for Solar Cell Characterization
Figure 4-1. SEAM signal detection from a mc-Si wafer 55
Figure 4-2. (a) SE and (b) SEAM images of a mc-Si wafer after SDE for 30 minutes. 57
Figure 4-3. (a) SE image of a mc-Si wafer after SDE for 5 minutes; (b) electron
backscatter diffraction (EBSD) grain map on the mc-Si wafer; and SEAM images of the
wafer at modulation frequencies of (c) 83 kHz, (d) 99 kHz, (e) 217 kHz and (f) 377 kHz.
59
Figure 4-4. (a) SE image and (b) SEAM image of a mc-Si wafer as-cut after sawing; (c)
SE image and (d) SEAM image of mc-Si wafer after saw-damage etch (SDE) for 20
minutes; (e) SE image and (f) SEAM image of wafer after SDE for 90 minutes. 61
Figure 4-5. (a) SE image and (b) SEAM image of another location of the mc-Si wafer as-
cut; (c) SE image and (d) SEAM image of mc-Si wafer after SDE for 20 minutes; (e) SE
image and (f) SEAM image of the wafer after SDE for 90 minutes. 63
Figure 5-1. SEAM and (b) EBIC signal detection of the mc-Si solar cell 67
Figure 5-2. (a) SE image at 30 keV. The parallel lines are the aluminium contacts. (b)
EBIC image at 20 keV; (c) SEAM amplitude image at 30 keV and modulation frequency
of 261 kHz 68
Figure 5-3. SE image and (b) EBIC image at 20 keV; (c) SEAM amplitude image and (d)
SEAM phase image at 30 keV and modulation frequency of 363 kHz 69

Figure 5-4. (a) SE image and (b) SEAM amplitude image of p- Si wafer before junction
formation at 30 keV and modulation frequency of 292 kHz. 70
Figure 5-5. (a) SE image at 30 keV; (b) EBIC image at 20 keV; (c) SEAM amplitude
image and (d) SEAM phase image at 30 keV and modulation frequency of 550 kHz. 71
Figure 5-6. (a) EBIC image; and (b) SEAM amplitude image at 30 keV and modulation
frequency of 391 kHz; (c) Line profile at X-X’ and Y-Y’ of EBIC and SEAM images. 73
Figure 5-7. (a) EBIC image; and (b) SEAM amplitude image at 30 keV and modulation
frequency of 292 kHz; (c) Line Profile at Z-Z’ of both EBIC and SEAM images. 74



xi
List of Figures
Figure 5-8. SE image of the cross-section at the location of the line-like defect. 75
Figure 5-9. Schematic of a-Si:H thin film solar cell 78
Figure 5-10. Schematic of LBIC imaging setup 80
Figure 5-11. LBIC images of (a) Sample 1; (b) Sample 2; and (c) Sample 3. 81
Figure 5-12. (a) LBIC image at the edge of Sample 1; and (b) LBIC line profile across
AA’. SE images of the cross section at (c) Location a1; and (d) Location a2 and a3. 83
Figure 5-13. (a) SE image; and LBIC images of Sample 2 (b) before and (c) after the FIB
cross-sectioning across BB’ and CC’. LBIC line profiles (d) at BB’; and (e) at CC’. 85
Figure 5-14. (a) FIB cross-sectioning across BB’ and CC’ shown in Figure 5-13. SE
images of these cross sections with different LBIC contrasts indicated in Figure 5-13(c)
and (d) at (a) Location b1; (b) Location c1; and (c) Location c2. 86
Figure 5-15. LBIC images of defective area in Sample 3 (a) before and (b) after the FIB
cross-sectioning across DD’ and EE’. (c) LBIC line profiles at DD’ and EE’. 87
Figure 5-16. FIB cross-sectioning at DD’ and EE’ shown in Fig. 6. SE images of the
cross section at (a) Location d1; (b) Location d2; (c) Location d3; and (d) Location e1. 88
Figure 5-17. (a) Optical and (b) Electroluminescence images of the mc-Si solar cell 91
Figure 5-18. (a) SE and (b) EBIC images of the mc-Si solar cell at the location that is

highlighted in red-dashed box in Figure 5-17. 92
Figure 5-19. (a) EL image; and the corresponding LBIC images of the device at a
wavelength of (b) 1064 nm; and (c) 633 nm 92
Figure 6-1. (a) SCEBIC configuration; (b) its equivalent circuit diagram of a typical p-n
junction solar cell 96
Figure 6-2. SPICE simulation of the SCEBIC transient response I
SCEBIC
(t) at a modulation
frequency of 200 Hz, where I
g
= 100 µA, R
sh
= 5 kΩ, R
s
= 1Ω, C
j
= 200 nF, C
s
= 100 pF.

xii
Advanced Electron-Beam Based Techniques for Solar Cell Characterization
The values assigned for each parameter are typical for solar cells with a sample size of
about 1 cm
2
. 98
Figure 6-3. SCEBIC transient characteristics of a typical single-junction solar cell using
SPICE simulations with the same model parameters as Figure 6-2, by varying only one
parameter each time: (a) shunt resistance R
sh

; (b) parasitic capacitance C
s
; (c) junction
capacitance C
j
; and (d) generation current I
g
. 99
Figure 6-4. Comparison of experimental (blue stars) and simulated (red-solid line)
SCEBIC transient responses to a pulsed electron beam (black dash) at the electron beam
energy of 30 keV and the modulation frequency of 60 Hz 101
Figure 6-5. Comparison of experimental single contact laser beam induced current
(SCLBIC) (red dot) and the same SCEBIC (blue dot) transient responses. 103
Figure 6-6. Experimental SCEBIC (colour dots) transient responses with different values
of generation current I
g
at the electron beam energy of 30 keV and the modulation
frequency of 60 Hz 104
Figure 6-7. (a) SE image; (b) conventional EBIC image; and SCEBIC image (c) without
and (d) with a metal enclosure (at 5.7 kHz electron beam modulation frequency) of a mc-
Si solar cell. All the images are taken at 30 keV electron beam energy. 105
Figure 6-8. SCEBIC image of the same mc-Si solar cell after removing the bottom
contact. The image was taken with a metal enclosure at the electron beam energy of 30
keV, and modulation frequency of 5.7 kHz. 106
Figure 7-1. Theoretical charge collection efficiency as a function of electron beam
penetration depth. L
diff
and z
j
are assumed to be 1 μm and 0.5 μm, respectively. 114

Figure 7-2. Electron-beam energy dissipation volume in silicon at 10 keV simulated by
CASINO software programme. 116
Figure 7-3. Schematic illustration of the cross-section of the (a) fully passivated and (b)
partially passivated bifacial n-type monocrystalline silicon (mono-Si) solar cell device
structure and the I
EBIC
connections. 117



xiii
List of Figures
Figure 7-4. The simulated carrier generation profiles (in the x-z plane when y = 0)
resulting from a 5 keV electron beam for an n-type silicon wafer solar cell (a) with, and
(b) without 80 nm AlO
x
/SiN
x
surface passivation film. The electron beam current and
radius are 300 pA and 10 nm, respectively. 118
Figure 7-5. Mono-Si solar cells after hot phosphorus acid bath for durations of (a) 0
minute, (b) 2 minutes, (c) 5 minutes, and (d) 10 minutes. 119
Figure 7-6. (a) SE and (b) EBIC images of the partially etched mono-Si solar cell. The
white region at the bottom left in the SE image and the corresponding dark region in the
EBIC image is the metal finger. 120
Figure 7-7. Plan view (a) secondary electron (SE) and (b) plan-view EBIC images of the
textured monocrystalline silicon solar cell at electron beam energy of 5 keV. Cross-
sectional view of (c) SE image, and (d) SE and EBIC overlap image of the same sample
at electron beam energy of 3 keV 121
Figure 7-8. Comparison of experimental and simulated EBIC gain (I

EBIC
/I
beam
) for the
solar cell without passivation. Surface recombination velocity is equal to the maximum-
possible value of 10
7
cm/s 123
Figure 7-9. Simulated EBIC gain (I
EBIC
/I
beam
) as a function of electron beam energy for a
n-type silicon wafer solar cell (a) with and (b) without a 80 nm AlO
x
/SiN
x
surface
passivation film, assuming various values of SRV. 125
Figure 7-10. Comparison of experimental and simulated EBIC gain (I
EBIC
/I
beam
) for the
solar cell with AlO
x
/SiN
x
passivation. Surface recombination velocity is 2.8 × 10
5

cm/s.
126


xiv
Advanced Electron-Beam Based Techniques for Solar Cell Characterization
List of Tables
Table 5-1. Summary of performance of the three solar cell samples 79





xv
List of Symbols
List of Symbols
C
j
Zero-bias junction capacitance
C
s
Parasitic capacitance
C
th
Specific heat
d
T
Thermal diffusion length
D Diffusion coefficient
E

b
Energy of electron beam (keV)
f Modulation frequency of electron beam
G Total charge induced due to SCEBIC phenomenon
G
eff
Effective generation strength
G
o
Generation strength of electron beam
I
b
Incident beam current
I
EBIC
Electron beam induced current (EBIC) signals
I
Diff
Diffusion current
I
g
Electron beam generation current
I
m
(t) Current transient as a function of time
I
max
Maximum value of current transient
I
s

Diode saturation current
I
sc
Short-circuit current
I
SCEBIC
Single contact electron beam induced current (SCEBIC) signals

xvi
Advanced Electron-Beam Based Techniques for Solar Cell Characterization
J
h
Current density of holes
J
sc
Short-circuit current density
k
1
, k
2
Constant
K Thermal conductivity of the material
L
b
Minority carrier bulk diffusion length
L
diff
Minority carrier diffusion length
n Diode ideality factor
Δp Excess minority carrier density (holes)

R Recombination rate
R
G
Gruen range of electron
R
s
Series resistance of solar cell
R
sh
Shunt resistance of solar cell
v
s
Surface recombination velocity
V
dd
Positive DC supply voltage terminal
V
oc
Open-circuit voltage
V
ss
Ground terminal (substrate of IC)
x Distance between electron beam and p-n junction
z
j
Depth of p-n junction
kT/q Thermal voltage
ξ Depth of electron-beam penetration
 Material density (gm/cm
3

)
τ Minority carrier lifetime



1
Introduction and Motivation
Chapter 1 Introduction and Motivation
1.1 Photovoltaic Technology and Challenges
Sunlight [1] is a vastly abundant and relatively evenly distributed source of renewable
energy. The key techniques to harvest solar energy include photovoltaics (PV) that use
solar cells to convert sunlight to electricity [2], solar-thermal relying on solar thermal
collectors to convert sunlight to heat [3], and solar-chemical, which uses photochemical
cells to collect sunlight to sustain chemical reactions such as electrolysis [4]. Compared
to the combustion of fossil fuels, these technologies are far less impactful on the
environment, especially in terms of the air pollutants and greenhouse gases emitted
during their life cycles. Despite holding great promise, solar energy, especially PV, is still
considered uneconomical due to its relatively low electricity-generating capacity, and
high production and installation costs. It currently contributes less than 1% to the global
energy supply [5]. The solar energy industry has thus been focusing on cost reduction to
add more competitive advantages to the various PV technologies. In particular, owing to
the growing environmental concerns and governmental supports for “green” energy,
interest and effort on PV research has intensified and significant developments have been
achieved recently. This has made the PV technologies a more economically viable
alternative energy option [6-10].
The main focus of the photovoltaic industry over the past decades has been on bulk
silicon crystalline solar cells, known as the “first generation” wafer-based technology.
Recently, major investments have been made in new manufacturing facilities for

2

Advanced Electron-Beam Based Techniques for Solar Cell Characterization
monocrystalline and multicrystalline wafer based solar cells, as well as for the closely
related silicon ribbon and sheet approaches. A “second generation” of thin-film solar cell
technology has also emerged during the past 15 years [11-13]. Thin-film solar cells offer
strong advantage as a major reduction in material cost by eliminating the need for
expensive wafer substrates. Thin film solar cells also offer other benefits, particularly in
terms of the increase in the unit of manufacturing from a wafer of about 100 cm
2
to a
glass sheet of about 1 m
2
. Thin film solar cells, however, suffer from substantially lower
energy conversion efficiencies as compared to those of bulk silicon crystalline cells.
The Carnot limit on the conversion of sunlight to electricity is 95% as opposed to the
theoretical upper limit of 33% [14] for a standard solar cell. This suggests that the
performance of solar cells could be further improved 2 - 3 times if different concepts are
used to produce a “third generation” of high-performance cells [15, 16]. For example,
novel structural design was employed to produce what is best known as a tandem cell,
where efficiency can be increased by adding more cells of different band gaps to a stack,
although it remains to be seen if this approach is economically viable for large scale
deployment. Apart from structural approaches, efficiency of a PV device can also be
enhanced through the use of new materials. Regardless of the approach, be it structural or
material or a combination of both, the success in enhancing PV efficiencies relies on a
clear understanding of the PV materials, optimization of the material quality and the
fabrication processes. All these require the right characterization techniques that are
effective and efficient in providing the corresponding useful information that would aid
in achieving PV cells with better conversion efficiencies.

3
Introduction and Motivation
1.2 Current Characterization Techniques for Solar Cells
According to the “Shockley-Queisser (SQ) limit” [14], also termed as the “radiative
recombination limit” for PV energy conversion, the efficiency of an ideal bandgap solar
cell whose charge carriers undergo solely radiative recombination is 33%. For real
devices, additional recombination channels come into play which further reduces the
efficiency. Ultimately, the performance of solar cells is limited by material properties
such as defect density [17], and p-n junction related parameters [18] like minority carrier
lifetime and surface recombination velocity. The abilities to characterize and optimize
these parameters are thus critical in achieving a cost effective PV process. There has been
much attention devoted in developing characterization procedures of solar cells recently
through a variety of techniques that include luminescence imaging [19, 20], which has
the advantage of high throughput and large-area imaging [21]; quasi-steady-state
photoconductance (QSSPC) [22] that is a widely used for the determination of minority-
carrier lifetime in semiconductor and trapping effects at a precisely calibrated carrier
density [23]; and surface photovoltage (SPV) [24], which calculates the minority-carrier
diffusion length and surface recombination velocity by measuring the illumination
induced change of the photovoltage at the surface of solar cells [25]. While these
techniques are straightforward and capable of large throughput characterization, they are
of relatively poor resolution due to the large spot size of the photon beam, and thus may
not be suitable for accurate analysis of microscopic material properties that are often the
fundamental origin behind a specific material characteristic.

4
Advanced Electron-Beam Based Techniques for Solar Cell Characterization
1.3 Strengths of Electron-Beam Based Techniques
Apart from the various methods above-mentioned, there has been an increasing trend of
extending electron-beam based techniques for characterization of PV material properties,
such as carrier recombination activities within defects, and device properties like excess-

carrier generation and illuminated current-voltage characteristics [26-30]. Electron-beam
based techniques rely on the bombardment of electrons [31] and their interaction with the
materials. Owing to the relatively small probe size of the electron beam (about few
nanometres), these techniques can be applied at localized areas of the device with a much
higher resolution than the conventional PV characterization methods that use photon as
an excitation source. For instance, electron beam induced current (EBIC), a technique
traditionally used for integrated circuit (IC) failure analysis, makes use of p-n junctions or
Schottky barriers to collect the charge carriers generated as a result of the electron-beam
interaction [32] within the selected scan areas of the devices. The flows of those charge
carriers are subsequently sensed as a current as a function of position, allowing accurate
mapping of recombination centres and electrically-active-defect locations of the devices.
Conventional EBIC techniques require electrical contacts to both sides of the p-n junction
for the measurement of the induced current so as to form a closed-loop in the external
circuit. This requirement poses a severe limitation to the application of EBIC imaging on
partially processed devices. In order to overcome this limitation, single contact EBIC
(SCEBIC) has been developed [33] for IC failure analysis. As the name suggests,
SCEBIC requires only one connection to the device and thus has the potential to
characterize a partially processed solar cell as long as one side of electrode is formed.
Moreover, another electron-beam based technique, scanning electron acoustic



5
Introduction and Motivation
microscopy (SEAM), which is based on the detection of electron acoustic signals [34]
generated within the materials by a periodic intensity-modulated electron beam, is a well-
established technique used for subsurface defect imaging and depth discrimination of
multi-level ICs.
While EBIC, SCEBIC and SEAM have been proven to be versatile techniques in IC and
other material systems, application of these techniques on solar cells remain relatively

less established. This project aims to adapt EBIC, SCEBIC and SEAM from the IC world
to the solar cells in the PV industry and to develop them for in-depth characterization
with both qualitative and quantitative analysis, where conventional EBIC techniques can
be employed for both material and device analyses and SCEBIC and SEAM methods can
be applied prior to the completion of the solar cells. Successful application of these
techniques on solar cells could offer a great potential in establishing the links between the
material properties and the performance of the corresponding photovoltaic devices made
from them.
1.4 Organization of thesis
This thesis consists of eight chapters describing a detailed comparative study of different
electron-beam based techniques such as EBIC, SCEBIC and SEAM for solar cell
characterization.
Following the present chapter (Chapter 1) on the background and motivation of the
project, Chapter 2 gives a detailed literature survey together with an in-depth discussion
on the theories and working principles of the key electron-beam based characterization

6
Advanced Electron-Beam Based Techniques for Solar Cell Characterization
techniques mentioned in this thesis whereas Chapter 3 describes briefly the overall
system design and setup of the key techniques, i.e. EBIC, SCEBIC and SEAM as well as
their typical imaging results of solar cells.
There are four main chapters that discuss the experimental findings. SEAM imaging on
silicon wafers processed at various solar cell fabrication stages is first presented in
Chapter 4. This is followed by an in-depth discussion on novel applications of EBIC,
SEAM and other complementary techniques on defect characterization of solar cells in
Chapter 5. Chapter 6 presents the first demonstration of SCEBIC imaging on solar cells
with a detailed theoretical explanation on its physical phenomenon and current
characteristics. The results are also supported with an analytical model that is verified by
SPICE simulations, which show a close match with the experimental results. The last
technical chapter (Chapter 7) illustrates the application of the EBIC technique for

quantitative analysis. A three-dimensional simulation is employed to fit the measured
EBIC data as a function of electron-beam energy, for the extraction of surface
recombination velocity of solar cells. The experimental data matches very well with the
simulation results.
Last but not least, Chapter 8 summarizes the findings reported in this project. The thesis
then concludes by suggesting a number of possible directions for future works.




7
Theory and Literature Review
Chapter 2 Theory and Literature Review
2.1 Introduction
The scanning electron microscope (SEM) is one of the most widely used scientific
instruments for characterizing morphology and topography of materials in various
technology fields ranging from engineering and physics to biology and medicine etc. For
morphology and topography imaging, SEM relies on the mechanism of secondary
electron emission from the sample surface as a result of the interaction between the
highly energetic electrons and the sample. Other than providing secondary electrons for
morphology and topography analysis, the interaction between the highly energetic
electrons and the sample also gives rise to other useful information/signals, whose
manifestation can be exploited for other applications. In this chapter, advanced electron-
beam based characterization techniques including scanning electron acoustic microscopy
(SEAM), electron beam induced current (EBIC) and single contact EBIC (SCEBIC) are
discussed in detail. When applied on solar cell characterization, these techniques are
capable for detailed localized analysis with a relatively high resolution given the small
probe size of the electron beam. In addition, these imaging techniques are often
performed simultaneously with the conventional SEM imaging procedure, thus allowing
meaningful comparison or correlation with the secondary electron (SE) images.

2.2 Electron Beam and Sample Interaction
The essence of electron beam-sample interaction is the generation of electron-hole pairs
within the materials and their subsequent behaviours. In brief, highly focused electrons