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The influence of CdSe surface ligands on hybrid inorganic/organic solar cell performance

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Content i

The Influence of CdSe Surface Ligands on Hybrid Inorganic/Organic Solar Cell
Performance

by

Blair A. Combs

A Thesis Presented to the FACULTY OF THE
USC GRADUATE SCHOOL UNIVERSITY OF
SOUTHERN CALIFORNIA
In Fulfillment of the
Requirements for the Degree
MASTER OF SCIENCE
(CHEMISTRY)

December 2014

Copyright 2014 Blair A. Combs
ii

Acknowledgements

I would like to thank my research advisor Professor Richard Brutchey for all of the advice
and support given. He has instilled many qualities in me that I will continue to utilize in
my future endeavors. I also thank Professor Surya Prakash and Professor Mark
Thompson for serving on my committee. Professor Thompson and his group have
provided assistance that has made this work possible. Finally, I thank all of the Brutchey
members past and present that were extremely helpful and a pleasure to work with.
iii

1. Introduction 1
2. Results and Discussion

2
2.1 Colloidal Ligand Exchange 2
2.2 Photovoltaic Effects from Ligands 5
2.2.1. Jsc Effect 11
2.2.2. Voc Effect 13
2.2.3. Morphology 16
3. Conclusion 21
4. Experimental 22
4.1 General Consideration 22
4.2 Synthesis of Cadmium Selenide Native Ligand (CdSeNL) 22
4.3 Ligand Exchanges 23
4.3.1. Pivalic Acid (PA) Exchange 23
4.3.2. Pyridine (Py) Exchange 24
4.3.3. Butylamine (BA) Exchange 24
4.3.4. tert-butylthiol (tBT) Exchange 25
4.3.5 Thiophenol (TP) Exchange 25
4.3.6 Tetrahydrothiophene (THT) Exchange 25
4.4. Device Fabrication 26
4.5. Characterization 27
5. References 28

Table of Contents
Acknowledgements ii
List of Tables and Equations iv
List of Figures v

iv

List of Tables and Equations

Table 1: The reported average and standard deviation of the device
parameters for 12-16 devices over multiple substrates 10

Table 2: Estimated Ered, HOMO/LUMO levels and ΔEDA,CV are
shown. The overall trends for Voc from I-V and ΔEDA,CV
from CV are well-correlated. 15

Equation 1: The diameter of the NCs was found by using the first
absorption peak wavelength for λ. 2

Equation 2: Estimated ELUMO values were obtained using the CV
determined Ered value. 14

v

List of Figures

Figure 1: Absorbance spectrum and TEM image of cadmium selenide
native ligand nanocrystal suspension in toluene are shown.
The histogram displays the size distribution of 100 CdSe NCs
that were found to have an average size diameter of 5.44 ± 0.28
nm, which is in agreement with the UV-Vis analysis. 3

Figure 2: Thermogravimetric analysis traces for (a) carboxylic acids,
(b) amines, and (c) sulfur-containing functionalized cadmium
selenide nanocrystals. 4

Figure 3: Fourier-transform infrared spectra for CdSeX (X = NL, PA, Py,
BA, tBT, TP, and THT) 4

Figure 4: The optimized device conditions were found by comparing
device parameters in (a), (b), (c), and (d) for PA hybrid
devices at different processing conditions. 6

Figure 5: The optimized device conditions were found by comparing
device parameters in (a), (b), (c), and (d) for Py hybrid
devices at different processing conditions. 6

Figure 6: The optimized device conditions were found by comparing
device parameters in (a), (b), (c), and (d) for BA hybrid
devices at different processing conditions. 7

Figure 7: The optimized device conditions were found by comparing
device parameters in (a), (b), (c), and (d) for tBT hybrid
devices at different processing conditions. 8
vi

Figure 8: The optimized device conditions were found by comparing
device parameters in (a), (b), (c), and (d) for TP hybrid
devices at different processing conditions. 8

Figure 9: The optimized device conditions were found by comparing
device parameters in (a), (b), (c), and (d) for THT hybrid
devices at different processing conditions. 9

Figure 10: Photoluminescence quenching shown for annealed neat P3HT
films and their annealed hybrid counterparts. Neat P3HT 175°C
(black), 200°C (gray), 225°C (Purple), and 250°C (wine) are
shown. The hybrid films shown are 18:3 CdSe(PA):P3HT
(200°C, red), 18:3 CdSe(Py):P3HT (175°C, green),
18:3 CdSe(BA):P3HT (200°C, blue), 12:3 CdSe(tBT):P3HT
(175°C, cyan), 09:3 CdSe(TP):P3HT (225°C, magenta),
and 06:3 CdSe(THT):P3HT (250°C, dark yellow). 11

Figure 11: I-V curves for optimized devices made with carboxylic
acid (PA), amines (Py, BA), and sulfur-containing
ligands (tBT, TP, THT). 12

Figure 12: External quantum efficiencies for optimized devices made with
(a) carboxylic acid (PA), (b) amines (Py, BA), and (c) sulfur-
containing ligands (tBT, TP, THT). 13

Figure 13: Cyclic voltammetry of CdSeX (X = NL, PA, Py, BA, tBT,
TP, THT) after ferrocene/ferrocenium redox couple corrections.
The reduction energy was measured from the onset of the
reduction potential wave. 14

vii

Figure 14: Dark I-V characteristics for each optimized hybrid device. 16

Figure 15: TEM images of (a) CdSe(NL):P3HT and (b) CdSe(PA):P3HT
annealed films show the difference between morphology. 17

Figure 16: TEM images of (a) CdSe(BA):P3HT and (b) CdSe(Py):P3HT
annealed films display a clear difference in morphology. 17

Figure 17: TEM images of (a) CdSe(tBT):P3HT, (b) CdSe(TP):P3HT,
and (c) CdSe(THT):P3HT annealed films reveal the difference
between morphology. 18

Figure 18: (a) Topological image of 18:3 CdSe(PA):P3HT was obtained
in a 5 μm x 5 μm window with a 123 nm maximum profile
height and an RMS roughness of 1.5 nm. (b) 3-D plot of
the PA hybrid film interface. 18

Figure 19: (a) Topological image for 18:3 CdSe(Py):P3HT hybrid film
was obtained in a 5 μm x 5 μm window with a 47 nm maximum
profile height and an RMS roughness of 0.86 nm. (b) 3-D plot
of the Py hybrid interface. (c) Topological image of 18:3
CdSe(BA):P3HT hybrid film was obtained in a 5 μm x 5 μm window
with a 222 nm maximum profile height and an RMS roughness of 5.8
nm. (d) 3-D plot BA hybrid film. 19

Figure 20: (a) Topological image for 12:3 CdSe(tBT):P3HT was obtained in a 5
μm x 5 μm window with a 91 nm maximum profile height and
an RMS roughness of 3.9 nm. A 3-D plot of the tBT hybrid film
is shown below. (b) Topological image of 09:3 CdSe(TP):P3HT was
obtained in a 5 μm x 5 μm window with a 40 nm maximum profile
height and an RMS roughness of 1.5 nm. A 3-D plot of the TP
viii

hybrid film is shown below. (c) Topological image of 06:3
CdSe(THT):P3HT was obtained in a 5 μm x 5 μm window with
a 54 nm maximum profile height and an RMS roughness of 1.84 nm.
A 3-D plot of the THT hybrid film is shown below. 20

1

The Influence of CdSe Surface Ligands on Hybrid
Inorganic/Organic Solar Cell Performance

1. Introduction

In recent years hybrid inorganic/organic bulk heterojunction (BHJ) solar cells have
attracted increasing attention because of their beneficial characteristics such as low
manufacturing costs via solution processing methods (e.g.“roll-to-roll” processing, ink-
jet printing and screen printing)
[1]
as well as the ability to produce lightweight and
flexible devices
[2]
. Furthermore, using inorganic semiconducting nanocrystals (NCs) as
the electron accepting phase provides high intrinsic carrier mobility
[3]
, tunable band gap
energies and absorption profiles due to quantum confinement effects
[4]
, and high
dielectric constants
[5]
. However, the champion power conversion efficiency (PCE) for
these hybrid systems is significantly lower than organic solar cells that reach PCEs up to
12%
[6]
. This substantial gap in PCE is in part due to charge transport limitations from
the electronically insulating native ligand shells
[7]
, surface traps or midgap states
[8]
, and
poor interparticle coupling
[9]
. Ligand exchanges with carboxylic acid
[11]
, amine
[12]
, and
sulfur-containing
[13]
functional groups have been successfully employed to replace the
long chain native ligand shell leading to improved charge transport in addition to other
beneficial effects such as passivation of trap states and increased electronic coupling.
These effects have a substantial influence on the overall device performance. However,
there is no fair and simple way to compare the effects that these commonly reported
ligands are having on overall device performance to the best of our knowledge.
Comparisons of ligand effects on PCE tend to be misleading in the current literature
2

because of differences in polymer batches, NC syntheses, environmental conditions,
device processing conditions, and inconsistencies in ligand exchanges.
Herein, we present a systematic study of commonly reported ligands and their influence
on PCE in poly(3-hexythiophene-2,5-diyl) (P3HT):CdSe NC hybrid BHJ solar cells
fabricated under conditions based on individually optimized loading ratios and annealing
temperatures. The optimization of each ligand exchange as well as loading ratio and
annealing temperature for each device ensures a reliable comparison that can be used to
investigate the contributions each ligand has on device performance. As a result, this
study can provide assistance in choosing an ideal ligand for specific design criteria and
serve as a benchmark for future device performances.

2. Results and Discussion

2.1 Colloidal Ligand Exchange

The CdSe NCs were prepared based on literature methods
[11]
. The purified NCs were found
to be approximately 5.44 ± 0.28 nm in diameter as determined by TEM and in agreement
with the diameter calculated from the first exciton peak at 625 nm in the absorption (Fig. 1)
by using the equation
[14]
:

D(nm) = 59.60816 - 0.54736λ + 1.8873 × 10
-3
λ
2
- 2.85743 × 10
-6
λ
3
+ 1.62974 × 10
-9
λ
4
(1)

3

Ligand exchanges were performed using a stock solution of CdSe(NL) in toluene and
refluxed or stirred for a given time. All treatments were followed by further purification
(see experimental section).

400 450 500 550 600 650 700
0.0
0.5
1.0
1.5
2.0
2.5

Absorbance (a.u.)
Wavelength (nm)
CdSeNL
4.4 4.6 4.8 5.0 5.2 5.4 5.6 5.8 6.0 6.2 6.4
0
5
10
15
20
25
30

Count
Diameter (nm)

Figure 1. Absorbance spectrum and TEM image of cadmium selenide native ligand nanocrystal suspension in
toluene are shown. The histogram displays the size distribution of 100 CdSe NCs that were found to have an
average size diameter of 5.44 ± 0.28 nm, which is in agreement with the UV-Vis analysis.

TGA and FT-IR were used to ensure the ligand exchanges were complete and effective.
Figure 2 shows the TGA traces for the CdSe NCs grouped by functionality. The traces
display the effects that increasing temperature has on the lability of the organic ligands until
all have been removed from the surface by 500°C. It can be seen that after every exchange
the amount of organic content has been reduced as evidenced by the lower mass loss
percentage at 500°C, which corresponds to the displacement of oleate and myristate native
ligands. This reduction in the native ligand shell helps increase charge transport in these
NCs
[15]
. Figure 2 shows the total mass loss percentages for the CdSe NCs to be 8% pivalic
acid (PA), 13% for both pyridine (Py) and butylamine (BA), 11% for both tert-butylthiol
(tBT) and thiophenol (TP), and 16% tetrahydrothiophene (THT). FT-IR spectra were
4

obtained for all the functionalized CdSe NCs and an internal standard was used to
normalize all the spectra in order to compare the efficacy of each exchange
[22]
.
150 200 250 300 350 400 450 500
50
60
70
80
90
100
CdSeNL
CdSePA

Mass (%)
Temperature (°C)
A
150 200 250 300 350 400 450 500
50
60
70
80
90
100
CdSeNL
CdSePy
CdSeBA
Mass (%)
Temperature (°C)
B
150 200 250 300 350 400 450 500
60
70
80
90
100
CdSeNL
CdSetBT
CdSeTP
CdSeTHT

Mass (%)
Temperature (°C)
C

Figure 2. Thermogravimetric analysis traces for (a) carboxylic acids, (b) amines, and (c) sulfur-containing
functionalized cadmium selenide nanocrystals.

Figure 3 shows the reduction in C-H stretching signals at 2850 cm
-1
and 2920 cm
-1
for all
the exchanged NCs. This reduction corresponds to the decrease in the number of C-H
stretches that would be observed if long chain organic native ligands were still left on the
surface of the NCs. These results further prove that the exchanges were all optimized and
can be used to fabricate devices without inefficiencies from inefficient ligand exchanges.

3200 3100 3000 2900 2800 2700 2600
CdSeTHT
CdSeTP
CdSetBT
CdSeBA
CdSePy
CdSePA
CdSeNL

Wavenumbers (cm
-1
)

Figure 3. Fourier-transform infrared spectra for CdSeX (X = NL, PA, Py, BA, tBT, TP, and THT)

5

2.2 Photovoltaic Effects from Ligands

In order to effectively display the effects that each ligand has on inorganic/organic BHJ
device performance, it is imperative that each device is optimized individually to
guarantee optimal device performance. Previous experience has shown that two
parameters have been found to significantly influence PCE, which include loading ratios
of CdSe NCs to P3HT and annealing temperatures. Figures 4-9, compare JSC, VOC, FF,
and PCE for all the devices made from different conditions in order to find processing
conditions that resulted in the highest PCE. After the ideal condition was found, devices
were replicated three times to confirm the results.

12:3 18:3 24:3
4.4
4.8
5.2
5.6
6.0
6.4
6.8
7.2
A
CdSePA:P3HT 200°C
CdSePA:P3HT (mg)
J
sc
(mA/cm
2
)
0.60
0.61
0.62
0.63
0.64
0.65
0.66
0.67
V
oc
(V)
12:3 18:3 24:3
0.46
0.48
0.50
0.52
0.54
0.56
0.58
0.60
B
CdSePA:P3HT 200°C
CdSePA:P3HT (mg)
Fill Factor
1.7
1.8
1.9
2.0
2.1
2.2
2.3
2.4
Power Conversion Efficiency (%)

6

175 200 225
4.4
4.8
5.2
5.6
6.0
6.4
6.8
7.2
Temperature (°C)
J
sc
(mA/cm
2
)
0.58
0.60
0.62
0.64
0.66
0.68
0.70
0.72
C
18:3 CdSePA:P3HT
V
oc
(V)
175 200 225
0.44
0.46
0.48
0.50
0.52
0.54
0.56
0.58
Temperature (°C)
Fill Factor
1.56
1.68
1.80
1.92
2.04
2.16
2.28
2.40
D
18:3 CdSePA:P3HT
Power Conversion Efficiency (%)

Figure 4. The optimized device conditions were found by comparing device parameters in (a), (b), (c), and (d) for
PA hybrid devices at different processing conditions.

12:3 18:3 24:3
5.4
5.6
5.8
6.0
6.2
6.4
6.6
6.8
A
CdSePy:P3HT (mg)
J
sc
(mA/cm
2
)
CdSePy:P3HT 175°C
0.52
0.54
0.56
0.58
0.60
0.62
0.64
0.66
V
oc
(V)
12:3 18:3 24:3
0.49
0.50
0.51
0.52
0.53
0.54
0.55
0.56
B
CdSePy:P3HT (mg)
Fill Factor
1.5
1.6
1.7
1.8
1.9
2.0
2.1
2.2
Power Conversion Efficiency (%)
CdSePy:P3HT 175°C

150 175 200
4.8
5.2
5.6
6.0
6.4
6.8
7.2
7.6
C
Temperature (°C)
J
sc
(mA/cm
2
)
18:3 CdSePy:P3HT (mg)
0.52
0.54
0.56
0.58
0.60
0.62
0.64
0.66
V
oc
(V)
150 175 200
0.50
0.51
0.52
0.53
0.54
0.55
0.56
0.57
D
18:3 CdSePy:P3HT (mg)
Temperature (°C)
Fill Factor
1.7
1.8
1.9
2.0
2.1
2.2
2.3
2.4
Power Conversion Efficiency (%)

Figure 5. The optimized device conditions were found by comparing device parameters in (a), (b), (c), and (d) for
Py hybrid devices at different processing conditions.
7

12:3 18:3 24:3
3.6
3.8
4.0
4.2
4.4
4.6
4.8
5.0
A
CdSeBA:P3HT 200°C
CdSeBA:P3HT (mg)
J
sc
(mA/cm
2
)
0.62
0.64
0.66
0.68
0.70
0.72
0.74
0.76
V
oc
(V)
12:3 18:3 24:3
0.49
0.50
0.51
0.52
0.53
0.54
0.55
0.56
CdSeBA:P3HT 200°C
CdSeBA:P3HT (mg)
Fill Factor
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
B
Power Conversion Efficiency (%)

175C 200C 225C
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
C
18:3 CdSeBA:P3HT (mg)
Temperature (°C)
J
sc
(mA/cm
2
)
0.62
0.64
0.66
0.68
0.70
0.72
0.74
0.76
V
oc
(V)
175C 200C 225C
0.44
0.46
0.48
0.50
0.52
0.54
0.56
0.58
D
18:3 CdSeBA:P3HT (mg)
Temperature (°C)
Fill Factor
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
Power Conversion Efficiency (%)

Figure 6. The optimized device conditions were found by comparing device parameters in (a), (b), (c), and (d) for
BA hybrid devices at different processing conditions.

9:3 12:3 18:3
4.6
4.8
5.0
5.2
5.4
5.6
5.8
6.0
CdSetBT:P3HT 175°C
CdSetBT:P3HT (mg)
J
sc
(mA/cm
2
)
A
0.64
0.68
0.72
0.76
0.80
0.84
0.88
0.92
V
oc
(V)
9:3 12:3 18:3
0.32
0.36
0.40
0.44
0.48
0.52
0.56
0.60
CdSetBT:P3HT 175°C
CdSetBT:P3HT (mg)
Fill Factor
B
1.7
1.8
1.9
2.0
2.1
2.2
2.3
2.4
Power Conversion Efficiency (%)

8

150 175 200
4.6
4.8
5.0
5.2
5.4
5.6
5.8
6.0
Temperature (°C)
J
sc
(mA/cm
2
)
C
0.70
0.72
0.74
0.76
0.78
0.80
0.82
0.84
12:3 CdSetBT:P3HT
V
oc
(V)
150 175 200
0.38
0.40
0.42
0.44
0.46
0.48
0.50
0.52

Temperature (°C)
Fill Factor
1.56
1.68
1.80
1.92
2.04
2.16
2.28
2.40
12:3 CdSetBT:P3HT
Power Conversion Efficiency (%)
D

Figure 7. The optimized device conditions were found by comparing device parameters in (a), (b), (c), and (d) for
tBT hybrid devices at different processing conditions.

200 225 250
2.4
3.2
4.0
4.8
5.6
6.4
7.2
8.0
A
9:3 CdSeTP:P3HT
Temperature (°C)
J
sc
(mA/cm
2
)
0.64
0.66
0.68
0.70
0.72
0.74
0.76
0.78
V
oc
(V)
200 225 250
0.40
0.42
0.44
0.46
0.48
0.50
0.52
0.54
B
9:3 CdSeTP:P3HT
Temperature (°C)
Fill Factor
0.4
0.8
1.2
1.6
2.0
2.4
2.8
3.2
Power Conversion Efficiency (%)

6:3 9:3 12:3
2.4
3.2
4.0
4.8
5.6
6.4
7.2
8.0
CdSeTP:P3HT (mg)
J
sc
(mA/cm
2
)
0.64
0.66
0.68
0.70
0.72
0.74
0.76
0.78
C
CdSeTP:P3HT 225°C
V
oc
(V)
6:3 9:3 12:3
0.40
0.42
0.44
0.46
0.48
0.50
0.52
0.54
D
CdSeTP:P3HT 225°C
CdSeTP:P3HT (mg)
Fill Factor
0.4
0.8
1.2
1.6
2.0
2.4
2.8
3.2
Power Conversion Efficiency (%)

Figure 8. The optimized device conditions were found by comparing device parameters in (a), (b), (c), and (d) for
TP hybrid devices at different processing conditions.
9

225 250 275
3.2
3.6
4.0
4.4
4.8
5.2
5.6
6.0
A
6:3 CdSeTHT:P3HT
Temperature (°C)
J
sc
(mA/cm
2
)
0.56
0.58
0.60
0.62
0.64
0.66
0.68
0.70
V
oc
(V)
225 250 275
0.48
0.49
0.50
0.51
0.52
0.53
0.54
0.55
B
6:3 CdSeTHT:P3HT
Temperature (°C)
Fill Factor
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
Power Conversion Efficiency (%)

3:3 6:3 9:3
4.0
4.5
5.0
5.5
6.0
6.5
7.0
CdSeTHT:P3HT (mg)
J
sc
(mA/cm
2
)
0.30
0.36
0.42
0.48
0.54
0.60
0.66
0.72
0.78
C
CdSeTHT:P3HT 225°C
V
oc
(V)
3:3 6:3 9:3
0.44
0.46
0.48
0.50
0.52
0.54
0.56
0.58
D
CdSeTHT:P3HT 225°C
CdSeTHT:P3HT (mg)
Fill Factor
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
Power Conversion Efficiency (%)

Figure 9. The optimized device conditions were found by comparing device parameters in (a), (b), (c), and (d) for
THT hybrid devices at different processing conditions.

Table 1 shows the average and standard deviation for each device parameter of all the
devices fabricated under their optimized conditions. It can be seen that devices made
from PA, Py, tBT, and TP yield power conversion efficiencies greater than 2.1%. It is
surprising that these four different ligands show similar device performances, despite their
difference in functionality and structure. This demonstrates the importance of
optimization when comparing the effects that different ligands have on their device
efficiencies. Without properly optimizing the processing conditions, inaccurate
10

comparisons would be made. The optimization process allowed for comparisons to be
made between all the different ligands and their respective device parameters.
Thiophenol had the highest JSC (6.78 mA cm
-2
) with pivalic acid and pyridine close
behind with JSC values of 6.67 and 6.59 mA cm
-2
, respectively. Although tert-butylthiol
had the lowest JSC, it had the highest VOC (0.819 V) with thiophenol behind it at 0.685 V.
Their FF values were 0.47 and 0.49 with similar PCE of 2.21% and 2.26%. Pyridine and
pivalic acid had lower VOC (0.600 V and 0.625 V), but higher FF values of 0.54 and 0.53.
The higher FF allowed for device efficiencies of 2.14% and 2.23%. These results showed
how different ligands affected different parameters, but the balance between all three
determined the overall PCE.

Ligand Jsc (mA/cm
2
) Voc (V) FF PCE (%)
Champion
PCE (%)
NL 1.02 ± 0.03 0.564 ± 0.046 0.45 ± 0.02 0.26 ± 0.03 0.292
PA 6.67 ± 0.16 0.625 ± 0.004 0.53 ± 0.01 2.23 ± 0.12 2.35
Py 6.59 ± 0.34 0.600 ± 0.005 0.54 ± 0.01 2.14 ± 0.10 2.26
BA 4.71 ± 0.23 0.710 ± 0.004 0.52 ± 0.01 1.72 ± 0.11 1.89
tBT 5.73 ± 0.08 0.819 ± 0.003 0.47 ± 0.01 2.21 ± 0.04 2.25
TP 6.78 ± 0.35 0.685 ± 0.004 0.49 ± 0.01 2.26 ± 0.14 2.49
THT 5.41 ± 0.04 0.625 ± 0.005 0.54 ± 0.01 1.82 ± 0.05 1.86

Table 1. The reported average and standard deviation of the device parameters for 12-16 devices over multiple
substrates.

11

2.2.1. JSC Effect

Steady-state photoluminescence (PL) and external quantum efficiencies (EQE) were
used to rationalize the short-circuit current-densities for each ligand. Figure 10 shows
the PL comparison of all hybrid films relative to their normalized neat P3HT films in
order to observe the effectiveness that each hybrid film has on quenching the P3HT
fluorescence due to efficient electron transfer from P3HT to NC
[16]
. The quenching
efficiencies were found to follow the trend: PA (98%) > Py (96%) > TP (94%) > tBT
(91%) > THT (82%) > BA (49%).
600 650 700 750 800
0.0
0.2
0.4
0.6
0.8
1.0
Normalized PL
Wavelength (nm)

Figure 10. Photoluminescence quenching shown for annealed neat P3HT films and their annealed hybrid
counterparts. Neat P3HT 175°C (black), 200°C (gray), 225°C (Purple), and 250°C (wine) are shown normalized
to 1.0 a.u. The hybrid films shown are 18:3 CdSe(PA):P3HT (200°C, red), 18:3 CdSe(Py):P3HT (175°C, green),
18:3 CdSe(BA):P3HT (200°C, blue), 12:3 CdSe(tBT):P3HT (175°C, cyan), 09:3 CdSe(TP):P3HT (225°C,
magenta), and 06:3 CdSe(THT):P3HT (250°C, dark yellow).

12

-0.8 -0.4 0.0 0.4 0.8
-10
-5
0
5
10
Potential (V)
Current density (mA/cm
2
)

CdSeNL
CdSePA
-0.8 -0.4 0.0 0.4 0.8
-10
-5
0
5
10
Potential (V)
Current density (mA/cm
2
)

CdSePy
CdSeBA
-0.8 -0.4 0.0 0.4 0.8
-10
-5
0
5
10
Potential (V)
Current density (mA/cm
2
)

CdSetBT
CdSeTP
CdSeTHT

Figure 11. I-V curves for optimized devices made with carboxylic acid (PA), amines (Py, BA), and sulfur-
containing ligands (tBT, TP, THT).

Within each functional group the JSC correlates with quenching efficiency and remains
consistent when compared to the values obtained from the I-V curves (Figure 11). It is
reasonable that the top performing devices have the highest PL quenching efficiencies.
PL shows electron transfer efficiency from P3HT to CdSe NCs by quenching, but the
ability to extract charges from excitons formed is observed by EQE
[17]
. Figure 12
shows the comparison of EQE between the devices in each functional group. The
spectra show that there is a spectra range from 300-700 nm where three distinct peaks
from P3HT are observed at higher energies with a maximum efficiency shown at 500
nm and the start of a NC contribution corresponding to its band edge around 700 nm.
The maximum EQE response for the top performing devices around 500 nm are TP >
50%, Py > 50%, tBT > 45% and PA > 40%. These results suggest that pyridine and
thiophenol are the best ligands at extracting charges from excitons that are formed in
the device.
13

300 400 500 600 700
0
10
20
30
40
50
60
70 18:3 CdSePA:P3HT
External Quantum Efficiency (%)

Wavelength (nm)
A
300 400 500 600 700
0
10
20
30
40
50
60
70 18:3 CdSePy:P3HT
18:3 CdSeBA:P3HT
External Quantum Efficiency (%)

Wavelength (nm)
B
300 400 500 600 700
0
10
20
30
40
50
60
70 C 12:3 CdSetBT:P3HT
09:3 CdSeTP:P3HT
06:3 CdSeTHT:P3HT
External Quantum Efficiency (%)

Wavelength (nm)

Figure 12. External quantum efficiencies for optimized devices made with (a) carboxylic acid (PA), (b) amines
(Py, BA), and (c) sulfur-containing ligands (tBT, TP, THT).

2.2.2. VOC Effect

It has been shown that by changing the surface of CdSe NCs, frontier orbitals can be
tuned
[18]
. Previously, Greaney
[13]
reported that tert-butylthiol effectively increased
the open circuit voltage by increasing the CdSe lowest unoccupied molecular orbital
(LUMO) relative to vacuum. The maximum theoretical VOC is determined by the
energy difference between the highest occupied molecular orbital (HOMO) of P3HT
and the LUMO of CdSe
[23]
. Electrochemical methods were employed to find the
difference in energy between donor and acceptor in order to determine if the VOC
values obtained from the x-intercepts in Figure 11 are correct. Cyclic voltammetry
(CV) was used to obtain the reduction energy found by measuring the onset potential
of the reduction wave (Figure 13)
[19]
.
14

0.0 -0.2 -0.4 -0.6 -0.8 -1.0 -1.2
NL
PA
Py
BA
tBT
TP
Voltage (V)
THT

Figure 13. Cyclic voltammetry of CdSeX (X = NL, PA, Py, BA, tBT, TP, THT) after ferrocene/ferrocenium
redox couple corrections. The reduction energy was measured from the onset of the reduction potential wave.

All CV traces were corrected relative to a ferrocene/ferrocenium redox couple
[20]
.
The energy of the LUMO level was calculated from putting the CV determined
reduction energy value into the following equation
[19-20]
:
ELUMO = − (Ered + 4.5) eV (2)
It was then possible to estimate the corresponding HOMO level by adding the
optical band gap of CdSe. Finally, the change in frontier orbitals, ΔE DA,CV, was
found by taking the difference between P3HT HOMO and CdSe LUMO. This value
reflects a change in the frontier orbitals and is known to affect the VOC, therefore, the
ligands affect the VOC. Table 2 shows the values obtained from CV and calculation.
15

The CV determined ΔE DA,CV trend correlates with the device VOC trend in Table 1.
Tert-butythiol possessed the highest ΔE DA,CV and correspondingly Voc. Butylamine
and thiophenol were significantly behind in both values. CV was used to estimate
the HOMO/LUMO levels and ΔE DA,CV in order to show that the device trends related
to VOC are consistent.

Ligand Ered (V) Elumo (eV) Eg (eV) Ehomo (eV) ΔEDA,CV (eV) I-V Voc (V)
NL -0.479 -4.02 2.00 -6.02 1.18 0.564 ± 0.046
PA -0.604 -3.90 2.00 -5.59 1.30 0.625 ± 0.004
Py -0.549 -3.95 2.00 -5.95 1.25 0.600 ± 0.005
BA -0.689 -3.81 2.00 -5.80 1.39 0.710 ± 0.004
tBT -0.924 -3.58 1.99 -5.57 1.62 0.819 ± 0.003
TP -0.630 -3.87 1.99 -5.86 1.34 0.685 ± 0.004
THT -0.605 -3.89 1.99 -5.88 1.31 0.625 ± 0.005

Table 2. Estimated E red, HOMO/LUMO levels and ΔE DA,CV are shown. The overall trends for V oc from I-V and
ΔEDA,CV from CV are well-correlated.

Figure 14 shows the dark I-V characteristics of all the optimized devices on a log
scale where the y-intercept corresponds to the reverse saturation current. Since all
the devices are within an order of magnitude the effect it would have on VOC is not
significant enough to alter the overall trend observed by the I-V curves. This proves
that the trends observed electrochemically with ΔE DA,CV and the VOC from I-V are
still consistent.
16

-0.5 0.0 0.5 1.0
10
-6
10
-5
10
-4
10
-3
10
-2
10
-1
10
0
18:3 CdSeNL:P3HT
18:3 CdSePA:P3HT
18:3 CdSePy:P3HT
18:3 CdSeBA:P3HT
12:3 CdSetBT:P3HT
09:3 CdSeTP:P3HT
06:3 CdSeTHT:P3HT

Current density (A/cm
2
)
Voltage (V)

Figure 14. Dark I-V characteristics for each optimized hybrid device.

2.2.3. Morphology

It is difficult to definitively describe the effects of ligand exchanges on morphology, but
there are some techniques that are commonly used to provide some insight to the subject.
The most qualitative method is by taking TEM images of hybrid films. In these images
individual NCs and domain networks can be observed allowing for suggestions to be
made about NC aggregation and effective mixing. TEM images of all the hybrid films
after being annealed at their optimal annealing temperature are seen in Figures 15-17.
Between the hybrid films in each functional group there are significant differences in
donor and acceptor networks formed. It can be clearly seen that CdSe(NL):P3HT hybrid
film (Fig. 15a) has an unfavorable morphology that has large amounts of aggregation, but
the CdSe(PA):P3HT hybrid film (Fig. 15b) has clearly defined domains. There is a mix
of different sized domains which likely helped charge separation and transport.
17

Figure 15. TEM images of (a) CdSe(NL):P3HT and (b) CdSe(PA):P3HT annealed films show the difference
between morphology.

The amines have films that seem to have domains that are more similar in size in their
respective films. The CdSe(BA):P3HT hybrid film (Fig. 16a) has good coverage, but
seems to have large phases formed. The CdSe(Py):P3HT hybrid film (Fig. 16b) has
complete coverage and uniform phases that would appear to be a favorable morphology.

Figure 16. TEM images of (a) CdSe(BA):P3HT and (b) CdSe(Py):P3HT annealed films displaying a clear
difference in morphology.

Finally, the thiol-containing ligands all have different looking films where the CdSe(tBT)
hybrid film (Fig. 17a) has a film with regular medium sized domains, the CdSe(TP)
A B
A B
18

hybrid film (Fig. 17b) has poor coverage with smaller sized domains, and the CdSe(THT)
(Fig. 17c) hybrid film has extremely poor coverage.

Figure 17. TEM images of (a) CdSe(tBT):P3HT, (b) CdSe(TP):P3HT, and (c) CdSe(THT):P3HT annealed films
reveal the difference between morphology.

Another technique commonly reported is atomic force microscopy (AFM). This
technique provides root mean square (RMS) roughness values, possible visual aggregates,
and 3-dimensional plots of the interface along with other topological parameters. The
AFM reveals that RMS values for the ligand set follow the trend: Py (0.86 nm) < PA = TP
(1.5 nm) < THT (1.84 nm) < tBT (3.9 nm) < BA (5.8 nm). These values were based on
the images seen in Figures 18-20.

Figure 18. (a) Topological image of 18:3 CdSe(PA):P3HT was obtained in a 5 μm x 5 μm window with a 123 nm
maximum profile height and an RMS roughness of 1.5 nm. (b) 3-D plot of the PA hybrid film interface.
A B C
A
B
19

AFM reveals that RMS values for the ligand set follow the trend: Py (0.86 nm) < PA = TP
(1.5 nm) < THT (1.84 nm) < tBT (3.9 nm) < BA (5.8 nm). These values were based on
the images seen in Figures 18-20. The topological images and 3-D plots for all the
hybrid films allow for possible explanations on NC aggregation, low shunt resistance
from device shorting, charge recombination from interfacial roughness, and implications
on series resistance. For example, Figure 19 shows how the morphology seen in
CdSe(Py):P3HT hybrid film is homogenous and corresponds to the low RMS value
obtained in AFM.

Figure 19. (a) Topological image for 18:3 CdSe(Py):P3HT hybrid film was obtained in a 5 μm x 5 μm window
with a 47 nm maximum profile height and an RMS roughness of 0.86 nm. (b) 3-D plot of the Py hybrid interface.
(c) Topological image of 18:3 CdSe(BA):P3HT hybrid film was obtained in a 5 μm x 5 μm window with a 222 nm
maximum profile height and an RMS roughness of 5.8 nm. (d) 3-D plot BA hybrid film.
A
B
C
D
20

Figure 20. (a) Topological image for 12:3 CdSe(tBT):P3HT was obtained in a 5 μm x 5 μm window with a 91
nm maximum profile height and an RMS roughness of 3.9 nm. A 3-D plot of the tBT hybrid film is shown below.
(b) Topological image of 09:3 CdSe(TP):P3HT was obtained in a 5 μm x 5 μm window with a 40 nm maximum
profile height and an RMS roughness of 1.5 nm. A 3-D plot of the TP hybrid film is shown below. (c)
Topological image of 06:3 CdSe(THT):P3HT was obtained in a 5 μm x 5 μm window with a 54 nm maximum
profile height and an RMS roughness of 1.84 nm. A 3-D plot of the THT hybrid film is shown below.

Although there is no universal standard on the effect of morphology on PCE, it has been
reported that smooth films indicate favorable mixing
[21]
which could increase charge
transport and reduce charge recombination resulting in higher overall PCEs. It is
important to have favorable mixing, but NC and P3HT percolation networks must be
maintained. If the non-NC networks exceed 10-20 nm, excitons formed in P3HT will not
A B C
21

be able to reach the NC to charge separate
[10]
and will lead to recombination. Thus,
morphology has many contributing factors that lead to a high overall PCE.

3. Conclusion

Power conversion efficiencies alone have shown that PA, Py, tBT, and TP are all ligands
that can be employed to produce P3HT-based hybrid solar cells with efficiencies greater
than 2.1%. The comparison between these four ligands reveals the balancing effect
needed between the different device parameters to ensure an overall efficient device. The
devices made with thiophenol possessed the highest short-circuit current-density. It is
possible that the negatively charged ligand binds strongly to the surface of the NC and
packed effectively with its planar conjugated ring. This conjugated ring might provide
better stabilization for separated charges reducing charge recombination while increasing
transport. Tert-butylthiol has proven to improve the open circuit voltage in these
optimized devices due to the change in frontier orbitals as shown by CV and I-V curves.
The increase in CdSeLUMO relative to vacuum causes an increase in VOC and results in
better device performance. Although pyridine doesn’t have the highest VOC, it does have
the highest fill factor, possibly due to the improved morphology as apparent from TEM
and AFM. There are a number of factors that contribute toward FF, but one possibility is
that the structure of pyridine helps form well-mixed domains as seen by the low RMS
value determined from AFM. Interestingly, it was not expected that these different
ligands would provide similar device efficiencies across the different functional groups.
However, it does show that it is possible to use different ligands to fabricate efficient
22

devices. It also demonstrates that optimization has a huge effect on device performance
and when designing new systems it is imperative to compare each system under their
respective optimized conditions. Although there is no clear optimal ligand choice for all
hybrid devices, this study shows that these four ligands produce efficient devices and can
serve as a benchmark to compare all future devices.

4. Experimental

4.1 General Consideration

Selenium (IV) oxide (99.8%) STREM Chemicals, cadmium acetate dihydrate (99.999%
metals basis) Alfa Aesar, pyridine EMD Chemicals, 2-methyl-2-propanethiol (99%)
Sigma Aldrich, tetramethylacetic acid (99%) Sigma Aldrich, 1-butylamine (99%) Alfa
Aesar, thiophenol Sigma Aldrich, tetrahydrothiophene (98%) Alfa Aesar, and poly(3-
hexylthiophene-2,5-diyl) electronic grade Rieke Specialty Polymers.

4.2 Synthesis of Cadmium Selenide Native Ligand (CdSeNL) Nanocrystals

The synthesis is based on literature methods
5
. In the synthesis, selenium (IV) oxide
(0.98g, 8.84 mmol), cadmium myristate (5.05g, 8.89 mmol), cadmium acetate dihydrate
(0.402g, 1.51 mmol), and 170 mL of octadecene were stirred under flowing nitrogen (.25
h). The reaction mixture was put under vacuum and held at 120°C (1h). It was ramped to
240°C under nitrogen, held for a desired reaction time, and a quenching solution (oleic
23

acid, tri-n-octylamine, and octadecene) was quickly injected and reaction flask was
removed from heat. The product was allowed to air cool until it reached room
temperature before purification.
The reaction product was separated into 45 mL centrifuge tubes. An acetone/IPA mixture
was added to flocculate the NCs and the tubes were centrifuged down (6,000 rpm, 4 min).
After the supernatant was discarded, 5 mL of toluene was used to redisperse the NCs.
Methanol (15 mL) was added and the tubes were centrifuged down (6,000 rpm, 2 min.).
The supernatant was discarded and the same washing procedure was repeated 3 more
times. After the final wash, the CdSe(NL) NCs were dispersed in 10 mL of toluene,
filtered through a 1 micron polytetrafluoroethylene (PTFE) filter and stored in the dark at
10°C.

4.3 Ligand Exchanges

4.3.1. Pivalic Acid (PA) Exchange

8 mL of a PA stock solution (20 mL triethylamine, 30 mL PA, and 40 mL DCB) was
added to a 320mg CdSe(NL) dispersion in toluene (4 mL) in two separate 15 mL
centrifuge tubes. The solutions were allowed to sit in the dark for 30 minutes, then
flocculated with methanol and centrifuged down (6,000 rpm, 2 min). The CdSe(PA) NCs
were redispersed in the PA stock solution and the exchange process was repeated four
more times. After the final wash, CdSe(PA) NCs were dispersed in DCB, filtered through
a 0.45 micron PTFE filter, and stored in the dark at 10°C.
24

4.3.2. Pyridine (Py) Exchange

A CdSe(NL) dispersion in toluene (80 mg CdSe; 5 mL) was added to 50 mL of pyridine
in a 150 mL round bottom flask fitted with a water-cooled condenser. The system was
purged with nitrogen for approximately 30 minutes and then placed in a heating bath. The
solution was under flowing nitrogen, stirring, and refluxed at 120°C. After 12-24 hours it
was removed from the heating source and allowed to cool to room temperature. The
reaction solution was separated into six 45 mL centrifuge tubes, pentane was added until
the 45 mL mark, and centrifuged down (6,000 rpm, 2 min). The supernatant was
discarded, the flocculated nanoparticles were dispersed in 5 mL of DCB and the washing
process was repeated four more times. The final dispersion was dispersed in 5 mL of
DCB, filtered through a 0.45 micron PTFE filter, and stored in the dark at 10°C.

4.3.3. Butylamine (BA) Exchange

The butylamine exchange was done in series from the same batch of CdSe(NL) as the
pyridine exchange. The procedure is the same except 50 mL of butylamine was used and
the reflux temperature was 80°C. After it was cooled to room temperature, the reaction
solution was flocculated with acetone and centrifuged down (6,000 rpm, 2 min). The
supernatant was discarded, the CdSe(BA) NCs were dispersed in 5 mL of DCB, filtered
through a 0.45 micron PTFE filter and stored in the dark at 10°C.

25

4.3.4 tert-butylthiol (tBT) Exchange

The tBT exchange was based on literature methods
[13]
. CdSe(NL) (4 mL),
tetramethylurea (4 mL) and tert-butylthiol (4 mL) were added together in two 15 mL
centrifuge tubes and allowed to sit in the dark for 30 minutes. Methanol was used as a
flocculant for the first two washes and the CdSe(tBT) solution was centrifuged down
(6,000 rpm, 2 min). After the supernatant was discarded, the NCs were redispersed in 4
mL of TMU and tBT. They were allowed to sit in the dark for 30 minutes. Pentane was
used as the flocculant for the last three washes and the solution was centrifuged down
(6,000 rpm, 2 min). The supernatant was discarded and on the final wash the CdSe(tBT)
NCs were dispersed in 4 mL of DCB, filtered through a 0.45 micron PTFE filter and
stored in the dark at 10°C.

4.3.5 Thiophenol (TP) Exchange

The thiophenol exchange was done in series with tBT and from the same batch of
CdSe(NL). It follows the same procedure except that thiophenol replaces tBT and in all 5
washes methanol was used as the flocculant.

4.3.6. Tetrahydrothiophene (THT) Exchange

The same CdSe(NL) (4 mL) used in the other thiol exchanges was used and added to THT
(40 mL). The solution was stirred in the dark for 24 h under nitrogen. The solution was
26

separated into four 45 mL centrifuge tubes where pentane flocculated the NCs that were
centrifuged down (6,000 rpm, 2 min). The supernatant was discarded and the CdSe(THT)
NCs were dispersed in DCB. This washing process was repeated four more times. After
the final wash, the NCs were dispersed in DCB, filtered through a 0.45 micron PTFE
filter and stored in the dark at 10°C.

4.4 Device Fabrication

Indium Tin Oxide (ITO) coated glass substrates (10 Ω cm
-2
, Thin Film Devices, Inc.)
were cleaned by sonication in 15 minute intervals. Tetrachloroethylene, acetone, and
isopropyl alcohol were used followed by UV-ozone cleaning for 30 minutes. A layer of
poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS, Sigma Aldrich)
was spun-cast onto the ITO (4000 rpm, 40s). The substrate was heated (120°C) under
vacuum for one hour. 15 mg mL
-1
solutions of P3HT in DCB were prepared by heating
and stirring until all P3HT was dissolved. The desired amount of CdSe was mixed with
P3HT to make active layer solutions of the specific loading ratios. These solutions were
spun-cast onto the PEDOT:PSS covered substrate (700 rpm, 50s) forming films with a
thickness between 70−100 nm. These films were dried in the dark under nitrogen (0.25h).
Once dried a 20 mg mL
-1
solution of zinc oxide NCs in ethanol was spun-cast (4,000 rpm,
40s) and dried in the dark under nitrogen (0.25h). The dried film was annealed at
different temperatures under flowing nitrogen on top of an aluminum block for 10
minutes and allowed to cool in the dark under nitrogen (0.25h). Al deposition was
performed at a rate of 2 Å s
−1
in a thermal deposition chamber (Kurt J. Lesker Co.) under
27

high vacuum (≈ 2 μTorr) for a final thickness of 100 nm. The device areas were 0.45
mm
2
.

4.5 Characterization

UV-Vis spectra were acquired using a quartz cuvette for liquid samples on a Shimadzu
UV-1800 spectrophotometer. Photoluminescence was conducted on a Horiba NanoLog
Spectrofluorometer System for all hybrid films. Each film was spun-cast onto a clean
glass substrate and processed in parallel with its corresponding device. Neat P3HT films
were cast using the same conditions as the devices with the same amount of P3HT in its
solution. TGA traces were obtained using dried CdSe (5 mg) in an alumina pan under
flowing nitrogen with a TA Instruments TGA Q50 model. The samples were dried under
flowing nitrogen at 80°C for several hours. CV measurements were conducted using a
BASi Epsilon-EC potentiostat. A 0.1M electrolyte solution containing tetra-n-
butylammonium hexafluorophosphate (98%, Sigma Aldrich) and dry acetonitrile was put
in a clean 25 mL three neck round bottom flask (rbf). A dilute CdSe solution was drop-
cast onto a Glassy Carbon (GC) electrode and allowed to dry. A platinum wire counter-
electrode and silver wire reference electrode were placed in the rbf along with the GC
electrode. The system was under flowing nitrogen when measurements were conducted.
The silver electrode was calibrated against a ferrocene/ferrocenium redox couple and all
potentials were reported relative to the normal hydrogen electrode. The electrolyte
solution was replaced and the GC electrode was cleaned before switching to a new ligand.
Current versus voltage curves were obtained under ambient conditions using a Keithley
28

2420 SourceMeter in the dark and under ASTM G173-03 spectral mismatch corrected
1000 W m
−2
white light illumination from an AM 1.5G filtered 300 W xenon arc lamp
(Newport Oriel). Chopped and filtered monochromatic light (250 Hz, 10 nm fwhm) was
used from a Cornerstone 260 1/4 M double grating monochromater (Newport 74125)
along with a lock-in amplifier EG&G 7220 to perform all EQE measurements.

5. References

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2006, 99.

(4) Yu, L. Q. W. W.; Guo, W.; Peng, X. Chem. Mater. 2003, 15, 2854-2860.

(5) Yang, J.; Tang, A.; Zhou, R; Xue, J. Sol. Energ. Mater. Sol. Cells 2011, 95,
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(8) Talapin, D. V.; Lee, J. S.; Kovalenko, M. V.; Shevchenko, E. V. Chem. Rev. 2009,
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(9) Zabet-Khosousi, A.; Dhirani, A. A. Chem. Rev. 2008, 108, 4072-4124.

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(11) Chen, O.; Chen, X.; Yang, Y.; Lynch, J.; Wu, H.; Zhuang, J; Cao, Y. C. Angew.
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(12) Lokteva, I.; Radychev, N.; Witt, F.; Borchert, H.; Parisi, J. r.; Kolny-Olesiak,
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Abstract (if available)

Abstract In recent years hybrid inorganic/organic bulk heterojunction solar cells have attracted increasing attention because of their beneficial characteristics such as low manufacturing costs as well as the ability to produce lightweight and flexible devices. However, the power conversion efficiencies for these devices, particularly CdSe:P3HT hybrid bulk heterojunction devices, have lower power conversion efficiencies (PCE) compared to organic solar cells. Different processing conditions and ligand exchanges have been employed to increase PCE, but no fair comparison in different functionalized CdSe nanoparticles and their influence on PCE exists. In this study commonly reported ligands and their influence on CdSe:P3HT hybrid devices fabricated under conditions based on individually optimized loading ratios and annealing temperatures are shown. This comparison can provide assistance in choosing an ideal ligand for specific design criteria and serve as a benchmark for future device performances.

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Asset Metadata

Creator Combs, Blair A. (author)

Core Title The influence of CdSe surface ligands on hybrid inorganic/organic solar cell performance

School College of Letters, Arts and Sciences

Degree Master of Science

Degree Program Chemistry

Publication Date 09/10/2014

Defense Date 09/10/2014

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag cadmium selenide,CdSe nanoparticle,hybrid solar cell,ligand exchange,nanotechnology,OAI-PMH Harvest,photovoltaic

Format application/pdf (imt)

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Brutchey, Richard L. (committee chair), Prakash, G. K. Surya (committee member), Thompson, Mark E. (committee member)

Creator Email blairacombs@gmail.com,blaircom@usc.edu

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c3-473840

Unique identifier UC11286879

Identifier etd-CombsBlair-2913.pdf (filename),usctheses-c3-473840 (legacy record id)

Legacy Identifier etd-CombsBlair-2913.pdf

Dmrecord 473840

Document Type Thesis

Format application/pdf (imt)

Rights Combs, Blair A.

Type texts

Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the a...

Repository Name University of Southern California Digital Library

Repository Location USC Digital Library, University of Southern California, University Park Campus MC 2810, 3434 South Grand Avenue, 2nd Floor, Los Angeles, California 90089-2810, USA