UV-vis and Transport Characterization of Degradation in Polymer Blend Photovoltaics
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Transcript of UV-vis and Transport Characterization of Degradation in Polymer Blend Photovoltaics
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UV-vis and Transport Characterization of Degradation in Polymer Blend Photovoltaics
American Physical Society March Meeting 2011
Dallas, TX
Emilee Sena, Justin Peel, Shreya Nathan, Devin Wesenberg, Marianne Wallis,
Thorsteinn Adalsteinsson, Brian McNelis, Richard Barber
Santa Clara University
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Overview•Description of organic photovoltaics
•How the devices work
•Device fabrication
•Device measurement
•Active layer molecules
•UV-vis spectroscopy
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What Are Organic Photovoltaic Devices?
O
O
SS
H3C(H2C)4H2C
CH2(CH2)4CH3
n
A
Multijunction
+
Charge Generator
Charge Acceptor
-
A
e-
Multijunction
ħω
Bulk Heterojunction
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What Are Organic Photovoltaic Devices?
ħω
O
O
SS
H3C(H2C)4H2C
CH2(CH2)4CH3
n
A
ħω
+
Charge Generator
Charge Acceptor
-
A
e-
Multijunction
ħω
Bulk Heterojunction
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What Are Organic Photovoltaic Devices?
ħω
SS
H3C(H2C)4H2C
CH2(CH2)4CH3
n
e-
A
ħω
+
Charge Generator
Charge Acceptor
-
A
e-
Multijunction
Bulk Heterojunction
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What Are Organic Photovoltaic Devices?
ħω
O
O
SS
H3C(H2C)4H2C
CH2(CH2)4CH3
n
e-
A
ħω
+
Charge Generator
Charge Acceptor
-
A
e-
Multijunction
Bulk Heterojunction
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What Are Organic Photovoltaic Devices?
ħω
O
O
SS
H3C(H2C)4H2C
CH2(CH2)4CH3
n
e-
e-
A
+
Charge Generator
Charge Acceptor
-
A
e-
Multijunction
Bulk Heterojunction
ħω
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Why Study Them?•Advantages over inorganic (silicon) solar cells:
•Inexpensive
•Flexible
•Lightweight
•Manufacturable
•Why aren’t they used now?
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Research Goals•Understand and improve
•Device lifetime
•Degradation
•Power conversion efficiency
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The ProjectPhysics:
•Sample Fabrication
•Measurement of Device Performance
Organic Chemistry:
•Molecule Synthesis
Physical Chemistry:
•Spectroscopic Analysis
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How Are the Devices Made?
•Indium-Tin-Oxide (ITO)•Indium-Tin-Oxide (ITO): “transparent” conductor
(Cross section)
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How Are the Devices Made?
•Indium-Tin-Oxide (ITO)
(Cross section)
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How Are the Devices Made?
SO3H
nPSS
(poly(styrenesulfonate))
O O
S nPEDOT (poly(3,4-
ethylenedioxythiophene))
•Indium-Tin-Oxide (ITO)
•PEDOT:PSS
•Indium-Tin-Oxide (ITO)
•PEDOT:PSS
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How Are the Devices Made?
SO3H
nPSS
(poly(styrenesulfonate))
O O
S nPEDOT (poly(3,4-
ethylenedioxythiophene))
•Indium-Tin-Oxide (ITO)
•PEDOT:PSS
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How Are the Devices Made?
Ph O(CH2)17CH3
O
P3HT (poly(3-hexylthiophene))
PCBOD ([6-6]-phenyl C61 butyric acid octadecyl ester)
•Indium-Tin-Oxide (ITO)
•PEDOT:PSS
•Active layer
PCBM ([6-6]-phenyl C61 butyric acid methyl ester)
O
O
(or)
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How Are the Devices Made?
•Indium-Tin-Oxide (ITO)
•PEDOT:PSS
•Active layer
•LiF
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How Are the Devices Made?
•Indium-Tin-Oxide (ITO)
•PEDOT:PSS
•Active layer
•LiF
•Al
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How Are the Devices Measured?
V varied & recorded
A
I recorded
Solar simulator illuminates from below
and I-V curves are generated
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Device Performance
|I*V|MAX
(V,I)
Power Conversion Efficiency:
ɳ = [(IV)max/area]/(Pin/area)
Pin = 100 mW/cm2
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Device Degradation
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The Active Layer•Electron donor and acceptor molecules
•Light-induced electron transfer
SS
H3C(H2C)4H2C
CH2(CH2)4CH3
n
Ph O(CH2)17CH3
O
+
P3HT
PCBOD
PCBM
O
O
(or)
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Meet the Molecules!•PCBM: [6,6]-phenyl-C61-butyric-acid-methyl-ester
•PCBOD: [6,6]-phenyl-C61-butyric-acid- octadecyl-ester
•Alkyl chain improves solubility, lifetime
PCBODPCBMC60
O
O
O
O
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UV-Vis Spectroscopy: Why Do We Use It?
•Insight into:
•Absorption of light energy to excite electrons
•Changes in chemical environment
•Absorption changes due to degradation
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How Does a Typical Spectrum Look?
•Ultraviolet and visible range
•Differential spectrum
•Time evolution
•MATLAB for analysis
300 nm 750 nm
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0 1 2 3 4 5 6 7 80
0.002
0.004
0.006
0.008
0.01
0.012
0.014
0.016
0.018
0.02x = 0.403 PCBOD:P3HT Blend, Annealed at 220C
Time (days)
Dif
fere
nti
al A
bso
rban
ce (
a.u
.)
554 nm
Finding the Characteristic Time (τ)
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0 1 2 3 4 5 6 7 8 9 10-10
-9
-8
-7
-6
-5
-4
-3
Time (days)
ln [
Am
ax -
A(t
)]
x = 0.403 PCBOD:P3HT Blend, Annealed at 220C
y = - 0.57*x - 4
554 nm linear
Exponential Fit, Solve for Characteristic Time (τ):
Ln [Amax – A (t) ] = -t / τ + constant
Result for 554 nm: τ = 1/0.57 = 1.75 days
IV data: τ = 1/0.56 = 1.79 days
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Variable Factors•Weight percent
•Blend stoichiometry
•Annealing temperature
•Most significant improvement in device performance
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Annealing
Anneal on hotplate
(up to 380°C)
Continue fabrication process with LiF and Al layers
(HEAT)
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Effects of Annealing: Device Performance
0.4 Molar Fraction PCBOD:P3HT Blend
O
O
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0.16 Molar Fraction PCBM:P3HT Blend
300 350 400 450 500 550 600 650 700 750-0.1
-0.05
0
0.05
0.1
0.15
0.2
0.25
Wavelength (nm)
Ab
sorb
ance
(a.
u.)
x=0.159 PCBM:P3HT Blend, Annealed at 220C
300 350 400 450 500 550 600 650 700 750-0.1
-0.05
0
0.05
0.1
0.15
0.2
0.25
Ab
sorb
ance
(a.
u.)
Wavelength (nm)
x=0.159 PCBM:P3HT Blend, Not Annealed
300 350 400 450 500 550 600 650 700 750-0.04
-0.035
-0.03
-0.025
-0.02
-0.015
-0.01
-0.005
0
0.005
Wavelength (nm)
Dif
fere
nti
al A
bso
rban
ce (
a.u
.)
x=0.159 PCBM:P3HT Blend, Not Annealed
300 350 400 450 500 550 600 650 700 750-0.04
-0.035
-0.03
-0.025
-0.02
-0.015
-0.01
-0.005
0
0.005
x=0.159 PCBM:P3HT Blend, Annealed at 220C
Dif
fere
nti
al A
bso
rban
ce (
a.u
.)
Wavelength (nm)Dif
fere
nti
al S
pec
tru
mIn
itia
l S
pec
tru
m
Not Annealed; PCEmax = 0.64 % Annealed at 220°C; PCEmax = 1.17%
450 nm
555 nm
λmax = 555 nm
O
O
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Effects of Annealing: Hypotheses
• Decreased quenching
• Morphological changes
• Crystallite formation
• Aggregation
• More stable structure
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Summary•Organic photovoltaics use light energy for electron transfer to produce current
•UV-vis spectroscopy can be used to indicate changes in structure and chemical environment, how absorption changes with degradation
•Annealing at high temperatures improves device performance
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Funding
Santa Clara University's BIN-REU - UCSC BIN-RDI, NASA Grant NNX09AQ44A
SCU, Science Technology and Society Grants
Grant from IntelliVision Technologies
AcknowledgementsMichael Sena