Transcript Document 7335079

Alvin Kwiram Symposium
Seattle, June 24, 2003
Plastic Solar Cells: current
status and future
prospects
Bernard Kippelen, Neal R. Armstrong, and
Seth Marder
Optical Sciences Center, and Department of Chemistry,
University of Arizona, Tucson, AZ 85721, USA
NREL, ONR, NSF
Collaborators
Kippelen Group
Armstrong Group
Marder Group
Benoit Domercq
Britt A. Minch
Steve Barlow
Seunghyup Yoo
Wei Xia
Yadong Zhang
Carrie Donley
Chet Carter
Prof. David O’Brien, deceased
Organic Electronics
Low temperature processing
of organic semiconductors,
metals and dielectrics on
flexible substrates: low cost
($0.01)
Metal deposition on plastics from solution, micro-size
features using soft lithography and transfer
A Complementary Material Platform
Light-weight, high versatility, low cost, large area
OLEDs
OFETs
PV
 Light
emission
 High
mobility
 High mobility
Memories
 High photogeneration
efficiency
 Very thin
 High
OK
luminescence
A convergence
of new material technologies for the
Electroefficiency

No

Absorb
visible
development of Application Specific Integrated Plastic
optics Chips
condition on
spectrum
 Low
(ASIPC)
optical
mobility OK
 Photoabsorption
Lasers and
stability
 Photoamplifiers
stability
Light-weight flexible substrate, barrier to O2 and H2O
Technology Opportunities
 Low cost scanners
Efficiency
 Optical isolators
Low
cost
Flexibility
Consumer
 Devices that take
advantage of the
integration of
photodetectors on lightweight flexible substrates
Outline
• Introduction to photovoltaic technologies
• Organic excitonic solar cells
• Requirements for conversion with high
efficiency
• An approach based on self-assembly
LUMO
Electrode
HOMO
Electrode
Semiconductor
Solar cell parameters
I
 short-circuit current ISC
V
 open circuit voltage VOC
 fill factor FF
Pmax
I SC VOC

 FF
Psolar
Psolar
I maxVmax
FF 
I SC VOC
Evolution of PV Technologies
AM 1.5 G, 25 C, 1 sun = 100mW/cm2
[A.M.: air mass; G: global, direct + scattered; angle of
48.2, zenith angle (sec(48.2) = 1/cos(48.2) = 1.5)]
State-of-the-art in organic photovoltaics
Organic
(Tang)
Silicon
Organic
(Forrest)
Organic
Graetzel
Polymer
(Shaheen)
Hybrid
nanorods
Open circuit voltage (Voc; V)
Short circuit current density (Jsc; mA/cm2)
Voc x Jsc (mW/cm2)
Fill factor (FF)
0.7
43
30.1
0.8
0.45
2.3
1.035
0.65
0.5
18
9
0.4
0.721
20.53
14.80213
0.704
0.82
5.25
4.305
0.61
0.7
5.7
3.99
0.4
Illumination intensity (mW/cm2)
1 sun = 100 mW/cm2
100
75
100
100
106.2
96.4
24.08
0.9
3.6
10.4
2.5
1.7
Conversion efficiency (%)
 Grätzel cell (liquid electrolyte, solid)
 Small molecules (bi-layers)
 Polymer blends (interpenetrated networks)
 Hybrid approaches (Inorganic sc doped in organic matrix)
The Challenge
5
4
60
J
total current density
integrated from 0 to 
maximum available value,
assuming 100% EQE.
2
J ( mA/cm )
020403a
40
3
2
20
0
1
500
1000
1500
2000
2500
Wavelength ( nm )
Harvesting the solar spectrum and…
0
Spectral Photon Flux Density
14
2
( 10 photons/sec-cm )
80
Maximum Current Density Available
in 1 Sun @ AM 1.5G
…maintain simultaneously high open circuit voltage and
high fill factor
 optimize absorption, charge generation, charge
collection: photocurrent
 optimize relative energy levels: built-in voltage
 optimize electrical characteristic: fill factor
Step #1: Achieve efficient dissociation of
excitons in organic materials
vacuum
vacuum
LUMO
electrode
electrode HOMO
Single layer
Double layer
Overcome exciton binding energy
Ansatz: the maximum value for Voc is the smallest band gap
minus the exciton binding energy (0.5 eV)
Maximum efficiency:
 = 28% x FF
0.6
1.5
JscxVoc
normalized by
2
100mW/cm
Jsc available
between 0 and  (nm)
0.5
0.4
0.3
897
1.0
Voc available
for Eg corresponding to 
Minimum band offset 0.5eV
assumed at the junction
( 1.38 eV )
0.2
0.5
0.1
0.0
500
1000
1500
2000
Cut-off Wavelength (nm)
0.0
2500
Voc ( V )
400 – 900 nm (1.4 eV)
020603c
2
Optimum harvesting:
Jsc/100 (mA/cm ), normailized Jsc Voc
0.7
Maximum Available JscVoc Product
for a given cut-off wavelength
High Efficiency: Maximize Light Harvesting and Fill
Factor
Energy Gap ( eV )
4 3
2
1
0.5
30
FF = 1.0
FF = 0.8
FF = 0.7
FF = 0.6
FF = 0.5
FF = 0.4
Efficiency (%)
25
20
15
021003b
10
5
0
500
1000
1500
2000
Cut-off Wavelength (nm)
2500
Equivalent Circuit Model
Understanding Key Factors for
Efficient Organic Photovoltaic
Cells
 Finite conductance of materials
and contact resistance : nonzero Rs
 Leakage path : finite Rp
J
 


V  JRS A
1
V
J
exp(
)

1

(
J

)

 0

ph
1  RS RP  
nVT
R
A
P


(J=0)
(V=0)
OPEN-CIRCUIT VOLTAGE
J ph


V
VOC  n VT ln  1 
 OC 
J 0 J 0 RP A 

SHORT-CIRCUIT CURRENT DENSITY
J SC RS  
J ph
R J 
nVT 
ln 1 
 (1  S ) SC 
A
J0
RP J 0 

What determines the fill factor ?
Effects of Rp
20
2
J ( mA/cm )
10
0
020603b
2
RpA = 50 cm
100
200
500
5000
50000
500000
2
2
Jph = 26 mA/cm , J0 = 30 pA/cm
n = 1.5, under 1 Sun ( AM 1.5G )
-10
FF=0.42,
=8.3%
-20
-30
0.0
0.60,
12.3%
0.71,
14.5%
0.2
0.4
Voltage ( V )
0.6
0.81,
16.8%
0.8
Effect of Rp on Voc at Different Photocurrents
021003c
Normalized Voc(Rp)
1.00
0.99
0.98
2
Jph = 26mA/cm
2
Jph = 2.6mA/cm
0.97
2
J0 = 30pA/cm , n=1.5
Voc normalized by the value
corresponding to Rp=infinite
0.96
0.95
2
10
3
10
4
10
2
RpA ( cm )
5
10
6
10
Effects of Rs
2
Current Density ( mA/cm )
5
020603a
2
0
J(RsA=0 cm )
2
J(RsA=2 cm )
2
-5
J(RsA=4 cm )
-10
J(RsA=8 cm )
-15
J(RsA=20 cm )
2
J0 = 30 pA/cm
2
Jph = 26 mA/cm
n = 1.5
2
J(RsA=6 cm )
2
2
J(RsA=10 cm )
2
-20
-25
FF=0.34,
=7.1%
-30
-35
0.0
0.2
0.54,
11.1 %
0.4
0.81,
0.64, 0.75, 16.8%
13.3% 15.6%
0.6
0.8
Voltage ( V )
A need for high mobility materials
Self-Assembly: a Path for Controlled Morphology in
Wet Processed Materials
Adapted from D. Haarer
Existing other approaches
C12H25
C12H25
C12H25
C12H25
C12H25
Choice of hexabenzocoronene
(HBC) driven by large core that
can lead to large mobility
max  3 exp( 83/ n) cm 2 / Vs
C12H25
Number of atoms
(C,O,N) in the core
O
O
N
N
O
O
Mixtures of HBC
and perylene
John Warman, Adv. Mater. 13, 130 (01)
Voc = 0.69 V; FF = 0.4; Jsc = 30 A/cm2
Saturation for illumination > 1 mW/cm2
K. Müllen, R. Friend et al. Science, 293,
1119, (01)
Our material’s choice: phthalocyanines

Good Thermal stability
N

Strong Molar absorptivity
N
NH
NH
N

N
N
N
Good Light stability
Alkoxy substituted Pc known to
form discotic hexagonally
ordered mesophases.
-
-
O
Problems: KI > 350°C,
difficult to align, no
photocurrent when combined
with PTCDI
e-
e
O
N N N
NH NH
N N N
O
e-
O
-e
O
e-
e
O
O
O
Skoulios et al. J. Am. Chem.Soc. 1982, 104, 5245-5247
e-
Metal:
Molecular design
Tuning of spectroscopic
and electronic properties
O
Core:
Provides large core for
strong -orbitals coupling
and cohesive forces
through Van der Waals
interactions
O
O
O
O
O
N N N
N Cu N
N N N
O
O
O
O
Arms:
Influence the solid-tomesophase (KDh)
and mesophase-toisotropic liquid (DhI)
transition temperatures
O O
O
O
O
O
Molecular optimization
O
O
O
O
O
O
O
O
O
S
N N N
N Cu N
N N N
O
O
O
O
O
O
S
S
S
N N N
N Cu N
N N N
O O
O
O
S
S
O
O
S S
O
O-Et-O-Bz CuPc
KDh 111°C; DhI > 400°C;
difficult to process
O
O
S-Et-O-Bz CuPc
KDh 134°C; DhI 320°C;
easy to process into thin films
by spin-coating (chloroform)
1.4
5
6.0x10
5
4.0x10
5
2.0x10
5
Film
Solution
1.2
Optical Density
8.0x10
1.0
0.8
0.6
0.4
0.2
0.0
0.0
400
500
600
700
Wavelength (nm)
800
Molar Extinction Coefficient
Optical properties
Material Characterization
Small-angle X-ray scattering
XRD of Spin-Coated Films on ITO
14000
020303a
Counts/Sec
12000
20nm
20nm Annealed
ITO BK
10000
8000
d
Substrate
6000
4000
d
Substrate
2000
0
3
4
5
2 theta
6
d
7
Substrate
Data show that Pc
form three
different types of
crystalline phases;
dependent on
surface treatment
AFM studies
Possibility to form
nanostructured surfaces
by thermal annealing to
create high area networks
for improved exciton
dissociation
Spin-coated at 4000 rpm on
PEDOT:PSS/ITO; 180°C for two
hrs.
Before annealing
After annealing
Device Configuration
C60
DLC-CuPc
* Energy scale in eV w.r.t vacuum
Experimental Results
 Annealing of DLC-CuPc
film resulted in 3.7-fold
increase in Jsc.
2
Current Density (mA/cm )
1.0
as-apun annealed
0.5
dark
light
 Estimation of RsA values by
inverse slopes of J-V curves at
V »Voc suggests increase of
mobility in annealed device.
0.0
-0.5
-1.0
-0.4
-0.2
0.0
0.2
0.4
Voltage (V)
Jsc
(mA/cm2)
FF
Voc (mV)
Rs A
( cm2)
Not Annealed
0.12
0.32
317
6.1
Annealed
0.44
0.39
222
1.6
 Reduction of Voc is
considered due to creation of
pinholes in DLC-CuPc film
caused by dewetting while
being annealed.
Result for device with ITO/PEDOT:PSS (30nm) /DL-CuPc (20nm)/C60 (40nm)/BCP (10nm)/Al, under 50mW/cm2 (AM1.5Direct illumination)
Self-assembled electron transport materials
-
-
e-
e
-e
OC8H17
C8 H17O
OC8H17
N
N
C8 H17O
C8 H17O
C8 H17O
N
O
N
O
N
O
e-
e
N
OC8H17
OC8H17
OC8H17
e-
e-
“Star-like” discotic LC
oxadiazole materials with
good electron mobility
TOF experiments
V
v  E  
L
L L2
 
v V
N2 laser, 337 nm, 6 ns
R = 102 –104 , C = 10 pF, RC << 
TOF mobility results at room
temperature
Cyclicvoltammetry of Discotic LCs
R3
R1
R3
N
N
O
O
N N
N N
O
R1
R2
O
R3
N N
R2
R1
0.6 volt shift
in reduction
potentional
R2
R2
R1
R3
R3
N
N
O
N
N
N
N N
R1
O
R2
R1
R2
R3
OE Testing Facilities
Fully automated high
vacuum deposition
system with four
organic sources and
two high power
sources for metals and
oxides (co-deposition
capabilities).
Integrated with double
glove box (one dry and
one wet with
integrated spincoater).
Conclusions and future work
 Transport properties of organic semiconductors often limit power conversion
efficiency in organic solar cells. High mobility required in materials that can be
processed from solution.
 DLC-CuPc is solution-processible, and we demonstrated that its transport
property can be improved in the discotic liquid crystalline phase.
 Photocurrents reaching mA/cm2, significant improvement over HBC-based
devices
 Development of discotic electron-transport oxadiazole-based materials.
 Optimization of parameters will require control of interfaces, relative orbital
energies, control of morphology through use of self-assembly.
Additional supporting information
Em
h E
is
s
t
i
c
x
io
n
Interfacial charge
collection
Hole and electron
transport
Charge separation
Energy transfer
Excited state
formation
n
it o
a
Interfacial charge
injection
Hole and electron
transport
Charge
Recombination
Excited state
formation
Energy transfer
h
e-/h+
Stabilization of geometry and patterning
Photo-crosslinking
between adjacent
side chains through
cyclobutane links;
> 50 % conversion
of styryl groups
O
O
254 nm
O
O
8 m 2m
a
bare Si
100nm
b
70nm
0nm
0nm
150m
8 m
100m
Effects of Rs and Rp on Fill Factor in High
Photocurrent Regime
5
020603a
0
2
J0 = 30 pA/cm
2
J(RsA=2 cm )
2
Jph = 26 mA/cm
n = 1.5
2
-5
20
2
J(RsA=4 cm )
2
J(RsA=6 cm )
10
2
J(RsA=8 cm )
2
2
-10
J ( mA/cm )
Current Density ( mA/cm )
2
J(RsA=0 cm )
J(RsA=10 cm )
2
-15
J(RsA=20 cm )
-20
2
FF=0.34,
=7.1%
-30
0.2
0.54,
11.1 %
0.4
Voltage ( V )
0.81,
0.75, 16.8%
0.64,
13.3% 15.6%
0.6
0.8
2
Jph = 26 mA/cm , J0 = 30 pA/cm
n = 1.5, under 1 Sun ( AM 1.5G )
-10
FF=0.42,
=8.3%
-20
-25
-35
0.0
0
020603b
2
RpA = 50 cm
100
200
500
5000
50000
500000
-30
0.0
0.60,
12.3%
0.71,
14.5%
0.2
0.4
Voltage ( V )
Minimize Rs and Maximize Rp
0.6
0.81,
16.8%
0.8
Excitonic Solar Cells: Energy Level Engineering
Vacuum level
EA
A)
B)
C)
D)
E
Ip
E
HTL
ETL
Band offset < exciton binding energy
Band offset > exciton binding energy
Working hypothesis: the maximum value for Voc is the smallest
band gap minus the exciton binding energy (0.5 eV)
1.0
Effect of J0 on PV Performance
20
021003e
Voc ( V ) , FF
0.9
18
0.8
16
0.7
14
2
Jph = 26 mA/cm , n = 1.5
0.6
10
100
1000
2
J0 ( pA/cm )
12
Efficiency (%)
Voc
FF
Efficiency