Organic Electronic Materials and Devices

Organic Electronic Materials and Devices

Photovoltaic Cells incorporating
Organic and Inorganic Nanostructures
Jiangeng Xue
Department of Materials Science and Engineering
University of Florida, Gainesville, FL, USA
([email protected], http://xue.mse.ufl.edu)
NSF/ONR OPV Workshop
Sept. 20-21, 2012
Outline
• History and status
• Recent development
– OPV cells with organic nanostructures
– Polymer:colloidal nanocrystal hybrid PV cells
– Optical management
• Challenges and opportunities
Rapid Development of the OPV Field
Publications in Each Year
Special issue on Organic Photovoltaics:
Polymer Reviews, Vol. 50, No. 4 (2010)
10000
1000
Updated (09/2012)
Original (07/2010)
30-35%
annual growth
100
10
Data from ISI Web of Science
1
1985 1990 1995 2000 2005 2010 2015
Year
Topic=((organic or polymer) and (photovoltaic or solar cell))
J. Xue, Polym. Rev. 50, 411 (2010).
Think about grass, trees, plants…
Vertical Molecular Nanostructures
BCP
Al
CuPc
(donor)
PCBM
CuPc
nanorods
ITO
With stationary
substrate
PCBM
(acceptor)
With rotational
substrate
Molecular Flux
Nanorod
α
100 nm
100 nm
100 nm
100 nm
Substrate
ω
Oblique Angle Deposition
Y. Zheng et al., Organic Electronics (2009); Polym. Rev. (2009);
IEEE J. Sel. Top. Quantum Electron. (2010).
CuPc Nanorod/PCBM Films and Devices
Spin-coating
Low PCBM loading
100 nm
100 nm
High PCBM loading
8
30
EQE (%)
2
J (mA/cm )
4
CuPc/PCBM bilayer
CuPc NR/PCBM
0
-4
-8
-1.0
20
10
Planar
Planar-NR
-0.5
0.0
0.5
V (V)
1.0
0
300
400
500
600
700
800
 (nm)
Y. Zheng et al., Organic Electronics (2009); Polym. Rev. (2009);
IEEE J. Sel. Top. Quantum Electron. (2010).
Crystalline Molecular Template
PbPc on pentacene on Si
PbPc
(a)
(b)
y (nm)
15
Monoclinic phase
Profile
10
5
10 nm
20 nm
30 nm
40 nm
0.9
Triclinic phase
0
500 nm
(d)
(c)
.2
.4
.6
x (μm)
(e)
0.6
0.3
PbPc thickness: (a) 0 Å, (b) 2 Å, (c) 5 Å, (d) 10 Å, (e) 20 Å
10 nm
20 nm
30 nm
40 nm
0.9
5
w/ pentacene
template
2
0.0
1.2
Absorbance
on bare ITO
J (mA/cm )
Absorbance
1.2
0.6
0.3
0.0
400
600
800
 (nm)
1000
0
None
Pentacene
CuPc
ZnPc
-5
-10
-1.0
-0.5
0.0
W. Zhao et al., Org. Electron. 13, 129 (2011). V (V)
0.5
.8
1.0
Hybrid Organic-Inorganic PV Cells: Motivation
• Incorporation of inorganic
semiconductors in polymer-based PV
devices may improve charge transport
and enhance environmental stability
• Inorganic semiconductor can also
compliment the absorption of polymer
• Colloidal-synthesized semiconductor
nanocrystals could be processed
together with polymers in solutions
W. U. Huynh, J. J. Dittmer, and A. P. Alivisatos, Science 295, 2425 (2002).
Hybrid Polymer:CdSe Nanocrystal Systems
for Photovoltaics
O=P
Al
CdSe O=P
P3HT:CdSe
Pyridine
ligand exchange
N
S
PEDOT:PSS
ITO
N
N
S CdSe S
O=P
O=P
N
S
Glass substrate
TOPO: a spacer of 11Å
3.2
Ligand exchange for CdSe nanocrystals
4.3-4.5
4.8
Dissolve in chlorobenzene and pyridine
(9:1 in volume) mixed solvent
5.2
ITO
PEDOT:
PSS
5.1
P3HT
6.2- 6.3
CdSe
4.3
Al
50
Effect of Nanosphere Size
40
EQE (%)
2 1 sun AM1.5G
2
J (mA/cm )
0
-2
-4
-8
-0.5
0.0
0.5
1.0
4.0 nm
400
500
600
Wavelength (nm)
700
3
10
Electron-only
devices: Al/P3HT:CdSe/Al
2
J (mA/cm )
10
2
J. Yang et al., Sol. Energy Mater.
Sol. Cells 95, 476 (2011).
20
0
300
V (V)
Larger size nanocrystals lead to
higher Jsc and ηP, likely due to
reduced defect density with fewer
atoms on nanocrystal surface .
30
10
4.0 nm
5.0 nm
6.1 nm
6.8 nm
-6
6.8 nm
6.1 nm, e~10-4 cm2/V·s
1
10
0
10
-1
10
-2
10
-3
10
0.01
4.0 nm, e~10-5 cm2/V·s
0.1
V (V)
1
P3HT/CdSe Film: Importance of Morphology
Show somewhat percolating structures of CdSe nanospheres, allowing for
good transport of photogenerated electrons.
J. Yang et al., Sol. Energy Mater. Sol. Cells 95, 476 (2011).
Hybrid PV Cells with a ZnO Nanosphere Layer
30
2
50
2(degree)
0
40
-2
30
-4
-6
-8
-1.0
(112)
(201)
(200)
60
Nano Today 5, 384 (2010);
Nat. Photon. 5, 543 (2011).
70
50
P3HT:CdSe
P3HT:CdSe/ZnO
EQE (%)
Current density (mA/cm )
4
2
(110)
(102)
40
(103)
(101)
ZnO NPs size ~3 nm
(002)
PEDOT:PSS
ITO
Glass substrate
ZnO NPs
Bulk ZnO
(100)
Intensity (a.u.)
Al
ZnO NPs
P3HT:CdSe
photoactive layer
P3HT:CdSe
P3HT:CdSe/ZnO
20
10
-0.5
0.0
0.5
1.0
Voltage (V)
P: 0.8%  1.5% (5.0 nm)
1.6%  2.5% (6.5 nm)
0
300
400
500
600
700
Wavelength (nm)
L. Qian et al., J. Mater. Chem. 21, 3814 (2011);
J. Yang et al., J. Appl. Phys. 111, 044323 (2012).
Efficiency Enhancement with ZnO NPs
P3HT:CdSe
Al
PEDOT:PSS
ITO
Wavelength (nm)
Combination of electronic, optical, and structural effects!
• Blocking hole leakage to cathode
400
• Preventing exciton quenching by cathode
• Transparent optical spacer for enhanced
500
light absorption
• Reducing damage to active layer during
cathode deposition (less defects formation
600
in active layer)
3.2
700
CdSe
500
600
7.4
700
ZnO
Al
ITO
PEDOT: P3HT ZnO
PSS 6.2- 6.3
P3HT:CdSe
5.1
400
PEDOT:PSS
5.2
4.3
Al
ITO
4.8
4.2
Wavelength (nm)
4.3-4.5
4
P3HT:CdSe/ZnO NPs
10
0
-4
PCE (%)
2
Current density (mA/cm )
Effect of ZnO Layer on Device Aging
After 70 days
0
10
-1
10
-2
10
-3
10
-4
Without ZnO
Thicker ZnO
Thinner ZnO
Fresh
-8
-0.8
-0.4
0.0
Voltage (V)
0.4
0.8
0
10
20
30
40
50
60
70
Times (day)
Without the ZnO layer, the devices have shelf lifetimes of a few hours; but
with the ZnO layer, the device retain >60% of the initial efficiency after 70 days
(stored in air)
2
0
-2
0.5
w/o ZnO
w/ ZnO
w/ ZnO, 60 days
-4
-6
0.3
0.2
w/o ZnO
0.1
-8
-10
-0.5
w/ ZnO
0.4
EQE
Current density (mA/cm2)
Hybrid PCPDTBT:CdSe NP/ZnO NP cells
0.0
0.5
Voltage (V)
PCPDTBT
(Eg = 1.5 eV)
1.0
0.0
300
400
500 600 700
Wavelength (nm)
800
• ηP = 3.5% in device with ZnO NPs under 1 sun
AM1.5G illumination, compared with 2.3% without ZnO.
• For the device with the ZnO NP layer, 30% loss in ηP
after 60 days of exposure to air without encapsulation
R. Zhou et al., Nanoscale 4, 3507 (2012).
Hybrid Solar Cells: Effect of EDT Treatment
HS
SH
Al
CdSe
Polymer:CdSe
NCs
PEDOT:PSS
20 nm
ITO
Current density (mA/cm2)
PCPDTBT:CdSe nanorods
0
-4
w/o EDT
w/ EDT
-8
-12
Glass
-16
-1.0
-0.5
0.0
0.5
1.0
Voltage (V)
Voc
(V)
FF
P3HT, w/o EDT
5.9
0.72
0.52
2.2
P3HT, w/ EDT
7.4
0.73
0.54
2.9
PCPDTBT, w/o EDT
9.3
0.72
0.49
3.3
PCPDTBT, w/ EDT
12.8
0.74
0.50
4.8
0.3
4
0.2
2
0
R. Zhou et al., submitted.
PCPDTBT:CdSe
0.1
w/ EDT
w/o EDT
10
100
2
PO (mW/cm )
0.0
JSC/PO (A/W)
6
ηp(%)
Jsc
(mA/cm2)
P (%)
Device
Hybrid Solar Cells: Effect of EDT Treatment
L-type (neutral) ligands
e.g. TOPO
HO
O
P
O
O
O O
P
P
O
OO
Cd2+
OH
O
++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++
P
O
P
O
CdSe
++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++
O
HO
OO
P
O
P
O
O
O O
P
O
P
OH
O
P
O
P
TOPO
Transmittance (a.u.)
P
EDT treated
Pyridine-exchanged
Purified
-C-H
-P=O
3500 3000 2500 2000 1500 1000
-1
Wavenumber (cm )
PA
1.2
Intensity (a.u.)
X-type (charged) ligands, phosphonic acid (PA)
0.8
Purified
Ligand-exchanged
EDT treated,
w/o annealing
EDT treated,
w/ annealing
P2p
0.4
0.0
138
136
134
132
130
Binding energy (eV)
Grafting Reative Oligomers on Nanocrystals
Anchoring
functional
group
e-
OPE-acid
(Eg = 2.9 eV)
Oligomer
or polymer
backbone
h+
Inorganic
nanorod
T7-acid
(Eg = 2.5 eV)
BTD-acid
(Eg = 1.8 eV)
With John Reynolds, Kirk Schanze, Paul Holloway
Jiangeng Xue
Nanocrystal:Grafted
Oligomers
ester (non-reactive)
acid (reactive)
ester
form
acid form
R. Stalder et al., Chem. Mater. 24, 3143–3152 (2012).
Light Trapping with Pyramidal Rear Reflector
Total
internal
reflection
SEM image of Pyramids
Improvement of Jsc (%)
Flat
60
Pyramidal
Exp.
Calculation
Ideal (no absorption loss)
40
20
0
40
60
80
100
Thickness of Active Layer (nm)
W. Cao et al., Appl. Phys. Lett. 99, 023306 (2011)
Polymer Microlens Array by Soft Lithography
PS microspheres
Norland
optical
adhesive
PS = 100μm
3˝ Si wafer
S.-H. Eom et al., Org. Electron. 12, 472 (2011);
E. Wrzesniewski et al., Small 8, 2647 (2012).
Jiangeng Xue
Enhancement (%)
OPV Cells with Microlens Array
60
SubPc/C60 (12/y nm)
Jsc
P
Simulation
40
20
0
20
40
60
80
100
C60 thickness, y (nm)
SubPc/C60 (12/40 nm)
Architecture
2
J (mA/cm )
0
-2
-4
P3HT:PCBM
P3HT:PCBM/ZnO
PCPDTBT:CdSe/ZnO
PCPDTBT:CdSe
PBnDT-DTffBT:PCBM*
w/o MLA
-6
-1.0
w/ MLA
-0.5
0.0
0.5
V (V)
ηp (%)
%
w/o MLA w/ MLA
incr.
3.4
1.9
2.8
2.2
6.2
3.9
2.4
3.3
2.9
7.0
15
26
18
32
13
1.0
J. D. Myers et al., Energy Environ. Sci. 5, 6900 (2012).
* Polymer from Wei You (UNC)
Challenges and Opportunities
• Control of molecular/polymer assembly
– Desired nanostructures or other hierarchical structures
– Molecular orientation and crystal structures
– Robust bulk heterojunction structure insensitive to molecular
structure modification and processing condition variation
• Organic-inorganic hybrid material interface: morphological,
chemical, and electronic properties
As-coated
–
–
–
–
–
Nanocrystal surface passivation
Nanorod alignment
Energy level alignment
Development of Cd-free nanocrystals
Near IR absorbing materials (organic & inorganic)
200 nm
• Optical management
Aligned
– Appropriate light trapping structure
– Large area manufacturability
50 nm
Acknowledgments
• Students and postdocs in my group at University of Florida:
Dr. Jihua Yang
Dr. Ying Zheng
Dr. Wei Zhao
Bill Hammond
Sang-Hyun Eom
Jason D. Myers
John Mudrick
Robel Bekele
Shuang Zhao
Yixing Yang
Weiran Cao
Renjia Zhou
Ed Wrzesniewski
Jiaomin Ouyang
Vincent Cassidy
• $$ from NSF CAREER and DOE SETP