Solution-Processed Bulk Heterojunction Solar Cells with Novel Acceptor Molecules Tommaso Giovenzana, Jason T. Bloking, Andrew T. Higgs, Andrew Ponec, Michael D. McGehee and Alan Sellinger Donor Acceptor Ca/Al PEDOT/ITO 2 3 3 HOMO Level (Valence Band) LUMO Level (Conduction Band) 1 How do organic solar cells work? How current is generated 1) Photon absorption and exciton generation 2) Charge transfer / exciton splitting 3) Charge carrier collection Ohmic electron contact (Ca/Al) Ohmic hole contact (PEDOT/ITO) Donor Acceptor Acceptor Materials in OPV Fullerene Derivatives (Up to 9.3% efficient) Drawbacks of Fullerene Derivatives Low singlet state energy (1.7 eV) limits V OC to 1.0 V Weak absorption of solar spectrum in visible region Higher cost ($50/g, $5.50/m 2 , $0.06/W) Alternative acceptor materials will be needed to produce high V OC top cells in a hybrid tandem configuration PC 61 BM PC 71 BM ICBA Synthesis of New Acceptor Materials X-Ray Diffraction shows no Miscibility Summary Future Directions X-ray diffraction scans show evidence of crystalline HPI-BT phase with only 15 wt.% HPI-BT (miscibility limit of HPI-BT in P3HT < 15 wt%) New small molecule electron acceptors fill some gaps left by fullerene derivatives Open-circuit voltages as high as 1.11 V targeting top cells in tandem devices Increased photocurrent from absorption in acceptor phase Potentially lower cost synthesis No energetic offsets caused by two-phase morphology (low miscibility of HPI-BT in donor polymers) results in lower charge collection efficiency Design rules for better non-fullerene acceptors Appropriate energy levels Good electron mobility (both local and device-level) Mixing between donor polymer and small molecule acceptor Good electron transport through mixed phase Investigate effect of local vs. device-level mobility in improving charge separation efficiency Synthesis of new acceptor architectures to control promote donor-acceptor mixing P3HT HPI-BT Device Performance with HPI-BT Current-Voltage Curves with HPI-BT Believed to be highest efficiency using non-fullerene based acceptor with P3HT as donor material BASF P3HT PDHTT BASF P3HT Acceptor HPI-BT HPI-BT PC 61 BM Jsc (mA/cm 2 ) 6.5 4.8 10.3 Voc (V) 0.94 1.11 0.58 FF 0.61 0.64 0.73 PCE (%) 3.7 3.4 4.4 η = 3.7% Lower quantum efficiency with HPI-BT Electric field dependence of internal quantum efficiency in combination with sufficient exciton quenching suggests strong geminate pair recombination from CT state -1 -0.5 0 0.5 1 1.5 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 Voltage (V) Current Density (mA/cm 2 ) PDHTT BASF P3HT BASF P3HT:PC 61 BM A B B B-A-B type molecular structure allows tailoring to obtain specific material properties HPI-BT PEDOT:PSS/ITO Donor Fullerene Derivative Ca/Al HOMO offset too small to split excitons on fullerene qV OC PEDOT:PSS/ITO Donor New Acceptor Ca/Al qV OC 300 400 500 600 700 800 900 0 0.2 0.4 0.6 0.8 Wavelength /nm External Quantum Efficiency P3HT:HPI-BT PDHTT:HPI-BT P3HT:PC 60 BM 350 400 450 500 550 600 650 0 0.2 0.4 0.6 0.8 Wavelength /nm Internal Quantum Efficiency PDHTT:HPI-BT P3HT:HPI-BT P3HT:PC 60 BM -10 -8 -6 -4 -2 0 2 0 0.2 0.4 0.6 0.8 1 Voltage /V Internal Quantum Efficiency P3HT:PCBM (600 nm) P3HT:HPI-BT (435 nm) PDHTT:HPI-BT (435 nm) 500 600 700 800 900 0 0.2 0.4 0.6 0.8 1 Wavelength /nm Photoluminescence /a.u. P3HT HPI-BT Blend Why do fullerenes work so well? Yin et al., ACS Nano. 2011 DOI: 10.1021/nn200744q Polymer-Fullerene Miscibility PC 60 BM Mixed PC 60 BM Mixed P3HT Energetic Offsets from Mixed Phase Two Phase Morphology HPI-BT P3HT No Energetic Offsets to increase IQE 0% HPI-BT in P3HT 100% HPI-BT 15% HPI-BT in P3HT Organic Photovoltaics Solar energy is a highly abundant, clean energy source. In 1.5 hours, enough solar energy hits the surface of the Earth to provide 100% of our global energy needs for an entire year (15 Tw × 1 year) Why organic photovoltaics? Using a 1.1V cell as a top cell in a tandem device with a CIGS cell could boost efficiency from 15% to 21%, cutting overall cost by ~ 30% Energy payback time for organic solar cells is only 2-3 months, compared to 2-3 years for other technologies Roll-to-roll processing techniques are potentially very low cost Roes et al., Prog. Photovoltaics., 2009, (17), 372-393 Varying Core Heteroatom HBT BO BSe Varying Arm Solubility OPI DPI EPI Varying Core Electronics DFBT Beiley, Z. and McGehee, M.D., Energy & Environ. Sci., 2012, (5), 9173
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Solution-Processed Bulk Heterojunction Solar
Cells with Novel Acceptor Molecules Tommaso Giovenzana, Jason T. Bloking, Andrew T. Higgs, Andrew Ponec, Michael D. McGehee and
Alan Sellinger
Donor
Acceptor
Ca/Al
PEDOT/ITO
2
3
3
HOMO Level (Valence Band)
LUMO Level (Conduction Band)
1
How do organic solar cells work?
How current is generated
1) Photon absorption and exciton generation
2) Charge transfer / exciton splitting
3) Charge carrier collection
Ohmic electron contact (Ca/Al)
Ohmic hole contact (PEDOT/ITO)
Donor
Acceptor
Acceptor Materials in OPV
Fullerene Derivatives (Up to 9.3% efficient)
Drawbacks of Fullerene Derivatives
Low singlet state energy (1.7 eV) limits VOC to 1.0 V
Weak absorption of solar spectrum in visible region
Higher cost ($50/g, $5.50/m2, $0.06/W)
Alternative acceptor materials will be needed to produce high VOC top cells
in a hybrid tandem configuration
PC61BM PC71BM ICBA
Synthesis of New Acceptor Materials
X-Ray Diffraction shows no Miscibility
Summary
Future Directions
X-ray diffraction scans show evidence of crystalline HPI-BT phase with only 15
wt.% HPI-BT (miscibility limit of HPI-BT in P3HT < 15 wt%)
New small molecule electron acceptors fill some gaps left by fullerene derivatives
Open-circuit voltages as high as 1.11 V targeting top cells in tandem devices
Increased photocurrent from absorption in acceptor phase
Potentially lower cost synthesis
No energetic offsets caused by two-phase morphology (low miscibility of HPI-BT in
donor polymers) results in lower charge collection efficiency
Design rules for better non-fullerene acceptors
Appropriate energy levels
Good electron mobility (both local and device-level)
Mixing between donor polymer and small molecule acceptor
Good electron transport through mixed phase
Investigate effect of local vs. device-level mobility in improving charge separation
efficiency
Synthesis of new acceptor architectures to control promote donor-acceptor mixing
P3HT
HPI-BT
Device Performance with HPI-BT
Current-Voltage Curves with HPI-BT
Believed to be highest efficiency using non-fullerene based acceptor with P3HT as
donor material
BASF P3HT
PDHTT BASF P3HT
Acceptor HPI-BT HPI-BT PC61BM
Jsc (mA/cm2) 6.5 4.8 10.3
Voc (V) 0.94 1.11 0.58
FF 0.61 0.64 0.73
PCE (%) 3.7 3.4 4.4
η = 3.7%
Lower quantum efficiency with HPI-BT
Electric field dependence of internal quantum efficiency in combination with
sufficient exciton quenching suggests strong geminate pair recombination from CT
state
-1 -0.5 0 0.5 1 1.5-12
-10
-8
-6
-4
-2
0
2
4
6
8
10
Voltage (V)
Cu
rre
nt D
en
sity (
mA
/cm
2)
PDHTT
BASF P3HT
BASF P3HT:PC61
BM
A B B
B-A-B type molecular structure allows tailoring to obtain specific material properties
HPI-BT
PEDOT:PSS/ITO
Donor Fullerene Derivative
Ca/Al
HOMO offset too small to split excitons on fullerene
qVOC
PEDOT:PSS/ITO
Donor New
Acceptor
Ca/Al qVOC
300 400 500 600 700 800 9000
0.2
0.4
0.6
0.8
Wavelength /nm
Ex
tern
al
Qu
an
tum
Eff
icie
nc
y
P3HT:HPI-BT
PDHTT:HPI-BT
P3HT:PC60
BM
350 400 450 500 550 600 6500
0.2
0.4
0.6
0.8
Wavelength /nm
Inte
rna
l Q
ua
ntu
m E
ffic
ien
cy
PDHTT:HPI-BT
P3HT:HPI-BT
P3HT:PC60
BM
-10 -8 -6 -4 -2 0 20
0.2
0.4
0.6
0.8
1
Voltage /V
Inte
rna
l Q
ua
ntu
m E
ffic
ien
cy
P3HT:PCBM (600 nm)
P3HT:HPI-BT (435 nm)
PDHTT:HPI-BT (435 nm)
500 600 700 800 9000
0.2
0.4
0.6
0.8
1
Wavelength /nm
Ph
oto
lum
ine
sc
en
ce
/a
.u.
P3HT
HPI-BT
Blend
Why do fullerenes work so well?
Yin et al., ACS Nano. 2011 DOI: 10.1021/nn200744q
Polymer-Fullerene Miscibility
PC60BM
P3HT
MixedPC60BM
MixedP3HT
Energetic Offsets from Mixed Phase
Two Phase Morphology
HPI-BT
P3HT
No Energetic Offsets to increase IQE
0% HPI-BT in P3HT 100% HPI-BT 15% HPI-BT in P3HT
Organic Photovoltaics
Solar energy is a highly abundant, clean energy source. In 1.5 hours, enough solar
energy hits the surface of the Earth to provide 100% of our global energy needs for
an entire year (15 Tw × 1 year)
Why organic photovoltaics?
Using a 1.1V cell as a top cell in a tandem device with a CIGS cell could boost
efficiency from 15% to 21%, cutting overall cost by ~ 30%
Energy payback time for organic solar
cells is only 2-3 months, compared
to 2-3 years for other technologies
Roll-to-roll processing techniques are
potentially very low cost
Roes et al., Prog. Photovoltaics., 2009, (17), 372-393
Varying Core Heteroatom
HBT
BO
BSe
Varying Arm Solubility
OPI
DPI
EPI
Varying Core Electronics
DFBT
Beiley, Z. and McGehee, M.D., Energy & Environ. Sci., 2012, (5), 9173
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