How Molecular Structure Influences Device
Performance in Organic Solar Cells
Fullerene Derivatives
Kirsten Parratt, Loo Lab, 11/9/2010
How it works
• Photons absorbed by the organic compounds in the active
layer create an exciton which diffuses randomly
• Upon reaching the acceptor and donor interface, the
electron dissociates from the hole
• Both electron and hole are transported to their respective
electrode
Al
Al
Al
ITO
ITO
ITO
Why Organic Solar Cells?
An alternative to silicon solar cells:
• Easier manufacturing
• Low temperature processing
• Solution processing
• Lower costs
• Flexible substrates
Electron Acceptor and Donor
• P3HT/PCBM cells currently have one of the highest efficiencies (~5-6%)
• PCBM: [6,6]phenyl-C61-butyric acid methyl ester, acceptor small
molecule
• P3HT: Poly(3-hexylthiophene), donor polymer
LUMO
3.7 eV
Al
P3HT
P3HT
PCBM
ITO
Al
PCBM
5.1 eV
ITO
Light
HOMO
Electron Acceptor and Donor
• P3HT/PCBM cells currently have one of the highest efficiencies (~5-6%)
• PCBM: [6,6]phenyl-C61-butyric acid methyl ester, acceptor small
molecule
• P3HT: Poly(3-hexylthiophene), donor polymer
Light
• Charge transport through pi orbitals
3.7 eV
Al
P3HT
ITO
5.1 eV
PCBM
P3HT
PCBM
Electron Acceptor and Donor
• P3HT/PCBM cells currently have one of the highest efficiencies (~5-6%)
• PCBM: [6,6]phenyl-C61-butyric acid methyl ester, acceptor small
molecule
• P3HT: Poly(3-hexylthiophene), donor polymer
3.7 eV
Al
P3HT
ITO
5.1 eV
PCBM
P3HT
PCBM
Electron Acceptor and Donor
• P3HT/PCBM cells currently have one of the highest efficiencies (~5-6%)
• PCBM: [6,6]phenyl-C61-butyric acid methyl ester, acceptor small
molecule
• P3HT: Poly(3-hexylthiophene), donor polymer
3.7 eV
P3HT
ITO
5.1 eV
PCBM
P3HT
PCBM
Al
Overview of Morphology-Length Scales
Molecular ordering
Crystal size
Phase separation
Structure/Function Relationship
• Systematically altered fullerene for better packing
• How the molecules pack effects device performance
CF3-TNPS-Tet-Fu
TNPS-Tet-Fu
TES-Tet-Fu
Large
Small
Side group
Side group
J. Anthony
Desired Stacking
Bad transfer
Good transfer
• Contact between fullerenes should have better charge transfer
• Fullerene-acene contact will be worse
• Best packing comes from the closest fullerenes
J. Anthony
Bad transfer
Stacking
Good transfer
CF3-TNPS-Tet-Fu
TNPS-Tet-Fu
TES-Tet-Fu
Good
Bad
Transport
Transport
J. Anthony
Single Carrier Diodes
• Composed of only a
fullerene
• No photocurrent generation
• Measure the transport of
charge through the active
layer
ITO
Fullerene
Pedot
Al
Mobility
ue= (J0.5/V)2* L3*e0*er*8/9
e0-permitivity of free space = 8.85418782 × 10-12 m-3 kg-1 s4 A2
er-dielectric constant = 3.9
- Measure of how fast charges can transport through the layer
[J (mA/cm^2)]^0.5
0.6
CF3-TNPS-Tet-Fu
TNPS-Tet-Fu
Tes-Tet-Fu
0.5
0.4
0.3
0.2
0.1
0.0
0
1
2
3
Voltage (V)
4
Efficiency
Jsc
2.0
Efficiency = max power
100 mW/cm2
J (mA/cm^2)
1.6
1.2
0.8
Maximum power
0.4
0.0
-0.4
-0.75
Voc
Bilayer
-0.50
-0.25
0.00
Voltage (V)
0.25
Bilayer Comparison
• Jsc shows same trend as mobilities in SCD
• CF3-TNPS-Tet-Fu shows worst Jsc and device performance
Efficiency (%)
0.24
0.18
J (mA/cm^2)
3.3E-2
CF3-TNPS-Tet-Fu
TNPS-Tet-Fu
Tes-Tet-Fu
0.12
1.6E-3
0.06
0.00
4.77E-5
-0.06
-0.9
-0.6
-0.3
0.0
Voltage (V)
0.3
Conclusion
• The observed mobilities and efficiencies show the
same trends
• Most likely this trends correlates to the size of the
side group
CF3-TNPS-Tet-Fu
TNPS-Tet-Fu
TES-Tet-Fu
Large Side group
Small Side group
Low efficiency
High efficiency
Future Work
Crystallized derivatives would allow us to determine if
the molecules are packing as planned
– More through testing of solvent vapor and thermal
annealling
– Thermal evaporation of the fullerene layer
Acknowledgements
•
•
•
•
Professor Loo
Stephanie Lee
Loo lab
PEI