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Learn how molecular structure impacts efficiency in organic solar cells, with focus on fullerene derivatives P3HT and PCBM. Discover why organic cells are a viable alternative to silicon, with insights on charge transport and morphology. Explore the relationship between molecular packing and device performance, along with future research directions.
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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 5.1 eV Al P3HT P3HT ITO PCBM PCBM Al HOMO ITO Light
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 • Charge transport through pi orbitals Light 3.7 eV 5.1 eV Al P3HT ITO PCBM PCBM P3HT
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 5.1 eV Al P3HT ITO PCBM PCBM P3HT
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 5.1 eV P3HT Al ITO PCBM PCBM P3HT
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 Side group Small 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
Stacking Bad transfer Good transfer CF3-TNPS-Tet-Fu TNPS-Tet-Fu TES-Tet-Fu Good Transport Bad 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
Efficiency Jsc Efficiency = max power 100 mW/cm2 Maximum power Voc
Bilayer Comparison • Jsc shows same trend as mobilities in SCD • CF3-TNPS-Tet-Fu shows worst Jsc and device performance Efficiency (%) 3.3E-2 1.6E-3 4.77E-5
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 Low efficiency Small Side group 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