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How Molecular Structure Influences Device Performance in Organic Solar Cells

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

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  1. How Molecular Structure Influences Device Performance in Organic Solar Cells Fullerene Derivatives Kirsten Parratt, Loo Lab, 11/9/2010

  2. 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

  3. Why Organic Solar Cells? • An alternative to silicon solar cells: • Easier manufacturing • Low temperature processing • Solution processing • Lower costs • Flexible substrates

  4. 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

  5. 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

  6. 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

  7. 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

  8. Overview of Morphology-Length Scales Molecular ordering Crystal size Phase separation

  9. 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

  10. 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

  11. Stacking Bad transfer Good transfer CF3-TNPS-Tet-Fu TNPS-Tet-Fu TES-Tet-Fu Good Transport Bad Transport J. Anthony

  12. Single Carrier Diodes • Composed of only a fullerene • No photocurrent generation • Measure the transport of charge through the active layer ITO Fullerene Pedot Al

  13. 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

  14. Efficiency Jsc Efficiency = max power 100 mW/cm2 Maximum power Voc

  15. 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

  16. 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

  17. 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

  18. Acknowledgements • Professor Loo • Stephanie Lee • Loo lab • PEI

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