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  • 1. Solar Energy: Organic Photovoltaics
    Courtney Klosterman
    Case Western Reserve University Physics Department
    REU Summer Program
  • 2. Outline of Presentation
    Big Picture Problem
    Background: Silicon/Organic Solar Cells
    What I’m looking at: Structures and Calculations
    Matching Calculations
    Future Work
  • 3. Big Picture Problem
    • Solar cells are much more expensive to produce than petroleum and coal
    • 4. Prevents their widespread use to generate electricity
    • 5. Most solar cells today are made of silicon in different states: crystalline, multicrystalline or amorphous
  • 6. Silicon Solar Cells
    • In semiconductor photovoltaics, a pn-junction is formed by bringing together doped p-type and n-type materials
    • 7. Recombination at the junction creates a depletion region with a large built-in electric field
    • 8. Electrons/holes are created when a photon is absorbed, and if in the depletion region, will be pushed in opposite directions by the electric field.
  • Organic Solar Cells
    • Organic solar cells are of interest due to their cheaper fabrication, installation and materials costs. However, they have lower efficiencies.
    • 9. The polymers poly-3-hexylthiophene (P3HT) and Phenyl-C61-butyric acid methyl ester (PCBM) are mixed together to form an interpenetrating network of p and n type material called a bulk heterojunction.
  • 10. Organic Solar Cells
    • When a photon is absorbed, an exciton, a bound electron-hole pair is created.
    • 11. When the exciton reaches an interface between the p and n-type materials, charge is transferred which breaks the exciton and allows charge to be extracted to the electrodes.
    • 12. In a bulk heterojunction the distance the electron-hole pair has to travel to an interface is minimized, which optimizes the absorbance and minimizes recombination of electrons and holes
  • The Main Problem
    Exciton Diffusion length=10 nm
    Optimal absorbance thickness=200 nm
  • 13. Structures
    • Calculations using transfer matrix theory show a significant change in absorption of the solar spectrum depending on the thickness of the polymer and the front electrode.
    • 14. The absorption peak would be optimized around 50nm layer thickness of the polymer photovoltaic.
    • 15. Similar to a laser cavity effect called frequency pulling
    ITO, PV, Al
    ITO, PV, no cavity effect
  • 16. Structures
    • This device, if realized experimentally could show that the light absorption and the efficiency will be higher with a thinner active layer. The efficiency improvement arises in large part because at 60nm active layer thickness there would be less recombination.
  • GOAL
    Verify the calculations that thinner samples can absorb as much as very thick samples depending on the thickness of the front electrode for regular and inverted structures
    LET’S DO IT!
  • 17. How we make slides
    Use commercial ITO glass, or Sputter-coat ITO
    Mix Polymers together, dissolve well
    Spin coat polymers at various speeds
    Evaporate Aluminum or Sputter-coat Silver
    Sputter-coater Some finished Samples Evaporator
  • 18. Silver, PV, ITO
    • These samples were made with sputtered Silver 65.6 nm thick, spin coated polymer, and sputtered ITO 101.3 nm thick
    • 19. Some of these graphs have features that match up nicely, but thicker silver is desired
  • Our ITO, PV, Silver
    • These samples were made with sputtered ITO 68.3 nm thick, spin coated polymer, and sputtered Silver 83.3 nm thick.
    • 20. The data vs. the calculations do not match up quite nicely, except for some bigger features. Could be an oddity in the ITO used
  • Commercial ITO, PV, Silver
    • These samples were made by Commercial ITO 100nm thick, spin coated polymers, and sputtered Silver 83.3 nm thick on top.
    • 21. Here, the red line (66.1 nm thick PV layer) shows the big features of the calculations. This is significant because it is absorbing more light out at longer wavelengths.
  • How does it further the big picture?
    • If more samples of the thinner PV can be made to match the calculations, then each different structure will absorb more light.
    Since the exiton has less room for recombination, it will more likely create work and thus give these organic solar cells greater efficiencies.
    With greater efficiencies, organic solar cells could be mass produced
  • 22. What’s next?
    • Make in Glove boxes
    • 23. More structures with Bilayer
    • 24. Look at electrical properties
    • 25. Find parameters that give good absorption and electrical properties
    Glove Box System at Ohio State
    Organic Solar Cell IV Curve
  • 26. Acknowledgements
    Kenneth D. Singer
    Brent Valle
    National Science Foundation