When a photon is absorbed, an exciton, a bound electron-hole pair is created.
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.
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
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.
The absorption peak would be optimized around 50nm layer thickness of the polymer photovoltaic.
Similar to a laser cavity effect called frequency pulling
ITO, PV, Al ITO, PV, no cavity effect
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!
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
Silver, PV, ITO
These samples were made with sputtered Silver 65.6 nm thick, spin coated polymer, and sputtered ITO 101.3 nm thick
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.
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.
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
Make in Glove boxes
More structures with Bilayer
Look at electrical properties
Find parameters that give good absorption and electrical properties
Glove Box System at Ohio State Organic Solar Cell IV Curve http://deibel.files.wordpress.com
Acknowledgements Kenneth D. Singer Brent Valle National Science Foundation Clips/SOURCE QUESTIONS?