1. yale university Photovoltaic Properties of a Revolutionary New Solar Cell Drew Mazurek Advisor: Jerry Woodall April 30, 2002

2. solar cells in real life Cost-effective way to provide power to remote areas

3. solar cells in real life Cost-effective way to provide power to remote areas Environmentally-friendly renewable energy source

4. solar cells in real life Cost-effective way to provide power to remote areas Environmentally-friendly renewable energy source Power source for outer space applications

5. how solar cells work The simple idea: photons in, current and voltage out

6. how solar cells work A closer look: photons above the semiconductor’s band gap energy generate hole/electron pairs… pn-junction p n - holes - electrons h > E g

7. how solar cells work which then diffuse across the cell’s concentration gradients pn-junction p n - holes - electrons

8. how solar cells work Some holes and electrons recombine before they can reach the other side of the junction. In good cells, however, there is very little recombination. pn-junction p n - holes - electrons

9. how solar cells work Most holes and electrons make it to the other side, resulting in a net charge increase on each side. This net charge increase is realized outside the cell as current and voltage, or power. pn-junction p n - holes - electrons

10. solar cells in space To go into space, solar cells must be efficient – want to produce as much power as possible lightweight – launching satellites into space costs $5,000 per pound Additionally, we’d like them to be inexpensive to manufacture – $$$ radiation-hard – Van Allen Belt ideal place for satellites, but high radiation environment

11. solar cells at yale Strong electric field (~1,000-10,000 V/cm) n ++ np Indium Phosphide drift-based design surface

12. solar cells at yale n ++ np h > E g Indium Phosphide drift-based design Strong electric field (~1,000-10,000 V/cm) surface

13. Strong electric field (~1,000-10,000 V/cm) solar cells at yale n ++ np Indium Phosphide drift-based design surface

14. solar cells at yale motion of carriers due to electric field not as susceptible to material defects motion of carriers due to concentration gradient material defects shorten carrier lifetime, causing more recombination Diffusion (theirs) Drift (ours) vs.

15. solar cells at yale Strong electric field… so what? holes are immediately swept into the junction, producing power fewer hole/electron pairs are lost due to recombination – no time to recombine! n ++ n p

16. solar cells at yale Strong electric field… so what? radiation damage decreases carrier lifetimes. Carriers swept by drift (electric) fields, however, aren’t affected as much. n ++ n p

17. solar cells at yale Why Indium Phosphide? very high ideal efficiency: ~37% at concentration of 1,000 suns

18. solar cells at yale Why Indium Phosphide? very high ideal efficiency: ~37% at concentration of 1,000 suns absorbs most light at small thicknesses – lightweight!

19. solar cells at yale Why Indium Phosphide? very high quantum efficiency across all wavelengths of visible light and UV – highly efficient and it makes good use of almost the entire spectrum

20. solar cells at yale Yale’s InP solar cells are ideal for outer space applications: lightweight radiation-hard highly efficient low cost (~$1/cm 2 vs. $10/cm 2 for current high- efficiency solar cells)

21. summary Solar cells are simply pn-junctions in which hole/electron pairs are created from photons. The holes/electrons diffuse into the junction, and are immediately swept to the other side. The net charge gain is seen outside the cell as current and voltage, or power.

22. summary At Yale, we have designed and perfected the first ever drift-dominated solar cell. By collecting carriers with an electric field, we are able to create solar cells that are robust in the strong radiation of outer space. Additionally, our cells are lightweight and inexpensive.

23. acknowledgements Many thanks to: Professor Jerry Woodall Professor Janet Pan Yanning Sun