1. A Study of the Effects of Processing Variations on the Performance of Silicon Solar Cells
J. Shawn Addington
David T. Wagner
Electrical and Computer Engineering Department
Virginia Military Institute

2. Outline Photovoltaic Effect Solar Cell Fundamentals
Solar Cell Design and Fabrication
Cell Performance
Conclusions

3. Photovoltaic Effect 1839 French physicist Becquerel
Conversion of light to electricity
Solar Cell

4. Solar Cell Structure and Operation
p-Si - E +
n-Si

5. Solar Cell Structure and Operation
Isc I
p-Si e- h+ +
Voc - - E + h+ e- h+ e- h+ + V - RL e- h+ e- h+ e- e- h+
n-Si

6. Electrical Model of a Solar CellRs RL I + V -

7. IV Characteristic of a Solar Cell

8. Series Resistance - Rs Rs = R front contact + Rp + Rn + R back contact
p-Si
n-Si
R front contact
R back contact Rp Rn
Rs = R front contact + Rp + Rn + R back contact

9. R front contact resistance is reduced by using a large contact area
BUT
large contact area blocks more incoming light and lowers power output of cell
Optimization?
(contact design)

10. R back contact Entire back surface
Large surface area minimizes resistance
Optimization?
(back side doping)

11. Rn and Rp Rn depends on the Si wafer characteristics
p-Si
n-Si
R front contact
R back contact Rp Rn
Rn depends on the Si wafer characteristics
Rp depends on the doping characteristics
Optimization?
(doping variations)

12. Effect of Processing on Rp
Dopant Diffusion
p-Si
n-Si

13. Effect of Doping Variations on Rs

14. IV Characteristic of a Solar Cell

15. Front Contact Design Optimization
A tapered design to address the trade-off between contact area and shadowing

16. Preliminary Results of Front Contact Design Optimization

17. Current Research in Back Side Doping
R front contact
p-Si
n-Si Rp Rn
n+ - Si
R back contact

18. Solar Radiation is Composed of a Range of Wavelengths

19. Solar Cell Performance as a Function of Incident Radiation
short wavelengths
long
wavelengths e- h+
p-Si - E +
h+ h+
e- e- e- h+
n-Si

20. Tuning the “Target Window” through Process Variation
shorter doping times
longer doping times
p-Si - E +
n-Si

21. Solar Cell Output Power vs Incident Wavelength
Maximum power output for 750nm incident wavelength
3 hour doping process
Expect sensitivity to shift as doping process varies

22. Current Research Maximum power wavelength vs doping variations
Back-side doping
Front contact design optimization
Antireflective coatings

23. Conclusions Thanks! How solar cells work Why solar cells work that way
How design and processing affect solar cell performance
Ongoing research