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

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

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

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

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

Electrical Model of a Solar Cell Rs RL I + V -

IV Characteristic of a Solar Cell

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

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)

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

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)

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

Effect of Doping Variations on Rs

IV Characteristic of a Solar Cell

Front Contact Design Optimization A tapered design to address the trade-off between contact area and shadowing http://pveducation.org/pvcdrom/design/metal-grid-pattern

Preliminary Results of Front Contact Design Optimization

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

Solar Radiation is Composed of a Range of Wavelengths http://zebu.uoregon.edu/~imamura/122/lecture-3/stellar_spectra.html

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

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

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

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

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