a-Si:H application to Solar Cells

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a-SiH application to Solar Cells

• Jonathon Mitchell
• Semiconductors and Solar Cells

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Overview

• Fundamentals
• Process
• Where were at

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Fundamentals

• Photovoltaic effect results from incident light
  on some materials
• PV effect promotes electrons into higher energy
  conduction bands, leaving holes behind
• Separation of carriers, electrons (-ve) and holes
  (ve) important to solar cells
• ve and ve carriers transported through material
  in all directions

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Fundamentals

• Recombination of these carriers occurs in a
  variety of ways
• End Result Nothing useful
• Need to separate these charges
• Recombination occurs at the surface as well due
  to free opposing charges (defects)
• Passivation of this surface is necessary to solar
  cells

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Surface Passivation

• High density of defects at surface of c-Si
• Defect passivating layers SiOx, SiNx, a-SiH

a-SiH Equal or better than others Good
conductor Temperatures lt250oC Doped
layers Heterojunctions possible Thin layers lt10nm
SiOx Best results Good insulator Temperatures
gt900oC High risk of impurity contamination
SiNx Very good results Good insulator Temperatures
?400oC Industrial BSF used Risk of impurities
Cheaper
Cheapest/Easiest
Expensive
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Process

• c-Si wafers etched, RCA cleaned
• a-SiH deposited by plasma enhanced chemical
  vapour deposition (PECVD)

homogenous layer quality and deposition
conditions improved
Non-homogenous Deposition difficult to
control Lower quality layer
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PECVD

• Deposition initiates reactions at surface
• Desorption/abstraction
• Absorption

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QSSPC

• Carrier lifetime measured within materials
• Quasi-Steady State Photoconductance (QSSPC)
• Transient Photoconductance (PCD)

? ? ? ? ?
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Where were at

• Post-deposition thermal anneal greatly improves
  passivation quality of a-SiH layer
• Carrier lifetimes equivalent or better than those
  reported by other groups
• Non-diffusion process defined for surface
  passivation

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Where were at

• Post-deposition thermal anneal
• Thermal annealing near deposition temperature
  significantly improved results
• Thermally stable once saturation reached
• Optimal a-SiH layer thickness 10-20nm
• Other thicknesses work well

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Where were at

• Non-diffusion surface passivation process
  measured and defined
• Surface passivation believed to occur from
  hydrogen diffusing from within these thin layers
  towards the surface
• Less energy needed for surface passivation than
  for diffusion
• A re-configuration of the surface fits these
  results

Energy 1.5eV
Surface Passivation Activation Energy 0.69
0.1eV
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Conclusion

• a-SiH thin film layers provide excellent surface
  passivation for c-Si solar cells
• Ultra-clean, state of the art, high power systems
  arent necessary for these results
• Bulk diffusion of hydrogen is insufficient to
  explain surface passivation 1.5eV
• Non-diffusion surface passivation reactions
  suggest surface reconfiguration is the underlying
  process 0.69 0.1eV
• Thermal annealing improves the surface
  passivation provided by the deposited a-SiH
• Solar cell are possible with the work that has
  been done so far

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Acknowledgements

• ARC for providing funding
• Murdoch University for use of PECVD