a-Si:H application to Solar Cells - PowerPoint PPT Presentation

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a-Si:H application to Solar Cells

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Overview Fundamentals Process Where we re at ... H thin film layers provide excellent surface passivation for c-Si solar cells Ultra-clean, state of the art, ... – PowerPoint PPT presentation

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Title: a-Si:H application to Solar Cells


1
a-SiH application to Solar Cells
  • Jonathon Mitchell
  • Semiconductors and Solar Cells

2
Overview
  • Fundamentals
  • Process
  • Where were at

3
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

4
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

5
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
6
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
7
PECVD
  • Deposition initiates reactions at surface
  • Desorption/abstraction
  • Absorption

8
QSSPC
  • Carrier lifetime measured within materials
  • Quasi-Steady State Photoconductance (QSSPC)
  • Transient Photoconductance (PCD)

? ? ? ? ?
9
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

10
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

11
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
12
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

13
Acknowledgements
  • ARC for providing funding
  • Murdoch University for use of PECVD
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