StaeblerWronski effect in amorphous silicon PV and procedures to limit degradation

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Staebler-Wronski effect in amorphous silicon PV
and procedures to limit degradation

• EY-1.1 28th October 2005
• Benjamin Strahm
• Ecole Polytechnique Fédérale de Lausanne
• Centre de Recherches en Physique des Plasmas

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Staebler-Wronski effect
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• Short after the development of the first solar
  cell based on amorphous silicon (a-Si) material
  in 19761, a light induced degradation of
  performances was reported by Staebler and Wronski
  in 1977, the Staebler-Wronski effect2 (SWE) was
  born.

1 Carlson, Wronski, Appl. Phys. Lett. 28 (1976)
671. 2 Staebler, Wronski, Appl. Phys. Lett. 31
(1977) 292.
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Mechanisms of light induced degradation
The photoconductivity is governed by
Mobility of free carrier
Free carrier life time
?pe?ne??G
Concentration of free e in the CB
e-hole pair generation rate
As the mobility ? and the generation rate G are
constant, the photodegradation is caused by
changes in the photocarrier lifetime ?, which is
directly related to defect density acting as
recombination centers and to the recombination
capability of such sites.
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Defects stability
The defects are not thermally stables. Thus, a
light degraded sample can recover its initial
performances after a thermal annealing.
Performance before and after annealing
From van den Heuvel et al.
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Types of defects

• Light induced defects can be of various kinds.
  Commonly we distinguish 3 sets of defects

With Ea the annealing energy and C the
recombination capability of the defect.
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Defect and annealing energy
Working temperature are high enough to be in a
regime where soft defect are unstable.
Stradins et al, Solar En. Mat. And Solar Cells 78
(2003) 349-367.
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Defects
Defects are not well identified.

• Soft defect can be related to small imperfections
  of the structure like mechanical stresses.
• Hard defect are more serious imperfections like
  Si dangling bonds.
• Stable defects are microstructural defects such
  as micro voids.

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Defects
The free carrier lifetime t is governed by the
defect density and their recombination
capability.
Many observations show that the relevant factor
that induces degradation is the defect density,
but others show that it is the recombination
capability of the defects.
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Lowering degradation (I) material improvement
Incorporate hydrogen (10 at) in using hydrogen
diluted silane (SiH4) during deposition (PE-CVD
or HW-CVD).
Deng Schiff Amorphous silicon based solar cells
Passivation of dangling bonds by H atoms.
Wronski, Solar Ene. Mat. And Solar Cells 41/42
(1996) 427-439.
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Lowering degradation (II) material improvement
Ordering the microstructure from a-SiH to
µc-SiH.
Franz et al, Thin Solid Films 383 (2001) 11-14.
Ea distribution becomes narrow and Ea decreases.
Working temperature is high enough to have only
stable defects.
Meier J et al, Appl. Phys. Lett. 65 (1994) 7.
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Lowering degradation (II) engineering improvement
Multi-junction solar cells
Reduction of the active layer thickness in order
to reduce free carrier losses due to low
diffusion length (??).
Single junction cell
Multi-junctions cell
??
??
??
Only a small fraction of free carriers are
collected.
Even if interface losses are added, the
collection of free carriers is more efficient.
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Future trends
The research is focused now on a mixture of
material improvement and engineering improvement
the tandem solar cell using a multi-junction cell
with both materials that have different
absorption spectrum.