Nonlinear Shunting

1
Nonlinear Shunting

• Presented by Victor Karpov
• Contributions by
• D. Shvydka,Y. Roussillon,
• A. D. Compaan and D. M. Giolando
• University of Toledo

Abstract a candidate major mechanism of
efficiency loss and degradation
2
Ohmic shunting

• Signature effects
• Loss in Voc
• Loss in FF
• Slope J(V) _at_ V0

CdS
3
Nonlinear shunting without Rsh
Small lift-off voltage (VLO) elements
J
J
J
VLO
VocVLO
Voc


V
V
V
Jsc
J
J
J
VLO
VocVLO
Voc


V
V
V
Jsc
Signature effects Low Voc and FF, no
Rsh Commonly observed in loss and degradation
4
Examples - Weak diodes
Low Voc, main junction problems
Partial shunts Say, through CdS only
Recombination pathways through depleted region
5
Examples Reach through diodes
Spots of strong back barrier
V0 Current blocked
J
VRT
VgtVRT Reach-through current
V
Reach-through diode
Genesis Poor doping, Contaminations, Metal
delamination
6
Correlated FF and Voc Modeling
E x a m p l e
1000 diodes, Gaussian disorder, dVoc/Voc0.2

• General trends
• Cell to cell variations
• Correlated FF and Voc

Ideal Voc1000, FF65 Random 1
Voc790, FF 58 Random 2 Voc720, FF 51
Could be misinterpreted as uniform junction
recombination
7
Correlated FF and Voc Experimental
N 1020 readings Fresh and degraded cells
(described in APL, 2002)
High correlation between FF and Voc
Could be misinterpreted as uniform junction
recombination
8
Nature degrades nonuniformlyStresses concentrate
on weak elements

• Electric Breakdown
• Mechanical Cracking
• Corrosion Spots
• Chemical Decomposition
• Health Problems
• PV should not be an exception

9
Discrete in space, continuous in time

• Stress concentrated on weak elements
• Degrades them further
• Weak element continuous degradation
• Drives the entire system down

Experimentally, various mappings (LBIC, EL, EBIC,
Thermography) Show increase in nonuniformity
Correlated with degradation
10
Shunting instability
Thin films are energetically unstable with
respect to shunting
Small thickness, l1mm, Large area, L 1cm, Large
capacitance, C 1nF, Voltage U 1V
Giant stored energy CU2/2 10 GeV !!! Suffices
to create multiple shunts Inevitable but can be
slowed down
11
Conclusions

• Commonly observed efficiency loss and degradation
  through FF and Voc explained by nonlinear
  shunting
• Caused by weak diode or strong back barrier spots
• Can be mistaken for uniform junction
  recombination
• If nothing else, occurs through electrostatic
  instability
• Inevitable, but can be slowed down