1) A study found that monocrystalline PERC solar modules typically suffer from 4-6% light-induced degradation (LID), but that modules left outdoors in open circuit conditions showed signs of natural regeneration from LID over time at field temperatures. 2) Laboratory tests confirmed that PERC cells can be regenerated at field temperatures of 50-60°C through simultaneous carrier injection and annealing. 3) However, field tests found that regeneration is much slower when modules are mounted and operating under maximum power point tracking conditions rather than open circuit. Separate regeneration treatment is still needed at the production stage for manufacturers.

1. Natural Recovery from LID:
Regeneration under Field Conditions?
EU PVSEC 2015, Hamburg, 16 September 2015
Kyumin Lee, Hyundai Heavy Industries, Co., Ltd.

2. Contents
1. Motivation
2. Regeneration at Field Temperatures
3. First Field Test with 60-Cell PERC Modules
4. The Twist: Open Circuit versus MPPT
5. Second Field Test with 60-Cell PERC Modules
6. Cell Regeneration: Module LID ~ 2%
7. Regenerating Lamination
8. Cell Regeneration vs. Regenerating Lamination
9. Summary
EU PVSEC 2015: Natural Recovery from LID? 2

3. Motivation
 Monocrystalline PERC modules typically suffer 4-6% LID.
Manufacturers need to “downgrade” the PERC modules when rating them.
Example: 3% LID penalty = 292 WP Pmax → 280 WP rating
 Permanent deactivation of BO defects by simultaneous carrier injection and
anneal, a.k.a. “regeneration”, is a promising solution (Herguth et al., 2006).
 We produce Cz Si PERC cells with ALD Al2O3 rear passivation.
 In the early stage of our work,
we observed a “natural recovery”
from LID. The reference 2×2 mini-
module (made with non-regenerated
PERC cells) recovered fully from
LID when left outdoors on a roof
for 50 days.
Could this be regeneration
occurring under field conditions?
EU PVSEC 2015: Natural Recovery from LID? 3

4. Regeneration at Field Temperatures
 In most of the existing literature,
regeneration experiments were carried out at temperatures above 70℃.
 How about 50-60℃?
Module temperatures reach the 50-60℃ range on hot days.
 Results of 1-sun-equivalent carrier injection at 50 and 60℃ (single cells):
PERC cells can be regenerated at field temperatures.
(Confirms the suggestion by Fertig et al., Solar Energy Materials & Solar Cells, 2014)
LID / Regeneration Test Setup
EU PVSEC 2015: Natural Recovery from LID? 4

5. First Field Test with 60-Cell PERC Modules
 A fresh 60-cell PERC module (“Test Module”) was installed next to
a stabilized 60-cell PERC module (“Reference”) at an outdoor facility
capable of monitoring the real-time Pmax.
 Relative Specific Yield (RSY) =
Specific YieldTest Module [W-h/WP] / Specific YieldReference [W-h/WP]
Regeneration seems to be occurring in the field.
The Test Module showed 3%p LID recovery in 50 days (5→2% LID).
EU PVSEC 2015: Natural Recovery from LID? 5

6. The Twist: Open Circuit versus MPPT
 The results so far were obtained from modules operating in open circuit.
• First hint from a mini-module left outdoors on a roof... in open circuit.
• First Field Test with a 60-cell test module installed at an outdoor facility...
that does a round-robin Pmax determination of a series of modules,
leaving each module in open circuit for 90% of the time.
 Comment from G. Hahn of U. Konstanz: “Voc conditions between monitoring
would result in higher excess carrier densities for the same irradiance
compared to mpp conditions.”
 The module temperature is also
higher for open circuit operation
than for MPPT operation.
 So...
we conducted the cell test again,
this time in MPPT mode.
Regeneration is 5-10 times slower
when MPP-tracked.
EU PVSEC 2015: Natural Recovery from LID? 6

7. Second Field Test with 60-Cell PERC Modules
 Two fresh 60-cell PERC modules, one made from non-regenerated cells
(“Standard”) and another made from regenerated cells (“Regenerated”),
were installed at the outdoor facility.
 This time, the facility was configured to track the two modules continuously.
(at the expense of the number of modules it can monitor simultaneously)
 The modules were MPP-tracked for
4 weeks. Every week, the modules
were characterized with a Class AAA
solar simulator at 25℃.
 The Standard PERC module did not
show any sign of LID recovery.
For MPP-tracked PERC modules,
field regeneration is too slow
to be useful for manufacturers.
Separate regeneration is needed at the production stage.
EU PVSEC 2015: Natural Recovery from LID? 7

8. Cell Regeneration: Module LID ~ 2%
 Currently, the most popular approach
for regeneration is light-induced
regeneration on finished cells.
 After regeneration, the PERC cells
show negligible LID (less than 0.5%).
But the modules show LID ~ 2%.
 The deactivated BO defects are
partially destabilized during
the high-temperature module
assembly steps (solder tabbing
and lamination).
Is there a better way to perform
regeneration?
EU PVSEC 2015: Natural Recovery from LID? 8

9. Regenerating Lamination
 Regeneration can be integrated
into the lamination process, by
flowing a current through
the cell strings with
a DC power supply
during lamination.
 Pros:
• No subsequent high T step
• No extra step in production
• Identical lamination process time
• Power supplies are cheaper and
last longer than light sources.
 Cons:
• Cell mismatches may occur after
lamination.
EU PVSEC 2015: Natural Recovery from LID? 9

10. Cell Regeneration vs. Regenerating Lamination
 For the Second Field Test,
we had in fact included a
60-cell PERC module made
with the regenerating
lamination.
 Results after 4 weeks of
MPPT operation:
Regenerating Lamination
is more effective, and it
can limit LID to below 1%.
Type LID [%]
Reference 4.3 ± 0.4
Cell Regen. 1.7 ± 0.4
Regen. Lam. 0.9 ± 0.4
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11. Summary
 Permanent deactivation of BO defects in PERC modules can occur naturally
in the field during the summer months, if the modules are in open circuit.
This may be a viable approach for PV system operators and installers.
 A module manufacturer cannot rely on the “natural recovery”, because the
field regeneration rate is very slow when the modules are MPP-tracked.
 Regeneration completes PERC.
 PERC modules made with regenerated cells show LID of ~2%.
With Cell Regeneration, the LID resistance is partially compromised during
the module assembly steps.
 With “Regenerating Lamination”, LID less than 1% is possible.
Thank You for Your Attention!
EU PVSEC 2015: Natural Recovery from LID? 11