NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

2009 SPIE Optics and 

Photonics Conference

San Diego, CA

J.A. del Cueto, 

C.A. Deline, 

D.S. Albin, 

S.R. Rummel, 

A. Anderberg

August 5, 2009

NREL/PR-520-46525

Striving for a standard protocol for 

preconditioning or stabilization of 

polycrystalline thin film photovoltaic modules
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Outline of Talk

 Rationale for stabilization/preconditioning procedure

 Proposed Study plan to probe transitory behavior

 Consists of preconditioning

 Dark 90°C anneal at open circuit

 emulates 85/85, but shorter and perhaps induce reversible changes

 Two branches of subsequent stabilization

 Light exposure at 1-sun, 60°C

 Dark exposure at 60°C, in forward bias between optimum power point 

voltage and open-circuit voltage

 Modules used in study

 Value of C-V profiling as signature

 C-V profiles

 on CIGS module, sweep directions, frequencies

 Carrier concentrations, depletion widths on CIGS & CdTe modules

 Performance changes in CIGS & CdTe modules with 

stabilization/preconditioning procedure

 Summary & conclusions

 CV -derived depletion width changes & hysteresis appear linked to stability

 Dark exposure in forward bias likely emulates light exposure at 1-sun
National Renewable Energy Laboratory Innovation for Our Energy Future

Rationale for Preconditioning Procedure

 Transient and metastable changes in CdTe/CIGS performance pose 

challenges in assessing performance: 

 Is a unique PV conversion efficiency/performance metric possible without 

specifying prior exposure history? (probably not)

 Time between exposure and I-V measurement is critical

 Current standard for thin-film PV certification (IEC 61646) stabilization: 

 calls for light-soaking until change in power ≤ 2% is achieved, after consecutive 

periods of at least 43 kW-h/m2 of integrated irradiance 

 designed with amorphous silicon (a-Si) in mind

 Dominant defect mechanism in a-Si: light-induced Staebler-Wronski effect 

 CIGS or CdTe devices most likely have different defect mechanisms 

 Current procedure may be inadequate when applied to CIGS / CdTe 

 Appropriate preconditioning or stabilization steps prior to performance 

testing would lead to reduced error in assessing long-term energy yield, 

service lifetime and/or reliability. 
National Renewable Energy Laboratory Innovation for Our Energy Future

Defects in CIGS or CdTe

 Ascribed major role in shaping transient or metastable 

phenomena

 Mobile ions

 CdTe 

 Cu moving between back contact and CdS/CdTe interface

 CIGS

 Cu - In vacancy complexes

 Se vacancy

 Presence of Traps

 CdTe

 Voc increases with time

 CIGS 

 Persistent photoconductivity

 Changes in acceptor concentration after thermal anneal

 Charging/discharging of donors at CdS/CIGS interface

 C-V profiling used as probe for defects in CIGS & CdTe
National Renewable Energy Laboratory Innovation for Our Energy Future

Proposed Stability Study Plan Flowchart

 Preconditioning step & Main Stabilization Sequence Study Plan 

 Main Stabilization: 

 dark 90°C anneal step, emulates and is shorter version of 85C/85% RH 1000-h 

certification test

 consists of two branches: light exposure & biased dark exposure, both @ 60°C

 Biased dark exposure is advantageous if successful because of ease and lower cost

Begin

Measure Baseline

'storage state'

Precondition: expose in

natural sunlight ~1-27

kW-h/m2, at open circuit

Quality

> min

Discard

No

Main

STAB

Yes

Measure

preconditioned state

End

Yes

Yes

No

dark-soak light-soak

*Note: light-soak on 1 CIS B

module done outdoors due to size

Dark thermal anneal 90°C 48 h, RH ~40%, OC, Measure

No

TPS-5 chamber, dark,

Tair = 60°C, Tmod ~ 60°±1°C

bias~Vmax-Voc, time: 24-48 h

XR-260 ~1-sun,*See Note

Tmod ~ 60°±5°C, load

~Vmax, time: 24 - 48 h

light or

dark soak

?

exposure

> 120 h?

∆ Pmax

< 2 % ?

Main

STAB

Measure
National Renewable Energy Laboratory Innovation for Our Energy Future

Modules Studied

 Diverse set of CdTe & CIGS modules 

 Some nascent or new, never used, or stored as controls

 Some pre-exposed in hot-humid climate

 Some light-soaked indoors

Module 

Type Quantity Exposure Conditions Pre-existing exposure conditions

CdTe A 2 One each: light soak, 

biased dark soak Yes, hot-humid outdoors 3 years

CdTe B 2 One each: light soak, 

biased dark soak No, nascent

CdTe C 1 biased dark soak Yes, indoor light-soak, 1130 kW-h 

in 2002

CIGS A 4 Two each: light soak, 

biased dark soak

Nascent: 3 controls from 2003; 

1 pre-exposed in hot-humid 

outdoors 3 years

CIGS B 2 One each: light soak, 

biased dark soak No, nascent
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Module characterization tests

 I-V

 Using large area continuous solar simulator (LACSS) apparatus

 Dark & light (STC), part into reverse bias

 Derive series resistances, shunt conductances, etc..., via standard diode analysis

 C-V profiling

 Used precision HP 4284A LCR meter

 If module cells are uniform, measuring C-V on module with Nc number of 

interconnected cells, produces signal as if the device under test were a cell 

sized 0.5-3 cm2, 

 due to series connection & magnitude cancellation of cell area (Ac) & number (Nc)

)1( 11 1 ... 1 ... 111

0

21 1 C

C

D

Mod

Mod j Nc Nc N

A

w

C

CCCCCC

⋅⋅⋅≈⇒+++++=

−

εε
National Renewable Energy Laboratory Innovation for Our Energy Future

Value of C-V profiling CdTe cells

 Albin, D.S. et al., “Degradation and Capacitance-Voltage Hysteresis in CdTe 

Devices,” ibid. these proceedings.

 Hysteresis in depletion width vs. bias and derived carrier densities as one sweeps 

into reverse then up to forward bias appears correlated to amount of Cu in devices

 No Cu in back contact  little hysteresis in C-V profile

 Cu in back contact  hysteresis in depletion width between reverse & forward bias scans

2

3

4

5

6

1014

2

3

4

5

6

1015

2

3

Charge Density (cm-3

)

1 2 3 4

Depletion Width, um

CdTe cell with no Cu in contact

 rev scan

 fwd scan

CdTe cell with Cu in contact

 rev scan

 fwd scan

4

3

2

1

0

Depletion Width, um

-0.5 0.0 0.5 1.0 1.5

Reverse Bias Voltage, V

CdTe cell with no Cu in contact

 rev scan

 fwd scan

CdTe cell with Cu in contact

 rev scan

 fwd scan
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C-V profiling on CIGS Module

 Mott-Schottky plot upper graph 

 A2/C2 vs. Vbias 

 C-V scan sweep details

 Start Vb = 0

 sweep to reverse bias, arrives at extrema 

 Vb dwells 5 mins. at extrema in reverse

 sweep reverses to forward bias

 Vb arrives at max fwd bias 

 Vb dwells 5 mins. at max fwd bias 

 sweep reverses & scans down to 0

 dwell for 5 mins. at 0 bias

 Multiple C-V profiles run across broad span of 

frequencies 100 Hz-800 kHz, (lower graph) to 

determine location of best signal

 best signal range ~ 50 – 100 kHz is Θ ≤ 20°

 Θ = angle between reactive component & 

total Z vector = arctangent of dissipation

A2/C2 (cm4/Fd2)

Vb (Volts/cell)

-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4

0.0

5.0.

1014

1.0 .

1015

1.5 .1015

50 kHz

100 kHz

start

Max Rev Vb

Max Fwd Vb

end

Zcros

X

X

Theta (°)

0

10

20

30

40

50

60

70

80

90

t=0

RevBias0

RevBias0 + 5m

Zcros

Fwd Bias0

Fwd Bias0 + 5m

End Vb0

End Vb0 + 5m

Excitation Frequency (Hz)

10 102 103 104 105 106
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CIGS A module, biased dark exposure branch: carrier 

density & depletion width

 Depletion width (Wd) vs. Vbias upper graph & 

carrier density (N(Wd)) lower graph shown 

Profiles at various stages of exposure: baseline, 

after precondition, after dark 90°C anneal, after 2 

voltage-biased dark soaks 

Wd profiles show variation & hysteresis 

Wd hysteresis between measurements sweeping 

into reverse bias & then forward bias

~16%-20% varying by exposure, 

 large increase in Wd and hysteresis after dark 

90°C anneal

 N(Wd) Carrier densities

At baseline start near ~ 1016/cm3 

Increase preconditioning outdoor exposure

Collapse after dark 90°C anneal

Increase after 120 h biased dark soak 60°C back 

up to level after preconditioning exposure

Similar changes for light-soaked CIGS A

Vb (Volts/cell)

-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4

Wd (

μm)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

baseline, never exposed

after 27 kW-h/m2 light

preconditioning outdoors

after dark 90°C anneal

after 24 h biased dark-soak

after 120 h biased dark-soak

1

N(Wd) (1/cm3) 1015

1016

1017

Wd (μm)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

baseline, never exposed

after 27 kW-h/m2 light preconditioning

after dark 90°C anneal

after 24 h biased dark-soak

after 120 h biased dark-soak
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CdTe C module, biased dark exposure branch: carrier 

density & depletion width

 Depletion width (Wd vs. Vb upper graph) & 

carrier density (N(Wd) lower graph) shown

Profiles at various stages of exposure, as in 

previous dark-soak example

 Wd profiles

Show less hysteresis ~5%-15% of total Wd 

and/or variations (as %) than CIGS during 

tests 

larger & span larger range than for CIGS

Dark 90°C anneal lowers Wd

Post-anneal exposure brings profiles close to 

levels after preconditioning

 N(Wd) carrier densities

show small or modest variation with exposure 

and dark anneal

Minima ~just under 1014/cm3, consistent with 

Albin data for device with no Cu

Vb (Volts/cell)

Wd (

μm)

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

-0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6

baseline, but pre-exposed

after 27 kW-h/m2 light

preconditioning outdoors

after dark thermal anneal

after 24 h biased dark-soak

after 120 h biased dark-soak

Wd (μm)

N(Wd) (1/cm3) 1013

1014

1015

1016

1017

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2

baseline, but pre-exposed

after 27 kW-h/m2 light

preconditioning outdoors

after dark thermal anneal

after 24 h biased dark-soak

after 120 h biased dark-soak
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CdTe A or B, light/dark exposure branches, depletion & 

carrier densities summarized, compared to CdTe C

 CdTe B

 Depletion widths (Wd)

 Wd vs. Vb profiles show variations & 

hysteresis similar to C

 somewhat larger than A

 exhibit similar changes with dark 

90°C anneal:

Wd values lowered, span vs. Vb larger

subsequent exposures raise & bring 

Wd vs. Vb profiles close to levels 

observed after preconditioning

 Carrier densities

 similar minima ~just under 1014/cm3

 mostly 1014-1015, for Wd 0.8-2 μm

 Show similar variation with exposure

 dark 90°C anneal has modest effect

 CdTe A

Depletion widths (Wd)

 Wd vs. Vb profile changes and 

hysteresis are smallest of all CdTe

little changes on dark 90°C anneal

subsequent exposures (light/dark) 

bring Wd vs. Vb profiles close to 

where they were at baseline

 Carrier densities

 similar minima ~just under 1014/cm3

mostly similar to B except spanned 

Wd is less than B modules
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Performance changes in CIGS A or B modules due to 

stabilization procedures

 I-V Changes plotted relative to 

baseline vs. 9 exposure categories 

 initial preconditioning outdoor 

exposures increase performance of 

A1-A3, slightly drops that of B1,B2

 dark 90°C anneal substantially 

degrades performance ~5%-12%, 

largely due to FF & Voc losses

Subsequent light/dark soaks only 

mitigate losses from dark anneal ≤ 1⁄2 

 Performance appear moderately 

stabilized after 3 exposures 

 ∆ ηSTC < ~2% after 3 exposures post 

dark anneal

Light or dark exposures appear 

equally capable of driving toward 

stabilized ηSTC

Module odd/even numbers & dark/light colors 

 dark/light exposures after dark 90 C anneal

∆ η (%) (STC)

∆ FF(%)

∆ Voc (%)

∆ Isc (%)

Preexisting

Baseline

1 kW-h/m2

outdoor

26 kW-h/m2

outdoor

90° C dark

anneal

24 kW-h/m2

or 24 h dark

48 kW-h/m2

or 48 h dark

48 kW-h/m2

or 48 h dark

48 kW-h/m2

light-soak

-15%

-10%

-5%

0%

5%

10%

-10%

-5%

0%

5%

10%

15%

-15%

-3%

-2%

-1%

0%

1%

2%

-4%

-3%

-2%

-1%

0%

1%

2%

3%

4%

5%

A1

A2

A3

B1

B2

A1

A2

A3

B1

B2

A1

A2

A3

B1

B2
National Renewable Energy Laboratory Innovation for Our Energy Future

Sources of performance changes in CIGS A or B modules 

due to stabilization procedures scrutinized

 Std. diode analysis for series resistance 

(Rse) & shunt conductance (Gsh)

 CIGS A

Rse dark/light increase after dark anneal, 

Rse (dark) drop in subsequent exposures, 

but stay somewhat elevated

Rse (light) diminish in latter exposures, are 

fairly low at end (0.4-0.8 Ω-cm2)

 Gsh are relatively small and drop after 

dark anneal and all exposures

 CIGS B

Rse dark/light increase slightly or 

modestly after dark 90°C anneal, but 

diminish in subsequent exposures

 Gsh increase for exposures after dark 

anneal for B2, but B1 shows no change 

after all exposures after dark anneal

 somewhat larger than for CIGS A, may be 

Baseline

more of source of FF loss 

1 kW-h/m2

outdoor

26 kW-h/m2

outdoor

90° C dark

anneal

24 kW-h/m2

or 24 h dark

48 kW-h/m2

or 48 h dark

48 kW-h/m2

or 48 h dark

48 kW-h/m2

light-soak

Rse light I-V 

Ω-cm2 Rse dark I-V (

Ω-cm2) Gsh dark I-V (milliS/cm2) 0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

0.0

0.5

1.0

1.5

2.0

2.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

A1

A2

A3

B1

B2

A1

A2

A3

B1

B2

A1

A2

A3

B1

B2

Module odd/even numbers & dark/light colors 

 dark/light exposures after dark 90 C anneal
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Performance changes in CdTe A, B, C modules due to 

stabilization procedures

 Changes plotted relative to baseline

 initial preconditioning outdoor 

exposures result in small changes

 dark 90°C anneal 

degrades performance of B’s due to FF 

recovers performance of A & C

increases Voc for all modules

 exposures subsequent to dark anneal

 keep A1,A2 gains in dark/light exposures

 B1 improves in dark soak, B2 improves 

at 1st then degrades in light soak

 C1 slowly loses gains in dark-soak

 Performance (∆ηSTC) post dark anneal

A2, B1 moderately stabilized (∆ηSTC< 2%) 

after 3 exposures

 rest not stabilized (∆ηSTC >2%)

dark exposures appear no worse than, & 

are equally capable of driving toward 

stabilized ηSTC as light exposures

Preexisting

Baseline

1 kW-h/m2

outdoor

26 kW-h/

m2

outdoor

 90°C dark

anneal

24 kW-h/m2

or 24 h dark

48 kW-h/m2

or 48 h dark

48 kW-h/m2

or 48 h dark

48 kW-h/m2

light-soak

0%

A1

A2

B1

B2

C1

A1

A2

B1

B2

C1

A1

A2

B1

B2

C1

∆ η % (STC) ∆ FF % ∆ Voc % ∆ Isc %

0%

-30%

-20%

-10%

10%

-5%

0%

5%

10%

-6%

-4%

-2%

0%

2%

-30%

-20%

-10%

10%

20%

Module odd/even numbers & dark/light colors 

 dark/light exposures after dark 90 C anneal
National Renewable Energy Laboratory Innovation for Our Energy Future

Sources of performance changes in CdTe A, B, C modules 

due to stabilization procedures scrutinized

 Std. diode analysis for Rse & Gsh

 all CdTe modules exhibit very low Gsh 

 ≤ 0.2 milliS/cm2, Gsh not a problem at all

 Rse dark/light

CdTe A: 

either slight change or increase initially 

drop close to baseline or lower with 

subsequent exposures

CdTe B

Rse (light) increase dramatically after 

dark anneal & stay high in subsequent 

exposures

 CdTe C

Rse not a problem

Rse dark increase after dark anneal 

but drop to lower than baseline in 

subsequent exposures

 Rse light stay low (≤ 1 Ω-cm2) 

throughout tests

Rse light I-V 

Ω-cm2

Baseline

1 kW-h/m2

outdoor

26 kW-h/m2

outdoor

90° C dark

anneal

24 kW-h/m2

or 24 h dark

48 kW-h/m2

or 48 h dark

48 kW-h/m2

or 48 h dark

48 kW-h/m2

light-soak

Rse dark-IV (

Ω-cm2)

0

20

40

60

80

100

120

140

160

180

Gsh dark-IV (milliS/cm2) 0.00

0.05

0.10

0.15

0.20

0

2

4

6

8

10

12

14

16

A1

A2

B1

B2

C1

A1

A2

B1

B2

C1

A1

A2

B1

B2

C1

Module odd/even numbers & dark/light colors 

 dark/light exposures after dark 90 C anneal
National Renewable Energy Laboratory Innovation for Our Energy Future

Summary

 CIGS A & B (unexposed previously)

FF losses dominate

 Rse increases are chief cause for loss in CIGS A, 

 reversible in exposures (light-soak and biased dark-soak) after dark 90°C anneal, 

so hard to reconcile that with TCO degradation

 more consistent with in changes in semiconductor as shown by changes in C-V 

carrier densities during tests

 Rse increases observed in CIGS B, appear somewhat reversible in 

subsequent exposures

 Shunt losses likely more of problem for CIGS B 

 Voc loses also present for both A & B, ~2%-4%

 Changes in and hysteresis in depletion width (C-V) suggest link to 

stability in CIGS A 

 not enough data measured for CIGS B
National Renewable Energy Laboratory Innovation for Our Energy Future

Summary

 CdTe A (pre-exposed)

 Relatively stable during tests, 

 modest increases in performance after dark anneal and subsequent exposures 

 but efficiencies are always lower than for CdTe B even after CdTe B degraded

Voc increases after dark anneal

 relative small changes in depletion width and hysteresis thereof during exposure 

tests is consistent with link to stability

 CdTe B (unexposed)

FF losses dominate after dark 90°C anneal

Rse (light) increases appears likely main mechanism

 do not reverse substantially in subsequent exposures, maybe irreversible changes after dark 90°C 

anneal at OC

 efficiencies are fairly larger than CdTe A even after degradation

 larger changes in depletion width and hysteresis thereof during exposure tests 

consistent with link to stability

 CdTe C (pre-exposed)

Slight FF, Voc increases with dark anneal & first few subsequent exposures 

Isc & Voc losses in latter exposures drop overall performance slightly

modest changes in depletion width and hysteresis thereof during exposure tests 

are somewhat consistent with the stability- depletion width link
National Renewable Energy Laboratory Innovation for Our Energy Future

Conclusions

 Devised stabilization / preconditioning study plan procedures 

that show promise in arriving at more stable performance 

values when implemented

Both types of exposure: light soaking at 1-sun & 60°C and voltagebiased dark-soaking at 60° C appear capable at driving similar 

performance changes for CdTe & CIGS

Have yet to more precisely quantify equivalency of exposure times between 

two types that produce stabilized performance values

C-V profiling and derived depletion widths changes plus hysteresis 

thereof likely provide valuable co-signature to potential metastability

details of hysteresis and link to stability is likely different for CdTe and CIGS
National Renewable Energy Laboratory Innovation for Our Energy Future

Acknowledgements

 Technical assistance and expertise with chambers & 

setup

 Kent Terwilliger,

 David Trudel,

 Edward Gelak,

 Jose Rodriguez

 This work was supported by the U.S. Department of 

Energy under Contract No. DOEAC36-08GO28308 

with the National Renewable Energy Laboratory.