Damp-Heat Induced Degradation of 

Transparent Conducting Oxides 

for Thin Film Solar Cells

John Pern, Rommel Noufi,* 

XiaoNan Li, Clay DeHart, and Bobby To

National Center for Photovoltaics

National Renewable Energy Laboratory

1617 Cole Blvd., Golden, CO 80401

* currently with SoloPower Inc.,

5981 Optical Court, San Jose, CA 95138

33rd IEEE PVSC, 5/12/2008, San Diego, CA

NREL/PR-520-43256

Presented at the 33rd IEEE Photovoltaic Specialist Conference held May 11-16, 2008 in San Diego, California
ACKNOWLEDGEMENTS

Dave Albin – Discussions

Steve Glick – SiOxNy coating 

Jie Zhou – XRD 

Joel Pankow – XPS

This work was supported by the U.S. 

Department of Energy under the Contract 

No. DE-AC36-99GO10337 with the 

National Renewable Energy Laboratory
OBJECTIVES

• Project Long-Term: To achieve a high long- term performance reliability for the thin-film 

CIGS PV modules with more stable materials, 

device structure designs, and moisture- resistant encapsulation materials and 

schemes. 

• Current work: To evaluate the DH stability of 

various transparent conducting oxides (TCOs)

– To identify the degradation mechanisms & quantify 

degradation rates.

– To seek chemical and/or physical mitigation 

methods, and explore new materials.

– Important Note:

Direct exposure to DH represents an extreme 

condition that a well-encapsulated thin film PV 

module may ever experience.
EXPERIMENTAL

Materials:

1. ZnO sputtered on 7059 glass plates:

Semicore:

• Single layer intrinsic ZnO (IZO), ~0.1 μm

• Single layer Al:ZnO (AZO), ~0.1 μm, 2% Al

• Bi-layer i-/Al:ZnO (BZO), ~0.2 μm (for NREL CIGS cells)

ATC 2200-V:

• Single layer Al:Zn1-xMgxO (ZMO, x=0~0.1), ~0.3~0.6 μm

2. ITO (1 commercial, AFL)

3. F:SnO2 (2 commercial, P-TCO & P-TEC, and 1 NREL, L6A3)

Accelerated Stress Exposures:

– Damp Heat (DH) -- 85oC / 85% RH

– DH with acetic acid vapor

– In a weatherometer at 2.5 UV suns, 60oC, 60% RH 

Characterization: – Optical (T%, R%); Electrical (Hall, 4-probe);

– Structural (XRD); Micro-imaging (optical and SEM)
Initial T% and 

R% spectra of 

as-deposited 

IZO, AZO, BZO, 

and ZMO

0

10

20

30

40

50

60

70

80

90

100

250 450 650 850 1050 1250 1450

Wavelength (nm)

Transmittance/Reflectance (%)

i-ZnO

i-ZnO

Al:ZnO

Al:ZnO

i-/Al:ZnO

i-/Al:ZnO

T% and R% Spectra for Semicore-Deposited ZnO Films

i-/Al:ZnO BZO: 

Ebg = 3.3~3.4 eV

0

10

20

30

40

50

60

70

80

90

100

250 450 650 850 1050 1250 1450

Wavelength (nm)

Transmittance/Reflectance (%)

T% and R% Spectra for ATC2200V-Deposited Al:ZnMgO Films

Al:Zn1-xMgxO

x = 0.0 x = 0.01 x = 0.1

insulating

Al:Zn1-xMgxO
0

10

20

30

40

50

60

70

80

90

100

250 450 650 850 1050 1250 1450

Wavelength (nm)

Transmittance/Reflectance (%)

0 h

480 h

242 h

120 h

480 h

0 h

(b)

0

10

20

30

40

50

60

70

80

90

100

250 450 650 850 1050 1250 1450

Wavelength (nm)

Transmittance/Reflectance (%)

480 h

242 h

0, 120 h

480 h

0 h

242 h

(c)

Optical Degradation of ZnO in DH

Al:ZnO

i-/Al:ZnO

0

10

20

30

40

50

60

70

80

90

100

250 450 650 850 1050 1250 1450

Wavelength (nm)

Transmittance/Reflectance (%)

480 h

0 h

242 h

480 h

0 h

242 h

(a)

0

10

20

30

40

50

60

70

80

90

100

250 450 650 850 1050 1250 1450

Wavelength (nm)

Transmittance/Reflectance (%)

480 h

Al:Zn0.9Mg0.1O

i-ZnO
Optical Stability 

of ITO & F:SnO2

at DH = 480 h

0

10

20

30

40

50

60

70

80

90

100

250 450 650 850 1050 1250 1450

Wavelength (nm)

Transmittance/Reflectance (%)

0

10

20

30

40

50

60

70

80

90

100

250 450 650 850 1050 1250 1450

Wavelength (nm)

Transmittance/Reflectance (%)

0

10

20

30

40

50

60

70

80

90

100

250 450 650 850 1050 1250 1450

Wavelength (nm)

Transmittance/Reflectance (%)

NREL F:SnO2

P-TEC F:SnO2

AFL ITO
Electrical Degradation

 Degradation Rate: ZnO >> ITO > F:SnO2

 For AZO:

 Thickness Effect - thicker film degraded slower

 Substrate Temperature Effect - 100oC better than ambient (<60oC)

 “Dry-out” Effect - “self-heal” somehow upon interval drying

• BZO => Highly resistive at DH = 242 h if continuous exposure without 

interruption

(assumed linear degradation)

Table 1. Sample Data and Sheet Resistance Changes upon Damp Heat Exposure

Sample ID IZO-11 AZO-21 BZO-31 AZO-383 AZO-471 AZO-501 AZO-681 AFL-1 L6A3-1 P-TCO1 P-TEC3

Composition int.-ZnO Al:ZnO i-/Al:ZnO Al:ZnO Al:ZnO Al:ZnO Al:ZnO Sn:In2O3 F:SnO2 F:SnO2 F:SnO2

MgO added 0% 0% 0% 0% 1% 10% 0%

Dep. Tsub (o

C) Ambient Ambient Ambient Ambient 100 100 100 N/A N/A N/A N/A

Thickness (μm) 0.1 0.1 0.2 0.3 0.5 0.5 0.6 N/A 0.5~0.6 0.26 N/A

DH Time (h) Rsh (ohm/sq)

0 HR* 85.71 64.03 68.19 73.30 HR* 29.34 83.00 8.30 17.92 12.53

120 967.00 107.70 155.60 165.50 39.11 77.87 7.98 17.15 12.82

242 15110.00 510.50 236.10 272.50 50.10 82.05 7.80 17.42 12.66

480 HR* HR* HR* HR* HR* 83.20 8.00 17.82 12.77

886 HR*: Highly resistive 84.34 8.24 17.38 12.69

Deg. Rate [(ohm/sq)/h] 6.21E+1 1.84E+0 6.94E-1 8.23E-1 8.58E-2 1.51E-3 -6.99E-5 -6.09E-4 1.80E-4
Structural Degradation of ZnO in DH

0

500

1000

1500

2000

30 31 32 33 34 35 36 37 38 39 40

2 Theta Degree

Intensity (cps)

XRD Peak and Pattern Changes as a Function of DH Exposure Time

0 h

120 h

242 h

480 h

480 h

ZnO 

(002)

ZnO 

(100) Zn(OH)2 

(201)

i-ZnO

0

500

1000

1500

2000

2500

3000

3500

33 34 35 36

2 Theta Degree

Intensity (cps)

i-/Al:ZnO

0

100

200

300

400

500

600

700

800

900

1000

30 32 34 36 38 40

2 Theta Degree

Intensity (cps)

Al:Zn0.99Mg0.01O

0

100

200

300

400

500

600

700

33 34 35 36

2 Theta Degree

Intensity (cps)

Al:ZnO
3D Images of Surface Morphology for i-ZnO 

upon DH Exposures (WYKO Optical Interferometer)

DH 242 h 50X DH 480 h 20X

DH 0 h 2.5X DH 120 h 20X DH 242 h 2.5X DH 242 h 20X

DH 242 h 50X

Intrinsic ZnO, 

~0.1μm
0 h, 20X 480 h 20X

0 h, 20X 120 h, 50X 242 h, 50X 480 h, 50X

Single-layer Al:ZnO, ~0.1 μm

120 h, 2.5X 242 h, 20X

Bi-layer i-/Al:ZnO, ~0.2 μm

3D Images of Surface Morphology for 

Al:ZnO &i-/Al:ZnO upon DH Exposures
Al:Zn0.99Mg0.01O

50X

Al:Zn0.9Mg0.1O

50X 20X

DH=242h 50X

50X

ITO

F:SnO2 (A) F:SnO2 (B)

DH=480h 50X

DH=886h 

2.5X DH=886h 50X

ITO

DH=242h 50X DH=242h 20X

More Morphological Degradations

DH=242h 20X DH=242h 20X

(surface data)
0.1 μm, 

Al:ZnO

5000X 50,000X 200,000X

0.1 μm, 

i-ZnO

0.2 μm, 

i-/Al:ZnO

5000X

5000X

50,000X

50,000X

200,000X

200,000X

SEM Surface Morphology of Unexposed ZnO Films
SEM Surface Micrographs for ZnO at DH=480 h

0.1 μm Al:ZnO

150X 500X 1000X 10,000X

150X 500X 5000X 50,000X

150X 1000X 5000X 30,000X

0.1 μm i-ZnO

0.2 μm i/Al:ZnO
10,000X 30,000X

Al:ZnO 

0.1 μm => 

10,000X 30,000X

SEM (10o tilt) Cross-Section/Surface Morphology of 

DH=480 h ZnO Films

i-ZnO 

0.1 μm => 

i-/Al:ZnO 

0.2 μm => 

20,000X 30,000X 50,000X
SEM Surface Micrographs for Al:ZnMgO at DH=480 h

0.5 μm 

Al:Zn0.99Mg0.01O

150X

5000X

0.3 μm 

Al:Zn1.0Mg0.0O

150X 1000X

500X 5000X

0.5 μm 

Al:Zn0.9Mg0.1O

150X 1000X 5000X
SEM Surface Micrographs for ITO & TCO at DH=886 h

AFL ITO

150X 1000X 5000X 30,000X

PTEC F:SnO2

150X 1000X 5000X 50,000X

NREL F:SnO2

150X 1000X 10,000X 50,000X
DH-Induced Degradation Mechanisms

The temporal process of DH-induced degradation is 

proposed to involve 

1. first hydrolysis of the oxides at some sporadic 

“weak” spots, 

2. swelling and popping of the hydrolyzed spots due 

to volume increase, 

3. “segregation” of hydrolyzed regions causing 

discontinuity of electrical path, 

4. hydrolysis of the oxide-glass interface, and 

5. finally formation of insulating M(OH)2 (& MO) along 

with visible delamination over larger areas.
CONCLUSIONS

• Clear evidence of degradation in the optical, electrical, 

and structural properties of the ZnO, ITO, and F:SnO2

on glass substrates upon direct damp heat exposures.

Degradation Rate in DH: ZnO >> ITO > F:SnO2

• ZnO & ZnMgO showed rapid degradation into highly 

resistive films in DH.

 Thickness effect (thicker film degraded slower)

 Substrate temperature effect (100oC better than ambient)

 Dry-out effect (“self-heal” somehow upon interval drying)

• Future Work to Investigate:

 interconnect stability of i-/Al:ZnO on Mo

 encapsulation effects

 chemical mitigations - PDMS silane treatment 

 physical mitigations - SiOxNy barrier oxide coating

 Experiment designs simulating actual device and module 

structures and encapsulation are required to better assess 

quantitatively the detrimental effects of the materials instability on 

the performance reliability of the CIGS solar cells.
Thanks for your attention!