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Stability Issues of 

Transparent Conducting 

Oxides (TCOs) for 

Thin-Film Photovoltaics

John Pern, Ph.D. 

National Center for Photovoltaics (NCPV)

National Renewable Energy Laboratory (NREL)

Golden, Colorado, USA

APP International PV Reliability Workshop

Dec. 4-5, 2008, SJTU, Shanghai, China

NREL/PR-520-44665
Outline

• Background 

– application of TCOs in all thin-film PV

• Stability Issues

-- Accelerated Tests and Results

-- ZnO, ITO, SnO2 by Damp Heat (IEC 61646)

-- Impact on thin film PV encapsulation/construction

• Solutions:

– New TCOs

– Mitigation Methods

• Conclusions
TCO in PV Applications

• Various TCOs are used in various thin-film 

PV technologies, ranging from a-Si to CIGS, 

CdTe, dye-sensitized, organic, and 

quantum-dot solar cells.

• Performance, Cost, and Thin-Film PV 

Fabrication Sequence (Substrate- or 

superstrate-type) are important factors in 

the selected use of TCO.
Example-1: ZnO, 

ITO, and SnO2 in 

a-Si & a-Si/μc-Si 

micromorph PV

Single p-i-n junction a-Si 

Oerlikon Solar

Sanyo Single

junction p-i-n a-/μcSi

MRS BULLETIN • VOL. 32 • 

S. Guha, Uni-Solar Triple- MARCH 2007

Junction a-Si, MRS, 2004 D. Domine et al., IMT, 2005
η = 19.9% η = 16.5% Substrate-Type

Superstrate-Type

Example-2: ZnO, ITO, SnO2, Cd2SnO4

in CIGS and CdTe PV
Example-3:

Nanowire DSSC

MRS BULLETIN • VOLUME 32 • MARCH 2007

Quantum Dot 

Solar Cell

Organic Solar Cell (OPV)

(Graetzel) Dye-Sensitized 

Solar Cell (DSSC)
Stability Issues of Thin Film PV

• Two aspects: Absorber vs. TCO

• Absorber: CIGS, CdTe, Organic (OPV)

– sensitivity to moisture at elevated temperature.

• TCO: stability concerns as contact electrode 

or buffer layer or internal reflector

– sensitivity to moisture and applied voltage at 

elevated temperature. 

(This talk focuses only on TCO)

Corrosion TCO Delamination

Glass

TCO

Na+

Na+ Na+

Na+ Na+

Glass

TCO

Na+

Na+ Na+

Na+ Na+

H2O H2O H2O H2O

H2O Na+ H2O Na+ H2O Na+

Na+ H2O Na+ Na+ H2O

Na+

Glass

TCO

H2O H2O H2O H2O

H2O Na+ H2O Na+ H2O Na+

Na+ H2O Na+ Na+ H2O

Na+

H2O H2O H2O H2O

H2O Na+ H2O Na+ H2O Na+

Na+ H2O Na+ Na+ H2O

Na+

H2O Na+ H2O Na+ H2O Na+

Na+ H2O Na+ Na+ H2O

Na+

Glass

TCO

Glass

TCO

K.W. Jansen, EPV, SPIE PV, 8/2008

Negatively biased
Material Stability of TCOs

TCOs:

1. ZnO for NREL CIGS: 

– single layer intrinsic ZnO (IZO)

– single layer Al-doped ZnO (AZO)

– Bilayer ZnO ( = i-ZO + AZO)

– Al-doped Zn1-xMgxO alloys (ZMO, x = 1 – 10 %)

2. In-Sn-O (ITO,, commercial product) 

3. F-doped SnO2 (commercial products and NREL)

Accelerated Stress Exposures:

– Mainly in Damp Heat (DH)--85oC/85%RH (IEC 61646)

– In DH with acetic acid vapor 

– in a weatherometer (WOM) at 2.5 UV suns/60oC/60%RH

Characterization:

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

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

=> To identify degradation mechanism and quantify 

degradation rates

Atlas Ci4000 WOM
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)

Al:ZnO (AZO)

i-/Al:ZnO (BZO)

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 (ZMO)

i-ZnO (IZO)

Optical Degradation of ZnO & ZMO in DH
Optical Degradation 

of Bi-layer i-/Al:ZnO 

(BZO)

Degrading Effect:

(DH + Acetic Acid) > DH > WOM

Acetic acid rapidly destroyed 

ZnO and likely accelerated 

conversion into non-crystalline 

Zn(OH)2

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% Changes for WOM-Exposed Bilayer ZnO #K5553SD1 

WOM

561 h

0, 144, 308 h

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 a Bilayer ZnO in (DH + HAc) at 85o

C

0 h

162.5 h

0 h

162.5 h

DH + Acetic Acid

0

10

20

30

40

50

60

70

80

90

100

250 450 650 850 1050 1250 1450

Wavelength (nm)

Transmittance/Reflectance (%)

0 h

288 h

144 h

T% and R% Changes for DH-Exposed Bilayer ZnO #K6023SD1 

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

Single-layer Al:ZnO (AZO), ~0.1 μm

0 h, 20X 120 h, 2.5X 242 h, 20X 480 h 20X

Bi-layer i-/Al:ZnO (BZO), ~0.2 μm

Surface Morphology Changes on Al:ZnO 

& i-/Al:ZnO upon DH Exposures 

(WYKO Optical Interferometer)
5000X

5000X

5000X

200,000X

200,000X

200,000X

Surface Morphology Changes on ZnO Films (SEM)

150X 500X

150X

150X

5000X

5000X

0.1 μm Al:ZnO

0.1 μm i-ZnO

0.2 μm i-/Al:ZnO

DH = 0 h DH = 480 h
Al:ZnO 0.12 μm => ~1.5 μm, 

porous, ring-like features

ZnO Films at DH=480 h Became Porous & 10~20X 

Thicker (10o tilt SEM)

i-ZnO 0.1 μm => ~2 μm, 

porous, segregated (ditched)

i-/Al:ZnO 0.2 μm => ~2 μm, 

porous, two layers

10,000X 30,000X

10,000X 30,000X

20,000X 50,000X
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
“Apparent” Optical 

Stability of ITO & 

F:SnO2 Coatings 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
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)
Electrical 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 (oC) 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

0

2

4

6

8

10

12

14

0 50 100 150 200 250 300

DH Exposure Time (h)

Mobility (cm2/V-s)

ZnO Mobility as a Function of DH Exposure Time

Al:ZnO 

(0.1 μm)

Al:Zn0.99Mg0.01O 

(0.5 μm)

Al:ZnO (0.6 μm) 

Al:ZnO 

(0.3 μm)

i-/Al:ZnO (0.2 μm)

Degradation Rate:

ZnO >> ITO > F:SnO2

For Al-doped ZnO:

• Thickness Effect

• Substrate Temperature Effect

• “Dry-out” Effect

• Transient Effect
Damp Heat Degraded CIGSSe Mini-Modules 

R. Feist et al., 33 IEEE PVSC, May 2008
Impact on Thin-Film PV 

Encapsulation and Construction

• More stringent requirements than c-Si PV

• Thin-film PV modules are mostly limited to 

glass/glass construction; some, e.g., CdTe 

and CIGS, with desiccant edge sealant to 

help block moisture ingress.

• Only a few flexible thin film modules (e.g., 

Uni-Solar’s a-Si with ITO) have passed the 

damp heat (85oC, 85%RH, 1000h) 

qualification test. 
CIGS with desiccant edge sealant

Seal

EVA

Circuit Plate

Cover Glass

Tedlar/Polyester/Al/Tedlar

Thin Films

EVA

Glass / Glass

Aluminum

Frame

Glass / TPAT

Ribbon Ribbon

Ribbon

Circuit Plate Circuit Plate

Seals

Aluminum

Frame

Seal

Ribbon

Ribbons

Screen

Cover Glass

Pottant Screen

Corrosion at the corner of a 

glass/glass laminate after 3000 h 

of damp heat exposure. The 

majority of the test modules 

failed at 4000 h.

Siemens Solar’s “TPAT” and “glass/glass” 

package and ST80 CIGS codule designs. 

Dale E. Tarrant, PVMaT QR, 8/2004 

Passed 85/85 1000 h, but ...
Solutions Being Investigated

CIGS, a-Si, and OPV (rigid/flexible package)

• Use ITO to replace Al-doped ZnO

• more DH-stable TCOs - InZnO, InGaZnO (NREL)

• Mitigation ZnO with Barrier Coating

-- Multilayer polymer/Al2O3 (PNNL)

-- CIGS mini-modules passed DH 1000 h test

-- Chemical with/without SiOxNy (NREL)

-- Other MO protective layers (NREL)

• Moisture-blocking polymeric topcover
PNNL’s 

Multilayer 

Moisture 

Barrier

Oxide layer

First polymer layer

Multilayer

Coating

H2O Enters

Through Defect

Substrate

L. Olsen, SPIE PV, 

8/2008

Uncoated CIGS Cells with ZnO TCO’s

0

2

4

6

8

10

0 500 1000 1500

Efficiency

Hours @ 85C/85%RH

SP-3827

Laminated SoloPower mini-module, 

25 cm2, on flexible metal foil

0

0.2

0.4

0.6

0.8

1

0 200 400 600 800 1000

Relative Efficiency

Environmental Test

60oC / 90%RH

85oC / 85%RH

Hours
Summary

• Degradation in the optical, electrical, and structural 

properties of ZnO, ITO, and F:SnO2 on glass 

substrates upon direct damp heat exposures.

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

• Instability of TCO, especially ZnO-based contact 

electrodes, (and absorber), imposes stringent 

requirements on thin-film module encapsulation and 

construction to minimize or eliminate the detrimental 

effects of moisture ingress.

• New DH-stable TCOs and moisture-blocking barrier or 

mitigation methods are under developments.
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