A national laboratory of the U.S. Department of Energy

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National Renewable Energy Laboratory 

Innovation for Our Energy Future 

Damp-Heat Induced Degradation 

of Transparent Conducting 

Oxides for Thin-Film Solar Cells 

Preprint 

F.J. Pern, R. Noufi, X. Li, C. DeHart, and B. To 

National Renewable Energy Laboratory 

Presented at the 33rd IEEE Photovoltaic Specialists Conference 

San Diego, California 

May 11–16, 2008 

Conference Paper 

NREL/CP-520-42525 

May 2008 

NREL is operated by Midwest Research Institute ● Battelle Contract No. DE-AC36-99-GO10337 
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Damp-Heat Induced Degradation of Transparent Conducting Oxides 

for Thin-Film Solar Cells 

F.J. Pern, R. Noufi1

, X. Li, C. DeHart, and B. To 

National Center for Photovoltaics, National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401 1

 currently with SoloPower Inc., 5981 Optical Court, San Jose, CA 95138 

ABSTRACT 

 The stability of intrinsic and Al-doped single- and bilayer ZnO for thin-film CuInGaSe2 solar cells, along with 

Al-doped Zn1-xMgxO alloy and Sn-doped In2O3 (ITO) and 

F-doped SnO2, was evaluated by direct exposure to damp 

heat (DH) at 85o

C and 85% relative humidity. The results 

show that the DH-induced degradation rates followed the 

order of Al-doped ZnO and Zn1-xMgxO >> ITO > F:SnO2. 

The degradation rates of Al:ZnO were slower for films of 

higher thickness, higher substrate temperature in sputterdeposition, and with dry-out intervals. As inferred from the 

optical micro-imaging showing the initiation and 

propagation of degrading patterns and regions, the 

degradation behavior appears similar for all TCOs, despite 

the obvious difference in the degradation rate. A 

degradation mechanism is proposed to explain the 

temporal process involving thermal hydrolysis. 

INTRODUCTION 

 Long-term performance reliability is required for all 

photovoltaic (PV) modules ranging from crystalline-Si 

based to thin films including CuInGaSe2 (CIGS), CdTe, 

amorphous Si, dye-sensitized, and organic PV that employ 

transparent conducting oxides (TCOs) as electrical 

contacts or electrodes. Achieving this goal requires not 

only reliable constituent component materials and device 

structure of the solar cells, but also reliable electrical 

interconnects and packaging. To assess the quality of a 

PV module, one of the qualification tests in the IEC61215 

or IEC61464 specifies a 1000-h exposure in damp heat 

(DH) at 85o

C and 85% relative humidity (RH). Most CIGS 

solar cells use a combination of intrinsic and Al-doped 

ZnO as the conducting buffer layer. Reliability of the bilayer i-/Al:ZnO (BZO), along with the Mo base electrode, 

can be the determining factor for durable long-term 

performance of a CIGS-based device/module. Similar 

reliability requirements of TCO-based contacts or 

electrodes are also applicable to other thin-film PVs. 

 In a previous paper, we reported some results from 

an exploratory study that demonstrated the instability of 

Mo and BZO when exposed to DH or to a 2.5 UV-sun at 

60o

C and 60% RH in a weatherometer (WOM) chamber 

[1]. The Mo was sputtered on 2.2-mm-thick soda lime 

glass. We found that Mo was rapidly corroded by the hot 

steam in DH condition, but was significantly less sensitive 

to the exposure in WOM and even less to the heating at 

85o

C in ambient relative humidity typically in the 

11%~15% RH range in our laboratory. The BZO samples 

in that study were old “witness” films deposited along with 

CIGS solar cells on thin 0.5-mm 7059 glass slides with 

some variations in film quality. Nonetheless, when 

exposed to the DH, the “witness” BZO films were all 

degraded by the hot steam, but more rapidly in the 

presence of acetic acid vapor [1]. The acetic acid was 

intentionally introduced to simulate the possible presence 

of the acid from thermal processing and photothermal 

degradation of the ethylene-vinyl acetate (EVA) 

encapsulant commonly used in PV module encapsulation 

[2]. There are also some other studies on the stability 

issues of Al:ZnO by DH exposure reported in the literature 

[3-5]. 

 In the present work, we used 1.1-mm-thick 7059 glass 

slides with single-layer intrinsic and Al-doped ZnO and 

bilayer i-/Al:ZnO that were specifically prepared with 

greater consistency in quality and property according to 

the “standard” conditions well established in our laboratory. 

The investigation was also extended to include Al-doped 

Zn1-xMgxO alloy, Sn-doped In2O3 (ITO), and F-doped SnO2

that can be employed as electrodes for various thin film 

solar cells. The Al-doped Zn1-xMgxO alloy, fabricated in 

house, was tested to evaluate its potential to replace the 

conventional Al:ZnO. The present study focused only on 

the DH exposure-induced degradations. It should be 

emphasized that the direct exposure of the TCO coatings 

on the glass substrates to the DH conditions represents a 

fairly extreme situation that normally may not be 

experienced by a well-encapsulated and packaged PV 

module. This study, therefore, represents primarily our 

efforts to evaluate and compare the stability of various 

TCO materials for thin-film PV applications. 

EXPERIMENTAL 

 To differentiate the differences among the three, 0.1-

μm single-layer intrinsic ZnO and Al-doped ZnO, and 0.2-

μm bi-layer ZnO were prepared separately by magnetron 

sputtering on 7.6-cm x 7.6-cm x 1.1-mm 7059 glass plates 

using a Semicore system, in which the in-situ substrate 

temperatures were determined to not exceed 60o

C with a 

temperature indicator strip. Another set of single-layer Aldoped ZnO with or without MgO (Al:Zn1-xMgxO, x= 0 to 0.1) 

at a thickness of 0.3~0.6 μm was made in an ATC 2200-V 

sputtering system from AJA International, Inc. The 

ambient substrate temperature inside the ATC system has 

1
not yet been definitely determined. The ITO glass plates 

were previously provided by Applied Film Laboratory 

(AFL). One of the F-doped SnO2 samples was made in 

house (ID# L6A3) and the other two were from glass 

manufacturers, one is a commercial product noted as PTCE and the other is an experimental product noted as PTCO. Specimens of ~2.0-cm x 2.8-cm were cut from the 

TCO-coated plates for exposures in a chamber operated 

at 85o

C and 85% RH. After baseline measurements, the 

TCOs were characterized periodically during the course of 

DH exposure by transmittance and reflectance 

measurements for optical properties, Hall measurements 

for sheet resistance, majority carrier mobility and 

concentration, x-ray diffraction (XRD) analysis for 

structural changes, and interference optical micro-imaging 

for morphological changes. Upon completion of the 

experiments, the exposed samples were examined with 

scanning electron microscopy (SEM). The systems used 

were a Cary 5G UV-Vis-NIR spectrophotometer with an 

integration sphere, an Ascent Hall550 Advance system, an 

X1 Advanced Diffraction System from Scintag Inc., a 

WYKO Optical Profiler Model 1100 system from Veeco, 

and a Nova Nano SEM 630 system from FEI, respectively.

RESULTS AND DISCUSSION 

The single-layer intrinsic and Al-doped ZnO showed 

apparent difference from their combined bi-layer i/Al:ZnO 

in transmittance and reflectance spectra as seen in Fig. 1a. 

The Al:Zn1-xMgxO also showed variation in T% and R% 

spectra, being influenced by the content concentration (x = 

0, 0.01, or 0.1) of MgO in the alloy, as seen in Fig. 1b. 

More detailed material information of the tested specimens 

is given in the upper half of Table 1, which includes type of 

TCO, MgO content, substrate temperatures during 

deposition, and film thickness if available. The initial and 

subsequent changes of sheet resistance (Rsh) from Hall 

measurements as a function of DH exposure time are 

given in the lower half. For the purpose of convenience to 

compare the degradation rate, as given in the bottom row, 

a linear degrading relationship was assumed. After 480 h 

DH exposure, all Al-doped ZnO films, including Al:Zn1-

xMgxO, became either insulating or highly resistive (> 1 

MΩ/square) and turned from initial clear to white hazy with 

visible delamination on some specimens. The calculations 

for the degradation rates in Rsh are therefore only up to 

242 h for Al-doped ZnO, and 886 h for ITO and F-doped 

SnO2. From the calculated degradation rates, it is clear 

that the ITO and F:SnO2 were about two to four orders of 

magnitude more stable than ZnO-based films. For the 

0

10

20

30

40

50

60

70

80

90

100

250 450 650 850 1050 1250 1450

Transmittance/Reflectance (%)

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

Fig. 1. Transmittance and reflectance spectra for (a) 

single-layer intrinsic, Al-doped, and intrinsic/Al-doped 

ZnO (i-ZnO, Al:ZnO, and i-/Al:ZnO), and (b) Al-doped 

Zn1-xMgxO films sputter-deposited on 1-mm-thick 7059 

glass plates. 

Wavelength (nm)

i-ZnO

i-ZnO

Al:ZnO

Al:ZnO

i-/Al:ZnO

i-/Al:ZnO

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

(a)

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

x = 0.1

Al:Zn1-xMgxO

x = 0.0 (b) x = 0.01

2
initially insulating intrinsic ZnO (ID# IZO-11) and 10%-

MgO Al:Zn0.9Mg0.1O (ID# AZO-501), degradations were 

determined by measuring changes in optical properties, 

XRD, and micro-imaging. The results show that DHinduced Rsh degradation rates of single- layer Al:ZnO films 

are faster for thinner films (e.g., AZO-21 vs. AZO-383). 

Also, Al:ZnO films deposited at ambient substrate 

temperature in the ATC system degraded faster than 

those deposited at 100o

C substrate temperature (e.g., 

AZO-383 vs. AZO-681). 

 The single-layer i-ZnO, Al:ZnO, and their combined 

bi-layer i-/Al:ZnO did not show substantial changes in 

transmittance and reflectance spectra until after 480 h DH 

exposure, whereas the hazy films exhibited essentially the 

same flat curves losing the characteristic features of Aldoped conducting ZnO as shown in Fig. 2. This is also 

evidenced by the corresponding decrease in the mobility, 

determined by Hall measurements, as seen in Fig. 3a. 

One notable observation is that the degradation rate would 

be greater if the Al:ZnO specimens (not given in Table 1) 

were exposed in the DH chamber continuously for 240 h. 

3

Fig. 2. Transmittance and reflectance spectra for (a) 

single-layer intrinsic ZnO (IZO-11), (b) single-layer Aldoped ZnO (AZO-21), and (c) bi-layer i/Al:ZnO (BZO31) before (T0 = 0h) and after accumulating 120 h, 242 

h, and 480 h DH exposure. 

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)

i-ZnO

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)

Al:ZnO

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)

(a)

Al:Zn0.99Mg0.01O 

(0.5 μm)

Al:ZnO (0.6 μm)

Al:ZnO 

(0.3 μm)

i-/Al:ZnO (0.2 μm)

0

10

250 450 650 850 1050 1250 1450

Wavelength (nm)

Tr

20

30

40

50

60

70

80

90

100 ansmittance/Reflectance (%)

480 h

242 h

0, 120 h

480 h

242 h

(c)

i-/Al:ZnO

0 h

Fig. 3. (a) Decrease in mobility of mono- and bi-layer 

Al:doped ZnO films as a function of DH exposure time, 

(b) change in XRD feature for IZO-11 DH-exposed 

upto 480 h, and (c) sheet resistance of ITO and 

F:SnO2 DH-exposed to 886 h. 

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 (b) (002)

ZnO 

(100) Zn(OH)2 

(201)

0

10

20

30

40

50

60

70

80

90

100

0 200 400 600 800 1000

DH Exposure Time (h)

Sheet Resistance (ohm/sq)

ITO & TCO Rsq as a Function of DH Exposure Time

(c) ITO

NREL 

F:SnO2

P-TEC 

F:SnO2

P-TCO 

F:SnO2
The sample set would become insulating with a white hazy 

appearance, the same as those in Table 1 for 480 h with 

periodic removal from the DH chamber for measurements. 

This suggests that the films removed from the DH 

chamber were capable of recovering their physical 

properties to some extent, as also indicated by the gradual 

change to a somewhat less cloudy appearance after ~2 

days in the air. One likely reason is the loss of water 

molecules from the hydrated or hydrolyzed film bulk and 

film/glass interface. Another interesting observation is that 

a bi-layer i-/Al:ZnO sample was highly resistive when it 

was still moist just after being taken out from a bottle filled 

with moisture in the DH chamber, and then became 

conducting as expected for a film with DH=94 h exposure 

after drying naturally in the air for some minutes. In 

comparison to the ZnO-based films, the optical property of 

the ITO and F:SnO2 films were essentially unchanged in 

transmittance and reflectance spectra after 886 h DH 

exposure. 

The structural changes by DH exposure are also 

reflected in the XRD patterns, per the example shown in 

Fig. 3b for the sample of intrinsic ZnO (IZO-11). Except for 

the bi-layer BZO-31, all single-layer ZnO films show an 

increase in (002) peak intensity at 34.50o

 (2 theta) up to 

DH 242 h, possibly a result of morphological changes (see 

below), before exhibiting a moderate to large decrease at 

DH 480 h due to structural disintegration. For the 

Al:Zn0.9Mg0.1O (AZO-501) alloy, a main peak appeared at 

34.12o

 with a long tail. The main peak shifted to ~33.95o

upon DH exposure up to 242 h and then back to 34.12o

 at 

480 h. IZO-11 and AZO-471 were the two samples that 

produced four new peaks at DH 480 h (Fig. 3b), possibly a 

consequence of structural degeneration effected by the 

hot moisture-induced degradation. The peaks at 31.38o

, 

33.22o

, and 35.38o are assigned, using PDF#89-0510, 65-

2880, and 75-1533 index references for ZnO, to be 

hexagonal (100), cubic (111), and hexagonal (101), 

respectively. The peak at 38.12o may be from cubic (200) 

or Zn(OH)2 (201), as referred to PDF #65-2880 and 89-

0138. Additionally, the peak width at half height for some 

samples showed notable narrowing at DH 480 h (e.g., Fig. 

3b), but its cause is not yet clearly understood. These 

results suggest the ZnO might have undergone structural 

degeneration from original highly (002) preferential 

orientation to a more disordered state with some 

transformation from original hexagonal to cubic, in addition 

to the formation of insulating ZnO and Zn(OH)2. 

Meanwhile, it is not clear yet on the fate of the Al dopant 

upon DH exposure, although it is possible that the Al was 

also readily transformed into Al(OH)3. 

 The surface morphology of the various TCO films was 

also examined periodically for the initiation and 

propagation of the defecting patterns and regions as a 

function of DH exposure time by using the WYKO 

interference optical microscope. More than 1000 microimages were taken. A few of the images are illustrated in 

Fig. 4 for the IZO-11sample at four stages, showing the 

progressive morphological degradation as the DH 

exposure time increased to 480 h. The initially grainy but 

fairly smooth surface started to show some small spikes 

that appeared in a random manner upon exposure to DH 

in the first ~100 h. The number of spikes increased along 

with the appearance of full or partial ring-like features that 

showed ditch-like cutting into the depth of the film as the 

DH time increased (e.g., 240 h). The ring features are 

suspected to arise from the finite water droplets 

condensed from the moisture, which was the case 

observed when the cool films were just loaded into the hot 

steamy DH chamber. As the degradation progressed 

further, the film disintegrated greatly into fabrics-like 

features at DH 480 h and was difficult to be microphotographed because of large loss of light reflection from 

the rough surface. This morphological degradation 

process and pattern development was essentially the 

Fig. 4. Optical micro-images for the sample of intrinsic ZnO (IZO-11), showing the progressive morphological 

degradation as the DH exposure time increased to 480 h. The magnification factors are also indicated. 

4
same for all ZnO-based films as observed in this study. 

While starting with grainy surfaces for the Semicoredeposited Al:ZnO, the DH exposure at 480 h resulted in 

obvious morphological differences for the single-layer iZnO and Al:ZnO, and bi-layer i-/Al:ZnO, as shown by SEM 

topographs in Fig. 5. Similar surface morphological 

degradations for the two samples of Al:Zn1-xMgxO alloy 

(top row), ITO and the experimental P-TCO F:SnO2

(bottom row) at DH 242 h are illustrated in Fig. 6. The TPEC F:SnO2 also exhibited a similar pattern of changes as 

ITO and P-TCO (not shown). At 10% of MgO, the 

Al:Zn0.9Mg0.1O (AZO-501) was originally insulating and, 

upon DH exposure, exhibited localized spot-wise material 

loss (see the third image on the top row of Fig. 6), 

suggesting the MgO-ZnO was not fully alloyed during cosputtered process. 

In comparison with Al-doped ZnO and Zn1-xMgxO alloy, 

the ITO and F:SnO2 were more stable with little or no 

changes in the overall optical and structural properties 

upon 886 h DH exposure (figures not shown), although 

optical micro-images revealed finite corrosions occurring 

at DH 242 h (Fig. 6, bottom row). All three F:SnO2

samples tested were definitely more stable than the ITO at 

886-h DH exposure. Although the low degradation rates 

and relatively flat curves for sheet resistance, Rsh, of ITO 

and F:SnO2, as seen in Table 1 and Fig. 3c, up to DH 886 

h may suggest their high stability against the damp heat, 

the truth is that the Rsh values at DH 480 h and 886 h were 

obtained by moving the four contact probes in the Hall 

measurements around the film surfaces to avoid the highly 

resistive regions or spots as illustrated in Fig. 6 (bottom 

row). This was especially required for the ITO sample 

(AFL-1) at DH 886 h having a number of large and small 

highly resistive regions or spots. The measured Rsh values 

are therefore for the surface areas that still possessed 

continuous conducting paths. Overall, when compared 

with ZnO and ITO, F:SnO2 films were more stable, as 

could be expected also from the literature [6]. Among all 

three, the NREL F:SnO2 (L6A3-1) showed the highest 

stability in all aspects against damp heat even with very 

little change in optical morphological micro-imaging 

observations. 

 Regardless of the film composition and the obvious 

difference in the degradation rate, the degradation 

behavior appears similar for all TCO films studied, as 

Fig. 5. SEM topographical microimages for the 

samples of Al:ZnO (AZO-21), i-ZnO (IZO-11), and 

bilayer i-/Al:ZnO (BZO-31) at DH exposure of 480 h. 

The magnification factors are also indicated. 

Fig. 6. Optical micro-images for the samples of (top row) Al:Zn1-xMgxO (x = 0.01 and 0.1); and (bottom row) ITO, 

and P-TCO F:SnO2, showing morphological degradations at DH 242 h exposure. The magnification factors are 

also indicated. 

5
inferred from optical micro-imaging observations. In 

general, the first sign of degradation started at a few small 

spots or areas with appearance of irregular circles and 

spikes (e.g., Figs. 4 and 6 at 120~242 h); as the DH 

exposure increased, the hydrolysis-induced corrosion 

circles, spikes, and pop-ups increased and became more 

widely spread (e.g., Figs. 4 and 6 at DH 242 h), and finally, 

especially for ZnO-based films, severe disintegration 

resulted (e.g., Fig. 4 at DH 480 h). This thermal hydrolytic 

process might have started from the surface of the film as 

well as at the grain boundary and progressed downward 

and inward. If this was the case, then it would take longer 

exposure time to allow the “ditches” (e.g., Fig. 4, leftmost 

figure at the bottom row) to grow deeper to break up the 

electrical conducting path on a thicker ZnO film, and to 

break up the better grain crystallites on a ZnO film made 

at higher temperature. In addition, visible delaminations 

were seen on some ZnO films after 480 h DH exposure, 

suggesting the destruction of the bonding at the interface 

between the ZnO layer and glass substrate. On the other 

hand, with extended DH exposure to 886 h, the ITO (AFL1) film showed two large and some small insulating 

regions, while F:SnO2 films remained visibly intact. 

Accordingly, based on all the optical, electrical, structural 

and morphological observations, the temporal process of 

DH-induced degradation is proposed to involve first 

hydrolysis of the oxides at some sporadic “weak” spots, 

swelling and popping of the hydrolyzed spots due to 

volume increase, “segregation” of hydrolyzed regions 

causing discontinuity of electrical path, hydrolysis of the 

oxide-glass interface, and finally formation of insulating 

M(OH)2 along with visible delamination over larger areas. 

CONCLUSIONS 

 We have shown the relative stability of a number of 

TCOs, including intrinsic and Al-doped ZnO, Al-doped Zn1-

xMgxO, ITO, and F-doped SnO2, on glass substrates when 

exposed to damp heat at 85o

C and 85% RH. From the 

electrical, optical, structural, and morphological analysis, 

the order of degradation rate is determined to be ZnO >> 

ITO > F:SnO2. With only one conducting Al-doped Zn1-

xMgxO alloy at x = 1% that failed in the DH test in the 

same way as other Al:ZnO samples, it is too premature to 

conclude whether the alloy has the potential with 

appreciable stability to replace Al:ZnO. New experiments 

will be conducted to examine the stability of single-layer iZnO and Al:ZnO, bilayer i-/Al:ZnO, and interconnect 

contact resistance of bilayer i-/Al:ZnO on Mo as a function 

of temperature and relative humidity in order to determine 

the activation energy of degradation. The protective 

effectiveness of moisture-blocking SiOxNy barrier coating 

deposited by PECVD process coupled with surface 

chemical treatment using PDMS silane [1] and DHresistant SnO2 on bi-layer i-/Al:ZnO will be also 

investigated. 

ACKNOWLEDGEMENTS 

 

 We thank NCPV staff D. Albin for useful discussions, 

and S. Glick, Jie Zhou, and J. Pankow for preliminary work 

on barrier oxide coatings, some XRD measurements, and 

XPS analysis, respectively. This work was performed at 

the National Center for Photovoltaics under DOE contract 

number DE-AC36-99-GO10337 with the National 

Renewable Energy Laboratory. 

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F1147-E(09/2007) 

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1. REPORT DATE (DD-MM-YYYY)

May 2008 

2. REPORT TYPE

Conference Paper 

3. DATES COVERED (From - To)

11-16 May 2008 

5a. CONTRACT NUMBER

DE-AC36-99-GO10337 

5b. GRANT NUMBER

4. TITLE AND SUBTITLE

Damp-Heat Induced Degradation of Transparent Conducting Oxides 

for Thin-Film Solar Cells: Preprint 

5c. PROGRAM ELEMENT NUMBER

5d. PROJECT NUMBER

NREL/CP-520-42525 

5e. TASK NUMBER

PVB76701 

6. AUTHOR(S)

J. Pern, R. Noufi, X. Li, C. DeHart, and B. To 

5f. WORK UNIT NUMBER

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

National Renewable Energy Laboratory 

1617 Cole Blvd. 

Golden, CO 80401-3393 

8. PERFORMING ORGANIZATION 

REPORT NUMBER

NREL/CP-520-42525 

10. SPONSOR/MONITOR'S ACRONYM(S)

NREL 

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

11. SPONSORING/MONITORING 

AGENCY REPORT NUMBER

12. DISTRIBUTION AVAILABILITY STATEMENT

National Technical Information Service 

U.S. Department of Commerce 

5285 Port Royal Road 

Springfield, VA 22161 

13. SUPPLEMENTARY NOTES

14. ABSTRACT (Maximum 200 Words)

The stability of intrinsic and Al-doped single- and bi-layer ZnO for thin-film CuInGaSe2 solar cells, along with Aldoped Zn1-xMgxO alloy and Sn-doped In2O3 (ITO) and F-doped SnO2, was evaluated by direct exposure to damp 

heat (DH) at 85oC and 85% relative humidity. The results show that the DH-induced degradation rates followed the 

order of Al-doped ZnO and Zn1-xMgxO >> ITO > F:SnO2. The degradation rates of Al:ZnO were slower for films of 

higher thickness, higher substrate temperature in sputter-deposition, and with dry-out intervals. As inferred from the 

optical micro-imaging showing the initiation and propagation of degrading patterns and regions, the degradation 

behavior appears similar for all TCOs, despite the obvious difference in the degradation rate. A degradation 

mechanism is proposed to explain the temporal process involving thermal hydrolysis. 

15. SUBJECT TERMS

CIGS; PV; transparent conducting oxide; damp- heat induced degradation; sputter deposition; thin film; optical microimaging; thermal hydrolysis; 

16. SECURITY CLASSIFICATION OF: 19a. NAME OF RESPONSIBLE PERSON 

a. REPORT 

Unclassified 

b. ABSTRACT 

Unclassified

c. THIS PAGE 

Unclassified

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UL 

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