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

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National Renewable Energy Laboratory 

Innovation for Our Energy Future 

Degradation of ZnO Window Layer 

for CIGS by Damp-Heat Exposure 

Preprint 

F.J. Pern, B. To, C. DeHart, X. Li, and S.H. Glick 

National Renewable Energy Laboratory 

R. Noufi 

SoloPower, Inc.

To be presented at 2008 SPIE PV Reliability Symposium 

San Diego, California 

August 10–14, 2008 

Conference Paper 

NREL/CP-520-42792 

August 2008 

NREL is operated by Midwest Research Institute ● Battelle Contract No. DE-AC36-99-GO10337 
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Degradation of ZnO-based window layers for thin-film CIGS 

by accelerated stress exposures 

F. J. Pern, R. Noufi1, B. To, C. DeHart, X. Li, and S. H. Glick 

National Center for Photovoltaics (NCPV) 

National Renewable Energy Laboratory (NREL) 

1617 Cole Blvd., Golden, CO 80401 

1

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

ABSTRACT 

The reliability of ZnO-based window layer for CuInGaSe2 (CIGS) solar cells was investigated. Samples of RF 

magnetron-sputtered, single-layer intrinsic and Al-doped ZnO and their combined bilayer on glass substrates were 

exposed in a weatherometer (WOM) and damp heat (DH) conditions with or without acetic acid vapor. Some 

preliminary samples of single-layer Al-doped Zn1-xMgxO (ZMO) alloy, a potential replacement for Al:ZnO with a wider 

bandgap, were also evaluated in the DH. The Al-doped ZnO and ZMO films showed irreversible loss in the conducting 

properties, free carrier mobility, and characteristic absorption band feature after <500-h DH exposure, with the originally 

clear transparent films turned into white hazy insulating films and the degradation rate follows the trend of (DH + acetic 

acid) > DH > WOM. The degradation rate was also reduced by higher film thickness, higher deposition substrate 

temperature, and dry-out intervals. The results of X-ray diffraction analysis indicate that the ZnO-based films underwent 

structural degeneration by losing their highly (002) preferential orientation with possible transformation from hexagonal 

into cubic and formation of Zn(OH)2. Periodic optical micro-imaging observations suggested a temporal process that 

involves initial hydrolysis of the oxides at 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 oxides/hydroxides with visible delamination over larger areas. 

Key words: transparent conducting oxides, intrinsic and Al-doped ZnO, ZnMgO alloy, accelerated exposure, damp heat, 

weatherometer, degradation, CIGS, thin-film solar cell. 

INTRODUCTION

Advancements in thin-film PV technologies coupled with a tight shortage of crystalline Si in the past years have enabled 

their market acceptance with a share of 10.4% in 2008, in which the amorphous and micromorph Si products take up 

5.2%, CdTe 4.7% with a large gain last year, and a small 0.5% for CuInGaSe2 (CIGS).1

 In the device construction of 

thin-film PV technologies, essentially all require the use of at least one transparent conducting oxide (TCO) as a contact 

electrode. Minami and Fortunato et al. reviewed and discussed the properties, requirements, and/or material design of 

various TCOs, including indium tin oxide (ITO), ZnO, and SnO2 for PV applications, and Gordon discussed the criteria 

for choosing the TCOs for different applications.2-4 Among a variety of TCOs, thin-film ZnO doped with different metals 

have been widely used as contact electrodes in thin-film PV technologies such as a-Si, micromorph Si, and CuIn1-

x(Ga)xSe2 (CIS or CIGS).3,5-7 ZnO is also used as an interlayer between the a-Si and microcrystalline Si in the 

micromorph Si solar cells,8

 and antireflection coating for spherical Si solar cells.9 ZnO nanowires are also being 

investigated as charge transport media in nano-hybrid polymer,3

 dye-sensitized,10 and core/shell structured solar cells.11 

Most of the present CIS- or CIGS-based PV modules employ ZnO as the conducting front contact electrodes, except for 

Global Solar, which uses ITO.12 The bilayer ZnO window layers are typically constructed with an intrinsic ZnO over the 

CdS layer, followed by an Al-doped ZnO. The intrinsic ZnO serves as a buffer layer that is believed to enhance the 

performance and thermal stability as reasoned by Roedern.13 The optical and electrical properties of ZnO, doped and 

1 
undoped, are strongly affected by its defect chemistry, as reviewed by Schmidt-Mende and MacManus-Driscoll for 

nanostructures and device applications.14 

At NREL, bilayer ZnO has been used on high-efficiency CIGS solar cells for years because of its high optical 

transmittance and low fabrication temperature; a new world record efficiency of 19.9% was recently reported by Repins 

et al.15 Recently, intrinsic and Al-doped Zn1-xMgxO (ZMO) alloys have been investigated for potential replacement of the 

ZnO based on their tunable wider-bandgap engineering to gain some current in the short-wavelength range and better 

conduction-band edge matching to minimize a conduction-band offset between ZnO and CIGS, which reduces the cell’s 

open-circuit voltage.16,17 These improvements in the short-circuit current density (Jsc) and Voc may also improve the fill 

factor (FF) and hence the CIGS cell efficiency. The band structure of ZnO can be engineered by alloying with MgO to 

form new compounds with appropriate optical and electronic properties.18,19 A concern of this approach is the stability of 

the alloys, as Ryoken et al. reported,20 that high concentration of Al in ZnO and Mg in Zn1-xMgxO alloys introduces nonequilibrium defects into ZnO films that are the cause of crystallinity degradation and thermal instability. 

As discussed above, ZnO-based contact electrodes and materials are widely used for PV devices (and other applications). 

Therefore, along with the Mo base electrode, their stability can become a critical determining factor in the long-term 

performance reliability of the PV module, which requires not only reliable constituent component materials and device 

structure of the solar cells, but also reliable electrical interconnects and packaging. Currently, 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). Passing the damp heat test is generally agreed to be the number one concern, 

especially for thin-film PV modules. As recently presented by Whitfield, there was a 42% failure by the DH exposure 

followed by a 40% failure by 200 thermal cycles in IEC 61215 qualification tests (for c-Si modules) conducted at TÜV 

Rheinland since 1998 and a ~28% failure by DH followed by 14% outdoors for thin-film technologies conducted at 

ASU-PTL between 1997 and 2005.21 Accordingly, a systematic evaluation of the performance reliability of ZnO as well 

as other TCOs is essential, not only to assess the material stability, but also to address the need for improvements by 

implementing effective mitigation methods or seeking more reliable new/replacement materials, and to develop reliable 

packaging technology. 

We have conducted stability studies for the Mo base electrode on soda lime glass and a number of instrinsic ZnO and Aldoped ZnO and Zn1-xMgxO alloys using accelerated stress tests that include DH with and without acetic acid, dry heat, 

and in a weatherometer. The results showing the severe degradation of Mo and ZnO in DH have been reported in two 

separate meetings.22,23 There are also some earlier reports on the stability issues of Al:ZnO by DH exposure in the 

literature.24-26 Lin and coworkers reported their observations of a greater electrical degradation for thinner Al:ZnO thin 

films sputtered on glass, but little change in optical transmittance measured in 300-800 nm range.27 Recently, Wiedeman 

et al. at Global Solar reported high stability of their rigid CIGS products in >3000-h DH exposure, but notable 

degradation for flexible package after ~550 h.28 Feist et al. reported a >50% efficiency loss for encapsulated CIGSS on 

soda lime glass from Shell Solar after 168 h of 85o

C/100%RH exposure and that the degradation of Al-doped ZnO could 

be the lifetime-limiting factor.29 In an effort to mitigate the DH attack on CIGS from Shell Solar (with Al:ZnO) and IEC 

(with ITO), Olson and coworkers demonstrated the efficacy of a multi-layer, transparent moisture barrier coating with 

varying degree of success that was dependent on the TCO type, roughness of CIGS, and encapsulation.30,31 Very recently, 

Vitex Systems reported that the flexible CIGS solar cells encapsulated with their multilayer “flexible glass” retained 98% 

of their original efficiency after 1100 h exposure in DH.32 It is not clear if the two groups used the same multilayer 

technology, however. Using a simple coating with PMMA, Kim et al. found the polymer coating was useful in blocking 

moisture and air to reduce the degradation of ZnO nanowire devices.33

 

This paper summarizes our work with more details and an emphasis on the DH-induced degradation of Al-doped ZnO 

and Zn1-xMgxO alloys. The other two TCOs, ITO and F:SnO2, are not included here.23 It should be noted that these 

studies, which allowed the bare ZNO-based coatings on the glass substrates to be exposed directly to the accelerated 

stress/weathering conditions, represent a fairly extreme situation that normally may not be experienced by a wellencapsulated and packaged PV module. Accordingly, these studies represent primarily our efforts to evaluate and 

compare the stability of various ZnO materials for thin-film CIGS PV applications. 

2 
EXPERIMENTAL 

Two experiments were conducted. In the first experiment, which was more exploratory in nature to establish/define the 

methodology and procedures for accelerated life tests, the bilayer i-/Al:ZnO samples were old “witness” films deposited 

along with CIGS solar cells in a Semicore/MRC603 system on thin 0.5-mm 7059 glass slides with some variations in 

films’ structural quality as observed in XRD. In the second experiment, consistent ZnO film overall quality was ensured 

as described in (a) below. 

(a) TCO samples. Two RF magnetron sputtering systems were used to prepare ZnO samples separately. On a 

Semicore/MRC603 system, ~0.1-μm single-layer intrinsic ZnO (IZO) and Al-doped ZnO (AZO), and ~0.2-μm bi-layer 

ZnO (BZO), were prepared on 7.6-cm x 7.6-cm x 1.1-mm 7059 glass plates. The IZO was made from a 99.99% pure 

ZnO target in a 5- mTorr deposition chamber pressure and a flow rate of 60 sccm of Ar with 1% O2 by using ~1 watt/cm2

RF power density. For AZO deposition, the target was a 99.99% ZnO with 2 wt% Al2O3 using a ~1.2 watts/cm2

 RF 

power density in Ar at a flow rate of 40 sccm. The in-situ substrate temperatures were determined to not exceed 60o

C 

with a temperature indicator strip. During the sputtering deposition, the substrate holder platform was translated 

(“scanned”) back and forth in front of the target at a rate of 25 cm/min. An ATC 2200-V sputtering system from AJA 

International, Inc., was employed to prepare the sample set of single-layer, Al-doped ZnO with or without MgO (Al:Zn1-

xMgxO, x= 0 to 0.33; ZMO), at a thickness of 0.3~0.6 μm. The ATC 2200-V system has the ability to co-sputter, where 

the vacuum system base pressure was typically maintained at 2 x 10-8 Torr. A rotatable substrate station is positioned 

over the two 3”-diameter targets that are tilted toward the substrate and positioned in line with each other, as recently 

described by Li and co-workers.16 The ZnO target was 99.999% pure with 98 wt% ZnO and 2 wt% Al2O3 (1.56 at% Al) 

and the MgO target was 99.95% pure, and both were used as received from Cerac Inc. The Al:Zn1-xMgxO (ZMO) films 

were sputtered on 6” x 6” 1737 Corning glass at a working pressure of 5 mTorr in Ar at a flow rate of 40 sccm. The RF 

power varied between 60 and 240 watts, depending on the desired Mg content and deposition rate. The substrate 

temperature could deviate somewhat from the programmed temperature as a result of ~1-cm gap between the substrate 

and the monitoring thermocouple. 

(b) Accelerated exposure tests (AET). In the first experiment, only BZO samples were studied and were exposed 

separately in (i) a Blue M environmental chamber that operated at the “damp heat” (DH) test condition of 85o

C and 85% 

RH; (ii) an Atlas Ci4000 weatherometer (WOM) chamber, running at 2.5 UV-sun, 60o

C, and 60% RH; or (iii) an oven 

set at 85o

C with samples placed in glass jars in which the RH level was controlled at 78% by saturated KCl with or 

without a small amount of glacier acetic acid (0.1 mL in a 250 mL jar). In the second experiment, all ZnO and ZMO 

samples were exposed only in the DH chamber.22,23

(c) Chemical and physical mitigations. Two preliminary studies were conducted. Some of the BZO samples in the first 

experiments were treated briefly in a diluted solution of a special polydimethylsiloxane (PDMS) to investigate the 

usefulness of the chemical treatment to stabilize the ZnO against DH by increasing hydrophobicity.22 In the physical 

mitigation approach, a thin layer (~0.4 ± 0.2 μm) of SiOxNy was PECVD-deposited on the BZO as moisture barrier.34 

(d) Analytical characterization. Specimens of ~2.0 cm x 2.8 cm were cut from the ZnO-coated plates for the 

convenience of testing and various measurements. After baseline measurements, the samples were characterized 

periodically during the course of DH exposure. Transmittance and reflectance measurements for optical properties were 

conducted on a Cary 5G or 6000i UV-Vis-NIR spectrophotometer with an integration sphere. Hall measurements were 

performed on an Ascent Hall550 Advance system for sheet resistance (R□), free-carrier mobility, and concentration. An 

X1 Advanced Diffraction System from Scintag, Inc. was used for the x-ray diffraction (XRD) analysis for structural 

changes. Interference optical micro-imaging for morphological changes were obtained on a WYKO Optical Profiler 

Model 1100 system from Veeco. Upon completion of the experiments, the exposed samples were examined with 

scanning electron microscopy (SEM) on a Nova Nano SEM 630 system from FEI. 

RESULTS AND DISCUSSIONS 

The results will be divided into several sections below to separately discuss degradation of ZnO-based thin films with 

regard to the optical, electrical, mechanical, and morphological properties. 

3 
I. Optical Property and Degradation 

The ZnO-based thin films exhibited different 

transmittance and reflectance spectra depending primarily 

on their carrier concentration and film thickness. Figure 1a 

shows the transmittance spectra for the Semicoredeposited IZO, AZO, and BZO. Doping with Al in AZO 

produced the free-carrier absorption band at >1000 nm. 

When the IZO and AZO were combined to BZO bilayer, 

which showed a bandgap of 3.3-3.4 eV, greater freecarrier absorption band and optical effect were observed 

as seen in Fig. 1a. The effects were more obvious when 

the film was thicker and R□ lower, as shown in Fig. 1b 

where the CIGS bandgap was also marked.35 As a note, 

NREL’s CIGS solar cell devices normally use the ~0.2-

μm BZO. The transmittance and reflectance spectra for 

three Al:Zn1-xMgxO (x = 0, 0.01, 0.1) ZMO films prepared 

on the ATC 2200-V sputtering system are given in Fig. 2. 

At 0.3 and 0.5 μm thick, the optical effect of interference 

patterns appeared, similar to those shown in Fig. 1b. 

Figure 3 illustrates the observed optical changes for some 

of the samples subjected to the three AET conditions. The 

BZO films that were in the first experiment showed that 

the degrading effect was the least for WOM exposure (Fig. 

3a) and the greatest for DH exposure in the presence of 

acetic acid vapor (Fig. 3b).22 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.36 Its presence in DH likely accelerated or 

facilitated the hydrolysis of ZnO into Zn(OH)2. When 

exposed in DH without acetic acid vapor, all of the ZnO 

and ZMO samples turned from initially clear into hazy 

white and became highly resistive after 480 h, and their 

transmittance and reflectance spectra became flat and 

almost identical (Figs. 3c-3f). The originally conductive 

ZnO (AZO and BZO) and ZMO films also lost their 

characteristic free-carrier absorption tail bands.23

II. Electrical Property and Degradation 

The typical sheet resistance (R□) as determined from Hall 

measurements was high at >l06

 ohm/sq for the IZO, ~85 

ohm/sq for AZO at ~0.1 μm, and 65~75 ohm/sq for BZO 

at ~0.2 μm. For the Al:Zn1-xMgxO (x = 0, 0.01, 0.1) ZMO 

films prepared on the ATC 2200-V sputtering system, the 

film at x = 0 was the same as a single-layer AZO, the 

ZMO film at x = 0.01 showed a R□ of ~70 ohm/sq, and the 

ZMO film became highly resistive at x = 0.1. Table 1 lists 

the information of the two sample sets used in the two 

experiments (top half for the first experiment and bottom 

Fig. 1. Transmittance and reflectance spectra for (a) singlelayer intrinsic, Al-doped, and bilayer intrinsic/Al-doped ZnO 

(i-ZnO, Al:ZnO, and i-/Al:ZnO) on glass plates, and (b) three 

bilayer i-/Al-ZnO films of different sheet resistance and 

thickness.35 

0

10

20

30

40

50

60

70

80

90

250 450 650 850 1050 1250 1450

Wavelength (nm)

Transmittance/Reflectance (%)

100

i-ZnO

i-ZnO

Al:ZnO

Al:ZnO

i-/Al:ZnO

i-/Al:ZnO

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

(a)

(b) 

61.6 ohm/square 

2300 angstroms 

7.1 ohm/square 

7800 angstroms 

3.3 ohm/square 

1.4 microns 

RF-sputtered

Target: ZnO, ZnO:Al 

Rate: 1.5Å/sec. 

CIGS bandgap

0

10

20

30

40

50

60

70

80

90

250 450 650 850 1050 1250 1450

Wavelength (nm)

Transmittance/Reflectance (%) 100

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

x = 0.01 x = 0.1

Al:Zn1-xMgxO

x = 0.0

Fig. 2. Transmittance and reflectance spectra for Al-doped 

Zn1-xMgxO films (x = 0, 0.01, and 0.1) sputter-deposited on 1-

mm-thick glass plates. 

4 
5 

Table 1. Sample Data and Degradation Rates upon Accelerated Exposures

Sample ID BZO-1A BZO-1B BZO-1C BZO-2A BZO-2B IZO-11 AZO-21 BZO-31 ZMO-383 ZMO-471 ZMO-501 ZMO-681

Composition i-/Al:ZnO i-/Al:ZnO i-/Al:ZnO i-/Al:ZnO i-/Al:ZnO int.-ZnO Al:ZnO i-/Al:ZnO Al:ZnO Al:ZnO Al:ZnO Al:ZnO

MgO (% wt) 1 10

Dep. Tsub (oC) Ambient Ambient Ambient Ambient Ambient Ambient Ambient Ambient Ambient 100 100 100

Thickness (μm) 0.2 0.2 0.2 0.2 0.2 0.1 0.1 0.2 0.3 0.5 0.5 0.6

Chem. Mitigation PDMS PDMS

AET Type DH DH DH DH WOM DH DH DH DH DH DH DH

Degrad. Rate 

[(ohm/sq)/h] 5.42 6.38 2.27 0.16 0.08 HR* 

at 0 h 62.08 1.84 0.69 0.82 HR* 

at 0 h 0.09

* HR: highly resistive

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 (%)

(b)

T0 = 0 h

162.5 h

T0 = 0 h

162.5 h

BZO in (DH + Acetic Acid)

(a)

T0 = 0 h

T1 = 144h

T2 = 308 h

T3 = 561h

BZO in WOM

561 h

0, 144 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 (%)

480 h

0 h

242 h

480 h

0 h

242 h

(c)

IZO in DH

0 h

480 h

242 h

120 h

480 h

0 h

(d)

AZO (Al:ZnO) in DH

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 (%)

480 h

242 h

0, 120 h

480 h

0 h

242 h

(e)

BZO 

(f) 480 h

(i-/Al:ZnO) in DH ZMO (Al:Zn0.9Mg0.1O) in DH

Fig. 3. Transmittance and reflectance spectra for various ZnO and ZMO films on glass upon accelerated exposure tests: (a) in the 

weatherometer, (b) in damp heat with acetic acid vapor, and (c-f) in damp heat. 
half for the second experiment) and their degradation rates upon accelerated exposures, mostly in DH and one in the 

WOM. The actual data of sheet resistance for the second experiment samples were given in ref. 23. The two samples 

treated briefly with PDMS were also indicated. By assuming a linear degradation, which appeared be valid as shown in 

Fig. 4 for most samples before they became highly resistive, their degradation rates were calculated and given at the 

bottom line of Table 1. The samples that were rapidly degraded in DH in the presence of acetic acid vapor (see Fig. 3b) 

are not tabulated here. 

0

500

1000

1500

2000

0 100 200 300 400 500 600

DH Exposure Time (h)

Sheet Resistance (ohm/sq)

(15110 at 

242 h)

BZO-1B

The results indicate that degradation rates of the BZO made 

by the Semicore/MRC603 system follow the trend of (DH + 

acetic acid) > DH > WOM, in parallel to the optical results 

(Fig. 3a). Three significant observations are made here in 

DH-induced degradation: 

1. Thickness effect: thicker Al-doped ZnO or ZMO films 

degraded more slowly than thinner ones. BZO-1C 

(+ PDMS)

AZO-21 BZO-1A

BZO-31

AZO-471

AZO-383 BZO-2A (+ PDMS) BZO-2B

AZO-681

2. Substrate-temperature effect: Al-doped ZnO or ZMO 

films made at higher substrate temperature degraded 

more slowly than those made at ambient temperature. 

3. “Dry-out” effect: Al-doped ZnO films with periodic 

removal from the DH chamber for measurements 

degraded more slowly than the specimens (not given in 

Table 1) exposed in the DH chamber continuously for 

240 h. 

The thickness effect suggests that DH-induced degradation 

more likely started from the surface of the ZnO or ZMO 

films and gradually deepened into the bulk and finally to the 

oxide/glass interfaces. Lin et al. also observed similar thickness effect for their Al:ZnO thin films from 95 nm to 320 nm 

thick upon DH exposure, and explained it by the larger grain size on the thicker film.27 The substrate-temperature effect 

suggests that a better grain crystallite of ZnO or ZMO produced at higher temperature may be more resistant to DH 

attack. This reasoning appears to be validated by a brief experiment, which resulted in a lower sheet resistance change 

percentage (28%) for a BZO post-annealed at 200o

-220oC for 15 min in a quartz tube in Ar flow than that for an unannealed specimen (40%) after 94.5 h of DH. The brief annealing increased the BZO specimen’s R□ from an original 64 

ohm/sq to 83 ohm/sq. More systematic study is required to fully explore the beneficial effect of slower DH-induced 

degradation for better crystallites made at higher temperature. The “dry-out” effect suggests that the films were capable 

of recovering their physical properties to some extent after being removed from the DH chamber, 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. A transient effect was noted on a BZO 

film that was highly resistive when it was still moist just after being taken out from a bottle filled with moisture in the 

DH chamber, but became conducting as expected for a film with DH = 94 h exposure after drying naturally in the air for 

some minutes. For those BZO films pretreated with the PDMS, the preliminary results indicate the hydrophobic surface 

treatment substantially reduced the DH-induced degradation rate (BZO-1C vs. BZO-1A and 1B in Table 1). This is not 

surprising because a hydrophobic surface, and perhaps also in the grain boundary, of the ZnO would be able to reduce 

the water molecules from attaching to the ZnO to initiate the subsequent hydration/hydrolysis reaction of degradation. 

Fig. 4. Sheet resistance changes for the samples in Table 1 

as a function of accelerated exposure time before the films 

became highly resistive 

Along with the increase in the sheet resistance, the ZnO and ZMO films also showed a decrease in mobility, as seen in 

Fig. 5 (a and b). Consistent with the order in sheet resistance degradation, thicker Al-doped ZnO films in general showed 

slower loss in mobility (Fig. 5b). Although PDMS treatment was able to reduce the degradation rate in sheet resistance, 

the mobility of those films exposed in DH decreased essentially the same as for the untreated, suggesting the current 

PDMS treatment was still inadequate in that moisture still could penetrate and impede the free-carrier movement. On the 

other hand, the free-carrier concentrations in most of the exposed films showed various degrees of decrease, as illustrated 

in Fig. 5 (c and d). However, a few samples exhibited an increase, such as DH-exposed Al:Zn0.99Mg0.01O (ZMO-471) and 

PDMS-treated BZO-1C, and WOM-exposed BZO-2B. The cause for such an increase is not really understood. 

6 
0

5

10

15

20

0 100 200 300 400 500 600

Exposure Time (h)

Mobility (cm2/V-s)

BZO-2B (WOM)

BZO-2A 

(+ PDMS)

BZO-1C 

(+ PDMS)

(a)

BZO-1A

BZO-1B

0

2

4

6

8

10

12

14

0 50 100 150 200 250 300

DH Exposure Time (h)

Mobility (cm2/V-s)

(b)

 

III. Structural Property and Degradation 

As noted in Fig. 6, the majority of the ZnO films (IZO, AZO, and BZO) prepared on the Semicore system possessed a 

highly (002)-preferred orientation (Fig. 6c-f), however some BZO films studied in the first experiment showed 

considerably less (002) preference (Fig. 6b) and some were in between (Fig. 6a). The peaks at 2θ 38.12o

 and 44.45o in 

Fig. 6 (a and b) are tentatively assigned to be (201) and (211) with some uncertainty, even compared extensively to a 

number of PDF index cards for ZnO and Al2O3. The cause for these structural variations has not been investigated, but is 

probably due to some variations during sputtering. A similar strong (002) peak preference was also observed for the 

ZMO films (not shown). Figure 5 also shows the DH-induced changes in the XRD peak intensity and pattern, which are 

fairly irregular from one sample to another. This is particularly obvious for the (002) peak originally at 2θ 34.3o

~34.5o, in 

which some films showed an early peak intensity increase (Fig. 6 (a and e)) and some showed a decrease (Fig. 6 (b, c, 

and f)), before exhibiting a moderate to large decrease at prolonged DH exposure due to structural disintegration. Except 

for the bilayer BZO-31 (Fig. 6f), all single-layer ZnO films showed an increase in (002) peak intensity up to DH 242 h. 

When rapidly degraded in the presence of acetic acid, the (002) peak in Fig. 6d almost completely disappeared, a clear 

indication of nearly total structural degeneration or destruction. 

Red-shift of the (002) peak was also observed for BZO in Fig. 6 (c and f) and for IZO in Fig. 6e, suggesting a certain 

degree of change in the lattice. In addition, the peaks at 2θ 38.12o

 and 44.45o either disappeared (Fig. 6a) or largely 

decreased (Fig. 6b), suggesting a faster degradation of the corresponding ZnO structure. Additionally, the ZMO films 

Al:ZnO (0.1 

μm), AZO-21

Al:Zn0.99Mg0.01O 

(0.5 μm), ZMO-471

Al:ZnO (0.6 μm), 

(ZMO-681)

Al:ZnO (0.3 μm), 

ZMO-383

i-/Al:ZnO (0.2 μm), 

BZO-31 

0

5

10

15

20

25

0 100 200 300 400 500 600

Exposure Time (h)

Carrier Concenration (/cm3 x 1020)

0

2

4

6

8

10

0 50 100 150 200 250 300

DH Exposure Time (h)

Carrier Concentration (1/cm3 x 1020)

(d)

Al:ZnO (0.3 μm), 

ZMO-383

Al:ZnO (0.6 μm), 

(ZMO-681)

Al:Zn0.99Mg0.01O 

(0.5 μm), ZMO-471

i-/Al:ZnO (0.2 μm), 

BZO-31 

(c)

BZO-2B (WOM)

BZO-2A (+ PDMS)

BZO-1A

BZO-1C 

(+ PDMS)

BZO-1B

Al:ZnO (0.1 μm), 

AZO-21

Fig. 5. Changes in (a and b) mobility and (c and d) carrier concentration for DH- and WOM-exposed ZnO and ZMO films. Curves 

for BZO films that were pretreated with PDMS are marked. All exposures, except that marked with WOM for weatherometer, 

were conducted in the damp heat chamber. 

7 
(ZMO-471 and ZMO-501 in Table 1) displayed unusual peak shifts. For example, the Al:Zn0.9Mg0.1O (ZMO-501) alloy 

exhibited a main peak at 2θ 34.12o

 with a long tail (not shown) that shifted to ~33.95o upon DH exposure up to 242 h and 

then back to 34.12o

 at 480 h. More study is needed to further verify this behavior. 

 

At DH = 480 h, the IZO-11 film in Fig. 6e and the Al:Zn0.99Mg0.01O film (ZMO-471 in Table 1, XRD figure not shown) 

have four new peaks that appeared in the 2θ 31o

–39o range, possibly a consequence of structural degeneration resulting 

from the hot moisture-induced degradation. The peaks at 31.38o

, 33.22o, and 35.38o are assigned, using PDF#89-0510, 

Fig. 6. X-ray diffractograms for (a, b, c, d, f) BZO films and (e) an IZO film upon damp heat exposures at 85oC and 85% RH, 

except for (c and d) as marked. The exposure times are indicated in each figure. Samples BZO-4 and 5 are not given in Table 1. 

0

500

1000

1500

2000

2500

3000

3500

33 34 35 36

2 Theta Degree

Intensity (cps)

0 h BZO-31

120 h

242 h

480 h

(f)

0

500

1000

1500

2000

30 32 34 36 38 40

2 Theta Degree

Intensity (cps)

0 h

120 h

ZnO 242 h

(002)

ZnO 

480 h

480 h

(100) Zn(OH)2 

(201)

IZO-11 (e)

0

500

1000

1500

2000

2500

3000

3500

30 32 34 36 38 40

2 Theta Degree

Intensity (cps)

Sat'd KCl + Acetic Acid

85o

C Oven (78% RH)

0 h

162.5 h

(d) BZO-4C

0

1000

2000

3000

4000

5000

30 32 34 36 38 40

2 Theta Degree

Intensity (cps)

Sat'd KCl only 

85o

C Oven (78% RH)

0 h

144 h

(c) BZO-4B

306.5 h

0

100

200

300

400

500

600

32 34 36 38 40 42 44 46

2 Theta Degree

Intensity (cps)

(b) BZO-5

0 h

0 h

0 h

288

288 h 288 h

0

1000

2000

3000

4000

5000

6000

7000

32 34 36 38 40 42 44 46

2 Theta Degree

Intensity (cps)

144 h

144 h

144 h h

(002)

BZO-2 (211)?

0 h

0 h

(a) 144 h

(201) (002)

8 
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. 

The observed XRD results are quite complicated and cannot be explained straightforwardly. However, coupled with 

SEM and optical micro-imaging results (see below), the (002) peak intensity increase in the early DH exposure may be 

explained by an increase in the film thickness and/or morphological changes. On the other hand, a decrease in (002) peak 

intensity in the early DH exposure may have resulted from early structural deterioration. However, the mechanism(s) that 

govern(s) such different structural changes is (are) not clear. Despite this perplexity, the XRD results in general suggest 

that the ZnO might have undergone structural degeneration from an 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. Feist et al. also proposed the formation of Zn(OH)2 to be the main cause of degraded CIGS 

mini-module performance upon DH treatment.29 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 ZMO alloys probably also 

underwent similar hydrolysis and turned into a mixture Zn(OH)2 and Mg(OH)2. 

IV. Morphological Degradation 

IV-1. Optical Micro-Imaging 

The ZnO and ZMO films were examined 

periodically during the course of DH 

exposures, in addition to the optical, electrical, 

and structural measurements described above, 

with optical micro-imaging using a WYKO 

interference microscope, which became 

available in the second experiment, for the 

initiation and propagation of the defecting 

patterns and regions as a function of DH 

exposure time. More than 1000 micro-images 

were taken. A few of the images are illustrated 

here. Figure 7 shows the progressive 

morphological degradation for the single-layer 

i-ZnO sample (IZO-11) at four stages, 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 DH 

exposure 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 were 

observed when the cool films were just loaded into the hot steamy DH chamber. As the degradation progressed further, 

the film disintegrated greatly into fabric-like features at DH 480 h and was difficult to be micro-photographed because of 

large loss of light reflection from the rough surface. This morphological degradation process and pattern development 

was essentially the same for all ZnO-based films as observed in this study, as further illustrated in Fig. 8 for the singlelayer AZO-21, bilayer BZO-31, and two ZMO films. The changing rate of the morphological images shows that bilayer 

BZO-31 at 0.2 μm degraded more slowly than the single-layer IZO-11 and AZO-21 at 0.1 μm, consistent with the 

electrical degradation rates given in Table 1. Similar surface morphological degradations were also observed for the two 

samples of Al:Zn1-xMgxO alloy at DH 242 h, as seen in Fig. 8 (bottom row). At 10% of MgO (x = 0.1), the 

Al:Zn0.9Mg0.1O (ZMO-501) was originally insulating and, upon DH exposure, exhibited localized spot-wise material loss 

(see the second image from right of the bottom row), suggesting the MgO-ZnO was not fully alloyed in the co-sputtering 

process. 

Fig. 7. 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 indicated. 

9 
These close observatons allow a generalization of the degrading process induced by the DH as follows. The first sign of 

degradation started at a few small spots or areas with the appearance of irregular circles and spikes (e.g., Figs. 7 and 8 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. 7 and 8 at DH 242 h). Finally, especially for ZnO-based films, severe 

disintegration resulted (e.g., Fig. 7 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., Figs. 7 and 8, 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.

IV-2. SEM Micro-Imaging 

The ZnO and ZMO films before and 

after DH exposures were also subject 

to scanning electron microscopic 

analysis for changes in surface 

morphology and cross-sectional 

structures. Figure 9 (a and b columns) 

demonstrates the large morphological 

changes for the single-layer i-ZnO 

and Al:ZnO, and bi-layer i-/Al:ZnO 

before and after 480-h DH exposure. 

The originally smooth surfaces with 

very small grains seen at very high 

magnification at 200,000X became 

strangely featured. Furthermore, the 

obvious morphological differences in 

the Fig. 9 (b column) also suggest the 

three ZnO films had probably 

experienced different reaction 

rates/paths with the hot steam that 

resulted in different features or 

structures from the degradation 

products. More strikingly observed 

from the cross-sectional SEM 

analysis are porous formations of the 

DH-degraded films (Fig. 9), and 

segregation of the film materials into 

round zones (e.g., AZO-21, not 

shown), seem to correspond to the 

partial or full cycles with ditching features seen in the WYKO optical micro-images (Figs. 7 and 8). The segregation 

resulted in some (ditched) areas becoming very thin and the film materials within swelling and becoming very thick, as 

shown in Fig. 9 (c column, middle) for IZO-11. Finally, as revealed from the cross-section images, all three exposed 

ZnO films are five to ten times thicker than their originals, as a result of DH-induced degradation. The increase in film 

thickness as well as surface morphological roughness may explain the initial increase of the (002) peak intensity in XRD 

analysis. The porous formation may have also contributed to the thickness increase. The surface morphologies of the 

DH-exposed, single-layer ZMO films (ZMO-383, 471, and 501 in Table 1) made on the ATC 2200-V sputtering system 

also showed varying changes, but are not shown here. 

Fig. 8. Optical micro-images for the samples of (top row) single layer Al:ZnO (AZO21), (middle row) bilayer i-/Al:ZnO (BZO-31), and (bottom row) Al:Zn1-xMgxO, x = 

0.01 and 0.1, (ZMO-471 and ZMO-501), showing morphological degradations at DH 

= 242 h or 480 h exposure. The magnification factors are indicated. 

10 
Fig. 9. SEM micrographs for (top row) single layer Al:ZnO (AZO-21), (middle tow) single-layer i-ZnO (IZO-11), and bilayer i-

/Al:ZnO (BZO-31) showing topographs before (a column) and after (b column) DH = 480 h exposure. Images in the (c column) 

are for the cross-sections of the samples, taken with a 10o

 tilt in view angle. The magnification factors are indicated. 

V. Proposed Damp Heat-Induced Degradation Mechanism 

As summarized from all observations of optical, electrical, structural, morphological changes induced by the accelerated 

DH exposures, the temporal process of DH-induced degradation is proposed to involve these steps: 

1. First, hydrolysis of the oxides at some sporadic “weak” (defect) 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 

5. Finally, formation of insulating metal hydroxide along with visible delamination over larger areas. 

VI. Chemical and Physical Mitigations 

Preliminary studies were conducted to investigate the feasibility of protecting the bilayer i-/Al:ZnO from DH 

degradation. The first was a chemical mitigation method that attempted to reduce the hydrophilicity of the ZnO by 

coating the surface, and perhaps the grain boundary too, with a hydrophobic silane that can form covalent bonding with 

the hydroxyl groups on the ZnO. PDMS, a widely used silane for surface coating, was tried in this study with limited 

success as discussed in Section II. An improvement in the treatment condition and silane solution formulation may 

improve the protective effect. Recently, Kim et al. reported that a simple coating of ZnO nanowires, which were very 

sensitive to ambient gas and humidity, with polymethylmethacrylate (PMMA) was useful to passivate the surface states 

on the nanowires and so reduce the moisture-induced degradation rate.33

11 
The second was a physical mitigation method that deposited on the BZO surface a moisture-blocking silicon oxynitride 

(SiOxNy) by the PECVD method. The SiOxNy as a “barrier oxide” coating has been studied several years at NREL to 

block moisture vapor transmission through polymer films of the “back foils” for PV module encapsulation 

applications.34,37,38 SiO2 is another barrier oxide commonly used in multi-layer films by the packaging industry. Figure 

10 shows the significant changes in 

transmittance and reflectance spectra 

of a BZO film upon coating with a 

thin layer (~0.4 μm) of SiOxNy. 

Whether the SiOxNy can be also used 

as antireflection coating, in addition 

to the physical protecting layer, 

remains to be investigated. Upon DH 

exposure, however, the SiOxNy 

coating layer was largely cracked, 

visibly flaked, and partially lost due 

to poor adhesion to the ZnO surface, 

and possibly internal stress also, as 

seen in the optical micro-images 

shown in Fig. 10. By chemically 

modifying the BZO surface, it is 

possible to improve the adhesion 

thereby reducing the stress of the 

SiOxNy. More work has been 

undertaken in the mitigation studies 

by improving the surface treatment conditions with silanes including PDMS for BZO and ZMO (with 2%-5% MgO) as 

well as for the SiOxNy coating layer.39 Initial observations suggest that it is possible to improve the adhesion of SiOxNy

coating on the BZO. Successful results will be reported in the near future.

CONCLUSIONS 

Accelerated exposure-induced degradation of single-layer and bilayer undoped and Al-doped ZnO and ZnMgO alloys for 

CIGS solar cells was investigated with optical, electrical, structural, and optical and SEM micro-imaging 

characterization. The results show that the degradation rate follows the trend of (damp heat in the presence of acetic acid) 

> (damp heat) > (in weatherometer). The degradation rate was also affected by film thickness, deposition substrate 

temperature, and periodic “dry-out.” A temporal degradation mechanism for the ZnO films in damp heat was proposed 

with a structural transformation and/or degeneration from originally high (002) preference by hydrolysis into highly 

resistive Zn(OH)2. Similar degradation consequences may be also true for the two ZMO alloys at 1% and 10% MgO in 

this study. Preliminary studies on chemical and physical mitigation methods were investigated. Limited success was 

observed for the chemical surface treatment using a hydrophobic PDMS. A physical mitigation method using a PECVDSiOxNy coating on the bilayer ZnO failed in damp heat by poor adhesion. More ZMO alloys with 2%-5% MgO have 

been fabricated and their stability was recently investigated along with further studies on the mitigation methods. 

ACKNOWLEDGEMENTS

 We thank NCPV staff D. Albin for useful discussion, J. Zhou for some XRD measurements, J. Pankow for XPS 

analysis, and T. Gessert for manuscript review and discussion, 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. 

0

10

20

30

40

50

60

70

80

90

250 450 650 850 1050 1250 1450

Wavelength (nm)

Transmittance/Reflectance (%)

100

T% and R% Spectra of Initial and SiOxNy-coated Bilayer ZnO 

Bilayer ZnO

BZO + PECVD SiOxNy

As-Deposited

DH = 120 h

Fig. 10. (Left) Transmittance and reflectance spectra for a bilayer ZnO film before 

and after coating with a SiOxNy layer by PECVD, (right) optical micro-images for the 

SiOxNy–coated BZO before (top) and after (bottom) 120 h exposure in damp heat. 

12 
REFERENCES 

[1] Photon International, March, pp. 152, 2008. 

[2] T. Minami, “New n-Type Transparent Conducting Oxides,” MRS Bulletin, 25, pp. 38-44, 2000. 

[3] E. Fortunato, D. Ginley, H. Hosono, and D. C. Paine, “Transparent Conducting Oxides for Photovoltaics,” 

MRS Bulletin, 32, pp. 242-247, 2007. 

[4] R. G. Gordon, “Criteria for Choosing Transparent Conductors,” MRS Bulletin, 25, pp. 52-57, 2000. 

[5] M. A. Green, “Thin-Film Solar Cells: Review of Materials, Technologies and Commercial Status,” J. Mater. 

Sci: Mater. Electron, 18, S15-S19, 2007. 

[6] R. W. Miles, G. Zoppi, and I. Forbes, “Inorganic Photovoltaic Cells,” Materials Today, 10, pp. 20-27, 2007. 

[7] R. E.I. Schropp, R. Carius, and G. Beaucame, “Amorphous Silicon, Microcrystalline Silicon, and Thin-Film 

Polycrystalline Silicon Solar Cells,” MRS Bulletin, 32, pp. 219-224, 2007. 

[8] (a) D. Domine, J. Bailat, J. Steinhauser, A. Shah, and C. Ballif, “Micromorph Solar Cell Optimization Using a 

ZnO Layer as Intermediate Reflector,” Proc. 30th IEEE PVSC, pp. 1465- 1468, 2006. (b) D. Domine, J. 

Steinhauser, L. Feitknecht, A. Shah, and C. Ballif, “Effect of ZnO Layer as Intermediate Reflector in 

Micromorph Solar Cells,” Proc. 20th European PVSEC, Barcelona, Spain, pp. 1600-1603, 2005. 

[9] T. Minemoto, T. Mizuta, H. Takakura, and Y. Hamakawa, “Antireflective Coating Fabricated by Chemical 

Deposition of ZnO for Spherical Si Solar Cells,” Solar Energy Materials & Solar Cells, 91, pp. 191-194, 2007. 

[10] P. Suri and R.M. Mehra, “Effect of electrolytes on the Photovoltaics Performance of a Hybrid Dye Sensitized 

ZnO Solar Cell,” Solar Energy Materials & Solar Cells, 91, pp. 518-524, 2007. 

[11] K. Wang, J. Chen, W. Zhou, Y. Zhang, J. Pern, Y. Yan, and A. Mascarenhas, “Synthesis and Optical 

Properties of Type II Core/Shell Nanowire Array for Solar Cell Application,” Proc. 33th IEEE PVSC, San 

Diego, CA, May 11-16, 2008. 

[12] H.S. Ullal and B. von Roedern, “Thin Film CIGS and CdTe Photovoltaics Technologies: Commercialization, 

Critical Issues, and Applications,” Proc. 22nd European PVSEC, Milan, Italy, Sept. 3-7, 2007. NREL/CP-520-

42058. 

[13] B. von Roedern, “How Do Buffer Layers Affect Solar Cell Performance and Solar Cell Stability,” Mat. Res. 

Soc. Symp. Proc., 668, H6.9.1-6, 2001. 

[14] L. Schmidt-Mende and J. L. MacManus-Driscoll, “ZnO – Nanostructures, Defect, and Devices,” Materials 

Today, 10, pp. 40-48, 2007. 

[15] Repins, M. Contreras, M. Romero, Y. Yan, W. Metzger, J. Li, S. Johnston, B. Egaas, C. DeHart, J. Scharf, B. 

E. McCandless, and R. Noufi, “Characterization Of 19.9%-Efficient CIGS Absorbers,” Proc. 33th IEEE PVSC, 

San Diego, CA, May 11-16, 2008.

[16] X. Li, H. Ray, C. L. Perkins, and R. Noufi, “Aluminum Doping Effects on The Zn1-xMgxO:Al Transparent 

Conducting Films Deposited by rf Sputtering,” Symposium KK5.15, MRS 2008 Spring Meeting, San 

Francisco, CA, March 24-28, 2008. 

[17] D. Schmid, M. Ruckh, and H.W. Schock, Solar Energy Materials and Solar Cells, 41/42, pp. 281. 1996. 

[18] S. Choopun, R.D. Vispute, W. Yang, R. P. Sharma, and T. Venkatesan, “Realization of Band Gap above 5.0 

eV in Metastable Cubic-Phase MgxZn1-xO Films,” Appl. Phys. Lett., 80, pp. 1529, 2002. 

[19] K. Matsubara, H. Tampo, H. Shibata, A. Yamada, P. Fons, K. Iwata, and S. Niki, “Band-gap Modified Aldoped Zn1-xMgxO Transparent Conducting Films Deposited by Pulsed Laser Deposition,” Appl. Phys. Lett.,

85, pp.1374–1376, 2004. 

[20] H. Ryoken, I. Sakagichi, T. Ohgaki, N. Ohashi, Y. Adachi, and H. Haneda, “Defect Structures in Undoped and 

Doped ZnO Films Studied by Solid State Diffusion,” Mat. Res. Soc. Symp. Proc., 829, B2.23.1-6, 2005. 

[21] K. Whitfield, “Practical Application of the Standard Qualification Test Sequences to New Thin-Film 

Products,” presentation file, 33th IEEE PVSC, San Diego, CA, May 11-16, 2008. 

[22] J. Pern and R. Noufi, “ An Investigation of Stability Issues of ZnO and Mo on Glass Substrates for CIGS Solar 

Cells upon Accelerated Weathering and Damp Heat Exposures,” Proc. DOE SETP Review Meeting, Denver, 

CO., April 17-19, 2007. 

[23] F. J. Pern, R. Noufi, X. Li, C. DeHart, and B. To, “Damp-Heat Induced Degradation of Transparent 

Conducting Oxides for Thin-Film Solar Cells,” Proc. 33th IEEE PVSC, San Diego, CA, May 11-16, 2008. 

13 
14 

[24] M. Bär, J. Reichardt, I. Sieber, A. Grimm, I. Kötschau, I. Lauermann, S. Sokoll, T.P. Niesen, M.C. LuxSteiner, and Ch.-H. Fischer, “ZnO Layers Deposited by the Ion Layer Gas Reaction on Cu(In, Ga)(S, Se)2

Thin Film Solar Cell Absorbers – Impact of ‘Damp Heat’ Conditions on the Layer Properties,” Progr. 

Photovolt: Res. Appl. 15, pp. 187–198, 2007. 

[25] J. Klaer, R. Klenk, A. Boden, A. Neisser, C. Kaufmann, R. Scheer, H.-W. Schock, “Damp Heat Stability of 

Chalcopyrite Mini-Modules: Evaluation of Specific Test Structures,” Proc. 31st IEEE PVSC, pp. 336-339, 

2005. 

[26] H. Stiebig, W. Reetz, C. Haase, T. Rapmann, and B. Rech, “Stability of Non-encapsulated Thin-Film Silicon 

Solar Cells in Damp Heat Tests,” Proc. 15th International PV Sci. & Eng. Conf. (PVSEC-15), Shanghai, China, 

pp. 561-562, 2005. 

[27] W. Lin, R. Ma, J. Xue, and B. Kang, « RF Magnetron Sputtered ZnO :Al Thin Films on Glass Substrates : A 

Study of Damp Heat Stability on Their Optical and Electrical Properties,” Solar Energy Materials and Solar 

Cells, 91, pp. 1902-1905, 2007. 

[28] S. Wiedeman, J. Britt, J. VanAlsburg, W. Stoss, U. Schoop, E. Kanto, S. Lundberg, “Scale-Up at Global Solar 

Energy using Roll-to-Roll Processes for Thin Film CIGS PV” and poster presentation, Proc. DOE SETP 

Review Meeting, Denver, CO., April 17-19, 2007. 

[29] R. Feist, S. Rozeveld, M. Mushrush, R. Kaley, B. Lemon, J. Gerbi, B. Nichols, R. Nilsson, T. Richardson, S. 

Sprague, R. Tesch, S. Torka, C. Wood, S. Wu, S. Yeung, and M. T. Bernius, “Examination of LifetimeLimiting Failure Mechanisms in CIGSS-based PV Minimodules under Environmental Stress,” Proc. 33th 

IEEE PVSC, San Diego, CA, May 11-16, 2008. 

[30] L. Olson, S. Kundu, M. Gross, and A. Joly, “Damp Heat Effects on CIGSS and CdTe Cells,” Proc. DOE 

SETP Review Meeting, Denver, CO., April 17-19, 2007. 

[31] L. Olson, M. E. Gross, and S. Kundu, “Properties of Encapsulated CIGS Cells in 85o

C/85%RH,” Proc. 33th 

IEEE PVSC, San Diego, CA, May 11-16, 2008. 

[32] Vitex Systems press release, “Vitex Systems Achieves Lifetime Record on Flexible Copper Indium Gallium 

Selenide Solar Cells,” http://www.vitexsys.com/new/index.php?action=release-detail&PRID=33. June 12, 

2008. 

[33] D.-W. Kim, S.-H. Choi, H.-J. Ji, S. W. Kim, S. E. Moon, S. J. Park, and G.-T. Kim, “Degradation of ZnO 

Nanowire Devices under The Ambient Conditions,” Mat. Res. Soc. Symp. Proc., 1080, 1080-O15-08, 2008. 

[34] S. H. Glick, J. A. del Cueto, K. Terwilliger, G. Jorgensen, B. M. Keyes, L. M. Gedvilas, and F. J. Pern, 

“Silicon Oxynitride Thin Film Barriers for PV Packaging,” Poster presentation, Proc. DOE SETP Review 

Meeting, Denver, CO, Nov. 7-10, 2005. 

[35] R. Noufi, “High Efficiency CdTe and CIGS Thin Film Solar Cells: Highlights of The Technologies 

Challenges,” presentation file, 2006 IEEE 4th World Conf. on PEC, Waikoloa, Hawaii, May 7-12, 2006. 

(NREL/PR-520-39773) 

[36] A. W. Czanderna and F. J. Pern, “Encapsulation of PV Modules using Ethylene Vinyl Acetate Copolymer as 

A Pottant: A Critical Review,” Solar Energy Materials and Solar Cells, 43, pp. 101-181, 1996. 

[37] G. Jorgensen, K. Terwilliger, S. Glick, J. Pern, and T. McMahon, “Materials Testing for PV Module 

Encapsulation,” Proc. DOE SETP Program Review Meeting, Denver, CO, March 24-26, 2003. 

[38] G. J. Jorgensen, K. M. Terwilliger, J. A. DelCueto, S. H. Glick, M. D. Kempe, J. W. Pankow, F. J. Pern, T. J. 

McMahon, “Moisture Transport, Adhesion, and Corrosion Protection of PV Module Packaging Materials,” 

Solar Energy Materials & Solar Cells, 90, pp. 2739–2775, 2006. 

[39] J. Pan, F. J. Pern, X. Li, S. H. Glick, and L. Kerr, unpublished work. 
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Degradation of ZnO Window Layer for CIGS by Damp-Heat 

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5a. CONTRACT NUMBER

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5d. PROJECT NUMBER

NREL/CP-520-42792 

5e. TASK NUMBER

PVB76701 

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-520-42792 

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR'S ACRONYM(S)

NREL 

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 reliability of ZnO-based window layer for CuInGaSe2 (CIGS) solar cells was investigated. Samples of RF 

magnetron-sputtered, single-layer intrinsic and Al-doped ZnO and their combined bilayer on glass substrates were 

exposed in a weatherometer (WOM) and damp heat (DH) conditions with or without acetic acid vapor. The Al-doped 

ZnO and ZMO films showed irreversible loss in the conducting properties, free carrier mobility, and characteristic 

absorption band feature after <500-h DH exposure, with the originally clear transparent films turned into white hazy 

insulating films and the degradation rate follows the trend of (DH + acetic acid) > DH > WOM. The degradation rate 

was also reduced by higher film thickness, higher deposition substrate temperature, and dry-out intervals. The results 

of X-ray diffraction analysis indicate that the ZnO-based films underwent structural degeneration by losing their highly 

(002) preferential orientation with possible transformation from hexagonal into cubic and formation of Zn(OH)2. 

Periodic optical micro-imaging observations suggested a temporal process that involves initial hydrolysis of the 

oxides at 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 oxides/hydroxides with visible delamination over larger areas. 

15. SUBJECT TERMS

PV; transparent conducting oxides; intrinsic and Al-doped ZnO; ZnMgO alloy; accelerated exposure; damp heat; 

weatherometer; degradation; CIGS; thin-film solar cell 

16. SECURITY CLASSIFICATION OF: 17. LIMITATION 

OF ABSTRACT

UL 

18. NUMBER 

OF PAGES 

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a. REPORT 

Unclassified 

b. ABSTRACT 

Unclassified

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