NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Contract No. DE-AC36-08GO28308

Thickness Effect of Al-Doped 

ZnO Window Layer on Damp 

Heat Stability of CuInGaSe2 

Solar Cells

Preprint

F.J. Pern, L. Mansfield, C. DeHart, S.H. Glick, 

F. Yan, and R. Noufi 

Presented at the 37th IEEE Photovoltaic Specialists Conference 

(PVSC 37)

Seattle, Washington

June 19-24, 2011

Conference Paper

NREL/CP-5200-50682

July 2011
NOTICE

The submitted manuscript has been offered by an employee of the Alliance for Sustainable Energy, LLC 

(Alliance), a contractor of the US Government under Contract No. DE-AC36-08GO28308. Accordingly, the US 

Government and Alliance retain a nonexclusive royalty-free license to publish or reproduce the published form of 

this contribution, or allow others to do so, for US Government purposes.

This report was prepared as an account of work sponsored by an agency of the United States government. 

Neither the United States government nor any agency thereof, nor any of their employees, makes any warranty, 

express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of 

any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately 

owned rights. Reference herein to any specific commercial product, process, or service by trade name, 

trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, 

or favoring by the United States government or any agency thereof. The views and opinions of authors 

expressed herein do not necessarily state or reflect those of the United States government or any agency thereof.

Available electronically at http://www.osti.gov/bridge

Available for a processing fee to U.S. Department of Energy

and its contractors, in paper, from:

U.S. Department of Energy

Office of Scientific and Technical Information

P.O. Box 62

Oak Ridge, TN 37831-0062

phone: 865.576.8401

fax: 865.576.5728

email: mailto:reports@adonis.osti.gov

Available for sale to the public, in paper, from:

U.S. Department of Commerce

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22161

phone: 800.553.6847

fax: 703.605.6900

email: orders@ntis.fedworld.gov

online ordering: http://www.ntis.gov/help/ordermethods.aspx

Cover Photos: (left to right) PIX 16416, PIX 17423, PIX 16560, PIX 17613, PIX 17436, PIX 17721

Printed on paper containing at least 50% wastepaper, including 10% post consumer waste.
1 

THICKNESS EFFECT OF Al-DOPED ZnO WINDOW LAYER ON DAMP-HEAT 

STABILITY OF CuInGaSe2 SOLAR CELLS

F.J. Pern, L. Mansfield, C. DeHart, S.H. Glick, F. Yan, and R. Noufi

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

ABSTRACT

We investigated the damp heat (DH) stability of 

CuInGaSe2 (CIGS) solar cells as a function of thickness 

of the Al-doped ZnO (AZO) window layer from the 

“standard” 0.12 μm to a modest 0.50 μm over an 

underlying 0.10-μm intrinsic ZnO buffer layer. The CIGS 

cells were prepared with external electrical contact using 

fine Au wire to the tiny “standard” Ni/Al (0.05 μm/3 μm)

metal grid contact pads. Bare cell coupons and sample 

sets encapsulated in a specially designed, Al-frame test 

structure with an opening for moisture ingress control 

using a TPT backsheet were exposed to DH at 85o

C and 

85% relative humidity, and characterized by currentvoltage (I-V), quantum efficiency (QE), and 

(electrochemical) impedance spectroscopy (ECIS). The 

results show that bare cells exhibited rapid degradation 

within 50-100 h, accompanied by film wrinkling and

delamination and corrosion of Mo and AlNi grid, 

regardless of AZO thickness. In contrast, the 

encapsulated cells did not show film wrinkling, 

delamination, and Mo corrosion after 168 h DH exposure; 

but the trend of efficiency degradation rate showed a 

weak correlation to the AZO thickness.

INTRODUCTION

Long-term performance reliability is critical for CIGS 

photovoltaic (PV) modules. The IEC 61646 qualification 

standard for terrestrial thin film modules requires a 

stringent DH test at 85o

C and 85% relative humidity (RH) 

for 1000 h. A high percentage of thin-film modules 

reportedly failed in this test. The CIGS cell component 

materials, including Mo, CIGS, and ZnO, have been 

shown by us [1–4] and others to be DH sensitive or 

unstable. For improving the performance reliability of thinfilm CIGS modules, several approaches can be employed 

to minimize the damaging impact of moisture ingress. 

Among them, use of desiccant-type edge sealant has 

been widely used for glass-glass module construction. 

Addition of a thin moisture barrier film to the 

encapsulation is being eagerly pursued for flexible CIGS 

modules. At NREL, we have investigated alternate 

approaches by using (i) a layer of DH-stable, transparent 

moisture barrier deposited directly on the CIGS solar cells, 

and (ii) a DH-stable TCO such as ITO or InZnO to replace 

the AZO window layer [5]. However the second method 

could increase the cost by greater use of expensive In 

metal. A much preferred approach is one that can 

preserve the high performance of the solar cell while not 

changing well-established fabrication process and 

causing significant cost increase. In a previous study on 

DH stability of various TCOs on glass substrates, we 

found that, as the AZO layer thickness increased from the 

“standard” 0.12 μm to 0.50 μm, its DH stability showed a 

large enhancement with little to no decrease in optical and 

electrical properties [1]. Although the exact mechanism for 

the improved DH stability has not been clear yet, we have 

engaged in an effort to apply thicker AZO layers to the

CIGS solar cells without changing any fabrication process 

conditions and materials. Their DH stability is the main 

subject of this paper. We also developed the means for 

making DH-stable external electrical contacts and test 

structure for encapsulation to ensure that the observed DH 

instability is from the CIGS solar cell and its component 

materials, rather that from the contact degradation. We 

employed I-V, QE, ECIS, PL, and EL to characterize the 

solar cells. With the potential for in-situ monitoring, ECIS is 

being evaluated as a sensitive, non-invasive, nondestructive analytical method for the CIGS solar cells [6–

12].

EXPERIMENTAL 

Solar cells and AZO layers. The solar cells were prepared 

on 0.7-μm Mo-coated soda lime glass with 1.5–2 μm CIGS 

absorber by NREL’s 3-stage thermal coevaporation 

process, followed by ~0.05-μm CBD CdS, 0.10-μm i-ZnO, 

and various thickness of AZO (0.12, 0.24, 0.36, and 0.50 

μm) deposited in a Semicore RF sputtering system. The 

four AZO thicknesses required 8, 16, 25, and 35 passes, 

respectively, of the substrate holder platform moving over 

the sputtering target. The temperature monitored with 

temperature indicator strips (Omega, P/N 8MA) showed

<65o

C for 0.12-μm AZO, >77oC for 0.24 μm, >121oC for 

0.36 μm, and >138o

C for 0.50 μm, respectively. 

External electrical contact. To avoid potential damage by 

repeated electrical measurements to the tiny AlNi square 

pads on the cells, external connection was made to the 

small contact pad on each device with a small dot of silver 

paste and a low-melting-point (m.p. 118o

C or 143oC) solder 

alloy followed by attaching a 4.5-mil Au wire by soldering. 

Depending on test needs, the other end of the Au wire was 

soldered on tab ribbon pieces secured either on edge 

sealant tapes over a 7.6-cm x 7.6-cm (3” x 3”) glass plate 

or outside the Al-frame test structure after being 

encapsulated. Au wire was also soldered to the In strip 

applied on the Mo surface of the cell coupon and a piece of 

tab ribbon.

Test Structure and Encapsulation. A specially designed

and machined 10.16-cm x 10.16-cm x 6.35-mm (4” x 4” x 

1⁄4”) Al plate with a 70-mm x 70-mm x 2.75-mm square cutoff “well” was used to secure cell samples. Encapsulation 

was made between a 3.2-mm thick borosilicate (borofloat) 
2 

glass plate on the top and a 10.16-cm x 10.16-cm x 1.6-

mm Al back plate on the bottom with two layers of 

TruSeal’s SET LP01 gray edge sealant tape (12.7 mm 

wide, 1.0 mm thick), creating a net chamber volume of 

~220 mm3 (~134 mm3 without edge sealant tapes) when 

laminated empty at 100o

C. Both the Al frame and plate 

had a 1.25-cm x 1.25-cm opening in the center for 

moisture ingress control by sandwiching a piece of 

multilayer film. Several films having different water vapor 

transmission rates (WVTR), measured on a Mocon 

Permatran-W 3/33 system at 100% RH, were chosen for 

moisture ingress control. This paper reports the results for 

the sample “Set 5” using Madico’s primed TPT 

(Tedlar/polyester/Tedlar) backsheet.

Characterization. Transmission and reflection spectra 

were measured on a Cary 6000i spectrophotometer. I-V 

measurements were performed on an Optical Radiation 

Corp. model 1000 solar simulator, and QE on a custombuilt system. ECIS used an SI 1260 impedance/gainphase analyzer and an SI 1286 electrochemical interface 

from Schlumberger Technologies, operated with the ZPlot 

and ZView software from Scribner Associates. Before and 

after wire soldering and encapsulation, and periodically 

during DH exposure, the CIGS cell coupons were 

measured with I-V, QE, and ECIS in the dark. Some 

samples were imaged with photoluminescence (PL) and 

electroluminescence (EL).

RESULTS AND DISCUSSION

I. DH stability and optical properties of AZO films

We reported previously that the DH stability of AZO on 

glass increased as the film’s thickness increased—the 

AZO films at ~0.5 μm showed essentially no decrease in 

optical and electrical properties after 778-h DH exposure, 

despite a small degree of morphological change on the 

film’s surface feature as revealed by SEM micro-images 

[1]. The reason for this increased DH stability may be due 

to increased temperature during film deposition from 

<65o

C for the standard 0.12 μm to >138oC for a 0.50 μm

film. The result suggests that if applied on a CIGS solar 

cell, the cell’s DH stability may be improved accordingly. 

Therefore, we prepared a series of samples of bilyer ZnO 

(BZO, i.e., AZO with the standard 0.10-μm i-ZnO 

underneath) on CIGS solar cells and glass substrates. 

The thicker BZO films on glass and CIGS solar cells 

produced stronger (002) peak intensity in XRD analysis 

(not shown), suggesting the presence of greater 

crystallinity from thickness and temperature effects. Their 

transmittance and reflectance spectra are shown in Fig. 1. 

As the AZO layer became thicker, there is some loss in 

T%, particularly in the 1050–1500-nm range due to 

ionized-carrier absorption. However, the net effect of 

optical loss due to increased AZO thickness on the 

BZO/CIGS solar cells is not so large, because the cells’

spectral responses are primarily in the 400–1050 nm 

range as illustrated in Fig. 2, showing the QE spectra of 

some cells with four different AZO thicknesses. This is 

significant because it is possible to use even thicker AZO 

window layer without severe spectral loss for the CIGS 

cells, if the thicker AZO is proved to greatly enhance the 

cell’s DH stability.

II. DH stability of bare cell coupons

Two bare (unencapsulated) Au-wired cell coupons, C2529-

12 and -23, were exposed directly in DH condition to test 

the cell DH stability, but also to test the reliability of 

external electrical contact structure and to provide 

feedback for improving the experimental design. The 

results showed that a 50~100-h DH exposure readily 

caused notable to substantial wrinkling and delamination of 

BZO layer and Al/Ni gridlines over the device areas, along 

with some blackened regions outside the device areas and 

corroded spot on the CIGS/Mo layer, as seen in the 

photographs of Fig. 3. The cell efficiency exhibited a large 

decrease, regardless of the AZO thickness, from an initial 

0

10

20

30

40

50

60

70

80

90

100

250 450 650 850 1050 1250 1450

Wavelength (nm)

Transmittance/Reflectance (%)

BZO_Group1-T0_%T

BZO_Group1-T0_%R

BZO_Group2-T0_%T

BZO_Group2-T0_%R

BZO_Group3-T0_%T

BZO_Group3-T0_%R

BZO_Group4-T0_%T

BZO_Group4-T0_%R

R% Spectra for BZO (0.10 um i-ZnO + Various AZO Thickness) on 7059

0.12 μm

0.36 μm

0.24 μm

~0.5 μm

Figure 1 Transmittance and reflectance spectra as 

a function of bilayer ZnO (BZO) with different 

thickness Al-doped ZnO on 0.10 μm i-ZnO.

0

10

20

30

40

50

60

70

80

90

100

350 450 550 650 750 850 950 1050 1150 1250

Wavelength (nm)

External Quantum Efficiency (%)

C2529-12W-Cell3-T0 (G1)

C2529-21W-Cell3-T0 (G2)

C2529-23W-Cell3-T0 (G3)

C2529-11W-Cell3-T0 (G4)

QE Spectra of CIGS Solar Cells with Various AZO Thickness

0.12 μm

0.36 μm

0.24 μm

~0.5 μm

Figure 2 External quantum efficiency spectra 

measured for CIGS solar cells with BZO of different 

thickness Al-doped ZnO on 0.10 μm i-ZnO.
3 

15~16% to 0%~8% in a fairly random fashion. The loss 

was mainly due to largely increased series resistance (Rs) 

and decreased shunt resistance (Rsh). Relatively gradual

decreases in current density Jsc were observed. These 

are illustrated by normalized I-V parameters for three 

cells on the bare C2529-12 coupon in Fig. 4. PL and EL 

imaging also showed largely dimmed light intensity for the 

cells on both coupons just after 50-h DH exposure (not 

shown). 

ECIS measurements indicated that the CIGS solar cells 

can be represented by a simple equivalent circuit, 

comprising a series resistance (Rs’) for contacts and bulk, 

and a parallel resistance (Rp) for recombination/diffusion 

and capacitance (C) for diffusion and depletion [7–9]. 

ECIS curve fitting also confirmed that the solar cells 

degraded with large increase in Rs’, large decrease in the 

Rp, and some change in C. However, the increase in Rs’ 

was not from the external wiring/soldering, since the DH 

stability of the Ag paste, solder alloy, and soldered contact 

with Au wires had been tested to be high in a separate 

study prior to applying them to the solar cells. To examine 

whether DH exposure degraded the CIGS solar cells in 

addition to electrical factors, external quantum efficiency 

(EQE) measurements were conducted. One of the results 

is shown in Fig. 5 for C2529-12 cell #1 before and after 

normalization at 550 nm for 0-, 50-, and 100-h DH 

exposure. The results suggest that there was certain level 

of degradation on both CdS-CIGS (400-500 nm range) and 

AZO (750-1050 nm range).

III. DH stability of encapsulated cell coupons

Figure 6 illustrates a borofloat/Al-frame encapsulated 

sample “Set 5” with four cell coupons inside, which have 

AZO thickness at 0.12 μm (C2354-4), 0.36 μm (C2354-5), 

and 0.50 μm (C2354-6 and C2529-13), respectively. Prior 

to encapsulation of Set 5, the soldered contacts on the 

cells, In strips on Mo as base contacts, and coupons’ 

edges were coated with epoxy to further ensure the 

electrical contacts would not degrade by DH. The yellow 

adhesive Kapton tapes were used to shield the soldered 

spots for Au wires on tab ribbon pieces from direct 

exposure to the DH moisture, and were removed before 

electrical measurements. The >10-mm wide edge sealant 

tape after lamination was found effective in blocking DH 

moisture when tested with two “dummy” sets that 

embedded a CoCl2-based RH indicator strip. The RH 

indicator strip would change from blue to pink within some

hours when TPT was used, but did not change color when 

a barrier film with WVTR < 10-3 g/m2

/day was used. This 

indicates that TPT would be the primary pathway for DH 

moisture vapor penetration. Therefore, with a high WVTR 

0

10

20

30

40

50

60

70

80

90

100

300 400 500 600 700 800 900 1000 1100 1200

Wavelength (nm)

Internal Quantum Efficiency (%)

C2529-12- Cell1 New_X

C2529-12W Cell1 DH50h_X

C2529-12W Cell1 DH100h-N_X

C2529-12 Cell#1 QE Spectra Normalized at 550 nm

CdS

ZnO

0

10

20

30

40

50

60

70

80

90

100

300 400 500 600 700 800 900 1000 1100 1200

Wavelength (nm)

Internal Quantum Efficiency (%)

C2529-12- Cell1 New

C2529-12W Cell1 DH50h

C2529-12W Cell1 DH100h-N

suggest CdS

degradation 0

10

20

30

40

50

60

70

80

90

100

300 400 500 600 700 800 900 1000 1100 1200

Wavelength (nm)

Internal Quantum Efficiency (%)

C2529-12- Cell1 New_X

C2529-12W Cell1 DH50h_X

C2529-12W Cell1 DH100h-N_X

C2529-12 Cell#1 QE Spectra Normalized at 550 nm

CdS

ZnO

0

10

20

30

40

50

60

70

80

90

100

300 400 500 600 700 800 900 1000 1100 1200

Wavelength (nm)

Internal Quantum Efficiency (%)

C2529-12- Cell1 New

C2529-12W Cell1 DH50h

C2529-12W Cell1 DH100h-N

suggest CdS

degradation

Figure 5 External quantum efficiency (EQE) 

normalized at 550 nm for C2529-12W cell#1 at 0-, 

50-, and 100-h DH exposure. Plots in the inset are 

for as-obtained EQE, top to bottom: 0, 50, 100 h.

Figure 3 Photographs of two bare cell coupons, 

C2529-12 with 0.12-μm AZO and C2529-23 with 

0.36-μm AZO, exposed in DH for 50 h (left) and 100 

h (right). “W” stands for Au wire being soldered. 

The white circles on C2529-12W indicate the 

corroded Mo spot.

Figure 4 Plot showing changes in I-V parameters 

normalized to the as-deposited state for three cells 

on bare C2529-12 coupon with 0.12 μm AZO in four 

stages: as-deposited, after Au wire soldered, 50-h 

DH, and 100-h DH exposure.

0.0

0.2

0.4

0.6

0.8

1.0

As dep - DH

0h

Au wired -

DH 0h

Au wired DH

50 h

Au wired DH

100 h

Sample State

Normalized -V Parameters

C2529112 Cell1 Voc

C2529112 Cell2 Voc

C2529112 Cell3 Voc

C2529112 Cell1 Jsc

C2529112 Cell2 Jsc

C2529112 Cell3 Jsc

C2529112 Cell1 FF

C2529112 Cell2 FF

C2529112 Cell3 FF

C2529112 Cell1 Eff

C2529112 Cell2 Eff

C2529112 Cell3 Eff

C2529112 Cell1 Rsh

C2529112 Cell2 Rsh

C2529112 Cell3 Rsh

C2529-12 Cells #1-3 I-V paprametrs Normalized to As-Deposited Dh 0h
4 

at 142.77 g/m2/day and breakthrough/equilibrium time at 

~3 min/~15 min measured at 86.8o

C, the TPT would 

ensure quick moisture vapor saturation of the ~220-mm3

chamber space, even only through the small surface area 

of 1.56-cm2 from the bottom opening. However, it was 

also recognized that the edge fringes of the sealant tape 

inside Set 5 could take up some of the moisture due to its 

desiccant content of metal oxides. Figure 7 shows the cell 

efficiency, normalized to that at DH 0h, for the four cell 

coupons in Set 5 as exposure time increased to 168 h. (A 

few cells that were poor initially at 0 h were excluded in 

the plots.)

Upon DH exposure, a major difference observed between 

the bare and encapsulated CIGS cell coupons was that 

there was no wrinkling and delamination of any film layer 

and no Mo corrosion hole on the DH-exposed cell 

coupons for 168 h. The CIGS/Mo areas outside the cells 

did show gradual slight blackening on some small areas. 

This major difference raises a question on whether the 

degradation mechanisms for direct exposure of bare 

CIGS cell coupons to the DH moisture vapor are same as 

for the cells encapsulated, or whether the latter just has a

slower rate due to TPT’s filtration effect. For the 

encapsulated test structure, cool-off and dry-out were 

allowed (typically a few hours to overnight) before I-V and

wirings made the measurements convenient. 

Figure 6 Photograph of sample “Set 5” after 168-h 

DH exposure. It has four cell coupons 

encapsulated in the Al frame with a borofloat glass 

plate on the top and a gray edge-sealant tape 

around the four sides.

Figure 7 Plots of cell efficiency degradation, as normalized to the efficiency at 0 h, as a function of DH 

exposure for the four cell coupons in the encapsulated Set 5.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 50 100 150 200

DH Exposure Time (h)

Efficieny Normalized to T=0h (%)

C2354-4 Cell2

C2354-4 Cell3

C2354-4 Cell4

C2354-4 Cell6

C2354-4 Cell7

C2354-4 Cell Coupon with 0.12 μm AZO

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 50 100 150 200

DH Exposure Time (h)

Eff. Normalized to T=0h (%)

C2354-5 Cell2

C2354-5 Cell3

C2354-5 Cell4

C2354-5 Cell6

C2354-5 Cell7

C2354-5 Cell Coupon with 0.36 μm AZO

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 50 100 150 200

DH Exposuer Time (h)

Efficiency Normalized to T=0h (%)

C2354-6 Cell3

C2354-6 Cell4

C2354-6 Cell5

C2354-6 Cell6

C2354-6 Cell7

C2354-6 Cell Coupon with 0.50 μm AZO

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 50 100 150 200

DH Exposuer Time (h)

Eff. Normalized to T=0h (%)

C2529-13 Cell1

C2529-13 Cell2

C2529-13 Cell3

C2529-13 Cell4

C2529-13 Cell5

C2529-13 Cell Coupon with 0.50 μm AZO

Cell3 became degraded and unsteady after 0.25V bias ECIS scan

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 50 100 150 200

DH Exposure Time (h)

Efficieny Normalized to T=0h (%)

C2354-4 Cell2

C2354-4 Cell3

C2354-4 Cell4

C2354-4 Cell6

C2354-4 Cell7

C2354-4 Cell Coupon with 0.12 μm AZO

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 50 100 150 200

DH Exposure Time (h)

Eff. Normalized to T=0h (%)

C2354-5 Cell2

C2354-5 Cell3

C2354-5 Cell4

C2354-5 Cell6

C2354-5 Cell7

C2354-5 Cell Coupon with 0.36 μm AZO

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 50 100 150 200

DH Exposuer Time (h)

Efficiency Normalized to T=0h (%)

C2354-6 Cell3

C2354-6 Cell4

C2354-6 Cell5

C2354-6 Cell6

C2354-6 Cell7

C2354-6 Cell Coupon with 0.50 μm AZO

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 50 100 150 200

DH Exposuer Time (h)

Eff. Normalized to T=0h (%)

C2529-13 Cell1

C2529-13 Cell2

C2529-13 Cell3

C2529-13 Cell4

C2529-13 Cell5

C2529-13 Cell Coupon with 0.50 μm AZO

Cell3 became degraded and unsteady after 0.25V bias ECIS scan
5 

Coupled with epoxy protection, it is quite reasonable to 

state that the monitored performance degradation of cells 

in Set 5 is primarily due to cell or materials deterioration, 

rather than being caused by degradation of the electrical 

contacts (i.e., Rs from contact points at AlNi pad/Ag 

paste/solder alloy/Au wire/solder/tab ribbon piece). This 

assessment was confirmed by the steady Rs’ resistance 

(ECIS measured) with similar contact preparations on 

AZO or BZO on glass, which was encapsulated and 

exposed to DH in the same manner but without epoxy 

coating. 

The (normalized) efficiency degradation plots for the 

multiple cells on the four cell coupons in Fig. 7 show that, 

except for the C2529-13 coupon, the other three would 

have one to three cells failed quickly. Most of the cells 

exhibited gradual degradation. However, a few cells 

displayed “improved” cell parameters upon DH exposure; 

its cause is not clear but likely due to heating (curing) 

effect on the contact spots. Similar to the bare cell 

coupons, the changes in Jsc were relatively gradual, 

whereas Rs and Rsh changes were significant and 

contributed to the large decreases in open-circuit voltage 

(Voc) and fill factor (FF) (figures not shown). Figure 8 

shows the EQE spectra for C2354-4 cell #4 before (inset) 

and after normalizing at 550 nm, showing a transmission 

increase in the 550–1100 nm range, which suggests a 

degraded AZO layer. Previous studies have shown that, 

upon DH exposures, AZO (or BZO) increased its 

transmission in the infrared/near-infrared regions due to 

loss in ionized-carrier absorption [1,2,5]. The transmission 

change (normalized at 550 nm) was also seen on C2354-

5 (AZO 0.36 μm) and C2354-6 (AZO 0.50 μm) but with 

decreasing extent, as one would expect for thicker AZO 

from the previous study [1]. Very little change was seen 

on the least degraded cells on C2529-13 (AZO 0.50 μm). 

The difference between C2354-6 and C2529-12 (figure 

not shown) suggests the 0.50-μm AZO layers deposited on 

separate runs could show some variation. 

ECIS results in Rs’, Rp and C also reflected the same trend 

of changes. For example, the semicircle in the complex 

plane plot for C2354-5 cell #3 became increasingly smaller 

as DH exposure time increase (not shown). Figure 9 shows 

a summary plot of Rs’, Rp and effective minority-carrier 

lifetime (or time constant, a product of Rp and C, [8,9]) for 

C2354-5 cells #3, 5, and 6 obtained from curve fitting of 

their semicircles with a 0.25 V bias (cells #3 and 5) or none 

(cell #6). Generally, the trends in Rs’ and Rp changes are in 

parallel to that for the Rs (increased) and Rsh (decreased) 

changes seen in I-V measurements. Capacitance C 

derived from curve fitting was typically in the range of high 

10-9 to mid 10-8 farads, and decreased as the cell degraded. 

The lifetime Rp*C for cells #3 and 5 decreased, but 

increased for cell #6 due to increased Rp, indicating the 

variation in cell’s response to DH-induced stress, which 

was not revealed from I-V measurements. Work attempting 

to correlate the ECIS parameters with I-V parameters is 

underway. 

By averaging the efficiency only for the gradually degrading 

cells on each of the four coupons (see Fig. 7), Fig. 10 

shows the hourly efficiency degradation rate in DH 

condition at 85o

C and 85% RH, along with the standard 

deviation, as a function of AZO thickness. With the 0.50-

μm AZO cells degraded at different rates (C2354-6 vs. 

C2529-13), but faster than 0.36-μm AZO cells, the results 

do not exhibit a strong correlation between cell DH stability 

and AZO thickness. The causes remain to be further 

investigated, but we suspect that the higher temperature (> 

138o

C) and longer time (35 passes) for the deposition of 

0.50-μm AZO than that for 0.36-μm AZO (>121o

C, 25 

passes) may have contributed in a yet-to-be-identified 

manner to the poorer performance DH stability. 

0

10

20

30

40

50

60

70

80

90

100

300 400 500 600 700 800 900 1000 1100 1200

Wavelength (nm)

External Quantum Efficiency (%)

C2354-4-Cell6 in Set5 DH0h

C2354-4-Cell6 in Set5 DH168h

Normalized at 550 nm

0

10

2 0

3 0

4 0

50

6 0

70

8 0

9 0

10 0

3 0 0 550 8 0 0 10 50 W avelengt h ( nm)

Figure 8 EQE spectra for C2354-4 cell #6 (AZO =

0.12 μm) in Set 5 before and after 168 h DH 

exposure and normalization at 550 nm. 

Figure 9 Plots of Rs’, Rp, and lifetime (Rp*C) for 

three cells on C2354-5 (AZO 0.36 μm) as a function 

of DH exposure time. 

1.0E+0

1.0E+1

1.0E+2

1.0E+3

1.0E+4

0 20 40 60 80 100 120 140 160 180

DH Exposure Time (h)

Rs' and Rp from Curve Fit (Ohm)

0.0E+0

1.0E-5

2.0E-5

3.0E-5

4.0E-5

5.0E-5

6.0E-5

7.0E-5

8.0E-5

Lifetime Rp * C (s)

C2354-5 Cell3 (Rs) C2354-5 Cell5 (Rs) C2354-5 Cell6 (Rs)

C2354-5 Cell3 (Rp) C2354-5 Cell5 (Rp) C2354-5 Cell6 (Rp)

C2354-5 Cell3 (Rp*C) C2354-5 Cell5 (Rp*C) C2354-5 Cell6 (Rp*C)

Rp

Rs'

Rp*C

C2354-5 Cell Coupon with 0.36 μm AZO

Cells 3 & 5: IS scanned at 0.25V 

and Cell 6 at 0.0V bias

Rp*C
6 

CONCLUSIONS

We have investigated the thickness effect of AZO window 

layer on CIGS cells’ DH stability using multiple 

characterization methods, whereas ECIS, PL, and EL

analyses were found complementary to the conventional 

I-V and QE measurements. The study was facilitated by 

the use of DH-stable external connections with soldered 

Au wires for electrical measurements. When exposed 

directly to DH, bare CIGS cell coupons degraded rapidly 

in 50–100 h with film wrinkling, delamination, and 

corrosion of Mo and Ni/Al metal grids, which were not 

observed on cell coupons encapsulated in a test structure 

using a TPT backsheet to control moisture ingress. The 

cell degradation is mainly attributed to series resistance 

increase, shunt resistance decrease, and AZO 

degradation. The encapsulated sample continues to 

undergo DH exposure and shows no wrinkling and 

delaimation at 270 h. With a limited number of 

encapsulated cell samples, the current results do not 

show statistically significant correlation between AZO 

thickness and cell DH stability as one would expect from 

the increased DH stability of AZO films on glass 

substrates. An immediate benefit from the study of bare 

cells is the replacement of the typical Ni/Al metal grid 

(0.5/3 μm), which was easily corroded in DH, with DHstable Ni (>0.2 μm) for newer cell sample sets. More work 

is underway and the results will be reported in the future.

ACKNOWLEDGEMENTS

The authors thank M. Contreras and T. Gessert for the 

earlier support of this work and S. Johnston for the 

support of PL and EL imaging by F. Yan. This work was 

performed at the National Center for Photovoltaics under 

DOE Contract No. DE-AC36-08GO28308 with the 

National Renewable Energy Laboratory.

REFERENCES

[1] F.J. Pern, S.H. Glick, X. Li, C. DeHart, T. Gennett, M. 

Contreras, and T. Gessert, “Stability of TCO Window 

Layers for Thin-Film CIGS Solar Cells upon Damp Heat 

Exposures – Part III,” Proc. SPIE PV Reliability Conference, 

8/2-6/2009, San Diego, CA.

[2] R. Sundaramoorthy, F.J. Pern, C. DeHart, T. Gennett, 

F.Y. Meng, M. Contreras, and T. Gessert, “Stability of TCO 

Window Layers for Thin-Film CIGS Solar Cells upon Damp 

Heat Exposures – Part II,” Proc. SPIE PV Reliability 

Conference, 8/2-6/2009, San Diego, CA.

[3] F.J. Pern, B. Egaas, B. To, C.-S. Jiang, J.V. Li, S. Glynn, 

and C. DeHart, “A Study on the Humidity Susceptibility of 

Thin-Film CIGS Absorber,” Proc. 34 IEEE PVSC, 6/7-

12/2009, Philadelphia, PA.

[4] R. Sundaramoorthy, F.J. Pern, and T.A. Gessert, 

“Preliminary Damp-Heat Stability Studies of Thin-Film 

CuInGaSe2 Solar Cells,” Proc. SPIE PV Conference # 

7773 “Reliability of Photovoltaic Cells, Modules, 

Components, and Systems III,” 8/1-5/2010, San Diego, CA.

[5] F.J. Pern, S.H. Glick, R. Sundaramoorthy, B. To, X. Li, 

C. DeHart, S. Glynn, T. Gennett, R. Noufi, and T. Gessert, 

“Damp-Heat Instability and Mitigation of ZnO-Based Thin 

Films for CuInGaSe2 Solar Cells,” Proc. 35 IEEE PVSC, 

6/20-25/2010, Honolulu, Hawaii.

[6] M.S. Suresh, “Measurement of Solar Cell Parameters 

using Impedance Spectroscopy,” Solar Energy Materials 

and Solar Cells, 43, 1996, pp. 21-28.

[7] J. Bisquert, “Theory of the Impedance of Electron 

Diffusion and Recombination in a Thin Layer,” J. Phys. 

Chem. B, 106, 2002, pp. 325-333.

[8] J. Bisquert, “Impedance Spectroscopy applied on solar 

cells,” a presentation at the Nordic Workshop on Solar 

Electricity, Sonnerupgaard Gods, Denmark, 27-29 April 

2004.

[9] I. Mora-Sero, G. Garcia-Belmonte, P.P. Boix, M.A. 

Vazquez, and J. Bisquert, “Impedance spectroscopy 

Characterisation of Highly Efficient Silicon Solar Cells 

under Different Light Illumination Intensities,” Energy 

Environ. Sci., 2, 2009, pp. 678–686.

[10] A. Meier, S.H. Glick, and F.J. Pern, “Impedance 

Spectroscopy as a Non-Invasive Analytical Method for 

Monitoring Solar Cell Degradation,” NCPV Photovoltaics 

Program Review, Proc. of 15th Conference, M. Al-Jassim, 

J.P. Thornton, and J. M. Gee ed., CP462, Denver, CO, 

Sept., 1999, pp. 661-666. American Institute of Physics, 

Woodbury, New York.

[11] H. Bayhan and A.S. Kavasoglu, “Admittance and 

Impedance Spectroscopy on Cu(In,Ga)Se2 Solar Cells, 

Turk J. Phys. 27, 2003, pp. 529-535.

[12] H. Bayhan and A.S. Kavasoglu, “Study of 

CdS/Cu(In,Ga)Se2 Heterojunction Interface Using 

Admittance and Impedance Spectroscopy,” Solar Energy,

80, 2006, pp. 1160-1164.

0.0E+0

1.0E-3

2.0E-3

3.0E-3

4.0E-3

5.0E-3

6.0E-3

7.0E-3

8.0E-3

9.0E-3

1.0E-2

C2354-4

(0.12 μm)

C2354-5

(0.36 μm)

C2354-6

(0.50 μm)

C2529-13

(0.50 μm)

Cell Coupon ID (AZO Thickness)

Hourly Eff. Degrad. Rate in DH (%/h)

Average

STD

Efficiency Degradation Rate for Gradually Degrading Cells

Figure 10 Average hourly efficiency degradation 

rates (and corresponding standard deviations, 

STD) for the gradually-degrading cells on the four 

coupons in Set 5.