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

Stability of CIGS Solar Cells and 

Component Materials Evaluated 

by a Step-Stress Accelerated 

Degradation Test Method

Preprint

F.J. Pern and R. Noufi 

Presented at SPIE Optics + Photonics

San Diego, California

August 12–16, 2012

Conference Paper

NREL/CP-5200-54187

October 2012
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1 

STABILITY of CIGS solar cells and component materials evaluated 

by a step-stress accelerated degradation test method

F.J. Pern* and R. Noufi 

National Center for Photovoltaics, National Renewable Energy Laboratory 

1617 Cole Blvd., Golden, CO 80401, USA

ABSTRACT

A step-stress accelerated degradation testing (SSADT) method was employed for the first time to evaluate the stability of 

CuInGaSe2 (CIGS) solar cells and device component materials in four Al-framed test structures encapsulated with an 

edge sealant and three kinds of backsheet or moisture barrier film for moisture ingress control. The SSADT exposure 

used a 15o

C and then a 15% relative humidity (RH) increment step, beginning from 40oC/40%RH (T/RH = 40/40) to 

85o

C/70%RH (85/70) as of the moment. The voluminous data acquired and processed as of total DH = 3956 h with

85/70 = 704 h produced the following results. The best CIGS solar cells in sample Set-1 with a moisture-permeable TPT 

backsheet showed essentially identical I-V degradation trend regardless of the Al-doped ZnO (AZO) layer thickness 

ranging from standard 0.12 μm to 0.50 μm on the cells. No clear “stepwise” feature in the I-V parameter degradation 

curves corresponding to the SSADT T/RH/time profile was observed. Irregularity in I-V performance degradation 

pattern was observed with some cells showing early degradation at low T/RH < 55/55 and some showing large Voc, FF, 

and efficiency degradation due to increased series Rs (ohm-cm2

) at T/RH ≥ 70/70. Results of (electrochemical) 

impedance spectroscopy (ECIS) analysis indicate degradation of the CIGS solar cells corresponded to increased series

resistance Rs (ohm) and degraded parallel (minority carrier diffusion/recombination) resistance Rp, capacitance C, 

overall time constant Rp*C, and “capacitor quality” factor (CPE-P), which were related to the cells’ p-n junction 

properties. Heating at 85/70 appeared to benefit the CIGS solar cells as indicated by the largely recovered CPE-P factor. 

Device component materials, Mo on soda lime glass (Mo/SLG), bilayer ZnO (BZO), AlNi grid contact, and 

CdS/CIGS/Mo/SLG in test structures with TPT showed notable to significant degradation at T/RH ≥ 70/70. At T/RH = 

85/70, substantial blistering of BZO layers on CIGS cell pieces was observed that was not seen on BZO/glass, and a 

CdS/CIGS sample displayed a small darkening and then flaking feature. Additionally, standard AlNi grid contact was 

less stable than thin Ni grid contact at T/RH ≥ 70/70. The edge sealant and moisture-blocking films were effective to 

block moisture ingress, as evidenced by the good stability of most CIGS solar cells and device components at T/RH = 

85/70 for 704 h, and by preservation of the initial blue color on the RH indicator strips. The SSADT experiment is 

ongoing to be completed at T/RH = 85/85.

Key words: Step-stress accelerated degradation test (SSADT), encapsulated test structure, CIGS solar cell, Mo on SLG, 

Al-doped ZnO window layer, AlNi contact grid, damp heat stability, performance reliability

1. INTRODUCTION

Recent research advance has resulted in an efficiency improvement of CuInGaSe2 (CIGS) thin-film solar cells from 20% 

to 20.3% [1-3]. Long-term performance reliability of thin-film PV modules, including CIGS, remains an important issue 

due to considerably high failure rate [4,5], particularly during the strenuous damp heat (DH) exposure test at 85o

C and 

85% RH for 1000 h according to the IEC 61646 qualification test requirement. CIGS solar cells and their device 

components, including Mo, ZnO, and AlNi contact grid, have been shown to be DH-sensitive or unstable [6-11]. DHinduced degradation of Al-doped ZnO (AZO) conductive window layer was reported to be performance-limiting factor 

for CIGSS mini-modules [12,13]. When tested in dry heat, e.g., 85o

C and ~10% RH, CIGS solar cells showed good 

stability [14].

To screen or determine a material or device performance stability within a reasonable time frame, accelerated 

degradation or life test (ADT or ALT) is typically employed using constant or steady conditions; for example, DH 

exposure at 85o

C and 85% RH has been commonly used in the IEC 61215 and IEC 61646 for PV modules. A constant 

ADT or ALT has the advantage of being simple and straight and is capable of screening or ranking test subjects in a 

* john.pern@nrel.gov; phone 1 303 384-6615; fax 1 303 384-6490; www.nrel.gov/pv
2 

fairly short period of time by using high stress conditions (e.g., temperature, RH, and pressure). However, the test may 

require either several test chambers or several runs and very long test time if working with one chamber to conduct

under different conditions, especially with the objective to acquire activation energy for a given degradation mode. In 

comparison, a step-stress ADT or ALT (SSADT or SSALT)—the two are essentially the same in terms of reliability 

testing because both use the cumulative damage model—increases the stress condition(s) typically in a step-up manner 

during the course of test. This can offer the benefits of using a single test chamber to complete a test using multiple 

condition changes within a reasonable period of time. Suhir has discussed the various aspects of ALT, model equations, 

and application of ALT in microelectronic and photonic devices [15,16]. Khamis compared the use of constant and stepstress test for Weibull models [17], and Tseng and Wen reported their work using SSADT for highly reliable products 

[18]. Jones, Lee and coworkers promoted at NREL the applications of SSADT/SSALT to evaluate performance 

reliability of PV devices and modules [19,20] and presented recently their modeling and testing plan [21,22]. 

A SSADT method was employed in this work for the first time, in part based on discussion of a previous proposal [19], 

to evaluate the stability of CIGS solar cells and component materials in four encapsulated, edge-sealed test structures. 

While the four sides of the test structures were sealed with a desiccant-type edge sealant to block moisture, the test 

structures incorporated a piece of backsheet or moisture barrier film on the bottom side that allowed moisture ingress 

control with known water vapor transmission rate (WVTR) [23,24]. Three films were used: a moisture-permeable 

primed Tedlar/PET/Tedlar (TPT) backsheet, a Tedlar/Al foil/PET/EVA (TFPE) backsheet with an embedded moistureblocking Al thin (0.7 mil) foil, and a transparent FG-200 moisture-barrier cover film. In addition to evaluating the 

usefulness of SSADT, the multiple purposes of this work and experiment design were to determine the threshold 

temperature (T) and RH that would induce significant material or cell degradation, the protection efficacy of moisture 

barrier film (TFPE and FG-200), the durability of the edge sealant, and, ultimately if successful, the T- and RHdependent degradation activation energy. The latter would be applied to perform service life prediction based on the 

Peck equation, which in effect was an Eyring equation expanded and modified for modeling the time-to-failure in the TRH bias conditions [15]. We reported previously the stability of CIGS solar cells, device components, and contact 

materials (Ag paste and solder alloy) exposed to constant DH at 85/85 either bare or in encapsulated test structure with 

TPT and FG-200 films [23,24]. At the moment of this writing, the SSADT experiment has progressed from 

40o

C/40%RH (simply 40/40) to 85oC/70%RH (85/70) with a total accumulated exposure time of 4204 h. The plan is to 

complete the test at 85o

C/85%RH (85/85). 

2. EXPERIMENTAL

Sample preparation. Details of making the encapsulated Al-framed test structures and electrical contacts and connections 

were described previously [23,24]. In this work, a group of four sample sets was prepared (Set-1 to Set-4) and each set 

contained a set of CIGS solar cells or device components. Table 1 gives the details of the sample sets and component

matrix. Basically, CIGS solar cells, Mo on soda lime glass substrate (Mo/SLG), AZO and bilayer ZnO (BZO) window 

films on glass substrates, and CdS/CIGS/Mo/SLG pieces were included. Except for the CdS/CIGS/Mo/SLG pieces, all 

the samples had thin Au wires soldered for electrical connections for current-voltage (I-V) and (electrochemical) 

impedance spectroscopy (ECIS) measurements. Three contact patterns were used: the “standard” trident grid on 0.42 

cm2 cells; a special interdigitated “bi-grid” on larger CIGS cells (1.8~2.2 cm2

/cell), AZO/glass, and BZO/glass samples

(see images in Figs. 2 and 3 below); and a simple, two narrow strips of solder alloy applied on four component samples, 

as indicated in the three grid-related columns in Table 1. The bi-grid contact pattern was designed to allow I-V and ECIS 

measurements for the solar cells as well as resistance measurements by ECIS for the AZO, BZO and Mo on glass 

substrates. “Standard 3.05-μm” AlNi grid metal and thin (0.1~0.2-μm) Ni grid on BZO/glass and CIGS solar cells were 

compared. The CIGS solar cells fabricated with the bi-grid contacts were low efficiency, being used mainly for testing 

BZO DH stability, although I-V and ECIS were still measured. WVTR data of the TPT backsheet from Madico were 7.8 

and 142.8 g/m2

-day measured at 39/100 and 83/100, respectively. WVTR for the TFPE backsheet, also from Madico, 

was < 0.05 g/m2

-day at 84.5/100, below the detection limit of the MOCON Permatran-W 3/33 system. WVTR for the 2-

dyad (two repeated layers) moisture barrier film, FG-200, from Materion Large Area Coatings based on Vitex Barix 

technology was < 5x10-3 g/m2

-day [25]. An edge sealant tape, gray SET LP101, from TruSeal was used. A RH indicator 

strip from Süd-Chemie Performance Packaging was included in Set-2, 3, and 4 as an in-situ humidity level indicator. 
3 

DH exposure. The term damp 

heat “DH” is used in a loose 

manner here to indicate 

humidity level and 

temperature, rather than refer 

to the specific condition at 

85/85. Prior to the DH 

exposures, the edges of the 

sample sets were covered with 

1”-wide Kapton tapes to 

minimize DH-induced 

corrosion on the solder spots 

and tab ribbon pieces. The 

SSADT process began at

40/40 with an increment step 

of 15o

C first and then 15% RH

(i.e., 40/40, 55/40, 55/55, etc.)

in an ESPEC environmental 

chamber [26]. Figure 1 

illustrates the T-RH-time step 

profile along with a linear plot 

of time line for each T/RH 

segment. 

Characterization. The T-RH 

stability of the samples was

monitored at least three times 

in each step segment. 

Photographs were taken each 

time the sample sets were 

removed from the ESPEC 

chamber and allowed to cool 

and “stabilize” (dry out) for at least two days. To ensure reliable ECIS and I-V measurements, the solder spots on the tab 

ribbon pieces were either cleaned with a blade or re-soldered, and the tab ribbon pieces were replaced with new ones and 

Figure 1. The T-RH step profile used in this 

study is shown in the top plot. Time for each 

step segment is indicated. The linear time line 

showing the T/RH, each step segment time, 

and the total accumulated time is given at the 

bottom. Time duration for 85/85 exposure is 

not yet available.

Total time 0 h 535h 1260h 2018h 2818h 3500h 4204h N/A 

0 h 535h 725h 758h 800h 682h 704h

T(oC)/%RH 40/40 55/40 55/55 70/55 70/70 85/70 85/85

Net DH time N/A

Total time 0 h 535h 1260h 2018h 2818h 3500h 4204h N/A 0 h 535h 1260h 2018h 2818h 3500h 4204h N/A 

0 h 535h 725h 758h 800h 682h 704h

T(oC)/%RH 40/40 55/40 55/55 70/55 70/70 85/70 85/85

Net DH time N/A

30

40

50

60

70

80

90

100

30 40 50 60 70 80 90 100

T (oC)

RH (%)

Step-Stess Test (SST) Temp-RH-Time Profile

535 h

725 h

758 h

On-going Experiment

(start)

(end)

800 h

704 h

682 h

(N/A)

Table 1. Sample Matrix of the Four Encapsulated Al-Framed Sets

Set ID Samples in Set Sample ID AZO Grid Grid Metal Back

(μm) Type Metal (μm) Sheet

Set-1 Std Mo/SLG Mo-6-1B (2 solder strips) Madico 

Std BZO/7059 BZO4-32 0.12 N/A (2 solder strips) TPT

CdS/CIGS/Mo/SLG C2529-22 (piece) 

CIGS Cells C2528-23 0.12 Std AlNi 3/0.05

CIGS Cells C2529-21 0.24 Std AlNi 3/0.05

CIGS Cells C2529-11 0.5 Std AlNi 3/0.05

Set-2 BZO/1737 BZO15 AlNi-1 0.12 Bi-Grid AlNi 3/0.05 Madico 

BZO/1737 BZO18 Ni-1 0.12 Bi-Grid Ni 0.2 TPT

BZO/1737 BZO24 Ni-1 0.5 Bi-Grid Ni 0.2

CdS/CIGS/Mo/SLG C2430 (piece)

CIGS Cell C2510-12A 0.12 Bi-Grid AlNi 3/0.05

CIGS Cell C2510-22A 0.12 Bi-Grid Ni 0.2

CIGS Cell C2510-32A 0.12 Bi-Grid Ni 0.2

RH Indicator Strip

Set-3 BZO/1737 BZO15 AlNi-2 0.12 Bi-Grid AlNi 3/0.05 Materion

BZO/1737 BZO18 Ni-2 0.12 Bi-Grid Ni 0.2 FG-200

BZO/1737 BZO24 Ni-2 0.5 Bi-Grid Ni 0.2

CdS/CIGS/Mo/SLG C2430 (piece)

CIGS Cell C2510-12B 0.12 Bi-Grid AlNi 3/0.05

CIGS Cell C2510-22B 0.12 Bi-Grid Ni 0.2

CIGS Cell C2510-32B 0.12 Bi-Grid Ni 0.2

RH Indicator Strip

Set-4 Std Mo/SLG Mo-6-1C (2 solder strips) Madico 

AZO/7059 AZO11 AlNi-3 0.12 Bi-Grid AlNi 3/0.05 TFPE

BZO/1737 BZO15 AlNi-3 0.12 Bi-Grid AlNi 3/0.05

AlNi Film/BZO AlNi Film-1/BZO15 AlNi 3/0.05

CdS/CIGS/Mo/SLG C2529-22 (piece)

CIGS Cell C2528-13 (3 cells) 0.12 Bi-Grid Ni 0.1

RH Indicator Strip

Note 1: All CIGS solar cells and bilayer ZnO have a 0.1-μm i-ZnO layer
4 

soldered when needed prior to ECIS measurements for both CIGS solar cells and component materials followed by I-V 

measurements for the solar cells [24]. Photoluminescence (PL) and electroluminescence (EL) imaging were also 

performed for CIGS solar cells and PL for CdS/CIGS/Mo/SLG pieces intermittently, due to limited availability of the 

systems as well as working personnel. The acquired images are still pending proper processing for normalization of the 

image light intensity and are therefore unavailable for discussion at the moment.

3. RESULTS AND DISCUSSION

Because the samples at DH = 4204 h have not been completed with all the measurements (except for the images 

photographed at DH 4204 h shown in Fig. 3 below) at the time of this writing, this paper will present only some of the 

main results from the large quantity of data that have been processed up to a total DH exposure time of 3956 h at 85/70

RH for 456 h. To simplify the expression, the exposure conditions, T, %RH and time, will be indicated as, for example, 

“a total DH = 3956 h (85/70 DH = 456 h)” in the following discussions. A more comprehensive report is expected after 

the experiment is completed with exposure at DH 85/85 in the near future. On the exposure T/RH condition, the SSADT 

experiment began at a low 40/40 to examine if CIGS cell and device components would show any notable sign of 

degradation. The exposure time for each step segment was somehow irregular. A relatively short period of 535 h at 

40/40 was used because degradation at this condition, if any, was not expected to be substantial. Longer exposure time 

was given in 55/55 to 70/55 segments in hope to “catch” the significant degrading points. At higher T/RH, 70/70 and 

85/70, a shorter time (~700 h) was used because degradation rate was expected to be greater. Although not yet conducted, 

final exposure at 85/85 is planned to complete in a similar period of ~700 h. The following discussions group the results 

into three main parts: Moisture ingress control, CIGS solar cells characterized by I-V and ECIS, and device components 

(Mo, ZnO, and AlNi contact) by ECIS. Effects of moisture ingress (i.e., via TPT) versus moisture blocking (FG-200 and 

TFPE) on the DH stability of test subjects will be compared in the discussion of each subject.

3.1 Moisture Ingress Control

Two materials were used on the four encapsulated Al-framed test structures to control the moisture ingress: an edge 

sealant (TruSeal LP101) around the perimeters between the glass cover plate and the Al frame, and a back sheet at the Al 

frame’s bottom side being secured also by the edge sealant with a piece of 1/16” Al plate. Both the Al frame and the 

1/16” Al plate had a 1.25-cm x 1.25-cm opening in the center. A RH indicator strip was included in Set-2, 3, and 4, but 

not in Set-1 because of space limitation. Although the RH indicator strip could not offer a precise reading of humidity

level inside the encapsulated test structures, it did provide a qualitative indication. As illustrated in Fig. 2 for Set-2 with a 

TPT backsheet, the initial blue color on the RH indicator strip progressively changed its color to pink-brown as the T/RH 

increased from 40/40 to 85/70 over a total DH = 3956 h. The pink-blue color change was based on Co(II) hydrationdehydration and scaled from 10% to 100% humidity level. However, the strip changed to brownish after prolonged 

heating at 70o

~85oC at 70%RH, likely affected by the strip paper, suggesting the shortcoming of using the RH strips in 

this kind of prolonged testing. 

Figure 2. Photographs showing color changes on the RH indicated strip inside Set-2 with a TPT backsheet. The T/RH conditions, 

along with the total and segmental exposure times, are indicated at the bottom of each image, increasing stepwise from 40/40 to 

85/70. Bi-grid contacts were used on the C2510 solar cells and BZO/glass samples.
5 

Figure 3 shows photographs of the four sets with sample layouts and compares the color on the RH indicator strips. As 

seen, the blue color on the RH strips in Set-3 with a FG-200 and in Set-4 with a TFPE moisture barrier film remained 

unchanged even at a total DH = 4204 h and 85/70 DH = 704 h. This observation indicates that the FG-200 and TFPE 

films were able to block the moisture effectively up to 70% RH at 85o

C. The results also indicate the sealing soundness 

and durability of the edge sealant, TruSeal LP101, which was able to withstand all the exposures as well as the handling 

over the course of 4204 h. The RH strip’s blue color would have changed readily if there were any sealing leak around 

the perimeters on either set. Therefore, the samples inside Set-3 and Set-4 could be considered as having been exposed to 

dry heat. In a previous study, the blue color of the RH indicator strip in a “Dummy-2” set with FG-200 remained 

essentially unchanged after exposing at a constant 85/85 DH for > 1000 h, but changed to pink-brown quickly within 

~250 h in a “Dummy-1” set with TPT [24]. It should be noted that the edge sealant, being a desiccant-type material, 

would absorb some moisture to certain level, therefore impeding the moisture saturation time inside the encapsulated 

structure. However, no quantitative evaluation was available or made for this hindrance.

3.2 Stability of CIGS Solar Cells in SSADT 

The SSADT stability of the CIGS solar cells in Set-1 is the primary focus of this part. As pointed out in the Experimental 

section, the CIGS cells with bi-grid contacts in Set-2, 3, and 4 were typically low efficiency and were used primarily for 

monitoring BZO degradation; although they were still I-V measured at each segment step, their results are not 

emphasized here. Figure 4 shows the Voc, Jsc, fill factor (FF), efficiency, and series Rs plots grouped for the five cells 

on C2528-23 coupon with 0.12 μm AZO as a function of total DH exposure time. (Cells with initially poor efficiency 

were not included.) Apart from cells # 1 and #5, the other three cells on C2528-23 displayed gradual decrease in Voc and 

stable Jsc until DH > 2818 h (70/55 DH = 800 h). Cell #4 was most stable, showing a small decrease until DH ≥ 3500 h 

(70/70 DH = 682 h) when series Rs started to increase. In comparison, cell #6 was less stable, showing an increase of Rs 

in early stage at DH =1494 h (55/55 DH = 234 h), resulting in corresponding drops in FF and efficiency. As seen in Fig. 

4b, when compared to the initial Jsc ~28 mA/cm2 at DH =0 h, cells #1 and #5 exhibited abnormal high Jsc (> 50 mA/cm2

) 

after being exposed to 40/40 for just 319 h, likely caused by “cross-talk” with neighboring regions or cells, either by 

incomplete device isolation during photolithographic processing and/or by certain “conductive link” induced by the 

moisture. Similar cross-talk contributing to unusual high Jsc was also observed in another study as reported [27]. The 

cross-talk consequentially affected other I-V parameters, more notably FF and efficiency, as seen in Fig. 4 (c and d). 

However the highly fluctuating curves in Rs plots for the two cells in Fig. 4e suggest the “conductive link” was unsteady. 

Figure 3. Photographs showing the four sets 

after a total DH = 4204 h and 85/70 DH = 

704 h. The photographs also show the layout 

of different samples in each set as marked by 

handwriting. The films used for moisture 

ingress control are also indicated at the lower 

left corners: Set-1 and Set 2: TPT; Set-3: FG200; Set-4: TFPE. 
6 

The two cells also degraded faster up to DH = 2818 h (70/55 DH = 800 h) and then failed in the next exposure at 70/70 

when total DH = 3028 h (Fig. 4d). 

Figure 5 shows the similar plots grouped for four cells on C2529-11 coupon with 0.50 μm AZO. While cell #6 was most 

stable with little change in Voc, Jsc, FF, and efficiency until DH ≥ 3500 h (70/70 DH = 682 h) when series Rs started to 

rise (Fig. 5e); cell #2 behaved similarly but started to show notable degradation in FF and efficiency at DH ≥ 2602 h 

(70/55 DH = 584 h) with a corresponding Rs increase. Compared to other three cells, cell #4 showed a lower FF and 

efficiency due to higher initial Rs, probably caused by soldering, but it was fairly stable until DH > 2602 h. Cell #7 was 

troubled with an early Rs increase (Fig. 5e) that decreased its FF and efficiency accordingly (Fig. 5c and 5d), but its Jsc 

Figure 4. Voc, Jsc, fill factor (FF), 

efficiency, and series Rs plots for the 

cells on C2528-23 coupon with a 0.12-

um AZO window layer in Set-1 with 

TPT as a function of total DH exposure 

time. Abnormal high Jsc data were 

obtained for cells #2 and #5 before DH = 

2818 h, likely due to cross-talk.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

Voc (V)

Cell-1

Cell-2

Cell-4

Cell-5

Cell-6

Voc Plot for All Devices on C2528-23 Upon DH Exposure

(a)

C2528-23: 

0.12 μm AZO

0

10

20

30

40

50

60

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

Jsc (mA/cm2)

Cell-1

Cell-2

Cell-4

Cell-5

Cell-6

Jsc Plot for All Devices on C2528-23 Upon DH Exposure

(cross-talk)

(b)

0

10

20

30

40

50

60

70

80

90

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

Fill Factor, FF (%)

Cell-1

Cell-2

Cell-4

Cell-5

Cell-6

FF Plot for All Devices on C2528-23 Upon DH Exposure

(c)

0

5

10

15

20

25

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

Efficiency, Eff. (%)

Cell-1

Cell-2

Cell-4

Cell-5

Cell-6

Efficiency Plot for All Devices on C2528-23 Upon DH Exposure

(cross-talk)

(d)

0

2

4

6

8

10

12

14

16

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

Rs (ohm-cm2)

Cell-1

Cell-2

Cell-4

Cell-5

Cell-6

Rs Plot for All Devices on C2528-23 Upon DH Exposure

(e)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

Voc (V)

Cell-1

Cell-2

Cell-4

Cell-5

Cell-6

Voc Plot for All Devices on C2528-23 Upon DH Exposure

(a)

C2528-23: 

0.12 μm AZO

0

10

20

30

40

50

60

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

Jsc (mA/cm2)

Cell-1

Cell-2

Cell-4

Cell-5

Cell-6

Jsc Plot for All Devices on C2528-23 Upon DH Exposure

(cross-talk)

(b)

0

10

20

30

40

50

60

70

80

90

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

Fill Factor, FF (%)

Cell-1

Cell-2

Cell-4

Cell-5

Cell-6

FF Plot for All Devices on C2528-23 Upon DH Exposure

(c)

0

5

10

15

20

25

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

Efficiency, Eff. (%)

Cell-1

Cell-2

Cell-4

Cell-5

Cell-6

Efficiency Plot for All Devices on C2528-23 Upon DH Exposure

(cross-talk)

(d)

0

2

4

6

8

10

12

14

16

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

Rs (ohm-cm2)

Cell-1

Cell-2

Cell-4

Cell-5

Cell-6

Rs Plot for All Devices on C2528-23 Upon DH Exposure

(e)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

Voc (V)

Cell-2

Cell-4

Cell-6

Cell-7

Voc Plot for All Devices on C2529-11 Upon DH Exposure

(a)

C2529-11: 

0.50 μm AZO

0

5

10

15

20

25

30

35

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

Jsc (mA/cm2)

Cell-2

Cell-4

Cell-6

Cell-7

Jsc Plot for All Devices on C2529-11 Upon DH Exposure

(b)

0

10

20

30

40

50

60

70

80

90

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

Fill Factor, FF (%)

Cell-2

Cell-4

Cell-6

Cell-7

FF Plot for All Devices on C2529-11 Upon DH Exposure

(c)

0

2

4

6

8

10

12

14

16

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

Efficency, Eff. (%)

Cell-2

Cell-4

Cell-6

Cell-7

Eff. Plot for All Devices on C2529-11 Upon DH Exposure

(d)

0

2

4

6

8

10

12

14

16

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

Rs (ohm-cm2)

Cell-2

Cell-4

Cell-6

Cell-7

Rs Plot for All Devices on C2529-11 Upon DH Exposure

(e)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

Voc (V)

Cell-2

Cell-4

Cell-6

Cell-7

Voc Plot for All Devices on C2529-11 Upon DH Exposure

(a)

C2529-11: 

0.50 μm AZO

0

5

10

15

20

25

30

35

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

Jsc (mA/cm2)

Cell-2

Cell-4

Cell-6

Cell-7

Jsc Plot for All Devices on C2529-11 Upon DH Exposure

(b)

0

10

20

30

40

50

60

70

80

90

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

Fill Factor, FF (%)

Cell-2

Cell-4

Cell-6

Cell-7

FF Plot for All Devices on C2529-11 Upon DH Exposure

(c)

0

2

4

6

8

10

12

14

16

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

Efficency, Eff. (%)

Cell-2

Cell-4

Cell-6

Cell-7

Eff. Plot for All Devices on C2529-11 Upon DH Exposure

(d)

0

2

4

6

8

10

12

14

16

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

Rs (ohm-cm2)

Cell-2

Cell-4

Cell-6

Cell-7

Rs Plot for All Devices on C2529-11 Upon DH Exposure

(e)

Figure 5. Voc, Jsc, FF, efficiency, and 

series Rs plots for four cells on the 

C2529-11 coupon with a 0.50-μm AZO 

window layer in Set-1 with TPT as a 

function of total DH exposure time.
7 

changed little even at DH = 3956 h while Voc, FF, and efficiency precipitated largely at DH = 3028 h (70/70 DH = 210 

h) but recovered briefly at 3500 h. The cell’s electrical Rs was lowered at higher T/RH condition, suggesting the cell’s 

large degradation at DH ≥ 3000h was not due to Rs but something else. The brief recovery at DH = 3500 h (70/70 DH = 

682 h) could be a result of heating effect; more on this possibility will be discussed below.

Only three of the five cells on C2529-21 with 0.24 μm AZO were useful after cell #1 was damaged by soldering, which 

recovered gradually by ~50% in efficiency at DH = 2189 h (70/55 DH = 171 h) (not shown), and cell #2 was 

accidentally “electrocuted” by a high bias spike in EL measurement at DH = 319 h. Among the three useful cells, I-V 

data of cell #4 fluctuated irregularly, perhaps a result of Rs fluctuation (not shown). The best two cells, #3 and #5, are 

plotted along with the best cells from C2528-23 and C2529-11, as illustrated in Fig. 6. More notable degradation was 

seen on C2529-11 cell #2 and C2529-21 cell #5 at DH ≥ 2818 h (70/55 DH = 800 h, Fig. 6e) due to increased Rs, which 

affected FF and efficiency (Fig. 6c and 6d). The results in Fig. 6 also indicate that DH stability for the best cells in the 

current SSADT study was nearly identical, independent of AZO thickness. Similar results — no AZO thickness effect on 

DH stability of CIGS solar cells — were also observed in another study [27]. Furthermore, with Jsc curves staying 

essentially flat, the Voc, FF, and efficiency curves at DH < 3000 h (70/70 DH = 210 h) in Fig. 6 gradually declined and 

did not show clear degrading steps as one would expect SSADT or SSALT to generate; only at higher T/RH at DH > 

~3000 h were more obvious degradation curves obtained. This observation raises an issue on what T/RH/time conditions 

should be chosen in a SSADT or SSALT experiment to produce the desired (or expected) stepwise degradation features 

for modeling analysis.

An interesting observation in a couple cases is that a CIGS solar cell, e.g., cell #1 on C2529-11 with 0.50 μm AZO and 

an initially poor efficiency, would gradually increase its Voc, Jsc, and efficiency significantly upon the lengthy DH 

exposure, while the Rs increase did not further reduce the initially low FF (figures not shown). The cause of this 

improvement is not clear, but probably due to heating effect on the cell. Additionally, apart from the “best cells” results

shown in Fig. 6, some cells on the three coupons with different AZO thickness exhibited a significant degree of 

irregularity in degradation trend (rate and pattern) — some are shown in Figs. 4 and 5 above and some are not shown 

with initially moderate efficiency that degraded in a similar random manner. This observation indicates the presence of a 

relatively high degree of variation in cell stability during the course of SSADT, which is essentially the same as that was 

observed in another study under constant ADT at DH 85/85 [27], presenting again the challenge of conducting such 

reliability studies. 

Figure 6. Voc, Jsc, FF, efficiency, and 

series Rs plots for the best cells on 

C2528-23 (cell #4), C2529-11 (cells #2 

and #6), and C2529-21 (cells #3 and #5) 

in Set-1 with TPT as a function of total 

DH exposure time. AZO thickness for 

each coupon is indicated in Fig. 6a.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

Voc (V)

C2528-23-Cell4

C2529-11-Cell2

C2529-11-Cell6

C2529-21-Cell3

C2529-21-Cell5

Voc Plot for Best Devices on 3 Coupons Upon DH Exposure

(a)

C2528-23: 0.12 μm AZO 

C2529-11: 0.50 μm AZO 

C2529-21: 0.24 μm AZO

0

5

10

15

20

25

30

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

Jsc (mA/cm2)

C2528-23-Cell4

C2529-11-Cell2

C2529-11-Cell6

C2529-21-Cell3

C2529-21-Cell5

Jsc Plot for Best Devices on 3 Coupons Upon DH Exposure

(b)

0

10

20

30

40

50

60

70

80

90

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

Fill Factor, FF (%)

C2528-23-Cell4

C2529-11-Cell2

C2529-11-Cell6

C2529-21-Cell3

C2529-21-Cell5

FF Plot for Best Devices on 3 Coupons Upon DH Exposure

(c)

0

2

4

6

8

10

12

14

16

18

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

Efficiency, Eff. (%)

C2528-23-Cell4

C2529-11-Cell2

C2529-11-Cell6

C2529-21-Cell3

C2529-21-Cell5

Eff. Plot for Best Devices on 3 Coupons Upon DH Exposure

(d)

0

2

4

6

8

10

12

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

Rs (ohm-cm2)

C2528-23-Cell4

C2529-11-Cell2

C2529-11-Cell6

C2529-21-Cell3

C2529-21-Cell5

Rs Plot for Best Devices on 3 Coupons Upon DH Exposure

(e)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

Voc (V)

C2528-23-Cell4

C2529-11-Cell2

C2529-11-Cell6

C2529-21-Cell3

C2529-21-Cell5

Voc Plot for Best Devices on 3 Coupons Upon DH Exposure

(a)

C2528-23: 0.12 μm AZO 

C2529-11: 0.50 μm AZO 

C2529-21: 0.24 μm AZO

0

5

10

15

20

25

30

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

Jsc (mA/cm2)

C2528-23-Cell4

C2529-11-Cell2

C2529-11-Cell6

C2529-21-Cell3

C2529-21-Cell5

Jsc Plot for Best Devices on 3 Coupons Upon DH Exposure

(b)

0

10

20

30

40

50

60

70

80

90

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

Fill Factor, FF (%)

C2528-23-Cell4

C2529-11-Cell2

C2529-11-Cell6

C2529-21-Cell3

C2529-21-Cell5

FF Plot for Best Devices on 3 Coupons Upon DH Exposure

(c)

0

2

4

6

8

10

12

14

16

18

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

Efficiency, Eff. (%)

C2528-23-Cell4

C2529-11-Cell2

C2529-11-Cell6

C2529-21-Cell3

C2529-21-Cell5

Eff. Plot for Best Devices on 3 Coupons Upon DH Exposure

(d)

0

2

4

6

8

10

12

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

Rs (ohm-cm2)

C2528-23-Cell4

C2529-11-Cell2

C2529-11-Cell6

C2529-21-Cell3

C2529-21-Cell5

Rs Plot for Best Devices on 3 Coupons Upon DH Exposure

(e)
8 

3.3 ECIS Analysis

All the CIGS solar cells and device components with external electrical contacts via the Au wires soldered on tab ribbon 

pieces on the four sample sets were also characterized with ECIS to derive the resistance and capacitance values. ECIS 

was employed in previous studies that produced useful information on cell characteristics and DH stability [23,24]. The 

basic principle, data acquisition and curve fit analysis were described in [24]. Essentially, the measured ECIS data files 

are curve fitted by a simple built-in “Randles cell” equivalent circuit (Rs–C//Rp) to obtain the series resistance Rs (ohm), 

parallel resistance Rp (ohm), and capacitance C or CPE-T (farad, F), where CPE is the constant phase element. The 

“overall” time constant is then calculated by the product of Rp*C (second, s), which is essentially equaled to the value 

calculated from 1/2πf using the frequency, f, at the top of the semicircle in Z” – Z’ complex plot and corresponds (or is 

related) to the (effective) minority carrier lifetime according to Bisquert and coworkers [28-30]. However, “chemical 

capacitance,” Cμ, being separated from the “double layer capacitance,” Cdl, beyond certain forward bias, should be used 

to more reliably derive the minority carrier lifetime [28]. Because the capacitance C (or CPE-T) derived from curve 

fitting of the Z”-Z’ semicircles in this work is an “overall” magnitude, without trying to separate the recombination and 

diffusion terms (i.e., Cμ and Cdl), therefore the Rp*C will be referred to simply as the overall time constant (or “lifetime”) 

here, similar to that used previously [24]. Furthermore, in curve-fitting the R-C circuit, a “capacitor quality” parameter, 

CPE-P, can be obtained: CPE-P = 1.0 for an ideal capacitor, < 1.0 for a “depressed” capacitor, and 0.0 for a resistor. 

More details on ECIS analysis and its applications for different types of solar cells including CIGS, as well as

explanations of CPE are available in the literature [28-35].

Figure 7 illustrates the ECIS parameter plots for three best cells on the three CIGS cell coupons in Set-1 obtained at 

0.25-V bias as a function of total DH exposure time; their I-V parameter plots vs. DH time are given in Fig. 6 above. In 

Fig. 7(a, c, and e), the three cells’ series resistance, Rs (derived from curve fitting), was very close to the measured 

impedance Z’ at 1M (106

) Hz, while the parallel Rp was essentially superimposed with Z’ at 1 Hz, which would be the 

sum of Rs and Rp. (Typically Rs << Rp on a solar cell.) All Rs (and Z’ at 1M Hz) showed a gradual increase over the 

Figure 7. ECIS parameter plots as a function of total DH exposure time for three best cells, cell #4 on C2528-23 with 0.12 μm 

AZO, cell #3 on C2529-21 with 0.24 μm AZO, and cell #6 on C2529-11 with 0.50 μm AZO. (a, c, d) impedance and resistance 

plots in semi-log scale: Z’ at 1M Hz = series resistance, Rs (ohm), and Z’ at 1 Hz = Rs + Rp, parallel resistance. Z’ (1 Hz) curves 

are not readily seen because they are overlapped with the Rp curves. (b, d, f) Plots of capacitance, C and CPE-T and time constant

(lifetime), Rp*C, in semi-log scale (left Y axis), and plots of “capacitor quality” factor, CPE-P, in linear scale (right Y axis). Z’ 

data were actually measured; Rs, Rp, C, and CPE-P were derived from curve-fit of the ECIS data files using a simple Rs–Rp//C 

equivalent circuit; and Rp*C data were calculated. The three cells’ I-V parameter plots are shown in Fig. 6.

1.0E+0

1.0E+1

1.0E+2

1.0E+3

1.0E+4

1.0E+5

1.0E+6

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

ECIS Parameters

Z' (1 Hz) (ohm)

Z' (1 MHz) (ohm)

Rs (ohm)

Rp (ohm)

C2528-23 Device #4 at 0.25V Bias

(0.12 μm AZO)

(a)

1.0E-9

1.0E-8

1.0E-7

1.0E-6

1.0E-5

1.0E-4

1.0E-3

1.0E-2

1.0E-1

1.0E+0

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

ECIS Parameters

0.80

0.85

0.90

0.95

1.00

1.05

CPE-P Factor

C/CPE-T (F)

Rp*C (s)

C (F/cm2) 

CPE-P

C2528-23 Device #4 at 0.25V Bias

(b)

1.0E+0

1.0E+1

1.0E+2

1.0E+3

1.0E+4

1.0E+5

1.0E+6

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

ECIS Parameters

Z' (1 Hz) (ohm)

Z' (1 MHz) (ohm)

Rs (ohm)

Rp (ohm)

C2529-21 Device #3 at 0.25V Bias

(c) (0.24 μm AZO)

1.0E-9

1.0E-8

1.0E-7

1.0E-6

1.0E-5

1.0E-4

1.0E-3

1.0E-2

1.0E-1

1.0E+0

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

ECIS Parameters

0.80

0.85

0.90

0.95

1.00

1.05

CPE-P Factor

C/CPE-T (F)

Rp*C (s)

C (F/cm2) 

CPE-P

C2529-21 Device #3 at 0.25V Bias

(d)

1.0E+0

1.0E+1

1.0E+2

1.0E+3

1.0E+4

1.0E+5

1.0E+6

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

ECIS Parameters

Z' (1 Hz) (ohm)

Z' (1 MHz) (ohm)

Rs (ohm)

Rp (ohm)

C2529-11 Device #6 at 0.25V Bias

(0.50 μm AZO) (e)

1.0E-9

1.0E-8

1.0E-7

1.0E-6

1.0E-5

1.0E-4

1.0E-3

1.0E-2

1.0E-1

1.0E+0

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

ECIS Parameters

0.80

0.85

0.90

0.95

1.00

1.05

CPE-P Factor

C/CPE-T (F)

Rp*C (s)

C (F/cm2) 

CPE-P

C2529-11 Device #6 at 0.25V 

Bi

(f)

1.0E+0

1.0E+1

1.0E+2

1.0E+3

1.0E+4

1.0E+5

1.0E+6

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

ECIS Parameters

Z' (1 Hz) (ohm)

Z' (1 MHz) (ohm)

Rs (ohm)

Rp (ohm)

C2528-23 Device #4 at 0.25V Bias

(0.12 μm AZO)

(a)

1.0E-9

1.0E-8

1.0E-7

1.0E-6

1.0E-5

1.0E-4

1.0E-3

1.0E-2

1.0E-1

1.0E+0

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

ECIS Parameters

0.80

0.85

0.90

0.95

1.00

1.05

CPE-P Factor

C/CPE-T (F)

Rp*C (s)

C (F/cm2) 

CPE-P

C2528-23 Device #4 at 0.25V Bias

(b)

1.0E+0

1.0E+1

1.0E+2

1.0E+3

1.0E+4

1.0E+5

1.0E+6

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

ECIS Parameters

Z' (1 Hz) (ohm)

Z' (1 MHz) (ohm)

Rs (ohm)

Rp (ohm)

C2529-21 Device #3 at 0.25V Bias

(c) (0.24 μm AZO)

1.0E-9

1.0E-8

1.0E-7

1.0E-6

1.0E-5

1.0E-4

1.0E-3

1.0E-2

1.0E-1

1.0E+0

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

ECIS Parameters

0.80

0.85

0.90

0.95

1.00

1.05

CPE-P Factor

C/CPE-T (F)

Rp*C (s)

C (F/cm2) 

CPE-P

C2529-21 Device #3 at 0.25V Bias

(d)

1.0E+0

1.0E+1

1.0E+2

1.0E+3

1.0E+4

1.0E+5

1.0E+6

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

ECIS Parameters

Z' (1 Hz) (ohm)

Z' (1 MHz) (ohm)

Rs (ohm)

Rp (ohm)

C2529-11 Device #6 at 0.25V Bias

(0.50 μm AZO) (e)

1.0E-9

1.0E-8

1.0E-7

1.0E-6

1.0E-5

1.0E-4

1.0E-3

1.0E-2

1.0E-1

1.0E+0

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

ECIS Parameters

0.80

0.85

0.90

0.95

1.00

1.05

CPE-P Factor

C/CPE-T (F)

Rp*C (s)

C (F/cm2) 

CPE-P

C2529-11 Device #6 at 0.25V 

Bi

(f)
9 

course of DH exposure, similar to the trends of series Rs (ohm-cm2) from light I-V measurements as seen in Fig. 6e.

Meanwhile, a gradual decrease was seen on the parallel Rp, an indicator of “diffusion and recombination resistance” of 

minority carriers at the p-n junction of a solar cell (represented as an Rs–C//Rp equivalent circuit) [28-30]. The effective 

lifetime Rp*C declined correspondingly, while the capacitance C or CPE-T (in F or F/cm2 if divided by the cell area 

0.42 cm2

) showed a slow decline to DH ~ 2500 h (Fig. 7b and 7d) or ~ 3000 h (Fig. 7f) before showing a gain then a 

drop again in later stage. In the meantime, the CPE-P factors remained steady (Fig. 7b, 7f) or exhibited a small decline 

(Fig. 7d) up to DH ~ 2600 h (70/55 DH = 584 h) for all three best cells, followed by a rapid drop between DH = 2600 h 

and 3500 h (70/70 DH = 682 h) before recovering significantly afterwards (85/70 DH = 456 h).

These results indicate that the I-V performance of the three best cells were degraded by an increase in series resistance 

and a decrease in Rp, C (and Rp*C), and CPE-P due to degraded and possibly “depressed” p-n junction and depletion 

width when exposing to DH. The degradation rate was fairly small or slow when T/RH < 70/55 (DH < 2600 h), but was 

much faster when T/RH = 70/70. The three cells’ capacitor quality was largely recovered after the cells were exposed to 

85o

C (DH > 3500 h), as indicated by improved CPE-P and C, suggesting the beneficial effect of heating to the solar cells

as also mentioned above. However, the gains in CPE-P and C were offset negatively by the continuously increasing Rs 

and decreasing Rp at 70%–85% RH, and could not result in a recovery in cell efficiency.

3.4 Stability of Device Components in SSADT

The SSADT DH stability of CIGS device components, namely Mo/SLG, Al-doped ZnO (AZO), bilayer ZnO (BZO), 

AlNi grid contact, and CdS/CIGS/Mo/SLG pieces, was also evaluated in encapsulated test structures seen in Fig. 3. 

3.4.1 Mo on SLG (Mo/SLG)

The stability of bare Mo/SLG in damp heat (85/85) was investigated previously [6]. Substantial flaking (as seen in SEM), 

corrosion, and color change on Mo were typically observed along with becoming highly resistive after 100~200 h at 

85/85 DH exposure. Dry heat (85/~10) normally did not cause notable degradation on Mo. The Z’ resistance (impedance) 

results for Mo/SLG in Set-1 (TPT) and in Set-4 (TFPE) are compared in Fig. 8. To measure the resistance across the 0.7-

μm thick Mo film, two narrow solder strips separated by ~18 mm were applied with Au wires attached. As seen in Fig. 

8a, the Mo/SLG in Set-1 (TPT) showed fairly stable resistance up to DH = 3716 h (85/70 DH = 216 h), but became 

highly resistive (not measurable) beyond that point. In the meantime, the color changed to darkening brown with small 

white dots starting to appear on the surface after DH > 3500 h, as illustrated by the photograph insert in Fig. 8a for the 

sample at DH = 3956 h (85/70 DH = 456 h). Similar white dots were also observed in a previous sample, “Dummy-1” 

(TPT), which was exposed to constant 85/85 DH = 1015 h [24]. The DH-induced corrosion and resistance increase was 

not observed for the Mo/SLG sample in Set-4 (TFPE), however. The difference clearly demonstrates that the TFPE film 

was effective to prevent the DH-sensitive Mo from being corroded up to 85/70 DH = 704 h. Whether TFPE is effective 

to block moisture at 85/85 remains to be verified when the experiment is completed later.

0

1

2

3

4

5

6

7

8

9

10

0 500 1000 1500 2000 2500 3000 3500 4000

DH Exposure Time (h)

Z' Resistance (Ohm)

Z' (1 Hz) (ohm)

Z' (1 MHz) (ohm)

Resistance Change on Mo6-1B/SLG in Set-1 upon DH Test

Very high 

resistance at 

DH=3956, not 

measurable.

White dots on 

the browned 

surface. (TPT)

(a)

0

1

2

3

4

5

6

7

8

9

10

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

Z' Resistance (Ohm)

Z' (1 Hz) (ohm)

Z' (1 MHz) (ohm)

(b)

(TFPE)

Resistance Change on Mo6-1C/SLG in Set-4 upon DH Test

Total DH=3956h 

85/70=456h

0

1

2

3

4

5

6

7

8

9

10

0 500 1000 1500 2000 2500 3000 3500 4000

DH Exposure Time (h)

Z' Resistance (Ohm)

Z' (1 Hz) (ohm)

Z' (1 MHz) (ohm)

Resistance Change on Mo6-1B/SLG in Set-1 upon DH Test

Very high 

resistance at 

DH=3956, not 

measurable.

White dots on 

the browned 

surface. (TPT)

(a)

0

1

2

3

4

5

6

7

8

9

10

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

Z' Resistance (Ohm)

Z' (1 Hz) (ohm)

Z' (1 MHz) (ohm)

(b)

(TFPE)

Resistance Change on Mo6-1C/SLG in Set-4 upon DH Test

0

1

2

3

4

5

6

7

8

9

10

0 500 1000 1500 2000 2500 3000 3500 4000

DH Exposure Time (h)

Z' Resistance (Ohm)

Z' (1 Hz) (ohm)

Z' (1 MHz) (ohm)

Resistance Change on Mo6-1B/SLG in Set-1 upon DH Test

Very high 

resistance at 

DH=3956, not 

measurable.

White dots on 

the browned 

surface. (TPT)

(a)

0

1

2

3

4

5

6

7

8

9

10

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

Z' Resistance (Ohm)

Z' (1 Hz) (ohm)

Z' (1 MHz) (ohm)

(b)

(TFPE)

Resistance Change on Mo6-1C/SLG in Set-4 upon DH Test

Total DH=3956h 

85/70=456h

Figure 8. Z’ resistance plots for Mo/SLG sample in (a) Set-1 with a TPT backsheet and (b) Set-4 with a TFPE backsheet. Z’ data 

at 1 Hz and 1 Mz were shown, where a small difference is present due to intrinsic capacitance on the contacts. The photograph 

insert in (a) shows the two solder strips/Au wire contacts and the small white dots on Mo surface at total DH = 3956 h (85/70 DH 

= 456 h). 
10 

3.4.2 BZO/AlNi and BZO/Ni on cells and glass substrates

The bilayer ZnO (BZO) was tested either as stand alone on glass substrates or on CIGS solar cells. A special “bi-grid” 

contact pattern was used on most of the BZO and CIGS cell pieces (Fig. 3) that allowed independent I-V and ECIS 

measurements. Electrical contact on one sample, BZO4-32/glass, used two solder strips, same as that on Mo/SLG, which 

would produce a resistance much higher than that with a bi-grid. To compare with the standard AlNi contact material, 

thin (0.1~0.2 μm) Ni on BZO/glass or BZO/CIGS solar cells was also investigated.

Figure 9 compares the Z’ resistance results for five BZO/glass in three sample sets: BZO4-32 with solder strip contacts 

in Set-1 (TPT), a pair of BZO-15 with standard 3.05-μm AlNi contact bi-grid and BZO-24 with 0.2 μm Ni in both Set-2 

(TPT) and Set-3 (FG-200), respectively (see Fig. 3 images with each sample ID marked). The results show that BZO-4 

in Set-1 (TPT) with solder strip contacts started to increase its resistance gradually at DH > 1000 h (55/40) and showed a 

large jump after DH > 3500 h (70/70 DH = 682 h) from ~100 ohms to ~145 ohms at DH = 3956 h (Fig. 9a). In 

comparison, BZO-15 in Set-2 (TPT) with standard AlNi grid contact showed an unsteady but increasing resistance after 

DH > 2818 h (70/55), Fig. 9b, but BZO-24 with thin Ni grid was fairly stable in Set-2 (TPT), despite some data 

scattering, Fig. 9c. The differences among the three BZO/glass samples indicate that the BZO resistance change was not 

clearly detected with the bi-grid contact likely due to small separation between the gridlines (1 mm, as compared with 

the 18-mm distance of the solder strips) and that the AlNi grid contact with BZO-15 also started to degrade unsteadily at 

DH = 3028 h (70/70 DH = 210 h) and more obviously at DH = 3716 h (85/70 DH = 216 h). We previously reported a 

very large resistance increase on bare (unencapsulated) BZO/glass samples with standard AlNi bi-grid exposed to

constant 85/85 DH = 100 h and nearly linear resistance increase with respect to time on a BZO/glass sample in a 

similarly encapsulated Dummy-1 (TPT) also exposed to constant 85/85 DH > 1200 h [24]. Corrosion of AlNi grid on the 

bare BZO/glass samples was observed but not on thin Ni grid [24]. In Set-3 with moisture barrier film FG-200, BZO-15 

(AlNi) and BZO-24 (Ni) samples remained essentially unchanged as seen in Fig. 9(d and e) at DH = 3956 h (85/70 DH = 

456 h). Similar stability was observed for AZO/glass and BZO/glass samples in a Dummy-2 (FG-200) exposed to 

constant 85/85 DH > 1200 h [24].

Figure 9. Z’ resistance plots for five 

BZO/glass samples as a function of total 

DH exposure time: (a) BZO-4 with 

solder strip contact in Set-1 (TPT), (b) 

BZO-15 with standard AlNi contact in 

Set-2 (TPT), (c) BZO-24 with 0.2 μm Ni 

contact in Set-2, (d) BZO-15 with 

standard AlNi contact in Set-3 (FG200), and (e) BZO-24 with 0.2 μm Ni 

contact in Set-3. Bi-grid pattern was 

used on samples in both Set-2 and Set-3.

0

5

10

15

20

25

30

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

Z' Resistance (Ohm)

Z' (1 Hz) (ohm)

Z' (1 MHz) (ohm)

Resistance Change on BZO-15 with Std AlNi Bi-Grid in Set-2

(b)

(TPT)

0

2

4

6

8

10

12

14

16

18

20

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

Z' Resistance (Ohm)

Z' (1 Hz) (ohm)

Z' (1 MHz) (ohm)

Resistance Change on BZO-24 with 0.2um-Ni Bi-Grid in Set-2

(c)

(TPT)

0

2

4

6

8

10

12

14

16

18

20

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

Z' Resistance (Ohm)

Z' (1 Hz) (ohm)

Z' (1 MHz) (ohm)

Resistance Change on BZO-15 with Std AlNi Bi-Grid in Set-3

(d)

(FG-200)

0

2

4

6

8

10

12

14

16

18

20

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

Z' Resistance (Ohm)

Z' (1 Hz) (ohm)

Z' (1 MHz) (ohm)

Resistance Change on BZO-24 with 0.2um-Ni Bi-Grid in Set-3

(e)

(FG-200)

(std AlNi bi-grid)

(std AlNi bi-grid)

(0.2-μm Ni bi-grid)

(0.2-μm Ni bi-grid)

80

90

100

110

120

130

140

150

160

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

Z' Resistance (Ohm)

Z' (1 Hz) (ohm)

Z' (1 MHz) (ohm)

Resistance Change on BZO4-32 in Set-1 Upon DH Test

(TPT)

(a)

(solder strips)

0

5

10

15

20

25

30

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

Z' Resistance (Ohm)

Z' (1 Hz) (ohm)

Z' (1 MHz) (ohm)

Resistance Change on BZO-15 with Std AlNi Bi-Grid in Set-2

(b)

(TPT)

0

2

4

6

8

10

12

14

16

18

20

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

Z' Resistance (Ohm)

Z' (1 Hz) (ohm)

Z' (1 MHz) (ohm)

Resistance Change on BZO-24 with 0.2um-Ni Bi-Grid in Set-2

(c)

(TPT)

0

2

4

6

8

10

12

14

16

18

20

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

Z' Resistance (Ohm)

Z' (1 Hz) (ohm)

Z' (1 MHz) (ohm)

Resistance Change on BZO-15 with Std AlNi Bi-Grid in Set-3

(d)

(FG-200)

0

2

4

6

8

10

12

14

16

18

20

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

Z' Resistance (Ohm)

Z' (1 Hz) (ohm)

Z' (1 MHz) (ohm)

Resistance Change on BZO-24 with 0.2um-Ni Bi-Grid in Set-3

(e)

(FG-200)

(std AlNi bi-grid)

(std AlNi bi-grid)

(0.2-μm Ni bi-grid)

(0.2-μm Ni bi-grid)

80

90

100

110

120

130

140

150

160

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

Z' Resistance (Ohm)

Z' (1 Hz) (ohm)

Z' (1 MHz) (ohm)

Resistance Change on BZO4-32 in Set-1 Upon DH Test

(TPT)

(a)

(solder strips)
11 

The SSADT DH stability of BZO window layer on CIGS solar cells was also monitored in-situ by ECIS measurements 

along with the typical I-V measurements for the solar cells. The ECIS measurements were conducted through the bi-grid 

contacts, which also offered the flexibility to measure I-V via either one of the two contacts. Similar to the BZO/glass 

samples above, standard 3.05-μm AlNi contact and 0.1-μm Ni contact in the bi-grid pattern were used on some CIGS 

solar cells that were encapsulated in Set-2, 3, and 4 (Fig. 3). Figure 10(a and b) show the I-V parameter plots for the cells

of C2510-12A in Set-2 (TPT) and C2510-12B in Set-3 (FG-200), and Fig. 10(c and d) show the Z’ resistance plots for 

the BZO layers on the two cells as a function of total DH exposure time. On C2510-12A in Set-2 (TPT), obvious 

fluctuation and degradation in I-V parameters were noted (Fig. 10a), and the Z’ resistance for the BZO measured via the 

AlNi bi-grid contacts displayed a small and gradual increase before DH ~ 3500 h but a quick rise afterward (Fig. 10c), 

similar to that seen in Fig. 9a. The FG-200 moisture barrier film on Set-3 provided good protection of C2510-12B cell 

and the BZO and AlNi grid on it — most of the I-V parameters and Z’ resistance remained stable as of DH = 3956 h as 

seen in Fig. 10(b and d), except for Voc that showed a little decrease. By comparing Fig. 10c with Fig. 9b, the difference 

in Z’ resistance changes in current SSADT exposure indicates that BZO sputtered on glass was relatively more DHstable than BZO on CIGS solar cells, which was in good accord with the previous results, attributing to higher 

crystallinity on the former [23,27]. Furthermore, the BZO layers on C2510 cell pieces in Set-2 (TPT) showed darkening 

from one side (closer to the center opening with TPT) at early DH < 1772 h (55/55 DH = 512 h) that progressed further 

onto more cell areas as T/RH/time increased, and finally showed a large number of small “blistering” (i.e., an early sign 

of flaking) on C2510-12A and 22A pieces at DH = 4204 h (85/70 DH =704 h). In contrast, C2510 cell pieces in Set-3 

(FG-200) did not show similar changes on the BZO layers.

The BZO on C2528-13 with 0.1-μm Ni bi-grid contacts in Set-4 (TFPE) exhibited a notable decrease in Z’ resistance on

cell #1 after DH > 1000 h (55/40) and stabilized at DH > 1772 h (55/55 DH = 512 h), Fig. 10e; the change was quite 

small on cell #3, Fig. 10f. It is not clear what caused the Z’ resistance to decrease, but the result seems to suggest that the 

BZO/Ni grid contact was improved by dry-heating so the overall resistance was decreased. Additionally, a thicker (0.2 

μm) Ni grid contact appeared to offer a lower resistance (~4 vs. 12 ohm) and better DH stability as well, when Fig. 10(e 

and f) were compared with Fig. 9(c and e). Also in Set-4 (TFPE), Z’ resistance measured for 0.12-μm AZO-11/glass via 

the 3.05-μm AlNi bi-grid contacts displayed a notable early decrease before DH = 1000 h followed by small fluctuations 

after DH > 1000 h, while BZO-15/glass was relatively stable until DH > 3000 h, when a moderate fluctuation was seen 

0

10

20

30

40

50

60

70

80

90

100

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

I-V Parameters

0.2

0.3

0.4

0.5

0.6

0.7

Voc (V)

Jsc (mA/cm2)

FF (%)

Eff.(%)

Rs (ohm-cm2)

Voc (V)

C2510-12 Device#A in Set-2; Right Grid

(cross-talk) (a)

(TPT)

0

10

20

30

40

50

60

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

I-V Parameters

0.4

0.5

0.6

0.7

Voc (V)

Jsc (mA/cm2)

FF (%)

Eff.(%)

Rs (ohm-cm2)

Voc (V)

C2510-12 Device#B in Set-3; Right Grid

(b)

(FG-200)

0

2

4

6

8

10

12

14

16

18

20

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

Z' Resistance (Ohm)

Z' (1 Hz) (ohm)

Z' (1 MHz) (ohm)

Resistance Change on C2510-12 Cell B Grid-Grid in Set3

(d)

(FG-200)

(Grid-Grid)

0

2

4

6

8

10

12

14

16

18

20

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

Z' Resistance (Ohm)

Z' (1 Hz) (ohm)

Z' (1 MHz) (ohm)

Resistance Change on C2510-12 Cell A Grid-Grid in Set-2

(c)

(TPT)

(Grid-Grid)

0

2

4

6

8

10

12

14

16

18

20

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

Z' Resistance (Ohm)

Z' (1 Hz) (ohm)

Z' (1 MHz) (ohm)

C2528-13 Device #1 in Set-4; Grid-Grid

(e)

(TFPE)

(Grid-Grid)

0

2

4

6

8

10

12

14

16

18

20

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

Z' Resistance (Ohm)

Z' (1 Hz) (ohm)

Z' (1 MHz) (ohm)

C2528-13 Device #3 in Set-4; Grid-Grid

(f)

(TFPE)

(Grid-Grid)

(std AlNi bi-grid)

(std AlNi bi-grid)

(0.1-μm Ni bi-grid)

(0.1-μm Ni bi-grid)

0

10

20

30

40

50

60

70

80

90

100

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

I-V Parameters

0.2

0.3

0.4

0.5

0.6

0.7

Voc (V)

Jsc (mA/cm2)

FF (%)

Eff.(%)

Rs (ohm-cm2)

Voc (V)

C2510-12 Device#A in Set-2; Right Grid

(cross-talk) (a)

(TPT)

0

10

20

30

40

50

60

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

I-V Parameters

0.4

0.5

0.6

0.7

Voc (V)

Jsc (mA/cm2)

FF (%)

Eff.(%)

Rs (ohm-cm2)

Voc (V)

C2510-12 Device#B in Set-3; Right Grid

(b)

(FG-200)

0

2

4

6

8

10

12

14

16

18

20

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

Z' Resistance (Ohm)

Z' (1 Hz) (ohm)

Z' (1 MHz) (ohm)

Resistance Change on C2510-12 Cell B Grid-Grid in Set3

(d)

(FG-200)

(Grid-Grid)

0

2

4

6

8

10

12

14

16

18

20

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

Z' Resistance (Ohm)

Z' (1 Hz) (ohm)

Z' (1 MHz) (ohm)

Resistance Change on C2510-12 Cell A Grid-Grid in Set-2

(c)

(TPT)

(Grid-Grid)

0

2

4

6

8

10

12

14

16

18

20

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

Z' Resistance (Ohm)

Z' (1 Hz) (ohm)

Z' (1 MHz) (ohm)

C2528-13 Device #1 in Set-4; Grid-Grid

(e)

(TFPE)

(Grid-Grid)

0

2

4

6

8

10

12

14

16

18

20

0 500 1000 1500 2000 2500 3000 3500 4000 4500

DH Exposure Time (h)

Z' Resistance (Ohm)

Z' (1 Hz) (ohm)

Z' (1 MHz) (ohm)

C2528-13 Device #3 in Set-4; Grid-Grid

(f)

(TFPE)

(Grid-Grid)

(std AlNi bi-grid)

(std AlNi bi-grid)

(0.1-μm Ni bi-grid)

(0.1-μm Ni bi-grid)

Figure 10. (a, b) I-V parameter plots for C2510-12A in Set-2 with a TPT backsheet and 12B cells in Set-3 with a FG-200 

moisture barrier film, respectively, as a function of total DH exposure time. (c, d) Z’ resistance plots for BZO on C2510-12A and 

12B cells measured via the standard 3.05-μm AlNi bi-grid contacts. (e, f) Z’ resistance plots for BZO on cells #1 and #3 of C2528-

12 with a TFPE backsheet measured via the 0.1-μm Ni bi-grid contacts.
12 

(figures not shown). In summary, no significant degradation was observed on any of the AZO, BZO/glass, and

BZO/CIGS cell sample in Set-4 with a FG-200 moisture barrier film. 

4. CONCLUSIONS

A step-stress accelerated degradation testing (SSADT) method was employed for the first time to evaluate the 

performance reliability of CIGS solar cells and device component materials in four encapsulated Al-framed test 

structures, using edge sealant and backsheet/moisture barrier film for moisture ingress control. The SSADT exposure 

used a 15o

C and then a 15%RH increment step, beginning from 40oC/40%RH (40/40) to 85oC/70%RH (85/70) for a total 

DH exposure of 4204 h as of the moment. The voluminous data processed as of DH = 3956 h produced the results as

summarized below: 

o The CIGS solar cells in Set-1 and 2 encapsulated with a moisture-permeable TPT backsheet exhibited some 

irregularity in I-V performance degradation pattern. A few cells started to degrade at low T/RH < 55/55, and crosstalk was observed on a few cells producing abnormal high Jsc, either due to incomplete device isolation and/or 

certain “conductive link” induced by moisture at T/RH < 55/40. Some cells showed large Voc, FF, and efficiency 

degradation due to increased series Rs (ohm-cm2

) at T/RH ≥ 70/70. Best cells showed nearly identical I-V 

degradation trend regardless of the AZO thickness on the cells ranging from standard 0.12 μm to 0.50 μm. No clear 

“stepwise” feature in the I-V parameter degradation curves corresponding to the SSADT T/RH/time profile was 

observed. 

o Results of ECIS analysis indicate degradation of the CIGS solar cells corresponded to increased series resistance Rs 

(ohm) and degraded p-n junction-related properties, i.e., decrease in parallel (minority carrier 

diffusion/recombination) resistance Rp, capacitance C, overall time constant (“lifetime”) Rp*C, and capacitor 

quality factor (CPE-P). Heating at 85/70 appeared to benefit the solar cells as indicated by the largely recovered 

CPE-P factor, but the benefit was offset negatively by Rs increase and Rp decrease. 

o Mo and BZO on glass substrates and BZO on CIGS solar cells in Set-1 and -2 (TPT) showed significant degradation 

with increased resistance at T/RH ≥ 70/70, as measured by ECIS. Exposed at T/RH = 85/70, (i) DH-induced 

corrosion caused Mo to became brownish with white dot growth and highly resistive; (ii) BZO layers on the CIGS 

cell pieces with “standard AlNi” bi-grid contact degraded more than on the glass substrates, and showed substantial 

blistering that was not seen on BZO/glass samples; and (iii) One CdS/CIGS/Mo/SLG sample displyed a small 

darkening and then flaking feature. Additionally, BZO with standard 3.05-um AlNi contact grid appeared to be less

stable than with thin (0.1–0.2 μm) Ni contact grid. 

o In Set-3 and 4 with moisture-blocking films (TFPE and FG-200), most CIGS solar cells and device components, i.e., 

Mo, AZO, BZO, AlNi grid and CdS/CIGS, appeared to be stable at T/RH = 85/70.

o The edge sealant (TruSeal LP101) and moisture-blocking films (TFPE and FG-200) were effective to block 

moisture ingress as evidenced further by preservation of the initial blue color on the RH indicator strips up to T/RH 

= 85/70. 

The SSADT study is currently ongoing and is to be finished at a final 85/85 DH exposure. A more comprehensive 

summary will be reported in the near future after the experiment is completed and the voluminous data are thoroughly 

processed and analyzed; the results may also allow a determination of whether T- and RH-dependent activation energy 

can be derived for use on the Peck equation.

ACKNOWLEDGEMENTS

The authors thank K. Terwilliger for the WVTR measurements and C. DeHart for assistance in some sample 

preparations. This work was performed at the National Center for Photovoltaics under DOE Contract No. DE-AC36-

08GO28308 with the National Renewable Energy Laboratory.
13 

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14 

[21] J. Lee, R. Elmore, C. Suh and W. Jones, “Step-Stress Accelerated Degradation Testing (SSADT) for Photovoltaic 

(PV) Devices and Cells,” Proc. 2010 IEEE ASTR (Accelerated Stress Testing & Reliability), October 6-8, 2010, 

Denver, CO. (Components, Packaging, and Manufacturing Technology Society, CPMT)

[22] J. Lee, R. Elmore, and W. Jones, “Statistical Modeling of Photovoltaic Reliability using Accelerated Degradation 

Techniques,” Proc. 2011 PV Module Reliability Workshop, NREL, Golden, CO

[23] F.J. Pern, L. Mansfield, C. DeHart, S.H. Glick, F. Yan, and R. Noufi, “Thickness Effect of Al-Doped ZnO 

Window Layer on Damp-Heat Stability of CuInGaSe2 Solar Cells,” Proc. 37th IEEE PVSC, June 21-25, 2011, 

Seattle, WA.

[24] F.J. John Pern and R. Noufi, “Characterization of Damp-Heat Degradation of CuInGaSe2 Solar Cell Components 

and Devices by (Electrochemical) Impedance Spectroscopy,” Proc. 2011 SPIE Conference: Reliability of PV Cells, 

Modules, Components, and Systems, August 22-25, 2011, San Diego, CA.

[25] Materion’s “Moisture Barrier Films” technical brochure LAC Barrier Films, Rev. 020311 (in which the unit uses 

gm/cm2

/day incorrectly). The WVTR data with correct unit is seen on “FG-200 Solar Module Vapor Barrier 

Films,” http://materion.com/Products/ProductGroups/ThinFilmCoatings/VaporBarrierFilms/FG-200SolarModuleVaporBarrierFilms.aspx.

[26] Jinsuk Lee, personal communications and discussions while Lee was still at NREL.

[27] F.J. Pern, F. Yan, K. Zaunbrecher, B. To, J. Perkins, and R. Noufi, “Investigation of Some Transparent Metal 

Oxides as Damp Heat Protective Coating for CIGS Solar Cells,” Proc. 2012 SPIE Conference: Reliability of PV 

Cells, Modules, Components, and Systems, August 12-16, 2012, San Diego, CA.

[28] 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. 

[29] J. Bisquert, “Impedance Spectroscopy Applied on Solar Cells,” a presentation at the Nordic Workshop on Solar 

Electricity, April 27-29 2004, Sonnerupgaard Gods, Denmark. 

[30] J. Bisquert, “Theory of the Impedance of Electron Diffusion and Recombination in a Thin Layer,” J. Phys. Chem. 

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

[31] Impedance Spectroscopy – Emphasizing Solid Sate Materials and Systems, J. Ross MacDonald ed., John Wiley & 

Sons, New York, 1987. Also see “Constant Phase Element” in Chapters 1 and 2.

[32] 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.

[33] 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.

[34] 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. AlJassim, J.P. Thornton, and J. M. Gee ed., CP462, Denver, CO, Sept., 1999, pp. 661-666. American Institute of 

Physics, Woodbury, New York.

[35] (a) “Constant phase element,” http://en.wikipedia.org/wiki/Constant_phase_element. (b) “Explaining a Constant 

Phase Element (CPE),” http://www.consultrsr.com/resources/eis/cpe2.htm, and the references therein. (c) Help 

menu on equivalent circuits, instant curve-fitting function and constant phase element in the ZView program (ver. 

3.2c) from Scribner Associates, Inc.