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

Characterization of Damp-Heat 

Degradation of CuInGaSe2 Solar 

Cell Components and Devices 

by (Electrochemical) Impedance 

Spectroscopy

Preprint

F.J. John Pern and Rommel Noufi

Presented at SPIE Optics + Photonics 2011

San Diego, California

August 21-25, 2011

Conference Paper

NREL/CP-5200-52564

September 2011
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1 

CHARACTERIZATION of damp-heat degradation of CuInGaSe2

solar cell components and devices by (electrochemical) impedance 

spectroscopy 

F.J. John Pern and Rommel Noufi 

National Center for Photovoltaics, National Renewable Energy Laboratory 

1617 Cole Blvd., Golden, CO, USA 80401 

ABSTRACT

This work evaluated the capability of (electrochemical) impedance spectroscopy (IS, or ECIS as used here) to monitor 

damp heat (DH) stability of contact materials, CuInGaSe2 (CIGS) solar cell components, and devices. Cell

characteristics and its variation of the CIGS devices were also examined by the ECIS. Bare and encapsulated sample sets 

were separately prepared and exposed in an environmental chamber at 85o

C and 85% relative humidity (RH). The ECIS

results from bare samples tested within 50-100 h of DH exposure allowed the determination of the use of a conducting 

Ag paste and a low-melting-point solder alloy for making a DH-stable external connection with Au wires. Bare Mo and 

AlNi grid degraded (corroded) rapidly while Ni was DH-stable. The moisture-dampened Al-doped ZnO (AZO) and 

bilayer ZnO (BZO) likely underwent hydrolytic “capacitor-forming” reaction by DH, resulting in "transient" behavior of 

very high resistance in ECIS that was not detected by four-point probe. Using an encapsulation test structure that 

allowed moisture ingress control, DH-induced degradation (resistance increase) rates of BZO on glass decreased from 

0.21 ohm/h using a moisture-permeable Tedlar/Polyester/Tedlar (TPT) backsheet to 1.0 x 10-3 ohm/h using a moisture 

barrier FG-200 film, while Mo on glass did not exhibit the same conducting degradation and corrosion as the bare 

samples after over 1270 h DH exposure. CIGS solar cells encapsulated with a TPT backsheet degraded irregularly over 

774 h DH exposure. Key resistance and capacitance parameters extracted by curve fitting of impedance data clearly 

showed the variation and impact of DH exposure on cell characteristics. Profound “depression” or shorting of the “p-n 

junction capacitor” by DH was evident. ECIS results are shown to correlate reasonably well with the solar cells’ currentvoltage (I-V) degrading trends. Furthermore, ECIS analysis was capable of differentiating cell degradation due to 

“junction capacitor” shorting, damage or breakdown from that due to electrical conduction failure on AlNi/BZO layers.

Keywords: (electrochemical) impedance spectroscopy (ECIS), damp heat degradation, CuInGaSe2 (CIGS) solar cell, 

AlNi contact grid, Al-doped ZnO, Mo base electrode, conducting silver paste. 

1. INTRODUCTION

Long-term performance reliability is critical for all kinds of photovoltaic (PV) modules, including thin-film CIGS. For 

terrestrial thin-film modules, the IEC 61646 qualification standard 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 [1]. The CIGS cell 

component materials, including Mo, CIGS, and ZnO, have been shown to be DH-sensitive or unstable [2–8]. The CIGS 

solar cells or modules have also been shown to degrade in DH exposures [9-11]. A number of analytical and 

characterization methods are conventionally employed to measure and determine the solar cell performance and 

component materials degradation. For example, current-voltage (I-V) and quantum efficiency (QE) measurements are 

used to characterize solar cells, permitting a direct assess of the performance and spectral response of the solar cell. 

Photoluminescence (PL) and electroluminescence (EL) can be used to examine the recombination activity of minority 

carriers, allowing also a direct evaluation of the quality of the solar cells or mini-modules [12]. These methods are in 

general complementary. However, despite the ability to determine the key electrical parameters (i.e., open circuit voltage 

Voc, short circuit current density Jsc, fill factor FF, efficiency, dark current density Jo, and diode ideality factor n, along 

with the series resistance (Rs) and shunt resistance (Rsh)), the light/dark I-V measurement and curve analysis may not be 

able to provide clear insight or quantitative evaluation of how/why a solar cell has degraded by, for example, DH 

exposure.
2 

In comparison, (electrochemical) impedance spectroscopy (IS or ECIS as preferably used here) is well known to be 

highly suitable for analyzing a variety of R-C circuits corresponding to a large number of chemical, electrochemical, and 

physical materials, devices, and/or systems [13]. Specifically, ECIS measurements can readily allow a quantitative 

evaluation of the electrical property of a solar cell’s p-n junction, which is typically represented by an equivalent circuit 

of a parallel resistor and capacitor (i.e., “R-C” circuit model), and its series (contact/bulk) resistance. ECIS was

employed to characterize the resistance and capacitance factors related to the minority carrier transport and performance 

of crystalline Si (c-Si) cells by Suresh [14] and Mora-Sero et al. [15]. A theoretical ECIS account of thin layer devices 

and extensive studies on the charge transfer dynamics of dye-sensitized solar cells have been given by J. Bisquert and his 

coworkers [15-17]. Along with the use of admittance spectroscopy, ECIS was used to characterize CIGS solar cells as 

reported by Bayhan and Kavasoglu [18,19]. Recently, we reported briefly its application to characterize CIGS solar cells 

and mini-modules as well as performance reliability of CIGS solar cells upon DH treatment [6,12]. Prior to this, its first 

use to characterize the DH stability of PV devices was reported for c-Si solar cells by Meier et al. at NREL [20]. 

Descriptions of the IS or ECIS technology are broadly available in literature, therefore no detailed elaboration is given

here. Briefly, in a simplest form of the (EC)IS measurements, a small amplitude sinusoidal AC signal is applied to the 

subject of study with or without bias sweeping over a range of frequency, and the monitored real (Z’) and imaginary (Z”) 

impedance responses are displayed in a complex plane (Z” vs. Z’) plot. A pure resistor will produce a vertical line, while

an R-C circuit generates a semicircle. From the intercepts of the semicircle on the Z’(real) axis, series/bulk and charge 

transfer resistances can be determined. Capacitance can be then extracted from curve fitting. Corresponding Bode plots

can be obtained as well. The majority of various solid-state solar cells 

including CIGS, where no mass transfer or diffusion of chemical or ionic 

species is involved so that slow-diffusion Warburg impedance is 

unimportant, can be modeled by a simple equivalent circuit as Scheme 1, 

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

resistance (Rp) for minority carrier recombination/diffusion and a 

capacitance (C) for diffusion and depletion [14-19]. (The symbol Rs’ is 

used here to differ from the series resistance Rs obtained from solar cell’s 

I-V measurement.) The circuit in Scheme 1 can also be called “Randles 

cell,” one of the simplest cell models, and the complex plane plot can be 

called Nyquist plot, as commonly in electrochemistry. These parameters 

can be extracted by using curve fitting with an appropriate equivalent 

circuit for a solar cell system. The effective minority carrier lifetime or 

time constant can be calculated simply by the product of Rp*C or from 

the ω = 1/2πf, where f is the maximum frequency derived from the 

semicircle of complex plane plot [16,17]. Degradation of the solar cells 

by environmental stresses can be therefore determined from changes in 

Rs’, Rp, C, and Rp*C. 

In this work, with the potential for in-situ monitoring, ECIS is being evaluated as a sensitive, non-invasive, nondestructive analytical method for monitoring the DH stability of the contact materials, CIGS solar cell components and 

devices. A quick screening was conducted for bare samples to determine the best materials and contact processing 

conditions prior to making DH-stable external contacts for samples to be encapsulated in a glass/Al fame test structure 

and subjected to longer-term stability study [6]. The obtained ECIS data are analyzed with curve fitting to extract key 

electrical IS parameters of degraded solar cells and correlate with I-V results as a function of DH exposure time, as well 

as to seek the possibility to differentiate or separate the degradation of the CIGS solar cells due to p-n junction 

shorting/breakdown from the degradation due to contact failure of bilayer ZnO and AlNi grid by DH.

2. EXPERIMENTAL

Sample preparations. Details for preparing the CIGS solar cells (cell area: 0.41 cm2), single layer Al-doped ZnO (AZO) 

or bilayer ZnO (BZO, i.e., AZO on an intrinsic ZnO layer), the contact materials and contact processing for making 

external electrical connections to the tiny AlNi pad on CIGS solar cells, and the glass/edge sealant/Al frame/backsheet 

test structure for encapsulation were described in [6]. In this work, two more encapsulated samples, Dummy-1 and -2, 

with AZO, BZO and Mo on glass substrates, but no CIGS cell coupon, and a RH indicator strip were prepared in 

Z’ (real)

Z” (imaginary)

Rs’ Rs’ + Rp

= Rp * C

High f Low f

Z’ (real)

Z” (imaginary)

Rs’ Rs’ + Rp

= Rp * C

High f Low f

Z’ (real)

Z” (imaginary)

Rs’ Rs’ + Rp

Lifetime (Time Constant) 

= Rp * C

High f Low f

A Complex Plane Plot

Z’ (real)

Z” (imaginary)

Rs’ Rs’ + Rp

= Rp * C

High f Low f

Z’ (real)

Z” (imaginary)

Rs’ Rs’ + Rp

= Rp * C

High f Low f

Z’ (real)

Z” (imaginary)

Rs’ Rs’ + Rp

Lifetime (Time Constant) 

= Rp * C

High f Low f

A Complex Plane Plot

Rs’

Rp

C

Rp

C

Scheme 1. Schematic representation of a 

simple equivalent circuit (insert), Rs’-(Rp-C), 

and its semicircle in the complex plane for a 

typical solar cell.
3 

addition to continue DH exposure for the “Set-5” with four CIGS cell coupons as reported in [6]. Dummy-1 used a 

Madico TPT backsheet for moisture ingress control, and “Dummy-2” used a moisture barrier film Barix FG-200, kindly 

provided by Techni-Met, Inc. All three were laminated at 100o

C with TruSeal’s SET LP01 gray edge sealant tape. The 

water vapor transmission rate (WVTR) is 142.77 g/m2

/day for the TPT backsheet, measured at 86.8oC with a 

breakthrough/equilibrium time at ~3 min/~15 min, and is < 10-3 g/m2

/day for the FG-200 barrier film (from product 

specifications). 

For quick-screening the DH stability of contact materials and contacts, a matrix of bare (unencapsulated) samples having 

different contact designs was prepared: AZO and BZO on glass substrates with standard AlNi (0.05 μm Ni + 3 μm Al) or 

Ni-only (0.1–0.2 μm) grid patterns using a special interdigitated “bi-grid” for electrical (ECIS) measurements. These 

grided-AZO and BZO samples, and samples of AlNi, Ni-only, Mo film, and plain Ag paste coating on glass substrates,

were first applied with a small dot of a Ag epoxy, Tra-Duct 2902 (TD2902) from Tra-Con, or a Ag paste, PV412 

Solamet from DuPont, heated to dry, and attached with 4.5-mil Au wires with a low-melting-point solder alloy

(Cerroseal35, or Cerro35 for short, In52/Sn48, from Cerro Corp.). Some BZO and Mo samples had the solder alloy 

directly applied on the sides and Au wires attached.

DH exposure. The exposure for the bare samples and the three encapsulated samples was conducted first in an ESPEC 

environment chamber at 85o

C and 85% RH. In later stage, the three encapsulated samples were transferred to an AES 

environment chamber operating at the same DH conditions. A quick-screen test for the bare samples was exposed for 

100 h. The encapsulated Set-5 and Dummy-1 and -2 were continued to 774 h and >1200 h, respectively. (The Set-5 was 

exposed for over 168 h when reported in [6].)

Characterization. As described previously in [6], I-V measurements were performed on an Optical Radiation Corp. 

model 1000 solar simulator, with QE on a custom-built system. ECIS measurements 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. A 10-mV amplitude AC voltage without a bias was used for all non-cell 

samples, and the scan frequency range started from 1M Hz to 1 Hz. For CIGS solar cells, the ECIS measurements were 

conducted in the dark. For the cells that showed a “jump” at 0.0V bias in the low-frequency range in the amplitude vs. 

frequency Bode plot, a 0.25V–0.30V forward bias was applied to obtain good Z” vs. Z’ semicircles (see Section 3.3.1

below for details). The two built-in “instant fit” functions, Rs’-(Rp-C) and Rs’-(Rp-CPE), in the ZView were used 

directly for curve-fitting manually and individually the numerous data files (several hundreds) measured for the CIGS 

solar cells in Set-5. CPE stands for “constant phase element,” a factor indicating the non-ideal quality of a capacitor

[13,21,22]. The reported resistance and capacitance data are used directly without subtracting off the values contributed 

from the cables, alligators and Kevin probes, which are very low (≤ 0.01 ohm). Bare samples were also measured with a 

four-point probe for sheet resistance and examined with optical micro-imaging as needed.

3. RESULTS AND DISCUSSION

3.1 DH Stability of External Electrical Contact

Two issues are involved in this part: the needs for DH-stable contact materials and a simple processing method for 

making reliable, low-resistance external electrical contacts to the AlNi grid pad on the CIGS solar cells, which upon 

encapsulation cannot be accessed for repair if anything happens to the contact spots. Although the 3-μm Al offers high 

electrical conductivity, it is well known that Al surface is very difficult to apply solder alloy with dependable adhesion. 

Furthermore, the processing temperature has to be relatively low (preferably under 150o

C) and the processing time 

relatively short (preferably under 20 min) in order to avoid causing thermal damage to the CIGS solar cells. Therefore, a 

conducting silver paste (PV412) and a two-part silver epoxy liquid (TD2902) were chosen and tested for their ability to 

deposit onto the surfaces of Al pad and other substrates at very small volume as a dot. The Ag paste PV412 was found 

more favorable for being easier to handle as well as its size control (without smearing or spread-out), and heat treatment

(130o

C for 8–10 min or 150oC for 5–8 min). Additionally, the dried Ag paste allows a solder alloy to be applied on with 

reasonable adhesion strength. A solder alloy, Cerro35, with a melting point of 118o

C was found to meet the need when 

spot-applied onto the PV412 dot at 177o

–200oC, the solder alloy dot then allowed the attachment of Au wire (4.5 mil 

diameter) by soldering with the same alloy. Another alloy with a melting point of 143o

C (WS143, In97/Ag3, Williams 

Advanced Materials) was also determined suitable and later used. The various contacts are essentially resistors in nature; 
4 

each has a resistance and produces a relatively vertical, linear curve in the ECIS measurements (capacitance is assumed 

to be negligible). The resistance can simply use the Z’(real) impedance value at 1 Hz. Figure 1a gives the quick-screen 

results for the PV412, TD2902, and Cerro35 attached with Au wires on different substrates and exposed to DH for 100 h, 

showing low resistance and good DH stability of PV412/Cerro35/Au wire combination. Figure 1b shows the good DH 

stability of PV412 Ag paste coated on glass microslides as measured with a four-point probe, confirming DuPont’s 

materials data sheet of high DH stability for PV412. The results hence established the simple processing conditions with 

materials that are DH-stable for making contact to the AlNi pad. The contact structure represented by layer would be:

Ni/Al/PV412/Cerror35/Au wire, first heated at 150o

C for ~6 min for PV412 and then soldered at 177-190oC for Cerro35

(or WS143). 

DH Stability of Cell Components

In previous studies [2-5], we reported that bare Mo and bilayer ZnO (BZO) on glass substrates rapidly degraded in DH. 

In this work, we compared the DH stability of both bare and encapsulated (Dummay-1 and -2) Mo, single layer Aldoped ZnO (AZO), and BZO on glass substrates, each was cut from the same large piece substrate. Some samples of the 

AZO and BZO on glass had either AlNi or Ni-only grids. The test species were processed with the contact materials and 

Au wiring described above, and exposed to DH. In addition to the ECIS measurements, the bare samples were also 

measured with four-point probe for sheet resistance. The results are presented separately in the following.

3.2.1 Base Contact Mo on SLG

The Mo on soda lime glass (SLG) samples (~1.5-cm x 2.5-cm) had three contact combinations and Au wires attached to 

the two ends for electrical ECIS measurements. The open areas allowed for sheet resistance by four-point probe. Bare 

samples degraded rapidly in DH and turned bluish or gray-bluish, same as observed before. The Mo surface already 

became rough and/or produced fine pits as corroded by only 50 h DH, as shown in Fig. 2a, when examined under a 

WYKO interference optical microscope. One of the Mo/SLG samples (contacts made with PV412) became highly 

resistive and un-measurable by ECIS or four-point probe after 100 h DH. The other two Mo/SLG samples remained low 

in Z’(1 Hz) resistance (by ECIS) across the film surfaces (Fig. 2b), but four-point probe measurements showed large 

increases in sheet resistance from original ~6x10-3 ohm/square to 20–90 ohms/square (Fig. 2c). In comparison, when 

encapsulated in the test structure with a moisture vapor permeable TPT backsheet (i.e., Dummy-1), no color change or 

significant resistance change (by ECIS, Fig. 2d) on the Mo/SLG sample was observed even after >1250 h DH exposure, 

although a number of small “dots” started to appear on the film surface after ~500 h DH. The Mo/SLG in Dummy-2 with 

FG-200 moisture barrier also remained constant in Z’ resistance without visible surface change (Fig. 2d). 

The above results raise two issues. Firstly, the large difference or irregularity in degradation rates as reflected in Fig. 2(b 

and c) indicates that the corrosion pathway of bare Mo by the hot steam in DH can be highly irregular – perhaps

penetrating to the bulk in the PV412-Mo/SLG sample through the initially formed pits or holes (Fig. 2a) and cutting off 

the electrical connection so that even ECIS could not get the resistance measured, or more limiting locally and/or to the 

surface layers on the other two Mo/SLG samples. Secondly, the large difference in degradation rate and corrosion 

Figure 1. (a) Contact resistance from the real Z’ impedance by ECIS measurements for seven samples of different contact 

materials made on microslide or stainless steel foil. The five curves at the bottom are DH-stable with low resistance. (b) Sheet 

resistance measured by four-point probe for two PV412 films coated on microslides and dotted with Cero35 solder alloy and Au 

wires attached.

0.000

0.005

0.010

0.015

0.020

0.025

0 20 40 60 80 100 120

DH Exposure Time (h)

Sheet Resistance (ohm/sq)

Au wire/Cerro35/PV412 Ag paste/Au wire

Au wire/Cerro35/PV412 Ag paste/Au wire

Sheet Resistance of PV412 Film Coated on microslide

(b)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0 20 40 60 80 100 120

DH Exposure Time (h)

Resistance Z' at 1 Hz (ohm)

Au wire/Cerro35/PV412 Ag paste/Au wire

Au wire/Cerro35/PV412 Ag paste/Au wire

Au wire/Cerro35/PV412/Au wire on SS foil

Au wire/Cerro35/Au wire on microslide

Au wire/Cerro35/Au wire on 302/304 SS foil

Au wire/TD2902/Au wire on SS foil

Contact Resistance of PV412, TD2902, and Cerro35 Solder by ECIS

TD 2902/SS foil 

100o

C/20 min

Cerro35/PV412/SS 

foil 150o

C/5.5 min

(a)

Cerro35/PV412
5 

between bare and encapsulated samples (Fig. 2a vs. 2d) suggests the reaction mechanism is different or the reaction rate 

is greatly reduced in encapsulated test structure, where moisture vapor had to diffuse through the TPT layer. The 

observation here, together with those observed on the Set-5 with CIGS cell coupons, indicates the better testing approach 

is to employ an encapsulation test structure, which is more realistic as it resembles actual PV modules.

3.2.2 Front Grid Contact and ZnO Window Layer

While the “standard” trident AlNi (0.05 μm Ni + 3 μm Al) contact grid offers good conductivity to the BZO/CIGS solar 

cell, at issue is its DH instability due to hydrolytic corrosion and the detrimental consequence on solar cell performance 

when it becomes highly resistive from DH. Another possible performance-degrading factor is the loss of good 

conduction at the interface of AlNi and BZO. For example, it has been observed recently that, on bare CIGS cell 

coupons exposed to 50–100 h DH, wrinkling of the BZO layer has resulted in buckling and breaking of the AlNi grid 

fingerlines, causing premature cell performance failure [6]. To investigate these factors, a special grid pattern was 

designed with two pairs of four fingerlines interdigitated toward each other and a large square contact pad on each side 

for easy soldering and wiring. The “bi-grid” was used to deposit AlNi or Ni-only (0.1–0.2 μm) grid on glass, AZO/glass, 

BZO/glass, and CIGS cell coupons as well. The bi-grid pattern serves dual functions, allowing resistance measurement 

of AZO or BZO on glass or on CIGS cells as well as the cell efficiency if used on the solar cells. 

Top portion of Fig. 3a illustrates the “bi-grid” contact pattern on four 0.22-μm BZO/glass pieces with 0.1 μm Ni-only 

(#A) and standard 3.05-μm AlNi (#B, C, D) grids. Sample pieces #A did not have Au wires attached, and pieces #B-#D 

were attached with Au wires using Cerro35, PV412/Cerro35, and TD2902 contact materials on the AlNi square pads. 

Upon quick-screen DH exposure to 100 h, sheet resistance measured by four-point probe of the Ni grid square pads 

showed a very small increase from ~8.5x10-3 ohm/square to 12.5x10-3 ohm/square, while the BZO films on #A–#D 

increased from 65–72 ohms/square to 74–85 ohms/square. However, the ECIS-measured Z’ impedance/resistance for the 

three BZO sample pieces with AlNi bi-grids increased from original ~1.85 ohms to 40–535 ohms at DH = 50 h (Fig. 3b), 

and then unmeasurable (actually, messy curves) one day after DH = 100 h. In fact, during the ECIS measurements, 

sample piece #B became heated and cracked, and #C had a brown spot appear and spread out from the heating, as seen 

in the photographs. Nine days later, the two “scorched” samples became measurable with Z’ (1 Hz) resistance at 56 

ohms for #B and 614 ohms for #C. Sample piece #D remained highly resistive after 10 days of dry-out. These results are

attributed to the “transient” behavior of the moisture-dampened BZO. DH-induced corrosion of the AlNi grids, which 

was clearly observed as shown in the bottom portion of Fig. 3a, could have also contributed to the magnitude of the 

transient phenomenon. However, similar transient behavior was observed on a large (1.5” x 3”) BZO/glass sample 

specially designed with two Ni-only electrode bands on the top and bottom of the BZO layer at DH = 50 h and 100 h

Figure 2. (a) 3D optical images of the bare Mo films on SLG substrates exposed to 

50 h DH, showing roughened surface and pits or holes due to corrosion. (b) 

Resistance across the Mo/SLG samples in (a) with three contact combinations, 

Cerro35, Cerro35/PV412 and Cerro35/TD2902, and Au wires attached. The heating 

conditions are indicated. (c) Sheet resistance of samples in (b), measured with fourpoint probe, upon DH exposure for 100 h. Cerro35/PV412/Mo sample was 

unmeasurable at DH=100 h. (c) Resistance across the Mo/SLG films with two 

Cerro35/PV412/Au-wire contact strips separated by ~20 mm and encapsulated in 

Dummy-1 and -2 test structures; the spike at 110h was due to loosen soldering on 

the tab ribbon piece outside the test structure, which was corrected by re-soldering. 

The resistances for samples in (b) and (d) were obtained directly from Z’ 

impedance at 1 Hz.

0

10

20

30

40

50

60

70

80

90

100

0 20 40 60 80 100 120

DH Exposure Time (h)

Sheet Resistance (ohm/sq)

Au/Cerro35/MoSLG/Au

Au/Cerro35/PV412/MoSLG/Au

Au/Cerro35/TD2902/MoSLG/Au

Sheet Resistance of Mo/SLG by 4-Point Probe

(c)

Cerro35/PV 412 

150o

C/5.5 min

Cerro35/TD2902 

100oC/20 min

Cerro35 

177o

C

0

1

2

3

4

5

6

7

8

9

10

0 200 400 600 800 1000 1200 1400

DH Exposure Time (h)

Resistance Z' at 1 Hz (ohm)

Dummy-1_Mo6-1A (TPT)

Dummy-2_Mo6-1E (FG-200)

Resistance of Mo/SLG in Dummy-1 & 2 

(d)

(TPT)

(FG-200)

0

1

10

100

1000

10000

100000

0 20 40 60 80 100 120

DH Exposure Time (h)

Resistance Z' at 1 Hz (ohm)

Au/Cerro35/MoSLG/Au

Au/Cerro35/PV412/MoSLG/Au

Au/Cerro35/TD2902/MoSLG/Au

Z' Resistance of Mo/SLG by ECIS with Three Contact Mat. 

Cerro35/PV 412 

150o

C/5.5 min

Cerro35/TD2902 100oC/20 min

Cerro35 177o

C

(b)
6 

(data/figure not shown). The moisture-soaked BZO film made it difficult to obtain good ECIS measurements on the 

same plane (top and bottom) and across the film, which became somewhat easier (but not completely) after allowing to

dry for several days. The implication of this transient behavior is that the dampened BZO by having absorbed moisture 

on the surface as well as at the grain boundary can greatly hinder the electron transfer or current flow. 

The transient behavior, reflected as a messy curve or unmeasurable in ECIS, has not been clearly observed for the 

BZO/glass samples with two narrow Cerro35 contact strips and Au wires soldered on two ends, which were separated by 

18 mm, and encapsulated in Dummy-1 and -2. As seen in Fig. 3c, the 0.22-μm BZO-4 in Dummy-1 with a moisturepermeable TPT backsheet shows a nearly linear increase in Z’ resistance over a period of DH 1270 h, giving a 

degradation (resistance increase) rate of ~0.21 ohms/h. For bare BZO/glass, previous studies have showed the BZO (and 

AZO) could become electrically insulating within 200–500 h DH exposure as measured by Hall method or four-point 

probe [2-5]. With a moisture barrier FG-200 film, the degradation rate dropped to 1.05 x 10-3 ohm/h, as reflected from 

the nearly flat (blue) curve in Fig. 3c for the BZO in Dummy-2. The result clearly indicates the moisture-blocking

effectiveness of the FG-200 barrier film. Single layer 0.12-μm AZO with bi-grid AlNi contacts was also studied. In 

Dummy-1 with TPT, the AZO-11 film showed a degradation rate of 0.14 x 10-3 ohms/h up to DH = 572 h, but became 

unmeasurable (“transient”) at DH = 767 h and 1015 h and then measurable again at 1270 h. The difference in the 

degradation rate between the BZO and AZO in Dummies with TPT backsheet was due to the difference in contact 

pattern and separation (18-mm for BZO with two Cerror35 strips vs. bi-grid pattern on AZO-11). For AZO-11 in 

Dummy-2 with FG-200 barrier film, (blue curve in Fig. 3d), a degradation rate of 1.06 x 10-3 ohm/h was obtained, 

essentially identical to that for the BZO-6 in Dummy-1. However, a transient behavior was observed at DH = 666 h, 

where a semicircle was obtained for the first time as shown in Fig. 3e, indicating the presence of an R-C circuit. The 

semicircle size decreased on a second scan 1 h later to confirm it was not an artifact. Curve fitting gives an Rs’ = 40 

ohms, Rp = 148610 ohms, and C = 1.44 x 10-9 Farads from the semicircle of first run, which decreased to 10, 67900, and 

8.06 x 10-10, respectively, for the second run, indicating the metastable nature of the R-C circuit. The Rp and C are most 

0

100

200

300

400

500

600

0 20 40 60 80 100 120

DH Exposure Time (h)

Resistance Z' at 1 Hz (ohm)

Au/Cerro35/AlNi JP Grid/BZO/Au (#B)

Au/Cerro35/PV412/AlNi JP Grid/BZO/Au (#C)

Au/TD2902/AlNi JP Grid/BZO/Au (#D)

"Transient" Resistance of BZO with Contact/JP AlNi Grid 

(No ECIS scan at T2=100h; 

save for 4-point probe)

heated/cracked by 

ECIS run at T2=100h

heated/browned by 

ECIS run at T2=100h

Cerro35

Cerro35/PV412 

135o

C/8 min

TD2902 100oC/20 min

(b)

0

50

100

150

200

250

300

350

400

0 200 400 600 800 1000 1200 1400

DH Exposure Time (h)

Resistance Z' at 1 Hz (ohm)

Dummy-1_BZO4-31 (TPT)

Dummy-2_BZO6-8 (FG-200)

Resistance of Grid/BZO/Grid Measurements in Dummy-1 & 2 

(TPT)

(FG-200)

(c)

0

5

10

15

20

25

30

35

40

0 200 400 600 800 1000 1200 1400

DH Exposure Time (h)

Resistance Z' at 1 Hz (ohm)

Dummy-1_AZO11-AlNi-1 (Bi-Grid) (TPT)

Dummy-2_AZO11-AlNi-2 (Bi-Grid) (FG-200)

Resistance of Grid/AZO-11/Grid Measurements in Dummy-1 & 2

Unmeasurable at 

DH=767 h and 

1015h

A semicircle at 

DH=666 h

(TPT)

(FG-200)

(d)

Rs (CPE-Rp)

Rs = -40.11 (±11.67)

CPE-T = 1.442E-9 (±6.0597E-11)

CPE-P = 0.93467 (±0.0034386)

Rp = 148610 (±1013.1)

Rs (CPE-Rp)

Rs = 10.41 (±3.2469)

CPE-T = 8.057E-10 (±1.1915E-11)

CPE-P = 0.97803 (±0.0011656)

Rp = 67898 (±150.06)

0 50000 100000 150000 200000

-150000

-100000

-50000

0

50000

Z'

Z''

Dum2-AZO11-AlNi-2-DH666h.z

Dum2-AZO11-AlNi-2-DH666h-2.z

FitResult

(e)

Figure 3. (a) Photographs of (top) four pieces of standard 0.22-μm BZO on glass with the “bi-grid” contact made of 0.1-μm Ni for 

#A and standard 3.05-μm AlNi for #B, C, and D; and (bottom) AlNi on the square pad area at DH = 0 h and 50 h. (b) “Transient” 

resistance for the bare samples of bi-grid AlNi/BZO in (a) at DH=50 h. The resistance was so high at DH = 100 h that samples #B 

and #C heated up and cracked (#B) or browned (#C) during ECIS scan. (c) Resistance of encapsulated 0.22-μm BZO/glass

samples with narrow Cerro35 strips soldered on two ends separated by 18 mm, and (d) bi-grid AlNi/0.12-μm AZO in Dummy-1 

and -2 as a function of DH exposure time. The sample in Dummy-1 was unmeasurable (messy curve) at DH = 767 and 1015 h. 

The sample in Dummy-2 also showed a transient behavior that produced a semicircle as shown in (e) for two scans at 0.0 V bias 

separated by 10 min.
7 

likely from low level moisture-dampened AZO film due to low WVTR of the FG-200 barrier film; and the Rs’ = 10–40

ohms is greater than the initial 3.92 ohms prior to DH exposure, suggesting the AlNi/AZO interface may have also been

affected. With higher moisture level through the TPT, the transient behavior for AlNi/AZO-11 in Dummy-1 was likely 

so great that it caused a messy curve (unmeasurable) at 767 h and 1015 h. However, how the transient behavior for 

AlNi/AZO or AlNi/BZO in the Dummies would later go away after longer DH exposure time remains unclear. On the 

other hand, the results shown in Fig. 3(c and d) also indicate that the contacts with Au wirings made on the Mo, AZO 

and BZO samples inside the encapsulated test structures for external electrical connections and ECIS measurements 

were DH-stable.

3.2.3 AlNi vs. Ni Contact Grid

As pointed out above, direct exposure of AlNi grid to DH resulted in hydrolytic corrosion within 50-100 h (e.g., Fig. 3a, 

bottom half photos), which can decrease CIGS cell performance by increased series (contact) resistance. The problem 

would become more serious if the ZnO bilayer wrinkled and cracked upon DH exposure, as observed on bare CIGS cell 

coupons [6]. Therefore, it is desirable to seek a DH-stable front contact metal to replace the AlNi so that concerns over 

the contact grid’s DH instability issues can be reduced or eliminated. In this regard, Ni is well known to be more stable 

than Al against oxidation and hydrolysis. Because it has already been used at a thin layer (0.05 μm) over BZO, it is 

natural to test a thicker Ni-only grid for front contact. Therefore, 0.1–0.2-μm Ni-only and standard 3.0μ5-μm AlNi grids 

and films were deposited on different substrate for quantitative comparison. The results show substantial wrinkling, popup, and even delamination occurred on the AlNi films on glass and CIGS device structure (corrosion of AlNi grid was 

already presented above), resulting in a sheet resistance increased from 8 x 10-3 ohm/square at DH = 0–50 h to >1 x 105

ohm/square at DH = 100 h. This is also reflected in large increase in the contact resistance (Z’ at 1 Hz) measured 

between the AlNi film layer and the CIGS cell devices as illustrated in Figure 4a and insert. In comparison, sheet 

resistance of the Ni film and grid contact pad remained essentially unchanged, as shown in Fig. 4b. The average sheet 

resistance of BZO on these samples increased from initial ~70 ohms/square to ~95 ohms/square after 100 h DH exposure

(not shwn). Accordingly, some CIGS solar cells with Ni-only grid 0.1–1.8 μm thick were prepared with mixed results. 

At a thin thickness of 0.2–0.3 μm, the Ni-only CIGS devices showed considerably lower efficiency due to relatively high 

resistance of the Ni gridlines than devices with standard AlNi grid. At 1.6–1.8 μm thick, the Ni-only CIGS devices 

offered comparable cell efficiency. However, due to high mechanical stress on thicker Ni layer, some of the 1.6–1.8-μm 

Ni-only contact grids exhibited cracking or peeled off, resulting in damaged or failed devices. More work is needed to 

optimize the sputtering approach and conditions for making thicker Ni contact grids with low stress and good adhesion.

3.2 DH Stability of CIGS Solar Cells

As stated earlier, this work was designed to evaluate ECIS technology as a sensitive non-invasive analytical method to 

characterize the CIGS solar cells fabricated by NREL’s three-stage process and monitor their DH stability in a specially 

designed encapsulation test structure. Characterization of DH stability of bare (unencapsulated) CIGS solar cells using 

conventional methods and one experiment using a test structure designed earlier were reported recently by us [6,9].

Figure 4. (a) Resistance across small CIGS cell pieces with AlNi as both contact electrodes to verify the DH-induced damages 

on AlNi films and contact resistance. Cerro35/PV412 and TD2902 were used on AlNi for Au wire attachment. The cell piece 

with Cerro35/PV412 contact was unmeasurable at D=100 h. Insert: Photograph of one of the cell piece after DH = 100 h, 

showing wrinkling and corrosion stain on the AlNi film. (b) Sheet resistance of 0.1-μm Ni on two glass substrates and three 

pieces of BZO/glass substrates upon 100 h DH exposure.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 20 40 60 80 100 120

DH Exposure Time (h)

Sheet Resistance (ohm/sq)

0.1 um Ni/std BZO/7059; piece A

0.1 um Ni/std BZO/7059; piece B

Narrow Ni/7059 strip scribed from edge

0.1 um Ni/std BZO/7059; piece C

Narrow Ni/7059 strip scribed from edge

Sheet Resistance of 0.1 um Ni by 4-Point Probe

Ni pad

Ni on BZO

(b)

0

1000

2000

3000

4000

5000

6000

0 20 40 60 80 100 120

DH Exposure Time (h)

Resistance Z' at 1 Hz (ohm)

Au/TD2902/AlNi Strip on cell piece-1/Au

Au/Cerro35/PV412/AlNi strip on cell piece-2/Au

Resistance of AlNi Strip on CIGS Cell Piece by ECIS

TD2902

Cerro/PV412

(a)
8 

3.3.1 Cell Characteristics from ECIS Measurements

An unusual phenomenon was observed for the first time when the as-fabricated CIGS solar cells were ECIS-measured. 

A “jump” in the amplitude-frequency Bode plot would show in the 10–40 Hz range on some devices, causing the 

subsequent data points in the complex plane to scatter randomly or to shift abruptly to high Z’(real) range. A messy or 

skewed curve would thus result and make data extraction unreliable or curve fitting difficult. This “jump” was highly 

repeatable in the 10–40 Hz range; but its occurrence was not universal on all CIGS solar cells. It appeared that the jump 

tended to happen on devices of good quality, but not necessarily of high efficiency. It would not occur on poor-quality, 

low-efficiency, or degraded devices. Figure 5a demonstrates the skewed and normal curves in the Z”-Z’ complex plane

for five devices on four different cell coupons, with the corresponding Bode plots and cell efficiency given in Fig. 5b. 

The “jump” seems to relate to yet-unidentified potential barrier in the devices. This potential barrier was overcome to 

obtain a good semicircle in the complex plane for most cells when a forward bias at 0.25 V was applied; some devices 

required a 0.30 V bias. However, the “jump” was also observed on pure capacitors, for example, a “jump” at 316 Hz 

appeared when a 5–20 pF or a 150–200 pF capacitor was measured in the same way, which caused similar messy curve 

in the complex plane plot. Therefore, the “jump” by a CIGS device may be a clear indicator of nearly pure-capacitor 

quality. Furthermore, ECIS analysis was found to be able to easily distinguish cells of different R-C quality/property but 

similar cell efficiency. For example, Fig. 5c compares the semicircles for five devices on the same cell coupon C2529-13, 

with two cells not biased and three biased at 0.30 V. The five devices had cell efficiency within a 13–15% range, but 

exhibited large variation in the ECIS responses. This kind of variation was observed for many of the CIGS cell coupons 

studied.

The semicircles were obtained in the ECIS scan because the CIGS solar cells, like many other kind of solar cells (e.g., cSi), are constituted with a p-n junction and a depletion width forming a capacitor and a parallel resistance, which, along 

with the contact series resistance, can be represented by a simple equivalent circuit as shown in Scheme 1 above, i.e., 

Rs’-(Rp-C). The three parameters can be extracted from the semicircle and by using an appropriate curve fitting.

Therefore, the large variations seen on the semicircles and the Bode plots in Fig. 5 clearly reflect the variations in the 

contact/bulk resistance and p-n junction details, or Rs’-(Rp-C), of the devices. These observations indicate two points 

that are mutually contradictory. On one hand, it shows that ECIS analysis is highly sensitive to the (Rp-C) difference 

among devices; on the other hand, the obvious variation for devices even on the same cell coupon indicates the nonuniformity of devices, a likely consequence of compositional and/or device quality non-uniformity. The latter

unwillingly imposes a restriction on the ECIS analysis for the CIGS solar cells; that is, individual analysis has to be 

performed for individual devices to monitor and track the performance and DH stability trend. A simple generalization 

of the analysis results may not be entirely accurate or reliable.

0 2.50e7 5.0

-1.0e8

-7.5e7

-5.0e7

-2.5e7

0

2.5e7 Z''

0 100000 200000 300000 400000

-400000

-300000

-200000

-100000

0

Z'

Z''

Set-5_C2354-4-Cell-4_DH0h Bias_0.0V.z

Set-5_C2354-5-Cell-2_DH0h Bias_0.0V.z

Set-5_C2354-6-Cell-4_DH0h Bias_0.0V.z

Set-5_C2529-13-Cell-2_DH0h Bias_0.0V.z

Set-5_C2529-13-Cell-5_DH0h Bias_0.0V.z

(a)

0 2.50e7 5.0

-1.0e8

-7.5e7

-5.0e7

-2.5e7

0

2.5e7 Z''

0 100000 200000 300000 400000

-400000

-300000

-200000

-100000

0

Z'

Z''

Set-5_C2354-4-Cell-4_DH0h Bias_0.0V.z

Set-5_C2354-5-Cell-2_DH0h Bias_0.0V.z

Set-5_C2354-6-Cell-4_DH0h Bias_0.0V.z

Set-5_C2529-13-Cell-2_DH0h Bias_0.0V.z

Set-5_C2529-13-Cell-5_DH0h Bias_0.0V.z

0 2.50e7 5.0

-1.0e8

-7.5e7

-5.0e7

-2.5e7

0

2.5e7 Z''

0 100000 200000 300000 400000

-400000

-300000

-200000

-100000

0

Z'

Z''

Set-5_C2354-4-Cell-4_DH0h Bias_0.0V.z

Set-5_C2354-5-Cell-2_DH0h Bias_0.0V.z

Set-5_C2354-6-Cell-4_DH0h Bias_0.0V.z

Set-5_C2529-13-Cell-2_DH0h Bias_0.0V.z

Set-5_C2529-13-Cell-5_DH0h Bias_0.0V.z

(a)

100 101 102 103 104 105 106 100

101

102

103

104

105

106

107

108

109

Frequency (Hz)

|Z|

Set-5_C2354-4-Cell-4_DH0h Bias_0.0V.z

Set-5_C2354-5-Cell-2_DH0h Bias_0.0V.z

Set-5_C2354-6-Cell-4_DH0h Bias_0.0V.z

Set-5_C2529-13-Cell-2_DH0h Bias_0.0V.z

Set-5_C2529-13-Cell-5_DH0h Bias_0.0V.z

100 101 102 103 104 105 106

-100

-75

-50

-25

0

25

Frequency (Hz)

theta

9.3%

9.7% 8.7% 14.6%

14.6%

“Jump

”

(b)

100 101 102 103 104 105 106 100

101

102

103

104

105

106

107

108

109

Frequency (Hz)

|Z|

Set-5_C2354-4-Cell-4_DH0h Bias_0.0V.z

Set-5_C2354-5-Cell-2_DH0h Bias_0.0V.z

Set-5_C2354-6-Cell-4_DH0h Bias_0.0V.z

Set-5_C2529-13-Cell-2_DH0h Bias_0.0V.z

Set-5_C2529-13-Cell-5_DH0h Bias_0.0V.z

100 101 102 103 104 105 106

-100

-75

-50

-25

0

25

Frequency (Hz)

theta

9.3%

9.7% 8.7% 14.6%

14.6%

“Jump

”

100 101 102 103 104 105 106 100

101

102

103

104

105

106

107

108

109

Frequency (Hz)

|Z|

Set-5_C2354-4-Cell-4_DH0h Bias_0.0V.z

Set-5_C2354-5-Cell-2_DH0h Bias_0.0V.z

Set-5_C2354-6-Cell-4_DH0h Bias_0.0V.z

Set-5_C2529-13-Cell-2_DH0h Bias_0.0V.z

Set-5_C2529-13-Cell-5_DH0h Bias_0.0V.z

100 101 102 103 104 105 106

-100

-75

-50

-25

0

25

Frequency (Hz)

theta

9.3%

9.7% 8.7% 14.6%

14.6%

100 101 102 103 104 105 106 100

101

102

103

104

105

106

107

108

109

Frequency (Hz)

|Z|

Set-5_C2354-4-Cell-4_DH0h Bias_0.0V.z

Set-5_C2354-5-Cell-2_DH0h Bias_0.0V.z

Set-5_C2354-6-Cell-4_DH0h Bias_0.0V.z

Set-5_C2529-13-Cell-2_DH0h Bias_0.0V.z

Set-5_C2529-13-Cell-5_DH0h Bias_0.0V.z

100 101 102 103 104 105 106

-100

-75

-50

-25

0

25

Frequency (Hz)

theta

9.3%

9.7% 8.7% 14.6%

14.6%

“Jump

”

(b)

0 50000 100000 150000

-100000

-50000

0

50000

Z'

Z''

Set-5_C2529-13-Cell-1_DH0h Bias_0.0V.z

Set-5_C2529-13-Cell-2_DH0h Bias_0.0V.z

Set-5_C2529-13-Cell-3_DH0h Bias_0.30V.z

Set-5_C2529-13-Cell-4_DH0h Bias_0.30V.z

Set-5_C2529-13-Cell-5_DH0h Bias_0.30V.z

14.7%

14.6%

14.4%

14.6%

13.2%

(c)

0 50000 100000 150000

-100000

-50000

0

50000

Z'

Z''

Set-5_C2529-13-Cell-1_DH0h Bias_0.0V.z

Set-5_C2529-13-Cell-2_DH0h Bias_0.0V.z

Set-5_C2529-13-Cell-3_DH0h Bias_0.30V.z

Set-5_C2529-13-Cell-4_DH0h Bias_0.30V.z

Set-5_C2529-13-Cell-5_DH0h Bias_0.30V.z

14.7%

14.6%

14.4%

14.6%

13.2%

0 50000 100000 150000

-100000

-50000

0

50000

Z'

Z''

Set-5_C2529-13-Cell-1_DH0h Bias_0.0V.z

Set-5_C2529-13-Cell-2_DH0h Bias_0.0V.z

Set-5_C2529-13-Cell-3_DH0h Bias_0.30V.z

Set-5_C2529-13-Cell-4_DH0h Bias_0.30V.z

Set-5_C2529-13-Cell-5_DH0h Bias_0.30V.z

14.7%

14.6%

14.4%

14.6%

13.2%

(c)

Figure 5. (a) Complex plane plots for five CIGS solar cells from four cell coupons, C2354-4, -5, -6 and C2529-13, at DH = 0 h, 

showing two skewed curves (insert) due to the “jump” at 39.8 Hz, one large, and three small semicircles. (b) Corresponding 

impedance amplitude (top) and phase angle (bottom) vs. frequency Bode plots, whereas the “jump” on two curves and efficiencies 

of the five cells are labeled. (c) Semicircles for five cells on the same coupon of C2529-13 at DH = 0 h, showing the large 

variation of impedance response due to variation of cell quality while cell efficiencies are within a narrow range of 13.2%–14.7%.
9 

3.3.2 DH-Induced Degradation and Curve Fitting with Equivalent Circuit Models

Although Rs’ values can be obtained from the Z’(real) at 1M Hz for convenience, the quality of the actual Z”-Z’ curve 

for a degrading solar cell may be quite noisy or non-linear in the low-frequency region, making determination of Rp 

from Z’(real) at 1 Hz (i.e., sum of Rs’ and Rp) unreliable. Besides, the purpose of this study is intended to seek the 

insight of what happens to the CIGS devices when they are degraded by DH exposure. Therefore, curve fitting is 

preferred so that more accurate or reliable values of Rs’, Rp and C can be obtained and compared to the values of Z’(real) 

at 1 Hz and 1M Hz. Due to the very large volume of data files (several hundreds), the two built-in “instant fit” functions, 

Rs’-(Rp-C) and Rs’-(Rp-CPE) equivalent circuits, were used directly for curve-fitting manually and individually with 

generally satisfactory results. The CPE stands for “constant phase element,” a factor indicating the non-ideal quality of a 

capacitor (or an R-C circuit) that can be affected by surface roughness, porous structures, varying thickness or 

composition, non-uniform current distribution, and/or inhomogeneous reaction rates on an electrode surface [13,21,22].

Quality of the curve fit was determined from the degree of curve superimposition as well as the error percentages of the 

parameters. The Rs’-(Rp-CPE) fit would produce two parameters: CPE-T, which is essentially the capacitance (C), and 

CPE-P, which typically is < 1.0 (1.0 for an ideal Rs’-(Rp-C) circuit or a pure capacitor, and 0 for a pure resistor) and can 

be viewed as a measure of “depression” degree on the “p-n junction capacitor” due to certain physical effects. For the 

initial and slightly degraded cells, curve fitting with either model was found satisfactory and gave similar results with 

CPE equal to or very close to 1.0 (not shown) because the semicircles (or the p-n junction capacitors) were not or little 

perturbed. As the cells degraded further and the semicircles became more “depressed,” curve fit using Rs’-(Rp-CPE) 

was required to obtain a good fit. Figure 6a illustrates the steady decrease in the size of Z”-Z’ semicircles for cell#5 on 

C229-13 coupon, which is common for most of the degrading cells, due to DH-induced degradation. However, a few

cells showed semicircle size increased first then largely decreased in later stage, as demonstrated in Fig. 6b for cell#7 on 

C2354-4 coupon. Figure 6c gives an example of curve fitting, comparing the fit quality using the two models Rs’-(Rp-C) 

and Rs’-(Rp-CPE) for a much-depressed semicircle for cell#7 on C2354-4 coupon at DH=774 h (Fig. 6b). For medium 

to highly degraded cells, the depressed semicircles are better fit with the CPE model than the Rs’-(Rp-C) (Randles cell) 

model, which is best for unexposed and lightly degraded cells.

3.3.3 Correlation of I-V and ECIS Results

Correlation between photovoltaic performance and impedance spectroscopy was reported for dye-sensitized solar cells

by Fabregat-Santiago et al. [23]. To correlate the DH-induced degradation on CIGS solar cell I-V performance and 

changes in ECIS parameters, cell#5 on the C2529-13 cell coupon (with 0.5-μm AZO) in the encapsulated Set-5 test 

structure was selected as an illustrative example given in Fig. 7. The six cells on the C2529-13 exhibited efficiency 

Figure 6. (a) Complex plane plots obtained with a 0.25 V forward bias for cell#5 on C2529-13 coupon in the encapsulated Set-5 

at DH = 0, 41, 90, 168, 270, 518, and 774 h, showing the continued decrease in the semicircle size. (b) Semicircles for cell#7 on 

C2354-4 coupon in Set-5 at the same DH exposure intervals. The semicircle size increased to DH = 518 h and then largely 

decreased at DH=774 h. In both cases, the cell efficiency steadily decreased as the DH exposure time increased. (c) Example of 

curve fitting quality of a “depressed” semicircle for cell#7 on C2354-4 coupon at DH = 774 h separately using the Rs’-(Rp-C) 

(top) and Rs’-(Rp-CPE) (bottom) equivalent circuit model with a constant phase element (CPE). The solid curves are fit results.

0 2500 5000 7500

-7500

-5000

-2500

0

Z'

Z''

Set-5_C2354-4-Cell-7_DH0h Bias_0.0V.z

Set-5_C2354-4-Cell-7_85-85_DH41h Bias_0.0V.z

Set-5_C2354-4-Cell-7_85-85_DH90h Bias_0.0V.z

Set-5_C2354-4-Cell-7_85-85_DH168h Bias_0.0V.z

Set-5_C2354-4-Cell-7_85-85_DH270h Bias_0.0V.z

Set-5_C2354-4-Cell-7_85-85_DH518h Bias_0.0V.z

Set-5_C2354-4-Cell-7_85-85_DH774h Bias_0.0V.z

(b)

270 h

774 h

168 h

518 h

41 h

0 2500 5000 7500

-7500

-5000

-2500

0

Z'

Z''

Set-5_C2354-4-Cell-7_DH0h Bias_0.0V.z

Set-5_C2354-4-Cell-7_85-85_DH41h Bias_0.0V.z

Set-5_C2354-4-Cell-7_85-85_DH90h Bias_0.0V.z

Set-5_C2354-4-Cell-7_85-85_DH168h Bias_0.0V.z

Set-5_C2354-4-Cell-7_85-85_DH270h Bias_0.0V.z

Set-5_C2354-4-Cell-7_85-85_DH518h Bias_0.0V.z

Set-5_C2354-4-Cell-7_85-85_DH774h Bias_0.0V.z

(b)

270 h

774 h

168 h

518 h

41 h

Rs (C-Rp)

Rs = 102.8 (±4.9403)

C = 4.727E-9 (±1.7894E-10)

Rp = 908.1 (±14.957)

0 250 500 750 1000 125

-500

-250

0

250

Z'

Z''

Set-5_C2354-4-Cell-7_85-85_DH774h Bias_0.0V.z

FitResult

Rs (CPE-Rp)

Rs = 54.12 (±1.5835)

CPE-T = 1.3674E-7 (±6.5075E-09)

CPE-P = 0.74235 (±0.0038246)

Rp = 1027 (±3.4277)

0 250 500 750 1000 1250

-500

-250

0

250

Z'

Z''

Set-5_C2354-4-Cell-7_85-85_DH774h Bias_0.0V.z

FitResult

(c)

Rs’

Rp

CPE

>>

Rp

CPE

>>

Rs’

Rp

C

Rp

C

Rs (C-Rp)

Rs = 102.8 (±4.9403)

C = 4.727E-9 (±1.7894E-10)

Rp = 908.1 (±14.957)

0 250 500 750 1000 125

-500

-250

0

250

Z'

Z''

Set-5_C2354-4-Cell-7_85-85_DH774h Bias_0.0V.z

FitResult

Rs (CPE-Rp)

Rs = 54.12 (±1.5835)

CPE-T = 1.3674E-7 (±6.5075E-09)

CPE-P = 0.74235 (±0.0038246)

Rp = 1027 (±3.4277)

0 250 500 750 1000 1250

-500

-250

0

250

Z'

Z''

Set-5_C2354-4-Cell-7_85-85_DH774h Bias_0.0V.z

FitResult

(c)

Rs (C-Rp)

Rs = 102.8 (±4.9403)

C = 4.727E-9 (±1.7894E-10)

Rp = 908.1 (±14.957)

0 250 500 750 1000 125

-500

-250

0

250

Z'

Z''

Set-5_C2354-4-Cell-7_85-85_DH774h Bias_0.0V.z

FitResult

Rs (CPE-Rp)

Rs = 54.12 (±1.5835)

CPE-T = 1.3674E-7 (±6.5075E-09)

CPE-P = 0.74235 (±0.0038246)

Rp = 1027 (±3.4277)

0 250 500 750 1000 1250

-500

-250

0

250

Z'

Z''

Set-5_C2354-4-Cell-7_85-85_DH774h Bias_0.0V.z

FitResult

(c)

Rs’

Rp

CPE

>>

Rp

CPE

>>

Rs’

Rp

C

Rp

C

0 100000 200000 300000 400000

-400000

-300000

-200000

-100000

0

Z'

Z''

Set-5_C2529-13-Cell-5_DH0h Bias_0.25V.z

Set-5_C2529-13-Cell-5_85-85_DH41h Bias_0.25V.z

Set-5_C2529-13-Cell-5_85-85_DH90h Bias_0.25V.z

Set-5_C2529-13-Cell-5_85-85_DH168h Bias_0.25V.z

Set-5_C2529-13-Cell-5_85-85_DH270h Bias_0.25V.z

Set-5_C2529-13-Cell-5_85-85_DH518h Bias_0.25V.z

Set-5_C2529-13-Cell-5_85-85_DH774h Bias_0.25V.z

(a)

41 h

0 h

270 h

90 h

168 h

774 h

“Jump”

0 100000 200000 300000 400000

-400000

-300000

-200000

-100000

0

Z'

Z''

Set-5_C2529-13-Cell-5_DH0h Bias_0.25V.z

Set-5_C2529-13-Cell-5_85-85_DH41h Bias_0.25V.z

Set-5_C2529-13-Cell-5_85-85_DH90h Bias_0.25V.z

Set-5_C2529-13-Cell-5_85-85_DH168h Bias_0.25V.z

Set-5_C2529-13-Cell-5_85-85_DH270h Bias_0.25V.z

Set-5_C2529-13-Cell-5_85-85_DH518h Bias_0.25V.z

Set-5_C2529-13-Cell-5_85-85_DH774h Bias_0.25V.z

(a)

0 100000 200000 300000 400000

-400000

-300000

-200000

-100000

0

Z'

Z''

Set-5_C2529-13-Cell-5_DH0h Bias_0.25V.z

Set-5_C2529-13-Cell-5_85-85_DH41h Bias_0.25V.z

Set-5_C2529-13-Cell-5_85-85_DH90h Bias_0.25V.z

Set-5_C2529-13-Cell-5_85-85_DH168h Bias_0.25V.z

Set-5_C2529-13-Cell-5_85-85_DH270h Bias_0.25V.z

Set-5_C2529-13-Cell-5_85-85_DH518h Bias_0.25V.z

Set-5_C2529-13-Cell-5_85-85_DH774h Bias_0.25V.z

(a)

41 h

0 h

270 h

90 h

168 h

774 h

“Jump”
10 

decrease from original 13%–16% (measured under the topcover glass) to 0%–6% in an irregular pattern after being 

exposed to the DH moisture vapor (permeated through the TPT backsheet) for 774 h. Cells#2, 4 and 5 showed vey

similar trend. Winkling and cracking of the BZO layer was extensive over the entire area of cells#1, 2 and 3, and partial 

on cell#4. BZO layer wrinkling was seen over the entire cell area of cells #1-4, but not on cells #5 and 6. For bare cell 

coupons, a 50–100 h DH exposure could induce extensive wrinkling, cracking, and delamination on the BZO layer and 

the AlNi grids over it [6]. Curve fitting using the Rs’-(CPE–Rp) equivalent circuit model was performed to extract the 

parameters of Rs’, Rp, CPE-T (i.e., C), CPE-P (see Fig. 6c), and maximum frequency from the Z”-Z’ semicircles and 

calculate the effective minority lifetime (time constant) by Rp*C [13,15-17,21,22]. However, the Rp*C is the “overall” 

magnitude, without trying to separate the recombination and diffusion terms.

Figure 7b shows the plot of I-V parameters (Voc, Jsc, FF, efficiency) normalized to that at DH=0 h, and Fig. 7c shows 

the plot of normalized Rs and Rsh from derived from light I-V measurements up to 518 h. The initial Rs and Rsh values 

were 1.96 and 2278 ohm-cm2 respectively, and became unreliably very large at DH=774 h as derived from three or four 

data points on the LIV curves. As seen in Fig. 7(b and c), cell FF and efficiency dropped largely after DH = 270 h, in 

parallel to the large increase in Rs and Rsh. Jsc was fairly steady till DH = 518 h, but dropped suddenly to 0.03 mA/cm2

at DH = 774 h, indicating a breakdown in the current flow pathway (i.e., electrical conduction) that also gave very high 

Rs. The Voc was more consistent over the course of 774 h DH exposure. On the ECIS side, Fig. 7d shows the plots of 

Rp (minority carrier recombination/diffusion resistance), CPE-T (the capacitance, same as C), CPE-P (the “depression” 

factor), and the Rp*C (i.e., CPE-T) lifetime in a logarithmic scale. Figure 7e shows the plots for the pairs of Rp and Z’(1 

Hz) and of Rs’ and Z’(1M Hz). Note that the initial data points at DH = 0 h for Rp, C, and Rp*C from curve fitting and 

Z’(1 Hz) from measurement are not used because of the “jump” (Fig. 6a). Rp from curve fitting matched quite well with 

Z’(1 Hz) and in a way was related to the Rsh, and the Rs’ deviated somewhat from Z’(1M Hz) by 0–6 ohms but was 

more dependable. The measurements were carried out at a 0.25 V forward bias. The ECIS results confirmed further the 

contact/bulk series resistance (Rs’) increased quickly, while Rp decreased steadily from original 4.18 x 105 ohms to 1.77 

x 104 ohms. The capacitance (CPE-T or simply C) decreased from an original 1.12 x 10-8 F to 7.03 x 10-9 F at DH = 168 

h and then increased to 2.20 x 10-8 F at DH =774 h, indicating the capacitance of the “p-n junction capacitor” gradually 

Figure 7. Correlation of I-V results with ECIS results for cell#5 of C2529-13 cell coupon exposed to DH for 774 h. (a) Efficiency 

degraded for all the six cells on the coupon, (b) normalized I-V parameters (Voc, Jsc, FF, and efficiency) for cell#5, (c) 

normalized series resistance (Rs) and shunt resistance (Rsh), (d) curve-fitting extracted ECIS parameters, Rp, CPE-T, CPE-P, and 

calculated lifetime (time constant), Rp*C (i.e., CPE-T), for cell#5, (e) curve-fitting extracted Rs’ and Rp plotted with the Z’ 

resistance obtained at 1 Hz and 1M Hz in the measurements, and (f) plots of DH-degraded CPE-P factor for two cells, #5 on 

C2529-13 and #7 on C2354-4 coupon. The ECIS measurements for C2529-13 cell#5 were performed with a 0.25 V bias to obtain 

the semicircles shown in Fig. 6a. Curve fitting used the Rs’-(CPE–Rp) equivalent circuit model.

0

100

200

300

400

500

600

700

800

900

1000

0 100 200 300 400 500 600 700 800

DH Exposure Time (h)

Normalized I-V Parameter (%)

Rs (ohm-cm2)

Rsh (ohm-cm2)

Normalized C2529-13 Cell-5 I-V Parameters upon DH Exposure

(Very large Rs & 

Rsh at 774 h)

Rs

Rsh

(c)

0.2

0.4

0.6

0.8

1.0

1.2

0 100 200 300 400 500 600 700 800

DH Exposure Time (h)

CPE-P Factor

DH-Induced Changes on CIGS Cell ECIS Parameters

C2354-4_Cell-7 (no bias)

C2529-13_Cell-5 (0.25V bias)

(f)

0

20

40

60

80

100

120

0 100 200 300 400 500 600 700 800

DH Exposure Time (h)

Normalized I-V Parameter (%)

Voc (V)

Jsc (mA/cm2)

FF (%)

Eff. (%)

Normalized C2529-13 Cell-5 I-V Parameters upon DH Exposure

Voc

Jsc

FF

Eff.

(b)

0

2

4

6

8

10

12

14

16

0 100 200 300 400 500 600 700 800

DH Exposuer Time (h)

Cell Efficiency (%)

Cell Efficiency Decreased by DH for Devices on C2529-13 Coupon

Cell-1

Cell-5

Cell-6

Cell-3

Cell-4

(in encapsulated Set-5)

Cell-2

(a)

1.0E+0

1.0E+1

1.0E+2

1.0E+3

1.0E+4

1.0E+5

1.0E+6

1.0E+7

0 100 200 300 400 500 600 700 800

85/85 DH Exposure Time (h)

Z' (1 Hz) or Rp (ohms)

0

2

4

6

8

10

12

14

Z' (1M Hz) or Rs' (ohms)

Z' (1 Hz)

Rp (Ohm)

Z' (1 MHz)

Rs' (Ohm)

DH-Induced Changes on C2529-13 Cell5 ECIS Parameters

Z' (1 Hz)

Rp

Rs'

Z' (1M Hz)

Bias 0.25V

(e)

1.0E-9

1.0E-7

1.0E-5

1.0E-3

1.0E-1

1.0E+1

1.0E+3

1.0E+5

1.0E+7

0 100 200 300 400 500 600 700 800

85/85 DH Exposure Time (h)

ECIS Parameters

CPE-T (F) CPE-P

Rp (Ohm) Lifetime Rp*C (s)

DH-Induced Changes on C2529-13 Cell5 ECIS Parameters

CPE-T

Rp

CPE-P

Rp * C

Bias 0.25V

(d)
11 

increased. The Rp*C lifetime (time constant) was decreased accordingly due to large change in Rp. Meanwhile, the 

CPE-P “depression” factor decreased steadily from 1.0 to 0.93 at DH = 774 h, as shown in Fig. 7(d and f). Greater 

“depression” of the CIGS solar cells’ “p-n junction capacitor” was obtained for other DH-degraded devices in Set-5, 

which decreased to 0.74 at DH = 774 h for the cell#7 on C2354-4 cell coupon with 0.12-μm AZO as illustrated by the 

blue curve in Fig. 7f. All the cells on C2354-4 coupon degraded faster than C2529-13. Although most of the degraded 

cells display trend of change in ECIS parameters similar to that of C2529-13 cell#5 above, there were a few cells that 

showed largely increased Rp, little decreased C, and hence increased Rp*C [6]. Overall the changes in Rs’, Rp, CPE-T, 

and Rp*C indicate that the BZO/AlNi electrical contact has degraded and the cell’s “p-n junction capacitor” (depletion 

layer) has undergone certain physical changes. The decreasing trend in CPE-P suggests the junction capacitor had been

affected probably by increased surface roughness, porous or varying thickness or composition, and non-uniform current 

distribution induced by DH hydrolysis at the CIGS/CdS layers and interface. The exact physical mechanisms are not 

clear, however.

Using the data for Fig. 7 above, we also attempt to seek correlation, and offer adequate physical interpretations, of the 

ECIS results with I-V results for the CIGS solar cells degraded by DH exposure by normalization of the data, which are 

then plotted to find the parallel trends or relationship among various parameters. The quality of correlation treated in this

manner will be obviously affected by the quality of the original data, especially the initial data at DH=0 h, however. This 

is simply because the value of initial data will determine the magnitude of the ratio. In this regard, the initial data points 

for Rp and Rp*C from curve fitting and Z’(1 Hz) from measurement are not used because of the “jump” (e.g., Fig. 6a); 

instead, the data at DH=41 h were used as the initial data for calculating the ratios. The processed results are grouped 

into three plots for easier viewing. In Fig. 8a, the two curves for Voc and CPE-P appear to match well, and the Rp*C 

curve aligns reasonably well with FF and efficiency (Eff.) curves and less with Jsc curve; but the five curves (including 

Rp) are generally in a similar trend of changes. These seem to be reasonable because CPE-P is a measure of 

“depression” of the junction capacitor (depletion width), a small decrease in the junction capacitor should result in also a 

small change in Voc. Because Rp is the minority carrier recombination/diffusion resistance and the Rp*C is the lifetime, 

they should impact directly on the current density and efficiency. Figure 8b shows that the curve-fit Rp is, and should be, 

in good parallel relationship with the measured Z’(1 Hz) because they are supposed to be the same. Their alignment with 

Rsh is not so satisfactory due to the fairly high uncertainty of Rsh values that were derived from only a few data points 

from the light I-V curves by the measurement program. The Rsh values were observed to change significantly when a 

device was measured repeatedly for three to four times. The Rp and Rsh should also be in parallel because they account 

for or reflect the minority carrier recombination/diffusion resistance. In Fig. 8c, the series resistances Z’(1M Hz) and Rs’ 

(from ECIS) are in parallel as expected, and Z’(1M Hz) is also in good agreement with Rs (from I-V). Although the 

CPE-T (or C) curve, capacitance for the p-n junction capacitor, seems to be in good agreement with the Rs’ curve in Fig. 

8c, it is probably better represented as in Rp*C that correlates well with FF and Eff. curves (Fig. 8a). Accordingly, as a 

first-time effort, the ECIS results are found to correlate reasonably well with the I-V results, although more in-depth 

material analysis and insightful physical interpretations are still needed. This is particularly true on the CPE-P factor – 

what and how chemical and physical changes can induce “depression” of the junction capacitor.

Figure 8. Normalized plots in three groups in an attempt to correlate I-V results with ECIS results for C2529-13 cell#5 given in 

Fig. 7. (a) Grouped plot for I-V’s Voc, Jsc, FF, efficiency with ECIS’s Rp, Rp*C, and CPE-P. (b) Grouped plot for I-V’s Rsh with 

ECIS’s Rp and Z’(1 Hz). (c) Grouped plot for I-V’s Rs with ECIS’s CPE-T, Rs’, and Z’(1M Hz). 

0

50

100

150

200

250

0 100 200 300 400 500 600 700 800

DH Exposure Time (h)

Normalized CPE-T (%)

0

200

400

600

800

1000

1200

1400

Normalized Z'(1M Hz), Rs, Rs' (%)

CPE-T (F)

Z' (1 MHz)

Rs' (ohm)

Rs (ohm-cm2)

Z'(1M Hz)

CPE-T

Rs

Rs'

Correlation of I-V and ECIS Results for C2529-13 Cell-5

(c)

0

20

40

60

80

100

120

0 100 200 300 400 500 600 700 800

DH Exposure Time (h)

Normalized Z'(1 Hz), Rp (%)

0

20

40

60

80

100

120

140

160

Normalized Rsh (%)

Z' (1 Hz)

Rp (ohm)

Rsh (ohm-cm2)

Z'(1 Hz)

Rsh

Rp

Correlation of I-V and ECIS Results for C2529-13 Cell-5

(b)

0

20

40

60

80

100

120

0 100 200 300 400 500 600 700 800

DH Exposure Time (h)

Normalized Parameters (%)

CPE-P

Rp (ohm)

Voc (V)

Jsc (mA/cm2)

FF (%)

Eff. (%)

Lifetime (Rp*C)

CPE-P

Voc

Jsc

FF

Eff.

Rp*C

Rp

Correlation of I-V and ECIS Results for C2529-13 Cell-5

(a)
12 

3.3.4 ECIS Differentiation of “Conduction-Failed” Cells from “Junction-Failed” Cells

An important issue related to the performance degradation of (CIGS) solar cells induced by accelerated environmental 

stresses (e.g., DH conditions) needs to be clarified. It will be useful to know whether the CIGS solar cells are truly dead, 

damaged, or shorted at the p-n junction level in addition to electrical conduction degradation/failure by DH exposure, or 

whether the solar cells’ I-V performance degradation is mainly due to electrical conduction degradation/failure at the 

ZnO/AlNi and Mo electrodes. Conventional I-V measurements may not be able to attain the differentiation. In contrast, 

ECIS analysis offers the possibility. Figure 9a shows the Z”-Z’ complex plane plots for some of the cells on the cell 

coupons in the encapsulated Set-5, indicating these cells were shorted, damaged, or highly degraded at different DH 

exposure time. These cells produced very low to zero efficiency. The degradation on these cells’ electrical contact parts 

varies, as inferred from the Z’(real) resistances (~2–27 ohms). Cell damages or shorting by DH on these cells are 

apparently more important than electrical contact degradation. However, ECIS analysis results indicate that, at DH=774

h, some devices still maintained good R-C semicircles with high to very high Rp and C, typical of good-quality cells, 

despite they lost a large percentage to 100% of efficiency. Figure 9b illustrates the Z”-Z’ complex plane plots obtained at 

0.25 V bias for four of such cells, two on C2354-5 with 0.36-μm AZO and two on C2529-13 cell coupon with 0.50-μm 

AZO. The insert in Fig. 9b shows the portion of Z’(1M Hz) resistance of these four cells, ranging from ~9 ohms to ~50 

ohms, indicating the presence of electrical conduction degradation. These cells were difficult to get good I-V 

measurements, hindered either by an odd I-V curve and/or by a very slow voltage scan, which produced very large Rs 

and Rsh, a clear indication of electrical conduction problem. Unless there was an electrical open circuit, ECIS 

measurements using a 10 mV AC amplitude were not hindered by this problem, however. These cells with low to zero 

efficiency but good R-C semicircles most likely continued to possess good p-n junction even as their electrical 

conductions were largely degraded. The degradation in electrical conduction is likely on BZO and AlNi (Fig. 3) and not 

on Mo because Mo/SLG in encapsulated Dummy-1 with TPT backsheet did not show significant change after >1270 h 

DH exposure (Fig. 2c). One way to confirm the assessment that the CIGS/CdS junction on these cells is still alive is, as 

we planned, to remove the four cell coupons from the Set-5 test structure and re-fabricate the cells with new CdS, BZO, 

and AlNi grids [9]. Finally, it is worth noting that an I-V dead cell (C2529-13 cell #6 at DH=774h) gave a double 

semicircle, which was observed for the first time, as shown in Fig. 9c and marked with curve fit parameters using both 

circuit models for the two scans at 0.0 V and 0.30 V bias respectively. The unusual behavior appears to provide further 

evidence of previously discussed points: the first (left) semicircle is attributed to the DH-degraded cell (as in Fig. 8a) and 

the second (right) semicircle is attributed to the capacitance-forming consequence on the BZO/AlNi layers (as in Fig. 3e). 

0 25 50

-40

-15

10

Z'

Z''

Set-5_C2354-4-Cell-1_85-85_DH270h Bias_0.0V.z

Set-5_C2354-6-Cell-6_85-85_DH270h Bias_0.0V.z

Set-5_C2354-5-Cell-1_85-85_DH518h Bias_0.0V.z

Set-5_C2354-5-Cell-2_85-85_DH518h Bias_0.0V.z

Set-5_C2354-4-Cell-2_85-85_DH774h Bias_0.0V.z

Set-5_C2354-5-Cell-2_85-85_DH774h Bias_0.0V.z

(a)

0 25 50

-40

-15

10

Z'

Z''

Set-5_C2354-4-Cell-1_85-85_DH270h Bias_0.0V.z

Set-5_C2354-6-Cell-6_85-85_DH270h Bias_0.0V.z

Set-5_C2354-5-Cell-1_85-85_DH518h Bias_0.0V.z

Set-5_C2354-5-Cell-2_85-85_DH518h Bias_0.0V.z

Set-5_C2354-4-Cell-2_85-85_DH774h Bias_0.0V.z

Set-5_C2354-5-Cell-2_85-85_DH774h Bias_0.0V.z

(a)

Rs (CPE-Rp)

Rs = 119210 (±494.48)

CPE-T = 2.2192E-8 (±1.9316E-09)

CPE-P = 0.87824 (±0.01237)

Rp = 62909 (±713.22)

Rs (C-Rp)

Rs = 87453 (±466.27)

C = 1.1269E-8 (±3.7288E-10)

Rp = 48248 (±691.63)

Rs (C-Rp)

Rs = 3436 (±703.71)

C = 1.043E-11 (±3.0143E-13)

Rp = 85130 (±1443.9)

Rs (CPE-Rp)

Rs = -103.5 (±898.91)

CPE-T = 4.994E-11 (±1.1256E-11)

CPE-P = 0.88716 (±0.016397)

Rp = 125480 (±2637.9)

0 50000 100000 150000 200000

-150000

-100000

-50000

0

50000

Z'

Z''

Set-5_C2529-13-Cell-6_85-85_DH774h Bias_0.0V-2.z

Set-5_C2529-13-Cell-6_85-85_DH774h Bias_0.30V.z

FitResult

(c)

Rs (CPE-Rp)

Rs = 119210 (±494.48)

CPE-T = 2.2192E-8 (±1.9316E-09)

CPE-P = 0.87824 (±0.01237)

Rp = 62909 (±713.22)

Rs (C-Rp)

Rs = 87453 (±466.27)

C = 1.1269E-8 (±3.7288E-10)

Rp = 48248 (±691.63)

Rs (C-Rp)

Rs = 3436 (±703.71)

C = 1.043E-11 (±3.0143E-13)

Rp = 85130 (±1443.9)

Rs (CPE-Rp)

Rs = -103.5 (±898.91)

CPE-T = 4.994E-11 (±1.1256E-11)

CPE-P = 0.88716 (±0.016397)

Rp = 125480 (±2637.9)

0 50000 100000 150000 200000

-150000

-100000

-50000

0

50000

Z'

Z''

Set-5_C2529-13-Cell-6_85-85_DH774h Bias_0.0V-2.z

Set-5_C2529-13-Cell-6_85-85_DH774h Bias_0.30V.z

FitResult

(c)

0 5000 10000 15000 20000

-15000

-10000

-5000

0

5000

Z'

Z''

Set-5_C2354-5-Cell-5_85-85_DH774h Bias_0.25V.z

Set-5_C2354-5-Cell-7_85-85_DH774h Bias_0.25V-2.z

Set-5_C2529-13-Cell-1_85-85_DH774h Bias_0.25V.z

Set-5_C2529-13-Cell-5_85-85_DH774h Bias_0.25V.z

0 25 50 75 100

-100

-75

-50

-25

0

Z'

Z''

(b)

0 5000 10000 15000 20000

-15000

-10000

-5000

0

5000

Z'

Z''

Set-5_C2354-5-Cell-5_85-85_DH774h Bias_0.25V.z

Set-5_C2354-5-Cell-7_85-85_DH774h Bias_0.25V-2.z

Set-5_C2529-13-Cell-1_85-85_DH774h Bias_0.25V.z

Set-5_C2529-13-Cell-5_85-85_DH774h Bias_0.25V.z

0 25 50 75 100

-100

-75

-50

-25

0

Z'

Z''

(b)

Figure 9. (a) Complex plane plots for six cells on three CIGS coupons exposed to DH at 270, 518 and 774 h, respectively, 

showing shorted (vertical lines), mostly shorted (red curve), depressed, and highly depressed cells. Efficiencies of these cells are 

in the order (from top down on the datafile list) of 1.73, 0.95, 0.0, 1.47, 0.0, and 2.39% (this showed an unreliable odd I-V curve). 

(b) Good semicircles obtained with 0.25 V bias for four cells on two cell coupons at DH=774 h. The insert is a blow-up to show 

the increased Rs’, Z’(1M Hz). The cell efficiencies are in the order (from top down) of 0.0, 1.55, 6.72, and 0.0%. (c) Two double 

semicircles obtained for cell#6 on C2529-13 coupon at DH=774 h. The cell was dead at 0.0% efficiency, and the two scans with 

0.0 V and 0.30 V bias were separated by 1 h. Curve fitting results for both pairs of double semicircle are included.
13 

4. CONCLUSIONS

We have conducted a fairly comprehensive study employing (electrochemical) impedance spectroscopy (ECIS) to 

characterize a large sample matrix, including various contact materials, processing conditions, CIGS cell components 

and devices, which used those contact materials for external electrical connection, for their DH stability in both bare and 

encapsulated test structures. The important results are outlined in the following: 

o DH-stable contacts are obtained with a Ag paste, processed briefly at 150o

C, and a low-melting-point solder alloy, 

realizing reliable external electrical connection with soldered Au wires on the encapsulated test structures.

o Mo on SLG shows significant difference in DH-induced reaction between bare and encapsulated states. Essentially 

little to no degradation was observed in encapsulated test structure with a moisture-permeable TPT backsheet that 

was exposed for over 1270 h.

o Bi-layer ZnO (BZO) in the same encapsulated test structure showed a degradation rate (resistance increase) of 

~0.21 ohm/h with a moisture-permeable TPT backsheet, which dropped to ~1.0 x 10-3 ohm/h when a moisture 

barrier film FG-200 was used. The BZO on CIGS solar cells tends to wrinkle and crack sooner or later upon DH 

exposure, whether in bare or encapsulated states.

o The standard 3.05-μm AlNi contact grids are readily degraded or corroded by DH, much faster on bare samples.

o Ni-only contact grids are DH-stable even on bare samples. At 1.6–1.8 um thick, Ni-only CIGS solar cells perform 

comparably to the standard 3.05-μm AlNi grids. However, cracking and/or peel-off of thicker Ni gridlines due to 

high mechanical stress remains an issue to be solved.

o The AlNi/BZO layers are the primary degrading factors on electrical conduction responsible for DH-induced CIGS 

performance loss. “Transient” behavior of very high resistance on dampened AZO and BZO is likely due to 

hydrolytic “capacitor-forming” reactions by DH on both the BZO layer and AlNi/(or Ni/)BZO interface. 

o ECIS is able to extract series resistance (Rs’), minority carrier recombination/diffusion resistance (Rp), and 

diffusion/depletion capacitance (C) via curve fitting with equivalent circuit models. These parameters show the 

large variation of individual CIGS cell characteristics and their changes provide insight of solar cell degradation.

o ECIS analysis show that the CIGS solar cells exposed to DH can result in shorting, damage, or depression of the 

cells’ p-n junction, in addition to the AlNi/BZO degradation. However, some cells with 0.36 or 0.50-μm AZO are 

likely to have performance degraded partly by electrical conduction loss on AlNi/BZO. ECIS is capable of 

differentiating such “conduction-failed” from “junction-failed” cell degradation.

o ECIS results appear to correlate reasonably well with the I-V results on performance degradation as a function of 

DH exposure time.

Accordingly, we have demonstrated the power of ECIS as a sensitive, non-invasive and non-destructive analytical 

technology as well as its highly complementary capability to conventional I-V measurements in the characterization of 

CIGS solar cell characteristics and performance reliability.

ACKNOWLEDGMENTS

The authors thank D. Gallapsie for lending the ECIS system. S. Glynn, C. DeHart and R. Sandaramoorthy are thanked 

for some sample preparations or processing. This work was performed at the National Center for Photovoltaics under 

DOE Contract No. DE-AC36-08GO28308 with the National Renewable Energy Laboratory.

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