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Contract No. DE-AC36-99-GO10337 

T.J. McMahon, T.J. Berniard, and D.S. Albin 

National Renewable Energy Laboratory 

S.H. Demtsu 

Colorado State University 

Prepared for the 31st IEEE Photovoltaics Specialists 

Conference and Exhibition 

Lake Buena Vista, Florida 

January 3–7, 2005 

February 2005 • NREL/CP-520-37380 

Excess Dark Currents and 

Transients in Thin-Film CdTe 

Solar Cells: Implications for 

Cell Stability and Encapsulation 

of Scribe Lines and Cell Ends 

in Modules 
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EXCESS DARK CURRENTS AND TRANSIENTS IN THIN-FILM CdTe 

SOLAR CELLS: IMPLICATIONS FOR CELL STABILITY AND 

ENCAPSULATION OF SCRIBE LINES AND CELL ENDS IN MODULES 

T.J. McMahon,1 T.J. Berniard,1 D.S. Albin,1 and S.H. Demtsu2 

1 National Renewable Energy Laboratory, Golden, CO 80401 2 Colorado State University, Fort Collins, CO 80523 

ABSTRACT 

We have isolated a non-linear, metastable, 

shunt-path failure mechanism located at the CdS/CdTe 

cell edge. In such cases, most performance loss, usually 

erratic, can be associated with the shunt path. We studied 

these shunt paths using dark current-transients and infrared (ir) imaging and find only one shunt path per cell and 

only at the cell corner wall, even in badly degraded cells. 

The effect on diminishing the cell's efficiency far exceeds 

what would be expected from the cell's linear shuntresistance value. We propose that current transients and ir 

imaging be used as a "fingerprint" of the source and magnitude of excess currents to evaluate the contribution of 

scribe-line edges and cell ends in thin-film module performance and degradation due to environmental stress. 

Protection afforded by, or contamination due to, new or 

currently used encapsulants can then be evaluated. 

INTRODUCTION 

Monolithic large-area modules based on thin-film 

(T-F) a-Si, CIGS, and CdTe have been on the market for a 

number of years. Most module structures have seriesconnected cells, each delineated by a set of three scribe 

lines that form the series interconnect. T-F module activearea performance is always below the active-area performance of cells made with correspondingly processed 

materials. The most commonly cited cause for this reduced performance is series resistance, due to the sheet 

resistivities of the transparent conducting oxides used for 

one of the contacts and the interconnect resistance. 

Here, we report on another, probably greater, 

loss mechanism that we have found in the CdS/CdTe T-F 

mesa-cell structure, namely, non-linear shunt paths at the 

cell edge, that can contribute to reduced initial performance loss as well as much of the degradation seen after 

stress [1,2]. Non-linear shunt paths reported here require 

two-dimensional (2-D) device modeling and are akin to, 

but much more devastating than the micro-nonuniformities 

reported in [3] that involve small micro-regions containing 

"weak diodes" that reside away from the cell edge. Our 

metastable, "very shunted, very weak" micro-diode regions appear at corners and probably at cusps, where the 

junction interfaces must intersect the surface at the cell 

edge where they are exposed to the effects of the environment, surface diffusion, and electromigration. 

These shunt paths are induced with the application of bias voltage and accelerated by temperature and 

humidity. The loss in performance due to these shunt 

paths far exceeds what 2-D discrete circuit-element device 

modeling would predict when only a simple linear shunt 

resistor is used to represent the defect. Smaller conductive shunt paths, by inference, must exist at cusps and 

grain-boundary corners along edges caused by the physical chipping during the scribing procedure. These can 

become more conductive with stress. 

At the sharp points on which a shunt is located, 

and especially at the cell corner where, additionally, localized electric-field enhancement can also occur, we observe significant and often times sporadic degradation. We 

use light and dark current-voltage (I-V) curves, darkcurrent transients [4]. and thermal imaging in the 3-5-mm 

wavelength range to identify these shunt losses. We propose that effects of humidity on cell ends and scribelines, 

with and without EVA or with new encapsulants, as well as 

edge-passivation processes, can be effectively screened 

for detrimental chemical reactions using the transients 

described in this paper. 

The effect of metastable shunt paths and darkcurrent transients has been well documented in the a-Si TF system [5]. Similar large excess dark currents and unsteady, minute-long, dark-current transients were found in 

CdTe cells [1,4]. Transients, measured by changing the 

voltage bias in a stepwise fashion, provide an ideal 

method to quantify the contribution of shunt paths. Excess dark currents on non-margined cells, i.e., cells with 

no border region between the back contact and the cell 

edge, were compared to currents on margined cells having a mm-wide border between the back contact and 

edge. Detailed descriptions of the margining process are 

given in these proceedings [2]. Cell preparation is described in [6]. The back contact for cells used in this study 

was graphite dag coated with a conductive Ag paste. 

CURRENT TRANSIENTS 

Ideally, dark currents are controlled by temperaturedependent carrier generation in reverse bias, and recombination in forward bias. In these cases, stepwise 

changes in bias produce either abrupt or smooth, steadily 

growing or decaying, current transients that occur as gapstate occupancy adjusts to the new bias condition. This 

type of normal transient current behavior for a margined 

cell before stress results in a smooth small 

1
600 

400 

200 

-200 

0 

-0.4 -0.2 

Volts (V) 

0.0 0.2 0.4 

800 

1000 

-400 

600 

400 

200 

-200 

0 

800 

1000 

-400 

-600 

-800 

1200 

Light 

25 50 75 100 

(a) 

(b) 

I (μA) 

-0.4 V -0.4 V 

I (μA) 

0 V +0.4 V +0.4 V 

Time (s) 

Fig. 1a. Erratic, shunted dark I-V for a non-margined, 0.778 

cm2, CdTe cell @ 25°C after stress; 1b. huge, abrupt, forwardcurrent transient steps of 0.2 to 0.3 mA from voltage switching 

between -0.4 and + 0.4 volts @ 25°C. Note: a discrete shunt 

path turns on at 24, 48, and 82 s while voltage is constant; the 

short exposure to light at 65 s has the effect of turning off the 

shunt while at -0.4 V. 

hysteresis loop in the forward bias dark I-V curve [1]. Also 

documented is the dark current transient at 90°C for a 

voltage bias switched from -0.3 V to + 0.3 V and back to -

0.3 V. A reverse bias of -0.3 V produces a very temperature-dependent current density: 20 nA/cm2 at 90°C and 

only 0.65 nA/cm2 at 25°C. That cell had an initial efficiency of 12.5% and dark currents measured before 

stress. 

For a non-margined cell with the same process 

steps as for the cell noted above, Fig. 1a shows an unstable, erratic dark I-V behavior during the 4-min voltage 

sweep cycles at 25°C. Voltage stress during measurement induces shunt path steps as large as 200 mA as the 

reverse bias increases to 0.4 V causing the large hysteresis loops. This cell also started at 12.5% efficiency, but 

was stressed for 1225 h at 100°C and one-sun exposure 

in open-circuit. This metastable shunt- path behavior in 

the I-V voltage sweep can be better studied by monitoring 

the transient behavior in the current due to stepwise 

changes in voltage. Figure 1b shows that switching between -0.4 and + 0.4 volt bias at 25°C produces large, 

minute-long current transients that contain the 200-mA 

step seen in Fig. 1a. One of the incremental changes in 

the current transient does turn off as the bias voltage 

sweeps through zero to the opposite sign. The short exposure to light at 65 s will also diminish the reverse bias to 

the shunt path and has the effect of turning it off. Sometimes, the shunt path was conductive enough to decrease 

the overall cell efficiency below 4.5%. 

These are discrete shunt current paths as identified 

by the step-wise increases in forward- and reverse-bias 

currents. It appears that a trapping or localized heating 

process at the shunt path may be modulating the conducting on-state. The sum of the shunt currents at -0.4 V is -

0.4 mA, which corresponds to 1000 ohms. As shunting 

worsens, discontinuous changes become larger. At T = 

90°C, these transient currents are similar in size, but the 

large steps are absent as though they switched immediately when the voltage was changed to the conducting 

"on-state," and the trap-like or localized heating transient 

is absent. 

THERMAL IMAGING AND CURRENT-VOLTAGE 

IR camera imaging studies were used before to 

produce temperature profiles caused by heat generated at 

point shunt paths in T-F CIGS cells [6]. Viewing the T-F 

directly was shown to be far more sensitive than viewing 

from the glass side. The film-side thermal image in the 3-

5 mm wavelength range of CdTe cell is shown in Fig. 2; 

The left-hand image shows the exact location of the large 

excess current shunt: The T rise is about 10 times the 

camera sensitivity and estimated to be about 1°C. The 

image was taken with a now, much larger, total shunt 

leakage current of 1.5 mA at a reverse bias of 0.45 V at 

25°C. This corresponds to a dark resistance of about 300 

ohms. The right-hand image was taken after removing 

with a razor blade a small piece of the corner containing 

the shunt. That procedure increased cell efficiency from 

4.23% to 9.0% and reduced the reverse current to a mere 

0.0009 mA at the 0.45 V reverse bias, unequivocally identifying that corner as the source of this cell's shunt loss. 

This result has been found on many of our shunted cells. 

Fig. 2. Thermal-image difference taken for cell #177D: 

(left) The reverse-bias image at -1.5 mA @ -0.45 V minus 

the zero-bias image. The light spot in the lower-right corner is the biggest heat source. (right) The reverse-bias 

image at -0.0009 mA @ -0.45 V minus the zero-bias image after removing that corner. Other contrasts are due to 

camera vibration. 

2
The position and electrical characteristics of the cell 

with and without the shuntpath are now known precisely. 

When the joule heating is large enough to see with this 

camera, we find them at a corner of the rectangular electrode. The effect of the removal of the shunted sub-mm2 

triangular corner (seen in Fig. 2 right) on cell performance 

can now be evaluated. Figure 3. shows the light and dark 

I-V performance before and after the removal of the 

shunted corner. Before removal of the shunt, Fig. 3 shows 

the devastating effect that this geometrically small shuntpath (apparently 300 ohms) has. Its removal results in 

greatly enhanced performance, with the shunt current reduced a thousand fold. Nearly all of the shunt current 

flows through one shunt path or group of shunt paths 

within the same small area. 

Fig. 3. Light and dark I-V performance; before and 

after scribing off the small piece of the defective corner. 

More careful consideration shows the loss in performance far exceeds what a shunt of this resistance would be 

expected to produce. The encircled circuit elements at the 

left in Fig. 4 show the equivalent circuit required to model 

the I-V performance of a normal, undegraded CdTe cell 

that includes a back-contact diode, DBack, opposing the 

main junction diode, DFront. The right-hand circle shows 

the circuit elements required to model the effects of the 

non-linear shunt. Initially, discrete circuit-element modeling with PSpice included the normal diode area producing 

20-mA light current plus the small shunted diode area with 

only a linear 300-ohm shunt resistor (RSH2) to simulate the 

shunt resistance and a series resistance (RSheet) to simulate the sheet resistance of the SnO2. This configuration 

showed no effect on open-circuit voltage (Voc) and only a 

small effect on the fill factor (ff). The large reduction in Voc 

and ff implies that this shunt path must be non-linear, i.e., 

an exceedingly leaky diode (DWeak) must be added in parallel to the shunt resistor to simulate the defective corner's 

micro defect. We show this in the encircled circuit elements at the left in Fig. 4. Using this circuit approach, we 

can model these I-V curves with PSpice. 

This defect has such a profound effect on the entire 

cell I-V because both junctions within the small defect area 

(the main front-junction that controls Voc and ff and the 

back barrier-junction that is the cause of roll-over and 

crossover) intersect the cell wall and are very vulnerable 

to contamination, especially at this sharp corner. The 

geometrical electric-field enhancement of junction and 

bias fields that lie in that exposed cell wall surface at this 

corner can only make matters worse. The enhanced field 

at a corner increases the current loss through the shunt 

path. 

 

Fig. 4 Equivalent circuit depicting unshunted cell area 

(left) and corner containing the non-linear shunt path (a 

parallel combination of a shunt resistor and weak diode in 

series with a connecting series resistor RSheet that takes 

into account the effect of the CTO sheet resistance (right). 

Ag, used in our CdTe contact, is known to be prone to 

electromigration [7], and the linear portion of the metastable shunt behavior may be the connecting and severing of 

small Ag tendrills on the exposed edge. The fact that humidity also accelerates shunting may be explained by envoking wet-electromigration [7]. The shunt paths in this 

report are turned on with a positive or negative voltage 

bias and turned off with a reduced or zero bias. It has 

been suggested that shunt transients in CdTe are due to 

Cu ion migration forming similar filaments [4]. We see that 

more is involved. Within the small shunt defect area, both 

the front and the back junction intersect the cell wall, exposing them to environmental stress and contamination 

from Ag electromigration, which diminishes the quality of 

both junctions. Lower Voc and lower ff are caused by the 

compromised front junction and the diminished crossover 

behavior caused by the compromised back contact. This 

small area degrades the entire cell's I-V performance. 

We also see evidence with ir imaging that these shunt 

losses are occurring at the scribe lines and cell ends in 

CdS/CdTe modules placed in the dark under forward bias; 

see Fig. 5. The edge wall of the CdS/CdTe cell's mesastructure used in this study can be compared to the edge 

wall of the back metal scribe lines of the seriesinterconnect region between cells and the cell ends of a 

full-sized module. Cusps and corners are the inevitable 

result of scribing with a razor or a laser. Bead-blast procedures are often used to isolate the module cell area at 

the edges, leaving the cell ends in a very ragged edge. 

Indeed, almost any T-F polycrystalline module has warm 

spots on edges and scribe lines when viewed with an ir 

3
camera while forward bias is applied in the dark. Mechanical and/or laser scribing are used to define interconnects, again leading to exposed cell walls. 

Fig. 5. IR image of a CdTe module forward biased to 

Voc in the dark. 

The effects of humidity on cell edges and scribe-lines, 

with and without EVA and with new encapsulants, can be 

effectively screened at the cell level for detrimental chemical reactions. Suitable water-vapor dielectric barriers may 

have the added effect of reducing the electric field at the 

cusps and corners present in scribed cells. 

Figure 6 shows the preliminary results of encapsulation studies. Non-margined and margined EVA encapsulated cells are stressed, along with a margined non- encapsulated cell. We see lamination loss on the two laminated cells at time = 0. No evidence of shunt-path formation is seen in any current transients up to 750 h. In addition to the initial loss, laminated cells degrade faster. 

More cells must be put into test, other encapsulants investigated, and stress conditions involvi idi ng hum ty added. 

Fig. 6. Performance of EVA-laminated, non-margined 

(NM) and margined (M) cells with stress time at 100°C 

and one-sun exposure in open-circuit. 

CONCLUSIONS 

We have isolated a non-linear, metastable, shuntpath failure mechanism located at the CdS/CdTe cell edge 

using dark-current transients and ir imaging and find only 

one shunt path per cell and only at the cell corner wall, 

even in badly degraded cells. They are induced by voltage 

and accelerated by temperature and humidity. The effect 

on diminishing the cell's efficiency far exceeds what would 

be expected from the cell's linear shunt-resistance value. 

To model the observed performance change, we must 

include a very weak diode in parallel with the shunt path, 

to fully describe how this small defect area can influence 

the entire cell. Exposure of the diode's front and back 

junctions at corners and edges makes it very vulnerable to 

environmental attack and electromigration. We are beginning to test encapsulated cells and preliminary results 

show EVA causes a lamination loss and an increased rate 

of degradation. Ultimate module efficiencies may be limited by this type of shunt path. 

ACKNOWLEGEMENTS 

We thank M. Kempe for lamination of the cells and C. Osterwald for providing the images in Fig. 5. This work was 

funded by the U.S. Department of Energy under Contract 

Number DE-AC-36-99G010337. 

REFERENCES 

[1] T.J. McMahon, T.J. Berniard and D.S. Albin, "Nonlinear shunt paths in thin-film CdTe solar cells," to be published in Feb 15, 2005 issue of J. Appl. Phys. 

[2] D.S. Albin et al., this volume. 

[3] V.G. Karpov, A.D. Compaan, and D. Shvdka, "Effects 

of nonuniformity in thin-film photovoltaics," Appl. Phys. 

Lett. 80, (2002) pp. 4256. 

[4] T.J. McMahon, "Dark current transients in thin-film 

CdTe solar cells," Proc. of the 29th IEEE Photovoltaic 

Specialists Conf., May 2002, New Orleans, LA., pp. 768-

771 

[5] T.J. McMahon and M.S. Bennett, "Metastable shunt 

paths in a-Si solar cells," Solar Energy Materials and Solar 

Cells, 41/42 (1996) pp. 465. 

[6] D. Rose, D.F. Hasoon, R. Dhere, D. Albin, R. Ribelin, 

X Li, Y. Mahathongdy, T. Gessert, and P. Sheldon. "Fabrication procedures and process sensitivities for 

CdS/CdTe solar cells," Progress in Photovoltaics: Research and Applications 1999; 7: pp. 331-340. 

[7] S.J. Krumbein, "Tutorial: Electrolytic models for metallic electromigration failure mechanisms," IEEE Trans. on 

Reliability, 44 no. 4, (1995) pp. 539-549. 

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4. TITLE AND SUBTITLE

Excess Dark Currents and Transients in Thin-Film CdTe Solar Cells: 

Implications for Cell Stability and Encapsulation of Scribe Lines and 

Cell Ends in Modules 

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T.J. McMahon, T.J. Berniard, D.S. Albin, and S.H. Demtsu 

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Colorado State University, Fort Collins, CO 80523 

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14. ABSTRACT (Maximum 200 Words)

We have isolated a non-linear, metastable, shunt-path failure mechanism located at the CdS/CdTe cell edge. In 

such cases, most performance loss, usually erratic, can be associated with the shunt path. We studied these shunt 

paths using dark current-transients and infrared (ir) imaging and find only one shunt path per cell and only at the cell 

corner wall, even in badly degraded cells. The effect on diminishing the cell's efficiency far exceeds what would be 

expected from the cell's linear shunt-resistance value. We propose that current transients and ir imaging be used as 

a "fingerprint" of the source and magnitude of excess currents to evaluate the contribution of scribe-line edges and 

cell ends in thin-film module performance and degradation due to environmental stress. Protection afforded by, or 

contamination due to, new or currently used encapsulants can then be evaluated. 

15. SUBJECT TERMS

PV; excess dark currents transients; thin film; solar cells; cell stability; encapsulation of scribe lines; cell-ends; 

module; infrared (ir) imaging; non-linear metastable shunt path; 

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