**When, Why and Where are CdTe/CdS Solar Cells Stable?**

K. D. Dobson,[^1] Iris Visoly-Fisher, R. Jayakrishnan, K. Gartsman, G.
Hodes and David Cahen

*Weizmann Institute of Science, Rehovot 76100, Israel.*

# 

# ABSTRACT

The role of Cu in CdTe/CdS solar cell instability remains the subject of
much debate. The investigation of a range of 'Cu'-contacted CdTe/CdS
cells, which had received various thermal stress treatments, is
described. Cells that were stressed in air exhibit strong
current-voltage (I-V) rollover and junction degradation. No such
degradation was observed for 'Cu'-contacted cells that had been stressed
in dry-N~2~ atmosphere. Cu is found to diffuse rapidly through the cell
structure during back contact annealing and to accumulate in the CdS
layer. With stress, significant levels of Cu dope the grain bulk,
producing (with Cl) high resistance, photo-conducting CdS. This behavior
is independent of stress atmosphere and is, therefore, unlikely to
(initially) be a dominating mechanism for cell degradation. Our results
suggest simple air oxidation of the back contact interface to be a
likely origin of I-V rollover in CdTe/CdS cells.

**INTRODUCTION**

CdTe/CdS solar cells show great promise for application to high-area,
low-cost photovoltaic systems. However, one of the inherent difficulties
in device processing is the inability, due to the high work function of
CdTe, to prepare ohmic contacts to p-CdTe. The addition of small amounts
of Cu to back contacts improves contact properties by p^+^-doping the
CdTe surface and creating a pseudo-ohmic contact. Often, however,
unencapsulated 'Cu'-contacted cells have exhibited variable stability
during stress testing in the laboratory. While such cell degradation has
been thought to be the result of Cu diffusion into the CdTe, such
effects are often not observed in encapsulated module devices under
field test conditions.

The role of Cu in improving contact properties and its possible
involvement in cell degradation is not known. An understanding of these
processes and the identification of cell degradation mechanisms will
allow the manufacture of devices with improved performance and long-term
stability. In a recent review \[[^2]\], we proposed a mechanism for the
behavior of Cu within the CdTe/CdS cell structure. Still, however, the
question if Cu is detrimental to cell performance remains. Here we
present results of an investigation of a series of 'Cu'-contacted
CdTe/CdS cells that have experienced various stress conditions
(including variation of stress atmosphere) using a range of electrical
and characterization techniques, and consider possible mechanistic
details for observed cell behavior.

# EXPERIMENTAL DETAILS

CdTe/CdS/TCO/glass (First Solar LLC) and CdTe/TCO/glass (University of
South Florida) substrates were contacted with graphite paste containing
Cu-doped HgTe ('Cu'-contacts). Substrate details and contacting
procedures have been presented elsewhere \[[^3]\]. Cells were stressed
by thermal treatment at 200°C in the dark under normal air or dry N~2~
(99.999% N~2~ passed through Oxyclear scrubber) atmospheres. Where
required, back contacts were removed by sonication in acetone. For
secondary ion mass spectroscopic (SIMS) investigations this was followed
by brief etching in Br~2~/methanol to remove high Cu levels from the
CdTe surface. CdS-only samples were prepared by careful removal of CdTe
layers from cells in 40% w/w aqueous solution of FeCl~3~ \[[^4]\].

Current-Voltage (I-V) curves were obtained using a computerized
home-made setup with a QTH lamp calibrated to \~ AM 1 intensity. Light
beam-induced current (LBIC) images were obtained by excitation through
the glass of cells mounted on a home-made stage, using an Olympus
confocal microscope, employing 1mW 568 nm laser light. SIMS measurements
were carried out using a 4 keV O~2~^+^ beam in a Cameca IMS4f ion
microscope at mass resolution of M/∆M\~3000. The primary beam was
rastered over a 200 x 200 μm^2^ area, analyzing an area 60 μm in
diameter. Photoluminescence (PL) spectra were recorded at 300 K,
employing the 514 nm line (\~50 mW intensity) of an Ar laser and a
standard PL setup equipped with 60 cm focal length double grating
monochromator (Jobin-Yvon HRD) and cooled photomultiplier (Hamamatsu
RG942).

# RESULTS AND DISCUSSION

Figure 1a shows illuminated I-V curves obtained from two 'Cu'-contacted
CdTe/CdS cells following 40 hours stress in air and dry-N~2~
atmospheres, respectively. The curve from the air-stressed cell shows
rollover at forward bias, as well as a decrease in V~oc~ and J~sc~ from
initial pre-stress values. Rollover usually appears following ∼20h
stress in air. By comparison, the I-V curve of the 40h N~2~-stressed
cell exhibited no rollover or degradation from pre-stress measurements.
Figure 1b shows the illuminated I-V curve of the 40h N~2~-stressed cell
with dark I-V curves obtained at various times during stressing. The
striking feature of the figure is the increasing cell series resistance,
producing a greater degree of dark/light I-V crossover and indicating
increasing cell photoconductivity.

Changes in cell junction properties were monitored using LBIC. Figure 2
shows LBIC images of a Cu/HgTe/graphite-contacted cell, following 0, 18
and 35 hours air stress. Prior to stress the image of the cell is bright
and reasonably homogeneous. Dark (low current) areas are often present,
representing areas of poor contact or mechanical damage. With stressing,
an increase in the frequency of dark areas along with a decrease in
overall image contrast is observed, indicating a loss of current in
localized areas (or loss of contact quality) and a decrease in current
over the whole sample area. However, no LBIC changes were observed from
pre-stress measurements for N~2~-stressed cells.

***Figure 1.** **(a)** Illuminated I-V curves obtained from two
'Cu'-contacted CdTe/CdS cells following 40h air and N~2~ (dotted)
stress, respectively. **(b)**Illuminated I-V curve of the 40h N~2~
stressed cell (dotted) plotted with dark I-V curves from the same cell
obtained following 0, 1, 3, 15 and 40h N~2~ stress. No changes were
observed in the light I-V characteristics with N~2~-stress.*

![](media/image1.png){width="2.269291338582677in"
height="1.8480314960629922in"}![](media/image2.png){width="2.2618110236220472in"
height="1.8480314960629922in"}![](media/image3.png){width="2.269291338582677in"
height="1.8480314960629922in"}

***Figure 2.** Front wall LBIC images obtained from 'Cu'-contacted
CdTe/CdS cell following 0, 18 and 35 hours air stress.*

SIMS investigations have been carried out to determine changes in the
distribution of species throughout the cell structure following stress.
Figure 3 shows the SIMS profiles of Cu in a series of cells that have
received different treatments. We, and others, have previously shown
that Cu from the back contact rapidly diffuses through the cell
structure during contact preparation to accumulate near the CdS region
of the cell \[Error: Reference source not found-Error: Reference source
not found[^5]\]. This can be clearly seen in figure 3 where, following
just contact preparation, a large increase in the amount of Cu in the
CdS region is observed, while the level of Cu in the CdTe shows only a
small increase by comparison. The accumulation profile of Cu has also
been corroborated using x-ray photoelectron and Auger electron
spectroscopies. An accumulation of Cl in the CdS region of non-contacted
substrate is also observed (see also \[Error: Reference source not
found,[^6],[^7]\]). SIMS investigations of 'Cu'-contacted CdTe/TCO/glass
cells detected no such accumulation profile within the cell structure
\[Error: Reference source not found\], indicating that Cu exhibits a
high affinity for polycrystalline-CdS \[Error: Reference source not
found,Error: Reference source not found\].

***Figure 3.** SIMS profiles of Cu and Cl in a CdTe/CdS substrate with
no contact and of Cu in 'Cu'-contacted CdTe/CdS cells following 0, 15
and 300 hours air stress and 15 hours N~2~-stress. Approximate cell
interface positions are indicated with cross-hatching to represent
degrees of interfacial intermixing and/or roughness*

From figure 3, the Cu content of the CdS layer is observed to further
increase with stressing, while in CdTe it increases only slightly.
Near-identical Cu profiles were obtained for cells stressed for 15h in
air or N~2~, indicating that the rate of Cu diffusion is independent of
atmosphere. This suggests that I-V rollover cannot be the direct result
of loss of favorable back contact by diffusion \[Error: Reference source
not found\] or of high Cu concentrations in the cell structure.

CdS-only samples were obtained from of a series of cells and their Cu
content measured by SIMS (not shown). This approach provides improved
depth resolution and avoids other technique-related artifacts. The
resultant Cu profiles again exhibit a large increase in the Cu content
of the CdS following just contact anneal (from 2.0x10^17^ to 2.5x10^19^
atoms cm^‑3^). With further stress, no change in the Cu level throughout
most of the CdS layer occurs, but a significant accumulation is observed
directly at the CdS/TCO interface (from 3.0x10^19^ atoms cm^‑3^ with no
stress to 3.5x10^19^ and 5.2x10^19^ atoms cm^‑3^ following 15 and 300h
stress, respectively). High Cu concentration at the front contact region
could affect cell behavior. As before, Cu distribution was found to be
independent of atmosphere.

The PL spectra from a set of CdS-only samples are shown in figure 4. The
spectrum of a CdS layer from a non-contacted substrate (figure 4a, top
plot) shows two broad bands of similar intensity centered at 680 and 750
nm. The 680 nm band is assigned to a Cl~S~-V~Cd~ complex \[[^8]\].
Following cell contacting (center plot), the 750 nm band appears to
shift to higher wavenumbers (∼770 nm), and with subsequent stressing
(figures 4a and b, bottom plots) further shifts to ∼790 nm (independent
of atmosphere). The relative intensity of the 680 nm peak decreases with
treatment and continues to do so with longer stress times (monitored up
to \>400h stress, not shown). The respective behaviors of the 790 and
680 nm bands are consistent with the formation of the Cu~Cd~ species
\[[^9]\] and corresponding loss of the Cl~S~-V~Cd~ complex,
respectively. Figure 4b shows the PL spectra obtained at two points on
the CdS layer from a cell that had received 16h air stress; one from a
position that had been beneath the back contact and the second from
outside the contacted area. The spectrum from outside the contact is
almost identical to those obtained for CdS from non-contacted substrates
(Fig 4a, top plot), while the spectrum from inside the contacted area is
consistent with spectra of CdS from stressed samples. Therefore, the
changes observed in the CdS PL spectra must be related to back contact
components and not to effects of stress conditions on the CdS. We also
note that very similar PL data were obtained, *in situ*, from the CdS
layers of CdTe/CdS cells (at 300 K using 514 nm excitation through the
cell glass).

A schematic of the proposed roles of Cu in CdTe/CdS cells, based on a
previous model \[Error: Reference source not found\], is presented in
figure 5. Improvements in initial cell performance are proposed to be
due to the formation of a favorable Cu~2~Te back contact from the
reaction of Cu with a Te-rich CdTe surface. This material is also an
excellent diffusion source for Cu into the cell structure.

SIMS analysis indicates that a significant level of Cu can diffuse
through the CdTe to the CdS region of the cell following just the back
contact anneal. With stress the Cu level continues to rise within the
CdS layer.

We have previously explained the high Cu affinity for polycrystalline
CdS over CdTe by direct comparison of bond energies (e.g. Cu-S \> Cu-Te)
\[Error: Reference source not found\]. Therefore, within a new cell, we
suggest the Cu will be more likely to reside on/in the CdS grain
surface/boundaries and only with stress does the Cu begin to penetrate
the grain bulk (evident from PL measurements). Cu doping will increase
the photo-conductivity (and resistivity) of CdS, however, this effect is
expected to be further enhanced in the presence of Cl \[[^10]\], which
also accumulates in the CdS \[Error: Reference source not found,Error:
Reference source not found,5\]. The increase in CdS photoconductivity is
clearly observed in the dark I-V curves of N~2~-stressed cells.
Therefore, this indicates that (at least initially) the
photoconductivity effect does not play a significant role in cell
stability (however, degradation may be expected if Cu-doping becomes
excessive \[Error: Reference source not found\]).

***Figure 4.** PL spectra of **(a)** CdS layers obtained from CdTe/CdS
substrate with no contact and from 'Cu'-contacted CdTe/CdS cells
following 0h and 16h N~2~ stress and of **(b)** CdS layer from
'Cu'-contacted cell following 16h air stress, recorded at positions on
the CdS which had been directly below the back contact and outside the
contacted area. Spectra were normalized and offset for clarity.*

***Figure 5.** Schematic model for the behavior of Cu(with Cl) in
CdTe/CdS solar cells.*

The appearance of rollover in the I-V characteristics of air-stressed
cells, along with the observation that Cu diffusion is unaffected by
stress atmosphere, indicates that the observed I-V rollover and cell
degradation is related to air oxidation of the cell, producing a Cd-
and/or Te-oxide barrier at the back contact interface (we note that
preliminary XPS studies have detected increased levels of Te- oxide on
CdTe from beneath the back contact of air stressed cells) and possibly
other changes deeper in the cell structure. This may account for why
encapsulated modules have been reported to exhibit superior stability
over non-encapsulated laboratory cells, and highlights the need for
cell/module protection from the atmosphere during working stress to
obtain high device stability.

**CONCLUSIONS**

We have investigated a series 'Cu'-contacted CdTe/CdS cells following
stress treatments to identify possible cell degradation mechanisms. I-V
rollover and (LBIC) junction degradation was observed in cells that had
been stressed in air, but did not occur in cells stressed in dry N~2~.
SIMS investigations highlighted fast diffusion of Cu in the cell
structure, with a significant level of Cu accumulating in the CdS layer
following just contact annealing, and increasing (independent of
atmosphere) with further stressing. While Cu is thought to initially
reside in the CdS grain boundaries, PL studies detected the formation of
Cu~Cd~ complexes within the grain bulk with stressing. The presence of
accumulated Cu (and Cl) is proposed to produce high resistance
photo-conducting CdS, evident from the increasing degree of light/dark
I-V crossover for N~2~-stressed cells. This effect, however, does not
initially appear to be detrimental to cell performance, suggesting
oxidation of the back contact interface as a likely origin for I-V
rollover and cell degradation.

# 

# ACKNOWLEDGEMENTS

This work was supported in part by the US DOE, via the NREL Thin-Film
Photovoltaics Partnership Program. We acknowledge D. Rose (FS) and C.
Ferekides (USF) for supplying substrates. We thank C. Cyterman
(Technion) for SIMS measurements and analysis, L. Burstein (Tel-Aviv
University) for AES measurements, H. Cohen and Y. Feldman (WIS) for XPS
measurements, and R. Tenne (WIS) for assistance with the PL
measurements.

# REFERENCES

[^1]: Current address: Institute of Energy Conversion, University of
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