### INTRODUCTION OF Cu IN CdS AND ITS EFFECT ON CdTe SOLAR CELLS

K. Barri^1^, M. Jayabal^1^, H. Zhao^1^, D. L. Morel^1^, S. Asher^2^, J.
W. Pankow^2^, M. R. Young^2^, and C. S. Ferekides^1^

^1^Clean Energy Research Center, Department of Electrical Engineering

^1^University of South Florida, Tampa, FL 33620

^2^National Renewable Energy Laboratory, Golden, CO, 80401

**ABSTRACT**

The use of copper during the fabrication of CdTe solar cells is a common
feature for nearly all processing schemes developed for these devices.
Copper is typically introduced in CdTe during the application of the
back electrode, to enhance device performance by facilitating the
formation of an ohmic back contact. However, Cu has also been associated
with observed instability in CdTe. For this work, although Cu was
utilized during the cell fabrication process, it was eliminated from the
back contact formation step, and instead introduced in the CdS film
prior to the deposition of the CdTe. The only fabrication step, where Cu
was intentionally introduced, was subsequent to the CdS deposition. Both
plain graphite and Sb~2~Te~3~/Mo were used as back contacts. Solar cell
results suggested that ohmic contacts to CdTe can be attained with
undoped graphite as the back electrode. For devices contacted with plain
graphite V~OC~'s and FF's in the range of 800-830 mV and 63-67% have
been obtained. For Sb~2~Te~3~-contacted cells, the incorporation of Cu
in CdS has lead to a significant increase in performance even though a
back barrier was present in these cells.

**INTRODUCTION**

The performance of CdTe solar cells depends greatly on the formation of
an ohmic back contact. Over the years the development of effective back
contact options has been based on the use of a key element, Cu. Although
a complete understanding of the role of Cu in CdTe cells is still the
subject of several studies, this element is believed to be beneficial
from a performance point of view, but evidence exists that suggests it
may also be responsible for observed instability in these devices
\[1,2\]. Most cell fabrication processes incorporate Cu during the
formation of the back contact. Copper is either evaporated to a small
thickness onto CdTe, or it is incorporated in the back electrode (as a
dopant) as is the case of the doped graphite-based approach. The
formation of Cu~2~Te on the surface of CdTe has been shown to be
important in achieving an ohmic contact. However, conversion of this
compound to CuTe is also believed to be the cause of device degradation
\[1\]. In addition, Cu has been found to accumulate at the CdTe/CdS
interface, and in the CdS layer \[3\]; the effect of Cu accumulating at
this device region is not well understood yet, but at large amounts it
is believed to be detrimental to device performance and stability. It is
therefore critical that studies on the role of Cu in CdTe devices
continue, in order to develop the understanding necessary for
effectively utilizing this element to achieve both high performance and
long-term stability. This paper reviews results obtained from a study,
where Cu was eliminated from the back contact process and was instead
incorporated in the CdS layer prior to several high temperature
processing steps, including the CdTe deposition (560-620°C) and CdCl~2~
heat treatment (390-410°C).

**EXPERIMENTAL**

Additional details on the procedures followed for the fabrication of
CdTe cells used for this study can be found elsewhere \[4\]. The devices
had the typical superstrate configuration, with borosilicate
glass/SnO~2~:F/SnO~2~ serving as the substrate. Both semiconductors were
prepared using the close-spaced sublimation (CSS) process, in an inert
or O~2~-containg ambient \[4,5\]. It should be noted that the thickness
of CSS-CdS films tends to vary significantly from run to run and to a
lesser extent over the deposition area (approx. 3 x 3 cm^2^). Following
the deposition of CdS, the substrates were dipped in a CuCl aqueous
solution, or were coated with a thin layer (typically less than 10 Å) of
Cu, deposited by sputtering; in both cases the objective was to
incorporate Cu in this device region, where previous studies have
indicated that it accumulates during the cell fabrication process, and
eliminate it from all other processing steps, including the back contact
process. After heat-treating the CdTe/CdS structures in the presence of
CdCl~2~, the CdTe surface was etched using a Br~2~/methanol (0.01% vol.)
solution. The back contact was formed by applying as-received graphite
paste (i.e. no dopants added), or by depositing Sb~2~Te~3~/Mo by
sputtering. Following the application of the back electrode the devices
were heat treated in inert ambient (T=150-300°C; t=10-25 min.). The
device structure indicating the "Cu-incorporation" step is depicted in
Fig. 1. Solar cells were characterized using J-V and SR measurements; a
series of CdS films dipped in CuCl were also characterized at the
National Renewable Energy Laboratory using SIMS and AES.

**RESULTS AND DISCUSSION**

The process of adding Cu to the CdS layer was optimized empirically
using device performance as the main criterion. Various CuCl
concentrations starting at approximately 0.1 M were utilized. In
addition to dipping CdS into CuCl solutions, Cu was also directly
deposited onto CdS by sputtering, even though controlling and varying
the amount of Cu was not very effective, especially when the Cu
thickness was below 10 Å. The solution approach was more effective as
several parameters such as the solution concentration and duration could
be used to control the amount of Cu incorporated in CdS. In both cases
Cu was added at the surface of the CdS, however for the CuCl solution
process, Cu could potentially penetrate deeper into the films.

![](media/image1.wmf){width="2.604330708661417in"
height="1.7334645669291338in"}

**Undoped Graphite Contacts**

Table 1 lists the best device performance obtained for several CuCl
concentrations. The cells listed in table 1 were contacted with undoped
graphite. Optimum performance was obtained for the cells for which the
CdS was "treated" in solution with concentration in the 10^-8^ M range;
this concentration was found to yield the best performance and
reproducibility, and was subsequently utilized as the "baseline optimum
concentration".

Table 1. The effect CuCl solution concentration on solar cell
performance

  -------------------------------- ------------------- -------------------
  CuCl Concentration \[M\]         V~OC~ \[mV\]        FF \[%\]

  6.0 x 10^-9^                     766                 60.1

  3.0 x 10^-7^                     772                 59.8

  6.0 x 10^-8^                     830                 67.4

  6.0 x 10^-7^                     733                 53.0
  -------------------------------- ------------------- -------------------

The time the CdS was treated with CuCl was also found to be an important
parameter, although in most cases not as critical as the concentration.
Table 2 lists solar cell characteristics for devices treated for
different times, using a solution with the optimum concentration shown
in table 1. The I-V characteristics for these devices are shown in Fig.
2. The device "treated" for 20 minutes (the longest time) shows signs of
shunting in its reverse bias characteristics. All others do not exhibit
shunting; the device treated for the shortest time has a low V~OC~ (not
shunting related). Based on this behavior, it can be speculated that
long treatment times (as well as high concentrations) introduce an
excess amount of Cu to the extent that it can cause shunts; in addition,
poor collection has also been found to be a significant factor for the
observed losses in V~OC~ and FF at high Cu levels (to be discussed
later). The device treated for the shortest time exhibits the lowest
V~OC~. This could be partially attributed to the effect of the "CuCl
treatment"; however it should be noted that this particular device also
had the thinnest CdS (see SR data in Fig. 3) among this group of cells.
Thin CdS is often the reason for low V~OC~'s; however, such devices also
exhibit significant "shunting" which is not evident in this case, and
therefore it is believed that the low V~OC~ is not simply due to the
"thin CdS".

![](media/image2.wmf){width="3.3437007874015747in"
height="3.0216535433070866in"}

Table 2. The effect of solution treatment time on solar cell performance

  ----------------- --------------- ------------- ------------------------
  Time \[min\]      V~OC~ \[mV\]    FF \[%\]      J~SC~ \[mA/cm^2^\]

  5                 710             55.5          23.9

  10                830             66.3          23.4

  15                830             65.3          23.9

  20                650             48.0          19.6
  ----------------- --------------- ------------- ------------------------

Figure 3 shows the SR of the devices listed in table 2 (also in Fig. 2).
As already mentioned the device treated for 5 minutes has the thinnest
CdS as evident from its high response below 500 nm. The device treated
for the longest time (i.e. highest Cu concentration) shows significant
collection losses resulting in low overall QE (and therefore J~SC~).
This poor collection characteristic was typical of devices for which Cu
was sputter-deposited on CdS. Based on these results it was concluded
that excess Cu, in addition to causing shunts, it can also affect the
collection efficiency in CdTe cells, most likely by affecting the
effective doping in the semiconductors; it was also concluded, that
sputter depositing Cu on CdS, is not an effective method to incorporate
small and controlled amounts of Cu in the device.

**Sb~2~Te~3~ Contacts**

In addition to undoped graphite, Sb~2~Te~3~ was also used as a back
electrode material during this work. Work with Sb~2~Te~3~ as a back
contact (for devices where Cu was completely eliminated from all
processing steps), has not produced the results reported by others
\[6\]. Typical V~OC~'s have been in the range of 600-700 mV, with the
highest FF ever achieved being only 64%, with a back contact barrier
being present in all cases. Table 3 lists the solar cell characteristics
for CdTe devices with CdS treated in a CuCl solution and contacted with
Sb~2~Te~3~/Mo; the solution concentration is in the 10^-8^ M range (see
results in table 1). These results clearly suggest that the
incorporation of Cu during the solar cell fabrication process improves
device performance with V~OC~'s reaching the 770-780 mV range and FF's
exceeding the 70% level; the highest efficiency achieved was 11.5%.

![](media/image3.wmf){width="3.3437007874015747in"
height="3.0216535433070866in"}Table 3. The effect of solution treatment
time on solar cell performance; devices contacted with Sb~2~Te~3~.

  --------------------- --------------------- ---------------------------
  Time \[min\]          V~OC~ \[mV\]          FF \[%\]

  5                     760                   64.0

  10                    770                   71.1

  15                    740                   61.0

  20                    780                   55.0
  --------------------- --------------------- ---------------------------

The J-V characteristics for the same devices are shown in Fig. 4. The
first quadrant behavior clearly indicates the formation of a back
contact barrier in these devices, which is believed to be the primary
reason for the relatively low V~OC~'s and to a less extent the FF. The
devices treated for the two longest times also exhibit significant
shunting as was the case for cells contacted with undoped graphite
described earlier. The improvement in these cells, over the ones
fabricated without Cu, is believed to be due to an increase in the
effective carrier concentration in CdTe, which is affected by the amount
of Cu incorporated into the CdS.

All results presented to this point provide indirect, empirical evidence
of the effect this process (Cu incorporation in CdS) has on device
performance. In general the results obtained suggest that Cu is indeed
important in improving device performance, however, above certain levels
it can: (a) lead to significant collection losses, and (b) cause
shunting.

![](media/image4.wmf){width="3.354724409448819in"
height="3.0216535433070866in"}

# SIMS and AES Analysis 

In order to directly verify that the above process can indeed be used to
vary the amount of Cu added to CdS, a series of CdS films were treated
with CuCl solution and analyzed using SIMS and AES at NREL. The films
were treated in CuCl solutions with three different concentrations: (a)
6.0 x 10^-9^, (b) 6.0 x 10^-8^, and (c) 6.0 x 10^-7^ M

Fig. 5 shows the AES depth profiles for O~2~ and Cu; as noted in the
experimental section the CSS ambient contains O~2~. Only the profiles
for the films treated with the two higher concentrations are shown, as
the Cu levels in the third one were below the detection limit of this
technique. These results suggest that that Cu is found primarily on the
surface of the CdS films, as one would expect based on the nature of the
process used. It is also clear that the amount of Cu present in CdS is
directly related to the CuCl solution concentration (i.e. higher
solution concentrations result in higher Cu levels in CdS). The
correlation between O~2~ and Cu is not understood at this time and
further studies are necessary. Based on SEM images obtained during this
analysis, it was noted that several patchy areas were present on the CdS
surface. This would suggest that the process must be further improved to
achieve better uniformity, and may also explain some of run-to-run
variations observed during this study.

A SIMS profile comparing all samples treated in the CuCl solutions
mentioned above is shown in Fig. 6; a reference CdS film was deposited
on Si and treated with solution (b). The SIMS data confirms the AES
analysis in that Cu is located primarily near the surface and that the
amount of Cu present depends on the CuCl solution concentration. For the
sample treated with the highest concentration solution, Cu seems to have
penetrated within 0.15 μm from the surface, which is approximately 1.5
times deeper than the two lower concentrations, where the Cu seems to
penetrate about 0.1 μm. The maximum concentration of Cu at the surface
of CdS varies from 1x10^19^ to just over 1 x 10^20^ cm^-3^. The total
amount of Cu present at the surface of the film (i.e. integrating the Cu
concentration not including the apparent background levels) correlates
relatively well with the concentrations of the CuCl solutions used (i.e.
it increases approximately by a factor of 10, as does the solution
concentration). The baseline Cu concentration does not represent the
lowest levels present in the films; it is due to Cu contamination
associated with the instrument.

![](media/image5.wmf){width="3.3437007874015747in"
height="3.0216535433070866in"}![](media/image6.wmf){width="3.3437007874015747in"
height="3.0216535433070866in"}

**SUMMARY**

An alternative process to incorporate Cu in CdTe solar cells has been
studied. The solar cell performance achieved (V~OC~ of 830 mV and FF's
in the high 60's), clearly indicate that Cu enhances device performance,
even when intentionally introduced in CdS. The absence of Cu from the
back contact fabrication process, also suggests that high performance
can be achieved without the formation of Cu~2~Te, which is know to be
unstable. Excessive amounts of Cu can lead to shunting and poor
collection. At this time the extent to which Cu diffuses through the
entire device is not known and future studies will focus on determining
its concentration in completed devices. The role of O~2~, the
concentration of which has been found to correlate with Cu also needs to
be further investigated. Eliminating Cu from the back contact process
suggests that the formation of Cu~2~Te may not be necessary to achieve
effective back contacts to CdTe; this may prove to be critical in
improving device stability.

**Acknowledgements**

This work was supported by the National Renewable Energy Laboratory
(NREL) under subcontract NDJ-2-30630-18.

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