**HIGH-EFFICIENCY CDTE AND CIGS THIN-FILM SOLAR CELLS:**

**HIGHLIGHTS AND CHALLENGES\***

Rommel Noufi and Ken Zweibel

National Renewable Energy Laboratory, Golden, CO 80401, USA

**ABSTRACT**

Thin-film photovoltaic (PV) modules of CdTe and Cu(In,Ga)Se~2~ (CIGS)
have the potential to reach cost-effective PV-generated electricity.
These technologies have transitioned from the laboratory to the market
place. Pilot production and first-time manufacturing are ramping up to
higher capacity and enjoying a flood of venture-capital funding. CIGS
solar cells and modules have achieved 19.5% and 13% efficiencies,
respectively. Likewise, CdTe cells and modules have reached 16.5% and
10.2% efficiencies, respectively. Even higher efficiencies from the
laboratory and from the manufacturing line are only a matter of time.
Manufacturing-line yield continues to improve and is surpassing 85%.
Long-term stability has been demonstrated for both technologies;
however, some failures in the field have also been observed, emphasizing
the critical need for understanding degradation mechanisms and packaging
options. These two thin-film technologies have a common device/module
structure: substrate, base electrode, absorber, junction layer, top
electrode, patterning steps for monolithic integration, and
encapsulation. The monolithic integration of thin-film solar cells can
lead to significant manufacturing cost reduction compared to crystalline
Si technology. The CdTe and CIGS modules share common structural
elements. In principle, this commonality should lead to similar
manufacturing cost per unit area, and thus, the module efficiency
becomes the discriminating factor that determines the cost per watt. The
long-term potential of the two technologies require R&D emphasis on
science and engineering-based challenges to find solutions to achieve
targeted cost-effective module performance, and in-field durability.
Some of the challenges are common to both, e.g., in-situ process control
and diagnostics, thinner absorber, understanding degradation mechanisms,
protection from water vapor, and innovation in high-speed processing and
module design. Other topics are specific to the technology, such as
lower-cost and fast-deposition processes for CIGS, and improved back
contact and voltage for CdTe devices.

**INTRODUCTION**

Rapid technical progress has occurred in both the CdTe and
Cu(In,Ga)Se~2~ (CIGS) thin-film PV technologies. Advances have been made
in the following areas: materials delivery and film growth, control of
film properties at the micro and nano levels, understanding how the
device works (device physics) and how to improve the properties of
individual layers, intrinsic device stability, and prototype module
reliability. The results from these advances have helped both
technologies evolve from the laboratory to the marketplace. The existing
industry, joined by new start-up entities supported by venture capital,
continues to work toward expanded capacity from pilot production to
first-time manufacturing and beyond. Perhaps the most impressive advance
is the success by First Solar, which is transitioning toward a 75-MW
capacity to produce commercial CdTe modules with power output greater
than 67 W.

In this paper, we present highlights of the CIGS and CdTe technologies,
and address key challenges that need to be overcome to accelerate the
commercialization of the two technologies.

# HIGHLIGHTS OF CIGS AND CdTe TECHNOLOGIES

**[Laboratory Devices.]{.underline}** The CIGS thin film belongs to the
multinary Cu-chalcopyrite system, where the bandgap can be modified by
varying the Group III (on the Periodic Table) cations among In, Ga, and
Al and the anions between Se and S \[1,2\]. A wide range of bandgaps can
be obtained using combinations of different compositions. The bandgap
range of interest for this technology is between 1 and 1.7 eV\[3\]. The
CdTe material in the device mostly exists as a binary with a slight
deviation from stoichiometry. Its bandgap is about 1.5 eV, which is a
good match to the solar spectrum. In the device, this bandgap may vary
somewhat as a result of its interaction with the CdS (\~2.4-eV bandgap)
heterojunction partner during processing \[4\]. Table 1I summarizes
champion efficiencies of CdTe devices and Cu(In,Ga,Al)(Se,S)~2~-based
devices of different compositions.

Table 2 compares champion efficiency and power of different
commercial-sized CIGS, CIGSS, and CdTe modules from leading companies.
The results show that the performance of the CIGS and CdTe modules are
now approaching that of polycrystalline silicon PV. In addition to
improved efficiency, we also see demonstrated high throughputs and/or
higher yields.

Figure 1 shows a schematic of the CdTe and CIGS device structure.
Individual layer thicknesses are approximate and may differ somewhat
among laboratories. For

![](media/image1.jpeg){width="3.251388888888889in"
height="3.2805555555555554in"}

Fig. 1. CdTe and CIGS Device Structure

comparison, cross sections of scanning electron micrographs are shown to
provide true physical perspectives of the structures. Note that the CIGS
device is a substrate configuration that starts with glass/base
electrode, whereas the CdTe device is a superstrate configuration that
starts with glass/transparent top electrode. The sequence of the growth
of the layers in both structures may influence the properties of the
front and back junctions---that is, the p/n interface and the back
contact---and, in turn, the efficiency of the devices.

The most common deposition methods for the CdTe device involve acquiring
commercial SnO~2~-coated glass, or the deposition of cadmium stannate
and zinc stannate by sputtering, followed by chemical-bath deposition
(CBD) of CdS. The CdTe thin-film absorber is usually applied by
close-spaced sublimation, vapor-transport deposition, or
electrodeposition, followed by CdCl~2~ treatment. The back contact is
then applied after a chemical preparation (etching) of the back surface
of CdTe. The nature of the back contact varies---from a carbon paste
containing Cu~x~Te and HgTe, to a combination of other metals with Cu.
The inclusion of a form of Cu, with the back contact, is necessary; its
effect on the performance and stability of the device is the subject of
intense investigations.\[5,6\]

The CIGS device starts with sputtered Mo on glass. The Mo film
properties have to be optimized for adhesion, sheet resistance, and
morphology where it allows sodium (Na) from the glass to diffuse through
to the CIGS layer. Sodium aids the CIGS grain growth and increases the
carrier concentration. The optimum concentration of Na is about 0.1%
(atomic). Growth on non-Na-containing substrates requires dosing of the
CIGS film by introducing a 60 to 120 Å NaF layer on the Mo back contact,
or introducing NaF during the CIGS deposition. The absence of Na in the
device reduces the efficiency by 2% to 3% (absolute). The CIGS absorber
is deposited using several methods of flux delivery: evaporation of
elements simultaneously or in a prescribed sequence, sputtering of
metals followed by selenization with H~2~Se, reactive sputtering of
metals with Se vapor, or printing of metals from ink precursors followed
by selenization. The latter method requires no vacuum. The CdS layer is
applied by CBD, followed by sputter deposition of a bilayer consisting
of intrinsic and conducting ZnO. The ZnO layer is also applied by using
the chemical-vapor deposition process. The industrial processes for both
technologies basically adopt combinations of the techniques, as
described above.

The CIGS and CdTe modules share common characteristics and device
structural elements. Therefore, in principle, the cost per unit area
should be similar, and, thus, the efficiency becomes a discriminating
factor for the cost/watt. However, in practice, production processes in
terms of throughput and yield can differ significantly and may offset
the advantage of higher performance. This is the case at this time,
where the cost of producing CdTe modules has an advantage over CIGS. In
future years, semiconductor costs may become more prominent drivers.

The long-term potential of the two technologies requires R&D emphasis on
science and engineering-based challenges to find solutions to achieve
targeted, cost-effective module performance and in-field durability.
Scientists and engineers in the thin-film community have demonstrated
some successes in this regard. Transitioning knowledge, especially in
the area of production processes, from the laboratory to manufacturing
has proven much more difficult than anticipated. Because of the inherent
complexity of the two compound semiconductors, much more research is
needed.

**[Challenges.]{.underline}** We list some key challenges that must be
addressed to accelerate progress and contribute to commercial success.
The list is not comprehensive and does not go into detail due to lack of
space.

**1. Science and Engineering Support.** There is a great need to enhance
the science and engineering knowledge base from which to (a) derive
measurable material properties that are predictive of device and module
performance, (b) model the relationship between film growth and material
delivery, and (c) couple this knowledge to industrial processes. The
beneficial impacts expected are higher throughput and yield at every
step of the process, and a higher degree of reliability and
reproducibility, which, of course, will lead to higher performance.

**2. Long-Term Stability.** Both technologies have shown long-term
stability. However, degradation of performance has also been observed.
So, why do some modules maintain stable performance, while others fail?
This question begs for a better understanding of degradation mechanisms
at the device level and prototype module level, to distinguish the
intrinsic device contributions from the extrinsic mechanisms that may
result from the packaging process. Infiltration of water vapor through
the encapsulation package has been shown to degrade performance. Hence,
developing a thin-film barrier to water vapor will boost in-field
durability. Much work has been done to monitor and investigate
performance of CIGS and CdTe modules in the outdoors. To date, the level
of understanding the causes of performance degradation is inadequate and
lacks the coupling of feedback from device- and module-level studies.
Recently, Albin et al. \[7,8,9\] at NREL investigated the
temperature-dependent degradation of CdTe devices. The findings point
out that different mechanisms dominate degradation at different
temperatures. From 90^°^ to 120^°^C, the degradation is dominated by Cu
diffusion from the back contact toward the electrical junction, whereas
the source of possible degradation from 60^°^ to 90^°^C is not currently
known, and may simply reflect straying outside a process window. Such
studies help to identify relevant and appropriate accelerated test
protocols. Another issue requiring further consideration is the need for
encapsulants that can be applied and cured at room temperature and that
are chemically inert toward the semiconductor layer with which they come
in contact.

**3. In-Situ Process Diagnostics and Control.** To date, very little
exists in the area of in-situ diagnostics and control for both the CIGS
and CdTe technologies. This situation is because science-based knowledge
of material properties is inadequate to serve as a solid foundation from
which diagnostics tools can be developed. These tools must be developed
such that they can respond to rapid processing and feedback for
adjusting real-time processes. The results will impact throughput and
yield, and will make the process reproducible and reliable. Currently,
only a few techniques are in practice, based on changes of emissivity
from the growing surface, and in-situ monitoring of composition using
X-ray fluorescence.

**4. Thinner CIGS and CdTe Absorbers.** This challenge is motivated by
concerns over the availability and price of In and Te. This concern
currently seems more severe for In than Te because of competing uses
(flat panel displays). For example, the availability of In will begin to
have a significant impact at a production capacity level of tens of
gigawatts. Reducing the absorber thickness also yields other benefits,
especially for CIGS---for example, higher throughput and less material
cost. The primary challenge will be to thin the absorber to below 0.5
µm, while maintaining state-of-the-art performance. Potential pitfalls
also exist for going very thin, including nonuniformity,
shunting/pinholes, lower yields, and a need to change device structure
from the current norm. Table 3 summarizes the status of performance for
laboratory devices for very thin CIGS and CdTe absorbers \[10,11\]. The
drop in performance, currently begins to become significant below a
thickness of 1 µm, but that is likely an artifact of our rudimentary
knowledge. Studies guided by device modeling are under way to understand
the loss mechanism for very thin absorbers \[12,13\].

**5. Need for High-Throughput, Low-Cost Processes.**

This challenge is more relevant to CIGS technology. Currently, the
best-performing devices and large modules are produced in two ways: by
evaporation of the elements in vacuum; and by sputtering of the metals,
followed by selenization with H~2~Se. These two processes suffer from
relatively slow throughput, poor material utilization, and relatively
high vacuum. Deposition by high-rate co-sputtering from cylindrical
magnetrons is now being pursued; however, to date, this approach has not
demonstrated state-of-the-art performance. A lower-cost process should
feature high deposition rates, high material utilization, and simpler
equipment capable of processing very large substrates. One such example
is a process that uses nano-components to make printable precursors that
are crystallized into CIGS \[14,15\].

  ------------------ ------------ --------------- --------- --------------
  t (µm)             V~OC~ (V)    J~SC~           FF (%)    Efficiency (%)
                                  (mA/cm^2^)                

  1.0 CIGS           0.676        31.96           79.47     17.16 NREL

  0.75 CIGS          0.652        26.0            74.0      12.5

  0.40 CIGS          0.565        21.3            75.7      9.1

  0.47 CIGS          0.576        26.8            64.2      9.9 EPV

  1\. CIGSS Module   25.26        2.66            69.2      12.8 Shell
                                                            Solar

  0.87 CdTe          0.772        22.0            69.7      11.8\
                                                            U. of Toledo
  ------------------ ------------ --------------- --------- --------------

Table 3. Summary-Thin Cells Efficiencies

**6. Improved Open-Circuit Voltage (V~oc~) in CdTe Devices.** The
primary parameter needed to improve and achieve high efficiency in CdTe
devices is V~oc~, whose magnitude is affected by junction properties,
bulk carrier concentration, and the back contact. Enhanced V~oc~ may be
achieved by increasing the net p-type doping of the bulk through
extrinsic doping, or, more likely, by better crystal growth conditions
that influence more favorable formation of native point defects.

## ACKNOWLEDGEMENTS

The authors acknowledge contributions to this paper from:
Polycrystalline Thin Film Group, Measurement and Characterization, K.
Zweibel, H. Ullal, B. von Roedern -- NREL, Dale Tarrant -- Shell Solar
Industries, Robert Birkmire -- IEC, U. of Delaware, Bernhard Dimmler --
Wurth Solar, Dennis Hollars -- MIASOLE, Jeff Britt and Scott Wiedeman --
Global Solar Energy, Tim Anderson -- U. of Florida, and W.S. Sampath --
AVA Tech, Peter Meyers -- First Solar. This work is support or funded
under DOE Contract No. DE-AC36-99GO10337.

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