THE POTENTIAL FOR HIGH PERForMANCE IN CIGS SOLAR CELLS: A LABORATORY
PERSPECTIVE

K. Ramanathan, J.C. Keane, B. To, R.G. Dhere, and R. Noufi

National Center for Photovoltaics, National Renewable Energy Laboratory,
1617 Cole Boulevard, Golden CO 80401\
e-mail: kannan_ramanathan@nrel.gov

ABSTRACT: We present a summary of our work with the preparation of CIGS
absorbers which has led to the fabrication of record-efficiency solar
cells. The use of the three-stage process in conjunction with
composition monitoring facilitates the fabrication of solar cell with
efficiencies between 18% and 19.5% for absorber bandgap in the range of
1.1--1.2 eV. It is possible to maintain high open-circuit voltages and
high fill factors, and the efficiency gains can be correlated to low
diode factors and low junction recombination current. In the second part
of this paper, we describe our recent results in reducing absorber
thickness and low-temperature deposition. Our preliminary results on
absorbers grown from low-purity source materials show promise of
reducing the cost of fabricating the absorber.

Keywords: Chalcopyrite, Cu(In,Ga)Se~2~, Solar Cell Efficiencies

1 INTRODUCTION

There is now a growing appreciation of the importance of photovoltaic
(PV) technologies in the world's energy production. Silicon-based
technologies continue to have a commanding lead in the market, and
polycrystalline thin-film solar cell technologies are expected to enter
the market as second-generation devices and strengthen the landscape.
Indeed, CdTe and CuInSe~2~ PV technologies have matured to the point of
large-scale commercial production. CuInGaSe~2~ (CIGS) thin-film solar
cells have demonstrated the highest efficiency in the laboratory and on
the industrial scale, and their long-term performance in the field shows
good stability. Sustained R&D effort in the USA, Europe, and Japan have
contributed to this success. The investment in R&D by the DOE PV Program
has paid dividends as the two prominent thin-film technologies continue
to make headway into the commercial market. However, much work remains
to be done to ensure continued growth and advancement: developing
faster, simpler processes for depositing the absorbers at high rates;
lowering process temperatures; simplifying junction-forming process
steps; reducing absorber thickness; and, maintaining high efficiency. In
the opinion of the authors, the community is addressing the above issues
with innovative approaches and finding solutions. We shall highlight
them in this paper.

Over the past five years, our group has devoted its efforts to two major
areas: reproducible fabrication of high-efficiency CIGS cells by the
three-stage process and the basic science of junction formation. Cell
efficiencies above 19% were reported \[1,2\] and the trend continues
\[3\]. The possibility of n-type doping during the CBD CdS growth was
proposed \[4\] and this culminated in the fabrication of efficient cells
without CdS \[5\]. Cadmium partial electrolyte treatment is still being
pursued as a potential junction-formation process step \[6\]. Surface
doping of CIGS absorbers was also demonstrated when Zn was incorporated
after the growth of the absorber, and solar cells with efficiency \>16%
were fabricated without using CdS or other window layers \[7\]. This
study provides a clear example of how the junction formation can be
achieved in an in-line mode using dry processes. Several alternative
approaches \[8,9\] being pursued demonstrate the options available to
further simplify the cell design and components. This is indeed
encouraging.

In this paper, we review our recent results on high efficiency CIGS
solar cells. We have begun to address the issue of cost reduction in a
multi-megawatt manufacturing scenario. Reducing the absorber thickness
would reduce the amount of In, the price of which has risen sharply. We
report our results on CIGS solar cells fabricated on absorbers that are
≤1 µm in thickness. Another approach to lowering the cost is to prepare
the absorbers from starting materials of lower purity than typically
used (99.9% vs. 99.999%). We also explore the deposition of thinner
absorbers at low temperatures and deposition at high rates. These
results are presented below.

2 EXPERIMENTAL DETAILS

CIGS absorbers were grown on soda-lime glass substrates with a
sputter-deposited Mo layer. The absorber is grown by first depositing an
(InGa)~2~Se~3~ layer and reacting it with Cu and Se. Compositional
control was achieved by detecting the temperature change of the
substrate during Cu-poor to Cu-rich transition at the end of the second
stage. The third stage consists of the evaporation of In and Ga in the
presence of Se. The absorber thickness for our standard runs was
2.5--2.75 µm. Thinner CIGS absorbers (0.4--1.0 µm) were deposited by
reducing the (In,Ga)~2~Se~3~ film thickness and making appropriate
modifications to the process. Thin absorbers were also prepared by
simultaneous deposition of all the elements in one step. Some absorbers
were deposited using 99.9% pure Cu, In, and Ga source materials. A few
CIGS absorbers were also fabricated by doubling the rates of all the
elements with a view to reduce the deposition time. CdS deposition was
performed using a solution consisting of 0.0015 M CdSO~4~, 1.5 M
NH~4~OH, and 0.075 M thiourea. The samples were immersed in the bath at
room temperature and the temperature of the bath was increased to 60˚C.
CdS thin films in the thickness range of 50--60 nm were deposited in 15
min. ZnO window layer was deposited in two stages. A 90-nm-thick,
undoped layer was first deposited from a pure ZnO target using Ar/O~2~
working gas, and a second layer of about 120 nm was deposited from an
Al~2~O~3~-doped ZnO target. The sheet resistance of the bilayer was
about 60 Ω/sq. Ni/Al grids were deposited by electron beam evaporation.
A 100-nm-thick MgF~2~ film was deposited to serve as an antireflection
coating. Current-voltage characteristics of the devices were measured
under AM 1.5 Global spectrum with the irradiance set to 1000 W/m^2^.

3 RESULTS AND DISCUSSION

3.1 High-efficiency solar cells

CIGS absorbers with a range of energy gaps were fabricated by adjusting
the Ga content of the (In,Ga)~2~Se~3~ precursor layer. Compositional
depth profiles show that the bandgap is graded through the depth of the
film. The energy gap was evaluated from spectral response measurements,
and the Ga/(In+Ga) ratio was derived from the electron microprobe data
obtained using 10-kV beam energy. This is indicative of the average
composition in a sample volume of approximately 1 µm. The energy gap
ranged from 1.1 to 1.21 eV, and the corresponding Ga/(In+Ga) ratios
ranged from 0.26 to 0.31. Table I shows a summary of the properties of
solar cells characterized under standard reporting conditions. Table II
lists the ideality factor and the reverse saturation current density of
the solar cells.

  ------------------ ------------ ------------ ------------- ---------------
  Sample Number      V~oc~ (V)    J~sc~        Fill factor   Eff (%)
                                  (mA/cm^2^)   (%)           

  S2212B1-3          0.701        34.60        79.65         19.3

  S2212B1-4          0.704        34.33        79.48         19.2

  S2212B1-5          0.703        34.08        79.23         19.0

  S2213A1-3          0.740        31.72        78.47         18.4

  S2213A1-4          0.737        31.66        78.08         18.2

  S2229A1-3          0.720        32.86        80.27         19.0

  S2229A1-5          0.724        32.68        80.37         19.0

  S2229B1-2          0.731        31.84        80.33         18.7

  S2229B1-4          0.728        31.87        80.16         18.7

  S2232A1-3          0.703        33.94        79.67         19.0

  S2232A1-4          0.704        33.83        80.09         19.1

  S2232B1-2          0.717        33.58        79.41         19.1

  S2232B1-3          0.713        33.38        79.54         18.9
  ------------------ ------------ ------------ ------------- ---------------

**Table I**: Parameters of high-efficiency CIGS solar cells.

  ---------------------------- ---------------------- -------------------
  Device Number                J~o~ (A/cm^2^)         n

  S2212-B1-3                   5x10^-11^              1.35

  S2212-B1-4                   6x10^-11^              1.36

  S2212-B1-5                   6x10^-11^              1.35

  S2213-A1-3                   4x10^-10^              1.57

  S2213-B1-4                   5x10^-10^              1.62
  ---------------------------- ---------------------- -------------------

**Table II**: Diode parameters determined from light current-voltage
curves. J~0~ is the reverse saturation current density, and n is diode
ideality factor.

The above results clearly demonstrate that high-efficiency CIGS solar
cells can be fabricated up to a bandgap of about 1.2 eV. The advantage
of using a higher bandgap is the higher open-circuit voltage and a lower
temperature coefficient. This also offers the possibility of reducing
the number of cells in a module. High voltages can be maintained along
with high fill factors, and the junctions are characterized by low
ideality factors and reverse saturation current density (J~o~).

3.2 Thinner absorbers

Reducing the thickness of the absorbers from 2.5 µm to ≤1 µm reduces the
quantity of In used in the cell. This must be done without adversely
affecting the efficiency. This aspect has been studied previously
\[10\], and the efficiency has been shown to decrease when the absorber
thickness is reduced below 1 µm. Recently, the price of In has risen,
and the concern about using it in large quantities resurfaces from time
to time. We have revisited the growth of thin absorbers by using a
modified three-stage process and a co-evaporation process similar to the
one used by Boeing \[11\]. To apply the three-stage process for thinner
films, the deposition rates of all the elements were reduced in the
first two stages, and the heating rates were also adjusted.
Compositional monitoring was still possible in spite of the reduced film
thickness. In the co-evaporation process, film growth was initiated in a
Cu-rich CuGaSe~2~ layer and the overall composition was converted to
device quality, Cu-poor Cu(InGa)Se~2.~ This type of growth often
resulted in some voids and poor adhesion at the Mo interface. These
observations are consistent with an earlier study by Kessler et al
\[12\]. However, compositional uniformity along the depth was easily
achieved, and the deposition time was considerably shorter (5--10 min)
than the three-stage process. With the latter, it was possible to
produce dense and smooth thin films. The distribution of Ga along the
depth was governed by the ramp rates and the kinetics of the reaction
between the binary selenides. Hence, we observed a difference in the
solar cell properties fabricated from absorbers made by the two methods.
Figures 1--3 show the microstructure of the thin absorbers.

![](media/image1.png){width="2.652083333333333in"
height="2.286111111111111in"}

**Figure 1**: SEM image of a 1-µm thick CIGS absorber deposited by
three-stage process.

![](media/image2.png){width="2.6590277777777778in"
height="2.292361111111111in"}

**Figure 2**: SEM image of a 0.75-µm CIGS absorber deposited by
co-evaporation.

![](media/image3.png){width="2.6590277777777778in"
height="2.292361111111111in"}

**Figure 3**: SEM image of a 0.5-µm CIGS absorber deposited by
co-evaporation.

Table III shows the properties of the best solar cells fabricated as a
function of absorber thickness.

+-------------------+----------+----------+-------------+-------------+
| Thickness         | V~oc~    | J~sc~    | Fill factor | Eff (%)     |
|                   | (V)      | (m       | (%)         |             |
| (µm)              |          | A/cm^2^) |             |             |
+-------------------+----------+----------+-------------+-------------+
| 1.0 (3-stg)       | 0.654    | 31.6     | 78.3        | 16.2        |
+-------------------+----------+----------+-------------+-------------+
| 1.0 (codep)       | 0.699    | 30.6     | 75.4        | 16.1        |
+-------------------+----------+----------+-------------+-------------+
| 0.75 (codep)      | 0.652    | 26.0     | 74.0        | 12.5        |
+-------------------+----------+----------+-------------+-------------+
| 0.5 (codep)       | 0.607    | 23.9     | 60.0        | 8.7         |
+-------------------+----------+----------+-------------+-------------+
| 0.4 (3-stg)       | 0.565    | 21.3     | 75.7        | 9.1         |
+-------------------+----------+----------+-------------+-------------+

**Table III**. Properties of thin CIGS solar cells.

The last sample in the above table was fabricated on a substrate
specially prepared to allow optical measurements to be made. In a
7.5x7.5 cm size substrate coated with Mo, 1x1cm size openings were made.
The CIGS film deposited on these windows was used for optical
measurements. Transmittance and reflectance were measured as a function
of wavelength, and the absorption was calculated. This was further
corrected for the reflectance of the solar cell made from the Mo coated
areas. The external quantum efficiency (QE) of the solar cell and the
absorption in the CIGS film are shown in Fig. 4. The shape of the
absorption and long wavelength edge of the QE are similar. The current
collection in the device is efficient because the QE curve nearly
matches the absorption curve. The difference between the two is the
collection loss in the long wavelength region, but it is surprisingly
low. To the extent that the thin film absorbs light, current is
efficiently generated and collected, but the consequence of not having
adequate thickness to absorb all the light (some light is transmitted)
is apparent. The short wavelength response is reduced by the window
layer absorption and loss of collection. The measured current density of
the cell is close to what is expected, and the fill factor is high. The
efficiency of the cell is reduced by a lower V~oc~ and it is likely that
the recombination in the back contact region is playing a role. This is
the major loss in thin cells. Current collection can be improved by
using a back reflector. Texturing the back surface can enhance the path
length of long wavelength light and help improve J~sc~.

3.3 Thin CIGS cells deposited at low temperature

It would be interesting to combine the challenges of thinning the cell
with performing the deposition at lower temperatures, say 400--450˚C,
where polymer substrates could be used. At the outset, such a
combination of deposition conditions would seem like a poor choice
because the material quality would be inferior, and the thickness
related losses would further decrease the performance. However, there
are practical reasons that make the study appealing. The ability to
fabricate device quality absorbers at low temperatures would facilitate
the exploration of monolithic thin film tandem solar cells. Our initial
attempts indicate that good performance can be obtained. For an absorber
thickness of 1.4 µm deposited at a substrate temperature of 450˚C, cell
efficiency was 12.9% without antireflective coating. The solar cell
parameters were: V~oc~= 0.585V; J~sc~ = 31.1 mA/cm^2^; and fill factor =
70.6%. This is a promising result, and we expect to improve on it by
incorporating Na doping.

3.4 Growth from low-purity elements

Impurities are generally detrimental to performance of electronic
devices. The standard practice in thin film deposition is to use source
materials of the highest purity available and affordable, typically
99.999% or better. Electronic-grade materials are expensive. One
approach to reducing cost would be to use lower-purity materials if we
know that the trace elements do not adversely affect the cell
performance. Impurities in the source materials may not evaporate at the
same rate as the elements. They may be less soluble, not at all soluble,
or segregate out of the solid film. If they do incorporate in the film,
the impurities may not be electrically active. A case in point is the
variety of impurities found in soda lime glass. In addition to Si, the
primary component, significant amounts of Na, K, Ca, Mg, Al and Fe are
present in the glass forming network as oxides, and the elements diffuse
through the semiconductor films. The electrical properties of II-VI and
I-III-VI~2~ semiconductors are dictated by native defects and complexes,
and only a few extrinsic impurities exhibit a strong effect in changing
the carrier concentration or type of conduction. Hence, one cannot
assume *a priori* that an increase in impurity content of the source
materials will lead to poor device performance. To test this hypothesis,
we purchased 99.9% pure Cu, In, and Ga and prepared CIGS absorbers from
them. Solar cells were fabricated and compared with cells made from
high-purity elements. At this time, we do not have a statistically
significant set of samples to draw firm conclusions from, and we have
not performed any analysis of the impurity content of the samples.
Nevertheless, a one-to-one comparison of the parameters shown in Table
IV suggests that the performance of solar cells fabricated from
low-purity elements is close to the control sample. Another factor that
can account for some of the differences are the differences in the
chemical composition of the two absorbers (random variables introduced
by growth apparatus, Ga variation). Indeed, our preliminary results are
encouraging, and the topic deserves to be investigated in
detail.![](media/image4.pct){width="2.2729166666666667in"
height="2.10625in"}

+-------------------+----------+----------+-------------+-------------+
| Sample details    | V~oc~    | J~sc~    | Fill factor | Eff (%)     |
|                   | (V)      | (m       | (%)         |             |
|                   |          | A/cm^2^) |             |             |
+-------------------+----------+----------+-------------+-------------+
| High Purity       | 0.671    | 33.1     | 77.3        | 17.2        |
|                   |          |          |             |             |
| (Control)         |          |          |             |             |
+-------------------+----------+----------+-------------+-------------+
| Low Purity        | 0.624    | 34.5     | 72.1        | 15.5        |
+-------------------+----------+----------+-------------+-------------+

**\
Table IV**: Properties of CIGS solar cells made from high-purity and
low-purity elements.

3.4 Three-stage process at high rates

Deposition time is an important factor in processing absorbers. Often, a
reduction in the time is achieved by increasing the rates of evaporation
of the elements or by moving the substrate at faster speeds. Both are
likely to affect the film quality. Deposition times on the order of
minutes are preferred. Our standard absorber deposition takes typically
40 min to complete. To find out whether the same level of cell
performance can be achieved at shorter deposition times, we doubled the
rates of all the elements during the three-stage run. Deposition time
was reduced to 20 min. Solar cells were completed and compared with our
standard absorber. The results of high-rate deposition are inferior, and
the major loss is in the V~oc~ as shown in Table V. It is likely that
the out-diffusion of Ga is limited by the reaction rate of the binary
selenides during the second stage.

  -------------------- ----------- ------------ -------------- --------------
  Sample details       V~oc~ (V)   J~sc~        Fill factor    Eff (%)
                                   (mA/cm^2^)   (%)            

  Normal run           0.671       33.1         77.3           17.2

  High rates           0.572       32.7         73.0           13.7
  -------------------- ----------- ------------ -------------- --------------

**Table V**: Effect of high deposition rates.

4 CONCLUSIONS

We have presented a summary of the properties of high-efficiency CIGS
solar cells. Absorber bandgap was varied in the range of 1.1--1.2 eV by
varying the Ga content of the alloy. Cell efficiency reaches a maximum
around 1.15 eV. The highest efficiency obtained so far (19.5%)
establishes a new world record for CIGS thin- film solar cells \[3\]. It
is possible to fabricate cells with high V~oc~ (0.74 V) and fill factors
(0.8). Consistent levels of high performance can be attributed to the
low recombination in the cells. Further improvements to cell efficiency
can be achieved by improving the short- wavelength collection with the
use of wide-bandgap window layers.

Thinner CIGS absorbers were grown by the three-stage process and a
modified co-evaporation process. Respectable performance was achieved at
film thickness of 1.0 µm, but decreased sharply when the thickness was
reduced further. Optical absorption and the quantum efficiency curves of
the 0.4-µm cell show that the photocurrent is generated efficiently but
only to the extent that the thin absorber is able to generate the
current. The reduction in V~oc~ is a serious problem. Thin absorbers
deposited at lower temperature yielded good solar cells. We also
examined the effect of higher deposition rates and found that the
performance degraded. CIGS absorbers grown from low purity elemental
sources yielded \> 15% efficient cells. Improvements in all areas can be
made by optimizing film growth.

5 ACKNOWLEDGMENTS

This work was performed for the U.S. DOE PV Program under Contract No.
DE-AC36-99GO10337 to NREL. The authors wish to thank M.A. Contreras,
F.S. Hasoon, J.S. Ward, R. Bhattacharya, J. Abushama, J. Dolan, and T.
Moriarty for their contributions to this work, and J. Benner, L.
Kazmerski, and B. Dimmler for their interest, encouragement and support.

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