**ZnO/ZnS(O,OH)/Cu(In,Ga)Se~2~/Mo SOLAR CELL with 18.6% efficiency**

^1^M.A. Contreras, ^2^T. Nakada, ^2^M. Hongo, ^3^A.O. Pudov, and ^3^J.R.
Sites

^1^National Renewable Energy Laboratory, Golden, CO, USA

^2^Aoyama Gakuin University, Tokyo, Japan

^3^Colorado State University, Fort Collins, CO, USA

# ABSTRACT![](media/image1.wmf){width="2.7756944444444445in" height="3.4506944444444443in"}

We report on recent enhancements to device performance leading to a
certified total-area energy conversion efficiency of 18.6% for
Cu(In,Ga)Se~2~ solar cells that incorporate a ZnS(O,OH) buffer layer as
an alternative to CdS. Along with information on device fabrication and
layer properties, we provide a comparative device analysis between this
type of solar cell and the slightly more efficient
ZnO/CdS/Cu(In,Ga)Se~2~/Mo solar cell structure. This comparative study
allows us to elucidate the areas for optimization in the quest for
conversion efficiency above 20% in thin-film polycrystalline solar
cells. It quantifies the gains in current generation due to superior
collection at short wavelengths, as well as the somewhat lower voltage
and infrared response.

## 1. INTRODUCTION

The interest in alternative buffer layers to CdS in Cu(In,Ga)Se~2~
(CIGS) photovoltaic technology is motivated primarily by the potential
to enhance solar cell current generation by the use of materials with
wider band-gap than CdS (E~g~ \~ 2.4 eV), and also by a desire for
Cd-free cells. In this regard, ZnS(O,OH) (E~g~ \~ 3.7 eV \[1\]), is an
ideal candidate for exploration. Previous work showed that
ZnS(O,OH)/CIGS cells could attain high efficiencies \[2\]. In this
contribution we report our most recent developments on the application
of such buffer layers to CIGS absorbers leading to the
highest-efficiency CdS/CIGS cells reported to date \[3\]. The best of
the cells reported here had a confirmed total-area energy conversion
efficiency of 18.6% under standard conditions for AM1.5 irradiation (see
Fig. 1)

**2. EXPERIMENTAL**

**2.1 CIGS Absorbers**

The CIGS absorbers used in this study have been grown on Mo-coated
soda-lime glass by evaporation from elemental sources following the
"3-stage" process developed at NREL. The Mo layer is \~1 µm thick and is
deposited by d.c. magnetron sputtering.

The \~2.5 µm thick CIGS absorbers have structural and compositional
qualities as described in \[3\] and their main characteristics are:

\(a\) atomic compositional ratios of Cu/(In+Ga)\~0.88 and
Ga/(In+Ga)\~0.3;

\(b\) a slight compositional gradient in the Ga content increasing
toward the back of the CIGS film and,

> \(c\) a strong \<110\> type of preferred orientation.

The optical bandgap of these materials is usually \~1.15 eV. Small
variations in this bandgap value can be expected from run to run due to
factors such as the non-uniform Ga distribution mentioned above and
unavoidable instrumental errors of the deposition rate control.

Since the CIGS surface morphology and structure have an effect on
nucleation and growth of the buffer layers, a few words on this issue of
morphology follow.

The surface morphology of \<110\> oriented CIGS films is shown in the
SEM micrograph displayed in Fig. 2. As it can be seen, this morphology
is quite different from \<112\> oriented films or even randomly oriented
films. Whereas \<112\> oriented CIGS films usually display smooth
triangular features arising from the 3-fold symmetry of the (112)
planes, the \<110\> oriented films display grains with a rather rough
character characterized by "steps" and "ridges" on the surface. It has
been argued that such ridges arise from the reconstruction or
decomposition of the (110) surfaces into two sets of polar (112) planes
(for additional arguments on this subject see ref. \[4\]. On the matter
of polarity, we point out that the (110) family of planes have a
non-polar character, that is, they present an equal number of anions and
cations on the ideally terminated surface.

**2.2 Chemical Bath Deposition for
ZnS**![](media/image2.wmf){width="2.6597222222222223in"
height="2.39375in"}

Deposition of ZnS(O,OH) films via a chemical-bath-deposition (CBD)
technique is reported in detail elsewhere \[1,5\]. The ZnS(O,OH) layers
for this work were grown on NREL CIGS absorbers using a ZnSO~4~ (0.16 M)
-- ammonia (7.5 M) -- thiorea (0.6 M) aqueous solution at 80°C for 15
min. The CBD layers were approximately 100 nm thick. XPS analysis
revealed that a large amount of oxygen was included in the form of
Zn(OH)~2~ and ZnO. The film composition was estimated to be Zn:S:O =
48:28:24. Best results so far have been achieved by air annealing
carried out at 200˚C for 10 minutes \[1\].

FTIR measurements revealed --N=C=S bonds, and thermal-deposition
spectroscopy showed that the material contained small amounts of C and
S, which probably originated form the CS(NH~2~) (thiorea) during the CBD
process. X-ray diffraction suggested that the crystal structure was
complex and included ZnS, ZnO, Zn(OH)~2~, and Zn(S,O) compounds. TEM
analysis confirmed the existence of wurtzite and sphalerite structures.
The optical absorption coefficient was derived from transmission and
reflection curves of the 100-nm-thick films, and the band-gap energy was
determined to be 3.7 eV from the extrapolation of (hν)^2^ vs. ν to zero.

The completed solar cells incorporated a ZnO window bi-layer with 60-80
Ω/sq sheet resistance, a Ni/Al top contact grid with 3-5% obscuration
loss, and a 100-nm MgF~2~ anti-reflection coating.

**3. MEASUREMENTS AND ANALYSIS**

In order to quantify the photocurrent gain (and loss) attained by the
use of ZnS(O,OH) alternative buffer layers, a comparative study was made
in which we contrast the electrical and optical characteristics of the
18.8% efficiency standard ZnO/CdS/CIGS/Mo cell reported in \[3\] with
the 18.6% Cd-free solar cell reported above.

**3.1 Quantum Efficiency**

As mentioned above, the interest of applying alternative buffer layers
with wider band-gap values than CdS has to do with the potential of
increasing current generation particularly in the spectral region for
wavelengths \<550 nm. A direct comparison of the quantum efficiency of
those two cells is shown in Fig. 2.
![](media/image3.wmf){width="3.022222222222222in" height="2.94375in"}

To quantify photocurrent density J over a selected wavelength range from
λ~1~ to λ~2~, we use the relationship:

~λ2~

> J = q∫*η~Q~*~\*~(dΓ/dλ)dλ, (1)

^λ1^

where q is the electron charge, *η~Q~* is the quantum efficiency, dΓ/dλ
is the global solar spectrum (AM1.5) and λ is the wavelength. From Fig.
2, we can observe the following:

(a) A net gain of \~3.3 mA/cm^2^ is attained in the ZnS(O,OH) cell
    compared to the CdS cell for wavelengths \<550 nm.

(b) Collection in the ZnS(O,OH) cell is 1.3 mA/cm^2^ lower in the
    wavelength range approaching the band-gap cutoff.

(c) The 30 mV band-gap difference between the two absorbers added 1.2
    mA/cm^2^ to the CdS cell's photocurrent.

An overall net gain in current density of \~ 1 mA/cm^2^ was indeed been
achieved by the utilization of the ZnS(O,OH) buffer layer, and would
have been greater than 2 mA/cm^2^ if there had been a common absorber
band gap. In any case, the potential of a significant gain (3.3
mA/cm^2^) in current for the UV region (\<550 nm) is clearly
demonstrated from this analysis.

**3.2 J-V(T) analysis**

Figure 3 shows two current-density vs. voltage vs. temperature (J-V-T)
characteristics, one for the high efficiency ZnS/CIGS cell from Fig. 1,
and the other for a high efficiency CdS/CIGS cell. It is seen from this
figure that both the alternative-buffer-layer ZnS cell and the standard
CdS cell are very well behaved in the range of temperatures used in this
experiment. Each set of curves on cell
efficiency.![](media/image4.wmf){width="3.0208333333333335in"
height="4.0569444444444445in"}

Figure 4 shows the short circuit current density (J~sc~), open circuit
voltage (V~oc~), fill factor (FF), and efficiency (Eff) of the ZnS(O,OH)
and CdS cells as a function of temperature from 240 to 320 K. J~sc~ is
consistently higher for the ZnS(O,OH) cell, and for both cells, it is
very constant over this temperature range. V~oc~ is well behaved in both
cases, and the extrapolation to 0 K matches well the low temperature
band gap values for the absorbers. The FF also increases as expected at
lower temperatures, except the lowest temperature point for the ZnS
cell, and is very nearly the same for both cells. And finally, the
efficiency of both cells increases at lower temperatures, but at a
slightly faster rate for the ZnS(O,OH) cell. In fact, below 250 K, its
efficiency exceeds 21%.

**4. CONCLUSIONS**

ZnS-based buffer layers have been shown to be a highly competitive
alternative to the traditional CdS used with CIGS absorbers. There is a
clear short-wavelength advantage in current collection with the
potential of increasing short-circuit current density by \~3.3 mA/cm^2^
when compared to CdS-based buffer layers. There is in the cells studied
to date, however, a somewhat faster red-falloff approaching the band gap
and a somewhat smaller voltage in comparison to the band gap. The poorer
response for wavelengths \>700 nm of the cells incorporating ZnS offset
the gains in the short-wavelengths and this problem, along with the
slightly lower V~oc~ attained, will be the focus of future optimizations
for these cells.![](media/image5.wmf){width="2.7881944444444446in"
height="2.645138888888889in"}![](media/image6.wmf){width="2.785416666666667in"
height="2.638888888888889in"}

The fact that the temperature dependence behavior of both type of cells
(with CdS-based or ZnS-based buffer layers) is somewhat similar, suggest
that the cells are likely dominated by the same recombination mechanism
and that that the junction formation phenomena occurring during the
buffer layer process is similar for Zn and/or Cd. That similar
temperature behavior also suggests that solar cells made with ZnS-based
buffer layers have the potential of being as reliable as the CdS-based
buffer layer cells.

The conduction-band offset, which should be considerably larger with the
ZnS-based buffer layers should also play a role on the characteristic
response or behavior of the cells, but it was not investigated in the
study reported here.

Future work to enhance performance of CIGS solar cells utilizing
ZnS(O,OH) buffer layers beyond the state-of-the-art reported here, will
cover optimizations of the annealing steps required for the buffer layer
and/or slight modifications to the absorber in terms of surface
composition both in Ga and Cu content.

Finally, we point out to the obvious benefits of a CIGS technology
employing a Cd-free device structure. Such benefits are not limited to a
direct impact on reducing manufacturing cost by eliminating the need of
handling and disposing toxic materials (mainly Cd compounds), but they
also have to do with the opening of markets where products containing
hazardous or toxic elements are simply banned or highly taxed.

**5. ACKNOWLEDGEMETS**

This work was supported by the New Energy and Development Organization
of Japan (NEDO), Team CIS-21, and by the U.S. Department of Energy under
Contract No. DE-AC36-99GO10337. The authors are grateful to colleagues
at NREL who assisted in completion of the cells studied.

### **REFERENCES**

\[1\] T. Nakada and M. Mizutani, "Improved Efficiency of Cu(In,Ga)Se2
Thin Film Solar Cells with Chemically Deposited ZnS Buffer Layers by Air
Annealing Formation of Homojunction by Solid Phase Diffusion",
*Proceedings of the 28^th^ IEEE Photovoltaic Specialists Conference*,
(Sept. 2000, Anchorage*)* pp. 529-534.

\[2\] A.O. Pudov, J.R. Sites, and T. Nakada, "Performance and Loss
Analyses of High-Efficiency Chemical Bath Deposition
(CBD)-ZnS/Cu(In~1-x~Ga~x~)Se~2~ Thin-Film Solar Cells", Jpn. J. Appl.
Phys. **41** (2002), Part 2, No 6B, L672-674.

\[3\] M.A. Contreras, B. Egaas, K. Ramanathan, J. Hiltner, A.
Swartzlander, F. Hasoon and R. Noufi, "Progress Toward 20% Efficiency in
Cu(In,Ga)Se~2~ Polycrystalline Thin-Film Solar Cells", Prog.
Photovoltaics **7(4)** (1999) 311.

\[4\] D. Liao and A. Rockett, "Effect of Surface Orientation on the
Growth and Properties of Cu(In,Ga)Se~2~", *Proceedings of the 29^th^
IEEE Photovoltaic Specialist Conference*, (2002, New Orleans, USA), pp.
515

\[5\] T. Nakada and M. Mizutani, "18% Efficiency Cd-Free Cu(In, Ga)Se~2~
Thin-Film Solar Cells Fabricated Using Chemical Bath Deposition
(CBD)-ZnS Buffer Layers Jpn. J. Appl. Phys. **41** (2002), Part 2, No.
2B, L165-167.