**Growth of Cu(InGa)(SeS)~2~ by the Reaction of Cu-Ga-In Precursors in
H~2~Se and H~2~S**

Gregory M. Hanket, Robert W. Birkmire, William N. Shafarman

Institute of Energy Conversion

University of Delaware, Newark, DE 19716

**ABSTRACT**

Critical issues for the formation of Cu(InGa)(SeS)~2~ films by the
selenization or sulfization reactions of Cu-Ga-In precursor films are
investigated. The phases present in the metal precursors after annealing
at 450°C were Cu, In, CuIn and Cu~9~(In~1‑x~Ga~x~)~4~ . Films reacted in
hydrogen selenide (H~2~Se) or hydrogen sulfide (H~2~S) were
characterized and unreacted intermetallic Cu-Ga or Cu-In phases
respectively were identified at the back of the films. This indicates a
reaction preference of Se with In, and S with Ga. Homogenization of the
Ga using a two-step selenization/sulfization process that takes
advantage of this reaction preference was confirmed and devices with
V~OC~ \> 0.64 V and eff. \> 13 % were demonstrated.

**1. Objectives**

The deposition of Cu(InGa)(SeS)~2~ films by the reaction of precursor
films containing Cu, In, and Ga is a promising process for the low cost
manufacture of PV modules being pursued by several companies. The Solar
Program Multi-Year Technical Plan identifies the need for improved
understanding of the chemistry and physics of active absorber materials
in the module-production process as a fundamental barrier to meeting
overall goals of improved efficiency at the module level and lower
manufacturing costs. An objective of this program is to improve the
understanding of the reaction chemistry leading to formation of
Cu(InGa)(SeS)~2~ by the precursor reaction process. This could lead to
improved composition control in the films which could increase the
efficiency in cells and modules and enable better optimization of the
reaction process.

**2. Technical Approach**

Cu-Ga-In precursor films are reacted in H~2~Se and/or H~2~S to form
Cu(InGa)(SeS)~2~. A complete understanding of the reaction process
requires characterizing phases in the as-deposited precursor films and
after annealing. The selenization/sulfization processes to form
Cu(InGa)(SeS)~2~ are characterized with respect to time-temperature-gas
concentration profiles used to control the reaction pathway. Processes
for controlling the through-film composition, particularly preventing
segregation of Ga to the back of the absorber layer which occurs with
just selenization and incorporating a S-gradient near the front, will be
developed to optimize device performance.

Experimentally, the precursors are prepared by sputtering onto Mo-coated
soda-lime glass substrates. The films are annealed or reacted in a
flowing hydride gas reactor at temperatures from 400 -- 550°C for times
up to 90 minutes. Characterization includes film composition and
structure and fabrication and characterization of solar cells.

**3. Results and Accomplishments**

Cu-Ga-In precursor bi-layers were prepared by sputtering 3490 Å of a
Cu~0.8~Ga~0.2~ alloy followed by 4890 Å of In. As-sputtered films were
found by XRD analysis to be comprised of Cu, Cu~3~Ga, CuIn, and In, as
shown in Fig. 1. Precursors were annealed for 10 minutes at 450°C in
atmospheric pressure flowing Ar. XRD of the annealed films, (Fig. 1)
indicates the presence of a Cu~9~(In~1‑x~Ga~x~)~4~ alloy with x = 0.36
from the peak position and assuming Vegard's law. Another film was
annealed under identical conditions except for the addition of 0.0035%
O~2~, the same concentration used in typical selenization reactions.
This had identical phases observed by XRD except for the additional
presence of an In~2~O~3~ peak.

![](media/image1.pct){width="3.375in" height="2.5861111111111112in"}

Fig 1. Cu-Kα XRD spectra of Cu-Ga-In precursors. Phases: 1 - In; 2 -
CuIn ; 3 - Cu~3~Ga; 4 - Cu; 5 - Cu~9~(In~0.64~Ga~0.36~)~4~; 6 - Mo; 7 -
In~2~O~3~.

Precursors were also reacted in H~2~Se at 450ºC for 10, 30, and 90
minutes in atmospheric pressure flowing Ar with 0.35% H~2~Se and 0.0035%
O~2~. The composition of the film determined by EDS with a sampling
depth of 1 -- 1.5 µm showed just Cu, In, and Se and only the ternary
phase CuInSe~2~ was identified by XRD. This is different than previously
reported results where both CuInSe~2~ and CuGaSe~2~ phases were
identified[^1]. In addition, the back sides of the films were
characterized after peeling them from the Mo contact. Analysis of XRD
and EDS measurements indicated the formation of a Cu~9~Ga~4~
intermetallic compound. Figure 2 shows islands on the Mo after a
Cu-Ga-In film selenized for 30 minutes was peeled off. The composition
at the point indicated by the arrow was 52.1 % Cu, 0.0 % In, 27.5 % Ga,
0.8 % Se, 0.0 % S, and 19.6% Mo. The primary difference between this
work and previous results was the use of a Cu-Ga target during
sputtering rather than elemental targets.

Films reacted under similar conditions but in H~2~S were also
characterized. The EDS composition in this case showed the presence of
In and Ga similar to the precursor. After peeling the sample from the
Mo, an unreacted Cu~16~In~9~ intermetallic phase was detected. These
results suggest a preference for In during reaction in H~2~Se and for Ga
in H~2~S.

![](media/image2.pct)

Fig. 2. Intermetallic inclusions remaining on the Mo films after peeling
off a selenized Cu-Ga-In layer. The arrows indicates the point whose
composition measured by EDS is in the text.

A process to form Cu(InGa)(SeS)~2~ by a two step reaction in H~2~Se at
400 -- 450°C followed by reaction in H~2~S at higher temperature has
been shown to produce a uniform Ga distribution[^2]^,^[^3]. The key
process variables to control the film composition are the gas
concentrations and the temperatures and durations of the two reaction
steps. EDS measurements of a film formed by a 15 min. selenization at
450°C followed by a 30 min. sulfization at 550°C gave Cu/(In+Ga) = 0.90,
Ga/(In+Ga) = 0.17, and S/(Se+S) = 0.28, matching the precursor. The
compositional depth profile, shown in Fig. 3, indicates a uniform Ga
profile, but a steep S gradient at the film surface.

Devices with a structure glass/Mo/Cu(InGa)(SeS)~2~/\
CdS/ZnO/ITO/Ni-Al grid were fabricated. Results are shown in Table 1
with this film and another reacted in H~2~S for only 15 min. In each
case, V~OC~ greater than 0.64 V was achieved. The lower FF with the
longer sulfization time may be due to a barrier to the collection of
electrons caused by the steep S gradient. Further investigation of this
process to determine the reaction pathways for film growth and the
homogenization of Ga will be used to optimize the time-temperature-gas
concentration profiles for improved device performance.

![](media/image3.pct){width="3.3409722222222222in" height="2.425in"}

Fig. 3. Auger depth profile of film processed by the two-step
selenization/sulfization reaction.

Table 1. Device parameters using absorbers processed by a two-step
selenization/sulfization process.

+-------------------+---------+------------------+----------+----------+
| H~2~Se/H~2~S      | V~OC~   | J~SC~            | FF       | Eff.     |
|                   |         |                  |          |          |
| (min.)            | \(V\)   | (mA/cm^2^)       | (%)      | (%)      |
+-------------------+---------+------------------+----------+----------+
| 15/15             | 645     | 29.6             | 69.6     | 13.3     |
+-------------------+---------+------------------+----------+----------+
| 15/30             | 641     | 31.6             | 65.1     | 13.2     |
+-------------------+---------+------------------+----------+----------+

**4. Conclusions**

The phases present in Cu-Ga-In precursors after annealing in Ar or Ar
with O~2~ were identified and included a Cu~9~(In~x~Ga~1‑x~)~4~ alloy.
Cu(InGa)(SeS)~2~ films formed by the reaction of Cu-Ga-In precursor
films in H~2~Se or H~2~S were characterized and unreacted intermetallic
Cu-Ga or Cu-In phases, respectively, were identified at the backs of the
films. This indicates a reaction preference between Se and In, and
between S and Ga. Homogenization of the Ga using a two-step
selenization/sulfization process that appears to take advantage of the
reaction preference was confirmed and devices with V~OC~ \> 0.64 V and
eff. \> 13 % were demonstrated.

**ACKNOWLEDGMENTS**

The authors thank Craig Perkins at NREL for Auger measurements. This
work was funded by NREL Subcontract #ADJ-1-30630-12.

**REFERENCES**

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    and J.M. Schultz, *Appl. Phys. Lett.* **67**, 3978 (1995).

[^2]: Y. Nagoya, K. Kushiya, M. Tachiyuki, and O. Yamase, *Solar Energy
    Mat. and Solar Cells* **67**, 247 (2001).

[^3]: V. Alberts, J. Titus, and R. Birkmire, *Thin Solid Films*
    **451-452**, 207 (2004).