Growth of WS2 Thin Photovoltaic Films Matthew Hilfiker,

Growth of WS2 Thin Photovoltaic Films Matthew Hilfiker,

Growth of WS2 Thin Photovoltaic Films

Matthew Hilfiker, Aaron Ediger, and Natale Ianno
Department of Electrical Engineering, University of Nebraska, Lincoln, NE 68508
Introduction

Electrical Characterization Measurements

W

W

Tungsten Target

W

W

+
Ar+

H2S

W
W

Ar+

S

S
W
S

W
S

S
W

WSX

The best results were produced by baking at 900C for 20hrs. The
XRD results are shown below along with the absorption plot. The
peaks seen in the XRD correspond with the (002), (004), and (006)
lattice for WS2 confirming WS2 was produced. The absorption plot
verifies the band gap of WS2 of 1.85eV and no absorption is occurring
below band gap indicating the absence of defect states in our
material. The problem with these films is they do not stick to the
quartz slide.
X-Ray Diffraction WS2 on Quartz
400000

S

WS2 (002)

350000

Film Growth Substrate

Intensity (cps)

300000
250000
200000
150000
100000
50000
WS 2 (004)
WS 2 (006)
0
7 1 5 9 3 7 1 5 9 3 7 1 5 9 3 7 1 5
.0 2.3 4.5 6.7 9.0 1.2 3.5 5.7 7.9 0.2 2.4 4.7 6.9 9.1 1.4 3.6 5.9 8.1
0
1 1 1 1 1 2 2 2 2 3 3 3 3 3 4 4 4 4

250000
200000

X-Ray Diffraction WS2 with Al2O3 Substrate

WS2 (002)

150000
100000
50000
WS2 (004)

WS2 (006)

0
09 .69 .29 .89 .49 .09 .69 .29 .89 .49 .09 .69 .29 .89 .49 .09 .69 .29 .89 .49
.
10 12 15 17 20 23 25 28 30 33 36 38 41 43 46 49 51 54 56 59
2-Theta/Omega (deg)

Conclusion
It was determined that 900C for 20 hrs with an Al 2O3 substrate
was necessary to produce the highest quality WS 2 films. Lower
baking temperatures caused defects to be seen in both the XRD and
absorption data. This production process is the first essential step
into better understanding the properties of WS 2 as a photovoltaic
material. Work will be continued to perform electrical
characterization measurements on these samples and the process of
creating a WS2 solar cell device will be researched.

2-Theta/Omega (deg)

Direct Band Gap Absorption
0.12

Acknowledgments
We acknowledge the UCARE program, the Department of
Electrical Engineering, and the University of Nebraska-Lincoln for
funding the research. I also thank Dr. Natale Ianno and his graduate
students for their guidance and expertise throughout this summer
research program.

0.1

(alpha*E)^.5 (eV/nm)^.5

To grow WS2 films, a reactive sputtering process is
implemented. In reactive sputtering, Argon atoms are ionized
causing them to accelerate towards a negatively charged target.
Upon hitting the Tungsten target, Tungsten atoms will be
discharged and they will react with the gas in the chamber,
Hydrogen Sulfide (H2S). This will then react and form WSX
molecules that will be uniformly deposited onto a quartz slide.
Between 1-5% H2S to Argon concentration and a pressure
between 5-30mT will be tested. These values are adjusted to
optimize the production process.

The XRD results are shown below for 20 hrs. at 900C with an Al 2O3
substrate. The XRD verifies that we can produce the same results as
before but with the different substrate. Using these substrates a film
that was able to fully stick to the substrate was produced.

The UV-VIS spectrophotometer will be used to measure the
absorption of the material. This will be used to verify the band gap of
the material and quality WS2 is being produced.

Results

Deposition Process

Ar+

X-Ray Diffraction (XRD) allows us to verify our film microstructure.
By calculating the intensity in which x-rays diffract across the sample,
we can determine the size and shape of the unit cell, which is unique
for each compound.

Intensity (cps)

Through research for a material that is low cost, earth abundant,
and highly efficient, Tungsten Sulfide (WS2) has been identified as a
promising solution. From literature, it has been shown that WS 2
possesses a band gap of 1.9 eV, which is ideal for a top layer
heterostructure cell. While this is a favorable value, other
properties needed to understand its value as a photovoltaic
material are unknown. Through research, WS2 films will be grown
through a sputtering deposition process and electrical
characterization measurements will be performed to study these
missing properties.

Al2O3 Substrate Results

0.08

0.06

0.04

0.02

4.08
3.93
3.78
3.65
3.52
3.41
3.3
3.2
3.1
3.01
2.93
2.85
2.77
2.7
2.63
2.56
2.5
2.44
2.39
2.33
2.28
2.23
2.18
2.14
2.1
2.05
2.01
1.98
1.94
1.9
1.87
1.84
1.8
1.77
1.74
1.71
1.69
1.66
1.63
1.61
1.58
1.56
1.54
1.51
1.49
1.47
1.45
1.43
1.41
1.39
1.37

0

Energy eV

Baking
The WSX films will be baked in a tube sealed at a pressure of
4.5E-7 Torr. This will be done to prevent oxygen from being
present during this process so the films will be baked to the
lowest energy state of hexagonal WS2. Baking temperatures
will be experimentally tested between 800 and 900C.

Aluminum Oxide (Al2O3) Substrate
Aluminum Oxide substrates will be used instead of quartz to help
the film in sticking. This is because Al2O3 has a thermal expansion
coefficient of /C which is closer to the expansion coefficient of WS 2
than quartz which doesnt expand.

References
Ellmer, K. (2008). Preparation routes based on magnetron sputtering for tungsten
disulfide (WS2) films for thin-film solar cells. Physica Status Solidi b 245 (9)
Ellmer, K., Stock, C., Diesner, K., Sieber, I. (1997) Deposition of C - oriented tungsten
disulfide (WS2) films by reactive DC magnetron sputtering from a W-target in
Ar/H2S. Journal of Crystal Growth. 182 (389-393)
Regula, M., Ballif, C., Remskar, M., Lvy, F. (1997) Crystallinity and texture promotion in
WS2 films. J. Vac. Sci. Technol. A15, 2323
Interlock Safety Inc.; 11 April 2015; Web; 10 April 2016; Image.

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