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Thin Film WSe2 for Use as a Photovoltaic Absorber Material

Article (PDF Available) inMRS Online Proceeding Library Archive 1670 · January 2014with153 Reads
DOI: 10.1557/opl.2014.739
Abstract
An excellent candidate for an earth abundant absorber material is WSe2 which can be directly grown as a p-type semiconductor with a band gap near 1.4 eV. In this work we present the structural, optical, and electrical properties of thin film WSe2 grown via the selenization of sputter deposited tungsten films. We will show that highly textured films with an optical band gap in range of 1.4 eV, and absorption coefficients greater than 105/cm across the visible spectrum can be easily achieved. In addition we will present Hall Effect and carrier density measurements as well, where will show densities in the 1017cm-3 range and p-type Hall mobilities greater than 10 cm2/V-s range can be obtained. We employ these results to numerically simulate solar cells based on this material, where we will show efficiencies greater than 20% are possible.
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Thin Film WSe2 for Use as a Photovoltaic Absorber Material
Qinglei Ma, Hrachya Kyureghian, Joel D. Banninga and N. J. Ianno
Department of Electrical Engineering, University of Nebraska, 209N WSEC Lincoln, NE
68588-0511
ABSTRACT
An excellent candidate for an earth abundant absorber material is WSe2 which can be directly
grown as a p-type semiconductor with a band gap near 1.4 eV. In this work we present the
structural, optical, and electrical properties of thin film WSe2 grown via the selenization of
sputter deposited tungsten films. We will show that highly textured films with an optical band
gap in range of 1.4 eV, and absorption coefficients greater than 105/cm across the visible
spectrum can be easily achieved. In addition we will present Hall Effect and carrier density
measurements as well, where will show densities in the 1017cm-3 range and p-type Hall
mobilities greater than 10 cm2/V-s range can be obtained. We employ these results to
numerically simulate solar cells based on this material, where we will show efficiencies greater
than 20% are possible.
INTRODUCTION
Tungsten selenide was first pointed out as a possible photovoltaic absorber in an early
compilation of band gaps for some materials. [1] Some of the early studies record a band gap of
1.35 eV, very close to the optimal value of 1.36 eV for a single-junction solar cell. Recently the
single crystal band gaps have been determined to be 1.545 eV and 1.486 eV for the direct and
indirect single crystal band gaps respectively. [2] In addition it has only one stable crystalline
structure at room temperature. [2] Also W and Se do not form a stable compound of any
stoichiometry other than WSe2. It should be mentioned that amorphous WSe3 decomposes to
WSe2 at 220 oC. [3] Bulk crystals have been doped p and n-type, leading to the possibility of
homojunction devices, and some heterojunction single crystal photovoltaic devices with
efficiencies in excess of 8 % have been fabricated. [4]
Several methods were adopted to prepare WSe2 thin films in previous research. [5-10] In this
paper, WSe2 thin films were prepared by selenization of tungsten films under 1 atmosphere
selenium pressure in a closed tube in a single-zone furnace. All WSe2 thin films synthesized
under different temperatures are highly oriented with the c axis predominantly perpendicular to
the substrate. The mobility of WSe2 thin films varied with processing temperature and rises to 30
cm2/V-sec at 875 oC.
In view of these favorable attributes we have employed PC1D to simulate the performance of
homojunction photovoltaic devices based on WSe2. The input parameters for the simulation are
obtained through a combination of existing literature and direct measurements performed on thin
films grown as described above. We will show that homojunctions devices have the potential to
exceed 20 % efficiency.
EXPERIMENT
Film growth
Tungsten films of about 100 nm thickness were DC-sputtered from a 99.5% pure tungsten
target onto 15 mm x15 mm quartz substrates at room temperature. The substrates were
thoroughly cleaned prior to insertion into the vacuum chamber with a base pressure of 1.33 x10-4
Pa. Before the tungsten films were deposited onto the substrates the target itself was pre-
sputtered for 5-10 minutes to clean away oxides on its surface. The operating argon pressure was
kept constant at 0.133 Pa during sputtering. The sputtering power was set to 40 Watt yielding a
deposition rate of about 3 nm/s. In order to obtain a uniform film, the substrate was rotated at 20
revolutions per minute.
A tungsten film and selenium powder (99.999%) were sealed into a quartz tube evacuated to
1.33x10-5 Pa. The amount of selenium was controlled to yield 1 atmosphere of Se pressure at the
process temperature. The sealed tubes were placed into a room temperature furnace and ramped
to the process temperature at a rate of 4 C/min, soaked for 20 hours and ramped down to room
temperature at the same rate. Processing temperatures from 825 oC to 900 oC with a step of 25 oC
were investigated. The results presented here clearly show that a single zone, single step process
can produce high quality WSe2 films.
Raman spectroscopy
In order to determine lattice dynamics of WSe2 thin films, Raman spectra of both a WSe2
commercially purchased single crystal and our films are investigated by Raman spectroscopy
employing a 633 nm excitation laser and 2400 mm-1 grating. Figure 1 shows the Raman spectra
of the single crystal and the films processed at 875 oC and 900 oC. Both the E2g1 and A1g modes,
are present in the crystal and the films. [11]
Figure 1. Raman Spectra of WSe2 crystal and thin films.
X-ray diffractometry
The x-ray diffraction patterns (CuKα anode) seen in Figure 2 indicate that the films exhibited
the 2H-WSe2 hexagonal structure. The three primary peaks in the spectrum correspond to the
(002), (006), (008), showing that the c axis is perpendicular to the substrate. It should be noted
that the double peak at the (002) plane is a result of the fact that the x-ray source is not
monochromatic so a peak from the CuKα2 emission line is observed. The average grain size as
determined by the Debye Scherrer formula is approximately 80nm for both films.
Figure 2. X-ray diffraction patterns of the WSe2 thin films.
Optical characterization
In this work, variable angle spectroscopic ellipsometry characterization of a WSe2 single
crystal sample and WSe2 thin films at three angles (55o , 65o , 75o) across the 350 nm-1700 nm
wavelength was performed. The thickness of the tungsten selenide film is determined by
profilometry and is about four times thicker than the original tungsten film. The measured film
thickness was employed in the ellipsometry analysis where the final film thicklness was within
10% of the measured value. The MSEs are between 12 and 17 for different WSe2 thin films. The
dispersion of the refractive index n and extinction coefficient k of WSe2 thin films, as well as
WSe2 crystal, are shown in Fig. 3. We can observe that all films share the similar n and k curve
shapes with WSe2 crystal. The extinction coefficient k is in good agreement with previous
results.[7]
Figure 3. The dispersion of the refractive index n (a) and extinction coefficient k (b) of WSe2
thin films, and single crystal.
Transmission data was also acquired for the films, and this data was used to generate the
abosprtion coefficient. The absorption coefficient was also generated from the extinction
coefficient obtained through ellipsometry. Tauc plots to determine the dierct and indirect band
gaps were generated from both sets of abosrption coefficients as seen in Figure 4.
Figure 4. Tauc plots of a WSe2 thin film from both transmittance and ellipsometry data. (a)
Direct band gap, (b) Indirect band gap. The WSe2 thin film is made at 875 oC.
As can be seen the figure the band gaps differ by about 5% with ellipsometry yielding a
smaller gap.
Electronic characterization
Electrical transport properties, including the mobility, resistivity and carrier concentration,
are investigated by performing Hall Effect and four point probe measurements. All tests are
performed at room temperature. A magnetic field of 1.3 T is employed while silver paint is used
as the metal contact. All films are as-grown p-type where those grown at 875 oC and 900 oC
exhibit Hall mobilities of 30 cm2V-1sec-1 and a carrier concentrations of around 1017 cm-3.
Photovoltaic device simulation
In order to obtain an idea of the potential device performance PC1D was employed to
simulate an ideal homojunction device consisting of a 1 micron thick p-type absorber layer,
p=1017 cm-3, µ = 30 cm2V-1sec-1 (data obtained from the as-grown films), with the band gap and
absorption coefficient also obtained from the as-grown films. A 50 nm WSe2 n-type layer, n=
1019 cm-3, µ = 1 cm2V-1sec-1 was employed as the window. The illumination was set at 1.5AMG.
All other needed parameters were taken from the literature, determined from as-grown films or
calculated from the literature data. [10-15] Figure 5 shows the results of the simulation where the
efficiency is plotted as a function of the carrier lifetime. For lifetimes in the ns range the
efficiency is between 10 and 15%. A well designed heterojunction may yield even higher
efficiency.
Figure 5. PC1D simulation of WSe2 homojunction photovoltaic device under 1.5AMG
illumination.
CONCLUSIONS
Thin films of WSe2 have been grown in a single step by single zone selenization of sputter
deposited tungsten films. When grown under 1 atmosphere of selenium pressure at 875 or 900 oC
p-type films where p=1017 cm-3, and µ = 30 cm2V-1sec-1 are reproducibly obtained. In addition
the absorption coefficient is above 105 cm-1 over the 400-900nm spectral range and the resultant
indirect band gap as determined by spectroscopic ellipsometry is 1.36 eV and the direct band gap
is 1.51 eV. Simulation of a homojunction device based on the parameters of the as-grown films
and literature values yields an efficiency of 10% for a 1 ns bulk carrier lifetime increasing up to
values greater than 20% for long lifetimes.
ACKNOWLEDGMENTS
This work is supported by NSF grant 1004094 and the Nebraska Center for Sciences
Research.
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