of 8
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et al
.,
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2019;
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: eaax6061 20 December 2019
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APPLIED PHYSICS
A new metal transfer process for
van der Waals contacts
to
vertical Schottky-junction transition metal
dichalcogenide photovoltaics
Cora M.
Went
1,2
, Joeson
Wong
3
, Phillip R.
Jahelka
3
, Michael
Kelzenberg
3
, Souvik
Biswas
3
,
Matthew S.
Hunt
4
, Abigail
Carbone
5
, Harry A.
Atwater
2,3,6
*
Two-dimensional transition metal dichalcogenides are promising candidates for ultrathin optoelectronic devices
due to their high absorption coefficients and intrinsically passivated surfaces. To maintain these near-perfect surfaces,
recent research has focused on fabricating contacts that limit Fermi-level pinning at the metal-semiconductor
interface. Here, we develop a new, simple procedure for transferring metal contacts that does not require aligned
lithography. Using this technique, we fabricate vertical Schottky-junction WS
2
solar cells, with Ag and Au as asym-
metric work function contacts. Under laser illumination, we observe rectifying behavior and open-circuit voltage
above 500 mV in devices with transferred contacts, in contrast to resistive behavior and open-circuit voltage
below 15 mV in devices with evaporated contacts. One-sun measurements and device simulation results indicate
that this metal transfer process could enable high specific power vertical Schottky-junction transition metal
dichalcogenide photovoltaics, and we anticipate that this technique will lead to advances for two-dimensional
devices more broadly.
INTRODUCTION
Two-dimensional (2D) semiconducting transition metal dichal-
cogenides (TMDs), including MoS
2
, WS
2
, MoSe
2
, and WSe
2
, are
promising for many optoelectronic applications, including high
specific power photovoltaics (
1
5
). With absorption coefficients one
to two orders of magnitude higher than conventional semiconductors,
monolayer (<1 nm thick) TMDs can absorb as much visible light as
about 15 nm of GaAs or 50 nm of Si (
6
). Both multilayer and mono-
layer TMDs can achieve near-unity broadband absorption in the
visible range (
7
,
8
). Because of their layered structure and out-of-plane
van der Waals bonding, TMDs have intrinsically passivated surfaces
with no dangling bonds and can form heterostructures without the
constraint of lattice matching.
To take advantage of the intrinsically passivated surfaces of TMDs,
gentle fabrication techniques are needed to form metal contacts
without damaging the underlying semiconductor. A number of new
contact techniques have been presented recently, including 1D edge
contacts (
9
), via contacts embedded in hBN (
10
), slowly deposited
In/Au contacts (
11
), and 2D metals (
12
). Recently, Liu
et al.
(
13
)
have shown that transferring rather than evaporating metal contacts
onto TMDs can yield interfaces with no Fermi-level pinning, where
the Schottky barrier height can be predicted by the ideal Schottky-
Mott rule. Their work demonstrates the use of transferring an arbitrary
3D metal onto a 2D material, forming a nondamaging van der Waals
contact. However, this technique requires a final aligned lithography
step to expose the contact under the polymer used for transfer, which
limits its scalability (
13
).
To date, the above techniques for gentle contact fabrication have
been applied to device geometries where carriers are collected laterally
rather than vertically. Laterally contacted TMD devices use contact
spacing on the order of 5
m, which would be prohibitively expensive
to fabricate for large-area, low-cost photovoltaic devices. Although
laterally contacted devices are important for electronic applications,
such as field effect transistors, vertically contacted devices are pref-
erable for optoelectronic applications that require scalable photo-
active areas, such as solar cells. Van der Waals contacts could have
an even greater advantage for these vertical device geometries, where
the ratio of contact area to device area is often higher than in lateral
device geometries.
Schottky-junction solar cells represent one specific device geometry
where van der Waals metal contacts could enable high performance in
vertical devices. Although vertical-junction solar cells are more aligned
with conventional photovoltaics (
14
), most Schottky-junction TMD
solar cells studied have been lateral-junction devices (
13
,
15
,
16
).
Vertical Schottky-junction TMD solar cells have been limited by
ohmic current-voltage (
I
-
V
) behavior, low external quantum effi-
ciencies, and low open-circuit voltages, likely due to Fermi-level
pinning induced by contact evaporation (
7
,
17
). New gentle contact
fabrication techniques have the potential to eliminate this Fermi-
level pinning, enabling high-efficiency vertical TMD solar cells in
the Schottky-junction geometry.
Here, we develop a simple technique for transferring metal
contacts, where all lithographic patterning is done on a donor
substrate rather than on the active device. We apply this technique
to vertical Schottky-junction solar cells with multilayer TMD ab-
sorber layers. Because of the trade-off between bandgap energy and
photoluminescence quantum yield, the theoretical maximum power
conversion efficiency achievable for multilayer and monolayer single-
junction solar cells is similar (
4
,
18
), and further, tunneling limits trans-
port in monolayer vertical devices (
19
), so we focus on multilayer
1
Department of Physics, California Institute of Technology, Pasadena, CA 91125, USA.
2
Resnick Sustainability Institute, California Institute of Technology, Pasadena, CA
91125, USA.
3
Thomas J.
Watson Laboratory of Applied Physics, California Institute
of Technology, Pasadena, CA 91125, USA.
4
Kavli Nanoscience Institute, California
Institute of Technology, Pasadena, CA 91125, USA.
5
Department of Materials Science
and Engineering, North Carolina State University, Raleigh, NC 27695, USA.
6
Joint
Center for Artificial Photosynthesis, California Institute of Technology, Pasadena, CA
91125, USA.
*Corresponding author. Email: haa@caltech.edu
Copyright © 2019
The Authors, some
rights reserved;
exclusive licensee
American Association
for the Advancement
of Science. No claim to
original U.S. Government
Works. Distributed
under a Creative
Commons Attribution
NonCommercial
License 4.0 (CC BY-NC).
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et al
.,
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2019;
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devices in this work. Ultrathin (10- to 20-nm) WS
2
forms the absorber
layer, while Ag and Au form the asymmetric work function contacts.
Devices made with transferred metal contacts show diode-like
I
-
V
behavior with a near-unity ideality factor and high
V
OC
, while similar
devices made with evaporated metal contacts show ohmic
I
-
V
be-
havior and near-zero
V
OC
. We demonstrate peak external quantum
efficiency (EQE) of >40% and peak active-layer internal quantum
efficiency (IQE
active
) of >90% in transferred-contact devices. Using
a solar simulator, we measure a photovoltaic power conversion effi-
ciency of 0.46%, comparable to what has been seen in ultrathin
vertical TMD photovoltaics with a p-n junction rather than a Schottky
junction (
20
,
21
). Device simulations of further-optimized geometries
suggest that this new metal transfer process has the potential to
enable Schottky-junction TMD solar cells with power conversion ef-
ficiencies greater than 8% and specific powers greater than 50 kW/kg.
RESULTS AND DISCUSSION
Fabrication of
vertical WS
2
Schottky-junction solar cells
We prepare vertical WS
2
Schottky-junction solar cells made from
16-nm-thick WS
2
absorber layers, with Ag (
Ag
≈ 4.3 eV) and Au
(
Au
≈ 5.1 eV) as asymmetric work function contacts (Fig. 1A) (
13
).
Template-stripped Ag, which exhibits a root-mean-square roughness
<0.5 nm, forms both the electron-collecting bottom contact and back re-
flector for all devices (
22
). We mechanically exfoliate WS
2
directly onto the
Ag substrate. The subwavelength-thick WS
2
achieves broadband,
angle-insensitive absorption on top of the highly reflective Ag, giving
the WS
2
a deep purple color (
7
,
23
). For transferred-contact devices,
we transfer thin Au disks from a thermally oxidized Si donor sub-
strate to form the semitransparent hole-collecting top contact,
using the process described in the following section. Both the top
surface of the template-stripped Ag and the bottom surface of the
transferred Au inherit the smoothness of the SiO
2
/Si donor substrate,
leading to near atomically sharp metal-WS
2
interfaces (
13
,
22
). Using
cross-sectional analysis by transmission electron microscopy (TEM),
we examine the interface between the transferred Au and the WS
2
(Fig. 1B). We find that in contrast to depositing Au via electron-beam
evaporation (
13
), transferring Au does not damage the intrinsically
passivated WS
2
layers, as evidenced by the columns of atoms visible
in the TEM image. Figure 1C shows an optical image of a completed
device. For comparison, we also fabricate devices by direct evapo-
ration of thin Au disks onto the WS
2
using standard photolithography
techniques.
The ideal band diagram of this Schottky-junction solar cell is
shown in Fig. 1D. We assume a doping concentration of 10
14
cm
−3
for WS
2
, as provided by the bulk crystal vendor. Since the length of
the depletion region at a Schottky junction between bulk WS
2
and
either Au or Ag is on the order of 1
m, the device is fully depleted.
We measure the final thicknesses of the WS
2
and the Au to be 16 and
19 nm, respectively, using atomic force microscopy.
Metal transfer process
We develop a new, simple process for transferring metal contacts onto
TMDs (Fig. 2). This process relies on a self-assembled monolayer (SAM)
A
B
D
Va
cuum level
φ
Au
φ
Ag
χ
TMD
E
F
E
C
E
V
Ag substrate
19 nm
Au
16 nm WS
2
C
WS
2
Au
Ag
WS
2
20 μm
2 nm
Transferred
Au
WS
2
Substrate
Fig. 1. Vertical Schottky-junction multilayer WS2 solar cells with transferred contacts.
(
A
) Bottom: Schematic of device structure. Top contact is a transferred gold
disk; bottom contact and back reflector is template-stripped silver. Top: 3D representation of multilayer WS
2
. W, blue spheres; S, gray spheres. (
B
) Cross-sectional image
of metal-semiconductor interface captured by transmission electron microscopy (TEM). (
C
) Optical image of device. (
D
) Solar cell band diagram obtained from electro-
static simulations.
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to reduce the adhesion between the Au and the SiO
2
/Si donor substrate
(
24
), a thermoplastic polymer to preferentially pick up or drop down
the metal (
25
), and a variable peeling rate to tune the velocity-
dependent adhesion between a metal and a viscoelastic stamp (
24
).
Briefly, we create a SAM on clean thermally oxidized Si chips in
a vacuum desiccator (
24
). We then deposit 20 nm of Au via electron-
beam evaporation. Using photolithography, we define the contact
areas with positive photoresist and a positive photomask. We etch
the Au outside the masked contact areas and then dissolve the
remaining photoresist in acetone, leaving Au disks on the SAM-
coated SiO
2
/Si substrates. We prepare a polydimethylsiloxane (PDMS)
stamp coated with the thermoplastic polymer polypropylene car-
bonate (PPC) on a glass slide (
25
). In a 2D transfer setup, we align
and slowly lower the stamp onto a contact at 60°C.
We set the tem-
perature to 40°C, and once the stage reaches that temperature, we
raise the transfer arm rapidly to peel the stamp and pick up the con-
tact. We align the contact with the target TMD and slowly lower the
stamp down at 60°C, and then slowly peel it away immediately after
contact at the same temperature. The contact delaminates from
the PDMS/PPC stamp and sticks to the TMD.
Further details of the
procedure are provided in section S1.
This metal transfer technique has worked in 15 of 16 devices fab-
ricated thus far (>90% yield). It works for both 20- and 100-nm-thick
Au and can likely be extended to other metals and to larger-scale
contacts [i.e., for contacts to chemical vapor deposition (CVD)–grown
TMDs]. A substantial advantage of this technique is that, whereas
prior metal transfer techniques require a final aligned electron-beam
lithography step to expose the contact area (
13
), this technique only
uses unaligned photolithography to define the initial contacts on the
SiO
2
/Si donor substrate. This allows for batch fabrication of an
array of contacts that can then be picked up, aligned, and printed
to form multiple devices. Further, this metal transfer process could
enable van der Waals contacts to air- and moisture-sensitive nano-
materials, such as lead halide perovskites or black phosphorus, to be
formed without removing the sample from an inert environment.
Comparison of
transferred and
evaporated metal contacts
We measure
I
-
V
curves under illumination with a 633-nm laser fo-
cused to a ~1-
m
2
spot in a confocal microscope at room temperature.
In devices with transferred metal contacts, we observe rectifying
I
-
V
curves and a pronounced photovoltaic effect (Fig. 3A). We measure
a
V
OC
of 510 mV under the maximum laser excitation. Short-circuit
current follows a power law as a function of incident power,
I
SC
=
P
inc
,
with
close to 1 (Fig. 3C). According to the diode equation,
V
OC
scales linearly with ln(
I
SC
) and can be fit with an ideality factor
n
= 1.2 (Fig. 3D). This near-unity ideality factor confirms the high
interface and material quality in these devices. The ideality factor,
diode-like behavior, and high open-circuit voltage suggest that a
Schottky junction is successfully formed in devices with transferred
contacts.
In contrast, we observe resistive behavior and a small photovoltaic
effect in devices with evaporated top metal contacts (Fig. 3B).
I
SC
versus
P
inc
follows a power law with
< 1 (Fig. 3E). As shown in
Fig. 3F, this device behaves as a resistor with
R
= 3.1 kilohms. At
comparable laser powers,
V
OC
is around 4 mV in evaporated-contact
devices and 400 mV in transferred-contact devices, and
J
SC
is three
to four times higher for transferred contacts than for evaporated
contacts. Previous work demonstrates that because of Fermi-level
pinning, evaporated Au and transferred Ag have effectively the same
barrier height for electrons and holes (
13
). Assuming an effective
work function difference between Au and Ag of 50 meV, device
simulations can predict the purely resistive behavior in an evaporated-
contact Schottky-junction device (fig. S1). This evidence points to
strong Fermi-level pinning in devices with evaporated contacts due
to interface states induced by the Au evaporation.
In devices with transferred contacts, the slope of the
I
-
V
curve at
short circuit increases linearly with increasing laser power, corre-
sponding to a decreasing shunt resistance (fig. S2A). This photo-
shunting effect occurs in solar cells without perfectly selective contacts
due to increased minority carrier conductivity across the device under
illumination (
26
,
27
). Device simulations can replicate this photoshunt
pathway without the addition of any external shunt resistance (fig. S2B).
In future devices, the introduction of contacts with greater carrier
selectivity or the combination of a p-n junction with a Schottky junction
could reduce or eliminate the photoshunting observed here.
Quantum efficiency and
photocurrent generation
Light beam induced current (or photocurrent) maps, acquired with
a 633-nm laser in a confocal microscope, show uniform current
generation under the entire Au disk contact, except where shaded
by the contact probe (Fig. 4A). The uniformity of the photocurrent
demonstrates that the Au is homogeneously semitransparent and in
(IV) Etch
Au, dissolve resist
(V) Peel
(VI) Print
Au
PDMS/PPC
Device
(I) Form SAM
(II) Deposit
Au
(III) Photolithography
SAM
SiO
2
/Si
Photoresist
Fig. 2. Metal transfer process.
Briefly, a self-assembled monolayer (SAM) is applied
to a clean SiO
2
/Si substrate (I). Au is deposited in an electron beam evaporator (II).
Disk contacts are defined using photolithography (III), and the surrounding Au is
etched away (IV). To peel the contacts, a polydimethylsiloxane (PDMS)/polypropylene
carbonate (PPC) stamp is laminated to the contacts, heated above the glass transition
temperature of PPC, and then cooled and removed quickly (V). To print the contacts,
the PDMS/PPC stamp is aligned and laminated onto the device and then peeled
away slowly above the glass transition temperature of PPC, leaving the contacts
behind (VI).
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RESEARCH ARTICLE
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good contact with the TMD.
This indicates that the area of the Au
disk can be used to accurately define the device active area (fig. S3) and
suggests that 1D device simulations are sufficient to describe the
behavior in these vertical devices (
28
). Further, it demonstrates that
there are no visible bubbles created during the metal transfer process.
The measured total absorption (Fig. 4C) matches well with the
absorption calculated using the transfer matrix method (Fig. 4B),
as has been previously demonstrated in TMD solar cells (
7
,
17
). To
calculate the active-layer absorption in the experimental WS
2
devices,
we subtract the simulated parasitic absorption (the sum of the Au
and Ag curves in Fig. 4B) from the experimentally measured total
absorption in Fig. 4C. The mean active-layer absorption from 450 to
650 nm is 39%. The reduced absorption in our devices relative to
what has been previously demonstrated in WS
2
on a metal back
reflector (~80% over this wavelength range) (
7
) is due to parasitic
absorption and reflection losses from the 19-nm-thick Au top contact.
Using a more transparent top contact could double our photogenerated
current, assuming identical work function and conductivity.
The EQE of the device follows the spectral shape of absorption
well, averaging 28% from 450 to 650 nm and reaching a peak of
above 40% around 550 nm (Fig. 4D). To accurately determine the
EQE, we multiply by a shading factor of 1.39 to correct for shading
from the probes (see Materials and Methods). The IQE remains
relatively flat across all wavelengths above the bandgap, averaging
49% from 450 to 650 nm (Fig. 4E). The IQE
active
, calculated by
dividing the EQE by the active-layer absorption rather than the
total absorption, is greater than 90% at its peak and averages 74%
between 450 and 650 nm (Fig. 4F). This high IQE
active
suggests efficient
collection of photogenerated carriers in transferred-contact devices.
Performance under one-sun illumination
Vertical Schottky-junction WS
2
solar cells with transferred top contacts
achieve reasonable photovoltaic performance when measured under
simulated AM1.5G illumination. Figure 5 shows the AM1.5G
I
-
V
behavior of a representative device. We divide the measured current
by the device active area to yield current density, and then further
divide by a factor of 0.67 to account for spectral mismatch between
our solar simulator calibration point and the true AM1.5G spectrum
(see Materials and Methods; fig. S4) (
29
). The spectral mismatch cor-
rection leads to a 50% increase in short-circuit current, so the
V
OC
and power conversion efficiency of the device are likely underestimated
here. We measure a
V
OC
of 256 mV, a corrected
J
SC
of 4.10 mA/cm
2
,
a fill factor of 0.44, and a power conversion efficiency of 0.46%. This
efficiency is in the range of what others have reported for ultrathin
TMD photovoltaics (
16
,
17
,
20
,
21
). Using the densities of Au, WS
2
,
and Ag, we estimate a specific power of 3 kW/kg for this device.
By fitting the one-sun
I
-
V
curve using the diode equation with
series and shunt resistances, we estimate a shunt resistance (
R
SH
)
of 231 ohm cm
2
and a negligible series resistance (
R
S
), as shown in
fig. S5. The shunt resistance is likely due to the photoshunting
behavior discussed above and could be reduced by design and
realization of contacts that are more carrier selective.
−0.01
−0.01
−0.005
00
.005
0.01
0.015
Voltage (V)
−10
−5
0
5
10
Current (μ
A)
Dark
970 W/cm
2
2400 W/cm
2
4800 W/cm
2
9700 W/cm
2
−0.6
−0.4−
0.20
0.20
.4
0.6
Voltage (V)
−20
−10
0
10
20
Current (μ
A)
Dark
970 W/cm
2
2400 W/cm
2
4800 W/cm
2
9700 W/cm
2
10
−2
10
−1
10
0
I
SC
(
μ
A)
0
5
10
15
V
OC
(mV)
R
= 3.11 kilohms
10
0
10
1
I
SC
(
μ
A)
350
400
450
500
V
OC
(mV)
n
= 1.2
I
0
= 1.31 pA
10
2
10
4
Power (W/cm
2
)
10
−1
10
0
10
1
I
SC
(
μ
A)
= 0
.86
10
2
10
4
Power (W/cm
2
)
10
−1
10
0
10
1
I
SC
(
μ
A)
= 0.98
Transferred contacts
AB
C
DE
F
Evaporated contacts
I
SC
R
I
SC
n
,
I
0
Fig. 3. Comparison of devices with transferred and directly evaporated top metal contacts.
(
A
and
B
) Power-dependent
I
-
V
characteristics of devices with trans-
ferred (A) and evaporated (B) Au top contacts taken under excitation with a 633-nm laser focused to a spot size of ~1
m
2
. (
C
and
E
) Short-circuit current of devices with
transferred (C) and evaporated (E) Au contacts. Symbols, measurements; line, power law fit. (
D
and
F
) Open-circuit voltage of devices with transferred (D) and evaporated
(F) Au contacts. Symbols, measurements; line, fit. Insets show representative circuit diagrams.
n
is the ideality factor, and
I
0
is the dark saturation current extracted from
the diode fit in (D).
R
is the resistance extracted from the linear fit in (F).
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This photovoltaic performance is consistent among multiple mea-
surements and devices. The
J
SC
of 4.10 mA/cm
2
that we measure
with the solar simulator is within 10% of the
J
SC
that we calculate by
integrating the EQE over the solar spectrum (4.55 mA/cm
2
). We
believe that probe shading, which we correct for in EQE measure-
ments but not in solar simulator measurements, accounts for the
10% discrepancy. Although the
J
SC
varies because of differences in
thickness and, therefore, absorption in exfoliated flakes, the
V
OC
is
replicable across all devices fabricated for this work. As shown in
figs. S6 and S7,
V
OC
is between 220 and 260 mV in all four devices
measured under one-sun illumination, and
V
OC
is greater than 220 mV
in six different devices measured under illumination with a halogen
lamp (~20 suns power density). The
I
-
V
curves show no hysteresis
when swept in the forward and backward directions (fig. S8).
Simulated performance of
optimized devices
To examine and further optimize the performance of these devices,
we simulate a variety of device geometries. The assumed material
parameters of the WS
2
are detailed in table S1. Simulating the same
450
500
550
600
65
07
00
Wavelength (nm)
0
0.2
0.4
0.6
0.8
1
IQE
Active
Total
450
500
550
6006
50
700
Wavelength (nm)
0
0.2
0.4
0.6
0.8
1
EQE
450
500
550
600
65
07
00
Wavelength (nm)
0
0.2
0.4
0.6
0.8
1
Absorption
Active
Total
450
5005
50
600
650
700
Wavelength (nm)
0
0.2
0.4
0.6
0.8
1
Absorption
Au
WS
2
Ag
Total
DE
A
20 μm
Experimental
C
B
Simulated
Reflection
Overlay
LBIC
Fig. 4. Photocurrent and quantum efficiency.
(
A
) Confocal reflection, photocurrent, and reflection/photocurrent overlay maps for the device shown in Fig.
1. The dark
spot in the lower left part of the device active area is a probe tip artifact. (
B
) Simulated total absorption in the device and absorption in each device layer. (
C
) Experimental
total absorption in the device and active-layer absorption calculated by subtracting the simulated parasitic absorption from the experimentally measured total absorption.
(
D
) Measured EQE. (
E
) Internal quantum efficiency (IQE) calculated from external quantum efficiency (EQE) and absorption; active-layer IQE calculated from EQE and
active-layer absorption.
AM1.5G Illumination
−0.1
0
0.1
0.
20
.3
Voltage (V)
−1
0
1
2
3
4
5
6
7
Current density (mA/cm
2
)
V
oc
= 256 mV
J
sc
= 4.10 mA/cm
2
FF = 0.44
PCE = 0.46%
Fig. 5. Photovoltaic performance under one-sun illumination.
I
-
V
characteristics of a
vertical Schottky-junction multilayer WS
2
solar cell measured using an AM1.5G solar simu-
lator, corrected for spectral mismatch. FF, fill factor. PCE, power conversion efficiency.
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device geometry as our experimental device yields the
I
-
V
curve in
Fig. 6A. The simulated
J
SC
of 5.7 mA/cm
2
is consistent with our
measured active-layer IQE of 74% and the
J
SC
of 4.55 mA/cm
2
estimated from the EQE.
The simulated
V
OC
of 646 mV and
R
SH
of 2240 ohms cm
2
are considerably higher than the
V
OC
of 256 mV
and
R
SH
of 231 ohms cm
2
observed in our one-sun measurements.
This demonstrates that with further optimization, our device geometry
could achieve higher voltages and less shunting than we currently
see (fig. S9). As a first improvement, we suggest replacing Au with a
different high work function metal, as Au is known to form thiol
bonds with sulfides that could affect the quality of the van der Waals
contact (
13
,
30
).
To identify a potential path toward high-efficiency vertical Schottky-
junction WS
2
solar cells, we simulate a series of optimized devices
(Fig. 6B). The use of an optimized thickness of WS
2
(26 nm) for max-
imum absorption under 20 nm of Au increases the
J
SC
to 7.1 mA/cm
2
.
The
J
SC
can be further increased to 12.5 mA/cm
2
by replacing Au with
a transparent top contact, assuming an identical work function to
Au and no parasitic absorption or reflection. By selecting metal work
functions that are optimally aligned to the conduction and valence
bands of WS
2
(
1
= 4.05 and
2
= 5.2 eV; e.g., In and Pd), we predict
a
V
OC
increase of 230 mV.
Combining transparent top contacts and
optimized metal work functions yields the device shown in Fig. 6C,
with a
V
OC
of 898 mV, a
J
SC
of 12.7 mA/cm
2
, a fill factor of 0.78, and a
power conversion efficiency of 8.9%. This simulated power conversion
efficiency in a device with a thickness <150 nm represents a specific
power of 58 kW/kg, demonstrating that this metal transfer process
has the potential to enable devices with an unprecedented power-
per-unit-weight ratio for transportation and aerospace applications.
CONCLUSIONS
We develop here a process for transferring metal contacts with near-
atomically smooth interfaces that has high yield, allows for batch
fabrication, and eliminates aligned lithography. We expect that this
procedure will be highly relevant and useful to the 2D community
and to researchers working on air-sensitive nanomaterials, as it
allows all processing to be done on the contacts rather than the de-
vice. By applying this new technique to vertical Schottky-junction
TMD solar cells, we demonstrate that transferred contacts are par-
ticularly advantageous for vertical device geometries, which are
important for photovoltaic and other optoelectronic applications due
to their scalable active areas. Recent advances in techniques such as
growth of wafer-scale 2D TMDs via CVD (
31
33
) and pickup and
stacking of large-area van der Waals materials (
34
) will enable the
scaling of TMD solar cells from the micrometer to the wafer scale.
The rectifying
I
-
V
curves shown in transferred-contact devices and
resistive
I
-
V
curves shown in evaporated-contact devices support the
hypothesis that transferring contacts can reduce Fermi-level pinning
and allow the work function asymmetry between the contacts to
define the maximum achievable
V
OC
. We observe active-layer ab-
sorption >55%, EQE >40%, and active-layer IQE >90% in these
devices, demonstrating efficient collection of photogenerated carriers.
Under one-sun illumination, we measure a
V
OC
of 256 mV, a
J
SC
of
4.10 mA/cm
2
, a fill factor of 0.44, and a power conversion efficiency
of 0.46%. We highlight areas for improvement by simulating the
behavior of optimized devices based on this architecture and show
8.9% simulated efficiency and 58-kW/kg simulated specific power
in a device with transparent top contacts, optimized thickness, and
ideal metal work functions for carrier extraction.




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Fig. 6. Simulated performance of optimized devices.
(
A
) Simulated
I
-
V
characteristics of the device geometry used in our experiments, assuming no external series/
shunt resistances. (
B
) Simulated
V
OC
,
J
SC
, fill factor (FF), and power conversion efficiency (PCE) for optimized devices. Apart from the final device geometry (“Optimized
WS
2
device”), optimizations are independent, not cumulative. (
C
) Simulated
I
-
V
characteristics of the fully optimized WS
2
device.
Went
et al
.,
Sci. Adv.
2019;
5
: eaax6061 20 December 2019
SCIENCE ADVANCES
|
RESEARCH ARTICLE
7 of 8
Given the proof-of-concept performance and the clear pathways
for improvement presented here for devices less than 150 nm thick,
ultrathin vertical Schottky-junction TMD solar cells with transferred
contacts are promising candidates for high specific power photo-
voltaic applications. We anticipate that this new metal transfer process
will enable similar advances for 2D TMD devices beyond Schottky-
junction solar cells and for nanomaterial-based devices more broadly.
MATERIALS AND METHODS
Device fabrication
Template-stripped silver substrates were prepared, as described
previously (
17
,
22
). WS
2
was mechanically exfoliated directly onto
template-stripped silver from the bulk crystal (HQ Graphene) using
Scotch tape. For transferred-contact devices, Au top contacts were
prepared and transferred using the metal transfer technique summa-
rized in the main text and described in detail in section S1. The SAM
used was trichloro(1
H
,1
H
,2
H
,2
H
-perfluorooctyl)silane (Sigma Aldrich),
the photoresist used was S1813, and the Au etchant used was Transene
Gold Etchant TFA.
For evaporated-contact devices, Au top contacts
were patterned using standard photolithography techniques as
described previously (
17
). Contacts were fabricated on WS
2
within
12 hours of exfoliation. Final WS
2
and Au thicknesses were con-
firmed using atomic force microscopy (Asylum Research).
Cross-sectional analysis by TEM
Site-specific, cross-sectional lamella samples were prepared near the
middle of metal contacts using a Nova 600 NanoLab (Thermo Fisher)
scanning electron microscope with a gallium focused ion beam and
an AutoProbe 200 sample lift-out system (Oxford Instruments).
TEM imaging was carried out in a Tecnai TF-30 (Thermo Fisher
Scientific) operated at 300 kV in high-resolution TEM mode.
Photocurrent and power-dependent
I
-
V
Photocurrent and power-dependent
I
-
V
were measured on a scan-
ning confocal microscope (Zeiss Axio Imager 2) using a long work-
ing distance objective [50×; numerical aperture (NA), 0.55]. Devices
were contacted using piezoelectrically controlled micromanipulators
(MiBots, Imina Technologies).
I
-
V
curves were measured with a
Keithley 236 Source-Measure Unit using custom LabView programs.
Laser powers were measured using a USB power meter (ThorLabs).
All measurements were performed under ambient temperature
and pressure.
Absorption and EQE
Absorption and EQE were measured using a home-built optical
setup with a long working distance objective (50×; NA, 0.55). A
supercontinuum laser (Fianium) was coupled to a monochromator
to produce a tunable, monochromatic light source. A chopper and
lock-in detection were used for all measurements. For absorption,
the sample reflectance was measured using a National Institute
of Standards and Technology (NIST)–calibrated photodetector
(Newport 818-ST2-UV/DB) with a beamsplitter. A protected silver
mirror (Thorlabs) was used to calibrate the reflectance based on its
reported reflectance curve, and a dark background was subtracted
from both measurements. For EQE, the current generated by the
sample was probed using MiBots and compared with the current
collected by the NIST-calibrated photodetector when placed at
the sample position, corrected by the photodetector’s responsivity.
Absorption and EQE measurements were both corrected by a shading
factor of 1.39 that corrects for the shading of the MiBot tips, which
was calculated by comparing absorption with and without the tips
in place and averaging over the spectral range 450 to 650 nm.
Solar simulator
One-sun
I
-
V
curves were measured using a 1-kW Xenon arc lamp
(Newport Oriel) with an AM1.5G filter (ABET Technologies). To
ensure 100-mW/cm
2
incident power, the lamp power was adjusted
to generate the correct current on an Si reference cell placed at the
same location as the sample. MiBots were used to contact the device,
and
I
-
V
curves were measured with a Keithley 2425 SourceMeter
using custom LabView programs. The current density was divided
by a spectral mismatch factor to account for the difference in band-
gap between our WS
2
sample and our Si reference cell and the dif-
ference in spectrum between our solar simulator and AM1.5G (
29
).
As no EQE data were available for our device below 400 nm, linear
extrapolation was used, leading to about a 5% error in the spectral
mismatch factor and the reported
J
SC
values. The device area was
assumed to be that of the Au disk, which has a diameter of 28
m.
Device simulations
Absorption and generation were calculated using the transfer ma-
trix method, with optical constants for WS
2
taken from literature
(
35
). We then used the calculated generation rate as an input into
a finite-
element device physics simulation software package that
solves the semiconductor drift-diffusion equations. All other device
simulations were performed using Lumerical CHARGE, a software
package that uses the finite-element drift-diffusion method to cal-
culate charge transport in semiconductor devices. The WS
2
doping
was specified by the bulk crystal vendor (HQ Graphene). Other
WS
2
parameters, including bandgap (
36
), work function (
37
), DC
permittivity (
38
), effective mass (
39
), out-of-plane mobility (
40
42
),
and photoluminescence quantum yield (
43
45
) were taken from the
literature and are listed in table S1. The radiative recombination
coefficient was calculated using the Roosbroeck-Shockley relation
(
46
), and the Shockley-Read-Hall lifetime for minority carriers was
then estimated using the photoluminescence quantum yield.
SUPPLEMENTARY MATERIALS
Supplementary material for this article is available at http://advances.sciencemag.org/cgi/
content/full/5/12/eaax6061/DC1
Section S1. Detailed metal transfer procedure
Fig. S1. Simulated
I
-
V
curve for evaporated devices.
Fig. S2. Photoshunting.
Fig. S3. Active area.
Fig. S4. Spectral mismatch.
Fig. S5. Fitting for one-sun
I
-
V
curve.
Fig. S6. Reproducibility.
Fig. S7. Microscope images of other fabricated devices.
Fig. S8. Forward/backward scans.
Fig. S9. Matching simulations to experimental device.
Table S1. WS
2
parameters for device simulations.
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Acknowledgments:
We thank S.
Nam for the useful discussions.
Funding:
This work was
supported by the DOE “Photonics at Thermodynamic Limits” Energy Frontier Research Center
under grant DE-SC0019140. C.M.W. and J.W. acknowledge support from the NSF Graduate
Research Fellowship under grants 1745301 and 1144469. C.M.W. acknowledges fellowship
support from the Resnick Sustainability Institute.
Author contributions:
C.M.W. fabricated
the devices, performed the measurements, and performed the simulations. C.M.W., J.W., P.R.J.,
and S.B. developed the metal transfer technique. J.W. and P.R.J. assisted with the simulations.
M.K. assisted with the solar simulator, absorption, and EQE measurements. M.S.H. and A.C.
assisted with the TEM sample preparation and imaging. H.A.A. supervised all the experiments,
calculations, and data collection. All authors contributed to the data interpretation,
presentation, and writing of the manuscript.
Competing interests:
The authors declare that
they have no competing interests.
Data and materials availability:
All data needed to
evaluate the conclusions in the paper are present in the paper and/or the Supplementary
Materials. Additional data related to this paper may be requested from the authors.
Submitted 8 April 2019
Accepted 30 October 2019
Published 20 December 2019
10.1126/sciadv.aax6061
Citation:
C.
M. Went, J.
Wong, P.
R. Jahelka, M.
Kelzenberg, S.
Biswas, M.
S. Hunt, A.
Carbone, H.
A. Atwater,
A new metal transfer process for van der Waals contacts to vertical Schottky-junction transition
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