of 15
advances.sciencemag.org/cgi/
content/full/5/12/eaax6061/DC1
Supplementary Materials for
A new metal transfer process fo
r van der Waals contacts to vert
ical
Schottky-junction transition meta
l dichalcogenide photovoltaics
Cora M. Went, Joeson Wong, Phillip R. Jahelka, Michael Kelzenbe
rg, Souvik Biswas, Matthew S. Hunt,
Abigail Carbone, Harry A. Atwater*
*Corresponding author. Email: haa@caltech.edu
Published 20 December 2019,
Sci. Adv.
5
, eaax6061 (2019)
DOI: 10.1126/sciadv.aax6061
This PDF file includes:
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 ot
her fabricated devices.
Fig. S8. Forward/backward scans.
Fig. S9. Matching simulations
to experimental device.
Table S1. WS
2
parameters for device simulations.
Section S1. Detailed metal transfer procedure
Complete details of the metal transfer procedure are described below.
Step 1: Metal contacts on SAM
-
coated Si/SiO
2
.
Commercially
-
available Si wafers coated with
285 nm thermal SiO
2
are diced into chips, cleaned in
N
anostrip for 5 minutes, then rinsed 3 times
in DI water. The chips are placed in a vacuum desiccator. 5 drops of trichloro(1H,1H,2H,2H
-
perfluorooctyl)silane (PFOTS, Sigma Aldrich) are placed in a cap in the bottom of the
desiccator. The desiccator is evacuated slowly, over the course of 3 minutes, then isolated from
the vacuum pump and left evacuated for 1 hour. The chips are removed from the vacuum
desiccator. 20 nm of Au is evaporated in an electron
-
beam evaporator at a
speed of 1 Å/s and a
base pressure below 5
10
-
7
Torr. Photolithography is performed using positive photoresist and a
positive photomask to define the contacts. For photolithography, S1813 is used according to the
following recipe: spin at 4000 rpm for 30 s
econds, soft bake at 115ºC for 1 minute, expose to
365 nm UV light at 15 mW/cm
2
for 8 seconds, develop in MF 319 for 50 seconds, and then rinse
in DI water for 10 seconds. The Au around the photoresist is etched by immersion of the chips in
Transene Gold E
tchant TFA for 10 seconds, then the sample is rinsed three times in DI water.
The photoresist is dissolved in slightly heated acetone (60ºC for 5 minutes).
Step 2: PDMS/PPC stamps.
A similar procedure is followed to that developed
in Ref.
(
25
)
.
PDMS (Sylgard 184) is
mixed in a glass petri dish and left in an oven at 60
-
70ºC overnight. PPC
is made by stirring 1.5 g PPC in 10 mL anisole on a hot plate at 60ºC for 1 hour. The PDMS is
cut into 1 cm by 1 cm squares with a razor blade. One square is removed from the petri d
ish,
rinsed in IPA for 20 seconds, then dried in nitrogen gas. The PDMS stamp is placed on one end
of a glass slide, and the thickest corner of the stamp is identified. The PDMS stamps are plasma
-
ashed at 300 mTorr and 120 W for 10 minutes. The PDMS stamp
is centered on the spinner, and
2 drops of PPC are placed on the PDMS stamp, and then spun at 1500 rpm for 1 minute. The
PDMS/PPC stamps are let sit for a few minutes, but not longer than 10 minutes. The edges of the
PDMS/PPC stamp are cut away with a fres
h razor blade until the stamp is ~2 mm by 2 mm.
Step 3: Transfer of metal contacts.
The stage of a 2D transfer setup is heated to 60ºC. A Si/SiO
2
chip containing metal contacts is loaded onto the stage, and the desired contact is centered in the
field of
view. The PDMS/PPC stamp is loaded onto the top arm of the transfer setup. The
thickest corner of the PDMS/PPC stamp is centered in the field of view. When the stamp is
lowered, the polymer front should originate from this corner. The PDMS/PPC stamp and th
e
desired contact are aligned so that the PPC completely covers the contact, but so that just the
corner of the stamp will make contact with the substrate. The PDMS/PPC stamp is lowered
slowly. Once the polymer front progresses just past the contact, the s
tage temperature is lowered
to 40ºC. After the temperature has reached 40ºC, the top arm of the transfer setup is raised
slowly. As the polymer front begins to move, but before it reaches the contact, the top arm of the
transfer setup and therefore the PDM
S/PPC stamp is raised very quickly, picking up the contact
with it. The stage of the transfer setup is heated to 60ºC again. The target substrate is loaded onto
the stage of the transfer setup and the target flake is centered in the field of view. Once the
temperature reaches 60ºC, the contact on the PDMS/PPC stamp is aligned with the flake. The
PMDS/PPC stamp is lowered slowly, until the polymer front progresses just past the sample.
Immediately, and with the stage still at 60ºC, the PDMS/PPC stamp is rais
ed very slowly. The
contact should delaminate from the stamp and stick to the flake. Remaining PPC that
occasionally sticks to the flake can be removed by rinsing in chloroform for 5 minutes, then
drying in nitrogen gas.
Fig. S1. Simulated
I
-
V
curve for evaporated devices.
Simulations can replicate the resistive
behavior of evaporated
-
contact devices, assuming the metal contacts have an effective work
function difference of 50 meV due to Fermi
-
level pinning at the Au contact.
A
B
Fig. S2
. Photoshunting.
(
A
)
R
SH
vs. I
SC
, experimental. R
SH
is extracted from the slopes of the
power
-
dependent I
-
V curves (Fig. 3
A
of the main text) at short
-
circuit. A power law fit yields
1.
(
B
)
R
SH
vs. I
SC
, simulated. A power law fit yields
=
1,
suggesting that the shunt
behavior is well
-
explained by increasing minority carrier conductivity at higher laser powers and
higher short
-
circuit currents.
Fig. S3. Active area.
The active area of the device presented in the main text is measured to be
615 μm
2
.
A
B
C
D
Fig. S4. Spectral mismatch.
(
A
,
B
)
C
urrent generated under AM1.5G (A) and solar simulator
lamp (B) for our silic
on reference solar cell. We adjust the lamp intensity until these integrated
currents are equal.
(
C
,
D
)
Same, but for the sample. We calculate the spectral mismatch factor M
= 0.67 by dividing the integrated
current in (D) by the integrated current in (C).
Fig. S5. Fitting for one
-
sun
I
-
V
curve.
The diode equation with shunt and series resistances is
us
ed to extract the parasitic resistances and diode parameters.
A
B
Fig.
S
6
.
Reproducibility
.
(
A
)
I
-
V curves taken from 4 different devices under one
-
sun
illumination. V
OC
is between 220 and 2
6
0 mV across all devices studied.
(
B
) I
-
V curves taken
from 6 different devices under illumination with a halogen lamp (~20 suns power density). V
OC
is greater than 220 mV across all devices studied. In both one
-
sun and halogen lamp I
-
V curves,
I
SC
varies due to the different thicknesses and active areas acros
s different devices, and therefore
current is normalized by the short
-
circuit current value for easier comparison.
Fig.
S
7
.
Microscope images of other fabricated devices.
The fir
st 5 images are Devices 1
-
5 in
f
ig.
S6
B
.
Fig.
S
8
.
Forward/backward
scans.
One
-
sun I
-
V curves swept in forward and reverse directions
show no hysteresis.
A
B
Fig. S
9
.
Matching simulations to experimental device.
(
A
)
An I
-
V curve with parameters close
to the experimentally
-
observed I
-
V curve can be sim
ulated if the Au work function is set to 4.6
eV, while all other parameters are kept the same. R
SH
for this simulation, approximated by the
inverse slope of the I
-
V curve at short
-
circuit, is 275
cm
2
. Au could have a lower effective
work function due to
contamination or thiol bonding at the Au/WS
2
interface. However, this
would also limit the V
OC
observed under laser illumination to ~400 mV and thus is not a
complete explanation. (
B
)
Increasing mobility reduces the V
OC
, but does not change the shunt
resistance. Here, mobility is set to 1000 cm
2
V
-
1
s
-
1
for both electrons and holes. R
SH
for this
simulation, approximated by the inverse slope of the I
-
V curve at short
-
circuit, is 2238
cm
2
.
Table
S1.
WS
2
p
arameters for
d
evice
s
imulations
.
Parameter
Value
Source
Bandgap
1.35 eV
Ref.
(
36
)
Work function
4.62 eV
Ref.
(
37
)
Electron effective mass
0.63m
e
Ref.
(
39
)
Hole effective mass
0.8
4
m
e
Ref.
(
39
)
Doping
10
14
cm
-
3
HQ Graphene
Out
-
of
-
plane mobility
(for holes & electrons)
0.01 cm
2
/Vs
Ref.
(
40
42
)
DC permittivity
6.7
Ref.
(
38
)
Radiative recombination
coefficient
1.64
10
-
14
cm
3
/s
Calculated using
optical constants in
Ref.
(
35
)
Shockley
-
Read
-
Hall
Lifetime
611 ns
Calculated using
PLQY from Ref.
(
43
45
)