nanomaterials
Article
The Graphene Structure’s Effects on the Current-Voltage and
Photovoltaic Characteristics of Directly Synthesized
Graphene/n-Si(100) Diodes
Šarūnas Jankauskas
, Rimantas Gudaitis, Andrius Vasiliauskas, Asta Guobienė
and Šarūnas Meškinis *
Institute of Materials Science, Kaunas University of Technology, K. Baršausko St. 59, LT-51423 Kaunas, Lithuania;
sarunas.jankauskas@ktu.lt (Š.J.); rimantas.gudaitis@ktu.lt (R.G.); andrius.vasiliauskas@ktu.lt (A.V.);
asta.guobiene@ktu.lt (A.G.)
* Correspondence: sarunas.meskinis@ktu.lt
Abstract: Graphene was synthesized directly on Si(100) substrates by microwave plasma-enhanced
chemical vapor deposition (MW-PECVD). The effects of the graphene structure on the electrical and
photovoltaic properties of graphene/n-Si(100) were studied. The samples were investigated using
Raman spectroscopy, atomic force microscopy, and by measuring current–voltage (I-V) graphs. The
temperature of the hydrogen plasma annealing prior to graphene synthesis was an essential parameter
regarding the graphene/Si contact I-V characteristics and photovoltaic parameters. Graphene n-type
self-doping was found to occur due to the native SiO2 interlayer at the graphene/Si junction. It was
the prevalent cause of the significant decrease in the reverse current and short-circuit current. No
photovoltaic effect dependence on the graphene roughness and work function could be observed.
Citation: Jankauskas, Š.; Gudaitis, R.;
Vasiliauskas, A.; Guobienė, A.;
Keywords: graphene; MW-PECVD; photovoltaics
Meškinis, Š. The Graphene
Structure’s Effects on the CurrentVoltage and Photovoltaic
Characteristics of Directly
Synthesized Graphene/n-Si(100)
Diodes. Nanomaterials 2022, 12, 1640.
https://doi.org/10.3390/
nano12101640
Academic Editors: Filippo Giannazzo
and Ivan Shtepliuk
Received: 22 March 2022
Accepted: 6 May 2022
Published: 11 May 2022
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Copyright: © 2022 by the authors.
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conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
1. Introduction
Graphene, the carbon 2D material, was discovered recently [1]. Notably, graphene’s
exciting abilities, such as 97.7% optical transparency [2], the high charge carrier mobility
of 200,000 cm2 V −1 s−1 [3], and Young’s modulus of 1 TPa [4], make it a perfect candidate material for optoelectronic device fabrication [5,6]. One of the prominent features of
graphene is that it can be used instead of metal to form a Schottky junction with semiconductors, e.g., silicon [7]. This enables the use of a graphene/silicon (Gr/Si) contact as a
base for solar cell production (see reviews [7–15]). Today, the highest power conversion
efficiency (PCE) reported for Gr/Si solar cells is 16.61% [16]. That is a result of the 9-year
development of the Gr/Si contact devices, from the 1.5% conversion efficiency reported
for the first graphene/silicon Schottky contact-based solar cell [17]. It means that solar
cells based on graphene can be very promising and achieve high PCE. Notably, according
to the simulations, it was suggested that the conversion efficiency of the graphene/Si
solar cell could potentially reach values higher than the conversion efficiency of the best
fabricated solar cells (see [18] and [13], respectively). High-efficiency graphene/silicon
solar cells were fabricated by combining silicon surface passivation with ultra-thin dielectric interlayers, graphene doping, and light management techniques such as Si substrate
micro/nanotexturing and, especially, antireflective films [7–15,19].
Further increase of the graphene/Si solar cell conversion efficiency requires optimization of all the functional parts of the solar cell. In most studies, graphene is synthesized by
chemical vapor deposition (CVD) on copper foil and then transferred to the silicon substrate [7–15]. The transfer is a prolonged process during which graphene is contaminated
by different adsorbates [20], and cracks can be induced in the transferred graphene [21]. It
can deteriorate the graphene/silicon junction device’s properties resulting in complicated
interface and solar cell property control [22,23]. It was reported that the use of the few-layer
Nanomaterials 2022, 12, 1640. https://doi.org/10.3390/nano12101640
https://www.mdpi.com/journal/nanomaterials
Nanomaterials 2022, 12, 1640
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graphene significantly increased graphene/Si solar cell efficiency up to 3–4 times [24,25].
However, on copper foil, usually, single-layer graphene is synthesized by chemical vapor
deposition [26]. Therefore, few-layer graphene for graphene/Si solar cells is fabricated by
the even more complex one-by-one transfer method [22,25,27,28].
The abovementioned problems can be solved using graphene directly grown on silicon
by plasma-enhanced chemical vapor deposition [29], although only a few studies have
been reported [30–37]. The polycrystalline nature of PECVD graphene increases its defect
density compared to that of the transferred graphene grown by CVD on the copper foil [20].
Vertical graphene is more widely used [30,31,34,37] as opposed to its planar counterpart
in terms of direct growth on Si. However, it poses additional light absorption issues
(see [30,31] and [26,38]). These effects should be considered while optimizing directly synthesized graphene-based solar cells. It is noteworthy that the high defect density transferred
graphene interlayer can improve the graphene/Si solar cell’s conversion efficiency compared
to the very low defect density transferred graphene monolayer/Si solar cell without the interlayer [39]. Despite increased sheet resistance and defect density, the graphene nanowall/Si
photovoltaic device’s open-circuit voltage increased with Schottky barrier height [37]. The
photovoltaic conversion efficiency of the transferred GNWs/n-Si solar cell reached up to
4.99% [40]. It was comparable to or even better than the efficiency of the transferred CVD
graphene/n-Si solar cells fabricated without the passivating interlayer, surface texturing,
doping, or antireflective film. That result was achieved despite much higher defect density
in graphene nanowalls compared to the planar graphene grown by CVD on copper foil
(4.98% in [25], 0.86% in [41], 3.5% in [13], 1.9–3% in [42]). The graphene nanowall/n-Si
solar cell open-circuit voltage increased with graphene layer number despite increased
defect density [30]. There are no studies regarding the graphene layer number and defect
density influence on photovoltaic properties of the directly synthesized planar graphene
and silicon solar cells. Meanwhile, graphene nanowall and transferred CVD graphene
cases have their specific peculiarities. Particularly, multilayer graphene fabricated using
layer-by-layer transfer results in different orientations of the carbon hexagons in different
layers. That may be a reason for the contradictory results concerning optimal graphene
layer number and the maximum conversion efficiency achieved [22,24,27,28,38]. Notably,
the optimal graphene layer number in different studies varied from two to four (2 in [38],
2–3 in [27], 3 in [28], 4 in [24]). A summarized benchmark showing PCE values and PCE
enhancement techniques of the CVD-synthesized graphene/Si solar cells investigated by
different research groups can be seen in Table S1.
Therefore, the present study investigates the effects of the directly synthesized graphene
structure on current–voltage characteristics and photovoltaic properties of the graphene/nSi photovoltaic devices. Various synthesis conditions were used to grow graphene samples
of different structures and surface morphologies. Only a small influence or no influence
of the graphene thickness, defect density, surface morphology, and work function was
found. The impact of substrate-induced self-doping and silicon surface pretreatment on
the graphene/n-Si device’s current-voltage and photovoltaics characteristics was revealed.
2. Materials and Methods
Samples were produced using a microwave PECVD system Cyrannus (Innovative
Plasma Systems (Iplas) GmbH, Troisdorf, Germany). Monocrystalline, double-side polished, n-type Si(100) (Sil’tronix Silicon Technologies, Archamps, France), with a resistivity
of 1–10 Ω·cm, was used as a substrate. A precursor gas mixture of hydrogen and methane
was used for graphene synthesis. Before the growth of graphene, hydrogen plasma was
ignited, and methane gas was only introduced when the target temperature was reached.
In some cases, the silicon substrates were plasma pre-annealed at higher temperatures
than the temperature of the subsequent graphene synthesis. A special enclosure was used
to protect from direct plasma that results in high etching rates of the sample (Figure S1).
Synthesis parameters for each sample can be seen in Table 1. Samples were grouped into
Sample Power,
Annealing Temperature,
H2,
CH4,
p, mBar T, °C t, min
No.
kW
sccm
sccm
°C
A1
0.7
75
25
10
700
60
700
A2
0.7
75
25
20
700
60
700
Nanomaterials 2022, 12, 1640
3 of 19
A3
0.7
75
35
10
700
60
700
A4
0.7
75
35
20
700
60
700
A5
0.7 three categories
75
35 based20
700 substrate
60 plasma pre-annealing
700 temperature
(A–C)
on the Si(100)
◦
sample size20was 1 × 700
1 cm.
A6
0.7 (700–900
75 C). The35
60
700
A7
0.7 Table 150
50
700
700
1. Synthesis conditions
for22
investigated
graphene60
samples.
A8
0.7
75
25
10
700
90
700
Sample No.
Power, kW
H2 , sccm
CH4 , sccm
p, mBar
T, ◦ C
t, min
Annealing Temperature, ◦ C
A9
0.7
75
35
20
700
90
700
A1
0.7
75
25
10
700
60
700
A10 0.7 0.7
75
25
10
700
150 60
700700
A2
75
25
20
700
A3
75 75
35
10
700
A11 0.7 0.7
25
10
700
150 60
700700
A4
0.7
75
35
20
700
60
B1 0.7 0.7
75
35
10
700
60 60
800700
A5
75
35
20
700
700
A6
75 75
35
20
700
B2 0.7 0.7
25
10
700
90 60
800700
A7
150
50
22
700
60
700
B3 0.7
0.7
75
35
20
800
60 90
800700
A8
0.7
75
25
10
700
C1 0.7 0.7
35
20
700
60 90
900700
A9
75 75
35
20
700
A10
0.7
75
25
10
700
150
C2
0.7
75
25
10
700
90
900700
A11
0.7
75
25
10
700
150
700
C3 0.7 0.7
50
22
900
20 60
900800
B1
75 150
35
10
700
B2
B3
C1
C2
C3
0.7
75
25
10
700
90
800
0.7
75
35
20
800
60
800
After
carried20out, diode
0.7 graphene75synthesis was
35
700fabrication
60 began with Al
900back con0.7
75
25
10
700
90
900+ acetone
formation (on the uncoated Si side) using e-beam technology. The DMF
0.7
150
50
22
900
20
900
tact
boiling and RCA 1 (1:1:5 solution of NH4OH + H2O2 + H2O), impurity removal (1:50 solution of HF + H2O), and RCA 2 (1:1:6 solution of HCl + H2O2 + H2O) treatments [43] were
After graphene
wasCr/Cu
carried electrodes
out, diode fabrication
began withon
Al back
contact
done prior to the deposition
of thesynthesis
Al layer.
were deposited
the graformation (on the uncoated Si side) using e-beam technology. The DMF + acetone boiling
phene through a mask
with 500 µm circular holes. The thicknesses of the Cr interlayer and
and RCA 1 (1:1:5 solution of NH4 OH + H2 O2 + H2 O), impurity removal (1:50 solution of
Cu layer were 20HF
and
200
nm, respectively. The schematic diagram is shown in Figure 1.
+H
2 O), and RCA 2 (1:1:6 solution of HCl + H2 O2 + H2 O) treatments [43] were done
prior
to
the
the Al to
layer.
electrodes
werecells
deposited
graphene
The structure of the devicedeposition
is more of
similar
theCr/Cu
real silicon
solar
than on
thetheusually
a mask
withan
500active
µm circular
holes.
thicknesses
the Cr interlayer
and Cu
used graphene/Sithrough
solar cells,
with
device
partThe
consisting
ofofgraphene
on silicon
layer were 20 and 200 nm, respectively. The schematic diagram is shown in Figure 1. The
in a hole opened in the silicon dioxide and metal electrodes on the graphene-coated SiO2
structure of the device is more similar to the real silicon solar cells than the usually used
[7–15]. It should graphene/Si
be noted that
solar
cellspart
ofconsisting
structure
similar to
ours were
solar graphene/Si
cells, with an active
device
of graphene
on silicon
in a hole
fabricated and investigated
[23,44–46].
Themetal
geometry
of on
the
metal electrodes
opened in thein
silicon
dioxide and
electrodes
thedevice’s
graphene-coated
SiO2 [7–15]. It
should be noted that graphene/Si solar cells of structure similar to ours were fabricated and
was not optimized.
investigated in [23,44–46]. The geometry of the device’s metal electrodes was not optimized.
1. Schematic
diagram of the graphene/n-Si(100)
Figure 1. SchematicFigure
diagram
of the graphene/n-Si(100)
fabricatedfabricated
diodes. diodes.
Thickness and defect characterization was carried out via Raman scattering spectroscopy using a Raman spectrometer, InVia (Renishaw, Wotton-under-Edge, UK). The
measurement was done just after the graphene synthesis and before the graphene/Si(100)
diode fabrication. We acquired Raman spectra at several different places on each sample,
considering possible differences in the graphene structure across the specimen. The beam
power was set to 1.5 mW, and the excitation wavelength was 532 nm. Several peaks were
Nanomaterials 2022, 12, 1640
4 of 19
analyzed for in-depth characterization and defect estimation (D, G, and 2D). The G peak
was separated into two components, with the actual G peak being at 1600 cm−1 and the
D’ peak (which was not analyzed) being at 1620 cm−1 . The Lorentzian function was used
for the best peak fit, considering Merlen et al. [47] who made observations to determine
peak intensities, positions, and full width at half maximums (FWHM). The well-known
ID /IG ratio was used to reveal the defectiveness of our produced samples [48]. In contrast,
the I2D /IG ratio contributed to the graphene thickness evaluation [49] (smaller ratios correspond to more graphene layers). The positions and FWHM of the G and 2D peaks were
analyzed to get information on graphene crystallite size, strain, and doping [50–52].
Atomic force microscopy (AFM) was employed to detect any structural peculiarities of
the graphene surface. The surface morphology was investigated at room temperature and
ambient air conditions using a NanoWizard III atomic force microscope (JPK Instruments,
Bruker Nano GmbH, Berlin, Germany). The measurements were done in tapping mode.
The silicon probes (CS Instrument, Harrislee, Germany) with a thin layer (25 ± 5 nm) of
Pt/Ir coating on both re-ex and tip sides of the probes were used. The probe parameters
were as follows: spring constant 2.7 N/m; 60 kHz frequency; 30 nm tip ROC; pyramidal
shape. Images of 2 µm × 2 µm size were acquired from the measured data using JPKSPM
Data Processing software (version spm-4.3.13, JPK Instruments, Berlin, Germany). Kelvin
probe measurements were carried out using the same instrumental setup to evaluate
graphene work function.
The current-voltage (I-V) characteristics were measured using a Keithley 6487 picoampere meter/voltage source. The measurements were done at several points on the sample to
evaluate the possible dispersion of the characteristics. Characteristics were investigated in
three different regimes to study the photovoltaic properties of the fabricated devices. These
were dark mode (sample was not illuminated), UV mode (when the sample was illuminated by 406 nm wavelength light-emitting diode (LED)), and IR mode (when the sample
was illuminated by 800 nm wavelength light-emitting diode). In all instances, the voltage
range was from −2 to +2 V. To ensure the same optical power (5.2 mW) between different
measurement modes, currents supplied to the LEDs were selected accordingly. The measurements were done at several different places on the samples to evaluate the dispersion
of the results. Diode behavior was studied by examining the I-V characteristic parameters
in the dark (reverse current at 0.3 V (IR (0.3 V)), forward current vs. reverse current at ±0.1V
(IR (0.1 V)/IF (0.1 V)), forward current vs. reverse current at ±0.3 V (IR (0.3 V)/IF (0.3 V))).
The photovoltaic parameters (short-circuit current (ISC ) and open-circuit voltage (UOC ))
were derived from current–voltage characteristics measured under illumination. The I-V
characteristic’s dependence on temperature was measured using a similar setup to the
photovoltaic parameter measurements. The same Keithley 6487 picoampere meter/voltage
source was employed, with thermal operational conditions being changed by a custommade Peltier element configuration. The temperature varied from −20 to 40 ◦ C. Each
measurement was made after the temperature value had settled down.
3. Results
3.1. Raman Spectra, Current–Voltage Characteristics of Produced Samples and Their AFM Micrographs
The Raman fingerprints of the synthesized samples were investigated, and graphenerelated peaks were confirmed (Figure 2a) [53]. The 2D peak was observed at ~2700 cm−1 .
The G peak of our samples lay at ~1600 cm−1 . All synthesized samples had a prominent
defect-related D peak at ~1350 cm−1 . The D′ band was detected at ~1620 cm−1 as a
shoulder of the G peak. This is also a significant feature showing the presence of the defects
in the graphene sample [47,54]. The defect-related peaks are due to the nanocrystalline
nature of the directly synthesized graphene [29,55]. This was also confirmed by the ID′ /ID
ratio, which was in the 2.62–4.6 range, indicating that the dominant defect source was
grain boundaries [54,56]. The further analysis of the selected samples will be discussed in
later sections.
Nanomaterials 2022, 12, 1640
defect-related D peak at ~1350 cm−1. The D’ band was detected at ~1620 cm−1 as a shoulder
of the G peak. This is also a significant feature showing the presence of the defects in the
graphene sample [47,54]. The defect-related peaks are due to the nanocrystalline nature
of the directly synthesized graphene [29,55]. This was also confirmed by the ID‘/ID ratio,
which was in the 2.62–4.6 range, indicating that the dominant defect source was grain
5 of 19
boundaries [54,56]. The further analysis of the selected samples will be discussed in later
sections.
(a)
(b)
Figure
typical II−V
Figure 2.
2. Typical
Typical Raman
Raman scattering
scattering spectra
spectra (a)
(a) and
and typical
−Vcharacteristics
characteristics(b)
(b)of
ofdirectly
directlysynthesynthesized graphene/Si(100) devices. The I−V characteristics of the device produced from the C2 sample
sized graphene/Si(100) devices. The I−V characteristics of the device produced from the C2 sample
exhibited diode behavior (red), ohmic contact was seen for the device produced from a sample B3
exhibited diode behavior (red), ohmic contact was seen for the device produced from a sample B3
(green), and the A1 sample had diode-like I-V features (blue).
(green), and the A1 sample had diode-like I-V features (blue).
The
The typical
typical current–voltage
current–voltage (I-V)
(I-V) characteristics
characteristics of
of the
the produced
produced photodiodes
photodiodes can
can be
be
seen
in
Figure
2b.
It
is
clear
that
even
though
directly
synthesized
graphene/n-Si(100)
deseen in Figure 2b. It is clear that even though directly synthesized graphene/n-Si(100)
vices
mostly
showcase
diode
behavior
devices
mostly
showcase
diode
behavior(as
(asisisexpected),
expected),exceptions
exceptionssuch
suchas
as ohmic
ohmic device
device
operation
regimes
were
found.
operation regimes were found.
The
The graphene
graphene AFM
AFM images
images and
and topography
topography parameters
parameters were
were studied
studied to
to supplement
supplement
our
spectroscopy findings
findings (Figures
(Figures S2–S10,
S2–S10, Table
Table2).
2).The
TheII2D
/IGGratio
2D/I
our Raman
Raman spectroscopy
ratiovalues
valuesindiindicated
the
presence
of
few-layer
graphene.
The
thickness
of
the
one
graphene
layer
cated the presence of few-layer graphene. The thickness of the one graphene layer was
was
~0.4
to to
thethe
roughness
values
larger
thanthan
several
nm, nm,
non-pla~0.4 nm
nm[57,58].
[57,58].Thus,
Thus,according
according
roughness
values
larger
several
nonnar
graphene
was
grown
in in
some
samples
(Table
planar
graphene
was
grown
some
samples
(Table2,2,Figures
FiguresS2,
S2,S5,
S5, S7–S9)
S7–S9) [27].
[27]. Sample
Sample
roughness
rangedfrom
from0.19
0.19toto
indicating
different
surface
morphologies.
The
roughness ranged
5.25.2
nm,nm,
indicating
different
surface
morphologies.
The work
work
functions
calculated
from measured
potential
averaged
at 4.820–
functions
calculated
from measured
contactcontact
potential
(VCPD)(VCPD)
averaged
at 4.820–4.826
eV
4.826
despite growth
differentconditions.
growth conditions.
workvariation
function was
variation
(TableeV
2) (Table
despite2)different
Thus, the Thus,
work the
function
tiny.
was tiny.
Table 2. AFM parameters of the directly synthesized graphene samples.
Table 2. AFM parameters of the directly synthesized graphene samples.
Sample No.
Highest Surface Point, nm
RMS Roughness, nm
Φ, eV
I2D /IG
Highest Surface
A1
~9
2.1
Sample
No.
RMS Roughness,
nm
Φ, eV4.82
I2D0.33
/IG
A7
1.3
0.295
4.824
0.42
point, nm
A8
0.35
A1
~9 0.9
2.1 0.19
4.82 4.824
0.33
A11
3.39
0.77
0.6
A7
1.3 1.8
0.295 0.42
4.8244.824
0.42
B1
0.34
A8
0.9 ~15
0.19 3.5
4.824 0.35
B2
0.47
B3
0.41
A11
3.39 ~6
0.77 1.36
- 4.824
0.6
C2
0.77
B1
1.8 22.9
0.42 5.2
4.824 0.34
C3
1.5
0.332
4.826
0.54
B2
~15
3.5
0.47
B3
~6
1.36
4.824
0.41
3.2. Raman Scattering Spectra Parameters and Synthesized Graphene Thickness, Defect Density,
C2
22.9
5.2
0.77
Doping, and Stress
C3
1.5
0.332
4.826
0.54
In most cases, graphene Raman scattering spectra were investigated for defect-free or
few-defect graphene (no Raman D peak). However, directly synthesized graphene usually
contains a significant number of defects [55]. This can affect several Raman D, G, and
2D peak parameters. In addition, graphene layer number, doping, and stress can also
significantly impact its Raman spectra [59,60]. Therefore, a more in-depth investigation of
several aforementioned peak parameters was carried out.
Remarkably, the decreased intensity of the 2D peak is commonly observed in defected
graphene, including that grown by direct synthesis [61,62]. However, no decrease in the
Nanomaterials 2022, 12, 1640
In most cases, graphene Raman scattering spectra were investigated for defect
or few-defect graphene (no Raman D peak). However, directly synthesized graphene
ally contains a significant number of defects [55]. This can affect several Raman D, G,
2D peak parameters. In addition, graphene layer number, doping, and stress can also
nificantly impact its Raman spectra [59,60]. Therefore, a more in-depth investigatio
6 of 19
several aforementioned peak parameters was carried out.
Remarkably, the decreased intensity of the 2D peak is commonly observed in
fected graphene, including that grown by direct synthesis [61,62]. However, no decr
I2D /IG ratio with
IDI/I
found
3a). found
Thus, the
I2D /I3a).
canthe
stillI2Dbe
G ratio
in the
/IGratio
ratiowas
with
ID/IG(Figure
ratio was
(Figure
Thus,
/IGused
ratio can sti
2DG
to evaluate graphene
layer number,
as suggested
in [49].
used to evaluate
graphene
layer number,
as suggested in [49].
(a)
(b)
(c)
(d)
(e)
(f)
Figure
3. Relation
between
different
Raman
scattering
spectra(a)
parameters:
I2DG/I;G vs. ID/IG
Figure 3. Relation
between
different
Raman
scattering
spectra
parameters:
I2D /IG vs.(a)
ID /I
FWHM
2D vs. I2D/IG; (c) Pos2D vs. I2D/IG; (d) Pos2D vs. PosG; (e) FWHM2D vs. Pos2D; (f) FWHM2
(b) FWHM2D vs. I2D /IG ; (c) Pos2D vs. I2D /IG ; (d) Pos2D vs. PosG ; (e) FWHM2D vs. Pos2D ;
(f) FWHM2D vs. FWHMG . Samples were grouped according to the temperature of Si(100) substrate
hydrogen plasma annealing before graphene growth: 700 ◦ C (red), 800 ◦ C (green), 900 ◦ C (blue).
The width of the 2D peak increases, and the peak position upshifts with increased
graphene layer number (decreased I2D /IG ratio) [49]. One can see only a weak tendency
of the FWHM2D decrease with the I2D /IG ratio increase in Figure 3b. Very different
FWHM2D values can be found for graphene samples of the same thickness. Thus, FWHM2D
depends on some other factors. The Pos2D , in our case, was upshifted with the I2D /IG ratio
(Figure 3c). In contrast, the 2D peak should downshift with decreased layer numbers [49].
Thus, no Pos2D dependence on graphene layer number was revealed. Therefore, doping or
Nanomaterials 2022, 12, 1640
7 of 19
strain effects can be the origin of the significant differences between the 2D peak position
and FWHM2D of the different graphene samples [60,63–65].
FWHMG is related to the ID /IG ratio of defective graphene [66]. However, no clear
FWHMG dependence on ID /IG ratio was found in our case (Figure S11a).
The FWHMG decreases with increased crystallite size [66–68] and graphene doping [63,69–71].
The latter case is accompanied by a PosG shift to the higher wavenumbers [70]. At the same
time, a slight narrowing of the G peak with PosG upshift was seen (Figure S11b). Thus,
the doping effects on G peak narrowing can be supposed. However, the influence of the
crystallite size changes cannot be rejected.
The Pos2D vs. PosG plot can be used to separate compressive and tensile stress and
p-type and n-type doping effects [60,65,69,72]. The downshift of the Pos2D with the upshift
of the PosG was found (Figure 3d). It is a signature of n-type doping [60,73]. The FWHM2D
decreased with an upshift of the Pos2D (Figure 3e). This is similar to the case in [73], where
such behavior was reported for n-type doped graphene. Thus, according to Figure 3d,e,
the synthesized graphene samples are n-type self-doped. The 2D peak is downshifted and
broadened with increased n-type dopant density [73].
It should be mentioned that the presence of the strain in graphene results in the
FWHM2D linear increase with FWHMG [63,74]. Meanwhile, in Figure 3d, FWHM2D increase with FWHMG can be seen only for three samples that were grown on Si(100) preannealed at 900 ◦ C. For the samples synthesized on the silicon pre-annealed at 700 ◦ C
temperature, the tendency of the FWHM2D to decrease with increased FWHMG was
found (Figure 3f). This supports the assumption of n-type self-doping of the studied
graphene [63,73]. Different sizes of the graphene crystallites can explain the significantly
different FWHM2D values seen for samples with nearly the same FWHMG values [50].
Thus, one can suppose that the charge transfer from the Si(100) substrate to the graphene
occurs during the graphene growth, resulting in the n-type self-doping of the graphene.
This explanation was provided in [75], taking into account [76–78].
3.3. Current–Voltage Characteristics’ Relation with Raman Parameters of Fabricated
Graphene/Si Devices
The relations between the current–voltage (I-V) characteristics of the graphene/Si(100)
heterojunctions and graphene structure were studied. The initial surface preparation significantly influences the Schottky and ohmic contact I-V characteristics [23,79–81]. Therefore,
we separately analyzed graphene samples synthesized on the silicon substrate, with hydrogen plasma-treated at different temperatures, to discern the graphene structure and the
graphene/Si interface effects. Hydrogen plasma’s silicon surface treatment was widely
studied and used for amorphous hydrogenated silicon and monocrystalline silicon heterojunctions. However, their mechanisms are far from the final description due to the
complexity of the competing effects. That is an increase of silicon surface roughness [82],
silicon etching [83] and etching rate dependence on temperature [84,85], Si surface amorphization [82], defect generation [86,87], and different silicon hydrides’ creation [86].
No clear dependence of the different I-V characteristic parameters (IR (0.3 V), IR (0.1 V)/
IF (0.1 V), IR (0.3 V)/IF (0.3 V)) on the main Raman peak ratios was found (Figure S12).
However, the G peak broadening influences the I-V characteristics’ shape (Figure 4a–c).
As FWHMG approaches higher values, indicating lowered self-doping level and, possibly,
graphene crystallite size decrease [50], the reverse current rises. The reverse and forward
current ratios approach 1, implying the ohmic behavior of the junction (Figure 4b,c). Differences between sample groups are not that noticeable, apart from samples annealed at
900 ◦ C, which resulted in a smaller current ratio. We noticed a general increase of reverse
current and reverse/forward current ratios, with 2D peak blueshift (ranging from 2653
to 2705 cm−1 ), when the Si(100) substrate was hydrogen plasma pre-annealed at 700 ◦ C
(Figure 4d–f). Considering the analysis provided in the Section 3.2, the reverse current and
IR /IF ratios decrease with increased n-type self-doping levels [59]. When looking at other
sample groups, results were inconclusive, although samples annealed at 800 ◦ C showcase a
Nanomaterials 2022, 12, 1640
current ratios approach 1, implying the ohmic behavior of the junction (Figure 4b,c). Differences between sample groups are not that noticeable, apart from samples annealed at
900 °C, which resulted in a smaller current ratio. We noticed a general increase of reverse
current and reverse/forward current ratios, with 2D peak blueshift (ranging from 2653 to
2705 cm−1), when the Si(100) substrate was hydrogen plasma pre-annealed at 700 °C (Fig8 of 19
ure 4d–f). Considering the analysis provided in the Section 3.2, the reverse current and
IR/IF ratios decrease with increased n-type self-doping levels [59]. When looking at other
sample groups, results were inconclusive, although samples annealed at 800 °C showcase
ratios
at 0.1
V, with
the current
ratioratio
dropping
whenwhen
Pos2D
a much
muchdifferent
differenttrend
trendinincurrent
current
ratios
at 0.1
V, with
the current
dropping
increases.
When
analyzing
FWHM
dependence
on
reverse
current
and
I
(0.3
V)/I
(0.3
2D
R
F
Pos2D increases. When analyzing FWHM2D dependence on reverse current and IR(0.3V)
ratio,
it is seen that values of the reverse current and IR (0.3and
V)/I
(0.3V)/I
V) Fof
the
V)/I
IRF(0.3
(0.3
V)samples
of the
F(0.3 V) ratio, it is seen that values of the reverse current
◦ C gradually decrease when FWHM
grown
after
annealing
in
700
decreases
(Figure
4g–i).
2D FWHM2D decreases
samples grown after annealing in 700 °C gradually decrease when
Thus,
it
supports
the
premise
that
an
increased
n-type
self-doping
level
decreases
the
(Figure 4g–i). Thus, it supports the premise that an increased n-type self-doping level dereverse
current
and
I
/I
ratio
[59].
R F and IR/IF ratio [59].
creases the reverse current
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
(i)
Figure
characteristic
V), IR(0.1 V)/IF(0.1 V), IR(0.3 V)/IF(0.3 V)) in relation
Figure4.4.I−V
I−V
characteristicparameters
parameters(I(IR(0.3
R (0.3 V), IR (0.1 V)/IF (0.1 V), IR (0.3 V)/IF (0.3 V)) in relation
with: (a–c) FWHMG; (d–f) Pos2D; and (g–i) FWHM2D. Samples were grouped according to the temwith: (a–c) FWHMG ; (d–f) Pos2D ; and (g–i) FWHM2D . Samples were grouped according to the
perature of Si(100) substrate hydrogen plasma annealing before graphene growth: 700 °C (red),
800
temperature of Si(100) substrate hydrogen plasma annealing before graphene growth: 700 ◦ C (red),
°C (green),
900
°C
(blue).
800 ◦ C (green), 900 ◦ C (blue).
Inclusions of the non-planar graphene, such as wrinkles, can significantly influence
charge transport properties [88,89]. However, results are somewhat inconsistent when
analyzing I-V parameters and their relation to roughness (Figure S13). The general tendency
of IR (0.3 V) decrease (IR (0.3V)/IF (0.3 V) increase) with increasing surface roughness can be
observed, although the plot’s strange “branching out” is seen. Due to this, it is impossible
to conclude whether this magnitude of roughness impacts device performance.
3.4. Photovoltaic Characteristics of Fabricated Graphene/Si Devices and Their Relation to the
Raman Parameters of the Produced Graphene
Typical I-V curves of produced photovoltaic devices under illumination can be seen
in Figure 4. Differences between different illumination regimes are minimal, with 800 nm
to conclude whether this magnitude of roughness impacts device performance.
Nanomaterials 2022, 12, 1640
3.4. Photovoltaic Characteristics of Fabricated Graphene/Si Devices and Their Relation to the
Raman Parameters of the Produced Graphene
9 of 19
Typical I-V curves of produced photovoltaic devices under illumination can be seen
in Figure 4. Differences between different illumination regimes are minimal, with 800 nm
excitations,
in most cases, contributing to a more significant photovoltaic effect, as preexcitations, in most cases, contributing to a more significant photovoltaic effect, as presumed
sumed
Figure
S14).
Theofshape
of characteristics
the I-V characteristics
in the
fourth is
quadrant
(Figure (Figure
5, Figure5,S14).
The
shape
the I-V
in the fourth
quadrant
is
typical
for
graphene/n-Si
solar
cells
grown
without
the
intentional
graphene
doping
and
typical for graphene/n-Si solar cells grown without the intentional graphene doping and
intentionally
depositedultra-thin
ultra-thin
dielectric
interlayers
[24,32,44,90,91].
No S-shaped
I-V
intentionally deposited
dielectric
interlayers
[24,32,44,90,91].
No S-shaped
I-V
characteristics
reported
for
some
graphene/Si
solar
cells
[92–94]
were
found.
characteristics reported for some graphene/Si solar cells [92–94] were found.
Figure 5.
V characteristics
of directly
synthesized
graphene/Si
heterojunctions
measured
Figure
5.Typical
TypicalI−I−V
characteristics
of directly
synthesized
graphene/Si
heterojunctions
measured
under the
byby
800800
nmnm
wavelength
(solid)
and 406
wavelength
(dashed)(dashed)
LEDs. LEDs.
under
theillumination
illumination
wavelength
(solid)
andnm
406
nm wavelength
To analyze the effects of the graphene structure on photovoltaic properties of the
To analyze the effects of the graphene structure on photovoltaic properties of the
graphene/Si(100) samples, ISC and UOC were investigated concerning Raman parameters
graphene/Si(100)
samples,
ISCI and
UOC were investigated concerning Raman parameters
(Figure 6). Figure 5a
shows an
SC of our fabricated Cu/Cr/Gr/Si/Al device in relation
(Figure
shows an graphene.
ISC of our The
fabricated
Cu/Cr/Gr/Si/Al
device
in relation to
to the I2D6).
/IGFigure
ratio of5asynthesized
same investigation
scheme
was chosen
the
I
/I
ratio
of
synthesized
graphene.
The
same
investigation
scheme
was
chosen due
2D
G
due to the previously mentioned effects of hydrogen plasma annealing before graphene
to
the previously
mentioned
effects ofbetween
hydrogen
plasma parameters
annealing and
before
growth.
Devices show
little to no correlation
photovoltaic
I2D /Igraphene
G.
◦ C exhibited some increase in I
Only samples
thatshow
were annealed
at 700
and UOC when
growth.
Devices
little to no
correlation
between photovoltaic
and I2D/IG.
SC parameters
I2D /IGsamples
increased
(layer
number
decreases)
6a,b). The
samples
annealed
900 ◦UCOC when
Only
that
were
annealed
at 700(Figure
°C exhibited
some
increase
in ISCatand
produced the lowest I and UOC . Thus,
the6a,b).
surface
pre-treatment
conditions
2D/IG increased (layer numberSCdecreases)
Idistinctly
(Figure
The
samples annealed
at 900 °C
are more critical than the graphene layer number regarding the photovoltaic parameters.
distinctly produced the lowest ISC and UOC. Thus, the surface pre-treatment
conditions are
Considering the changes of the UOC and ISC in the samples grown using 700 ◦ C temperature
more
critical
than
the
graphene
layer
number
regarding
the
photovoltaic
pre-treatment, the graphene layer number effects can be explained by changes inparameters.
the
Considering
the changes
of the Uand
OC and ISC in the samples grown using 700 °C temperature
reflectance, optical
transmittance,
graphene work function [27]. In the graphene/Si
pre-treatment,
the graphene
number
effects by
cantheberise
explained
by changes
solar cell, the open-circuit
voltagelayer
increase
was explained
in the Schottky
barrier in the
reflectance,
optical
transmittance,
and
graphene
work of
function
[27]. In the
height and work
function
[24,30]. In our
case,
no dependence
the graphene/Si
solargraphene/Si
cell
short-circuit
current
and
open-circuit
voltage
on
graphene
work
function
was
found.
solar cell, the open-circuit voltage increase was explained by the rise in theAsSchottky
mentioned
earlier
in this
article,
the graphene
number
for maximization
of
barrier
height
and
work
function
[24,30]. layer
In our
case, necessary
no dependence
of the graphene/Si
the graphene/Si
solar cell
photovoltaic
characteristics
was reported
by different
solar
cell short-circuit
current
and open-circuit
voltage
on graphene
workauthors
function was
to be from two to four [24,27,28,38]. In our case, the lowest graphene layer number used,
found. As mentioned earlier in this article, the graphene layer number necessary for
according to the I2D /IG ratio analysis, was 1–2 layers. Thus, our results are close to the data
reported in [28,38], where no graphene work function influence was revealed. Noteworthily,
ISC exhibited a noticeable decrease with increased FWHM2D (Figure 6c) when samples
were annealed at 700 ◦ C. Similar results were not reproduced when looking at the UOC –
FWHM2D relation (Figure 6d), with samples occupying similar values of UOC throughout
the whole range of FWHM2D .
Nanomaterials 2022, 12, 1640
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Figure 6. Structural effects on graphene/Si(100) diode’s photovoltaic parameters at 800 nm illumination: (a) ISC vs. I2D /IG plot; (b) UOC and I2D /IG relation; (c) ISC relation with FWHM2D ; (d) UOC
vs. FWHM2D plot; (e) ISC with respect to Raman 2D peak position; and (f) ISC and PosG correlation.
Samples were grouped according to the temperature of Si(100) substrate hydrogen plasma annealing
before graphene growth: 700 ◦ C (red), 800 ◦ C (green), 900 ◦ C (blue).
We also analyzed the ISC correlation with Pos2D and PosG (Figure 6e,f). Interestingly
enough, almost all analyzed samples followed an increasing ISC trend with a shift of the
Nanomaterials 2022, 12, 1640
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Pos2D to the higher wavenumbers and PosG to the lower wavenumbers. It means that the
graphene n-type self-doping could be the predominant phenomenon, affecting photovoltaic
properties [75] (Table S2). Thus, the graphene n-type self-doping results in decreased shortcircuit current. That is in accordance with numerous studies because graphene p-type
doping is used to increase graphene/n-Si solar cell efficiency by raising the graphene/Si
contact potential barrier height [10]. The same distribution could not be recorded for UOC
due to very dispersive data (Figure S15).
The photovoltaic properties’ relation with ID /IG and FWHMG plots was employed
to examine changes in electric properties due to defects or grain size effects (Figure S16).
Relatively high dispersion can be seen when analyzing short circuit current deviation
due to defects (in terms of ID /G ) (Figure S16a,b), and data distribution gives no concrete
answer. When observing defect influence on open-circuit voltage, higher UOC values did
not correlate to the aforementioned parameters (Figure S16). It should be mentioned that,
in [31,39,40], graphene/Si solar cell conversion efficiency was improved by inserting a
highly defective graphene interlayer. While in [30], the lowest UOC and ISC were found
for directly synthesized graphene/Si solar cells fabricated using graphene with the lowest
defect density. It is also hard to stress any presence of photovoltaic parameter variation
due to grain size [50] after analyzing G band broadening (Figure S16c,d). Data points are
too dispersive to conclude. When considering sample topography and its significance on
photovoltaic parameters, it is essential to note that only a small ISC reduction can be seen
due to the increase in roughness (Figure S17). The most notable case is samples grown
on the Si(100) annealed at 800 ◦ C. In the case of the 700 ◦ C annealing, no correlation can
be observed due to predominant roughness effects. UOC and RMS roughness relation
indicate relatively high dispersion, thus omitting roughness as a detrimental parameter of
open-circuit voltage. It should be mentioned that transferred CVD graphene/Si solar cell
efficiency can be improved by inserting a graphene nanowall interlayer [38]. At the same
time, the transferred graphene nanowalls and n-Si solar cell efficiency were comparable to
the efficiency of the transferred CVD graphene/n-Si solar cells [40].
3.5. I-V and Photovoltaic Parameter Relation
The photovoltaic parameter’s relation with IR (0.3V) was analyzed. An increase in the
ISC following a rise in IR (0.3V) can be seen at least in two groups of samples (Figure 7a).
Curve shape investigation was carried out using reverse and forward current ratios at
0.1 V and 0.3 V, respectively, as the diode nature of samples may impact photovoltaic
parameters. In samples that were annealed at 700 ◦ C, an increase in ISC can be seen
when IR (0.1 V)/IF (0.1 V) increases (Figure 7b), with other groups following that tendency
dubiously. When IR (0.3 V)/IF (0.3 V) is taken into account (Figure 7c), the dispersion of
data became broad, hence limiting conclusiveness. When analyzing the aforementioned I-V
parameters with respect to UOC , the results were even more dispersive (Figure S18). The
UOC vs. ISC /IR (0.3 V) plot was employed to show that UOC tends to increase with a shortcircuit and reverse current ratio increase, although it branches out when the ratio reaches
a value of ~1 (Figure 7d). While annealing temperatures had an impact on ISC /IR (0.3V),
which tends to be minimal (<0.183 V) when annealing was carried out at 900 ◦ C, different
illumination regimes show that IR irradiation yields higher UOC and ISC . As in many
discussed relations, samples that were annealed at 700 ◦ C also had the most significant
spread of ISC /IR (0.3 V), with values situated in a range of 0.336–3.375. UOC in samples that
had been annealed at 800 ◦ C before graphene growth tended to increase with ISC /IR (0.3V),
although moderate dispersion of values was observed.
Nanomaterials 2022, 12, 1640
12 of 19
Figure 7. Diode I−V and photovoltaic parameter (at 800 nm illumination) relation: (a) ISC vs. IR (0.3 V)
plot; ISC correlation with diode’s I-V curve shape estimated using reverse and forward current ratio
measured at 0.1 V (b) and 0.3 V (c); UOC vs. ISC /IR (0.3 V) plot (d) (solid and hollow markers indicate
device illumination at 800 and 406 nm, respectively).
4. Discussion
The electron transfer from the n-Si(100) to the graphene should result in decreased
graphene/Si contact barrier and, hence, increased reverse current [95,96]. In our case,
the opposite tendency was found. However, the native oxide layer can be present at the
graphene and silicon interface because silicon surface reoxidation after direct graphene
synthesis was reported in [62]. Graphene placed on the silicon dioxide can be electrondoped due to the positive silanol groups on the SiO2 surface [97,98].
The charge exchange at the graphene/SiO2 interface results in a dipole formation, and
charge redistribution imposes n doping in the graphene [98], although no chemical bonds
form at the graphene–SiO2 interface [99]. The graphene placed on the amorphous SiO2 can
also be n-type doped [99].
It should be mentioned that the single-layer graphene Fermi level and work function
vary equally [100]. Nevertheless, in the present study, the graphene samples’ work function
changed in a very narrow range despite different graphene n-type doping levels found
while evaluating Pos2D (Figure 3d).
Graphene work-function shift with doping significantly decreased when the graphene
layer number increased [101]. The main decline occurs with changing from single-layer to
two-layer graphene [101]. The work function of the 4–5-layer graphene was the same as that
of the pristine undoped ultra-thin graphite [101]. The work function of graphene placed on
SiO2 decrease (increase) with graphene dopant concentration is significantly suppressed by
increasing the graphene layer number [102]. This is because of the charge transfer from
Nanomaterials 2022, 12, 1640
13 of 19
SiO2 to the graphene and subsequent charge redistribution within the graphene [103]. The
charge in graphene decays exponentially with distance from the substrate resulting in
suppressed changes in the few-layer graphene work function [101]. Numerous defects
found in the directly synthesized graphene by Raman scattering spectroscopy (Figure 2a)
can also reduce the graphene’s work function shift [104].
The analysis of samples’ I-V characteristics measured at different temperatures revealed the flow of the tunneling and thermionic emission currents (Supplementary Materials S4). At lower measurement temperatures, the tunneling current dominated (Supplementary Materials S4, Figures S19 and S20). For I-V characteristics measured at higher
temperatures of 30 and 40 ◦ C, the current is dominated by the thermionic emission at low
reverse biases, and at higher voltages, the tunneling current prevailed (Supplementary
Materials S4). The tunneling current via ultra-thin dielectric grown on the n-type semiconductor can be decreased by a fixed positive charge induced in the dielectric layer [94]. The
graphene Pos2D should downshift and the FWHM2D should increase with an increase in
doping and, hence, increased native oxide surface positive charge density. Thus, the reverse
current and IR /IF ratios decrease with the graphene substrate-induced self-doping seen in
Figure 3d–i is in good accordance with this assumption. In such a way, the ISC increase with
IR /IF ratio and with IR can be explained by the flow of the tunneling photocurrent similarly
to the quantum dot and superlattice solar cells where the tunneling effect was used to raise
the short-circuit current [105,106]. It should be mentioned that, in the graphene/ultra-thin
dielectric/Si solar cells, short-circuit current increases with tunneling current [32]. In addition, graphene/ultra-thin dielectric/Si photodiodes photoresponsivity also increases with
increased tunneling current [107,108].
It was revealed that the silicon substrate hydrogen plasma pre-annealing was a very
important technological parameter regarding the photovoltaic parameters. An increase in
the annealing temperature to 900 ◦ C resulted in suppression of the photovoltaic effect. The
AFM study revealed no clear morphology and phase changes due to the silicon surface
treatment by hydrogen plasma at both 700 and 900 ◦ C (Table S3). Si(100) surface plasma
annealing at 700 ◦ C resulted in no work function changes. However, plasma treatment
at 900 ◦ C decreased the substrate surface work function by ~0.05 eV, indicating a silicon
surface electronic structure change (Table S3). Thus, in the present study, the effects of
initial substrate surface electronic structure on graphene/Si device photovoltaic properties
were more significant than differences in the graphene structure.
UOC did not depend on the ISC and increased with ISC /IR ratio for ratios up to 1–1.5
(Figure 7). It can be explained by relatively large dark reverse currents found in studied
samples [109]. That is because UOC , differently from the ISC , usually is decreased due
to the tunneling [110]. Reduced UOC with increased leakage current was reported for
multi-crystalline silicon [111], organic [112,113], and graphene/GaAs [114] solar cells.
5. Conclusions
In conclusion, the graphene synthesis conditions, structure, and substrate treatment’s effects on directly synthesized graphene/n-Si(100) photovoltaic devices properties were revealed.
The graphene n-type self-doping due to the charge transfer from the native SiO2
interlayer to the graphene was the main reason for the notable reverse current (IR ) and
short-circuit current (ISC ) decrease. Due to the tunneling photocurrent flow, the UOC increased with a short-circuit current, and the reverse current ratio increased. Significant hydrogen plasma pre-treatment effects on the current-voltage characteristics and photovoltaic
parameters were observed, revealing the importance of the graphene/silicon interface.
It was found that the graphene samples’ work functions were nearly the same (4.820–4.826 eV),
even though the graphene structure and properties of the photovoltaic devices varied dramatically. No effects of graphene surface morphology and defects on the electrical and
photovoltaic characteristics were found. The short-circuit current and open-circuit voltage
only slightly increased with graphene layer number.
Nanomaterials 2022, 12, 1640
14 of 19
Thus, directly synthesized graphene/n-Si solar cells can be improved by preventing ntype self-doping and optimizing the graphene/silicon interface, whereas graphene defects,
layer number, work function, and morphology are much less critical.
Supplementary Materials: The following supporting information can be downloaded at https:
//www.mdpi.com/article/10.3390/nano12101640/s1: Figure S1: Schematic of an enclosure that was
used during the MW-PECVD process to prevent direct plasma effects; Figure S2: AFM image (a),
AFM phase image (b), and height profile (c) of A1 sample; Figure S3: AFM image (a), AFM phase
image (b), and height profile (c) of A7 sample; Figure S4: AFM image (a), AFM phase image (b),
and height profile (c) of A8 sample; Figure S5: AFM image (a), AFM phase image (b), and height
profile (c) of A11 sample; Figure S6: AFM image (a), AFM phase image (b), and height profile (c) of
B1 sample; Figure S7: AFM image (a), AFM phase image (b), and height profile (c) of B2 sample;
Figure S8: AFM image (a), AFM phase image (b), and height profile (c) of B3 sample; Figure S9: AFM
image (a), AFM phase image (b), and height profile (c) of C2 sample; Figure S10: AFM image (a), AFM
phase image (b), and height profile (c) of C3 sample; Figure S11: FWHMG vs. ID/IG (a) and FWHMG
vs. PosG (b) plots; Figure S12: I-V characteristic parameters: (a,d) IR(0.3 V); (b,e) IR(0.1 V)/IF(0.1 V);
(c,f) IR(0.3 V)/IF(0.3 V); in relation with (a-c) I2D/IG; (d-f) ID/IG; Figure S13: I-V characteristic
parameters: (a) IR(0.3 V); (b) IR(0.3 V)/IF(0.3 V) in relation with surface roughness; Figure S14: ISC vs.
ID/IG (a) and UOC vs. ID/IG (b) plots showing a difference between devices measured at 800 nm
illumination (solid) and 406 nm illumination (hollow); Figure S15: UOC vs. PosG plot; Figure S16: ISC
(a,c) and UOC (b,d) relation with respect to ID/IG (a,b) and FWHM2D (c,d) under 800 nm illumination; Figure S17: ISC (a) and UOC (b) and sample roughness relation under 800 nm illumination;
Figure S18: Diode I-V and UOC (at 800 nm illumination) relation; Figure S19: Different charge
transport mechanisms estimated from typical fabricated diode I-V graphs under various thermal
conditions: (a) Poole–Frenkel mechanism; (b) Image-force-induced charge transport; (c) Thermionic
emission; Figure S20: Diode operating regimes in terms of temperature: (a) typical I-V characteristics
measured in the dark at different temperatures (253–313 K); (b) The Arrhenius plot; (c) ln(σ/T2)
vs. 1000/T plot; Table S1: Summarized benchmark showing PCE values and PCE enhancement
techniques of the CVD-synthesized graphene/Si solar cells investigated by different research groups;
Table S2: Probable doping and strain effects governing main graphene’s Raman peak (G, 2D) positions and their FWHM; Table S3: Hydrogen plasma pre-treatment effects on Si(100) substrate surface.
References [115–125] are cited in the Supplementary Materials.
Author Contributions: Conceptualization, R.G. and Š.M.; investigation, Š.J., R.G., A.V. and A.G.;
writing—original draft preparation, Š.J. and Š.M.; writing—review and editing, Š.M., Š.J. and A.G.;
visualization, Š.J. and A.G.; project administration, Š.M.; funding acquisition, Š.M. All authors have
read and agreed to the published version of the manuscript.
Funding: The research project No. 09.3.3-LMT-K-712-01-0183 is funded under the European Social
Fund measure “Strengthening the Skills and Capacities of Public Sector Researchers for Engaging in
High Level R&D Activities” administered by the Research Council of Lithuania.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Acknowledgments: The research project No. 09.3.3-LMT-K-712-01-0183 is funded under the European Social Fund measure “Strengthening the Skills and Capacities of Public Sector Researchers for
Engaging in High Level R&D Activities” administered by the Research Council of Lithuania. The
authors acknowledge other participants of the research project No. 09.3.3-LMT-K-712-01-0183—A.
Vasiliauskas, A. Guobienė, K. Šlapikas, V. Stankus, D. Peckus, E. Rajackaitė, T. Tamulevičius, A.
Jurkevičiūtė, and F. Kalyk.
Conflicts of Interest: The authors declare no conflict of interest.
Nanomaterials 2022, 12, 1640
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References
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
28.
29.
Novoselov, K.S. Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306, 666–669. [CrossRef] [PubMed]
Nair, R.R.; Blake, P.; Grigorenko, A.N.; Novoselov, K.S.; Booth, T.J.; Stauber, T.; Peres, N.M.R.; Geim, A.K. Fine Structure Constant
Defines Visual Transparency of Graphene. Science 2008, 320, 1308. [CrossRef] [PubMed]
Morozov, S.V.; Novoselov, K.S.; Katsnelson, M.I.; Schedin, F.; Elias, D.C.; Jaszczak, J.A.; Geim, A.K. Giant Intrinsic Carrier
Mobilities in Graphene and Its Bilayer. Phys. Rev. Lett. 2008, 100, 016602. [CrossRef] [PubMed]
Lee, C.; Wei, X.; Kysar, J.W.; Hone, J. Measurement of the Elastic Properties and Intrinsic Strength of Monolayer Graphene. Science
2008, 321, 385–388. [CrossRef]
Bonaccorso, F.; Sun, Z.; Hasan, T.; Ferrari, A.C. Graphene Photonics and Optoelectronics. Nat. Photonics 2010, 4, 611–622.
[CrossRef]
Li, X.; Tao, L.; Chen, Z.; Fang, H.; Li, X.; Wang, X.; Xu, J.-B.; Zhu, H. Graphene and Related Two-Dimensional Materials:
Structure-Property Relationships for Electronics and Optoelectronics. Appl. Phys. Rev. 2017, 4, 021306. [CrossRef]
Huang, K.; Yu, X.; Cong, J.; Yang, D. Progress of Graphene-Silicon Heterojunction Photovoltaic Devices. Adv. Mater. Interf. 2018,
5, 1801520. [CrossRef]
Wirth-Lima, A.J.; Alves-Sousa, P.P.; Bezerra-Fraga, W. Graphene/Silicon and 2D-MoS2 /Silicon Solar Cells: A Review. Appl. Phys.
A 2019, 125, 241. [CrossRef]
Bhopal, M.F.; Lee, D.W.; ur Rehman, A.; Lee, S.H. Past and Future of Graphene/Silicon Heterojunction Solar Cells: A Review.
J. Mater. Chem. C 2017, 5, 10701–10714. [CrossRef]
Kong, X.; Zhang, L.; Liu, B.; Gao, H.; Zhang, Y.; Yan, H.; Song, X. Graphene/Si Schottky Solar Cells: A Review of Recent Advances
and Prospects. RSC Adv. 2019, 9, 863–877. [CrossRef]
Song, L.; Yu, X.; Yang, D. A Review on Graphene-Silicon Schottky Junction Interface. J. Alloy. Compd. 2019, 806, 63–70. [CrossRef]
Shin, D.H.; Choi, S.-H. Use of Graphene for Solar Cells. J. Korean Phys. Soc. 2018, 72, 1442–1453. [CrossRef]
Abdullah, M.F.; Hashim, A.M. Review and Assessment of Photovoltaic Performance of Graphene/Si Heterojunction Solar Cells.
J. Mater. Sci. 2019, 54, 911–948. [CrossRef]
Ju, S.; Liang, B.; Wang, J.-Z.; Shi, Y.; Li, S.-L. Graphene/Silicon Schottky Solar Cells: Technical Strategies for Performance
Optimization. Opt. Commun. 2018, 428, 258–268. [CrossRef]
Cui, K.; Maruyama, S. Multifunctional Graphene and Carbon Nanotube Films for Planar Heterojunction Solar Cells. Prog. Energy
Combust. Sci. 2019, 70, 1–21. [CrossRef]
Shin, D.H.; Kwak, G.Y.; Kim, J.M.; Jang, C.W.; Choi, S.-H.; Kim, K.J. Remarkable Enhancement of Stability in High-Efficiency
Si-Quantum-Dot Heterojunction Solar Cells by Employing Bis(Trifluoromethanesulfonyl)-Amide as a Dopant for Graphene
Transparent Conductive Electrodes. J. Alloy. Compd. 2019, 773, 913–918. [CrossRef]
Li, X.; Zhu, H.; Wang, K.; Cao, A.; Wei, J.; Li, C.; Jia, Y.; Li, Z.; Li, X.; Wu, D. Graphene-On-Silicon Schottky Junction Solar Cells.
Adv. Mater. 2010, 22, 2743–2748. [CrossRef]
Wirth-Lima, A.J.; Alves-Sousa, P.P.; Bezerra-Fraga, W. N-Graphene/p-Silicon-Based Schottky Junction Solar Cell, with Very High
Power Conversion Efficiency. SN Appl. Sci. 2020, 2, 246. [CrossRef]
Badhulika, S.; Terse-Thakoor, T.; Villarreal, C.; Mulchandani, A. Graphene Hybrids: Synthesis Strategies and Applications in
Sensors and Sensitized Solar Cells. Front. Chem. 2015, 3, 38. [CrossRef]
Haigh, S.J.; Gholinia, A.; Jalil, R.; Romani, S.; Britnell, L.; Elias, D.C.; Novoselov, K.S.; Ponomarenko, L.A.; Geim, A.K.; Gorbachev,
R. Cross-Sectional Imaging of Individual Layers and Buried Interfaces of Graphene-Based Heterostructures and Superlattices.
Nat. Mater. 2012, 11, 764–767. [CrossRef]
Liang, X.; Sperling, B.A.; Calizo, I.; Cheng, G.; Hacker, C.A.; Zhang, Q.; Obeng, Y.; Yan, K.; Peng, H.; Li, Q.; et al. Toward Clean
and Crackless Transfer of Graphene. ACS Nano 2011, 5, 9144–9153. [CrossRef] [PubMed]
Ihm, K.; Lim, J.T.; Lee, K.-J.; Kwon, J.W.; Kang, T.-H.; Chung, S.; Bae, S.; Kim, J.H.; Hong, B.H.; Yeom, G.Y. Number of Graphene
Layers as a Modulator of the Open-Circuit Voltage of Graphene-Based Solar Cell. Appl. Phys. Lett. 2010, 97, 032113. [CrossRef]
Suhail, A.; Pan, G.; Jenkins, D.; Islam, K. Improved Efficiency of Graphene/Si Schottky Junction Solar Cell Based on Back Contact
Structure and DUV Treatment. Carbon 2018, 129, 520–526. [CrossRef]
Li, Y.F.; Yang, W.; Tu, Z.Q.; Liu, Z.C.; Yang, F.; Zhang, L.Q.; Hatakeyama, R. Schottky Junction Solar Cells Based on Graphene with
Different Numbers of Layers. Appl. Phys. Lett. 2014, 104, 043903. [CrossRef]
Li, X.; Xie, D.; Park, H.; Zeng, T.H.; Wang, K.; Wei, J.; Zhong, M.; Wu, D.; Kong, J.; Zhu, H. Anomalous Behaviors of Graphene
Transparent Conductors in Graphene-Silicon Heterojunction Solar Cells. Adv. Energy Mater. 2013, 3, 1029–1034. [CrossRef]
Das, S.; Pandey, D.; Thomas, J.; Roy, T. The Role of Graphene and Other 2D Mater. in Solar Photovoltaics. Adv. Mater. 2019,
31, 1802722. [CrossRef]
Shin, D.H.; Kim, J.H.; Jung, D.H.; Choi, S.-H. Graphene-Nanomesh Transparent Conductive Electrode/Porous-Si SchottkyJunction Solar Cells. J. Alloy. Compd. 2019, 803, 958–963. [CrossRef]
Lin, Y.-K.; Hong, Y.-T.; Shyue, J.-J.; Hsueh, C.-H. Construction of Schottky Junction Solar Cell Using Silicon Nanowires and
Multi-Layered Graphene. Superlattices Microstruct. 2019, 126, 42–48. [CrossRef]
Chugh, S.; Mehta, R.; Lu, N.; Dios, F.D.; Kim, M.J.; Chen, Z. Comparison of Graphene Growth on Arbitrary Non-Catalytic
Substrates Using Low-Temperature PECVD. Carbon 2015, 93, 393–399. [CrossRef]
Nanomaterials 2022, 12, 1640
30.
31.
32.
33.
34.
35.
36.
37.
38.
39.
40.
41.
42.
43.
44.
45.
46.
47.
48.
49.
50.
51.
52.
53.
54.
55.
16 of 19
Jiao, T.; Liu, J.; Wei, D.; Feng, Y.; Song, X.; Shi, H.; Jia, S.; Sun, W.; Du, C. Composite Transparent Electrode of Graphene Nanowalls
and Silver Nanowires on Micropyramidal Si for High-Efficiency Schottky Junction Solar Cells. ACS Appl. Mater. Interfaces 2015, 7,
20179–20183. [CrossRef]
Liu, J.; Sun, W.; Wei, D.; Song, X.; Jiao, T.; He, S.; Zhang, W.; Du, C. Direct Growth of Graphene Nanowalls on the Crystalline
Silicon for Solar Cells. Appl. Phys. Lett. 2015, 106, 043904. [CrossRef]
Rehman, M.A.; Akhtar, I.; Choi, W.; Akbar, K.; Farooq, A.; Hussain, S.; Shehzad, M.A.; Chun, S.-H.; Jung, J.; Seo, Y. Influence of an
Al2 O3 Interlayer in a Directly Grown Graphene-Silicon Schottky Junction Solar Cell. Carbon 2018, 132, 157–164. [CrossRef]
Rehman, M.A.; Roy, S.B.; Akhtar, I.; Bhopal, M.F.; Choi, W.; Nazir, G.; Khan, M.F.; Kumar, S.; Eom, J.; Chun, S.-H.; et al.
Thickness-Dependent Efficiency of Directly Grown Graphene Based Solar Cells. Carbon 2019, 148, 187–195. [CrossRef]
Rehman, M.A.; Roy, S.B.; Gwak, D.; Akhtar, I.; Nasir, N.; Kumar, S.; Khan, M.F.; Heo, K.; Chun, S.-H.; Seo, Y. Solar Cell Based on
Vertical Graphene Nano Hills Directly Grown on Silicon. Carbon 2020, 164, 235–243. [CrossRef]
Bhopal, M.F.; von Lee, D.; Lee, S.H.; Lee, A.R.; Kim, H.J.; Lee, S.H. Selective Nickel/Silver Front Metallization for Graphene/Silicon
Solar Cells. Mater. Lett. 2019, 234, 237–240. [CrossRef]
Meng, J.-H.; Liu, X.; Zhang, X.-W.; Zhang, Y.; Wang, H.-L.; Yin, Z.-G.; Zhang, Y.-Z.; Liu, H.; You, J.-B.; Yan, H. Interface Engineering
for Highly Efficient Graphene-on-Silicon Schottky Junction Solar Cells by Introducing a Hexagonal Boron Nitride Interlayer.
Nano Energy 2016, 28, 44–50. [CrossRef]
Zhou, Q.; Liu, X.; Zhang, E.; Luo, S.; Shen, J.; Wang, Y.; Wei, D. The Controlled Growth of Graphene Nanowalls on Si for Schottky
Photodetector. AIP Adv. 2017, 7, 125317. [CrossRef]
Jiao, T.; Wei, D.; Song, X.; Sun, T.; Yang, J.; Yu, L.; Feng, Y.; Sun, W.; Wei, W.; Shi, H.; et al. High-Efficiency, Stable and
Non-Chemically Doped Graphene–Si Solar Cells through Interface Engineering and PMMA Antireflection. RSC Adv. 2016, 6,
10175–10179. [CrossRef]
Gnisci, A.; Faggio, G.; Lancellotti, L.; Messina, G.; Carotenuto, R.; Bobeico, E.; Delli Veneri, P.; Capasso, A.; Dikonimos, T.; Lisi, N.
The Role of Graphene-Based Derivative as Interfacial Layer in Graphene/N-Si Schottky Barrier Solar Cells. Phys. Status Solidi A
2019, 216, 1800555. [CrossRef]
Zhang, L.; Huang, F.; Li, S.; He, S.; Yu, M.; Fu, J.; Yang, Q.; Huang, R.; Cheng, Q. Interface Engineering for Graphene
Nanowalls/Silicon Schottky Solar Cells Prepared by Polymer-Free Transfer Method. J. Appl. Phys. 2020, 128, 025301. [CrossRef]
Chandramohan, S.; Janardhanam, V.; Seo, T.H.; Hong, C.-H.; Suh, E.-K. Improved Photovoltaic Effect in Graphene/Silicon Solar
Cell Using MoO3 /Ag/MoO3 Multilayer Coating. Mater. Lett. 2019, 246, 103–106. [CrossRef]
Miao, X.; Tongay, S.; Petterson, M.K.; Berke, K.; Rinzler, A.G.; Appleton, B.R.; Hebard, A.F. High Efficiency Graphene Solar Cells
by Chemical Doping. Nano Lett. 2012, 12, 2745–2750. [CrossRef] [PubMed]
Kern, W. The Evolution of Silicon Wafer Cleaning Technology. J. Electrochem. Soc. 1990, 137, 1887–1892.
Pour-mohammadi, Z.; Amirmazlaghani, M. Asymmetric Finger-Shape Metallization in Graphene-on-Si Solar Cells for Enhanced
Carrier Trapping. Mater. Sci. Semicond. Process. 2019, 91, 13–21. [CrossRef]
Kalita, G.; Wakita, K.; Umeno, M.; Tanemura, M. Fabrication and Characteristics of Solution-Processed Graphene Oxide-Silicon
Heterojunction. Phys. Status Solidi Rapid Res. Lett. 2013, 7, 340–343. [CrossRef]
Behura, S.K.; Nayak, S.; Mukhopadhyay, I.; Jani, O. Junction Characteristics of Chemically-Derived Graphene/p-Si Heterojunction
Solar Cell. Carbon 2014, 67, 766–774. [CrossRef]
Merlen, A.; Buijnsters, J.G.; Pardanaud, C. A Guide to and Review of the Use of Multiwavelength Raman Spectroscopy for
Characterizing Defective Aromatic Carbon Solids: From Graphene to Amorphous Carbons. Coatings 2017, 7, 153. [CrossRef]
Childres, I.; Jauregui, L.A.; Tian, J.; Chen, Y.P. Effect of Oxygen Plasma Etching on Graphene Studied Using Raman Spectroscopy
and Electronic Transport Measurements. New J. Phys. 2011, 13, 025008. [CrossRef]
Hwang, J.-S.; Lin, Y.-H.; Hwang, J.-Y.; Chang, R.; Chattopadhyay, S.; Chen, C.-J.; Chen, P.; Chiang, H.-P.; Tsai, T.-R.; Chen, L.-C.;
et al. Imaging Layer Number and Stacking Order through Formulating Raman Fingerprints Obtained from Hexagonal Single
Crystals of Few Layer Graphene. Nanotechnology 2013, 24, 015702. [CrossRef]
Mallet-Ladeira, P.; Puech, P.; Toulouse, C.; Cazayous, M.; Ratel-Ramond, N.; Weisbecker, P.; Vignoles, G.L.; Monthioux, M. A
Raman Study to Obtain Crystallite Size of Carbon Materials: A Better Alternative to the Tuinstra–Koenig Law. Carbon 2014, 80,
629–639. [CrossRef]
Casiraghi, C.; Pisana, S.; Novoselov, K.S.; Geim, A.K.; Ferrari, A.C. Raman Fingerprint of Charged Impurities in Graphene. Appl.
Phys. Lett. 2007, 91, 233108. [CrossRef]
Vinchon, P.; Glad, X.; Robert-Bigras, G.; Martel, R.; Sarkissian, A.; Stafford, L. A Combination of Plasma Diagnostics and Raman
Spectroscopy to Examine Plasma-Graphene Interactions in Low-Pressure Argon Radiofrequency Plasmas. J. Appl. Phys. 2019,
126, 233302. [CrossRef]
Ferrari, A.C.; Meyer, J.C.; Scardaci, V.; Casiraghi, C.; Lazzeri, M.; Mauri, F.; Piscanec, S.; Jiang, D.; Novoselov, K.S.; Roth, S.; et al.
Raman Spectrum of Graphene and Graphene Layers. Phys. Rev. Lett. 2006, 97, 187401. [CrossRef] [PubMed]
Eckmann, A.; Felten, A.; Mishchenko, A.; Britnell, L.; Krupke, R.; Novoselov, K.S.; Casiraghi, C. Probing the Nature of Defects in
Graphene by Raman Spectroscopy. Nano Lett. 2012, 12, 3925–3930. [CrossRef] [PubMed]
Khan, A.; Islam, S.M.; Ahmed, S.; Kumar, R.R.; Habib, M.R.; Huang, K.; Hu, M.; Yu, X.; Yang, D. Direct CVD Growth of Graphene
on Technologically Important Dielectric and Semiconducting Substrates. Adv. Sci. 2018, 5, 1800050. [CrossRef] [PubMed]
Nanomaterials 2022, 12, 1640
56.
57.
58.
59.
60.
61.
62.
63.
64.
65.
66.
67.
68.
69.
70.
71.
72.
73.
74.
75.
76.
77.
78.
79.
80.
81.
82.
17 of 19
Stubrov, Y.; Nikolenko, A.; Strelchuk, V.; Nedilko, S.; Chornii, V. Structural Modification of Single-Layer Graphene Under Laser
Irradiation Featured by Micro-Raman Spectroscopy. Nanoscale Res. Lett. 2017, 12, 297. [CrossRef]
Nemes-Incze, P.; Osváth, Z.; Kamarás, K.; Biró, L.P. Anomalies in Thickness Measurements of Graphene and Few Layer Graphite
Crystals by Tapping Mode Atomic Force Microscopy. Carbon N Y 2008, 46, 1435–1442. [CrossRef]
Yao, Y.; Ren, L.; Gao, S.; Li, S. Histogram Method for Reliable Thickness Measurements of Graphene Films Using Atomic Force
Microscopy (AFM). J. Mater. Sci. Technol. 2017, 33, 815–820. [CrossRef]
Kim, S.; Ryu, S. Thickness-Dependent Native Strain in Graphene Membranes Visualized by Raman Spectroscopy. Carbon 2016,
100, 283–290. [CrossRef]
Lee, J.E.; Ahn, G.; Shim, J.; Lee, Y.S.; Ryu, S. Optical Separation of Mechanical Strain from Charge Doping in Graphene. Nat.
Commun. 2012, 3, 1024. [CrossRef]
Gayathri, S.; Jayabal, P.; Kottaisamy, M.; Ramakrishnan, V. Synthesis of Few Layer Graphene by Direct Exfoliation of Graphite
and a Raman Spectroscopic Study. AIP Adv. 2014, 4, 027116. [CrossRef]
Tai, L.; Zhu, D.; Liu, X.; Yang, T.; Wang, L.; Wang, R.; Jiang, S.; Chen, Z.; Xu, Z.; Li, X. Direct Growth of Graphene on Silicon by
Metal-Free Chemical Vapor Deposition. Nano-Micro Lett. 2018, 10, 20. [CrossRef] [PubMed]
Neumann, C.; Reichardt, S.; Venezuela, P.; Drögeler, M.; Banszerus, L.; Schmitz, M.; Watanabe, K.; Taniguchi, T.; Mauri, F.;
Beschoten, B.; et al. Raman Spectroscopy as Probe of Nanometre-Scale Strain Variations in Graphene. Nat. Commun. 2015, 6, 8429.
[CrossRef] [PubMed]
Tang, B.; Guoxin, H.; Gao, H. Raman Spectroscopic Characterization of Graphene. Appl. Spectrosc. Rev. 2010, 45, 369–407.
[CrossRef]
Moon, J.-Y.; Kim, M.; Kim, S.-I.; Xu, S.; Choi, J.-H.; Whang, D.; Watanabe, K.; Taniguchi, T.; Park, D.S.; Seo, J.; et al. LayerEngineered Large-Area Exfoliation of Graphene. Sci. Adv. 2020, 6, eabc6601. [CrossRef]
Wu, J.-B.; Lin, M.-L.; Cong, X.; Liu, H.-N.; Tan, P.-H. Raman Spectroscopy of Graphene-Based Materials and Its Applications in
Related Devices. Chem. Soc. Rev. 2018, 47, 1822–1873. [CrossRef]
Ribeiro-Soares, J.; Oliveros, M.E.; Garin, C.; David, M.V.; Martins, L.G.P.; Almeida, C.A.; Martins-Ferreira, E.H.; Takai, K.; Enoki,
T.; Magalhães-Paniago, R.; et al. Structural Analysis of Polycrystalline Graphene Systems by Raman Spectroscopy. Carbon 2015,
95, 646–652. [CrossRef]
Pillet, G.; Sapelkin, A.; Bacsa, W.; Monthioux, M.; Puech, P. Size-controlled Graphene-based Materials Prepared by Annealing of
Pitch-based Cokes: G Band Phonon Line Broadening Effects Due to High Pressure, Crystallite Size, and Merging with D′ Band.
J. Raman Spectrosc. 2019, 50, 1861–1866. [CrossRef]
Das, A.; Pisana, S.; Chakraborty, B.; Piscanec, S.; Saha, S.K.; Waghmare, U.V.; Novoselov, K.S.; Krishnamurthy, H.R.; Geim, A.K.;
Ferrari, A.C.; et al. Monitoring Dopants by Raman Scattering in an Electrochemically Top-Gated Graphene Transistor. Nat.
Nanotechnol. 2008, 3, 210–221. [CrossRef]
Casiraghi, C. Probing Disorder and Charged Impurities in Graphene by Raman Spectroscopy. Phys. Status Solidi Rapid Res. Lett.
2009, 3, 175–177. [CrossRef]
Fates, R.; Bouridah, H.; Raskin, J.-P. Probing Carrier Concentration in Gated Single, Bi- and Tri-Layer CVD Graphene Using
Raman Spectroscopy. Carbon 2019, 149, 390–399. [CrossRef]
Lee, U.; Han, Y.; Lee, S.; Kim, J.S.; Lee, Y.H.; Kim, U.J.; Son, H. Time Evolution Studies on Strain and Doping of Graphene Grown
on a Copper Substrate Using Raman Spectroscopy. ACS Nano 2020, 14, 919–926. [CrossRef] [PubMed]
Khalil, H.M.W.; Nam, J.T.; Kim, K.S.; Noh, H. Controlled N-Doping in Chemical Vapour Deposition Grown Graphene by
Antimony. J. Phys. D Appl. Phys. 2015, 48, 015307. [CrossRef]
Bissett, M.A.; Tsuji, M.; Ago, H. Mechanical Strain of Chemically Functionalized Chemical Vapor Deposition Grown Graphene.
J. Phys. Chem. C 2013, 117, 3152–3159. [CrossRef]
Gudaitis, R.; Lazauskas, A.; Jankauskas, Š.; Meškinis, Š. Catalyst-Less and Transfer-Less Synthesis of Graphene on Si(100) Using
Direct Microwave Plasma Enhanced Chemical Vapor Deposition and Protective Enclosures. Materials 2020, 13, 5630. [CrossRef]
[PubMed]
Kiraly, B.; Jacobberger, R.M.; Mannix, A.J.; Campbell, G.P.; Bedzyk, M.J.; Arnold, M.S.; Hersam, M.C.; Guisinger, N.P. Electronic
and Mechanical Properties of Graphene–Germanium Interfaces Grown by Chemical Vapor Deposition. Nano Lett. 2015, 15,
7414–7420. [CrossRef]
Banszerus, L.; Janssen, H.; Otto, M.; Epping, A.; Taniguchi, T.; Watanabe, K.; Beschoten, B.; Neumaier, D.; Stampfer, C. Identifying
Suitable Substrates for High-Quality Graphene-Based Heterostructures. 2D Mater. 2017, 4, 025030. [CrossRef]
Kang, Y.-J.; Kang, J.; Chang, K.J. Electronic Structure of Graphene and Doping Effect on SiO2 . Phys. Rev. B 2008, 78, 115404.
[CrossRef]
Tung, R.T. Recent Advances in Schottky Barrier Concepts. Mater. Sci. Eng. R Rep. 2001, 35, 1–138. [CrossRef]
Ali, M.Y.; Tao, M. Effect of Sulfur Passivation of Silicon (100) on Schottky Barrier Height: Surface States versus Surface Dipole.
J. Appl. Phys. 2007, 101, 103708. [CrossRef]
Tao, M.; Udeshi, D.; Agarwal, S.; Maldonado, E.; Kirk, W.P. Negative Schottky Barrier between Titanium and N-Type Si(001) for
Low-Resistance Ohmic Contacts. Solid-State Electron. 2004, 48, 335–338. [CrossRef]
Martín, I.; Vetter, M.; Orpella, A.; Voz, C.; Puigdollers, J.; Alcubilla, R.; Kharchenko, A.V.; Roca i Cabarrocas, P. Improvement of
Crystalline Silicon Surface Passivation by Hydrogen Plasma Treatment. Appl. Phys. Lett. 2004, 84, 1474–1476. [CrossRef]
Nanomaterials 2022, 12, 1640
83.
84.
85.
86.
87.
88.
89.
90.
91.
92.
93.
94.
95.
96.
97.
98.
99.
100.
101.
102.
103.
104.
105.
106.
107.
108.
109.
18 of 19
Soman, A.; Antony, A. A Critical Study on Different Hydrogen Plasma Treatment Methods of A-Si: H/c-Si Interface for Enhanced
Defect Passivation. Appl. Surf. Sci. 2021, 553, 149551. [CrossRef]
Yamada, T.; Ohmi, H.; Okamoto, K.; Kakiuchi, H.; Yasutake, K. Effects of Surface Temperature on High-Rate Etching of Silicon by
Narrow-Gap Microwave Hydrogen Plasma. Jpn. J. Appl. Phys. 2012, 51, 10NA09. [CrossRef]
Ishii, M. Effects of Substrate Temperature and Bias Potential on Hydrogen Plasma Etching of Silicon. J. Vac. Sci. Technol. B
Microelectron. Nanometer Struct. 1994, 12, 2342. [CrossRef]
Schüttauf, J.W.A.; van der Werf, C.H.M.; van Sark, W.G.J.H.M.; Rath, J.K.; Schropp, R.E.I. Comparison of Surface Passivation of
Crystalline Silicon by A-Si:H with and without Atomic Hydrogen Treatment Using Hot-Wire Chemical Vapor Deposition. Thin
Solid Film. 2011, 519, 4476–4478. [CrossRef]
Lavrov, E.V.; Weber, J. Evolution of Hydrogen Platelets in Silicon Determined by Polarized Raman Spectroscopy. Phys. Rev. Lett.
2001, 87, 185502. [CrossRef]
Zhu, W.; Low, T.; Perebeinos, V.; Bol, A.A.; Zhu, Y.; Yan, H.; Tersoff, J.; Avouris, P. Structure and Electronic Transport in Graphene
Wrinkles. Nano Lett. 2012, 12, 3431–3436. [CrossRef]
Zhong, H.; Liu, Z.; Shi, L.; Xu, G.; Fan, Y.; Huang, Z.; Wang, J.; Ren, G.; Xu, K. Graphene in Ohmic Contact for Both N-GaN and
p-GaN. Appl. Phys. Lett. 2014, 104, 212101. [CrossRef]
Capasso, A.; Salamandra, L.; Faggio, G.; Dikonimos, T.; Buonocore, F.; Morandi, V.; Ortolani, L.; Lisi, N. Chemical Vapor
Deposited Graphene-Based Derivative as High-Performance Hole Transport Material for Organic Photovoltaics. ACS Appl. Mater.
Interfaces 2016, 8, 23844–23853. [CrossRef]
Yavuz, S.; Kuru, C.; Choi, D.; Kargar, A.; Jin, S.; Bandaru, P.R. Graphene Oxide as a P-Dopant and an Anti-Reflection Coating
Layer, in Graphene/Silicon Solar Cells. Nanoscale 2016, 8, 6473–6478. [CrossRef] [PubMed]
Larsen, L.J.; Shearer, C.J.; Ellis, A.V.; Shapter, J.G. Optimization and Doping of Reduced Graphene Oxide–Silicon Solar Cells.
J. Phys. Chem. C 2016, 120, 15648–15656. [CrossRef]
Adhikari, S.; Biswas, C.; Doan, M.-H.; Kim, S.-T.; Kulshreshtha, C.; Lee, Y.H. Minimizing Trap Charge Density towards an Ideal
Diode in Graphene–Silicon Schottky Solar Cell. ACS Appl. Mater. Interfaces 2019, 11, 880–888. [CrossRef] [PubMed]
Song, Y.; Li, X.; Mackin, C.; Zhang, X.; Fang, W.; Palacios, T.; Zhu, H.; Kong, J. Role of Interfacial Oxide in High-Efficiency
Graphene–Silicon Schottky Barrier Solar Cells. Nano Lett. 2015, 15, 2104–2110. [CrossRef] [PubMed]
Zhong, H.; Xu, K.; Liu, Z.; Xu, G.; Shi, L.; Fan, Y.; Wang, J.; Ren, G.; Yang, H. Charge Transport Mechanisms of
Graphene/Semiconductor Schottky Barriers: A Theoretical and Experimental Study. J. Appl. Phys. 2014, 115, 013701.
[CrossRef]
Zhang, X.; Zhang, L.; Chan, M. Doping Enhanced Barrier Lowering in Graphene-Silicon Junctions. Appl. Phys. Lett. 2016,
108, 263502. [CrossRef]
Wittmann, S.; Aumer, F.; Wittmann, D.; Pindl, S.; Wagner, S.; Gahoi, A.; Reato, E.; Belete, M.; Kataria, S.; Lemme, M.C. Dielectric
Surface Charge Engineering for Electrostatic Doping of Graphene. ACS Appl. Electron. Mater. 2020, 2, 1235–1242. [CrossRef]
Shi, Y.; Dong, X.; Chen, P.; Wang, J.; Li, L.-J. Effective Doping of Single-Layer Graphene from Underlying SiO2 . Phys. Rev. B 2009,
79, 115402. [CrossRef]
Miwa, R.H.; Schmidt, T.M.; Scopel, W.L.; Fazzio, A. Doping of Graphene Adsorbed on the A-SiO2 Surface. Appl. Phys. Lett. 2011,
99, 163108. [CrossRef]
Samaddar, S.; Coraux, J.; Martin, S.C.; Grévin, B.; Courtois, H.; Winkelmann, C.B. Equal Variations of the Fermi Level and Work
Function in Graphene at the Nanoscale. Nanoscale 2016, 8, 15162–15166. [CrossRef]
Leenaerts, O.; Partoens, B.; Peeters, F.M.; Volodin, A.; van Haesendonck, C. The Work Function of Few-Layer Graphene. J. Phys.
Condens. Matter 2017, 29, 035003. [CrossRef] [PubMed]
Ziegler, D.; Gava, P.; Güttinger, J.; Molitor, F.; Wirtz, L.; Lazzeri, M.; Saitta, A.M.; Stemmer, A.; Mauri, F.; Stampfer, C. Variations
in the Work Function of Doped Single- and Few-Layer Graphene Assessed by Kelvin Probe Force Microscopy and Density
Functional Theory. Phys. Rev. B 2011, 83, 235434. [CrossRef]
Renault, O.; Pascon, A.M.; Rotella, H.; Kaja, K.; Mathieu, C.; Rault, J.E.; Blaise, P.; Poiroux, T.; Barrett, N.; Fonseca, L.R.C. Charge
Spill-out and Work Function of Few-Layer Graphene on SiC(0 0 0 1). J. Phys. D Appl. Phys. 2014, 47, 295303. [CrossRef]
Akada, K.; Terasawa, T.; Imamura, G.; Obata, S.; Saiki, K. Control of Work Function of Graphene by Plasma Assisted Nitrogen
Doping. Appl. Phys. Lett. 2014, 104, 131602. [CrossRef]
Sugaya, T.; Numakami, O.; Furue, S.; Komaki, H.; Amano, T.; Matsubara, K.; Okano, Y.; Niki, S. Tunnel Current through a
Miniband in InGaAs Quantum Dot Superlattice Solar Cells. Sol. Energy Mater. Sol. Cells 2011, 95, 2920–2923. [CrossRef]
Wang, Y.; Wen, Y.; Sodabanlu, H.; Watanabe, K.; Sugiyama, M.; Nakano, Y. A Superlattice Solar Cell with Enhanced Short-Circuit
Current and Minimized Drop in Open-Circuit Voltage. IEEE J. Photovolt. 2012, 2, 387–392. [CrossRef]
Yin, J.; Liu, L.; Zang, Y.; Ying, A.; Hui, W.; Jiang, S.; Zhang, C.; Yang, T.; Chueh, Y.-L.; Li, J.; et al. Engineered Tunneling Layer with
Enhanced Impact Ionization for Detection Improvement in Graphene/Silicon Heterojunction Photodetectors. Light Sci. Appl.
2021, 10, 113. [CrossRef]
Xu, J.; Liu, T.; Hu, H.; Zhai, Y.; Chen, K.; Chen, N.; Li, C.; Zhang, X. Design and Optimization of Tunneling Photodetectors Based
on Graphene/Al2O3/Silicon Heterostructures. Nanophotonics 2020, 9, 3841–3848. [CrossRef]
Zhou, Y.; Khan, T.M.; Shim, J.W.; Dindar, A.; Fuentes-Hernandez, C.; Kippelen, B. All-Plastic Solar Cells with a High Photovoltaic
Dynamic Range. J. Mater. Chem. A 2014, 2, 3492. [CrossRef]
Nanomaterials 2022, 12, 1640
19 of 19
110. Varghese, A.; Yakimov, M.; Tokranov, V.; Mitin, V.; Sablon, K.; Sergeev, A.; Oktyabrsky, S. Complete Voltage Recovery in Quantum
Dot Solar Cells Due to Suppression of Electron Capture. Nanoscale 2016, 8, 7248–7256. [CrossRef]
111. Nishioka, K.; Sakitani, N.; Uraoka, Y.; Fuyuki, T. Analysis of Multicrystalline Silicon Solar Cells by Modified 3-Diode Equivalent
Circuit Model Taking Leakage Current through Periphery into Consideration. Sol. Energy Mater. Sol. Cells 2007, 91, 1222–1227.
[CrossRef]
112. Yang, W.; Luo, Y.; Guo, P.; Sun, H.; Yao, Y. Leakage Current Induced by Energetic Disorder in Organic Bulk Heterojunction
Solar Cells: Comprehending the Ultrahigh Loss of Open-Circuit Voltage at Low Temperatures. Phys. Rev. Appl. 2017, 7, 044017.
[CrossRef]
113. Tang, Y.; Bjuggren, J.M.; Fei, Z.; Andersson, M.R.; Heeney, M.; McNeill, C.R. Origin of Open-Circuit Voltage Turnover in Organic
Solar Cells at Low Temperature. Sol. RRL 2020, 4, 2000375. [CrossRef]
114. Li, Y.; Yu, M.; Cheng, Q. Improved Performance of Graphene/n-GaAs Heterojunction Solarcells by Introducing an ElectronBlocking/Hole-Transporting Layer. Mater. Res. Express 2018, 6, 016202. [CrossRef]
115. Armano, A.; Buscarino, G.; Cannas, M.; Gelardi, F.M.; Giannazzo, F.; Schilirò, E.; Agnello, S. Monolayer Graphene Doping and
Strain Dynamics Induced by Thermal Treatments in Controlled Atmosphere. Carbon 2018, 127, 270–279. [CrossRef]
116. Bissett, M.A.; Izumida, W.; Saito, R.; Ago, H. Effect of Domain Boundaries on the Raman Spectra of Mechanically Strained
Graphene. ACS Nano 2012, 6, 10229–10238. [CrossRef]
117. Frank, O.; Mohr, M.; Maultzsch, J.; Thomsen, C.; Riaz, I.; Jalil, R.; Novoselov, K.S.; Tsoukleri, G.; Parthenios, J.; Papagelis, K.; et al.
Raman 2D-Band Splitting in Graphene: Theory and Experiment. ACS Nano 2011, 5, 2231–2239. [CrossRef]
118. Shiwakoti, N.; Bobby, A.; Asokan, K.; Antony, B. Interface and Transport Properties of Gamma Irradiated Au/n-GaP Schottky
Diode. Mater. Sci. Semicond. Processing 2018, 74, 1–6. [CrossRef]
119. Becker, J.A.; Brattain, W.H. The Thermionic Work Function and the Slope and Intercept of Richardson Plots. Phys. Rev. 1934, 45,
694–705. [CrossRef]
120. Lin, T.; Xie, J.; Ning, S.; Ma, Z.; Mu, Y.; Sun, W.; Yang, S. Effect of Annealing Process Parameters on N-GaAs Ohmic Contacts.
Microelectron. Eng. 2022, 258, 111772. [CrossRef]
121. Lin, T.; Xie, J.; Ning, S.; Li, Q.; Li, B. Study on the P-Type Ohmic Contact in GaAs-Based Laser Diode. Mater. Sci. Semicond.
Processing 2021, 124, 105622. [CrossRef]
122. Latreche, A. Combined Thermionic Emission and Tunneling Mechanisms for the Analysis of the Leakage Current for Ga2 O3
Schottky Barrier Diodes. SN Appl. Sci. 2019, 1, 188. [CrossRef]
123. Arslan, E.; Çakmak, H.; Özbay, E. Forward Tunneling Current in Pt/p-InGaN and Pt/n-InGaN Schottky Barriers in a Wide
Temperature Range. Microelectron. Eng. 2012, 100, 51–56. [CrossRef]
124. An, Y.; Behnam, A.; Pop, E.; Ural, A. Metal-Semiconductor-Metal Photodetectors Based on Graphene/p-Type Silicon Schottky
Junctions. Appl. Phys. Lett. 2013, 102, 013110. [CrossRef]
125. Tomer, D.; Rajput, S.; Hudy, L.J.; Li, C.H.; Li, L. Carrier Transport in Reverse-Biased Graphene/Semiconductor Schottky Junctions.
Appl. Phys. Lett. 2015, 106, 173510. [CrossRef]