مرکزی صفحہ Journal of Molecular Structure Graphene for Si-based solar cells

Graphene for Si-based solar cells

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Journal of Molecular Structure
DOI:
10.1016/j.molstruc.2019.127055
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Graphene for Si-based solar cells
A. Altuntepe, A. Seyhan, R. Zan
PII:

S0022-2860(19)31155-X

DOI:

https://doi.org/10.1016/j.molstruc.2019.127055

Reference:

MOLSTR 127055

To appear in:

Journal of Molecular Structure

Received Date: 23 February 2019
Revised Date:

24 August 2019

Accepted Date: 9 September 2019

Please cite this article as: A. Altuntepe, A. Seyhan, R. Zan, Graphene for Si-based solar cells, Journal of
Molecular Structure (2019), doi: https://doi.org/10.1016/j.molstruc.2019.127055.
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Graphene for Si-based Solar Cells
1

1,2

1,2*

A. Altuntepe , A. Seyhan , R. Zan
1

Niğde Ömer Halisdemir University, Nanotechnology Application and Research Center, 51200, Niğde,
Turkey
2

Niğde Ömer Halisdemir University, Department of Physics, 51200, Niğde, Turkey

*recep.zan@ohu.edu.tr

ABSTRACT
In this paper, we report on the single layer graphene synthesis to establish the growth conditions and
improve the opto-electronic properties that can be employed in silicon based heterojunction solar
cells. To do this, the effect of hydrogen and methane flow on the graphene growth on copper foil in a
CVD system was investigated. The analyses were conducted by changing either the hydrogen or the
methane flow by keeping all the other growth parameters constant. Single layer graphene growth
recipe was established in order to have the optimum optical transmission and sheet resistance values
via amen; ding the graphene growth conditions. It was found that the sheet resistance values of the
single layer graphene should be lowered further to be used as transparent conductive electrode.
However, the combination of graphene with indium tin oxide film functioned well as transparent
conductive electrode in the silicon based the solar cells. Additionally, the cell efficiency increased by
about 10% as a result of incorporating it with the single layer graphene.

Keywords: Graphene, CVD, Growth, Copper, Raman, Solar Cell

1. Introduction
Graphene, which is one of the most up-to-date and heavily researched topics of recent years, is
the first two-dimensional material that can be synthesized. It is one of the allotropes of carbon and the
basic building block of some other well-known carbon forms like fullerene, carbon nanotube and
graphite[1]. Graphene has attractive magnetic properties and recently most of the researches have
been directed into this aspect. Particularly, the relation between zigzag-edge states and the magnetic
2

properties of nanoribbons was figured out. Graphene exhibits sp hybridization and this contributes to
attractive properties of graphene [2, 3]. Thanks to its almost perfect physical, chemical, electronic,
optical and photonic properties, graphene has a wide range of applications from energy field to sensor
technology and also from electronic applications to photonic ones [4, 5]. Thus, graphene is expected
to replace many conventional materials in various applications. Even though it stands out with its
characteristics like high electrical and heat conductivity, high optical transparency, large surface area,
flexibility and durability, it has a few drawbacks like zero bandgap and high sheet resistance [6, 7].
These flaws limit the electronic applications of graphene and they might even make it inappropriate
for some applications. On the other hand, graphene, along with its high conductivity and optical
transparency, is considered as one of the most promising materials to be used as transparent

conductive electrode (TCE) in solar cells in particular to replace commonly used indium tin oxide (ITO)
[8, 9]. Although the ITO is fragile and expensive its usage in the solar cell is common. However, this
make solar cell expensive and less durable. Single layer graphene film can transmit 97% of the visible
light, which is much higher than 100nm thick ITO transmission value, 85% [10, 11]. However, as
stated earlier, the sheet resistance of the graphene film should be lowered to be competitive with the
ITO for solar cell and other optoelectronic applications as graphene sheet resistance is much higher
than that of the ITO value. Thus, rather than using solely graphene as TCE in solar cells, graphene
was used along with ITO as TCE in this study.
To synthesize graphene, there are various techniques such as mechanical exfoliation, epitaxial
growth, chemical exfoliation and chemical vapor deposition (CVD), and each technique has its own
advantages [9, 12]. The chosen technique depends on its applications in terms of size, thickness,
quality, uniformity and crystal type of the synthesized graphene film. Graphene growth conditions in
CVD system are related to many parameters, and changing even a single parameter can affect such
qualities of the graphene layers as homogeneity, thickness, optical transmission and sheet resistance
of the thin film [13, 14].
In this paper, graphene growth was conducted with different growth conditions on copper foils in
the CVD system to find out the best growth recipe for single layer graphene that can be implemented
into solar cell. The main reasons to choose copperas substrate is that the copper is cheaper than
other possible substrates and it has low carbon solubility that help to control thickness of the
graphene films. The reason to utilize the CVD technique was to produce one layered, cost efficient,
large-scaled and easy synthesis of the graphene film. Methane (CH4) and hydrogen (H2) gases were
used for the synthesis and the effect of gas ratio (CH4/H2) was investigated regarding the quality of
the graphene so that single layer or a few layer graphene growth can be amended by changing the
flow rate. Other parameters such as the growth time, growth temperature, annealing time and the
pressure were kept constant for all the recipes. The best recipes for the synthesis of single and
homogenous graphene layers have been identified via Light microscopy, Raman spectroscopy,
ellipsometry and four-point probe characterizations. After the growth optimization, the obtained single
layer graphene was incorporated into the silicon-based heterojunction (Si-HIT) solar cell with the idea
of increasing the efficiency of the solar cell and other photovoltaic parameters. It was found that the
efficiency of the cell, which was graphene transferred on top, was increased by about 10% compared
to the cell without graphene

2. Experimental
All the graphene samples were grown 50µm thick double sided copper foil in a three zone CVD
furnace. Ahead of loading the copper foil into the CVD furnace, the foils were pre-cleaned with a
standard procedure where the foils were subjected respectively to the acetone, deionized water and
isopropanol alcohol for a few minutes in the ultrasonic bath. The aim was to reduce any possible
contamination on the copper surfaces that might mitigate possible homogenous growth of the
graphene layer. Following the loading of the foils to the CVD system, Argon was introduced for a few

minutes to remove any oxides/impurities from the quartz tube. The waiting time was about 15 minutes
for good base pressure and then the processes started. As is stated earlier, during the growth, the
same procedures were applied except for the gas flow rates, which were investigated. Since
hydrogen flow rate is one of the most effective parameters for graphene growth that can affect the
thickness of the graphene film and sheet resistance, the methane flow (40 sccm) was kept constant
and hydrogen flow rate was changed from 15 to 40 sccm and the impact was observed. Then, the
hydrogen flow was kept constant at 20 sccm and methane flow was changed from 10 to 40 sccm. The
o

typical growth process is illustrated in Figure 1a). The furnace was heated up to 1000 C for about 40
mins, and then annealed for 30 mins, in addition to another 30 mins. for growth. Following the
process, the system was cooled down for a few hours. Hydrogen was let to flow at constant rate
during whole process even during the cooling. Methane was only introduced to the system during the
growth (Argon was also used to vent the system).
For graphene characterization, light microscopy for topographic, Raman spectroscopy for the
structural, ellipsometry for the optical, and four-point probe for the electrical characterizations were
employed. Raman spectroscopy was used more frequently than the other techniques, as it is a fast
and non-destructive technique to assess the quality of the graphene. Graphene has three main
Raman peaks called D, G and 2D. G peak represents crystalline structure of the graphene; D peak is
used to determine defects for graphene. 2D peak is another important peak used for determination of
thickness. In addition single layer graphene has single sharp 2D peak and it consists of two phonons
with opposite momentums [15, 16]. Positions, sharpness and intensities of the characteristic peaks
(2D and G) of the graphene were utilized for the characterization[17]. The I2D/IG intensity ratio of the
film helped to determine the film thicknesses. Single-layer and double-layer graphene were
determined by I2D/IG > 2 ratio, but multilayer graphene typically had I2D/IG < 2 [18-20]. The full width at
half maximum (FWHM) can also help to decide the quality of the graphene layer. It is clear that there
are consistent, substantial, and distinguishable ranges for single-, bi-, tri-, four, and five-layer
-1

-1

-1

-1

-1

graphene at around 28 cm , 51 cm , 56 cm , 63 cm , and 66 cm respectively[16]. Therefore, the
FWHM and I2D/IG intensity ratio were employed to determine graphene layer thickness and this is also
provided information about the quality of the graphene.

a

b

Figure 1. Graphene growth procedure on Copper foil (a) and c-Si structure (b)

Si-based heterojunction solar cells were fabricated by employing the plasma enhanced chemical
vapor deposition (PECVD) system. The reason why graphene was applied to Si-based solar cells that
this type of solar cell are used commercially as is more economical than other type of solar cells. The
structure and film thicknesses of the heterojunction solar cell is shown in Figure 1b). The temperature
during the cell fabrication was kept around 200°C. Hydrogenated amorphous silicon (a-Si:H) thin films
have been deposited on textured n-type crystalline silicon wafer. High purity (6N) silane, phosphine,
trimethlyboron and hydrogen were used as precursor gases to obtain other solar cell layers on the
wafer, such as absorber and windows layers. Afterwards, ITO were deposited in the Physical Vapor
Deposition (PVD) system. A front grid and back contact was prepared by using silver paste. On the
other hand, by using the same solar structure, graphene was transferred onto the cell over the ITO
film ahead of front grid deposition. Graphene was transferred via polymer assisted transfer technique
that the polymer was removed in acetone bath for about 10 mins. The fabricated solar cells were
characterized by using solar simulator to find out the photovoltaic parameters such as, efficiency and
I-V curves.

3. Results
The pre-cleaning procedure was applied to all the copper foils ahead of the growth. The
cleaning aimed to remove mainly any organic contaminants that might hinder the growth and affect
the quality of the graphene layer.
Many hydrogen flow rates which range from 15 to 40 sccm by 5 increments were investigated
during the growth. The other parameters were also kept the same during the growth to be able to
make a sound comparison. Figure 2. shows Raman spectrums, which indicate the impact of hydrogen
flow on the growth for different H2 flows by keeping methane at 40 sccm. So, 20 sccm and 35 sccm
hydrogen flows provided the best results in terms of graphene quality and homogeneity. I2D/IG ratio,
which is known as quality checker, was found to be 2,91 and 3,67 for graphene grown with 20 sccm
and 35 sccm hydrogen flow, respectively. The ratios for the graphene grown with the other H2 flows
were found to be smaller that resembles few layer graphene. Based on this output, 35 sccm hydrogen
flow rate demonstrated better quality than 20 sccm hydrogen flow rate for single layer graphene
growth. Still, other features should also be considered regarding the hydrogen flow due to its effect on
the other properties of the graphene especially optical and electrical ones[21]. So, 20 sccm hydrogen
flow rate is advised for single graphene synthesis as graphene layer sheet resistance increases
compared to the graphene grown with 35 sccm hydrogen flow, which will make graphene unsuitable
to be used as a transparent conductive electrode. It is known that the hydrogen was used for
decomposition of the hydrocarbon gases and plays a crucial role for synthesis of high quality of
graphene. Usage more hydrogen gas can decompose more active carbon species that can
accumulate on the growth surface and effect the film thickness. Additionally, the ratio between
hydrogen and hydrocarbon gases and the partial pressure of the hydrogen gas itself have a
considerable effect on the growth conditions [22, 23].

Figure2.Ramanspectrumswithchanging hydrogen flow (methane constant at 40 sccm).

Additionally, methane was used as a carbon source during the graphene growth and its flow rate was
another important parameter for the graphene synthesis. Once methane flow rate was changed, the
graphene growth was affected [14, 24]. In this study, methane flow ranging from 20 to 60 sccm was
investigated to grow single layer graphene on copper foil by keeping all the other parameters the
same including the hydrogen flow that was kept constant at 20 sccm and this was selected according
experiment conducted and explained above. The best graphene quality was found for 40 sccm
methane flow since the single layer graphene was grown homogenously. However, the graphene film
grown with 30 sccm methane was also found to have a good quality. This can be seen in Raman
spectrums in Figure 3. I2D/IG ratio is 3,26 and 3,54 for 30 sccm and 40 methane flow rate respectively
and this indicates the presence of the single layer graphene. It was found that the effect of the
methane flow on the quality of the graphene was not effective as hydrogen flow.

Figure 3.Raman spectrums with changing methane flow (hydrogen constant at 20 sccm).

Moreover, Raman mapping was employed to be able to confirm the single layer graphene
homogeneity since I2D/IG ratio or FWHM would not be enough to determine the homogeneity and
continuity of the graphene film as the Raman spectrums were collected from single point, which was
proportional to the size of the laser spot (~600nm). Raman mapping of graphene layer grown on the
copper foil with 20 sccm hydrogen and 30 sccm methane is shown in Figure 4. This mapping was
performed on graphene on the copper surface and the maps were obtained according to the 2D and
G peak positions. The optical image of the copper surface and the maps collected area are shown in
figure4 (a). 2D and G peaks Raman maps are illustrated in figure4 (b) and (c), respectively. The
homogeneity of the graphene films is clearly seen in the Raman maps.

(a)

(b)

(c)

Figure 4. Raman mapping for single layer graphene on the copper surface

Another check regarding the quality and thickness of the graphene films is the optical
transmission experiments. Graphene film thickness can be deduced from the optical transmission

values since the transmission decreases with the increasing number of layers[25]. To be able to
perform graphene optical and electrical experiments, the graphene film needs to be transferred from
the copper foil to a transparent substrate for transmission and to an insulator substrate for sheet
resistance measurements [26, 27]. Furthermore, the technique used for graphene transfer plays an
important role because some residues may have remained on the graphene surface, which can affect
the accuracy of the measurement [28]. So, graphene films were transferred from the copper foil to a
glass lamella by using wet transfer technique with the help of PMMA and the PMMA were cleaned in
an acetone bath after the transfer. The optical measurements were performed in the visible region
(400-700nm) by using the ellipsometer [27, 29].The optical transmission of graphene synthesized with
different hydrogen and methane flow is given in Table 1.As seen in the table, the highest transmission
value was obtained for 30 sccm methane and 20 sccm hydrogen flows.
Table 1. Graphene transmission values of graphene on glass substrate.
CH4 Flow (sccm)

H2 Flow (sccm)

Optical Transmission (%)

20

20

96

30

20

97

40

20

96

50

20

92

60

20

95

The sheet resistance of graphene films was measured by the van der Pauw method via
Jandel four-point probe. Initially, graphene grown on copper foil was transferred to Si/SiO2 wafer with
the same way that is explained above. Following the transfer, the sample was cleaned and annealed
for a short time. The results indicated that the lowest sheet resistance of single layer graphene was
around 500 ohm/square for graphene grown with 20 sccm H2 and 30 sccm CH4 and increased with
the risingH2 flow up to 1200 ohm/square in our experiments. The sheet resistance of 80 nm ITO was
also measured and it was found to be around 60 ohm/square, which is much lower than the graphene
value. On the basis these results one can conclude that although its high optical transmission
graphene usage as TCE and taking ITO’s place is not possible at the current stage. Thus,
combination of ITO and graphene as TCE were investigated in terms of both electrical, optical
properties and then applied on the solar cell to find out its performance. To do this, 80nm ITO was
coated onto glass substrate and single layer graphene was transferred onto it. It was found that the
sheet resistance was 65 ohm/square, which is slightly higher than ITO and the optical transmission
was 91 %for 633nm wavelength light that is slightly lower than ITO value, which was measured to be
92 %.
Since ITO/graphene film showed only a small deterioration compared to the ITO, it was
applied to the Si-based solar cell to find out how graphene incorporation affects the photovoltaic
parameters. In order to investigate this issue, 6 inch solar cell was fabricated using PECVD system as
a first step and then was cut into pieces that are about 1 inch. Following this, all the pieces were

coated with 80nm ITO film and a single layer graphene was transferred onto a few of them for precise
comparison between ITO/graphene and only ITO in terms of cell performance. The cell structure was
finalized by making metal grids (contacts) that were placed on the top and back surface of the cell.
The performance of the cell was investigated by using a solar simulator that provides I-V curves and
information about efficiency, Voc, Jsc, FF and so forth. The I-V curves of the cells only with ITO and
with ITO/graphene as TCE are given in figure 5.

Figure 5. The I-V curves of the cells only with ITO and ITO/graphene as TCE.

The table that includes the cell Voc, Vmp, Jsc,Jmp FF and the efficiency of both with graphene
and without graphene are given in Table 2. Based on these results, it can be seen that all photovoltaic
parameters of graphene-incorporated cell were improved. This improvement can be attributed to the
graphene that serves as an anti-reflective layer, which helps to keep the light longer inside the cell.
Voc increase can also be related to higher work function of graphene (∼4,65 eV) compared to the ITO
(∼4,45 eV). Thanks to this and conductivity of the graphene contributes to transfer extracted electrons
to the contact. Overall, the cell efficiency increased by 8% with graphene insertion on top of the ITO
layer. This improvement could be higher with a better transfer procedure of graphene and using a
relatively smaller wafer to fabricate the cell rather than cutting a large cell that could be damaged
(micro cracks) during cutting.

Table 2. Photovoltaic parameters of the cells with and without graphene.

Cell
nm

2

2

Voc(V)

Vmp(V)

Jsc(A/cm )

Jmp(A/cm )

Filling Factor

Efficiency%

with

80

0,363

0,303

0,035

0,031

0,747

9,5

Cell with
nm
ITO
graphene

80
+

0,379

0,321

0,035

0,032

0,773

10,2

ITO

4. Conclusion
In the present study, the effect of hydrogen and methane flow on graphene growth on the copper foil
in the CVD system was investigated. The analyses were conducted by changing only the hydrogen

flow from 15 to 40 sccm and keeping all the other growth parameters the same. The procedure was
conducted by changing only the methane flow from 20 to 60 sccm and keeping all the other growth
parameters constant. As a result, single layer graphene growth recipe for copper foil was established
so as to get the optimum optical transmission and sheet resistance values. Since the sheet resistance
values were improved via amending the graphene growth conditions, the sheet resistance values
should be lowered further to be able competitive with those of the ITO. However, combining ITO and
graphene films works well as TCE in the Si-based heterojunction solar cell. Presence of graphene on
the cell improve the cell efficiency by 8%. This can even be intensified by improving the graphene
transfer technique, using cells without breaking and by doping the graphene films that reduces the
sheet resistance. In order to improve solar cell efficiency, the metallization (contacts) of the cell could
be advanced and other TCE could be employed. Amount of the improvement in the cell efficiency and
possibility to increase is a great hope towards use of graphene in the solar cell and efficiency
improvement in the field of solar cell.

Acknowledgement
The authors gratefully acknowledge the funding from The Scientific and Technological Research
Council of Turkey (TÜBİTAK-117M401).
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Highlights
•
•


Large-scale graphene growth by changing the growth parameters in three-zone CVD system.
Si-based solar cell with different ITO thickness.
Graphene applied to Si-based solar cell that improve the cell efficiency