chapter 4 fabricating graphene by chemical exfoliation of...
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Chapter 4
Fabricating Graphene by Chemical Exfoliation of
Graphite and Combined Experimental and Theoretical
Study on rGO/p-Si Heterojunction Solar Cell
Combined experimental and theoretical investigations on the heterojunctions of chemically
derived graphene with Si have been presented. The stability study of graphene oxide (GO)
and reduced GO (rGO) in aqueous medium were performed by visual observation and
surface charge measurement. The detailed characterizations by FT-IR, UV-Vis and Raman
exhibited the formation of rGO with a high optical band gap of 3.6 eV. The rGO was spin-
coated on the p-Si substrate for fabrication of a heterojunction device, with the structure of
rGO/p-Si. In the fabricated device, incident light was transmitted through the thin rGO film
to reach the junction interface, generating photoexciton, and thereby a photo-conversion
efficiency of 0.02% was achieved experimentally and its (rGO/p-Si heterojunction device)
theoretical simulation using SCAPS 1-D tool showed the efficiency of 1.32%. Such large
deviations in efficiency between experiment and theory have been discussed in details. In
addition to the material stability test, the device stability has also been verified both
experimentally and theoretically.
The contents of this chapter have been published in Carbon and Journal of Nanoscience and
Nanotechnology:
S. K. Behura, S. Nayak, I. Mukhopadhyay, O. Jani, “Junction characteristics of chemically-
derived graphene/p-Si hetero-junction solar cell,” Carbon, Vol. 67, p. 766-774 (2014).
K. Batra, S. Nayak, S. K. Behura, O. Jani, “Optimizing Performance Parameters of
Chemically-Derived Graphene/p-Si Heterojunction Solar Cell,” Journal of Nanoscience and
Nanotechnology, doi:10.1166/jnn.2014.9818 (2014).
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4.1 Introduction
Graphene due to its high room temperature carrier mobility (15000 cm2/V.s) [1], tunable
band gap [50], ballistic transport with a mean free path of 300-500 nm [51] and transparency
of 97.7% [52], makes it a super candidate material for next generation electronic applications.
Among the various potential applications, graphene especially has shown great potential for
creating photovoltaic solar devices. Several attempts have already been made to incorporate
graphene into various solar cells as transparent electrodes [53], electron acceptors [54], hole
acceptors [55], counter electrodes [56] and photoactive promoters [57]. The first prediction
by Tongay et al. [58] on single-layer graphene contacted to a semiconductor substrate and
later, the formation of graphene/semiconductor Schottky barriers was experimentally
verified, opening the opportunity for graphene/p-Si heterojunction solar cells [27].
The fabrication of graphene/p-Si heterojunction solar cell from chemically-derived
graphene at room temperature is an excellent solution to the problems related to high-
temperature junction formation and graphene can be used as the junction emitter, as the
passivation layer, and anti-reflective layer. The simple planar structure of the graphene/p-Si
heterojunction device also helps in controlling the processing costs. Herein, graphene film not
only serves as a transparent electrode for light transmission on semiconductor photovoltaic
device [59, 60], but also as an active layer for electron/hole separation and hole transporting
medium. A maximum efficiency of 10.30% has been recently achieved using doped few-
layer graphene/silicon nanoarray configuration [61]. It was found that surface charge
recombination as well as graphene conductivity along with work function played important
roles in determining the solar cell performance. Nitric acid (HNO3) has been widely used to
dope graphene film to enhance the cell performance [61, 62]. This is due to the p-type
chemical doping effect of HNO3 which increases the work function, the carrier density of
graphene (decreasing the series resistance) and the built-in potential (increasing the open
circuit voltage).
An understanding of the physical limits for conversion of radiation into electrical
power of semiconductors is important for designing electronic devices and for understanding
their function and performance. Application of graphene in this field is a very open question
and requires investigation on different aspects of the material and the devices. The recent
works based on graphene-on-semiconductors heterojunction solar cells have been
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summarized in Table 4.1. Most of the past reports demonstrate the heterojunction device
fabricated from a transferred CVD and mechanically exfoliated graphene film on Si [63, 64].
The metallic characteristics of the CVD and exfoliated graphene hinder the possibility of
fabricating p-n heterojunction based devices. Keeping in view the above mentioned
shortfalls, the chemically-derived rGO with high optical band gap can be suitable for a
heterojunction device fabrication. In addition, this will also avoid high-cost deposition
techniques and complicated processing, which are essential for Si-based p-n and p-i-n type of
devices. Therefore, an attempt has been taken to study the simplest heterojunction made of
rGO-on-p-Si without any doping or other configurations.
In this work, we conducted a comprehensive study on chemically-derived graphene-
on-Si-based Schottky junction solar cells using both experimental and computational
techniques. Chemically-derived graphene was synthesized using modified Hummers method,
subsequently reduced using NaBH4 and spin-coated on p-Si substrates. The present results
suggest great potential of the graphene/Si as high-efficiency and low-cost photovoltaic
devices. SCAPS (A Solar cell Capacitance Simulation) program was used to simulate the
model rGO/p-Si device of the present study using experimental results for chemically-derived
graphene.
Table 4.1: List of graphene-on-semiconductor based solar cells in the literature.
Sr. Device Type VOC (V) JSC (mA/cm2) FF η (%) References
1 Graphene-P3HT/C60 0.43 3.5 0.41 0.61 [65]
2 GS/n-Si 0.48 6.5 0.56 1.70 [63]
3 Graphene/Si 0.517 13.2 0.58 3.93 [66]
4 FLG/Graphite - - 4.35 [67]
5 Graphene/n-SiNWs 0.462 9.2 0.30 1.25 [68]
6 G/Si Pillar Array 0.487 16.03 0.45 3.55 [69]
7 SLG/SiNWs 0.19 0.154 0.25 2.15 [70]
8 SLG/n-Si 0.54 25.3 0.63 8.6 [71]
9 rGO/n-Si 0.254 4.28 0.23 0.25 [72]
10 Graphene/Si 0.51 24.28 0.6 7.5 [73]
11 FLG/P3HT/SiNH 0.48 38.86 0.552 10.3 [61, 62]
12 TiO2/Graphene/Si 0.612 32.7 0.72 14.5 [74]
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4.2 Experimental Methodology
For characterization and device fabrications, the experimental studies began with the
synthesis of graphene oxide (GO) from pure graphite (purity ~99.9%) by modified Hummers
method [8, 75]. The schematic of the whole chemical synthesis process is demonstrated in
figure 4.1. GO contains a range of reactive oxygen functional groups, which renders it a good
candidate for use in the electronic device applications. Therefore, it is very much essential to
reduce the highly resistive GO. It was chemically reduced by sodium borohydride (NaBH4), a
strongly reducing agent.
The aqueous solutions of GO and reduced GO (rGO) were prepared in DI water with
a concentration of 1 mg/ml. The solutions were spin-coated on p-Si and glass substrates for
device fabrication and characterization. The morphology of graphite flakes, GO and rGO
were characterized using scanning electron microscopy (SEM) (JEOL, JSM-6010LV) and
atomic force microscopy (AFM) (Pico-IC). The transmission electron microscopy (TEM)
images were recorded using FEB Tecnai G2 20, instrument operated at 200 kV (The
Netherland) to observe the nanoscale structures. Raman spectrometer (Invia Reflex/514,
Incoterm, UK) was used to confirm GO and its reduction. Raman spectroscopy was carried
out with laser excitation energy of 514 nm. The optical characterizations were carried out
using Fourier transform infrared spectroscopy (Varian 800, Japan) and UV-Vis (UV-2600,
Shimadzu, Japan). The thermo-gravimetric analysis (TGA) of the materials was performed
under continuous argon atmosphere on a PerkinElmer (USA) Pyris 1 analyser. The samples
were scanned from 50 oC to 1000
oC at a heating rate of 10
oC/min. GO samples were heated
from 50 oC to 500
oC at 1
oC/min to avoid thermal expansion due to rapid heating. Particle
charge detector (Mutek) was used to determine the specific surface charges of GO in different
solvents. Current-voltage (I-V) characterizations of the fabricated devices were done with the
Photo Emission Tech. solar simulator in dark and under air mass (AM) 1.5 simulated solar
radiations. The computational study was carried out using SCAPS simulation software, which
is one dimensional simulation program developed at University of Ghent. This program was
designed basically for the simulation and studying the properties of photovoltaic devices.
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Figure 4.1: Scheme showing the chemical route for the synthesis of graphene. 1: Oxidation
of graphite to graphite oxide with oxygen function groups on the surface. 2: Exfoliation of
graphite oxide in water by sonication to obtain GO colloids that are stabilized by electrostatic
force of repulsion. 3: Controlled conversion of GO colloids to conducting graphene colloids
through deoxygenation by sodium borohydride reduction.
4.3 Characterization of Graphene
There are many reports which represent the stability of GO in aqueous medium rather than
organic medium [76]. So, first we tested the stability of the GO and rGO by measuring the
surface charge (zeta potential) of as-prepared GO sheets in aqueous as well as in organic
medium (ethanol, propanol and dimethyl formamide). The results show that these sheets are
highly negatively charged (40 C/g ± 20) when dispersed in water rather than organic medium
[Figure 4.2 (a)], as a consequence of ionization of the carboxylic acid and phenolic hydroxyl
groups that are already exist on the GO sheets [76]. This result suggests that the formation of
stable GO colloids should be attributed to electrostatic repulsion, rather than just the
hydrophilicity of GO as previously assumed. The inset of figure 2a shows the stability of GO
and rGO in water for 0 and 24h. After 24 h, the rGO was settled down, while GO was
dispersed with brownish colour. This is due to the fact that, carboxylic acid groups are likely
to be reduced by borohydride under the given reaction conditions [77], therefore these groups
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are absent in the reduced product as confirmed by the FT-IR analysis [Figure 4.2 (b)]. The
oxygen containing functional groups of GO generated bands at 870 cm-1
(C=C, conjugation),
1090 cm-1
(C-O stretching), 1234 cm-1
(C-OH stretching), 1420 cm-1
(C-O-H deformation),
1635 cm-1
(C=C stretching) and 1750 cm-1
(C=O stretching) [78]. However, the oxygen
containing functional groups were almost entirely removed during reduction except few
groups, which further confirms the oxidation and reduction of graphite flakes.
Figure 4.2: (a) Surface charge with stability analysis (inset) and (b) FT-IR spectra of GO and
rGO.
Furthermore, the identification rGO formation was confirmed by Raman analysis,
which is the fingerprint for carbon materials identification. Figure 4.3 shows the Raman
studies of GO and rGO, presenting a disorder induced D-band at 1348 cm-1
and a graphitic G-
band at around 1574 cm-1
. The strong D-band is associated with vibrations of carbon atoms
with dangling bonds or formation of sp3 hybridization with oxidation. At higher wavenumber,
a small peak at 2692 cm-1
and a broader peak at 2905 cm-1
is observed corresponding to the
2D and D + G combinational mode, respectively. The D+G-band is the combination of D and
G mode [79, 80]. Further detailed physics of Raman modes are demonstrated pictorially in
Appendix II. The slight reduction in 2D peak and the presence of a broad D + G peak signify
well oxidation of the graphitic material. However, there is no significant difference of the
Raman spectra for the GO and rGO, rather only a small enhancement in the IG/ID ratio is
observed which is shown in the inset of Figure 4.3.
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Figure 4.3: Raman spectra of graphite, GO and rGO taken at Raman excitation wavelength
of 514 nm. Inset indicates the reduction of D- and G-band intensity, which confirms the
formation of rGO.
As the morphology of the GO and rGO are known to govern their optical and
electrical properties, these synthesized sample were subjected to detailed morphological
characterization by SEM, TEM and AFM. Figure 4.4 (a) shows SEM image of starting
graphite powder. Flakes of 40-50 µm are clearly observed. The inset depicts a digital image
of graphite powder. Figure 4.4 (b) shows the morphology of the prepared GO sample. A
smooth and flat overlapping structure reveals that the thin sheets of graphene oxides were
stacked randomly, and few layers are folded. Figure 4.4 (c) and (d) shows the surface
morphology and TEM image of rGO which is transparent in nature. The inset of figure 4.4
(d) depicts selected area electron diffraction pattern, which is typical of few layer with
crystalline structure. These samples were very stable under electron beam.
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Figure 4.4: SEM micrograph of (a) graphite powder with its digital image in the inset, (b)
GO, (c) rGO and TEM micrograph of (d) rGO with its SAED pattern in the inset.
To evaluate the size and thickness of the rGO flake, AFM analysis was subjected to
detail. Figure 4.5 (a) shows an AFM image of the rGO flake. Figure 4.5 (b) shows the
thickness profile image, presenting the thickness of the rGO flake as around 10 nm. Here, it
should be noted that a pure single layer graphene has a thickness of 0.34 nm, however, a rGO
sheet is ~1 nm thick with presence of functional groups, defects and absorbed water
molecules on rGO surfaces [81]. The AFM studies show that the lateral size of rGO flakes
are of the order of 1-5 μm, consisting of 10 layers of graphene. The thickness of the GO and
rGO thin film can be controlled with the concentration of the solution during spin coating,
spinning speed and spinning time.
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Figure 4.5: (a) AFM topography image and (b) thickness profile of an rGO showing flake of
thickness ≈ 10 nm.
Thermal stability is an important criterion for the use of graphene-based devices under
high temperature conditions. For this purpose, graphite, GO and rGO were investigated using
TGA, as presented in figure 4.6. The TGA curves of pristine graphite show a very negligible
weight loss around 1.5% of its total weight. On the other hand, GO shows a weight loss of
10% at 120 oC and then 25% at 200
oC corresponding to the removal of physically adsorbed
water, moisture and COOH groups, respectively. Next, the fast weight loss of GO takes place
until 440 oC due to pyrolysis of oxygen bearing functional groups associated with GO. It
clearly shows that thermal stability of GO is very poor compared to graphite. Furthermore,
rGO shows very steady weight loss with temperature, which is around 30% of its total
weight. This apparently shows removal of oxygen bearing functional groups after reduction.
It also shows that thermal stability of rGO is better than GO. This observation also indicates
the reduction of GO to rGO by removing the carboxyl, epoxide and hydroxyl groups. DTA
curve (figure 6 (b) also shows a strong exothermic peak at around 600 oC in case of rGO
corresponding to the combustion of rGO molecule.
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Figure 4.6: (a) TGA and (b) DTA plot for graphite, GO and rGO.
In addition to the thermal properties, optical properties, in particular bandgap, is the
key factor which governs the photo-conversation efficiency. GO due to its insulating nature
possesses high optical band gap, which can be optimized using the ratio of sp2 and sp
3
carbon. Since we focused on the heterojunction device based on rGO; the transmittance of
rGO-coated on glass substrates was measured (Figure 4.7). The transmittance increases
throughout the visible region and slightly decreases near infrared light region. The rGO-
coated glass shows a transmittance of 80.92% at 550 nm wavelength. The optical gap of rGO
can be obtained from the Tauc plot, using the relation αhν = (hν – Eg)1/2
, where α is the
absorption coefficient, hν is the photon energy and Eg is the optical gap. The inset depicts a
Tauc plot for the as-synthesized rGO. The optical band gap calculated to be 3.62 eV.
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Figure 4.7: Transmittance of rGO on glass substrate with the inset shows the Tauc plot.
The main challenge in graphene for optoelectronics research is the optimized value of
transmittance (Tr) and sheet resistance (Rs’). Therefore, rGO films of various thicknesses
were spin-coated on SiO2/Si substrate and the Tr and Rs’ were measured as a function of
thickness and shown in Figure 4.8. Lowest sheet resistance of 1 kΩ/□, though at a low
transparency of ~ 72%, was achieved for rGO films with thickness of ~ 15 nm. The best
optoelectronic properties were obtained for lower thicknesses with a thin film of 10 nm
exhibiting sheet resistance of ~ 1.2 kΩ/□ at a transmittance of ~ 81%, while 5 nm thin films
exhibited transmittance of ~ 90% at a sheet resistance of ~ 20 kΩ/□ Since for single-layer
graphene, the transmittance value may approach 97%, while its sheet resistance is very high
of the order of several kΩ/□ For this reason, doping of the graphene film with HNO3, has
emerged for the high photo-conversion efficiency of these types of devices. Doping has
advantage of decreasing the Rs’ of the film.
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Figure 4.8: Transmittance and sheet resistance of rGO thin films as a function of thickness.
4.4 Fabrication and Characteristics of rGO/p-Si Heterojunction
Solar Cell
To investigate the potential application of chemically-derived graphene (CDG) in
optoelectronic devices, we have fabricated CDG-based heterojunction solar cell by spin-
coating solution processed graphene on p-Si. Spin-coating of CDG on p-Si has advantages
over other carbon nanostructures of being naturally compatible with thin film processing,
making large device areas. As it is described in the experimental methodology, the aqueous
solutions of rGO were prepared in DI water with a concentration of 1 mg/ml and spin-coated
on p-Si followd by baking in 80 oC for 10 minutes.
4.4.1 Current-Voltage Characteristics: Figure 4.9 (a)-(d) shows the schematic
diagram, the I-V curve both under dark and illumination of 1000 W/m2, the equivalent circuit
and the energy-band diagram of the fabricated rGO/p-Si heterojunction device, respectively.
I–V characteristic (Figure 4.9 b) in dark condition shows very good rectification with small
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leakage current in the reverse bias. For a Schottky barrier diode with assumption that the
current is due to thermionic emission, the relation between the applied forward bias and
current can be expressed as [27]:
I = IS [exp (VD/nkBT)-1]...................................................................................................... (4.1)
Where I is the diode current, IS is the reverse bias saturation current, VD is the voltage across
the diode, kBT is the thermal voltage, (kB is the Boltzmann constant and T is the temperature
in Kelvin), and n is the ideality factor. IS can be extracted by extrapolating the straight line of
Ln (I) to intercept the axis at zero voltage:
IS = AA*T2 exp (-qφSBH/kBT).............................................................................................. (4.2)
Where A is the effective area of the device (0.25 cm2), A* is the Richardson constant (32
A/(cm2K
2) for p-Si substrates.
Under illumination, the fabricated device with rGO shows a photovoltaic action.
Photo excited electrons and holes are generated in the Si substrate which are separated and
collected by means of a built-in electric field at the heterojunction of rGO/Si. The device
characteristics are open-circuit voltage (Voc) of 0.27 V, short-circuit current (Isc) of 0.11 mA
and fill factor (FF) of 0.12 with power conversion efficiency (η) 0.02%. This efficiency value
is one order magnitude higher than the previously reported rGO/p-Si device efficiency by
Mohammed et al. [82]. Comparing with another device rGO/n-Si work [72], our device
shows a magnitude decrease in efficiency. This might be due to the partial reduction of GO as
it is confirmed from the measurement of the optical band gap (rGO) ≈ 3.62 eV. According to
G. Kalita et al. [72], their GO and partial reduced GO show an optical band gap of 3.6 and 2.8
eV, respectively. The lower conductivity of the synthesized rGO in our case, gave rise to
higher series resistance and ultimately lowers the Isc of the fabricated device.
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Figure 4.9: (a) Schematic view of prototype rGO/p-Si heterojunction device, (b) I-V curve
under light and dark, (c) equivalent circuit model and (d) energy-band diagram.
The equivalent circuit shows that the current density is greatly influenced by the
series resistance (Rs) of the device. The shunt resistance (Rsh) takes into account all parallel
resistive losses across the photovoltaic device including leakage current. The RS is mainly due
to the bulk and contact resistance of the device. By neglecting the shunt resistance (Rsh) of the
device, the forward bias current is mainly affected by RS.
Rectification behaviour is observed at the interface of graphene/p-Si device with the
appearance of a barrier height, which is calculated experimentally below. Considering the
work function for graphene (ΦG) = 4.70 eV, and electron affinity for Si (χp-Si) = 4.05 eV, the
calculated barrier height (ΦBP) and built-in potential (Vbi) are given in Eq. 4.3 and 4.4.
BP = Eg - G + χp-Si = 0.47 eV ..…………………..………………………………......(4.3)
The difference in potential between Fermi level and valance band edge (Vp) = 0.12 eV
Now, Vbi = Bp – Vp= 0.47 – 0.12 = 0.35 eV...………………………………......………. (4.4)
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4.4.2 Capacitance-Voltage Characteristics: A capacitance-voltage (C-V)
measurement was employed to probe the rGO/p-Si heterojunction system for the knowledge
on associated changes in the density of ionized donors (ND) and built-in-potential (Vbi) which
is presented in Eq. 4.4. Now, the solution to the Poisson’s equation for a specific charge
distribution in the depletion region gives rise to a relationship (Eq. 4.5),
1/C2 = 2 (Vbi + V)/(qNDεs)…………………………………………………………………(4.5)
Here, the parameter q is the charge of electron and εs is the dielectric constant of the
semiconductor. Figure 4.10 displays C-V and 1/C2-V plots at 100 kHZ for a rGO/p-Si
heterojunction at room temperature. The resulting 1/C2 analysis confirmed the Si to be p-type
with a hole concentration of 2 x 1014
cm-3
. The built-in potential was extracted to be
approximately ≈ 0.3 eV, which is consistent with the band bending that would be required for
the Fermi levels of the graphene and Si to align.
Figure 4.10: C vs. V and 1/C2 vs. V (inset) plot of rGO/p-Si at 100 kHz and 300 K.
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4.5 Computational Methodology and Simulations
After evaluating the structural, optical, stability and electrical parameters of chemically-
derived graphene, the possibility of implementation as a heterojunction solar cell and its
stability was verified. The simulation work was carried out using SCAPS program. This
simulation package is equipped with advanced tools for I–V, QE (quantum efficiency) and
carrier transport simulations [83]. Simulation models are generated by digital description of
physical parameters of each structure layer, including contacts. Program SCAPS is constantly
developed since 1990 and available free of charge for scientific research. SCAPS uses
simultaneous of Poisson’s equation, the continuity equation and the boundary condition to
model one-dimensional solar cells. The program output includes simulated capacitance
values.
4.5.1 Simulation and Stability of rGO/p-Si Heterojunction Solar Cell: To
understand our experimental results, a thorough computational study was undertaken to
investigate the influence of rGO thickness on the efficiency of this simplest device. An
efficiency of 1.32% was found for rGO/p-Si device, while considering our experimental
results such as: Si thickness of 500 µm and rGO thickness of 10 nm with transparency of
80.92%, which were about 2 orders of magnitude higher than our experimental value. This
discrepancy may be due to the partial reduction of GO, which hinders its intrinsic property. A
maximum efficiency of 6.74% was achieved, while considering a single-layer graphene.
Figure 4.11 (a) represents the J-V curve and (b) shows the variation of the efficiency with the
graphene thickness.
Figure 4.11: (a) J-V characteristic plot of rGO/p-Si heterojunction and (b) Variation of the
simulation efficiency vs. rGO thickness.
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Moreover, the effect of temperature on the performance of rGO/p-Si heterojunction
solar cell has been tested with an aim to study its stability. Figure 4.12 (a) represents the J-V
curve and (b) shows the variation of the efficiency with the temperature. As it can be clearly
understood from figure 4.12 (b), the stability of the device needs to be improved as there is
sudden fall in efficiency with increasing temperature from room temperature to 350 K.
Figure 4.12: (a) J-V characteristic plot of rGO/p-Si heterojunction and (b) Variation of the
simulation efficiency vs. temperature.
4. 6 Summary
To conclude, we have demonstrated the fabrication of a solid state heterojunction with
chemically-derived graphene-on-Si configuration. High transparency in the visible and near
infrared light was obtained by coating the rGO layer, which allowed light to reach the Si
interface. In the fabricated device, a built-in electric potential was created at the junction, by
which photoexcited electrons and holes were transported and collected to cause a
photovoltaic action. The simple fabrication technique of the rGO/Si heterojunction device can
be exploited for other applications replacing high cost fabrication techniques. The fabricated
heterojunction device showed an efficiency of 0.02%, which can be enhanced by optimizing
our device structure. The computational analysis predicted efficiency of 1.32% using the
derived experimental value. According to the computational study, a better and efficient
device (efficiency of 6.74%) can be achieved using single-layer and high-quality rGO. This
unique combination of experimental and simulation study opens up new direction for
graphene-based low-cost photovoltaic cells.