Development of Graphene Oxide and TiO
2
Heterojunctions for
Hybrid Solar Cells
P. Custódio, P. A. Ribeiro, M. Raposo and S. Sério
CEFITEC, Departamento de Física, Faculdade de Ciências e Tecnologia da Universidade Nova de Lisboa,
2829-516 Caparica, Portugal
Keywords: Hybrid Solar Cells, Layer by Layer Films, Graphene Oxide, Pei, TiO
2
.
Abstract: This work reports the development of hybrid devices composed of organic and inorganic thin films,
deposited on fluorine doped tin oxide coated glass substrates (FTO). The organic layers, graphene oxide
(GO) and poly (ethylenimine) (PEI) were deposited by the layer by layer technique (LbL), through the
aerosol spray variant. The inorganic layer, titanium dioxide (TiO
2
), was deposited by sputtering and the
aluminium electrode by thermal evaporation. To characterize these devices was used UV-Visible
spectrophotometry to observe the films growth and optical microscopy to analyze the surface morphology.
Finally, the electrical measurements were performed by measuring the I-V characteristic curves. The final
device (FTO/PEI/GO/ TiO
2
/Al)
20
showed a significant change in the behaviour when interacting with light.
1 INTRODUCTION
The industrial revolution has marked the beginning
of a new technological stage, which was characteri-
zed by the use of fossil fuels and minerals as the
main source of energy.
Since then, several studies have shown the
harmful impact of these forms of energy production
have on planet Earth, causing depletion of the ozone
layer and increasing of global warming. Currently,
about 80% of CO
2
emissions come from the energy
sector, thus demonstrating the need to develop new
approaches of generating energy in a sustainable and
clean way. Therefore, it is important and urgent the
development of devices capable of generating
energy without the need of use fossil fuels, such as
solar cells, biomass, wind turbines, among others.
However, this type of technology has two inherent
limitations, the price of the materials used and their
efficiency, leading to the prevalence of fossil fuels.
Nevertheless, great progress has been made in
these technological areas, making its use more
feasible, being presently around 8% of the energy
generated in the United States coming from
renewable sources (Serrano 2009).
This growing demand for alternative methods for
energy production has led to the development of
new architectures as well as research of new
materials in order to increase the efficiency of these
devices. One of the materials that has attracted
significantly the researchers' attention is graphene
and its derivatives (graphene oxide (GO) and
reduced graphene oxide (rGO)). These materials
have been extensively studied due to their electrical,
mechanical, optical and thermodynamic properties
and are presently used in several applications such
as: solar cells, solar fuels, lithium ion batteries,
supercapacitors, among others.
In the particular case of solar cells, these
compounds have been used as transparent and non-
transparent electrodes, in photoactive layers and in
electron transport layers and gaps (Yin 2014).
Based on the technological progress made in this
area during the last decades solar cells with hybrid
heterostructures have emerged in order to overcome
some disadvantages of organic solar cells such as
low optical absorption and degradation of the
compounds used (Roland 2015 and Wright 2012).
In this context, with this work it is aimed to find
solutions to capture solar energy, based on the
knowledge acquired in the last two decades under
organic conductive polymers, photoluminescent and
photochromic (Ferreira 2013, Ferreira 2007 and
Ferreira 2007) and also on semiconductors oxides
films, in particular titanium dioxide, TiO
2
(Sério
2011 and Sério 2011).
Custøsdio P., Ribeiro P., Raposo M. and SÃl’rio S.
Development of Graphene Oxide and TiO2 Heterojunctions for Hybrid Solar Cells.
DOI: 10.5220/0006324103860390
Copyright
c
 2017 by SCITEPRESS – Science and Technology Publications, Lda. All rights reserved

2 EXPERIMENTAL DETAILS
The layer-by-layer films were prepared from
aqueous solutions of poly(ethyleneimine) (PEI)
(Mw) 750 000 g/mol) and graphene oxide (GO) 2
mg/mL, dispersion in H
2
O, with concentrations of
3×10
-3
M and 10
-2
M, respectively, using the LbL
technique, implemented through aerosol spray
variant, instead of adsorption from bulk solution.
The chemicals were obtained from Aldrich and the
corresponding molecular structures are depicted in
Figure 1.
(a)
(b)
Figure 1: (a) poly(ethyleneimine) (PEI) (b) graphene oxide
(GO).
The ultrapure water with a resistivity of 18 MΩ
cm was supplied by a Millipore system (Milli-Q,
Millipore GmbH). Accordingly, both cationic and
anionic solutions, as well as washing ultra-pure
water, were placed in aerosol spray dispensers and
were ordered with the following sequence: cationic
solution, ultra-pure water, anionic solution and ultra-
pure water. The films were sprayed onto Fluorine-
doped tin oxide (FTO) coated glass substrates
(TEC15, 12-14 Ω/). The solid supports were
placed at a distance of 6 cm from the spray dispenser
and the solutions were alternately sprayed following
the order aforementioned. Each dispenser was
pressed twice to guarantee a satisfactory amount of
ejected solution towards the supports. After the
adsorption of the cationic layer solution, the
substrate becomes positively charged. It is then
washed with water to remove the amount of cationic
molecules solution that wasn’t satisfactorily
adsorbed on the substrate. Then the same procedure
was used for the anionic layer, washed again with
water and finally dried with nitrogen flow. After this
sequence, the first bilayer film was formed and, the
steps abovementioned were repeated until obtain the
desired number of bilayers. The lag time between
each solution spraying was of 10 s and all the films
were produced with 20 bilayers: (PEI/GO)
20
.
After, the inorganic layer (TiO
2
) was deposited
by DC-magnetron sputtering. The sputtering was
carried out at room temperature (RT) using a
titanium disc (99.99% purity, GoodFellow) with
64.5 mm of diameter and 4 mm of thickness as
sputtering target. A turbomolecular pump was used
to achieve a base pressure of 10
-4
-10
-5
Pa (before
introducing the sputtering and reactive gases).
Before the deposition, a movable shutter was
interposed between the target and the substrates, and
the target was pre-sputtered in Ar atmosphere for 5
min to clean the target surface. The deposition of
TiO
2
was carried out in 100% O
2
atmosphere
(99.999% purity) at constant total gas pressure of 1.2
Pa, a sputtering power of 500 W during 10 min. The
target-to-substrate distance was kept constant at 100
mm. No external substrate heating was used during
the depositions. The substrate temperature was
measured by a thermocouple passing through a small
hole in a copper piece, which was placed in contact
with the substrate. During the deposition process the
sample temperature increased up to 60 °C due to the
plasma particle‘s bombardment of the substrate.
Finally, to obtain the desired solar cell device (FTO /
PEI / GO / TiO
2
/ Al), an aluminium (Advent
Research Materials, 99.5%) electrode was deposited
by thermal evaporation, in a vacuum chamber at a
pressure between 10
-6
and 10
-5
mbar, over an area of
approximately 0.95 cm
2
. The films were
characterized by optical microscopy using a Nikon
Eclipse LV100.
The UV-vis spectroscopy for the films was
carried out with a Shimadzu UV b - 2101PC
UV/VIS spectrophotometer at room temperature
within the wavelength range 200-900 nm. The
electric measurements (I-V characteristic curve) of
the final device were carried out using a
programmable DC power supply model Rigol
DP811A in absence of light, at ambient light and
with light from a 250 watts halogen lamp positioned
at a distance of 40 cm from the device. All the I-V
measurements were performed by changing the

voltage between 0V and ~1.5 V, with an increment
of 50 mV at room temperature, which was
guaranteed by a vent placed in the measurement
system.
3 RESULTS AND DISCUSSION
In figure 2 are depicted some representative images
obtained by optical microscopy for the PEI/GO LBL
films with 20 bilayers without and with TiO
2
film. In
general it can be observed that the films are
homogeneous in both situations, although can be
detected some aggregates (with and without TiO
2
).
This homogeneity remains even for bigger
magnifications.
(a)
(b)
Figure 2: Optical microscopic images for (a) (PEI/GO)
20
without TiO
2
(b) (PEI/GO)
20
with TiO
2
.
In figure 3 a) and b) is shown the ultraviolet-
visible absorbance spectra of different number of
bilayers of PEI/GO LBL films and the absorbance
intensity at 380 nm as a function of the number of
bilayers, N, respectively. It can be observed that the
absorbance at maximum increases with the number
of bilayers indicating a linear film growth (see figure
3b).
(a)
(b)
Figure 3: (a) Absorbance spectra of PEI/GO LBL films as
a function of the number of bilayers, N. (b) Absorbance
intensity at 380 nm as a function of the number of
bilayers, N.
Figure 4 shows the I-V curves for three devices with
the structure (FTO/PEI/GO/TiO
2
/Al)
20
after
production and without interaction with radiation.
Although, it is expected that the LBL films have the
same thickness, the presence of aggregates as
revealed by optical microscopy may have led to
regions with different thicknesses. Considering this
possible difference in the thickness of the deposited
films, this fact can lead to a short circuit and
therefore explaining the erratic behavior of the I-V
curves observed in Figure 4 a). However, with a
another similar device produced using the same
conditions, I-V curves were obtained for different
experimental conditions (without light, strong light,
ambient light), which are depicted in Figure 4 b).
The analysis of the figure shows an increase of the

current for positive voltages when the device is
exposed to the light in comparison to the other
experimental conditions. The same behavior is
verified for negative voltages, however when the
device interacts with ambient light there is an
increase in current in the circuit, indicating an
increase of the charge carriers.
(a)
(b)
Figure 4: Electrical characterization for three devices with
the architecture (FTO/PEI/GO/TiO
2
/Al)
20
a) without the
interaction with light b) ambient light, strong light and in
absence of light.
Presently, more studies are in progress in order
to avoid the short circuit of the devices, increasing
the number of the bilayers.
4 CONCLUSIONS
In this work we report the development of hybrid
solar cells with the configuration
(FTO/PEI/GO/TiO
2
/Al)
20
. The organic layers, PEI
and GO, were deposited by layer-by-layer technique
through the aerosol spray variant and it was revealed
by optical microscopy that the deposited LBL films
are homogeneous, although are detected some
aggregates.
Considering the I-V characteristic curves for
several devices developed with this architecture, it is
observed a change of the behavior for the different
experimental conditions, increasing the conduction
in the following order: absence of light, ambient
light and strong light, for positive voltages.
However, for negative voltages the devices exhibit
increased conduction when exposed to ambient light,
indicating an increase in charge carriers. It was
further observed that some devices with this
architecture, the I-V curves performed without the
interaction with radiation presented an erratic
behavior, possibly due to differences in the thickness
of the films leading to the short circuit of the
devices. Moreover, this study also evidences that the
inorganic layer prevents the degradation of the
organic layers when exposed to the atmospheric
conditions.
Therefore, this work allows to conclude that this
device not only reacts to light but also that the
combination of materials and techniques used for its
manufacture are appropriate.
ACKNOWLEDGEMENTS
The authors acknowledge the financial support from
FEDER, through Programa Operacional Factores de
Competitividade − COMPETE and Fundação para a
Ciência e a Tecnologia − FCT, for the project
UID/FIS/00068/2013.
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