Development of Hybrid Solar Cells based on TiO
2
or ZnO- Graphene
Oxide Heterojunctions
D. Carreira
1
, P. A. Ribeiro
2a
, M. Raposo
2 b
and S. Sério
2c
1
Departamento de Física, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal
2
CEFITEC, Departamento de Física, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa,
2829-516 Caparica, Portugal
Keywords: Hybrid Solar Cells, Graphene Oxide, Zinc Oxide, DC- Magnetron Sputtering, Layer-by-Layer.
Abstract: Nowadays it is becoming increasingly necessary to find alternatives to fossil fuels in order to produce energy
from renewable sources that do not have a negative impact on the environment. In this work, two types of
hybrid solar cells devices were produced, in which the photoactive layer is made of poly (allylamine chloride)
(PAH) and graphene oxide (GO) organic films and titanium dioxide (TiO
2
) or zinc oxide (ZnO) inorganic
films. These films were deposited on fluoride-doped tin oxide (FTO) glass substrates, being the organic layer
deposited by the layer-by-layer (LbL) technique and the inorganic layer by DC-reactive magnetron sputtering.
The aluminum electrodes were deposited by thermal evaporation. The final device configuration was
FTO/(PAH/GO)
x
/TiO
2
/Al and FTO/(PAH/GO)
x
/ZnO/Al, where x is the number of bilayers deposited.
(PAH/GO)
x
films were characterized by ultraviolet-visible spectrophotometry, which revealed a linearity in
the growth of the films with the number of bilayers. Scanning electron microscopy (SEM) showed that the
morphology of the inorganic layer is homogeneous and is dependent on the number of layers of the organic
layer. The SEM cross section images further revealed the desired architecture. The electrical properties were
characterized by constructing current-voltage curves. The FTO/(PAH/GO)
50
/TiO
2
/Al,
FTO/(PAH/GO)
30
/ZnO/Al and FTO/(PAH/GO)
50
/ZnO/Al devices were the only ones to exhibit a diode
behavior, although they did not show any reaction when exposed to light. The FTO/ (PAH/GO)
50
/ZnO/Al cell
experienced a decrease in current when characterized in a low humidity environment, revealing that humidity
is a key factor in the conduction of the organic films.
1 INTRODUCTION
Today we live in a world completely dependent on
the use of fossil fuels such as coal, oil and natural gas.
This dependence manifests itself in the most diverse
sectors of society, especially in the energy sector. The
use of these fuels leads to a number of inherent
problems, because these resources are limited and
therefore there is an increasing difficulty in satisfy the
energy demands of a constantly growing population.
In addition, they can contribute significantly to the
increase of greenhouse gases in the planet, which
inevitably leads to the global warming. These
implications undoubtedly represent a dangerous
threat to the development of a sustainable global
a
https://orcid.org/0000-0001-9665-7610
b
https://orcid.org/0000-0003-4710-0693
c
https://orcid.org/0000-0002-8086-7792
society.
In this sense, it is imperative to find mechanisms to
produce and store energy efficiently, which present a
reduced environmental impact. The first strategy to
achieve this goal is to obtain energy from renewable
sources such as the sun, wind and water and
effectively converted into electricity or fuel. In this
context there has been an increasing research on
alternative materials and/or solutions such as solar
cells and photoelectrochemical cells (Hu 2015). The
area of solar cells has been constantly changing since
the discovery of the photovoltaic effect in 1839 and
considers the existence of four distinct generations of
such devices, ranging from silicon-based solar cells
to hybrid solar cells.
Carreira, D., Ribeiro, P., Raposo, M. and Sério, S.
Development of Hybrid Solar Cells based on TiO2 or ZnO-Graphene Oxide Heterojunctions.
DOI: 10.5220/0009380801850191
In Proceedings of the 8th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2020), pages 185-191
ISBN: 978-989-758-401-5; ISSN: 2184-4364
Copyright
c
 2022 by SCITEPRESS – Science and Technology Publications, Lda. All rights reserved
185

One of the materials that has attracted a huge interest
in the scientific comunity 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 also in electron transport
layers and gaps (Yin 2014).
Considering the technological progress made in
this area during the last decades solar cells with
hybrid heterostructures have emerged in order to
overcome some drawbacks of organic solar cells such
as low optical absorption and degradation of the
compounds used (Roland 2015 and Wright 2012).
In this sense and within the scope of the
knowledge and work carried out 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, such as TiO
2
and ZnO
(Sério 2011, Sério 2011 and Siopa 2016), the present
work was undertaken in order to find solutions to
capture and convert the solar energy.
2 EXPERIMENTAL DETAILS
The layer-by-layer films were prepared from aqueous
solutions of poly-(allylamine hydrochloride) (PAH)
(Mw) 50 000-65 000 g/mol g/mol) and graphene
oxide (GO) 2 mg/mL, dispersion in H
2
O, with
concentrations of 10
-2
M and 0.510
-2
M,
respectively, using the LbL technique. The ultrapure
water with a resistivity of 18 MΩ cm was supplied by
a Millipore system (Milli-Q, Millipore GmbH). The
chemicals were obtained from Aldrich and the
corresponding molecular structures are depicted in
Figure 1.
(a)
(b)
Figure 1: (a) poly-(allylamine hydrochloride) (PAH) (b)
graphene oxide (GO).
Accordingly, thin films of PAH/GO that were
deposited onto Fluorine- doped tin oxide (FTO)
coated glass substrates (TEC15, 12-14 /) were
obtained by adsorbing alternate layers of electrically
charged polyelectrolytes at solid/liquid interface,
carefully washing with water the already adsorbed
layers after immersion in the polyelectrolyte solution
to remove the polyelectrolyte molecules that were not
completely adsorbed. The adsorption time period of
each layer (immersion time in each polyelectrolyte
solution) was 60 s and the thin film was dried, using
a nitrogen flow after the adsorption of each layer.
After this sequence, the first bilayer was formed and
the steps aforementioned were repeated until obtain
the desired number of bilayers.
After, the inorganic layer (TiO
2
or ZnO) was
deposited by DC-magnetron sputtering. Titanium and
zinc discs (Goodfellow, 99.99% purity) with 64.5 mm
of diameter and 4 mm of thickness each were used as
the sputtering targets. A turbomolecular pump
(Pfeiffer TMH 1001) was used to achieve a base
pressure of 10
-4
-10
-5
Pa (before introducing the
sputtering gas). Before the sputter-deposition step of
the films, a movable shutter was interposed between
the target and the substrates. The target was pre-
sputtered in the Ar atmosphere for 2 min to clean the
target surface. The target-to-substrate distance was
kept constant at 100 mm. Gases in the system were
pure Ar and O
2
and needle valves separately
controlled their pressures. TiO
2
and ZnO depositions
were both carried out in 100% O
2
atmosphere. For the
TiO
2
film the total pressure was kept constant at 2 Pa,
the sputtering power was 530 W, and the deposition
time was 25 min. In the case of ZnO film, the total
pressure was fixed at 4.8 Pa, the sputtering power was
300 W, and the deposition time was 30 min. The
target-to-substrate distance was kept constant at 100
PHOTOPTICS 2020 - 8th International Conference on Photonics, Optics and Laser Technology
186

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 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 characterization of the thin films thickness
and morphology was performed by a field emission
scanning electron microscope (FEG-SEM JEOL
7001F) operating at 15 keV. A gold thin film was
coated on the films surface before SEM analysis to
charge build-up prevention. The images of the cross
section allowed for the estimation of the films’
thickness. 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
devices were carried out using a programmable DC
power supply model Rigol DP811A (programmable
DC power supply, LX1) 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
3.1 UV-vis Spectrophotometry
In order to study the growth of a PAH/GO film and
the variation of its absorbance with the number of
adsorbed bilayers, films with 20, 30 and 50 bilayers
were produced using the LbL technique. In figure 2
and 3 is shown a representative ultraviolet-visible
absorbance spectra of different number of bilayers of
PAH/GO LBL films and the absorbance intensity at
380 nm as a function of the number of bilayers, N,
respectively.
Figure 2: Absorbance spectra of PAH/GO LBL films as a
function of the number of bilayers, N.
It can be observed that the absorbance at maximum
increases with the number of bilayers indicating a
linear film growth (see figure 3). This behaviour was
detected for all the developed devices.
Figure 3: Absorbance intensity at 380 nm as a function of
the number of bilayers, N.
3.2 Morphological Characterization
The SEM images reveal for all samples a uniform
surface with no apparent defects, such as the presence
of cracks. However for TiO
2
it can be noticed the
formation of aggregates, ranging from 260 nm to 400
nm for the devices with 20 bilayers and from 200 nm
to 340 nm for the 30 bilayers devices. The same is not
true for the device with 50 bilayers, which are much
smaller in the order of 20-30 nm (figure 4). The
surface of the alumimum electrodes also were
analysed and the surface was also homogeneous with
no sign of cracks. The morphology was similar to the
one detected for the TiO
2
films (images not shown).
Development of Hybrid Solar Cells based on TiO2 or ZnO-Graphene Oxide Heterojunctions
187

Figure 4: SEM images for three devices with the
architecture (FTO/PAH/GO/TiO
2
/Al)
x
a) x= 20 bilayers b)
x= 30 bilayers and c) x= 50 bilayers. The surface
corresponds to the TiO
2
film.
In the case of the devices with ZnO, for all samples
an uniform is observed with no cracks (figure 5) as
well for the aluminium electrode. Also for these
devices the SEM images reveal that these films are
formed by aggregates, which do not present
significant differences in their dimensions between
the analyzed devices, being their dimensions between
approximately 50 nm and 200 nm. These values are
considerably lower than those observed for TiO
2
films.
Figure 5: SEM images for three devices with the
architecture (FTO/PAH/GO/ZnO/Al)
x
a) x= 20 bilayers b)
x= 30 bilayers and c) x= 50 bilayers. The surface
corresponds to the ZnO film.
3.3 Electric Characterization of
FTO/(PAH/GO)
x
/TiO
2
/Al for
x = 20, 30 and 50 Bilayers
The analyzed devices correspond to samples with the
architectures: FTO/(PAH/GO)
20
/TiO
2
/Al and FTO/
(PAH/GO)
30
/TiO
2
/Al.
It is important to point out, that the samples with
20 and 30 PAH/GO bilayers were analyzed at room
temperature, at a temperature ranging from 20°C to
25°C, with a relative humidity between 50% and
60%. The samples were characterized only at ambient
light and in an environment without light interaction.
Figure 6 shows the current-voltage curves obtained
for the FTO/(PAH/GO)x/TiO
2
/Al samples, where x
indicates the number of bilayers.
Analyzing the figure, it is verified that these
samples do not present the expected behavior, that is,
they do not show a typical semiconductor behavior,
but rather a linear dependence of the current with the
applied voltage- a resistive behavior. This may be
due to several reasons, such as the high resistance at
the photoactive electrode layer interface, which will
block a large portion of the charge carriers, and the
existence of a donor-acceptor junction not efficient,
which may lead to an inefficient transport of charges
to the electrodes. For these reasons, and since they did
not exhibit the desired behavior, these samples were
not analyzed when exposed to a light spot.
It should be noted that in the sample with 30 PAH/GO
bilayers (Figure 6 (b)) there were no measurable and
significant changes with respect to exposure to
ambient light. Furthermore, this device did not show
any hysteresis in consecutive measurements.
On the other hand, the device with 20 bilayers
shows a decrease in its conduction when it is deprived
of ambient light. Given its resistive behavior, it
cannot be said to be a photovoltaic phenomenon, and
must rather be interpreted as a consequence of the
degradation of its constituent films.
In an attempt to understand why the previously
presented devices exhibited a poor performance, a
device with the following architecture was
developed: FTO/(PAH/GO)
50
/TiO
2
/Al. The number
of bilayers of the organic part was increased in order
to increase its thickness and to prevent the aluminum
migration inside the device, as occurred in previous
works carried out by the group (Magalhães-Mota
2018). Since parameters such as temperature and
humidity could also influence the characterization of
the sample, it was analyzed inside a desiccator, where
the temperature ranged from 20°C to 23°C, with a
very low relative humidity, ranging from 10%. and
15%. Although this experimental setup allowed a
PHOTOPTICS 2020 - 8th International Conference on Photonics, Optics and Laser Technology
188

more accurate control of temperature and humidity, it
was impossible to characterize the device under the
action of a light spot. The current-voltage curve of
this device is shown in Figure 6 c). Contrary to the
previously analyzed devices, the sample consisting of
50 PAH/GO bilayers has a curve I (V) similar to the
characteristic curve of a diode, also revealing a
hysteresis effect. This phenomenon is quite
noticeable from the first to the second measurement,
which suggests that there may have been a
degradation of the organic films, making the device
less conductive. Moreover, from the analysis of
Figure 6 c) it is visible that this sample is more
conductive when positively polarized.
Figure 6: Electrical characterization for three devices with
the architecture (FTO/PAH/GO/TiO
2
/Al)
x
a) x= 20 bilayers
b) x= 30 bilayers and c) x= 50 bilayers, for different light
conditions.
3.4 Electric characterization of
FTO/(PAH/GO)
x
/ZnO/Al for
x = 20, 30 and 50 Bilayers
The analyzed devices correspond to the samples
FTO/(PAH/GO)
x
/ZnO/Al, where the x correspond to
the number of bilayers. This study was not performed
under the same conditions for all samples. The
analysis of the first two samples was carried out under
ambient atmosphere, at a temperature ranging from
20°C to 25°C, with a relative humidity between 50%
and 60%. The latter was analyzed inside a desiccator,
where the temperature ranged from 20° C to 23° C,
with a relative humidity ranging from 10% to 15%.
Figure 7: Electrical characterization for three devices with
the architecture (FTO/PAH/GO/ZnO/Al)
x
a) x= 20 bilayers
b) x= 30 bilayers and c) x= 50 bilayers, for different light
conditions.
Development of Hybrid Solar Cells based on TiO2 or ZnO-Graphene Oxide Heterojunctions
189

All samples were characterized at ambient light and
in an environment without interaction with light. The
sample FTO/(PAH/GO)
30
/ZnO/Al was also exposed
to a light spot from a halogen lamp.
The sample corresponding to Figure 7 (a) shows a
typical resistance behavior where the current depends
linearly on the voltage applied to the device terminals.
Four measurements were performed, the first two at
ambient light and the next without light, with no
significant hysteresis effect noticeable. This sample
also does not reveal any change in its electrical
behavior when exposed to ambient light.
In turn, the sample with 30 PAH/GO bilayers - Figure
7 (b) - presents a behavior very close to what would
be expected for a diode. Unlike the other devices
studied so far, the behavior of this latter device
seemed to change when exposed to the light spot from
a halogen lamp. However, when constructing its
current-voltage characteristic curve, it was concluded
that the slight increase in current in the presence of
light was not significant enough to be considered a
photosensitivity phenomenon. FTO/(PAH/
GO)
50
/ZnO/Al device was analyzed inside a
desiccator, where the relative humidity value was
considerably lower than that recorded for the devices
analyzed in this subsection. Its curve I (V) is shown
in Figure 7 c). For this device, four consecutive
measurements were performed, in environments
without light and in the presence of ambient light,
alternately. In this sample, it is possible to observe a
current behavior against the applied voltage similar to
what would be expected for a diode type device.
Between the first and second measurements and
between the third and fourth, the hysteresis effect is
reduced, with no significant current decreases
between them. However, between the second and
third measurements, this effect is quite noticeable.
Again, although this device has the desired behavior,
no current increase has been recorded in the presence
of light and is therefore not photosensitive.
Looking at the quotient between the applied voltage
values and the obtained current values, it is apparent
that this device proved to be much less conductive
than the others. It is important to remember that this
device was analyzed in an environment with 10%
relative humidity, and this parameter can play a key
role in the conduction of the fabricated solar cells. In
fact, according to the literature (Raposo 1999), the
presence of moisture and / or oxygen is a major factor
in the conduction of organic films, making them more
conductive.
4 CONCLUSIONS
This work reports the development of two types of
solar cells devices: FTO/(PAH/GO)x/TiO
2
/Al and
FTO/(PAH/GO)x/ZnO/Al, where x, corresponds to
the number of bilayers. The organic films were
deposited by layer-by-layer technique, while
inorganic films were deposited by DC-reactive
magnetron sputtering. The aluminum electrode was
deposited by thermal evaporation.
The characterization of the organic films was
carried out by ultraviolet-visible spectrophotometry,
which revealed a linear film growth with the number
of bilayers. This means that the same amount of
PAH/GO per unit area was adsorbed in each bilayer,
so their number is proportional to the thickness of the
films.
Scanning electron microscopy (SEM) was used to
study the surface morphology of the samples, as well
as to estimate the thickness of each layer which
constitutes the developed devices. In the case of TiO
2
samples, larger aggregates were detected for the
devices with 20 and 30 PAH/GO bilayers, with sizes
ranging between 260 nm and 400 nm and between
200 nm and 340 nm, respectively while for the
devices with 50 bilayers the aggregates were
significantly smaller (20-30 nm). In the case of the
samples with ZnO, the observed aggregates were
smaller in size than those observed for TiO
2
(between
50 nm and 200 nm), with no change in size with
varying number of bilayers, unlike for devices with
TiO
2
, where SEM measurements showed that the size
of the aggregates decreased with increasing number
of bilayers of the organic layer. None of the SEM
images analyzed revealed cracks in the samples, so it
is excluded that aluminum migration may have
occurred at the time of electrode deposition.
The electrical characterization of the
FTO/(PAH/GO)x/TiO
2
/Al and FTO/(PAH/GO)x/
ZnO/Al samples was performed based on the
construction of their current-voltage characteristic
curves. The samples with TiO
2
and in which 20 and
30 PAH/GO bilayers were deposited were analysed in
air, with ambient light, showing a resistive behavior,
without photosensitivity. This may be due to high
resistance between the photoactive layer and the
electrode or to an inefficient donor-acceptor interface.
For the TiO
2
-based device with 50 bilayers of
PAH/GO (analysed inside a desiccator to reduce
moisture) presented a characteristic I (V) curve of a
diode, which is more conductive when positively
polarized. In the case of the devices with ZnO for the
device with 20 bilayers of PAH / GO an ohmic
behavior was detected and for the device with 30
PHOTOPTICS 2020 - 8th International Conference on Photonics, Optics and Laser Technology
190

bilayers a semiconductor behaviour was revealed.
The latter sample was exposed to a spotlight from a
halogen lamp and a slight increase in current was
recorded. However, it was not a significant enough
increase to consider it to be photosensitive. In the
device with 50 bilayers, a typical diode behavior was
detected, although it was much less conductive than
all the other samples analyzed. In fact, it was studied
inside a desiccator, with relative humidity close to
10%, so it is concluded that humidity is a major factor
in the conduction of organic films. This work further
evidenced that the use of zinc oxide as an electron
acceptor material in a solar cell, appears to be more
suitable for the performance of such devices than
titanium dioxide.
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/00068/2019.
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