2. 2
ABSTRACT
In this study, a procedure to synthesize Strontium Titanate (SrTiO3) paste was developed,
in order to use SrTiO3 as an alternative material to Titanium Dioxide (TiO2) for semiconductor
material for dye-sensitized solar cells (DSSCs). This method yielded a viscous paste that was
able to adhere to the substrate well. Moreover, when thin layers of Alumina is deposited onto a
DSSC photoanode, the back reaction, known as recombination, where injected electrons in the
semiconductor transport back to the dye, becomes hindered. Therefore, it is expected that SrTiO3
treated with Alumina will have a higher open circuit voltage. In this study, we investigate the
effects varying levels of deposited Alumina on SrTiO3 have on DSSC performances. SrTiO3
based DSSCs that were deposited with two cycles of Alumina via Atomic Layer Deposition
techniques achieved a high voltage characteristic, comparable to that of TiO2 based DSSCs.
3. 3
INTRO
Every year, the global demand and usage of energy steadily increases. It is common
knowledge that fossil fuels make up a major portion of the energy source relied on today, despite
several severe drawbacks that include limited availability and a harmful contribution to the
climate change phenomenon. For these reasons, great attention has been placed on the
development of an alternative, clean and economically viable source of renewable energy.
With approximately 1017
Joules of energy striking the Earth’s surface every second, the
sun has the capability to easily provide more than one thousand times the worldwide energy
demand.1
However, harnessing from this vast energy source has proved to be a challenge in
terms of cost-efficiency and power conversion efficiency, preventing solar energy from being a
commercially feasible energy source in comparison to fossil fuels today. One form of solar cell
technology, the dye-sensitized solar cell, is gathering interest in the scientific community for
research and is beneficial due to the fact that dye-sensitized solar cells have low production cost,
are relatively easy to make, as well as a multitude of minor aesthetic characteristics.2,3
As of now, the rigid standard model of dye-sensitized solar cells (DSSCs) incorporates
titanium dioxide (TiO2) as the photoanode material, due to its superior electrical and chemical
properties. Since Gratzel’s contribution to the DSSC research field in the late 20th
century with
the nanocrystalline TiO2 based DSSC, the record for highest efficiency obtained has plateaued to
a little over 12%. While researchers have studied rigorously each independent major component
of the photo-related part of the DSSC for over a decade, the standard model for most optimized
DSSC is still essentially unchanged from Gratzel’s model, indicating a model gridlock and a
necessity to investigate new materials that is compatible with other materials to open up new
research opportunities.2
Although undoubtedly proven to be efficient in its respective function to adsorb dye and
transport electrons, TiO2 as a semiconductor has been nearly optimized, shown by the fact that
alterations to TiO2 based DSSCs haven’t significantly improved efficiency over the last two
decade. Strontium titanate (SrTiO3) has been reported as having similar titanium structural
similarities to anatase, the material most DSSCs use for semiconductors, with an even higher
flatband potential. For this reason, it is expected that SrTiO3 based DSSCs will have an open
circuit voltage comparable to, or even greater than that of TiO2 based DSSCs.4-5
4. 4
METHODOLOGY
Materials: Sr-Ti metal alkoxide precursor solutions for the synthesis of SrTiO3
nanocrystals were supplied by Gelest Inc. Ethyl cellulose (Aldrich) and α-Terpineol 96%
(SAFC/USA) were employed to help produce the viscous quality in the SrTiO3 paste. For the
solar cells, cis-Bis(isothiocyanato) bis(2,2'-bipyridyl-4,4'-dicarboxylato ruthenium(II)(N3) dye
(Dyesol) was employed, as well as Lithium Iodide (Alfa Aesar) and Iodine (Aldrich) were used in
the electrolyte solution.
Fig. 1: The apparatus used consists of a three-neck flask to be used as the reaction
chamber, a rotameter, and a bubbler with 200mL of 0.75M HCl. The arrows depict direction of
N2 gas flow in and out of the system.
Apparatus: The apparatus employed for SrTiO3 nanocrystals synthesis via vapor
diffusion is shown in Fig. 1. This consists of a glass bubbler with 200-mL of 0.75 M HCl
solution, and a 100-mL three-neck flask for the reaction. The setup has been described prior in
literature.6
Basically, the input of the bubbler is connected to an N2 gas source and the output is
connected via glass gas adapter to a stopcock. The three-neck flask has two separate
configurations: configuration 1, for during the vapor diffusion sol-gel (VDSG) reaction, where the
flask has an output stopcock B closed, an opened stopcock A connected to an N2 source, and a
rubber suba septum (C), and configuration 2 (shown in Fig 1.), for precursor injection, where the
flask has output stopcock B open, the N2 stopcock A closed, and the rubber suba septum replaced
with the output stopcock C connected to the bubbler’s gas adapter (also open).
Experimental: For our experiment, Strontium Titanate paste was synthesized through the
Vapor Diffusion Sol-Gel (VDSG) route, explained in literature elsewhere.7
First, N2 gas is
5. 5
bubbled through the HCl solution for approximately 30 minutes while configuration 1 is set up to
create an N2 atmosphere inside the flask. 5 mL of SrTi precursor is transported into the three-neck
flash through the suba septum via syringe. After 30 minutes of bubbling, the reaction flask is
changed to configuration 2. The Nitrogen flow from the bubbler provides continuous N2/HCl/H2O
vapor flow over the precursor solution. This flow is maintained at a vigorous rate for 48 hours, to
obtain off-white colored particles.
Within ten minutes, the precursor started to show visible signs of turning into a solid gel
with folds. After being under N2 flow for roughly 48 hours, dry SrTiO3 particles were
synthesized. This is removed from the flask and crushed using a mortar and pestle to obtain a
more powdery form with greater surface area. The powder is then sonicated in ethanol until
completely dispersed, and centrifuged back out to clean the SrTiO3 and remove impurities; a
process that is repeated thrice.
The paste is once again transported to a round-bottom flask and ethanol is added to
dissolve the paste. α-terpineol and ethyl-cellulose is added while stirring vigorously. The solution
is sonicated for thirty minutes, stirred for ten minutes, and then sonicated/stirred two more times.
The majority of the ethanol is then evaporated off in a rotavap until only a viscous paste remains.
This final paste is stirred for an additional hour, and prepared with a mortar and pestle prior to
usage.
Dye-sensitized solar cells were made with this paste using doctor-blade techniques onto
conductive FTO glass. Traditional dye-sensitized solar cell construction methodology follows: the
films were heat treated at 500°C, and soaked in a solution of N3 dye overnight. Counter
electrodes were made out of conductive FTO by drilling two holes (for injecting electrolyte) and
depositing platinum catalyst via H2PtCl6 solution. The two electrodes were sandwiched together
using a 25 um ionomer Surlyn 1702 (Dupont) film. I3
-
/I-
electrolyte was then injected into the
counter electrode hole to cover the active area. SrTiO3 films were also adapted with various
number of Atomic Layer Deposition cycles of Al with Trimethyl Alumina being used as the
precursor.
Photoelectrochemical measurements were performed with an Autolab PGSTAT
potentiostat interfaced with a Xenon Arc Lamp. An AM 1.5 solar filter was employed to simulate
sunlight at 100 mW cm-2
. Additionally, a 400 nm long-pass filter was used in all light
measurements.
6. 6
RESULTS
Characterization of SrTiO3 paste: To ensure the purity of the SrTiO3 made by this new
procedure and to detect exactly what materials were synthesized, an X-ray diffraction
measurement was taken in the 20°-80° 2θ range. As seen in Fig. 2, the nanoparticles produced
from the reaction have the definitive peaks that correspond to SrTiO3.
Fig. 2: X-ray diffraction patterns for SrTiO3.
The physical characteristics of the SrTiO3 nanoparticles were observed under a Scanning
Electron Microscope (SEM). Fig. 3 shows the SEM images of the nanoparticles, which measured
out to be approximately 15-22nm in diameter.
Fig. 3: SEM pictures of synthesized SrTiO3 nanoparticles after 24 hours of N2 flow (left) vs after
48 hours of N2 flow (right).
7. 7
In this project, it was discovered that high quality SrTiO3 nanoparticles could be
synthesized by vapor diffusion under a slow gas flow of N2, HCl vapor and water vapor. It was
found that when placed under flow for a prolonged time of 48 hours and under a not very
vigorous flow rate that the nanoparticles sizes became both smaller and more consistent, to
around 15-22nm, as can be seen in Fig. 3.
During the synthesis process, the SrTi(OR)6 precursor undergoes a slow reaction to
become SrTi and then form SrTiO3: The slow reaction rate allows the metals to make
homogenous nanostructures by slow diffusion. This reaction and diffusion continues until
homogenous bimetallic oxide nanocrystals are formed.
Optical and electrical properties of SrTiO3 based DSSCs: Transmittance and
reflectance was measured on a UV-VIS spectrometer with an integrated sphere setup and is
shown in Fig. 4. The transmittance of this SrTiO3 paste in the visible range was measured to be
roughly 70%, while the reflectance was under 20% for the 450+ nm wavelengths range. This all
indicates that the film does not inhibit light from reaching the dye.
Fig. 4: Plot of Transmittance and Reflectance for unmodified SrTiO3 films.
When compared to a standard TiO2-based DSSC, SrTiO3 shows photoelectrical
characteristics that are significantly less than TiO2. Key parameters demonstrating this is shown
in Fig. 5. It is notable that although the current density for the SrTiO3 DSSCs was <1% of the
current density of TiO2, the voltage for the former was found to be only about half of that of the
latter.
8. 8
DSSC VOC (V) JSC (A) FF
SrTiO3 -0.300 0.028 45%
TiO2 -0.627 7.320 70%
Fig. 5: Open circuit voltage (VOC), short circuit current (JSC), and fill factor (FF) values for a
SrTiO3 based DSSC and a TiO2 based DSSC are shown. The latter demonstrates superior
characteristics in all regards, especially with current density.
Fig. 6 shows the current density, J, versus applied voltage, V, curves comparing DSSCs
of SrTiO3 film with 2 layers of deposited Alumina versus the DSSCs of SrTiO3 films without
Alumina. In our experiment, the SrTiO3 films were optimized with two atomic layers of
Alumina. As seen in the graph, the short circuit current (JSC) remained relatively unchanged
when adding single atom layers of Alumina, whereas the open circuit (VOC) from the cells
increased because of limited back recombination of electrons from the photoanode to the ground
state dye. But the addition of a third layer of Al led to a significant decrease in JSC, mainly due to
the Alumina blocking electrons from injecting into the photoanode more than it blocked
electrons from recombining back to the dye. The measured JSC values for 0, 1, and 2 layers of Al
gravitate around 0.03A, and the VOC values are consistent, gradually increasing until the VOC
peaks at ~0.54V for 2 layers. Due to the large jump in open circuit voltage from zero to one layer
of Al, as well as the large drop in JSC from two layers to three layers of Al without very
noticeable changes at other major points, it can be deduced that for this particular configuration
of N3 dye, SrTiO3, and I3
-
/I-
electrolyte, 2 ALD cycles of Al deposited on the photoanode yields
optimal electrochemical results in light settings.
9. 9
Fig. 6: Plot of J-V curves measured under AM 1.5 illumination for DSSCs with 0, 1, 2, & 3
atomic layers of Alumina
DSSC
Layers of Al
VOC (V) JSC (A) PMAX
0 -0.300 0.028 0.38
2 -0.546 0.030 0.73
Fig. 7: Table of the Open Circuit Voltages (VOC), Short Circuit Currents (JSC), and Maximum
Output Power (PMAX) values of two solar cells of each varying amount of Al
Fig. 8 shows a plot of multiple dark J-V curves, for the same DSSCs plotted for light J-V
curves in Fig. 6. It is evident from this that the layers of Alumina have an indirect correlation
with the amount of dark current that occurs in the DSSC. For example, when the potential of -0.3
V is applied, the DSSC with 3 layers of Al only has a current density of -0.002, whereas the
DSSC with no Al has a J value of approximately -0.018, signifying more recombination takes
place in this cell. It is reasonable to assume that based on this data, the SrTiO3 solar cells with
deposited Alumina has increased blocking of back electron transfer of injected electrons in
the semiconductor back to the ground state dye. When additional layers of Al were added,
the achieved voltage started to drop again, signifying a possible decrease in initial injection
of electrons into the semiconductor.
10. 10
Fig. 8: Dark J-V curves of the same cells that are plotted in Fig. 7 & 12
As can be seen in Fig. 9, the lifetime of the Alumina added DSSCs demonstrate a longer
lifetime characteristic. When the applied potential is at 0.3V, the DSSC with 0 layers of Al
demonstrated a lifetime characteristic of ~0.1 seconds, less than one tenth the lifetime of the
DSSC with 2 atomic layers of Al at the same applied potential. The increase in lifetime with the
addition of thin layers of Alumina is explained by the reduced rate of injected electrons
transporting back to the ground state dye. It is important to note that excessive additional layers
of Alumina hindered performance with a decreased rate of injection of excited electrons from the
dye to the semiconductor material, cutting current.
11. 11
Fig. 9: Open-circuit voltage decay plot of a DSSC with 0 layers and with 2 layers of Alumina.
CONCLUSION
We have shown the preparation method of SrTiO3 films and the effect of a thin layer of
Aluminum. Although the overall photovoltaic performance of these solar cells was hindered due
to low current, the open circuit voltage is comparable to and very similar to that of TiO2 film
based DSSCs. Furthermore, deposition of Al proved to slow recombination and increase voltage.
12. 12
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