A Thesis submitted to the Faculty of Science, School of Chemistry at
University of the Witwatersrand, in fulfilment of the requirements for the
degree of Doctor of Philosophy. Johannesburg, 2015. / Photovoltaic cells offer a good alternative to the fossil fuels. Several approaches are being
analysed in order to have solar cells that are capable to conquer the energy market all around
the world. Quantum dots (QDs) have already proven features that can be taken into account to
improve the properties of solar cells. Metal selenide nanoparticles (NPs) possess
semiconducting behaviours that can vary with their structural and optical properties evolving
from their synthesis. The reaction parameters such as the method, time, solvent and precursors
can affect the growth and nucleation of particles and thus impose on the properties of the
synthesized materials. The performance of solar cells made of the synthesized metal selenides
will then be dependent upon the properties of the NPs used as active layer. Furthermore, the
electrical current generation also depends on the structure of the deposited active layer and its
interface with other films to be assembled for the device. The binary copper selenide, ternary
copper indium selenide (CISe), quaternary copper indium gallium selenide (CIGSe) and
quinary copper zinc tin sulphur selenide (CZTSSe) NPs were synthesized via conventional
colloidal method (CCM) and microwave assisted method (MAM). The MAM has a particular
interest as it is less time consuming and can easily be a large scale synthesis. Photovoltaic
devices were fabricated from the synthesized materials as proof of concept for photovoltaic
activities. The CCM was used to optimize various parameters for the synthesis of each type of
the chalcogenide materials as this is easily controllable than the ones from the sealed vessel
from MAM. The dependency of properties of all copper chalcogenide NPs on the time,
precursor concentration, temperature and solvent of synthesis have been demonstrated via
various characterization techniques including ultraviolet-visible-near infrared spectroscopy,
photoluminescence spectroscopy, X-ray diffractometry and transmission electron microscopy.
The binary copper selenide was first synthesized and considered as a template for evaluation
of the use of copper chalcogenide materials in solar cells. Relatively smaller copper selenide
NPs with average sizes of 4.5 and 6.0 nm were obtained from conventional colloidal and
microwave assisted methods respectively. The sample yielded from the microwave assisted
method possessed less polydispersed NPs. The later had better crystallinity in which prevailed
a single cubic Cu2Se phase. To the best of our knowledge this is the first evidence of defined
shapes and nearly single phase of small sized copper selenide NPs synthesized by mean of the
MAM. The copper selenide particles synthesized via this method were used to fabricate a
Schottky device. The conditions of copper selenide synthesis were optimized to 250 oC, 30
min of CCM synthesis using oleylamine (OLA) and a Cu/Se ratio of 1:1. Nearly hexagonal
facets with blue-shifted absorption band edge of monodispersed NPs sizing 4-8 nm in
diameter were obtained. The synthesized copper selenide showed better crystallinity with a
single cubic Cu2Se phase. A Schottky device using MAM synthesized copper selenide NPs as
the semiconducting layer was fabricated at room temperature. The diode effect was
demonstrated with the electrical parameters such as the ideality factor, barrier height and the
series resistances extracted from the experimental current-voltage data using the thermionic
theory and Cheung’s modification. The thermionic theory resulted in the ideality factor of
4.35 and the barrier height of 0.895 eV whilst the Cheung’s method resulted in the ideality
factor, barrier height and series resistance of 1.04, 2.59 10-3 eV and 0.870 Ω respectively.
The ternary copper indium selenide NPs showed that the MAM allowed the formation of
copper rich NPs alongside secondary products. The synthesis of the ternary sample via CCM
was optimized using uncapped precursors (no TOP was added) in OLA at 220 oC for 30 min.
The synthesized CuInSe2 NPs possessed a large blue-shift in their absorption band edges and
emission peaks. The nearly stoichiometric CuInSe2 particles with diameter sizes of 5-9 nm
were found in tetragonal crystalline orientation. The cyclic voltametry (CV) and the
absorption spectra showed a large blue-shifted energy gap, about 0.95 eV, an increase from
the bulk, proving the quantum confinement effects of synthesized copper indium selenide
quantum dots. The CuInSe2 NPs were thus used as absorbing materials in the quantum dot
sensitized solar cell devices (QDSSCs). The QDSSC devices were assembled via treatment of
the titanium oxide, quantum dot layers and their interface. This was done by the treatment of
copper indium selenide surface with mercapto-propionic acid (MPA) and ethanedithiol (EDT)
during the deposition of the quantum dots onto TiO2 films. The MPA treatment did not reveal
positive effects on copper indium selenide thin film and the assembled device under our
optimized working conditions. However the use of EDT allowed the improvement of electron
transport. The short circuit current (Jsc), open circuit voltage (Voc) and fill factor (FF)
obtained from the current-voltage (J-V) curves reached the values of 324 μA cm-2, 487 mV
and 43% respectively, indicating that the investigated quantum dots possess electrical
properties.
For the quaternary copper indium gallium selenide, relatively small sized NPs were
synthesized via CCM and MAM. The CCM synthesized CIGSe NPs were less agglomerated
with a shorter tailing in absorption than those from MAM. The stoichiometric
CuIn0.75Ga0.25Se2 showed less agglomerated and highly crystalline particles with a large blueshifted
absorption band edge and a smaller full width at halth maximum (FWHM) of the
emission peak compared to CuIn0.5Ga0.5Se2 and CuIn0.25Ga0.75Se2. The use of OLA as solvent
of synthesis improved the growth and dispersivity of copper indium gallium selenide NPs.
The particles with a large blue-shifted absorption band edge, a lattice of tetragonal phase,
more monodispersed CIGSe and possessing an average size of 6.5 nm were obtained from
CCM synthesis using OLA. The OLA as-synthesized CIGSe NPs were used in thin film for
the assembly of QDSSC. The device exhibited electrical properties with the Jsc, Voc and FF
of 168 μA cm-2, 162 mV and 33% respectively. The overall device performance was poor but
may further be improved for further photovoltaic application.
The quinary CZTSSe NPs possessed large blue-shifted absorption band edges of 450-460 nm
than the bulk material (827 nm). The emission peak at 532 nm and similar FWHM of less
than 50 nm were observed in samples from both CCM and MAM. More monodispersed
crystals were obtained with both methods whilst the average particle sizes of 10 and 9 nm
were yielded from MAM and CCM respectively. The nanoparticles crystallized in tetragonal
lattices between copper zinc tin sulphide and copper zinc tin selenide crystals. However, the
MAM gave more crystalline phases. The CV and the absorption spectra showed a blue shifted
energy gap, about 0.21 eV increase from the buk which is located at 1.51 eV. This is
indicative of the quantum confinement effects of synthesized NPs. The evidence of electrical
properties was also shown in the QDSSCs fabricated using the MAM synthesized quinary
QDs. This was done following the same treatments as for copper indium selenide devices.
The Jsc, Voc and FF were found at the maxima of 258 μA cm-2, 395 mV and 38%
respectively. The MPA and EDT treatments did not improve the device performance under
our working conditions. Nevertheless, the electrical properties observed in the assembled
device were indicative of promising efficient solar cells from synthesized CZTSSe NPs.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/19309 |
| Date | January 2015 |
| Creators | Kalenga, Pierre Mubiayi |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
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