Fabrication of dye-sensitized solar cells using heteroatom-doped reduced graphene oxide, and metal oxide-based nanocomposites as a counter electrode or photoanode.
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Date
2021
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Abstract
Recent advancements in technology have led to an exponential increase in global energy
demand. Non-renewable energy sources have been widely used to meet this demand; however, they are
prone to depletion and have side-effects such as the emission of greenhouse gases that lead to
environmental pollution and climate change. Thus, there is a need to develop and
advance inexhaustible and clean energy sources that maximize the electrical power generated. This
will be beneficial for reducing greenhouse gas emissions, excess power fluctuations and electricity
bills. Therefore, renewable energy has been a centre of attention in the past few years. The
improvement in renewable energy sources, particularly in terms of quality, efficiency, and
accessibility to all, is a significant step towards addressing the challenge of failing to meet
the ever-increasing energy demand. Thus, renewable energy sources, particularly solar
energy, have evolved as a suitable alternative.
Among the photovoltaic devices that convert solar into electrical energy, dye-sensitized solar
cells (DSSCs) offer a cost-effective route towards a reliable energy supply. However,
the performance of DSSCs is limited by factors, such as poor optical absorption,
inefficient electron transport and severe instability. Thus, the innovation and
development of novel materials are required to improve the power conversion efficiency (PCE) and
sustainability of DSSCs. Graphene has been recently investigated as a suitable material to replace
metal-based electrodes and to enhance the properties of electronic materials in different
fields, such as photovoltaics and sensors. However, a defect-free graphene sheet is not
suitable as a counter electrode (CE) or photoanode in DSSCs because of its neutral polarity, which
often restricts efficient charge transfer at the graphene/liquid interface, irrespective of
the high in-plane charge mobility. Thus, the focus on graphene has been based on its
alteration by chemical doping with various heteroatoms, such as nitrogen, oxygen, boron, sulfur,
and phosphorus.
Chemical doping assists in improving the electronic properties of graphene to unlock potential
characteristics that are advantageous in DSSCs. Thus, heteroatom-doped reduced graphene oxide
(heteroatom-doped rGO) serves as a good material to replace conventional electrodes and enhance the
PCE in DSSCs. Heteroatom-doped rGO-based CEs and heteroatom-doped rGO-metal oxide-based
photoanodes are being developed to enhance the reduction activity of the redox couple, and to
increase the electrical conductivity for efficient charge transfer, respectively. Hence,
the focus is to reduce the cost, and to improve the performance and stability of DSSCs.
This study involved the synthesis of GO, rGO, nitrogen-doped reduced graphene oxide (N- rGO),
boron-doped reduced graphene oxide (B-rGO), boron and nitrogen co-doped reduced graphene oxide
(BN-rGO), and heteroatom-doped rGO-metal oxide-based nanocomposites: reduced graphene
oxide-titanium oxide (rGO-TiO2), N-rGO-TiO2, B-rGO-TiO2, reduced graphene oxide-bismuth
oxide (rGO-Bi2O3), N-rGO-Bi2O3, B-rGO-Bi2O3, and their application in DSSCs as a CE or
photoanode. The synthesized nanomaterials were further characterized by standard instrumental
techniques to elucidate their structural, morphological, optical, physicochemical, electrical
conductivity, and electrochemical properties.
The effect of doping temperatures (600, 700 and 800 °C), and various nitrogen precursors (4-
nitroaniline, 4-aminophenol, and 4-nitro-ο-phenylenediamine), on the physicochemical,
optical, and conductivity properties of N-rGO were investigated. The lowest doping
temperature (600 °C) resulted in less thermally stable N-rGO, yet with higher porosity, while the
highest doping temperature (800 °C) produced the opposite results. The choice of nitrogen
precursor significantly impacted the atomic percentage of nitrogen in N-rGO. The nitrogen- rich
precursor, 4-nitro-ο-phenylenediamine, provided N-rGO with favourable physicochemical properties
(larger surface area of 154.02 m² g⁻¹) and an enhanced electrical conductivity (0.133 S cm⁻¹).
Pyrrolic-N doping was achieved as the main constituent of nitrogen moieties in N- rGO.
In the study of B-rGO, where the boron concentration was varied, the elemental
analysis revealed that the highest boron content incorporated into the rGO framework was 7.12%. An
electronic band structure with a low charge resistance of 20.23 Ω and an enhanced electrical
conductivity of 5.920 S cm⁻¹ was observed and noted to be dependent on the concentration of boron
incorporated. All the B-rGO samples demonstrated a p-type conductivity behaviour, which is
attributed to an increase in the density of states near the Fermi level. Thus, the studies of
N-rGO and B-rGO revealed that by adjusting the doping temperatures, dopant precursors, and dopant
concentration, one could tailor various properties of heteroatom-doped rGO.
This study also determined the effect of the metal oxide (TiO2 and Bi2O3) on the heteroatom- doped
rGO-metal oxide-based nanocomposites. The integration of TiO2 with heteroatom- doped rGO
significantly enhanced the photon absorption, exciton generation and charge carrier separation, as
well as reducing the recombination effect, resulting in the efficient transfer of photogenerated
electrons from the photoanode to the collecting electrode. The low content of TiO2 in N-rGO-TiO2
rendered the DSSCs with excellent optoelectronic properties, leading to the fabrication of DSSCs
with a relatively higher PCE of 3.94% compared with 1.78 and 2.55%for devices based on rGO-TiO2 and B-rGO-TiO2, respectively. Similar to the TiO2-based nanocomposites, Bi2O3-based nanocomposites also exhibited a higher PCE due to good
electrochemical properties. Thus, the synergistic effect between the high
electrical conductivity of TiO2- and Bi2O3-based nanocomposites and the formation of a good ohmic
contact at the anode/cathode layer interface facilitated charge carrier transport,
while suppressing their recombination, to yield high PCEs.
It was also discovered that dual heteroatom-doped rGO is a promising material that can
enhance the electrical conductivity compared with other materials, such as GO, rGO, and single
heteroatom-doped rGO. When inves tigating the effect of single or dual heteroatom-doped rGO as CEs
for DSSCs, a precursor-dependent electrical conductivity behaviour is demonstrated in BN-rGO
samples. The BN-rGO-3 with polyaniline (PANi) polymer as a CE exhibited the best electrical
conductivity, yielding a PCE of 4.13%. Several traits that linked the physicochemical properties
and the PCE were successfully elucidated.
This affirms the hypothesized potential of heteroatom-doped rGO nanomaterials in DSSCs
through understanding and controlling both (i) nano-structural parameters and
(ii) physicochemical, optical, and electrical properties.
Description
Doctoral Degree. University of KwaZulu-Natal, Durban.