A Dissertation Submitted in Partial Fulfilment of the Requirements for the Award of the Degree of Doctor of Philosophy in Materials Science and Engineering of the Nelson Mandela African Institution of Science and Technology
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Date
2025-07
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NM-AIST
Abstract
Dye-sensitized solar cells (DSSCs) are promising third-generation photovoltaic cell technology
due to their easy fabrication and environmental friendliness. In DSSCs, a photosensitizer is
crucial for photon absorption, and electron transfer. Natural pigments are investigated as
photosensitizers in DSSCs due to their abundance and environmental friendliness. However,
their narrow solar spectrum absorption and weak bonding with the semiconductor limit their
wide application, necessitating the design of novel photosensitizers. This work focuses on the
theoretical design of Braz-Bd-ether, Braze-Bd-ether, Aliz-Bd-ether, Braz-Bd-oxane, Aliz-Bd
oxane, Quinaliz-Bd- oxane molecules via etherification and bi-etherification reactions.
Moreover, Braz 01, Braz 01b, Braz 01t, Braz 01tb, Braz 02, Braz 02b, Braz 02t and Braz 02tb
molecules were obtained from the chemical structure modification of brazilin dye. The
photoelectrical, optoelectronic and structural properties of the isolated dye, dye@(TiO2)9H4
and dye@I2, were examined using DFT and TD-DFT methods. The DSSCs’ performance was
simulated using TiberCAD software. The results reveal that Braz-Bd-oxane, Braze-Bd-ether,
Aliz-Bd-ether, Aliz-Bd-oxane and Quinaliz-Bd-oxane complexes exhibited improved non
linear optical properties (NLO), reduced energy gap, lower exciton binding energy (Eb), lower
total reorganization energy (λt), and red-shifted absorption spectra by 10.29 nm ‒ 54.83 nm
regarding betanidin dye. The frontier molecular orbital (FMO) plots, ∆Ginj (-0.248 ‒ -0.453
eV), ΔGreg, and energy level alignment plots indicate that all designed complex dyes inject
electrons into TiO2 and are regenerated by I‒/I3‒ redox electrolyte, satisfying the photosensitizer
requirement. For Braz 01-Braz 02tb, are predicted to have improved charge transfer due to
reduced λtotal (0.951 eV ‒ 0.528 eV), increased EA (1.021 ‒ 3.472 eV), and decreased IP (5.591
5.252 eV). Additionally, the designed dyes (Braz01‒Braz02 tb) exhibited high current density
and minimal charge recombination compared to the brazilin dye, attributed to their red-shifted
absorption spectra, reduced Eg (5.074‒2.46 eV) and Eb (0.591‒0.137 eV). Negative Eads for the
dyes@(TiO2)9H4 and dye@I2 were noticed for all dyes, signifying spontaneous chemisorption
adsorption. Moreover, the 1D-DSSCs sensitized with designed complex dyes exhibit greater
power conversion efficiency (PCE) than natural dyes forming these complexes. Aliz-Bd-oxane
and Braz-Bd-oxane exhibited higher PCEs of 14.78% and 14.74% among the designed
complexes. Also, Braz 01-Braz 02tb demonstrated higher PCE than brazilin dye. The 1D
DSSC-Braz 01tb dye has the highest PCE of 13.44%. Therefore, the designed molecules Braz
Bd-oxane, Aliz-Bd-oxane, Braze-Bd-ether, and Braz 01tb are better photosensitizers in DSSC.
Sustainable Development Goals
SDG-7:Affordable and Clean Energy
SDG-9:Industry, Innovation, and Infrastructure
SDG-12: Responsible Consumption and Production
SDG-13: Climate Action