الفهرس 
Chapter 1: IntroductionOnly 14 pages are availabe for public view
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Abstract
In this thesis, we carried studies on the performance of metal-free
organic dyes (bis-N,N-dimethylaniline-based dyes) and natural dyes
(anthocyanin-based dyes) as sensitizers for dye sensitized solar cells
(DSSCs).
The thesis consists of three chapters as follows
Chapter 1 presents types of energy resources and the need for
renewable and clean energy sources, we briefly gave a short background
on solar cell (DSSCs). That is called “Grätzel cells”, which emerged as an
alternative inexpensive and environmental friendly method for converting
sunlight to electricity. Such devices become more attractive when the dye
molecules are synthesis or natural which don’t contain rare, toxic, metals.
The biggest challenges for developing DSSCs are low efficiency and
limited long term stability. Both factors depend sensitively on the
mechanism of dye and semiconductor binding that works as
photoelectrode. Tuning theses parameters can greatly enhance the device
performance, leading to higher the scale for solar energy conversion into
electric energy. Our attention in this work was paid to two types of dye
that can loaded into semiconductor (photoelectrode), organic dye has bis-
N,N-dimethylaniline as a donor group and natural dye derived from
anthocyanin structure.
Comparing the acyclic and cyclic linker between the donor group
(bis-N,N-dimethylaniline) and acceptor group (cyanoacrylic acid) is a
useful strategy to determine the effect of sensitizers linker on the cell
efficiency. This help us to tune the highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels
of bis-N,N-dimethylaniline-based dye for better cells efficiency.
Anthocyanins are plant pigments that have been used as safe food
colorant, antioxidant as well as DSSCs. The main drawback of the
anthocyanins is their instability. So, we studied the stability of
anthocyanin in presence of some 3-O-subsitution and reviewed their
performance as DSSCs. Furthermore, we studied the stabilizing effect of
co-pigments on the studied 3-O-subsitution anthocyanin.
Chapter 2 includes a short background on quantum chemical
calculations and a detail description of the procedures used throughout this
work. Geometry optimizations of bis-N,N-dimethylaniline-based dyes and
their dye/(TiO2)38 have been performed using density functional theory
(DFT) at the B3LYP/6-31G(d,p) levels of theory. Electronic absorption
spectra for the isolated dyes and dye/(TiO2)38 were calculated at the
structures thus obtained with time-dependent DFT (TD-DFT; TDB3LYP).
The electronic and optical properties including light harvesting
efficiencies (LHE), free energy of injection (ΔGinject ), free energy of
regeneration (ΔGregen), electronic open circuit photovoltage (eVoc),
strength of coupling and interaction between dye and surface of metal
oxides are good indicators of DSSCs efficiency. Furthermore, mechanism
of electron injection is clarified, to judge the sustainability and efficiency
of the investigated dyes. The DFT method at B3P86-D2/cc-pVDZ level
with dispersion has been used to optimize the most stable conformer of
the simplest anthocyanin (cyanidin) and some co-pigmented structure
with flavocommeline.
The experimental procedures for preparation of 3-Oglucosylcyanidin
and cyanidin (has hydroxyl group at the 3- position),
and the synthesis of 3-O-methylcyanidin have been presented. The UVVis
spectra, kinetic analysis and color measurements have been used to
determine the stability of studied 3-O-subsitution anthocyanin in absence
and presence of flavocommeline and rutin copigments. Also, the method
of DSSCs fabrication has been also given.
Chapter 3 summarized the results and discussion of bis-N,Ndimethylaniline-
based dyes (part I) and 3-O-subsitution cyanidin (part II)
as sensitizers for DSSCs.
Part I:
For the study of some bis-N,N-dimethylaniline-based dyes performance
as sensitizers in DSSCs, the results obtained can be summarized as
follows:
1- Shorter acyclic π-conjugated linkers help in electron injection and dye
regeneration processes.
2- Increasing the length of acyclic π-conjugated linkers lowers free
energy of injection, decreases probability of dye regeneration,
induces the probability of dye aggregation, and enhances only the
indirect injection.
3- Cyclic π-conjugated linkers exhibit better performance regarding
LHE, high free energy of injection, decrease probability of dye
aggregation, and coexistence of direct and indirect mechanisms.
4- The dye with pyrrole linker and the one with the shortest acyclic
linker (-CH=CH-) that called P1 illustrate direct electron transfer
from the dye to the photoanode.
5- The better performance of P2 that have two ethylene group
(butadiene) as a linker group over P1 could be explained in terms of
better free energy of regeneration, participation of direct and indirect
injections, lower exciton binding energy (EB), and higher LHE in P2
relative to P1.
6- All binding modes for the dye/TiO2 systems carried out in this study
are energetically stable and give comparable data.
In an aqueous solution at pH 1, where the flavylium cation form has
to be dominated, 3-O-substituion plays an important role on the stability of
anthocyanin. The stability of 1 (3-O-glucosylcyanidin) and 3 (3-Omethylcyanidin)
exhibits similar stabilities, and 2 (cyanidin) was less stable
than 1 and 3. The instability of 2 might be due to its isomerization to the 3-
keto structure. In contrast, 3-O-glycosyl and 3-O-methycyaidin cannot
generate such structure. This may be the major reason for the enhanced
stability of 3-O-substitution. The difference between the stabilities of 1 and
3 may be ascribed to the steric hinderance and number of OH moieties on
the 3-O residue.
B3P86-D2/cc-pVDZ calculation reveals that 3- Keto structures of
cyanidin is thermodynamically preferable reaction than hydration reaction
and chalcone isomerization to 3-keto structure is thermodynamically and
kinetically preferable than flavylium cation isomerization to 3-keto form.
The bathochromic shift of λvismax and stability of the color by addition
of flavocommelin (4) co-pigment was greater than that of rutin (5)
copigment. The difference in the co-pigment effects of 4 and 5 may be
attributed to their structural difference; the glycosyl residues at 6 and 4’ in 4
gave it greater water-solubility and made its aromatic chromophore stack
more tightly with the anthocyanidin. The 3-O-rutinose residue in 5 is larger
than the sugar residue in 4 and inhibits efficient hydrophobic interactions
with the anthocyanidin chromophore. Thus, 4 can stack closer and stronger
to the anthocyanidin chromophore than 5. The high stability brings out
efficient dyes for dye sensitized solar cells. Thus, 1 and 3 have higher
conversion efficiency than 2. In our finding, the conversion efficiency was
greatly reduced with the addition of 4-Tert- Butylpyridine (TBP) to the
electrolyte. Therefore, TBP cannot be used for this system.