![]() | Only 14 pages are availabe for public view |
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. |