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Title: Theoretical design and characterization of sensitizers for dye-sensitized solar cells
Other Titles: Ran liao min hua tai yang neng dian chi min hua ji fen zi de li lun she ji yu biao zheng
Authors: Lu, Xiaoqing ( 魯效慶)
Department: Department of Physics and Materials Science
Degree: Doctor of Philosophy
Issue Date: 2011
Publisher: City University of Hong Kong
Subjects: Dye-sensitized solar cells.
Notes: CityU Call Number: TK2960 .L84 2011
xvi, 161 leaves : ill. (some col.) 30 cm.
Thesis (Ph.D.)--City University of Hong Kong, 2011.
Includes bibliographical references (leaves 143-159)
Type: thesis
Abstract: The growing worldwide demands for energy along with increasing concerns over global warming have stimulated the interest in seeking for renewable energy sources. Dye-sensitized solar cells (DSSCs) have gained widespread attention for their potential as low-cost solar energy conversion devices. One of key issues for the improvement of DSSCs is the design of light-absorbent dyes with cheaper, safer, more efficient, and more sustainable materials. In this thesis, we present theoretical investigations on various sensitizers using density functional theory (DFT) and time-dependent DFT (TD-DFT). The research covered a broad range of materials systems, including Ru(II)-, Cu(I)-, and Fe(II)-based metal-centered sensitizers and metal-free organic sensitizers. Ru(II)-based sensitizers containing functionalized bithiophene (btp) ligands (CYC-B1 and CYC-B11) have been investigated in both the gas phase and dimethylformamide (DMF) solution. The results characterized the role of the functionalized btp ligands with respect to the absorption properties of the dye. Frontier orbital analysis has shown that the three highest HOMOs are composed of nonbonding combinations of the Ru t2g orbitals with the p orbital and lone pairs of the SCN ligands, while the six lowest LUMOs are the π* combinations of the 4,4'-dicarboxy-2,2'-bipyridine (dcbpy) and/or btp-functionalized bipyridine (bpy) ligands calculated in the gas phase. The inclusion of solvent has resulted in great changes of the energies and compositions of the molecular orbitals of these complexes. It was found that the spectra are assigned to the intraligand π → π* transitions of the dcbpy ligand in the ultraviolet range, whereas spectra in the visible range show multitransition characteristic of metal-to-ligand charge transfer (MLCT), interligand π → π*, and intraligand π → π*. The functionalized btp ligands have provided additional contributions of intraligand π → π* in the visible range due to enhancement in the electron-donating capability and electron-accepting capability by the extension of the π-conjugation. The molecular geometries, electronic structures, and optical absorption spectra of a series of polypyridyl Cu(I)-based complexes have been investigated in both the gas phase and methyl cyanide (MeCN) solution. The frontier orbital analysis showed that the five HOMOs have predominantly Cu 3d character, resulting from a set of distorted degenerate orbitals, while the four LUMOs were composed of the antibonding combination of the C and N 2p orbitals of bipyridines. The absorption spectra of Cu(I)-based complexes displayed multitransition characters of Cu → bipyridine MLCT and ligand-to-ligand charge transfer (LLCT) transitions in the range of 350 - 700 nm. The solvent effect resulted in sharply upshifts of molecular orbital energies of Cu(I)-based complexes. Structural optimizations by enhancing the π-conjugation and introducing the heteroaromatic groups on ancillary ligands lead to the upshifts of molecular orbital energies, increase in oscillator strength, and red shift of the absorption spectra. When compared with Ru(II) sensitizers, polypridyl Cu(I)-based complexes showed similar optical properties and improving trend of DSSC performance along with the optimizations of structures. The results of this work have highlighted the point that polypyridyl Cu(I)-based complexes could provide promising sensitizers for efficient next-generation DSSCs. Fe(II)-based complexes incorporating polypyridyl ancillary ligands of [FeL2(SCN)2], [FeL3]2+, [FeL'(SCN)3]-, [FeL'2]2+ and [FeL''(SCN)2] (L = 2,2'-bipyridyl-4,4'-dicarboxylic acid, L' = 2,2',2''-terpyridyl-4,4',4''-tricarboxylic acid, L'' = 4,4'''-di-methyl-2,2':6',2'':6'',2'''-quaterpyridyl-4',4''-bis-carboxylic acid) have been investigated. The molecular geometries, electronic structures, and optical absorption spectra were predicted in both the gas phase and MeCN solution. The protonated/deprotonation effect was slight, while the isomerization effect and the solvent effect had great influence on the spectra characteristics of Fe(II)-based complexes. The spectra showed LLCT characteristic at shorter wavelength region, whereas the spectra display multitransition characters of Fe → polypyridine MLCT and LLCT transitions at the longer wavelength of the visible region. Fine tuning of electronic properties by proper choice of functionalized chromophores produced a series of bipyridyl Fe(II)-based complexes with general formula [FeL2(NCS)2] (L = 4,4'-substituted-2,2'-bipyridine), which leads to alterations of the spectral response ranges with different oscillator strengthes. The introduction of conjugated hydrocarbon at the 4,4'-substituted positions can enhance spectral response range and increase molar extinction coefficient yielding. The extension of the π-conjugation system of the bipyridine and conjugated electron-rich heteroaromatics in ancillary ligands can add donor groups and adjust the molecular orbital energy levels significantly. When compared with Ru(II) sensitizers, Fe(II)-based complexes showed similar characteristics and improving trend of optical absorption spectra along with the introduction of different polypridyl ancillary ligands. Metal-free organic sensitizers incorporating triphenylamine (tpa)-based donors and binary π-conjugated bridge have been investigated in the gas phase, DMF and chloroform solutions. The geometrical and electronic structures of the tpa-based sensitizers were used to unravel the relation between structural optimizations and optical properties. Based on the reference sensitizer of C219, novel organic sensitizers were designed by, (1) the extension of the donors by addition of a tpa subunit; (2) the alteration of the bridge groups by electron-rich furan and selenophene containing binary π-conjugated groups. By Substituting Na+ for H+ on the cyanoacrylic acid, it was confirmed the fact that the protonation/deprotonation effects had great effect on the spectra. The solvent effect leads to minor changes in the ground state geometries but great alterations in the excitation energies. The absorption spectra of tpa-based complexes exhibited intra-ligand LLCT characteristic with better response at the near-infrared region.
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