A high molar extinction coefficient charge transfer sensitizer and its application in dye-sensitized solar cell

This thesis contains complementary synthetic and computational studies of transition metal complexes with polypyridyl ligands for use either as water oxidation catalysts or for application in dye-sensitised solar cells (DSSCs). Chapter 1 introduces the reasons for researching water splitting catalysts and describes a number of current techniques used to do so; from photoelectrochemical cells to the use of transition metal polypyridyl complexes. It also introduces three commercially available types of solar cells; silicon, thin film and the dye-sensitised solar cell. Chapter 2 describes the synthesis of seven ruthenium(II) complexes with substituted 4'-(4-pyridyl)-2,2':6',2''-terpyridine ligands and their photophysical and electrochemical properties. Density Functional Theory (DFT) calculations were used to explore the compositions of the highest occupied- and lowest unoccupied molecular orbitals (HOMO and LUMO, respectively) and Time Dependent DFT (TD-DFT) was used to predict the absorption spectra of the complexes. Chapter 3 contains information on water soluble ruthenium(II) complexes, their synthesis, photophysical and electrochemical properties and their activity as water splitting co-catalysts. A mechanism to explain the variable activities of the complexes is also put forward. Chapter 4 describes the synthesis of two homoleptic Cu(I) complexes. One complex involves a simple 6,6'-dimethyl-2,2'-bipyridine ligand. The other complex contains a ligand with extended ?-conjugation. The properties of the Cu(I) complexes are studied in terms of their suitability for use in DSSCs. A strategy of ligand-exchange on the surface of titanium dioxide (TiO2) is then utilised to form surface-bound heteroleptic Cu(I) complexes and efficiences of these complexes in DSSCs were measured. Chapter 5 details the development of a suitable basis set to be used in both DFT and TD-DFT to predict the absorption spectra of the homoleptic Cu(I) complexes in Chapter 4; the accuracies of the predicted spectra are assessed. The properties of the uncharacterised, heteroleptic Cu(I) complexes were then predicted and the effects of the anchoring ligands on the overall properties of the complexes were assessed. Chapter 6 describes the synthesis of two mono-substituted bipyridine-based ligands and their corresponding homoleptic chiral copper(I) complexes. Variable temperature nuclear magnetic resonance (VT-NMR) experiments are described, along with the photophysical properties of the ligands and complexes. Chapter 7 consists of the overall conclusions and an outlook.

A new high molar extinction coefficient ruthenium(II) bipyridyl complex ‘cis-Ru(L1)(2,2′-bipyridine-4,4′-dicarboxylic acid) (NCS)2, BDF’, where L1=4,4-bis(9,9-dibutyl-9H-fluorene-2-yl)-[2,2] bipyridine, has been synthesized and characterized by Fourier transform infrared (FTIR), hydrogen nuclear magnetic resonance (1 H-NMR) and electrospray ionization mass (ESI–MASS) spectroscopes. The dye, upon anchoring onto mesoporous nano-crystalline TiO 2 solar cells, exhibited a broader photocurrent action spectrum, with a solar-to-electric energy conversion efficiency (η) of 6.58% (J SC =14.66 mA cm −2, V OC =640 mV, fill factor=0.71) under sunlight at air mass (AM) 1.5, larger than the reference Z907 sensitized solar cell fabricated under similar conditions, which exhibited an η-value of 4.65% (J SC =11.52 mA cm −2, V OC =566 mV, fill factor=0.72). Absorption measurements and time-dependent density functional theory (TDDFT) calculations show that the increased conjugation length by introducing 9,9-dibutyl-9H-fluorene moiety relatively enhances the spectral response of the ancillary ligand and the corresponding BDF complex. The calculated dipole moments for BDF and Z907 are 17.71 and 16.34 Debye, respectively. The first three highest occupied molecular orbitals (HOMOs) of BDF have a t 2g character, as observed in Z907, while HOMO-4 and HOMO-5 have considerable sizable mixing from Ru-NCS with π-orbitals of L1.

Phenothiazine conjugated bipyridine as ancillary ligand in Ru(II)-complexes for application in dye sensitized solar cell

A new high molar extinction coefficient ruthenium(II) bipyridyl complex “cis-Ru(4,-bis(9,9-dibutyl-7-(3,6-di-tert-butyl-9H-carbazol-9-yl)-9H-fluoren-2-yl)-2,-bipyridine)(2,-bipyridine-4,-dicarboxylic acid)(NCS)2, BPFC” has been synthesized and characterized by FT-IR, -NMR, and ESI-MASS spectroscopes. The sensitizer showed molar extinction coefficient of M−1cm−1, larger as compared to the reference N719, which showed M−1cm−1. The test cells fabricated using BPFC sensitizer employing high performance volatile electrolyte, (E01) containing 0.05 M I2, 0.1 M LiI, 0.6 M 1,2-dimethyl-3-n-propylimidazolium iodide, 0.5 M 4-tert-butylpyridine in acetonitrile solvent, exhibited solar-to-electric energy conversion efficiency (η) of 4.65% (short-circuit current density () = 11.52 mA/cm2, open-circuit voltage () = 566 mV, fill factor = 0.72) under Air Mass 1.5 sunlight, lower as compared to the reference N719 sensitized solar cell, fabricated under similar conditions, which exhibited η-value of 6.5% ( = 14.3 mA/cm2, = 640 mV, fill factor = 0.71). UV-Vis measurements conducted on TiO2 films showed decreased film absorption ratios for BPFC as compared to those of reference N719. Staining TiO2 electrodes immediately after sonication of dye solutions enhanced film absorption ratios of BPFC relative to those of N719. Time-dependent density functional theory (TD-DFT) calculations show higher oscillation strengths for 4,-bis(9,9-dibutyl-7-(3,6-di-tert-butyl-9H-carbazol-9-yl)-9H-fluoren-2-yl)-2,-bipyridine relative to 2,-bipyridine-4,-dicarboxylic acid and increased spectral response for the corresponding BPFC complex.

A series of organic sensitizers with the direct electron injection mechanism and a high molar extinction coefficient comprising double donors, a π-spacer, and anchoring acceptor groups (D–D−π–A type) were synthesized and characterized by experimental and theoretical methods for dye-sensitized solar cells. (E)-2-Cyano-3-(5″-(4-((4-(3,6-di-tert-butylcarbazol-9-yl)phenyl)dodecylamino)phenyl)-[2,2′:5′,2″-terthiophene]-5-yl)acrylic acid showed performance with a maximal incident photon to electron conversion efficiency of 83%, Jsc value of 10.89 mA cm–2, Voc value of 0.70 V, and fill factor of 0.67, which correspond to an overall conversion efficiency of 5.12% under AM 1.5G illumination. The molecular geometry, electronic structure, and excited states were investigated with density functional theory, time-dependent density functional theory, and the symmetry-adapted cluster-configuration interaction method. The double donor moieties not only contribute to enhancement of the electron-donating ability, but also inhibit aggregation between dye molecules and prevent iodide/triiodide in the electrolyte from recombining with injected electrons in TiO2. Detailed assignments of the UV–vis spectra below the ionization threshold are given. The low-lying light-harvesting state has intramolecular charge transfer character with a high molar extinction coefficient because of the long π-spacer. Our experimental and theoretical findings support the potential of direct electron injection from the dye to TiO2 in one step with electronic excitation for the present D–D−π–A sensitizers. The direct electron injection, inhibited aggregation, and high molar extinction coefficient may be the origin of the observed high efficiency. This type of D–D−π–A structure with direct electron injection would simplify the strategy for designing organic sensitizers.

A computational finding on the effect of π-conjugated acceptors in thiophene-linked coumarin dyes for potential suitability in DSSC application

Rational design of metal-free organic D-π-A dyes in dye-sensitized solar cells: Insight from density functional theory (DFT) and time-dependent DFT (TD-DFT) investigations

Four novel metal-free organic sensitizers based on 5,7-dihexyl-6,12-diphenyl-5,7-dihydroindolo[2,3-b]carbazole (DDC) donors have been synthesized. Their optical/electrochemical properties, dye-sensitized solar cell (DSSCs) performances and photo-stability upon successive irradiation for 30 min have been investigated. The optical data indicates that all these dyes showed a high molar extinction coefficient (5.6–6.3 × 104 M−1 cm−1). Under standard global AM 1.5 solar light condition, the DDC4 sensitized cell gave a short circuit photocurrent density (Jsc) of 14.81 mA cm−2, an open circuit voltage (Voc) of 0.688 V, and a fill factor (FF) of 0.69, corresponding to an overall efficiency (η) of 7.03%. The benzothiadiazole units and thieno[3,2-b]thiophene (TT) in the π spacer were found to increase the photo-stability of the corresponding dyes and these dyes based on this donor are promising candidates for the further application in DSSCs.

Dye-sensitized solar cells (DSSC) is a device which converts a solar energy to electrical energy. Different with semiconductor thin film based solar cell, DSSC utilize the sensitized-dye to absorb the photon and semiconductor such as titanium dioxide (TiO2) and zinc oxide (ZnO) as a working electrode photoanode. In this report, the preparation of TiO2 film using a facile method of spray deposition and its application in DSSC have been presented. TiO2 photoanode was synthesized by growing the droplet of titanium tetraisopropoxide diluted in acid solution on the substrate of conductive glass flourine-doped tin oxide (FTO) with variation of precursor volume. DSSC was assemblied by sandwiching both of photoanode electrode and platinum counter electrode subsequently filling the area between these electrodes with triodine/iodine electrolite solution as redox pairs. The characterization of the as prepared DSSC using solar simulator (AM 1.5G, 100 mW/cm2) and I-V source meter Keithley 2400 showed that the performance of DSSC was affected by the precursor volume.. The overall conversion efficiency of DSSC using the optimum TiO2 film was about 1.97% with the open circuit voltage (Voc) of 0.73 V, short circuit current density (Jsc) of 4.61 mA and fill factor (FF) of 0.58.

We synthesized hydrophobic ruthenium(II) sensitizers (SY-04 and SY-05) with high molar extinction coefficient by extending the π-conjugation of 3,4- or 3-alkylthiophene-substituted bipyridine ligands. Both dyes displayed a remarkably high molar extinction coefficient of 21.7 × 103 M−1 cm−1 arising from red-shift of their metal-to-ligand charge transfer band when compared to a commonly used N3 sensitizer. The solar-to-electrical energy conversion efficiency (η) of the SY-04 based dye-sensitized solar cell (DSC) was 7.70%, which is 27% higher than that (6.05%) of the N3-based DSC under the same cell fabrication conditions. The increased η was attributed to the increase in life time and recombination half-life measured by electrochemical impedance and transition absorption spectroscopy, respectively. Density functional theory and time-dependent density functional theory calculations of two dyes in both gas phase and solution were performed. The calculated values of the highest occupied and the lowest unoccupied molecular orbitals and absorption spectra are in good agreement with the experimental results.

Enhancing the performance of dye-sensitized solar cells based on an organic dye by incorporating TiO 2 nanotube in a TiO 2 nanoparticle film

In our quest to develop good materials as photosensitizers for photovoltaic dye-sensitized solar cells (DSSCs), cis-dithiocyanato-4-(2,3-dimethylacrylic acid)-2,2'-bipyridyl-4-(9-anthracenyl-(2,3-dimethylacrylic)-2,2'-bipyridyl ruthenium(II) complex, a high molar extinction coefficient charge transfer sensitizer, was designed, synthesized and characterized by spectroscopy and electrochemical techniques. Earlier studies on heteroleptic ruthenium(II) complex analogues containing functionalized oligo-anthracenyl phenanthroline ligands have been reported and documented. Based on a general linear correlation between increase in the length of π-conjugation bond and the molar extinction coefficients, herein, we report the photophysical and electrochemical properties of a Ru(II) bipyridyl complex analogue with a single functionalized anthracenyl unit. Interestingly, the complex shows better broad and intense metal-to ligand charge transfer (MLCT) band absorption with higher molar extinction coefficient (λmax = 518 nm, ε = 44900 M−1cm−1), and appreciable photoluminescence spanning the visible region than those containing higher anthracenyl units. It was shown that molar absorption coefficient of the complexes may not be solely depended on the extended p-conjugation but are reduced by molecular aggregation in the molecules.

The influence of π-linkers configuration on properties of 10-hexylphenoxazine donor-based sensitizer for dye-sensitized solar cell application – Theoretical approach

A series of perylene-based novel metal-free organic dye sensitizers are designed and optimized for dye-sensitized solar cell (DSSC) applications. The electronic and optical properties are analyzed through density functional theory (DFT) and time-dependent density functional theory (TD-DFT) approach. For perylene-based donors, the effects of additional donor units and different π-spacer positions were investigated. Cyanovinyl and thiophene are used as π-spacers, dimethylamine (DM) and N-N-dimethylaniline (DMA) are used as additional donors, and cyanoacrylic acid is used as mono acceptor unit for the designed sensitizers. Natural bonding orbitals (NBOs), frontier molecular orbitals (FMO), UV-Vis, and nonlinear orbital analysis were predicted to find the net electron transfer, energy gap, absorption spectra, and electronic charge distribution for perylene-based dye sensitizers, respectively. The electron injection and electron regeneration properties were also analyzed for perylene-based sensitizers.

Properties of four indoline dyes were studied by means of density functional theory (DFT) and time- dependent density functional theory (TD-DFT) with the goal of finding an excellent photosensitizer for use in dye-sensitized solar cells. Theoretical results showed that the frontier molecular orbital structures of indoline dyes are suitable for electron injection from the excited states of indoline dyes to a TiO2 electrode. Calculated UV-visible absorption spectra of indoline dyes in vacuum match well with solar radiation spectra. The calculated energy levels of these dye molecules demonstrate that indoline dyes can be used as photosensitizers for TiO2 nanocrystalline solar cells together with the I-/I3- electrolyte. The lowest unoccupied molecular orbital (LUMO) energy levels of indoline dyes are higher than the conduction band edge of the TiO2 crystal, which ensures a high efficiency of electron transfer from indoline dyes to TiO2 electrodes. As the highest occupied molecular orbital (HOMO) energy levels of indoline dyes are lower than those of I-/I3- , molecules that donated electrons can receive electrons from the electrolyte. By comparison with the experimental data, the transfer efficiency of dye-sensitized solar cells may be determined mainly by the LUMO energy levels. The working lifetime of a dye-sensitized solar cell depends mainly on the stability of the dye molecule. From an analysis of the bond length of chemical bonds, we find that the stability of the four indoline dye molecules is basically the same. We further show that indoline dye 1 (ID1) has the highest LUMO energy level and the highest molecular stability. Its absorption spectra match solar radiation spectra well in an ethanol solution, therefore, it is the best photosensitizer among the analyzed dyes for application in dye-sensitized solar cells.