FLUORENE BASED ORGANIC DYES FOR DYE SENSITIZED SOLAR CELLS

Abstract

Among the techniques available for the conversion of solar energy to electricity, dye sensitized solar cells (DSSCs) have attracted huge attention due to the variety of sensitizers available for exploration and functional alternations. A sensitizer is deemed to play an important role to harvest light energy in DSSC, therefore it has attracted a lot of attention in the research and development. Several organic dyes having different donor and acceptor moieties conjugated with various rigid polyaromatic/heteroaromatic segments and possessing D-π-A architecture have been developed. They have exhibited impressive photovoltaic performance (ca.10%) when applied as sensitizers in DSSCs. Efficiency of DSSCs depends on the Fermi level of the semiconductor, the redox potential of the redox mediator and particularly the light-harvesting property of the dye. Light-absorption and charge separation in the dyes can be efficiently tuned by structural engineering with appropriate chemical modifications. As the electronic structure of the organic dyes respond to slight structural modifications significantly therefore, structureproperty relationships in organic dyes have to be evaluated actively. This thesis is divided into nine chapters. First chapter introduces aim and scope of the work. Though several polyaromatic hydrocarbon based organic dyes have been developed for use in DSSCs, dipolar compounds featuring a fluorene unit have shown promising potential as light harvesting component. In second chapter, we present a comprehensive survey of the organic dyes containing fluorene either in the donor part or as a π-linker in the conjugation pathway in a conventional donor–π–acceptor molecular configuration. Structural features of the molecular materials are correlated with the functional properties, such as optical absorption and oxidation potential. Third chapter describe the design and synthesis of metal free organic dyes based on diphenylaminofluorene/trifluorenylamine donor and cyanoacrylic acid acceptor for application as sensitizer in dye-sensitized solar cell. The optical, electrochemical, theoretical and photovoltaic properties of the sensitizers have been investigated and critically analyzed. It is found that the trifluorenylamine donor significantly red-shifts the absorption and facilitates the oxidation property of the dyes when compared to the triphenylamine donor. The longer wavelength absorption corresponds to the charge transfer from the trifluorenylamine donor to cyanoacrylic acid acceptor and is confirmed by the TDDFT theoretical computations. The variations in the photovoltaic performance of the dyes have been corroborated by the dye loading data, IPCE and v the interfacial kinetic parameters estimated from the intensity modulated photovoltage/photocurrent spectral measurements. A dye with fluorene and bithiophene segments in the π-linker exhibited device efficiency up to 5.8%. The enhanced power conversion efficiency exhibited by this dye when compared to its counterpart containing diphenylamine donor is attributed to its superior anti-aggregation ability on and comparatively prolonged electron lifetime. Chapter 4 deals with the synthesis and characterization of two types of novel organic dyes (T-shape/rod shape) based on phenothiazine donors, cyanoacrylic acid acceptors and fluorenylbenzene/fluorenyl-oligothiophene π-spacers for DSSCs. Crystal structure analysis of one of the dye revealed by single crystal XRD shows co-facial slip-stack columnar packing of the molecules. The effects of substitution position of donor unit as well as the influence of different linking segments on the optical, electrochemical and on the photovoltaic properties are investigated. The optical properties of the dyes are modeled using TDDFT simulation employing B3LYP level of theory and demonstrated a correlation with the experimental results. The evaluated results demonstrated that in rod shape configuration phenothiazine help in increasing the conjugation length and enhance the light harvesting ability. Whereas, the DSSC based on Tshape dyes display ~2 fold increase in the efficiency due to the favorable LUMO energy level and effective surface blocking to suppress the recombination of electrons between the electrolyte I3ˉ and TiO2. The Electron Impedance Spectroscopy (EIS) investigations provide a support to interpret the solar cell efficiency alternations. In chapter 5 organic dyes containing 2,7-diaminofluorene-based donor, cyanoacrylic acid acceptor and various aromatic conjugation segments composed of benzene, fluorene, carbazole and thiophene units as π- bridge have been synthesized and characterized by optical, electrochemical and theoretical investigations. The trends observed in the absorption and electrochemical properties of the dyes are in accordance with the electron-donating ability of the conjugating segment. Consequently, the dyes containing 2,7-carbazole unit in the π-spacer exhibited red-shifted absorption and low oxidation potentials than the corresponding fluorene and phenylene bridged dyes. However, the enhanced power conversion efficiency exhibited by the fluorene-bridged dyes in the DSSCs is attributed to the broader and intense absorption. Despite the longer wavelength absorption and reasonable optical density, carbazole bridged dyes vi exhibited low power conversion efficiency in the series which is ascribed to the poor alignment of dye LUMO level leading to the inhibition of electron injection into the TiO2 conduction band. In chapter 6 we implicate one or two fluorenylidene moieties in the donor part of triarylamine organic dyes containing fluorene-oligothiophene spacer in a D-π-A architecture and used them as sensitizers for nanocrystalline TiO2-based DSSCs. Their optical, electrochemical and photovoltaic properties are compared with electron accepting dicyanovinyl unit containing organic sensitizer. Incorporation of fluorenylidene moiety dominates the optical properties of the dyes in terms of relatively broad and high molar extinction coefficient absorption when compared to the dicyanovinyl derivative. Theoretical investigations using TDDFT simulations indicates that the trends in the excitation energies are consistent with the solution spectral data for higher wavelength absorption and the lower wavelength absorptions attributed to the amine to auxiliary acceptor charge transfer. The electrochemical properties are influenced by the number of fluorenylidene chromophore and the electron richness of the linking segment. The dye-sensitized solar cells fabricated using fluorenylidene-based sensitizers showed higher power conversion efficiency than the dicyanovinyl derivative attributed to their higher photocurrent density. A fluorenylidene-based dye exhibited high power conversion efficiency of 6.13% under full sunlight (AM 1.5G, 100 mW cm-2). Chap 7 illustrate a comparative study of novel difluorenylamine-based organic dyes containing carbazole as auxiliary donor, oligothiophene linker and cyanoacrylic acid acceptor/anchoring group featuring two different architectures (D-D-π-A and D-D-(π-A)2). Absorption measurements indicate that monoanchoring dyes with D-D-π-A configuration is beneficial for charge transfer transitions while the dianchoring dyes with D-D-(π-A)2 architecture lead to intense π-π* transitions suggesting more delocalized electronic structure. Incorporation of bithiophene in the conjugation slightly lowers the LUMO level of the dyes when compared to the corresponding thiophene derivatives. The TDDFT computations are performed to rationalize the trends in the optical and electrochemical properties of the dyes. The DSSC based on monoanchoring dyes showed 2 to 5-fold increase in the efficiency than the corresponding dianchoring congeners. Also the use of CDCA as an additive increases the efficiency of the devices reasonably. Chap 8 elucidate the synthesis and characterization of new metal free dianchoring organic dyes featuring A–π–D–π–D–π–A (acceptor – π bridge – donor – π bridge – donor – π bridge – vii acceptor) configuration incorporating fluorene and oligothiophene units. Elongating the conjugation pathway between the donor and acceptor units altered the distance between the anchoring sites besides the absorption and redox properties. These dyes exhibit broad and intense absorption when compared to the corresponding monoanchoring donor–π–acceptor congeners. Though the dye containing bithiophene unit exhibit comparatively low VOC due to low electron life time and facile back electron transfer, shows high power conversion efficiency arising from the good light-harvesting capability attributable to the intense absorption peak in the visible region and enhanced interfacial electron transfer rate. This work demonstrates that the smaller distance of separation between the anchoring units increases the insulating capacity of the molecular layer which retards the back electron transfer. In chapter 9 a summary of the work accomplished during the dissertation work is presented. The relationship between the absorption wavelength, short circuit current density (JSC) and open circuit voltage (VOC) of the sensitizers reported in this work is studied with their efficiency. The role of molecular structure of the organic dyes on the photophysical and photovoltaic properties is established for conceptually designing organic sensitizers to conquer the urgent challenges in DSSCs research.