ORGANIC MATERIALS CONTAINING NEW HETEROAROMATIC BUILDING BLOCKS

Abstract

In order to efficiently function the dye molecule that is the heart of DSSC needs to meet a number of the necessities which includes low energy band gap, appropriate LUMO energy level relative to the conduction band of the TiO2 semiconductor for higher electron injection, and sufficiently lower HOMO energy level as compared to the electrolyte redox potential for fast regeneration of the dye. Thus the development of innovative and stable organic dyes with optical absorptions extending into red and/or near-IR regions of the solar spectrum is a challenging endeavor. In a D-π-A dye, modification of the donor (D) and conjugation pathway (π) is a useful strategy to alter the photophysical, electrochemical and photovoltaic performances. Therefore, the major approaches adopted for the improvement are the alternation of donor-acceptor motifs and modulation of the conjugation pathway to facilitate the intermolecular charge transfer (ICT). However, this approach suffers due to large twist angle between the aromatic units in the conjugation pathway which affect the ICT. The way to sort out this problem is the rigidification of the neighboring aromatic units into planar segments. Thus fused aromatic chromophores composed from the small aromatic units into planar and big π-conjugated system facilitate the charge transfer and light harvesting in DSSCs. These rigid structures also possess less reorganization energy and reduce the steric congestion that allows the packing of dye more compact on TiO2. In this regard, a plethora of novel organic sensitizers containing numerous fused polyaryl/heteroaryl derivatives is known. This thesis focused on importance of these heteroatoms in the field of DSSCs. Research on organic conjugated materials increased tremendously in the last few decades due to their potential application in organic electronics. In the growing smart world, all the domestic usable things becoming smart devices such as smart television, smart phone, etc. To develop vi such smart devices organic conjugated materials are essential because it provides facilely tunable functional properties and miniaturization opportunities. The functional properties of organic materials can be tuned by modifying the molecular structure and configuration by subtle variation of chromophores. The linearly conjugated molecule suffer due to the disadvantage of being aggregated in the solid, which generally affects the electronic device performance. The functional properties of organic of organic conjugated materials possessing bithiophene, thienoimidazole, fluorine, carbazole, dithienopyrrole, pyrene, quinaxaline, benzothiadiazole, etc, core have studied by several researchers. Among them bithiophene and thienoimidazole derivatives displayed promising optical and conduction properties suitable for application in electronic devices photovoltaics and organic light-emitting diodes. The thesis is bundled with six chapters. First chapter reveals the scope of the work presented in the thesis. The dyes are designed with an aim to study the functional properties such as optical, electrochemical, thermal properties and their application in electronic devices. In second chapter we introduce the importance of heteroaromatic chromophore in photovoltaic applications. A comprehensive survey of the dyes containing thienoimidazole either in the donor part or as a π-linker in the conjugation pathway in a conventional donor- π-acceptor molecular configuration. From the literature background, it is understood that molecule allow a more straightforward and reliable analysis of the structure-property relationship, an approach that remain the key tool for the design of new materials specifically designed for photovoltaic applications. Therefore, herein we believe that thienoimidazole chromophore can be explored towards systematic molecular engineering of organic materials for photovoltaic applications. The second part of the this chapter explores the literature background about bithiophene and phenanthroimidazole based derivatives and their electro-optical properties. vii In third chapter, we have designed and synthesized two different types of thienoimidazole-based organic sensitizers featuring (mono and di anchoring) D-π- A and D-D-(π-A)2 molecular configurations. The effect of different conjugated donors such as triarylamine, carbazole on the photophysical, electrochemical and photovoltaic properties is investigated. The optical and electrochemical properties of the dyes are strongly influenced by the nature of conjugating donor. The dye containing triaryl amine as donor exhibit longer wavelength absorption and lowest oxidation potential in the series. These T-shaped based sensitizers were appropriate for effective charge separation and recombination resistance than the similar linear dyes. The DSSC fabricated using di-anchoring donor-based sensitizer showed promising higher power conversion efficiency in the series attributable to the high recombination resistance (Rrec) and low charge transfer resistance (Rct2). The absorption maxima for the dyes realized from HOMO to LUMO transitions. Among the dyes, 4C displayed highest power conversion efficiency due to high photocurrent density and open circuit voltage. The EIS studies infer the superior performance of 4C originating from large recombination resistance and small charge transfer resistance. This work suggests that the strategy of synthesizing mono and di-anchoring dyes with the choice suitable aromatic system can be promising strategy to improve the efficiency of DSSCs. In fourth chapter, a series of bithiophene based dyes are synthesized. Their functional properties such as photophysical, electrochemical, and thermal are explored and correlated with the substitution pattern. The triarylamine based dyes showed red shifted absorption and emission spectra while their isomeric analogue show blue shifted absorption and emission. There is twisting in the isomeric analogues led to larger Stokes shifts, dipole moment and low fluorescence quantum yield. All dyes exhibited good thermal stability. viii In Chapter 5, a series of thienoimidazole based dyes substituted with phenanthroimidazole were synthesized. Their functional properties such as photophysical, electrochemical, and thermal are explored and correlated with the substitution pattern. These dyes showed emission in red-orange region. All dyes showed good thermal stability with thermal decomposition greater than 300 ˚C. In the sixth chapter, a summary of the work accomplished during the thesis work is presented. The relationship between the parameters such as absorption wavelength, short circuit current density (JSC) and open circuit voltage (VOC) is established. The role of molecular structure of the organic dyes on the photophysical and photovoltaic properties has clearly presented which may help the conceptual advancement in the development of future organic sensitizers to conquer the urgent challenges in DSSCs research. The newly designed bithiophene and thienoimidazole based dyes find application in OLEDs.