DITHIENOPYRROLE-BASED ORGANIC MATERIALS FOR PHOTOVOLTAICS

Abstract

In order to efficiently function the dye molecule which is the heart of DSSC needs to fulfill some of the requirements such as low energy band gap, suitable LUMO energy level relative to the conduction band of the TiO2 semiconductor for better 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 the 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 neighboring aromatic units into planar segments. Thus, fused aromatic chromophores composed from the small aromatic units into planar and big π-conjugated systems 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. The coplanar π-chromophore due to the intramolecular annulation results in promising properties such as photoluminescence efficiency and charge mobility. Thus, restriction for the twist and rotation allow the better electronic communication and better photo-induced charge transfer in the donor-acceptor dyads. However, more planarity for the chromophores also improves tendency of dye aggregation, low solubility in organic solvents, quenching of excited state sensitizers and recombination of the injected electrons. One way to alleviate this problem is to sterically demanding substituents or long hydrocarbon chains on rigidified chromophores, which retard dye aggregation and improve the solubility. In this regard, a plethora of new organic sensitizers containing several fused polyaryl/heteroaryl derivatives is registered. This thesis focuses on the importance of fusion of aryl groups by heteroatom in special context with dithienopyrrole and its use in DSSCs. This thesis is divided into six chapters. First chapter introduces the importance of rigidification of chromophores by a heteroatom, a comprehensive survey on the organic vi dyes containing dithienopyrrole either in the donor part or as a π-linker in the conjugation pathway in a conventional donor-π-acceptor molecular configuration and then presents the aim and scope of the work. From the literature background, it is understood that from fundamental viewpoint, molecules allow a more straightforward and reliable analysis of the structure-property relationships, an approach that remains the key tool for the design of new materials specifically designed for photovoltaic applications. Therefore, herein we believe that electron rich dithienopyrrole (DTP) unit can be explored towards systematic molecular engineering of organic materials for photovoltaic applications. In second chapter, organic dyes based on dithienopyrrole (DTP) as a donor were synthesized in which thienyl/DTPs as π-linkers and cyanoacrylic acid acceptor were demonstrated. In comparison to thienyl, DTP-linkers showed remarkably improved light harvesting efficiency and reduced oxidation potentials for the dyes. The sufficient spatial separation between HOMO and LUMO orbitals establishes the presence of charge transfer on electronic excitation in the visible region for the DTP-linked dyes. The presence of bulky and electron rich DTP units in conjugating bridge for DTP donor dyes is found to be beneficial to retard the charge recombination processes. Therefore, the dye containing one DTP-linker showed the highest efficiency of 5.94% in the series attributable to its high JSC and VOC values. Irrespective of intense absorption, low efficiency was observed for the dye containing two DTP-linkers owing to its low charge collection efficiency and reduced electron lifetime. In third chapter, DTP π-linker based organic dyes containing different arylamine/heterocyclic units as conjugating donors and cyanoacrylic acid as acceptor were synthesized and characterized. The effect of different conjugated donors such as triarylamine, carbazole and phenothiazine 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 phenothiazine donor exhibited longer wavelength absorption and lowest oxidation potential in the series. The time dependent density functional calculations performed on the dye models reveal charge transfer character for their longer wavelength absorption. The dye-sensitized solar cells fabricated using a dye containing fluorenyldiphenylamine donor displayed highest power conversion efficiency (6.81%) in the series originating from the high short circuit current density (JSC = 14.01 mA cm-2) and high open circuit voltage (VOC = 738 mV). vii In fourth chapter, organic dyes featuring dithienopyrrole as a donor in D-A-π-A structural organization by employing different electron-demanding linkers such as benzothiadiazole, quinoxaline and benzotriazole besides phenyl/thienyl π-spacer and cyanoacrylic acid acceptor were synthesized. Further incorporation of TPA and DPF auxiliary donor on BTD dye with phenyl-linker extended the conjugation length and improved the spectral response for the dyes. Moreover, the position of BTD unit in the DD- A-π-A to D-A-π-A molecular configurations adequately altered the absorption and electrochemical characteristics. The dyes, which possess BTD after DTP show negative solvatochromism in the absorption spectra, while the dyes containing BTD before DTP exhibit positive solvatochromism in the emission spectra. This clearly indicates that the former dyes are polar in ground state while the later are polarized only on electronic excitation. Theoretically, it was established that sufficient overlap of HOMO and LUMO of the dyes featuring BTD between donor and DTP units facilitate the charge transfer propensity form arylamine donor to acceptor and ensure high molar extinction coefficients in absorption. Among the dyes, the highest power conversion efficiency (η = 7.57%) for one of the dye is owed to its high photocurrent density and open circuit voltage. The superior performance of most efficient dye is attested to the sufficiently low charge transfer resistance by EIS studies. From the work presented in Chapters 3 and 4, it is evident that the insertion of auxiliary donor increases electron lifetime and charge collection efficiency; however, introduction of BTD unit between arylamine and DTP linker (D-A-D-A) leads to low electron lifetime and high recombination resistance. This helps to realize high power conversion efficiency for this class of dyes. In the fifth chapter, two sets of λ- and H-shaped dianchoring dyes are described which were synthesized either by tri-functionalization of dithienopyrrole or bridging two DTPs by fluorene. The λ- and H-shaped shaped dyes showed red shifted absorption maxima in comparison to previously known mono-anchoring congeners. Further, in case of λ-shaped dyes, decoration of conjugated electronic rich units at C7 position of fluorene was found to be beneficial for broadening the absorption. Also the insertion of different aromatic chromophores exerted similar effect on raising the HOMO and LUMO energy levels as a result of which energy band gap remained almost constant for all the dyes except for one dye. Theoretical calculation established that the HOMOs for λ-shaped dye remained spread over the electronic rich aromatic segments while LUMOs are viii delocalized between two cyanoacrylic acids through DTP segments. The absorption maxima for the dye lacking auxiliary donor realized from HOMO to LUMO transitions while for the dyes λ-shaped dyes originated from HOMO-2 to LUMO transitions. Among the dyes, a fluorene containing H-shaped dye displayed highest power conversion efficiency due to high photocurrent density and open circuit voltage. The EIS studies revealed the large recombination resistance and small charge transfer resistance is responsible for the relatively superior performance. This work suggests that the strategy of synthesizing multi-anchoring dyes with the choice suitable aromatic system can be promising method to improve the efficiency of DSSCs. 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.