BENZOTHIADIAZOLE DERIVATIVES FOR PHOTOVOLTAIC DEVICES

Abstract

Organic materials containing low band-gap chromophores such as benzothiadiazole, benzotriazole, diketopyrrolopyrrole, isoindigo, quinoxaline, phthalimide, thiazolo[5,4-d]thiazole, and benzo[1,2-d:4,5-d′]bisthiazole.etc have been attracted wide interest among academicians and industrialists owing to their potential application in dye sensitized solar cells (DSSCs), bulk hetero-junction solar cells (BHJSCs), light-emitting diodes (OLEDs) and related opto-electronic devices. Benzothiadiazole has been regarded as a promising π-conjugation unit for the construction of organic semiconductor materials due to its high electron-accepting capabilities and facile functionalization possibilities at 4,7-positions. The chemical modifications allow to tune the photophysical and electrochemical properties. Particularly, it has been recognized that conjugating the electron-donating group with benzothiadiazole helps to balance the hole and electron mobility in the molecular layer. It also modulates the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the molecules and ensure longer wavelength absorption. Electron-rich chromophores can be integrated with benzothiadiazole by several palladium catalyzed cross-coupling reactions leading to the formation of vinyl (Heck), acetylene (Sonogashira), C-C (Stille or Suzuki or Negishi) and C-N (Buchwald-Hartwig) linkages. Though several dipolar compounds featuring benzothiadiazole acceptor and donors such as triphenylamine, indoline, carbazole, fluorene, etc. have been known in the literature, the best of our knowledge, compounds containing amines directly attached to benzothiadiazole and benzothiadiazole functionalized with phenothiazine, pyridine or ferrocene are not yet explored. It is our intention to use phenothiazine and ferrocene as donor components in the construction of dipolar compounds suitable for application in BHJSCs. Also suitably designed molecules with cyanoacrylic acid or pyridine as end groups can be exploited as sensitizers for DSSCs. Both the vinylic and C-C linkages have been used to tether the benzothiadiazole with the functional chromophores such as phenothiazine and amine donors. This allows us to evaluate the effect of slight alternations in the structure on the electronic properties and thermal stability of the molecules. The thesis is divided into eight chapters. First chapter presents the aim and scope of the work. Second chapter outlines the review of the available literature related to the synthesis, optical and electrochemical characterization and electronic applications of benzothiadiazole-based compounds used in DSSCs and BHJSCs. vi Figure 1. Structures of the amine directly integrated benzothiadiazole sensitizers. Chapter 3 describes the synthesis, photophysical, electrochemical and photovoltaic performance of the amine directly integrated benzothiadiazole featured sensitizers (Figure 1). The dyes were constructed by C-N, C-C bond forming reactions followed by a Knoevenagel condensation reaction. They exhibited excellent light harvesting properties as identified from their absorption spectra. All the dyes showed two prominent bands originating from the charge transfer and π–π* transitions respectively. These assignments were confirmed by the time-dependent density functional theoretical calculations. Effects of protonation/deprotonation on the absorption profile of the sensitizers have been investigated by electronic absorption measurements in the presence of TFA/TEA. The dyes showed blue shifted absorption on addition of trifluoroacetic acid and triethyl amine, due to the protonation of the amine group and deprotonation of the cyanoacrylic acid groups, respectively. Time-dependent density functional theoretical calculations were performed to evaluate the nature of the electronic excitations occurred by the absorption of light. The torsion angles (θ) between the benzothiadiazole moiety and thiophene, bithiophene were less when compared with the phenyl spacer, allows the high electronic communication and resulted to the larger red-shift in absorption bands in former dyes. The DSCs sensitized by the dyes 6a and 6b obtained the η values 1.68 % (Jsc = 7.54 mA cm-2, Voc = 0.36 V and ff = 0.62) and 3.07 % (Jsc = 12.4 mA cm-2, Voc = 0.40 V and ff = 0.63) respectively. It is ascertained that direct attachment of donor to the BTD-unit is not jeopardized, to obtain better solar cell performance of these dyes further molecular design is needed. vii We have developed four novel D-A-π-A organic dyes (Figure 2) incorporating benzothiadiazole/benzotriazole as the auxiliary acceptor units, phenothiazine as the electron donor, cyanoacrylic acid as the anchoring group and thiophene, benzene are used as the π-linker between auxiliary acceptor and cyanoacrylic acid acceptor in Chapter 4. These sensitizers were unambiguously characterized by spectroscopic and TD-DFT techniques. The photophysical and electrochemical properties of the dyes were conveniently tuned by alternating the auxiliary acceptor and the π-linker. Incorporation of a phenothiazine donor group on the benzo(thiadiazole/triazole) conjugation segment bathochromically shifted the absorption wavelength probably due to the ICT, which results in a pronounced donor-acceptor interaction. The absorption and electrochemical properties dyes reveals that the both auxiliary acceptor analogs play exactly different roles. The TDDFT calculations proved that, the photoexcited electrons could be successively transferred from the phenothiazine donor to the benzo(thiadiaozle/triazole) auxiliary acceptor unit, then transferred to the cyanoacetic acid subunit, and finally to nanocrystalline TiO2 surface. It was consistent with our proposed auxiliary acceptor cascaded role. The molecular engineering approach demonstrated in this chapter provides an important guide for choice of the auxiliary acceptors for the development of novel materials for use in dye sensitized solar cells as well as BHJ solar cells. Figure 2. Structures of the benzothiadiazole/benzotriazole based sensitizers. The synthesis and electro-optical properties of the organic dyes derived from benzothiadiazole and pyridine connected by a vinyl bridge featured with different donors (Figure 3) are presented in the Chapter 5. The donors piperidine, morpholine were directly attached and carbazole, phenothiazine, ferrocene were attached by vinyl linkage to benzothiadiazole, respectively. The effect of the protonation/deprotonation of the pyridine unit on absorption spectra of the dyes was studied by addition trifluoroacetic acid/triethylamine. The viii addition of TFA leads to a significant red-shift in the absorption spectra, owing to the protonation occurred on nitrogen atom of the pyridine ring. And red shifted absorption spectra attained the indigene position by deprotonation of the dyes on addition of TEA. From DFT calculations it was confirmed that, when these pyridine anchoring dyes applied in DSSCs, electrons from donor part sequentially transferred to benzothiadiazole moiety, then shifted to the pyridine anchoring unit, and finally to nanocrystalline TiO2 surface. Figure 3. Structures of pyridine anchoring sensitizers featured with benzothiadiazole. The synthesis and characterization of a new family of phenothiazine-benzothiadiazole vinyl hybrids (Figure 4) were described in the chapter 6. The dyes were synthesized by palladium-catalyzed Heck cross coupling reactions and isolated in good yields. The extended conjugation in the dyes 28a, 28b drafted to display the red shifted absorptions with high molar extension coefficients than 26a, 26b. All the new dyes displayed one-electron reversible oxidation, irreversible reduction peaks in cyclic voltammetry which are attributable to the oxidation of the phenothiazine and reduction of the benzothiadiazole units, respectively. The dye 27 exhibited the low band gap in the series due to the presence of two strong electron donating phenothiazine units. There is a reasonable correlation between the experimental and calculated optical data from TDDFT. The thermal decomposition temperatures of the dyes are greater than 400 ºC, which is good enough to apply in any optoelectronic device. The photophysical and electrochemical properties of the dyes are highly dependent on the conjugation length and the pattern of the linkage mode between benzothiadiazole and phenothiazine. In this chapter we ix have established the structure-property relationship in the new phenothiazine-benzothiadiazole conjugates extended though vinyl linkage. Figure 4. Structures of the benzothiadiazole-phenothiazine vinyl conjugated derivatives. Chapter 7 describes the synthesis, photophysical and electrochemical properties of the organic-inorganic hybrid materials containing benzothiadiazole-ferrocene (Figure 5). The complexes were thoroughly characterized by the routine spectroscopic techniques, the structure of complex 30a was confirmed by single crystal X-ray diffraction analysis. The dyes were exhibited excellent light absorbing properties and non-fluorescent in nature. All these complexes exhibited one-electron reversible oxidation, irreversible reduction peaks in cyclic voltammetry which are attributable to the removal of electron from ferrocene and reduction of the benzothiadiazole units, respectively. The electronic properties of these complexes were also examined by the DFT calculations and found to be in agreement with the observed trends. All the complexes showed the high thermal stability. It was demonstrated that the optical and electro chemical properties of the complexes are tuned by modulation of electro-active group on the benzothiadiazole unit. x Figure 5. Structures of the benzothiadiazole-ferrocene complexes. In Chapter 8, a summary of the work accomplished during the dissertation work is presented. A comparison of the optical, electrochemical and DSSC characteristics of the benzothiadiazole based dyes reported in this dissertation with that known in the literature is taken into account. The role of direct attachment of amine directly connected to benzothiadiazole for DSSC, benzothiadiazole linked with double bond to phenothiazine, pyridine and ferrocene, which are suitable in the design for organic solar cells (OSC) is unraveled. Similarly the role of auxiliary acceptors benzothiadiazole/benzotriazole, in phenothiazine donor based dyes for DSSCs is seriously analyzed.