CARBAZOLE-BASED ORGANIC DYES FOR DYE-SENSITIZED SOLAR CELLS

Abstract

Organic materials suitable for application in electronic devices such as dye-sensitized solar cells (DSSCs) and organic solar cells (OSCs) have received immense attention in recent years due to the demand for alternate energy harnessing technologies. Particularly, DSSC using organic and inorganic sensitizers have attracted huge research focus due to favorable efficiency to cost tradeoff. Efficiency of the device containing organic dye is largely dependent on its electronic and structural characteristics. Till to date, power conversion efficiencies greater than 11% have been achieved with ruthenium-based sensitizers. However, rarity, high cost and environmental concern may limit the extensive usage of ruthenium-based dyes. Porphyrins have unique properties such as fast electron injection, good photophysical and thermal stability which make them ideal candidates for photovoltaic applications. But, organic sensitizers have drawn the attention due to their advantages such as high molar absorption coefficients, facile molecular tailoring and cost-effectiveness. Most of the organic sensitizers have a common architecture of donor-π bridge-acceptor (D-π-A). Generally the dyes are constructed using triarylamine donors, oligoarene or oligoheteroarene π-linkers and cyanoacrylic acid acceptor/anchor. Besides triarylamines nitrogen heterocyclic moieties such as indoline, phenothiazine and carbazole also have been used as donors. Carbazole is unique core unit due to its good charge transporting property and it offers many nuclear sites for chromophore incorporation which can be used to fine-tune the functional properties of the materials. For instance, many carbazole derivatives with functional chromophores attached via C-2, C-3, C-6, C-7, and N-9 have been reported for application in organic light emitting diodes and photovoltaic devices. In this thesis, organic dyes featuring carbazole as donor or 2,7-disubstituted carbazole as π-linker have studied in detail. In brief, the effect of conjugation and electron richness of donor on the optical, electrochemical and photovoltaic properties were elucidated carefully with appropriate molecular designs. The thesis is divided into seven chapters. A survey of carbazole-based organic dyes reported in the literature is presented in Chapter 1. The carbazole-based materials are conveniently classified into various classes based on the nuclear substitution. It is found that the position of attachment of carbazole significantly affects the functional properties of the materials. The electro-optical and photovoltaic properties were discussed thoroughly for better understanding. The structure property relationship of the dyes are critically analyzed and discussed in detail. v Chapter 2 describes the aim and scope of the work. Carbazole is an electron rich heterocyclic compound and offer many nuclear sites at various positions and linear conjugation can be obtained by 2,7-disubstitution. Chart 1 Structure of the dyes containing 2,7-disubstituted carbazole π-linker. In Chapter 3, carbazole dyes (Chart 1) containing diphenylamine donor, 2,7-carbazole and thiophene in the conjugation pathway and cyanoacrylic acid acceptor are described. They possess red-shifted absorption with high molar extinction coefficient when compared to the corresponding dyes with fluorene or phenyl units in the conjugation. The dyes with oligothiophene conjugation shows bathochromically shifted absorption when compared to the dye containing phenyl linker attributable to the electron richness of the former one. The paraconjugation of phenyl linker red shifted absorption compared to meta-congener due to delinking of donor and acceptor interactions for the later one. The introduction of fluorene (11d) and carbazole (11e) linker enhances the molar extinction coefficients while terthiophene (11c) redshifted the absorption when compared to the dye (7a) with a thiophene linkage. The DSSC fabricated using the dyes 7b as sensitizer exhibited power conversion efficiency of 6.8% which increased to 7.2% when chenodexoycholic acid (CDCA) is used to impede dye aggregation. The DSSCs with ionic liquid electrolytes displayed marked stability over 1000 h for the dye 7b. For dye 11d, two ethyl chains of fluorene linker helps to minimize the intermolecular interactions and resulted in higher open circuit voltage (VOC) compared to phenyl and carbazole containing vi dyes. Electrochemical impedance spectroscopy was used to characterize the interfacial charge transfer and electron recombination kinetics. Chart 2 Structure of the dyes featuring donor structure modifications. In Chapter 4, we study the effect of the alternations in the structure of the donor segment. Substitution of phenyl groups with fluorene in the donor unit of carbazole dyes (5, 7a and 7b) produced several dyes. The structures of the dyes are shown in Chart 2. Replacement of phenyl unit with the fluorene shifts the charge transfer transition peak to the longer wavelength region progressively which reflects changes in the donor unit. The dyes containing bithiophene displayed red shifted absorption compared to the thiophene containing dyes due to elongation of conjugation and electron richness of thiophene units. Moreover, the introduction of fluorene decorated donor raises the HOMO due to facile oxidation which helps to increase the VOC. Also due to the extensive accumulation of electrons in the conduction band of TiO2 due to the favorable LUMO level which inject electrons efficiently led to negative shift in the Fermi level of TiO2. Among these dyes 18b showed the highest efficiency (6.4%). The factors affecting the efficiency of the devices and the manifestations of the molecular structure on it are critically analyzed and discussed at the end. Generally, the fluorene containing dyes improved lightharvesting ability and electron life time (τ). vii Chart 3 Structures of the alkoxy substituted carbazole based dyes. In Chapter 5, the organic dyes (Chart 3) containing alkoxy substitution on the donor unit are described. We have used both butoxy and 2,4-dimethophenyl substituents on the diphenylamine donor to maximize the donor strength and retard the intermolecular interactions by providing hydrophobic environment on the donor side. When compared to the parent dyes (5, 7a and 7b), all these dyes showed red-shifted absorption owing to the introduction of peripheral units which increases the donor strength and facilitate strong donor-acceptor interactions. The oxidation potentials of the dyes are reflective of the electron releasing effect of the peripheral units (4- butoxy > 2,4-dimethoxyphenyl). The excitation energies of the dyes are calculated by TDDFT with two different models B3LYP and MPW1K and the best correlations obtained from later one. Introduction of alkoxy units raised the HOMO and lowered the LUMO. This resulted in the poor dye regeneration and low electron injection efficiency. Among this class of dyes, a dye with 2,4-dimethoxyphenyl peripheral unit and bithiophene in conjugation (34b) showed relatively high power conversion efficiency attributable to the favorable electron injection from the excited dye into the conduction band of TiO2. The suitable LUMO levels of the dyes hike the efficiency while the mismatch of the levels produces inferior efficiency. The charge transport properties of the devices are scrutinized by electrochemical impedance spectroscopy. viii Chart 4 Structure of the organic dyes containing carbazole as donor and π-linker. In Chapter 6, a set of dyes (Chart 4) using carbazole as donor and π-linker have been synthesized and characterized as effective sensitizers for TiO2-based DSSCs. The donor properties of the dyes increased on introduction of tert-butyl groups at C3 and C6 of carbazole and insertion of thiophene moiety in the conjugation pathway. These structural modifications fine-tuned the optical and electrochemical properties of the dyes. Additionally, the presence of tert-butyl groups on the carbazole nucleus minimized the intermolecular interactions. The dyes served as efficient sensitizers in dye-sensitized solar cells and the efficiency ranged from 4.2- 6.0%. The tert-butyl groups are found to suppress the recombination of injected electrons which contributed to the increment in the photocurrent generation and VOC. A dye (44a) with tert-butyl groups on the carbazole nucleus and the conjugation bridge composed of carbazole and thiophene fragments exhibited high VOC value. Overall the dye 43b showed high efficiency of 6.0% with JSC of 15.78 mA cm-2. We unraveled the structural modifications required on the carbazole nucleus on using it as a donor and linker for the optimization of the performance of the DSSCs. In Chapter 7, the salient features of the work are summarized and the future prospects also addressed. The unique outcome of the work is discussed and a comparative evaluation of optical, electrochemical and photovoltaic properties with those of the known compounds is presented.