Synthesis, Characterization and Device Studies of Conjugated Foldamers

Abstract

Conjugated polymers have found a wide range of electronic applications in last few decades due to their superior combination of optoelectronic, mechanical properties, and solution processability. In the past few years, significant advancement has been achieved in developing organic semiconductors with high charge mobility, which are used in solar cells, organic light emitting diodes, and organic field effect transistors. Two of the main challenges of organic semiconductors, for commercialization are their efficiency and lifetime. Although the power conversion efficiency (PCE) of organic solar cells has improved to above 10%, device stability has remained a major issue. Conjugated foldamers can be considered as a viable option to achieve the desired stability. Conjugated foldamers are generally driven by secondary interactions such as π-π stacking, H-bonding, and solvophobic interactions. We have designed foldamers based on polycyclic aromatic systems (PCA) that have large aromatic sizes. PCA containing conjugated polymers may form π-stacked columns after folding. These conjugated foldamers exhibit efficient charge transport that is an essential property for optoelectronic devices. We envision that in a composite containing a conjugated foldamer and doped electron acceptors (such as PCBM), the dopants may be encapsulated inside the folded polymers, leading to improved morphological stability, and thus, device stability. We have synthesized triphenylene-based conjugated foldamers 58 and 59 with and without imide functionalization and studied their optoelectronic properties. Their optical data in different solvents showed interesting results that provided folding possibilities of 59 also supported by their theoretical calculations. In the next chapter, we report the synthesis and characterization of a new conjugated foldamer based on imide-functionalized naphthodithiophene (INDT) building blocks. We also synthesized naphthalene dithiophene (NDT)-based D/A conjugated polymers 31 and 36 for comparison with INDT based polymers. Their UV/vis absorption and fluorescence emission properties with respect to solvents and concentrations have been carefully studied to shed light on their folding process. CD spectroscopy was used to confirm the presence of helical folding present in the polymers. The device properties of INDT-based polymers were found to be 0.026 and 0.18% for polymers 20 and 22, respectively. In the last chapter, we have synthesized organic-inorganic hybrid conjugated copolymer 56 wherein hexamolybdate clusters are covalently embedded in the main chain with diene and have characterized their properties using 1H-NMR, GPC and CV spectroscopy.