Applications of conjugated polymer (CPs) in optoelectronic devices are critically depend on nature of thin film morphology. In thin film of CPs, the distribution of conjugation length is highly heterogeneous because of conformational defects, distortions of polymer chain and aggregates. A greater understanding of the self-assembly properties of polymer in solution, in particular control over aggregation leads to richer description of electronic properties and hence reproducible fabrication of thin film devices. Recently, chiral CPs have attracted profound interest because of their promising chiroptical properties in thin films and easy control over the selective agglomeration process. In this thesis, we have investigated the role of chiral side-chains on a series of thiophene diketopyrrolopyrrole (TDPP)-benzodithiophene (BDT) based copolymers. Chiral 3,7-dimethyloctyl chain was introduced as an asymmetric chain to incorporate chirality on one of the repeating unit (TDPP) of copolymers. Two polymers with side-chains of identical chirality (S),(S)-PTDPP-BDT; (R),(R)-PTDPP-BDT and a third polymer with similar side-chains of opposite chirality (R),(S)-PTDPP-BDT were synthesized. The chiroptical properties were investigated by UV-visible and circular dichroism (CD) spectroscopy. Figure 1: The structure of the TDPP-BDT copolymers. The copolymers dissolved in a good solvent (e.g. chloroform, chlorobenzene) in which polymers adopts random coil conformation, no chiral response has been observed. However, a critical addition of non-solvent (methanol), the copolymers stack in a chiral fashion and leads to typical bisignate Cotton effects. It is noteworthy that the two polymers, (S),(S)-PTDPP-BDT and (R),(R)-PTDPP-BDT exhibiting a nearly ideal mirror-image relationship in CD spectra (Figure 2a) whereas the (R),(S)-PTDPP-BDT lacks chiropticity even with the addition of methanol. The aggregation induced CD phenomena are dependent on the temperature of solution and do not exhibit reversibility in a heating-cooling cycle. Figure 2: (a) The mirror image Cotton effects of (S),(S)-PTDPP-BDT and (R),(R)-PTDPP-BDT (b) No CD signal was observed for the (R),(S)-PTDPP-BDT polymer. Figure 3: The variation of (a) UV-vis and (b) CD spectra of (R),(R)-PTDPP-BDT polymer with thickness of the solid film. To investigate the role of thickness and annealing temperature on optical and chiroptical properties of polymer films, thin films were prepared using drop-casting method from a solution of chlorobenzene. Both the polymer showed gradual enhancement of CD signal with the increase of film thickness but we did not see any such order with temperature (Figure 3). Figure 4: The morphology observed for the film by (a) AFM; (b); (c) FESEM. The thin film morphology of polymers is characterized by atomic force microscopy (AFM) and field emission scanning electron microscopy (FESEM) (Figure 4). AFM studies show the polymer molecules self-assembled and formed interconnected nanofibers. Whereas FESEM images clearly revealed that, the nanofibers of polymers are predominantly stack in a chiral fashion and mimic a one-handed helix which leads to bisignate Cotton effects. The (S),(S)-PTDPP-BDT and (R),(R)-PTDPP-BDT form fibers with opposite handedness whereas (R),(S)-PTDPP-BDT do not have such preferred handedness. The research described in this thesis aims to explore the role of chiral side-chains to impose chiral stacking and hence resulting chiral expression. Chirality in this class of polymers may endows them promising optoelectronic properties. (For figures pl see the abstract pdf file)

The present thesis is focused on the rational design of Diketopyrrolopyrrole based π- conjugated polymers for organic electronics. The thesis is organized into six different chapters and a brief description of the individual chapters is provided below. Chapter 1 briefly describes the physics governing the electronic processes occurring in organic photovoltaics (OPVs) and organic field-effect transistors (OFETs) followed by design rules for the synthesis of conjugated polymers for organic electronics. Diketopyrrolopyrrole (DPP) based π-conjugated materials and their development in OPVs and OFETs have been highlighted. Chapter 2 discusses the synthesis and characterization of a series of small molecules of DPP derivatives attached with different alkyl chains. Influence of side chains on the photophysical properties of these DPP derivatives have been studied by UV-visible spectroscopy and DFT calculations. Crystal structure studies revealed the effect of alkyl chains on the torsional angle, crystal packing, and intermolecular interactions such as π-π stacking. Chapter 3 reports the synthesis of novel diketopyrrolopyrrole-diketopyrrolopyrrole (DPPDPP) based conjugated copolymers and their application in high mobility organic field-effect transistors. Effect of insulating alkyl chains on polymer thin film morphology, lamellar packing and π-π stacking interactions have been studied in detail. Investigation of OFET performance of these DPP-DPP copolymers with branched alkyl chains (N-CS2DPP-ODEH) resulted in low charge carrier mobilities as compared to the polymers (N-CS2DPP-ODHE) with linear alkyl chains. Polymer with triethylene glycol side chains (N-CS2DPP-ODTEG) exhibited a high field-effect electron mobility value of ~3 cm2V-1s-1 with a very low threshold voltage of ~2 V. Chapter 4 investigates the effect of torsional angle on the intermolecular interactions and charge transport properties of diketopyrrolopyrrole (DPP) based polymers (PPDPP-OD-HE and PPDPP-OD-TEG). Grazing incidence x-ray diffraction studies shows the different orientation of the polymer crystallites and lamellar packing involved in polymer thin films. Investigation of OFETs evidenced the effect of torsional angle on the charge transport properties where the polymer with higher torsional angle PPDPP-OD-TEG resulted in high threshold voltage with less charge carrier mobility compared to the polymer with lower torsional angle (N-CS2DPP-OD-TEG). Chapter 5 investigates the effect of photoactive material morphology on the solar cell device performance, and charge transfer kinetics by adding high boiling point processing additives. DPP based donor-acceptor (D-A) type low band gap polymers (PTDPPQ and PPDPPQ) have been synthesized and employed in bulk-heterojunction (BHJ) solar cells with the acceptor PC71BM. Addition of processing additive 1,8-diiodooctane (DIO) resulted in three order improvements in power conversion efficiency (PCE). Chapter 6 describes the design and synthesis of two diketopyrrolopyrrole based copolymers (PPDPP-BBT and PTDPP-BBT) for their application in organic devices such as field-effect transistors and bulk-heterojunction solar cells. Investigation of OFET performance of these DPP based copolymers displayed hole mobilities in the order of 10-3 cm2V-1s-1. The semiconductor-dielectric interface has been characterized by capacitance-voltage, and Raman scattering methods. In summary, the work presented in this thesis describes the synthesis and characterization of diketopyrrolopyrrole based new polymeric semiconductors. Effects of insulating side chains and torsional angle on the charge transport properties of these polymers in OFETs have been investigated. This work also describes the effect of solvent additives on the active layer morphology and BHJ solar cell device performance. The results described here show that these materials have potential application as active components in plastic electronics.