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Studies on Correlation between Microstructures and Electronic Properties of Organic Semiconductors

Electronic Theses of Indian Institute of Science

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Title Studies on Correlation between Microstructures and Electronic Properties of Organic Semiconductors
 
Creator Mukhopadhyay, Tushita
 
Subject Organic Semiconductors
Organic Electronics
Diketopyrrolopyrrole-based Copolymers
Diketopyrrolopyrrole-based Oligomers
Diketopyrrolopyrrole Based Chromophores
Organic Field Effect Transistors (OFETs)
OFET Measurements
Charge Transport Measurements
Thin Film Transistors (TFTs)
Diketopyrrolopyrrole-diketopyrrolopyrrole (DPP-DPP)
Solid State and Structural Chemistry
 
Description The work carried out in this thesis systematically investigates the correlation between microstructures and electronic properties of organic semiconductors. The major directions that were pursued in this thesis are: (i) studies on structure-property relationship by rational design and synthesis of monodisperse oligomers with varying chain-lengths (ii) role of electronic properties and aggregation (microstructures) in governing singlet fission (SF). In the first part of the thesis, the optical, structural and charge transport properties of Diketopyrrolopyrrole (DPP)-based oligomers, as a function of the chain length, has been discussed. The energy bands became wider with an increase in chain length and a gain in backbone electron affinity was observed, with an offset in microstructural order. With an increase in chain length, the tendency to form intramolecular aggregates increased as compared to intermolecular aggregates due to the onset of backbone conformational defects and chain folding. An insight into the solid-state packing and microstructural order has been obtained by steady-state and transient spectroscopy, grazing incidence small angle x-ray scattering (GISAXS), atomic force microscopy (AFM) and transmission electron microscopy (TEM) studies. The charge-carrier mobilities varied in accordance with the degree of microstructural order as: dimer > trimer > pentamer. A library of DPP-DPP based trimers was also generated by modifying the donor chromophore (phenyl, thiophene and selenophene) in the oligomer backbone. Highest n-channel mobility of ~0.2 cm2V-1s-1 was obtained which validated that: (a) the effect of solid-state packing predominates the effect of backbone electronic structure on charge carrier mobility. Although oligomers possess lesser backbone defects than polymers in general, their charge carrier mobilities were not comparable to that of 2DPP-OD-TEG polymer, which forms highly oriented and isotropic edge-on crystallites/microstructures in the thin film, shows high n-channel mobility of 3 cm2V-1s-1 and band-like transport ;(b) although delocalized electronic states are achieved at greater chain lengths, the degree of solid-state microstructural order drastically reduces which leads to lower charge carrier mobilities; (c) conformational collapse resulted in lower electron mobilities and an increase in ambipolarity.
The later part of the thesis debates on the relative contribution of electronic structure and aggregation (microstructures) in governing singlet fission (SF). Motivated by the recent SF model in carotenoid aggregates, a DPP-DPP based oligomer was synthesized by incorporating a vinylene bridge to imbue “polyene” character in the chromophore. Transient Spectroscopy (TA) measurements were carried out to monitor the formation of triplet states in the oligomer and to probe the occurrence of singlet fission. Although the oligomer exhibits “polyene” character like a typical “carotenoid aggregate”, it did not show singlet fission because of the additional stabilization of the singlet (S1) state which reduces the ∆EST. This study rationalized the importance of judicious control of band structures as well as microstructures to observe the SF phenomenon in this category of chromophores. The novel synthetic protocol provides the scope to tailor DPP-DPP based materials with desired effective conjugation lengths and side chains and can foreshow great prospects for future generation of organic electronics.
 
Contributor Patil, Satish
 
Date 2018-07-05T15:00:13Z
2018-07-05T15:00:13Z
2018-07-05
2017
 
Type Thesis
 
Identifier http://etd.iisc.ernet.in/2005/3794
http://etd.iisc.ernet.in/abstracts/4665/G28572-Abs.pdf
 
Language en_US
 
Relation G28572