BITS Faculty Publications
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Item Electronic Structures and Optical Absorption of N-Type Conducting Polymers at Different Doping Levels(ACS, 2019-06) Ghosh, SarbaniTheoretical understanding of the electronic structure and optical transitions in n-doped conducting polymers is still controversial for polaronic and bipolaronic states and is completely missing for the case of a high doping level. In the present paper, the electronic structure and optical properties of the archetypical n-doped conducting polymer, double-stranded benzimidazo-benzophenanthroline ladder (BBL), are studied using the density functional theory (DFT) and the time-dependent DFT method. We find that a polaronic state in the BBL chain is a spin-resolved doublet where the spin degeneracy is lifted. The ground state of two electrons corresponds to a triplet polaron pair, which is in stark contrast to a commonly accepted picture where two electrons are postulated to form a spinless bipolaron. The total spin gradually increases until the reduction level reaches cred = 100% (i.e., one electron per monomer unit). With further increase of the reduction level, the total spin decreases until it becomes 0 for the reduction level cred = 200%. The calculated results reproduce the experimentally observed spin signal without any phenomenological parameters. A detailed analysis of the evolution of the electronic structure of BBL and its absorption spectra with increase in reduction level is presented. The calculated UV–vis–NIR spectra are compared with the available experimental results. The electronic structure and optical absorption for different reduction levels presented here are generic to a wide class of conducting polymers, which is illustrated by the corresponding calculations for another archetypical conducting polymer, poly(3,4-ethylenedioxythiophene) (best known as PEDOT).Item Electronic Structures and Optical Properties of p-Type/n-Type Polymer Blends: Density Functional Theory Study(ACS, 2020-04) Ghosh, SarbaniA blend made of p-type and n-type polymers can act as bipolar/ambipolar material composites that transport both electrons and holes. Although several experimental efforts are currently devoted to p-/n-type blends of conducting polymers, theoretical studies of these systems are missing to a large extent. In the current paper, using the density functional theory (DFT) and the time-dependent DFT, we calculate electronic and optical properties of a p-type/n-type polymeric blend, where we have chosen the poly(3,4-ethylenedioxythiophene)/benzimidazo-benzophenanthroline ladder (PEDOT/BBL) as a model composite system. We demonstrate that in the blend, PEDOT acts as an electron donor and BBL acts as an electron acceptor under doped conditions. However, no charge transfer between the chains takes place for an undoped composite system. Due to a significant difference in the electron affinities and the ionization energies of PEDOT and BBL, the electronic properties of a negatively (positively) doped PEDOT/BBL blend are primarily governed by the chains where negative (positive) charges are localized, i.e., the BBL chains (the PEDOT chains). However, this is no longer valid for the optical absorption where the electronic transition occurs between the two chains and, therefore, the calculated UV–vis–near-infrared (NIR) absorption spectra of the negatively (positively) doped PEDOT/BBL blend are rather different compared to the corresponding spectra of the single BBL chains (PEDOT chains). The electronic coupling between the photoexcited state and the final charge-transfer state of the blend was calculated to be ∼0.08 eV. The results presented here are generic to a wide class of p-type/n-type combinations, which was further confirmed by calculations performed on the polythiophene (PT)/BBL blendItem Ordered and disordered microstructures of nanoconfined conducting polymers(RSC, 2023) Ghosh, SarbaniWe probe the microstructural differences of conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) derivatives under geometrical nanoconfinement using a high-resolution electron microscopy (HRTEM) technique. Highly ordered domains of poly(3,4-ethylenedioxythiophene):tosylate PEDOT:Tos, which is polymerized within alumina nanochannels, are observed. These features are in contrast to those of the polymer blend poly(3,4-ethylene dioxythiophene):poly(styrenesulfonate) PEDOT:PSS inserted into the nanopores. The extent of the order–disorder parameter in terms of surface crystallization and the number of ordered domains of the long-chain polymers strongly depends on the dopant environment, processing conditions and structural confinement. Atomic force spectroscopy of individual PEDOT nanochannels highlights counterion-dependent surface adhesive factors. The molecular dynamics (MD) simulation of these systems reveals similar polymer chain configurations and the resulting morphology.