Browsing by Author "Puttaraju, B."

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    ItemOpen Access
    Air-stable n-channel diketopyrrolopyrrole-diketopyrrolopyrrole oligomers for high performance ambipolar organic transistors
    (American Chemical Society, 2016) Mukhopadhyay, T.; Puttaraju, B.; Senanayak, S. P.; Sadhanala, A.; Friend, R.; Faber, H. A.; Anthopoulos, T. D.; Salzner, U.; Meyer A.; Patil, S.
    n-channel organic semiconductors are prone to oxidation upon exposed to ambient conditions. Herein, we report design and synthesis of diketopyrrolopyrrole (DPP)-based oligomers for ambipolar organic thin-film transistors (OFETs) with excellent air and bias stability at ambient conditions. The cyclic voltammetry measurements reveal exceptional electrochemical stability during the redox cycle of oligomers. Structural properties including aggregation, crystallinity, and morphology in thin film were investigated by UV-visible spectroscopy, atomic force microscopy (AFM), thin-film X-ray diffraction (XRD), and grazing incidence small-angle X-ray scattering (GISAXS) measurements. AFM reveals morphological changes induced by different processing conditions whereas GISAXS measurements show an increase in the population of face-on oriented crystallites in films subjected to a combination of solvent and thermal treatments. These measurements also highlight the significance of chalcogen atom from sulfur to selenium on the photophysical, optical, electronic, and solid-state properties of DPP-DPP oligomers. Charge carrier mobilities of the oligomers were investigated by fabricating top-gate bottom-contact (TG-BC) thin-film transistors by annealing the thin films under various conditions. Combined solvent and thermal annealing of DPP-DPP oligomer thin films results in consistent electron mobilities as high as ∼0.2 cm2 V-1 s-1 with an on/off ratio exceeding 104. Field-effect behavior was retained for up to ∼4 weeks, which illustrates remarkable air and bias stability. This work paves the way toward the development of n-channel DPP-DPP-based oligomers exhibiting retention of field-effect behavior with superior stability at ambient conditions.
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    ItemOpen Access
    Spin density encodes intramolecular singlet exciton fission in pentacene dimers
    (Nature Publishing Group, 2019) Krishnapriya, K. C.; Roy, P.; Puttaraju, B.; Salzner, Ulrike; Musser, A. J.; Jain, M.; Dasgupta, J.; Patil, S.
    The formation of two triplet excitons at the cost of one photon via singlet exciton fission in organic semiconductors can potentially enhance the photocurrent in photovoltaic devices. However, the role of spin density distribution in driving this photophysical process has been unclear until now. Here we present the significance of electronic spin density distribution in facilitating efficient intramolecular singlet exciton fission (iSEF) in π-bridged pentacene dimers. We synthetically modulate the spin density distribution in a series of pentacene dimers using phenyl-, thienyl- and selenyl- flanked diketopyrrolopyrrole (DPP) derivatives as π-bridges. Using femtosecond transient absorption spectroscopy, we find that efficient iSEF is only observed for the phenyl-derivative in ~2.4 ps while absent in the other two dimers. Electronic structure calculations reveal that phenyl-DPP bridge localizes α- and β-spin densities on distinct terminal pentacenes. Upon photoexcitation, a spin exchange mechanism enables iSEF from a singlet state which has an innate triplet pair character.