Article pubs.acs.org/Macromolecules
Naphthalene Diimide Copolymers with Oligo(p‑phenylenevinylene) and Benzobisoxazole for Balanced Ambipolar Charge Transport Nagesh B. Kolhe,† A. Z. Ashar,‡ K. S. Narayan,*,‡ and S. K. Asha*,† †
Polymer & Advanced Material Laboratory, CSIR-NCL, Pune 411008, Maharashtra, India Chemistry and Physics of Material Unit, Jawaharlal Nehru Centre for Advanced Scientific Research Jakkur, Bangalore 560064, Karnataka, India
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S Supporting Information *
ABSTRACT: A series of alternating and random donor (D)− acceptor (A) copolymers based on naphthalene diimide (NDI) as the acceptor and oligo(p-phenylenevinylene) (OPV) or benzobisoxazole (BBO) as the strong and weak donor, respectively, were designed and synthesized by Suzuki coupling and Horner−Wadsworth−Emmons polymerization. The effect of the varying donor strength of OPV and BBO on the photophysical, electrochemical, and semiconducting properties of the polymers was investigated. Absorption and emission spectra recorded for dilute chloroform solution and thin film showed increased intramolecular charge transfer for NDI-alt-OPV polymer compared to NDI-alt-BBO polymer. Cyclic voltammetry studies along with DFT (density functional theory) studies at the B3LYP/6-31g* level gave insight into the energy level (HOMO/LUMO) and molecular orientation of donor and acceptor along the polymer backbone. NDI-alt-OPV polymer exhibited rigid coplanar structure with extended πconjugation which induced backbone planarity and crystallinity to the polymer. The inherent poor solubility of the NDI-alt-BBO prevented further device characterization of this polymer. Random copolymer having maximum 30% incorporation of BBO comonomer in NDI-r-OPV/BBO was found to be soluble for further characterization. Compared to NDI-alt-OPV, lowering of both energy levels LUMO (∼0.2 eV) and HOMO (∼0.5 eV) was observed for both NDI-alt-BBO and the NDI-r-OPV/BBO. Bottom gate−top contact organic field effect transistors (OFETs) of NDI-alt-OPV exhibited balanced ambipolar charge transport with average electron and hole mobility of 3.09 × 10−3 cm2 V−1 s−1 and 2.1 × 10−3 cm2 V−1 s−1, respectively, whereas the random copolymer incorporating both OPV and BBO units NDI-r-OPV/BBO showed dominant n-type charge transport with moderate 4 × 10−4 cm2 V−1 s−1 average electron mobility. The present work thus highlights the structure−property relationship and the electronic tunability required in this class of NDI-based polymers to produce ambipolar transistors.
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INTRODUCTION Organic semiconducting materials based on π-conjugated oligomers1 and polymers2 have proved to be promising for application in optoelectronic devices such as organic field effect transistor (OFETs), light-emitting diode (OLEDs), and solar cells due to their unique properties like low fabrication cost, lightweight, flexibility, and solution processability. They are widely classified as electron (n-type), hole (p-type), and electron−hole (ambipolar) transporting material, depending upon the nature of their charge transport.3 Although the majority of materials reported are p-type with high performance parameters in terms of hole mobility and air stability, the scenarios for n-type and ambipolar materials are also improving steadily, due to accomplishment of rational molecular design and solution processable polymeric semiconductors.4 Ambipolar materials based on polymeric semiconductor with high and balanced electron and hole mobilities are highly desired5 due to their application in complementary-like circuits6 and in lightemitting diodes.7 The criteria for any material to exhibit © 2014 American Chemical Society
ambipolarity is that they should possess lower-lying LUMO (lowest unoccupied molecular orbital)
