Ion insertion and Ion-host Interactions in Diketopyrrolopyrrole-based π-Conjugated Polymers
π-conjugated organic semiconductors are a class of technologically important materials with broad applicability in areas as diverse as energy conversion and storage, bioelectronics, and consumer electronics. This is because their properties can be tailored to suit a host of applications by, for example, subtle changes to their structure, conjugation length, side chains, crystallinity, and doping. Diketopyrrolopyrrole (DPP)-based π-conjugated polymers exhibit high ambipolar carrier mobilities and high performance in organic field effect transistors and organic photovoltaics.1 Owing to their low band gap, redox activity and favorable redox potentials, DPP-based π-conjugated polymers could also be suitable as channel materials for organic electrochemical transistors, electrodes for ion-insertion batteries, neuromorphic devices and sensing elements in bioelectronic applications. Electrochemical potential-dependent control of polymer redox coupled with ion-insertion, and the subsequent electrochemical doping-induced changes to the electronic conductivity of the semiconducting host, are central to the realization of these functional devices. In this work, we used organic electrochemical transistors (OECTs) as a platform to simultaneously probe the mechanistic aspects of ion-insertion and electrochemical doping in DPP-based π-conjugated polymers.2 Based on OECT measurements, we discuss the role of ion-size, polymer side chains and electrolyte composition in dictating ion-insertion and transconductance of OECT devices. We corroborate these findings with spectroelectrochemical and electrochemical impedance measurements. Strikingly, our results show a polymer side-chain dependent asymmetry for the insertion of cations and anions. These cannot be fully understood when the electrochemical dopant is assumed to be in a polymer host matrix with a uniform dielectric constant. We discuss whether electrochemical dopant and host polymer interactions should be treated like ion-solvent interactions in classical electrolyte solutions. Based on these mechanistic insights, we developed DPP-based OECTs that have balanced n- and p-type OECT device performance. We then configured arrays of single-component DPP-based OECTs into complementary like logic circuits to demonstrate NOT, 2-input NAND and 2-input NOR logic gates.3 Since the NAND and NOR circuits are configured from identical single-component OECTs, the NAND and NOR operations are fully voltage reconfigurable. Our work is a significant step towards building OECT-based bioelectronics and polymorphic circuits. Further, owing to their fast redox kinetics, we show that DPP-based polymers could be suitable as cathode materials in Li-ion batteries. We demonstrate fast and stable cycling of DPP-based cathodes in a Li-metal based half-cell with an average voltage of 2.2 V, 70% capacity retention at 500 C and high cycling stability of up to 1000 cycles.4 Our work is an important step towards a mechanistic understanding of ion-insertion and electrochemical doping in π-conjugated organic semiconductors. This understanding of mixed ionic and electronic conduction of these materials will be critical for a host of applications such as for example, i) interfacing circuits that mediate communication between an ionic biological circuitry and a fully electronic circuitry; ii) electrodes for energy storage devices; iii) electrochromic windows, iv) neuromorphic devices and v) polymorphic circuits with concealed logic operations. References: 1. Catherine Kanimozhi, , Nir Yaacobi-Gross, Kang Wei Chou, Aram Amassian, Thomas D. Anthopoulos, and Satish Patil. “Diketopyrrolopyrrole–Diketopyrrolopyrrole-Based Conjugated Copolymer for High-Mobility Organic Field-Effect Transistors.” Journal of the American Chemical Society 134, no. 40 (2012): 16532–35. 2. Jonathan Rivnay, Sahika Inal, Alberto Salleo, Róisín M. Owens, Magnus Berggren, and George G. Malliaras. “Organic Electrochemical Transistors.” Nature Reviews Materials 3, no. 2 (2018): 17086. 3. Jibin J. Samuel, Ashutosh Garudapalli, Aiswarya Abhisek Mohapatra, Chandrasekhar Gangadharappa, Satish Patil, and Naga Phani B. Aetukuri. “Single-Component CMOS-Like Logic Using Diketopyrrolopyrrole-Based Ambipolar Organic Electrochemical Transistors.” Advanced Functional Materials 31, no. 45 (2021): 2102903. 4. Jibin J. Samuel, Varun Kumar Karrothu, Ram Kumar Canjeevaram Balasubramanyam, Aiswarya Abhisek Mohapatra, Chandrasekhar Gangadharappa, Varun Ravi Kankanallu, Satish Patil, and Naga Phani B. Aetukuri. “Ionic Charge Storage in Diketopyrrolopyrrole-Based Redox-Active Conjugated Polymers.” The Journal of Physical Chemistry C 125, no. 8 (2021): 4449–57.