Theoretical Investigation of Electronic and Optical Properties of NDI-Fused-Bithiophene (NDI-f-BT) Copolymer at Different Redox States for Single-Component Ambipolar Transistors

Naphthalene diimide (NDI) copolymerized with thiophene-based donor moieties has the potential to be used as an ambipolar conducting polymer to transport both charge carriers, viz, electrons and holes, at different redox states. The p-type conductivity in these copolymers is not up to the mark compared to the n-type conductivity, and there is scope for improvement by strategically modifying the donor moieties. So, replacing the nonfused thiophene donor moieties with fused thiophene moieties can lead to an increase in the π-conjugation length, which can improve the p-type electronic and optical properties. Here, we have studied the electronic and optical properties of the NDI-fused-bithiophene (NDI-f-BT) donor–acceptor polymer and their evolution at different redox states (up to 200% redox levels) using density functional theory (DFT) and time-dependent density functional theory (TD-DFT). The electron affinity and ionization potential of NDI-f-BT, considering the first redox states, are compared with the experimentally reported lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO), respectively, measured through electrochemical switching, and they are in good agreement. We note that the TD-DFT calculated optical properties of NDI-f-BT are qualitatively in agreement with the experimental findings and can be used to understand the changes in optical properties during oxidation and reduction. The absorption spectra indicate a red shift up to the 100% redox state, indicating that NDI-f-BT has a good potential to be used in an ambipolar field effect transistor. We also observed the chemical alteration of the donor moieties beyond 100% oxidation level, which leads to an increase in the π-conjugation length to accommodate the bipolaron. This finding indicates that increasing the π-conjugation length can be a strategy to have a balanced p-type conductivity compared to that of the n-type, aiming for ambipolar conductivity of the donor–acceptor copolymer.