Intrinsic Charge Transport for DTzTI-Based All-Acceptor Homopolymer n-Type Organic Semiconductors: Roles of Conjugation Length and Orbital Delocalization

Abstract

2,2′-Bithiazolothienyl-4,4′,10,10′-tetracarboxydiimide (DTzTI), a novel imide-functionalized thiazole, is envisioned as a candidate for an excellent building block for constructing all-acceptor homopolymers, and the resulting PDTzTI, which is the polymer of DTzTI, demonstrated unipolar n-type transport with an exceptional electron mobility (μe) of 1.61 cm2 V–1 s–1. Density functional theory (DFT) and the incoherent charge-hopping model at the molecular level are used to design and investigate the model compounds DTzTI and two novel fluorine- or selenium-substituted analogues, DTzTI-2F and DTzTI-4Se, in order to better understand the roles of conjugation length and orbital delocalization for intrinsic charge transport as well as to increase the electron mobility and ambient stability of DTzTI-based polymers. According to the DFT results, increasing the conjugation length (n, number of haploids) of homopolymer molecules could significantly lower the recombination energy, decrease the ELUMO–HOMO, improve the delocalization of the frontier molecular orbitals, and raise the electron’s transfer integral (Ve) between adjacent neighboring homopolymer molecules. This would make it easier to delocalize and transport charge carriers between chains, increasing the electron-transfer efficiency. Additionally, lowering the lowest unoccupied molecular orbital (LUMO) level below −4 eV with the substitution of fluorine or selenium would be very advantageous to ambient stability. 8DTzTI, 8DTzTI-2F, and 8DTzTI-4Se are anticipated to have μe values of 23.87, 19.44, and 29.07 cm2 V–1 s–1, respectively. The performance of all the three analogues is unipolar n-type. The bigger orbital delocalization and larger transfer integral resulting from the face-to-face π–π stacking produce significant electron mobility for DTzTI-4Se, demonstrating that larger delocalization of molecular orbitals will improve intermolecular conjugation and boost charge transport characteristics. A straightforward mathematical model of mobility and conjugation length is discussed, enabling a rapid computation of the theoretical mobilities for specific homopolymers of all-acceptor n-type semiconductor materials. Another method for enhancing the electron mobility and environmental stability of DTzTI-based unipolar n-type polymer semiconductors is selenium substitution.