Benzodipyrrolidone (BDP)-Based Polymer Semiconductors Containing a Series of Chalcogen Atoms: Comprehensive Investigation of the Effect of Heteroaromatic Blocks on Intrinsic Semiconducting Properties

In order to determine the effects of actual ‘chalcogen atoms’ on semiconducting properties for application in a variety of optoelectronic devices, a class of donor (D)–acceptor (A) polymer semiconductors, namely PBDP-Fu, PBDP-Th, and PBDP-Se, containing the recently formulated benzodipyrrolidone (BDP) accepting unit and furan (Fu), thiophene (Th), or selenophene (Se) as a donating unit has been synthesized, characterized, and used in an active layer of organic field-effect transistors (OFETs). With the LUMO levels being comparatively consistent for all three polymers (−3.58 to −3.60 eV) due to the dominant BDP contribution to the polymer backbone, the HOMO energies are somewhat sensitive to the structurally distinctive feature of the donor counits used. Utilizing a combination of X-ray diffraction (XRD) and atomic force microscopy (AFM), it is apparent that further crystalline domains occur with edge-on orientation for the polymers (PBDP-Th and PBDP-Se) with relatively heavier chalcogen atoms such as Th and Se, compared with PBDP-Fu which has a rather amorphous nature. Investigation of their OFET performance indicates that all the polymers show well balanced ambipolar operations. The desirable morphological structures of both the PBDP-Th and PBDP-Se result in higher mobilities in OFETs than those of PBDP-Fu. In particular, 200 °C annealed PBDP-Se OFETs results in ambipolarity being mobile for both holes of up to 1.7 × 10^–2 cm²/V·s and electrodes of up to 1.9 × 10^–2 cm²/V·s. In addition, OFETs with PBDP-Th show nearly equivalent charge carrier mobilities for both holes (μ_h = 1.2 × 10^–2 cm²/V·s) and electrons (μ_e = 1.1 × 10^–2 cm²/V·s). Consequently, we systematically demonstrate how the manipulation of existing heteroaromatics can modulate the electronic properties of conjugated D–A polymers, elucidating structure–property relationships that are desirable for the rational design of next generation materials.