n-Type conjugated polymers based on 3,3′-dicyano-2,2′-bithiophene: synthesis and semiconducting properties

Three new alternating copolymers containing 3,3′-dicyano-2,2′-bithiophene (BT2CN), i.e., DPPTh-BT2CN, DPPPy-BT2CN and 2FIID-BT2CN based on diketopyrrolopyrrole (DPP) and isoindigo (IID) derivatives, were synthesized. The properties of these three polymers were compared with those of the reference polymer without a cyano (CN) group, DPPTh-BT. The introduction of CN groups dramatically lowered both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of the polymers. DPPTh-BT2CN exhibited HOMO and LUMO energy levels of −5.41 and −3.67 eV, respectively, which are much lower than those of the analogue polymer DPPTh-BT comprising a biothiophene (BT) segment. DPPPy-BT2CN and 2FIID-BT2CN showed even lower HOMO and LUMO levels, which are −5.83 and −3.75 eV for DPPPy-BT2CN and −6.15 and −3.92 eV for 2FIID-BT2CN. Organic thin-film transistors (OTFTs) were fabricated to evaluate the semiconducting properties of the polymers. DPPTh-BT2CN-based OTFTs exhibited electron-dominant transport characteristics, similar to the devices based on the benchmark n-type conjugated polymer N2200. By contrast, DPPPy-BT2CN and 2FIID-BT2CN-based devices displayed pure n-channel characteristics due to their lower HOMO and LUMO levels. All three polymers displayed an electron mobility (μe) of ca. 0.3 cm2 V−1 s−1, which is comparable to that of N2200 when the same device structure and measurement conditions were adopted.