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Bithienopyrroledione (bi-TPD) based polymers P1 and P2 are designed and synthesized. Photovoltaic devices based on P1:PC71BM and P2:PC71BM blend films show power conversion efficiencies (PCEs) of 8.22% and 9.08%, respectively, whereas devices based on their thienopyrroledione (TPD)-based analogue P3 and PC71BM blend films display a PCE of 5.10%.

Bi-thieno[3,4-c]pyrrole-4,6-dione (bi-TPD) and oligothiophene copolymer semiconductors P1–P4, with different alkyl side chain densities and orientations, were synthesized. Their physicochemical properties were systematically characterized. With trimethoxy(octadecyl)silane (OTMS) modified SiO2 as substrates, the as-spun thin films of P1, P3 and P4 adopted the face-on packing structure while the P2 films displayed the edge-on packing arrangement. The packing differences of P1–P4 were ascribed to their different alkyl chain densities and orientations. The higher alkyl chain density is favorable to the face-on packing structure. After thermal annealing, the packing orientation of P2 remained and the packing structures of P1, P3 and P4 changed. P1 and P4 based films exhibited the coexistence of the edge-on and face-on packing structures in which the edge-on structure was predominant, and P3 based thin films were converted to the edge-on packing arrangement. The relationship between the aggregation structures and charge carrier transport properties of these films was explored through thin film transistors. All devices exhibited p-channel behavior with a maximum mobility larger than 1.0 cm2 V−1 s−1. Interestingly, the P3 films with face-on packing structures displayed a high mobility up to 1.18 cm2 V−1 s−1 (average mobility of 1.02 cm2 V−1 s−1), very close to the edge-on packed films which showed a maximum mobility of 1.4 cm2 V−1 s−1 (average mobility of 1.16 cm2 V−1 s−1). These results demonstrated, similar to edge-on packing structures, the face-on packing arrangement with π–π interactions facilitated charge carrier transport.

Two novel polymers PDPP3T-CN and PDPPTPT-CN containing nitrile-substituted thienyl and phenyl units were designed and synthesized. The influence of nitrile groups on the electronic properties and charge-carrier transport of the polymers was thoroughly investigated and highlighted by comparing with the corresponding non-nitrile substituted polymers. Experimental results showed that the introduction of nitrile groups lowered the HOMO and LUMO energy levels of polymers by about 0.3 eV and 0.2 eV, respectively. Thin film transistor characteristics displayed that both polymers exhibited electron-dominated ambipolar behavior under ambient conditions. The highest electron/hole mobilities were 0.18/0.0083 cm2 V−1 s−1 for PDPPTPT-CN and 0.09/0.0017 cm2 V−1 s−1 for PDPP3T-CN, one of the highest electron mobilities for DPP-based polymers measured under ambient conditions with bottom-gate/top-contact device configurations.

A solution-processable tetrathienoquinoidal semiconductor CMHT was synthesized and characterized. Single crystal diffraction results showed that CMHT adopted slipped π–π stacking in the crystal structure. Multiple intermolecular interactions, such as S⋯N and S⋯C (where C is the carbon on the cyano group), existed among neighboring molecules, which formed a 2-dimensional charge transport network. Solution-processed thin film transistors of CMHT displayed a high electron mobility of up to 0.22 cm2 V−1 s−1 under ambient conditions, one of the highest electron mobilities for quinoidal semiconductors. The correlation between the molecular packing of the CMHT and transistor performance was studied by AFM and XRD.

A donor–acceptor–donor (D–A–D) type small molecule DHB-QDTB, with dicyano-substituted quinoidal benzo[1,2-b:4,5-b′]dithiophene as the acceptor, was designed and synthesized. A high electron mobility up to 0.88 cm2 V−1 s−1 was observed for DHB-QDTB based transistors under ambient conditions, suggesting quinoidal benzo[1,2-b:4,5-b′]dithiophene is an excellent acceptor unit in D–A type n-channel organic semiconductors.

A novel solution-processable dicyano-substituted quinoidal oligothiophene molecule CDT-2T(CN)4 was synthesized and characterized. CDT-2T(CN)4 displayed high thermal stability and broad thin film absorption in the near IR region. Electrochemical results showed that CDT-2T(CN)4 exhibited reversible oxidation and reduction peaks with the LUMO energy level at −4.2 eV and the HOMO energy level at −5.3 eV. The solution processed thin film transistors of CDT-2T(CN)4 displayed electron-dominant ambipolar transport behavior under ambient conditions. The optimal saturation hole and electron mobilities were 0.003 and 0.023 cm2 V−1 s−1, respectively, one of the highest values for solution processed quinoidal based ambipolar organic semiconductors with good ambient stability.