n-Channel Polymers by Design: Optimizing the Interplay of Solubilizing Substituents, Crystal Packing, and Field-Effect Transistor Characteristics in Polymeric Bithiophene-Imide Semiconductors

Electron transporting (n-channel) polymer semiconductors for field-effect transistors are rare. In this investigation, the synthesis and characterization of new electron-depleted N-alkyl-2,2′-bithiophene-3,3′-dicarboximide-based π-conjugated homopolymers and copolymers containing the 2,2′-bithiophene unit are reported. A novel design approach is employed using computational modeling to identify favorable monomer properties such as core planarity, solubilizing substituent tailorability, and appropriate electron affinity with gratifying results. Monomeric model compounds are synthesized to confirm these properties, and a crystal structure reveals a short 3.43 Å π−π stacking distance with favorable solubilizing substituent orientations. A family of 10 homopolymers and bithiophene copolymers is then synthesized via Yamamoto and Stille polymerizations, respectively. Two of these polymers are processable in common organic solvents: the homopolymer poly(N-(2-octyldodecyl)-2,2′-bithiophene-3,3′-dicarboximide) (P1) exhibits n-channel FET activity, and the copolymer poly(N-(2-octyldodecyl)-2,2′:5′,2′′:5′′,2′′′-quaterthiophene-3,3′-dicarboximide) (P2) exhibits air-stable p-channel FET operation. After annealing, P1 films exhibit a very high degree of crystallinity and an electron mobility > 0.01 cm2 V−1 s−1 with a current on−off ratio of 107, which is remarkably independent of film-deposition conditions. Extraordinarily, P1 films also exhibit terracing in AFM images with a step height matching the X-ray diffraction d spacing, a rare phenomenon for polymeric organic semiconductors. Another fascinating property of these materials is the air-stable p-channel FET performance of annealed P2 films, which exhibit a hole mobility of ∼0.01 cm2 V−1 s−1 and a current on−off ratio of 107.

用于场效应晶体管（Field-Effect Transistor, FET）的电子传输型（n沟道）聚合物半导体极为稀缺。本研究报道了一类新型缺电子型N-烷基-2,2′-联噻吩-3,3′-二甲酰亚胺基π共轭均聚物与共聚物的合成与表征，此类聚合物均含有2,2′-联噻吩单元。研究采用计算模拟建模的创新设计策略，以筛选具备核心平面性、可溶性取代基可修饰性与适宜电子亲和势的单体，最终获得了令人满意的结果。通过合成单体模型化合物验证了上述单体特性，其晶体结构显示出3.43 Å的短程π-π堆积距离，且可溶性取代基取向良好。随后分别通过Yamamoto聚合与Stille聚合，合成了包含10种均聚物与联噻吩基共聚物的材料体系。其中两种聚合物可在常见有机溶剂中加工：均聚物聚(N-(2-辛基十二烷基)-2,2′-联噻吩-3,3′-二甲酰亚胺)（P1）表现出n沟道场效应晶体管活性；共聚物聚(N-(2-辛基十二烷基)-2,2′:5′,2′′:5′′,2′′′-四噻吩-3,3′-二甲酰亚胺)（P2）则具备空气稳定的p沟道场效应晶体管性能。经退火处理后，P1薄膜展现出极高的结晶度，电子迁移率大于0.01 cm² V⁻¹ s⁻¹，电流开关比达10⁷，且该性能几乎不受薄膜沉积条件的影响。尤为特别的是，P1薄膜的原子力显微镜（Atomic Force Microscopy, AFM）图像中呈现出台阶形貌，其台阶高度与X射线衍射d间距相匹配，这在聚合物有机半导体中实属罕见。此类材料的另一项引人关注的特性在于，退火后的P2薄膜具备空气稳定的p沟道场效应晶体管性能，其空穴迁移率约为0.01 cm² V⁻¹ s⁻¹，电流开关比同样达到10⁷。