Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM) , Nanjing Tech University , 30 South Puzhu Road , Nanjing 211816 , Jiangsu , China.
Department of Materials Science and Engineering, The Shenzhen Key Laboratory for Printed Organic Electronics , Southern University of Science and Technology (SUSTech) , No. 1088, Xueyuan Road , Shenzhen 518055 , Guangdong , China.
ACS Appl Mater Interfaces. 2019 Mar 13;11(10):10089-10098. doi: 10.1021/acsami.8b22457. Epub 2019 Mar 1.
Polythiophenes, built on the electron-rich thiophene unit, typically possess high-lying energy levels of the lowest unoccupied molecular orbitals (LUMOs) and show hole-transporting properties. In this study, we develop a series of n-type polythiophenes, P1-P3, based on head-to-head-linked 3,3'-dialkoxy-4,4'-dicyano-2,2'-bithiophene (BTCNOR) with distinct side chains. The BTCNOR unit shows not only highly planar backbone conformation enabled by the intramolecular noncovalent sulfur-oxygen interaction but also significantly suppressed LUMO level attributed to the cyano-substitution. Hence, all BTCNOR-based polymer semiconductors exhibit low-lying LUMO levels, which are ∼1.0 eV lower than that of regioregular poly(3-hexylthiophene) (rr-P3HT), a benchmark p-type polymer semiconductor. Consequently, all of the three polymers can enable unipolar n-type transport characteristics in organic thin-film transistors (OTFTs) with low off-currents ( Is) of 10-10 A and large current on/off ratios ( I/ Is) at the level of 10. Among them, polymer P2 with a 2-ethylhexyl side chain offers the highest film ordering, leading to the best device performance with an excellent electron mobility (μ) of 0.31 cm V s in off-center spin-cast OTFTs. To the best of our knowledge, this is the first report of n-type polythiophenes with electron mobility comparable to the hole mobility of the benchmark p-type rr-P3HT and approaching the electron mobility of the most-studied n-type polymer, poly(naphthalene diimide- alt-bithiophene) (i.e., N2200). The change of charge carrier polarity from p-type (rr-P3HT) to n-type (P2) with comparable mobility demonstrates the obvious effectiveness of our structural modification. Adoption of n-hexadecyl (P1) and 2-butyloctyl (P3) side chains leads to reduced film ordering and results in 1-2 orders of magnitude lower μs, showing the critical role of side chains in optimizing device performance. This study demonstrates the unique structural features of head-to-head linkage containing BTCNOR for constructing high-performance n-type polymers, i.e., the alkoxy chain for backbone conformation locking and providing polymer solubility as well as the strong electron-withdrawing cyano group for lowering LUMO levels and enabling n-type performance. The design strategy of BTCNOR-based polymers provides useful guidelines for developing n-type polythiophenes.
聚噻吩基于富电子噻吩单元，通常具有较高的最低未占据分子轨道（LUMO）能级，并表现出空穴传输性能。在这项研究中，我们基于头对头连接的 3,3'-二甲氧基-4,4'-二氰基-2,2'-联噻吩（BTCNOR）设计了一系列具有不同侧链的 n 型聚噻吩 P1-P3。BTCNOR 单元不仅具有由分子内非共价硫-氧相互作用实现的高度平面骨架构象，而且由于氰基取代而显著抑制了 LUMO 能级。因此，所有基于 BTCNOR 的聚合物半导体都表现出较低的 LUMO 能级，比基准 p 型聚合物半导体规正聚（3-己基噻吩）（rr-P3HT）低约 1.0 eV。因此，所有三种聚合物都可以在有机薄膜晶体管（OTFT）中实现单极 n 型传输特性，具有低关态电流（Is）为 10-10 A 和在 10 数量级的大电流开关比（I/Is）。其中，具有 2-乙基己基侧链的聚合物 P2 提供了最高的薄膜有序性，导致在非中心旋涂 OTFT 中具有优异的电子迁移率（μ）为 0.31 cm V s 的最佳器件性能。据我们所知，这是首次报道具有与基准 p 型 rr-P3HT 的空穴迁移率相当且接近最受研究的 n 型聚合物，即聚（萘二酰亚胺-交替-噻吩）（即 N2200）的电子迁移率的 n 型聚噻吩。从 p 型（rr-P3HT）到 n 型（P2）的电荷载流子极性的变化以及相当的迁移率表明了我们结构修饰的明显有效性。采用正十六烷基（P1）和 2-丁基辛基（P3）侧链会降低薄膜有序性，并导致μs 降低 1-2 个数量级，表明侧链在优化器件性能方面起着关键作用。本研究证明了包含 BTCNOR 的头对头键合的独特结构特征，用于构建高性能 n 型聚合物，即用于骨架构象锁定和提供聚合物溶解性的烷氧基链以及用于降低 LUMO 能级并实现 n 型性能的强吸电子氰基基团。基于 BTCNOR 的聚合物的设计策略为开发 n 型聚噻吩提供了有用的指导。
