Department of Energy Science and Engineering , DGIST , Daegu 42988 , Korea.
GREMAN, UMR 7347, CNRS , Université de Tours , 37200 Tours , France.
J Am Chem Soc. 2018 Apr 25;140(16):5375-5384. doi: 10.1021/jacs.7b10306. Epub 2018 Apr 17.
Poly-3,4-ethylenedioxythiophene:polystyrenesulfonate (PEDOT:PSS) is a water-processable conducting polymer with promise for use in transparent flexible electrodes and thermoelectric devices, but its conductivity is not satisfactory. Its low conductivity is attributed to the formation of hydrophilic/insulating PSS outer layers encapsulating the conducting/hydrophobic p-doped PEDOT cores. Recently a significant conductivity enhancement has been achieved by adding ionic liquid (IL). It is believed that ion exchange between PEDOT:PSS and IL components helps PEDOT to decouple from PSS and to grow into large-scale conducting domains, but the exact mechanism is still under debate. Here we show through free energy calculations using density functional theory on a minimal model that the most efficient IL pairs are the least tightly bound ones with the lowest binding energies, which would lead to the most efficient ion exchange with PEDOT:PSS. This spontaneous ion exchange followed by nanophase segregation between PEDOT and PSS, with formation of a π-stacked PEDOT aggregate decorated by IL anions, is also supported by molecular dynamics performed on larger PEDOT:PSS models in solution. We also show that the most efficient IL anions would sustain the highest amount of charge carriers uniformly distributed along the PEDOT backbone to further enhance the conductivity, providing that they remain in the PEDOT domain after the ion exchange. Hence, our design principle is that the high-performance IL should induce not only an efficient ion exchange with PEDOT:PSS to improve the PEDOT morphology (to increase mobility) but also a uniform high-level p-doping of PEDOT (to enhance intrinsic conductivity). Based on this principle, a promising (electron-withdrawing, but bulky, soft, and hydrophobic) new IL pair is proposed.
聚 3,4-亚乙基二氧噻吩：聚苯乙烯磺酸盐（PEDOT：PSS）是一种可水加工的导电聚合物，有望用于透明柔性电极和热电设备，但导电性不理想。其低导电性归因于形成亲水性/绝缘 PSS 外层，封装了导电/疏水性 p 掺杂 PEDOT 核。最近，通过添加离子液体（IL）实现了显著的电导率增强。人们认为 PEDOT：PSS 和 IL 成分之间的离子交换有助于 PEDOT 与 PSS 解耦并生长成大尺度的导电畴，但确切的机制仍存在争议。在这里，我们通过使用密度泛函理论在最小模型上进行自由能计算表明，最有效的 IL 对是结合能最低、结合最不紧密的 IL 对，这将导致与 PEDOT：PSS 最有效的离子交换。这种自发的离子交换，然后是 PEDOT 和 PSS 之间的纳米相分离，形成由 IL 阴离子修饰的π堆积 PEDOT 聚集体，也得到了在溶液中更大的 PEDOT：PSS 模型上进行的分子动力学的支持。我们还表明，最有效的 IL 阴离子将在 PEDOT 主链上均匀分布并保持最高数量的载流子，以进一步提高电导率，前提是它们在离子交换后仍留在 PEDOT 域中。因此，我们的设计原则是，高性能 IL 不仅应诱导与 PEDOT：PSS 的有效离子交换以改善 PEDOT 形态（提高迁移率），还应均匀高水平地对 PEDOT 进行 p 掺杂（增强本征电导率）。基于这一原则，提出了一种有前途的（吸电子但庞大、柔软且疏水）新的 IL 对。
