D‐A₁‐D‐A₂ Backbone Strategy for Benzobisthiadiazole Based n‐Channel Organic Transistors: Clarifying the Selenium‐Substitution Effect on the Molecular Packing and Charge Transport Properties in Electron‐Deficient Polymers

Abstract

Unipolar n‐type semiconducting polymers based on the benzobisthiadiazole (BBT) unit and its heteroatom‐substituted derivatives are for the first time synthesized by the D‐A1‐D‐A2 polymer‐backbone design strategy. Selenium (Se) substitution is a very effective molecular design, but it has been seldom studied in n‐type polymers. In this study, within the similar conjugated framework, the Se substitution effects on the optical, electrochemical, solid‐state polymer packing, electron mobility, and air‐stability of the target unipolar n‐type polymers are unraveled. Replacing the sulfur (S) atom in the thiadiazole heterocycles with the Se atom leads to narrower bandgaps and deeper lowest unoccupied molecular orbital (LUMO) levels of the n‐type polymers. Furthermore, the Se‐substituted polymer (pSeN‐NDI) shows shorter lamellar packing distances and stronger edge‐on π–π stacking interactions than its S‐counterpart (pSN‐NDI), as observed by the two‐dimensional grazing‐incidence wide‐angle X‐ray scattering (GIWAXS) patterns. With the deeper LUMO level and thin‐film microstructures suitable for transistors, pSeN‐NDI exhibits four‐fold higher electron mobilities (μe) than pSN‐NDI. However, the other Se‐containing polymer, pSeS‐NDI, forms rather amorphous film structures, which is caused by its limited thermal stability and decomposition during the thermal annealing processes, thus giving rise to a lower μe than its S‐counterpart (pBBT‐NDI). Most importantly, pBBT‐NDI demonstrates an electron mobility of 0.039 cm2 V−1 s−1, which is noticeable among the unipolar n‐type polymers based on the BBT and its analogs.

High‐Performance n‐Channel Organic Transistors Using High‐Molecular‐Weight Electron‐Deficient Copolymers and Amine‐Tailed Self‐Assembled Monolayers

“…These copolymers were composed of NDI and benzothiadiazole (BT) derivatives based on the newly developed regioregular donor–acceptor 1 –donor–acceptor 2 (D–A 1 –D–A 2 ) polymer backbone design strategy (pBTTz, pPTT, and pSNT, Scheme ) . It should be noted that the backbone of pSNT is similar to that of a previously reported polymer, pNDI–SN . However, most importantly, these polymers have different alkyl side chains (Chart S1, Supporting Information).…”

Fluoro-Modulated Molecular Geometry in Diketopyrrolopyrrole-Based Low-Bandgap Copolymers for Tuning the Photovoltaic Performance

“…In addition, the dominant absorption peak of PCFB (~740 nm) is slightly red-shifted compared with that of other polymers. This result can be explained by the shorter lamellar distance existing in PCFB film as validated from X-ray diffraction (XRD) data ( vide infra , see Figure S1, Table S2), which indicates that the polymer chains were packed more tightly (Wang Y. et al, 2017). The electrochemical properties were also investigated by cyclic voltammetry (CV).…”

“…Another way to adjust the energy levels of polymers is to fabricate copolymers with different units. Recently, terpolymers which comprise three various components in the backbone have emerged as a new design strategy for donor polymers (Qin et al, 2014; Duan et al, 2016; Wang X. et al, 2017; Wang Y. et al, 2017; Huo et al, 2018). Among these copolymers, regioregular ones provide natural advantages, such as well-defined molecular structure, highly reproducibility and better molecular packing (Qin et al, 2014; Kim et al, 2015; Lee et al, 2015).…”

Effects of Different Unsaturated‐Linker‐Containing Donors on Electronic Properties of Benzobisthiadiazole‐Based Copolymers

“…The lowest unoccupied molecular orbital (LUMO) energy levels are calculated from the onset of the reduction peaks, and their LUMO energy levels are in the range of −3.97 to −4.14 eV. The deep LUMO energy levels of the four polymers are suitable for electron injection …”

“…Donor–acceptor (D–A)‐based copolymers have been widely utilized in organic field‐effect transistors (OFETs) due to their suitable molecular orbital energy levels and strong π–π interactions . Benzobisthiadiazole (BBT) has a quite planar skeleton, strong electron‐drawing ability as well as easy intermolecular stacking tendency, which make this unit a potential building block for high‐performance materials for both hole and electron transport . Since the donor is also important for the device performance of D–A copolymers, the selection of donors is one way to design new BBT‐based semiconductors …”