Precise Side-Chain Engineering of Thienylenevinylene–Benzotriazole-Based Conjugated Polymers with Coplanar Backbone for Organic Field Effect Transistors and CMOS-like Inverters

Abstract

Two donor-acceptor (D-A) alternating conjugated polymers based on thienylenevinylene-benzotriazole (TV-BTz), PTV6B with a linear side chain and PTVEhB with a branched side chain, were synthesized and characterized for organic field effect transistors (OFETs) and complementary metal-oxide-semiconductor (CMOS)-like inverters. According to density functional theory (DFT), polymers based on TV-BTz exhibit a coplanar and rigid structure with no significant twists, which could cause to an increase in charge-carrier mobility in OFETs. Alternating alkyl side chains of the polymers impacted neither the band gap nor the energy level. However, it significantly affected the morphology and crystallinity when the polymer films were thermally annealed. To investigate the effect of thermal annealing on the morphology and crystallinity, we characterized the polymer films using atomic force microscopy (AFM) and 2D-grazing incidence X-ray diffraction (2D-GIWAXD). Fibrillary morphologies with larger domains and increased crystallinity were observed in the polymer films after thermal annealing. These polymers exhibited improved charge-carrier mobilities in annealed films at 200 °C and demonstrated optimal OFET device performance with p-type transport characteristics with charge-carrier mobilities of 1.51 cm/(V s) (PTV6B) and 2.58 cm/(V s) (PTVEhB). Furthermore, CMOS-like inorganic (ZnO)-organic (PTVEhB) hybrid bilayer inverter showed that the inverting voltage (V) was positioned near the ideal switching point at half (1/2) of supplied voltage (V) due to fairly balanced p- and n-channels.

Effect of Semi‐Fluorinated Alkyl Side Chains on Conjugated Polymers with Planar Backbone in Organic Field‐Effect Transistors

“…The UV–Vis absorption spectra revealed that the polymer has an optical band gap of 1.69 eV as shown in Figure . The band gap is similar to the ≈1.71 eV gap of PTV6B as previously reported by our group, which is referred to compare with PCTV‐BTzF because they have the same backbone structure. As expected, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of PCTV‐BTzF are found to have energy levels of −4.86 eV/−3.17 eV, similar to those (−4.90 eV/−3.19 eV) of nonfluorinated PTV6B.…”

“…Recent advances in π‐conjugated polymer semiconductors have provided considerable improvements for many future applications, such as field‐effect transistors (FETs), light‐emitting diodes, and solar cells in low‐cost, large‐area printed and flexible electronics . In particular, high performance of FETs, surpassing that of FETs using amorphous inorganic semiconductors has been realized through the rational design of the backbone structures and engineering of alkyl side chains in π‐conjugated polymer . Recent studies demonstrated that π‐conjugated polymers with rigid and planar backbone structures provided high charge‐carrier mobilities in FETs .…”

“…Herein, we introduce a donor–acceptor (D–A) type conjugated polymer with SFA side chains (PCTV‐BTzF) comprising alternatingly coupled thienylenevinylene (TV) and benzotriazole (BTz) units, to investigate the effect of SFA chains on the highly coplanar backbone. The high planarities of TV and BTz were investigated in our previous study . To provide self‐organization properties, SFA side chains are substituted onto the BTz unit, while dodecyl hydrocarbon alkyl side chains are introduced to the TV units.…”

“…It is noted that the as‐cast film shows β‐phase dominant reflection peaks, while 200 and 300 °C annealed films exhibit co‐existing β‐phase and α‐phase, resulting from the change of self‐assembled structure afforded by the SFA side chains and thermal annealing. Meanwhile, peaks along the backbone direction are not observed due to the strongly assembled property of SFA side chains. To quantitatively analyze the crystalline structures, crystallographic parameters including the lamellar d ‐spacing, π–π stacking distance of the out‐of‐plane and in‐plane direction were determined using the Bragg equation (see the details in Table S1, Supporting Information) The lamellar d ‐spacing and π–π stacking distance in the annealed films (29.14 Å/3.63 Å) are smaller than those of the as‐cast film (29.98 Å/3.67 Å), indicating that the crystallinity is increased and that much more closely packed structures are formed in the 200 °C annealed film compared to those in the as‐cast film.…”

The Effects of Side Chains on the Charge Mobilities and Functionalities of Semiconducting Conjugated Polymers beyond Solubilities

“…The effects of the side chains can be complex. For instance, the pristine thin film of P122 , in which the linear chains are attached to the benzotriazole units, shows higher charge mobility than that of P123 , which entails branching chains on the benzotriazole units . Interestingly, after thermal annealing P123 shows better semiconducting property than P122 .…”