Taming Charge Transport in Semiconducting Polymers with Branched Alkyl Side Chains

Abstract

The solid‐state packing and polymer orientation relative to the substrate are key properties to control in order to achieve high charge carrier mobilities in organic field effect transistors (OFET). Intuitively, shorter side chains are expected to yield higher charge carrier mobilities because of a denser solid state packing motif and a higher ratio of charge transport moieties. However our findings suggest that the polymer chain orientation plays a crucial role in high‐performing diketopyrrolopyrrole‐based polymers. By synthesizing a series of DPP‐based polymers with different branched alkyl side chain lengths, it is shown that the polymer orientation depends on the branched alkyl chain lengths and that the highest carrier mobilities are obtained only if the polymer adopts a mixed face‐on/edge‐on orientation, which allows the formation of 3D carrier channels in an otherwise edge‐on‐oriented polymer chain network. Time‐of‐flight measurements performed on the various polymer films support this hypothesis by showing higher out‐of‐plane carrier mobilities for the partially face‐on‐oriented polymers. Additionally, a favorable morphology is mimicked by blending a face‐on polymer into an exclusively edge‐on oriented polymer, resulting in higher charge carrier mobilities and opening up a new avenue for the fabrication of high performing OFET devices.

Challenge and Solution of Characterizing Glass Transition Temperature for Conjugated Polymers by Differential Scanning Calorimetry

“…DSC measurements for PDPP‐T3 and PNDI‐C0, which have highly planar and rigid backbone structures, are presented in Figure (d,e), the step change in c p , thus the glass transition, cannot be readily seen from the DSC scans. A linear curve is observed and is expected based on various previous reports . PDPP‐T3 shows a small curvature at the vicinity of 0 °C.…”

The Critical Role of Electron‐Donating Thiophene Groups on the Mechanical and Thermal Properties of Donor–Acceptor Semiconducting Polymers

“…[61] Thus, we used two techniques that are sensitive to the T g , and the results were summarized in Table 1. [39] From Figure 2a-c, we observed a noticeable increase of backbone T g from −3.96 °C (DPP-T) to 11.95 °C (DPP-T2), and 18.98 °C (DPP-T3) as the number of thiophene units increased (Table 1). Here, we identified the major peak on the tan δ curve as the backbone T g .…”

“…All five samples displayed similar nanofibrillar textures. [39,47] The aggregation behavior was investigated by comparing the relative intensity of peak 0-1 and peak 0-0, the result indicated slight difference in the aggregation behavior for the polymer chain, e.g., DPP-T3 showed 11% decrease in short-range aggregation when compared with DPP-T. There is no direct correlation between the mechanical property and surface morphology of conjugated polymers.…”

“…[25][26][27][28] Similar methods were later applied to D-A polymers, a class of conjugated polymers with superior electrical performance relative to poly(3-alkylthiophenes) (P3ATs). [21,30,[32][33][34][35][36][37][38][39] Backbone engineering of the DPP polymer comes from two strategies, either by tuning conjugated donor or acceptor groups or flexible nonconjugated linker groups. Encouraged by its promising electronic properties, several methods have been explored to improve the mechanical properties of DPPbased polymers and unravel the role of backbone and side chain structure in their intrinsic stretchability and charge mobility in order to achieve the best of two worlds.…”

“…[21,30,[32][33][34][35][36][37][38][39] Backbone engineering of the DPP polymer comes from two strategies, either by tuning conjugated donor or acceptor groups or flexible nonconjugated linker groups. [39,43] Savagatrup and co-workers showed that by introducing flexible groups like alkyl chains to the conjugated backbone, the crack onset strain increased from 4% without a spacer to 12% with 70% ratio of spacer while retaining a decent charge mobility around 0.05 cm 2 V −1 s −1 . [40] Similar conclusions were drawn by Lu and co-workers, where DPP polymers with branched side chains were noticed to be less stiff and more stretchable than linear ones.…”