Comparative Study of Charge-Transport Behavior of Edge-on- and Face-on-Oriented Diketopyrrolopyrrole-Based Conjugated Copolymers Bearing Chalcogenophene Units

Abstract

Herein, we present comparative device analyses that help understand the charge-transport behavior in polymer field-effect transistors (PFETs) in which synthesized diketopyrrolopyrrole-based (DPP-based) conjugated copolymers bearing chalcogenophene units are used as semiconducting channel materials. The copolymer backbone orientations were successfully modified by tailoring the spacer units of the conjugated copolymers to be face-on-oriented for DPP-based copolymers bearing methylthiophene-furan-methylthiophene units (PDPP-MT-F-MT) and edge-on-oriented for the corresponding thiophene (PDPP-MT-T-MT)- and selenophene (PDPP-MT-Se-MT)-containing copolymers. Higher backbone planarity and higher field-effect mobilities were expected for the edge-on-oriented DPP-based copolymers (PDPP-MT-T-MT and PDPP-MT-Se-MT) based on density functional theory calculations; however, the face-on-oriented PDPP-MT-F-MT-based PFET exhibited superior electrical performance in terms of field-effect mobility. Charge-transport analyses based on the Gaussian disorder model, focusing on the charge-transport behavior of the disordered (amorphous) domains in the DPP-based polymer films, revealed that the localized states in the disordered domains of the face-oriented PDPP-MT-F-MT become more delocalized than those of the edge-oriented PDPP-MT-T-MT and PDPP-MT-Se-MT polymers when a gate bias is applied to their PFETs. Therefore, we attribute the electrical performance of a DPP-based PFET to the degree of delocalization of the disordered domains of its DPP-based polymer, which has a significant impact on the charge-transport behavior.

Synthesis of Asymmetric Benzotrithiophene/Benzotriselenophene Building Blocks and Their Donor–Acceptor Copolymers: Chalcogen Effect on Face-on/Edge-on Orientations and Charge Transport

“…The orientation of conjugated polymers on substrates can be controlled by various structural factors including the composition of the main-chain chalcogenophene, the regioregularity of the side chains, − the density of side-chain attachment, , the placement of side chains, − and the molecular weight. − In general, polymers exhibiting strong intermolecular interactions tend to adopt an edge-on orientation while those with weaker interactions tend to adopt a face-on orientation. Grazing-incidence wide-angle X-ray scattering (GIWAXS) was used to gain a deeper understanding of the correlation between charge transport in OFETs and molecular packing of polymers.…”

Enhancement of Conjugated Polymer Microstructure and Mixed-Conducting Properties via Chalcogenophene Heteroatom Substitution

“…Such chalcogenophene heteroatom directed crystallite orientation was not wholly unexpected as it has been observed before in other systems. 41,42 The p(g3T2-Te) GIWAXS presents a distinct case. The outof-plane lamellar scattering is stronger than previous three polymers (with clear three orders of scattering peaks) and manifests not as arcs of scattered intensity, but instead as nearly perfectly round spots indicating well-oriented crystallites with little variation about perfectly edge-on orientation, Figure 2e.…”

From Solution to Thin Film: Molecular Assembly of π-Conjugated Systems and Impact on (Opto)electronic Properties

“…346 This is consistent with conformation-molecular orientation relationship reported in two other works. 344,345 Based on the concentration-dependent solution-state absorption spectroscopy, the authors propose that the planar PDPP-T polymer readily aggregates as concentration increases during evaporative assembly, leading to edge-on orientation guided by solution aggregation; the lack of aggregation of PDPP-BT chains in solution is responsible for a preferential face-on orientation. This works has suggested that molecular conformation is not the sole factor that governs the molecular orientation.…”

“…Currently, there is a controversy regarding the dependence of out-of-plane molecular orientation (i.e., face-on vs edge-on) within crystallites on molecular conformation: some works have demonstrated that a planar, linear conformation facilitates development of face-on molecular orientation, − whereas others have suggested the opposite. − For example, Chen and Fréchet et al have observed an increased preference for edge-on orientation with improving chain planarity for IID-based conjugated polymers (Figure c) . The chain planarity is systematically improved by substituting thiophenes for phenyl rings within the IID unit, which is supported by DFT calculations and UV–vis absorption spectroscopy.…”