Control of Polymer-Packing Orientation in Thin Films through Synthetic Tailoring of Backbone Coplanarity

Abstract

Controlling solid-state order of π-conjugated polymers through macromolecular design is essential for achieving high electronic device performance; yet, it remains a challenge, especially with respect to polymer-packing orientation. Our work investigates the influence of backbone coplanarity on a polymer’s preference to pack face-on or edge-on relative to the substrate. Isoindigo-based polymers were synthesized with increasing planarity by systematically substituting thiophenes for phenyl rings in the acceptor comonomer. This increasing backbone coplanarity, supported by density functional theory (DFT) calculations of representative trimers, leads to the narrowing of polymer band gaps as characterized by ultraviolet-visible-near infrared (UV-vis-NIR) spectroscopy and cyclic voltammetry. Among the polymers studied, regiosymmetric II and TII polymers exhibited the highest hole mobilities in organic field-effect transistors (OFETs), while in organic photovoltaics (OPVs), TBII polymers that display intermediate levels of planarity provided the highest power conversion efficiencies. Upon thin-film analysis by atomic force microscropy (AFM) and grazing-incidence X-ray diffraction (GIXD), we discovered that polymer-packing orientation could be controlled by tuning polymer planarity and solubility. Highly soluble, planar polymers favor face-on orientation in thin films while the less soluble, nonplanar polymers favor an edge-on orientation. This study advances our fundamental understanding of how polymer structure influences nanostructural order and reveals a new synthetic strategy for the design of semiconducting materials with rationally engineered solid-state properties.

Aggregates of diketopyrrolopyrrole dimers in solution

“…[12,14,16] However, this explanation contradicts the general understanding for the effect of π−conjugation length on spectroscopic behaviours. Moreover, a recent study by Fritzner et al on the influence of thiophenes between 1-(1,1-dicyanomethylene)-cyclohex-2-ene (DCC) units has clearly showed that the expected bathrochromic shift in absorption is actually observed for their materials with longer thiophene bridges.…”

Effects of branching position of alkyl side chains on ordering structure and charge transport property in thienothiophenedione- and quinacridone-based semiconducting polymers

“…3 In the development of semiconducting polymers, the design of the conjugated backbone is of primary importance because it substantially affects the molecular orbital energies, [4][5][6] bandgaps, 7,8 and ordering structure 9,10 that determine the device performance of the polymers. To date, enormous effort has been focused on the exploration of backbone structures (that is, new conjugated building units and their combinations).…”

Control of Phase Separation of Benzothienoisoindigo-Benzodithiophene Copolymer for Organic Photovoltaics

“…To this end, we [25] and Chen et al [26] reported that BTIDG copolymers, a hybrid of IDG and TIDG units, boosted the PCEs over 4%. It should be emphasized that BTIDG is the unsymmetrical acceptor, leading to drastic improvement in solubility compared to the symmetrical TIDG.…”

“…The expanded d IL of BDT-BTIDG compared to BDT-TIDG is readily understood by the longer side alkyl chain (hexyldecanyl) than the latter (ethylhexyl). BTIDG itself is able to keep a short d - (3.56−3.66 Å), if it is coupled with appropriate donor unit such as benzobisthiazole [25] and oligothiophene [26]. However, the use of BDT as the donor could not maintain the strong - stacking.…”