Tuning Intra and Intermolecular Interactions for Balanced Hole and Electron Transport in Semiconducting Polymers

Charge transport in conjugated polymers depends critically on the chemical structure of the polymer chain, morphology, aggregation, and the complex microstructure in the solid state. Recently, molecular planarity and intramolecular electron transport were associated with J-type aggregation, while coplanar stacking and intermolecular hole transport were correlated with H-type aggregation. This fundamental observation suggests that the degree of H- or J-aggregation could be a handle to tune carrier mobility toward desirable device performances. Here, we use a diketopyrrolopyrrole copolymer as a model semiconducting polymer and tune the type and degree of aggregation through film thickness. Optical absorption measurements, grazing incidence wide angle X-ray scattering, and polarized optical microscopy reveal that thin films compose mainly fibrelike J-aggregated structures, and as the films become thicker, the degree of crystallinity and H-aggregation increase. Thickness-dependent charge mobility values, extracted from corresponding organic field effect transistors, confirm that J-aggregated polymer chains are generally preferable for electron mobility, while polymer crystalline H-aggregates support better hole transport. To obtain perfectly balanced ambipolar OFETs, we optimize the microstructure through film thickness and reduce contact resistance by inserting an interlayer of mixed additives at the organic/contact interfaces. A complementary-like voltage inverter combining two identical ambipolar DPP-T-TT OFETs with a common gate as the input voltage and symmetrical performance confirms that DPP copolymers are a promising candidate for applications in ambipolar devices and integrated circuits.