Structure-Property Relationships Directing Transport and Charge Separation in Isoindigo Polymers

Since being introduced to the open literature in 2010, the isoindigo heterocycle has been extensively studied as a novel electron deficient building block for organic electronic materials in conjugated polymers, discrete length oligomers, and molecular systems, particularly targeting high charge mobility values and ambipolar transport in organic field effect transistors, along with high power conversion efficiencies in organic photovoltaic devices. This article introduces results obtained on copolymers of isoindigo with thiophene and alkylated terthiophenes to highlight fundamental characteristics in isoindigo-based polymers and the resulting organic field-effect transistors and photovoltaic devices. By comparing and contrasting the optoelectronic properties, thin film morphology, organic field-effect transistor (OFET) mobilities, and organic photovoltaic (OPV) performance to previously reported polymers, structure-processing-property relationships were uncovered. In particular, isoindigo-containing polymers with more rigid backbones and higher coherence lengths in thin-films lead to increased charge mobility in OFET devices. In OPV devices, efficiencies over 6% can be obtained by balancing high ionization potentials typically dictating the open-circuit voltage, and the charge transfer state energy and blend morphology impacting short-circuit currents. Furthermore, the impact of polymer structure on solubility and on phase separation in blends with PC71M is discussed, with isoindigo-based polymers exhibiting lower solubility possibly leading to more fiber-like morphologies due to both kinetic and thermodynamic effects, either stemming from polymer dissolution in the casting solvent or from polymer self-assembly during film formation. This fiber-like polymer morphology remains unaffected by the presence of processing additives, such as 1,8-diiodooctane. These structure-property relationships developed for isoindigo-based polymers can also be discussed in the broader context of diketopyrrolopyrrole (DPP) and thienoisoindigo (TiI) as electron-deficient moieties that can also be doubly substituted on their amide functionality.