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

Abstract

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 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 stemming either 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.

Functionalization of poly(bis‐thiophene methine)s via facile C–C bulk polymerization and their application as chemosensors for acid detection

“…Compounds of 4‐(octyloxy)benzaldehyde, 2‐bromo‐3‐(hexyloxy)thiophene and 2‐bromo‐3‐hexylthiophene were synthesized following published procedures as shown in Supporting Information, Scheme S1.…”

Multifluorination toward High‐Mobility Ambipolar and Unipolar n‐Type Donor–Acceptor Conjugated Polymers Based on Isoindigo

“…To date, almost all the reported ambipolar transport CPs with both hole and electron mobility (μ h and μ e ) >1 cm 2 V −1 s −1 were built with diketopyrrolopyrrole (DPP), and the ambient‐stable n‐type CPs with μ e > 1 cm 2 V −1 s −1 were based on either naphthalene diimide or benzodifurandione derivatives . On the other hand, isoindigo (IID) has been recognized as an ideal electron‐deficient building block for making D–A CPs soon after its first use in the design of donor materials for organic solar cells by Reynolds and co‐workers and IID‐based CPs have been widely used as semiconducting materials in OFET and organic photovoltaic devices . Ambipolar CPs based on IID have been demonstrated by introducing electron withdrawing groups on IID unit or using different comonomers .…”

“…On the other hand, isoindigo (IID) has been recognized as an ideal electron‐deficient building block for making D–A CPs soon after its first use in the design of donor materials for organic solar cells by Reynolds and co‐workers and IID‐based CPs have been widely used as semiconducting materials in OFET and organic photovoltaic devices . Ambipolar CPs based on IID have been demonstrated by introducing electron withdrawing groups on IID unit or using different comonomers . Particularly, Pei and co‐workers found that introducing two fluorine (F) or chlorine (Cl) atoms in IID core could lower the frontier molecular orbital energy levels and promote the self‐organization ability of the polymers, resulting in ambipolar CPs with μ e up to 0.66 cm 2 V −1 s −1 under the ambient conditions .…”

Recent Advances in Isoindigo‐Inspired Organic Semiconductors

“…The fill factor also increased slightly, from 0.63 to 0.69, helping to offset the decrease in V oc from 0.89 to 0.70 V. The good performance of 4 : PC 71 BM devices was an important milestone for isoindigo‐based polymers, and prompted further isoindigo‐oligothiophene copolymer research. Copolymers of isoindigo and oligothiophenes (e. g., 5 and 6 ) would go on to become some of the most popular isoindigo‐based polymer semiconductors owing to their ease of synthesis, excellent optoelectronic properties, and OPV performance …”