Ambipolar Charge Transport in Isoindigo-Based Donor–Acceptor Polymers

Abstract

A series of donor−acceptor isoindigo (iI)-based copolymers synthesized with increasing numbers of thiophene rings in the repeat unit (from zero to three thiophene rings, including silole and germole-bridged fused bithiophene units) is applied toward solutionprocessed OFET devices. Differential pulse voltammetry on thin films of the polymers recorded LUMO energy levels confined within a 0.1 eV range around 3.9 eV, while their bandgaps are estimated at 1.5 to 1.7 eV. The interchain π-stacking distance of each sample was evaluated from the 2D-WAXS diffraction patterns of annealed extruded filaments and the GIWAXS patterns of thin films, and were found to be all in the same range, between 3.65 and 3.75 Å for the thin films. Both p-type and n-type charge transport in thin film bottom gate, bottom contact transistor devices were recorded. In particular, the copolymer P(T-iI) containing one thiophene ring afforded wellbalanced ambipolar p-type and n-type mobilities of 0.04 cm 2 /(V s) and 0.1 cm 2 /(V s), respectively. Under our processing conditions, the charge transport properties evolved from exclusively n-type to solely p-type as the number of thiophene rings within the repeat unit is increased to three rings in the case of P(T3-iI). This was observed despite all polymers displaying similar LUMO energy levels, interchain π-stacking distances, and microscopic thin film morphology (all face-on arrangement on the dielectric surface). This prompted a molecular-scale morphological analysis of P(T-iI) and P(T3-iI) in particular, using solid-state NMR spectroscopy in order to further investigate the stark difference in n-type mobilities between these two polymers. Using the complete assignment of solution 2D-NMR spectra of a thiophene-iI-thiophene model compound as guideline, the analysis of proton−carbon correlations in the solid-state 2D 13 C{ 1 H} FSLG-HETCOR NMR spectra of P(T-iI) and P(T3-iI) revealed differences in the molecular environment surrounding each iI unit. The latter suggests a stronger correlation of neighboring iI units in P(T-iI), whereas a stronger intermixing of iI and thiophenes prevails in P(T3-iI). We conclude that, in this study, the choice of the donor unit length within the primary structure of the D−A polymer can be responsible for hindering its n-type character.

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

“…This structure consists of two isoindigo units joined by a single bond at their 6 positions . Shortly before we started this work, others had elucidated structure‐property relationships relating the number of donor units in a donor‐acceptor copolymer to optoelectronic properties such as carrier mobility and optical band gap . We felt that this isoindigo dimer would be an effective way to develop similar relationships from the perspective of the acceptor.…”

“…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 …”

Benzo[1,2‐b:4,5‐b′]Dithiophene–6,7‐Difluoroquinoxaline Small Molecule Donors with >8% BHJ Solar Cell Efficiency

“…[38,39] To probe carrier transport across optimized BHJ active layers with SM1 and SM2, we turned to space charge limited current (SCLC) mobility measurements (cf. [38,39] To probe carrier transport across optimized BHJ active layers with SM1 and SM2, we turned to space charge limited current (SCLC) mobility measurements (cf.…”