Approaching disorder-free transport in high-mobility conjugated polymers

Abstract

Conjugated polymers enable the production of flexible semiconductor devices that can be processed from solution at low temperatures. Over the past 25 years, device performance has improved greatly as a wide variety of molecular structures have been studied. However, one major limitation has not been overcome; transport properties in polymer films are still limited by pervasive conformational and energetic disorder. This not only limits the rational design of materials with higher performance, but also prevents the study of physical phenomena associated with an extended π-electron delocalization along the polymer backbone. Here we report a comparative transport study of several high-mobility conjugated polymers by field-effect-modulated Seebeck, transistor and sub-bandgap optical absorption measurements. We show that in several of these polymers, most notably in a recently reported, indacenodithiophene-based donor-acceptor copolymer with a near-amorphous microstructure, the charge transport properties approach intrinsic disorder-free limits at which all molecular sites are thermally accessible. Molecular dynamics simulations identify the origin of this long sought-after regime as a planar, torsion-free backbone conformation that is surprisingly resilient to side-chain disorder. Our results provide molecular-design guidelines for 'disorder-free' conjugated polymers.

Very Large π-Conjugation Despite Strong Nonplanarity: A Path for Designing New Semiconducting Polymers

“…distortions from planarity of 60° cause a decrease of the intra-chain coupling of ~15% rather than the expected 50%. The torsional barrier around  1 is ~0.25 eV, close to the value reported for common polymers, 4 implying that the high barriers seen for the dimers are not essential to see reduced angular dependence of the coupling.…”

“…As discussed more extensively in 5 a reduced disorder can be either a geometric or an electronic feature with ref. 4 being an example of the former and the effect discussed in this paper of the latter. The two approaches are complementary and can be possibly combined in future designs.…”

“…The IDTBT polymer was shown in ref. 4 to display a low level of disorder due to its high level of conformational order along the conjugated chain, not to the insensitivity to the disorder. As discussed more extensively in 5 a reduced disorder can be either a geometric or an electronic feature with ref.…”

“…To explain the high mobility observed in polymers lacking any long-range order it was proposed that they could possess some sort of short-range ordering due to aggregation. [1][2] Several independent studies have put forward the idea of tolerance 3 or resilience 4 to disorder, i.e. different mechanisms by which the amount of structural disorder in polymeric semiconductors is lower than expected, e.g.…”

“…In Figure 4 we illustrate this point considering two models of high mobility isoindigo-based polymer discussed in refs. 4 and 20 respectively and referred to as IDTBT and IIDT in these works. The coupling along the chain displays the expected ~cos( ) behaviour in these cases.…”