Elucidating structure-property relationships of indacenodithiophene-copolymers for the development of soft and stretchable electronics

Abstract

Indacenodithiophene (IDT) copolymers are a promising class of copolymers for use as semiconducting charge transport layers in organic electronic devices. Certain IDT-copolymers have achieved hole mobilities that surpass 3.0 cm2 V–1 s–1 in spite of its near-amorphous thin-film morphology. Their near-amorphous morphologies and high charge makes them prime candidates for use in stretchable electronics. Despite this potential, some IDT-copolymer systems including PIDTC16-BT and PIDTC16-TPDC1 undergo brittle failure and have high elastic moduli at low molecular weight, while at the same molecular weight others including PIDTC16-BPDC1 undergo ductile failure with a comparatively smaller elastic modulus. The two studies described herein are focused understanding the how structural differences between IDT-copolymers impact their electronic and mechanical properties, to guide the design of IDT-copolymers for use in stretchable electronics. First, a series of PIDTC16-BT-r-BPDC1 copolymers is investigated. It is found that the side chain extension profile of the hexadecyl side chains that extend from the IDT monomer, which is modulated by the degree of backbone coplanarity, has a deterministic role in the ability of an IDT-copolymer to plastically deform. Unfortunately, the increased hole mobility comes at the sacrifice of hole mobility. Second, side chain substitution was performed on both BT-containing and TPD-containing IDT copolymers; all of which have coplanar backbones. Inclusion of larger alkyl side chains on the TPD monomer led to plasticly deforming IDT-copolymers with remarkably low elastic moduli. This improvement arises because of an inhibited ability for individual polymer chains to interdigitate alkyl their side chains with the side chains of neighboring polymer chains, which makes reorganization more facile. Unfortunately, the hole mobility is decreased due to the impact of interdigitation on the microstructural arrangement of IDT-copolymers. These results highlight the extensive role side chains play in determining the materials properties of IDT-copolymers and indicate the promise that side chain substitution can have in tuning the mechanical properties of IDT-copolymers.