Structure-thermal/mechanical property relationship of semiconducting polymer thin films*

The past decades have witnessed surging developments of semiconducting polymer thin films (<100 nm) in the flexible/wearable electronics field. Despite much efforts in enhancing their electronic performances, the fundamental understanding of their thermal and mechanical behaviors has been limited. This is mainly because of their heterogeneous rigid backbone/soft side-chain structure and a lack of characterization techniques. Here, we deconvoluted the backbone and side-chain effect on thin-film thermal and mechanical properties through thin-film dynamic mechanical analysis (DMA) and film-on-water (FOW) tensile tests. A model diketopyrrolopyrrole (DPP)-type polymer is selected to show the effect of chemical moiety choice on thin-film glass transition temperature (Tg) and modulus. It is noticed that the inclusion of thiophene groups in the polymer backbone increases the backbone Tg, while a longer side-chain length can sufficiently depress the backbone dynamics. A Tg predictive model is developed to capture such an effect and provide a design strategy for the engineering of new semiconducting polymers.