Impact of polymorphism on the optoelectronic properties of a low-bandgap semiconducting polymer

Polymorphism of organic semiconducting materials exerts critical effects on their physical properties such as optical absorption, emission and electrical conductivity, and provides an excellent platform for investigating structure-property relations. It is, however, challenging to efficiently tune the polymorphism of conjugated polymers in aggregated, semi-crystalline phases due to their conformational freedom and anisotropic nature. Here, two distinctly different semi-crystalline polymorphs (beta(1) and beta(2)) of a low-bandgap diketopyrrolopyrrole polymer are formed through controlling the solvent quality, as evidenced by spectroscopic, structural, thermal and charge transport studies. Compared to beta(1), the beta(2) polymorph exhibits a lower optical band gap, an enhanced photoluminescence, a reduced pi-stacking distance, a higher hole mobility in field-effect transistors and improved photocurrent generation in polymer solar cells. The beta(1) and beta(2) polymorphs provide insights into the control of polymer self-organization for plastic electronics and hold potential for developing programmable ink formulations for next-generation electronic devices.