Improving Charge Mobility of Polymer Transistors by Judicious Choice of the Molecular Weight of Insulating Polymer Additive

In this work, we have carefully examined the morphology of semiconducting polymer:insulating polymer blends, which were deposited from inkjet printing. We attempted to study the impact of molecular weight (MW) of insulating polymer on the nanoscale morphology and function of the blends. The morphology of all of the inkjet-printed samples was characterized by small-angle neutron scattering (SANS), grazing incidence X-ray diffraction (GIXD), and atomic force microscopy (AFM). The SANS results show that the domain size of the blends increases by increasing the MW of insulating polymer, while the domain purity reaches the maximum with proper molecular weight of insulating polymer. AFM images show that the connectivity of semiconducting polymer domains is disrupted with addition of polystyrene (PS) with low molecular weight (M-w = 2.5K and 20K), while well interconnected domains are observed with addition of PS with high molecular weight (M-w = 182K and 2000K). GIRD results indicate that the pi-pi stacking distance of semiconducting polymer can be shortened with addition of PS and decreases with an increase of PS molecular weight from 2.5K to 182K Further increasing molecular weight of PS to 2000K results in very weak pi-pi stacking ordering. This work demonstrates that the domain purity, connectivity of semiconducting polymer domains, and molecular packing are crucial for the charge transport. The judicious choice of the MW of insulating polymer could carefully control the nanoscale morphology of semiconducting polymer:insulating polymer blends, which could provide blend morphology with high domain purity, well-connected domains, along with reduced pi-pi stacking distance, all of which facilitate charge transport, resulting in a significant improvement of charge mobility.