Formation and Charge Transport Properties of P(NDI2OD-T2) Thin Films with End-on Oriented Form III Crystallites

The molecular packing and orientation of conjugated polymer chains in thin films have considerable influence on the charge transport properties and performance of polymer-based optoelectronic devices. Understanding and controlling the formation of different packing geometries and textures are important for understanding structure–function relationships and the purposeful optimization of device performance. Here, we extensively study the origin of a recently discovered third crystalline form (“form III”) of the well-studied electron transporting polymer P(NDI2OD-T2) that also exhibits pronounced end-on orientation. The effects of various processing conditions on the thin-film microstructure are studied with grazing-incidence wide-angle X-ray scattering. We find that end-on oriented form III crystallites directly evolve from face-on oriented form I crystallites upon melting under the specific conditions of high molecular weight, optimum annealing temperature, and optimum film thickness. Furthermore, by studying the charge transport properties of P(NDI2OD-T2) thin films in electron-only diodes and field-effect transistors, we find that films with end-on oriented form III crystallites demonstrate a 5-fold increase in carrier mobility in diodes (vertical transport direction) and a 1.5-fold increase in electron mobility in transistors (horizontal transport direction) compared to films with either face-on form I crystallites or edge-on form II crystallites.