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.