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 polymerchainsin thin films have considerable influence on the charge transportproperties and performance of polymer-based optoelectronic devices.Understanding and controlling the formation of different packing geometriesand textures are important for understanding structure-functionrelationships and the purposeful optimization of device performance.Here, we extensively study the origin of a recently discovered thirdcrystalline form ("form III") of the well-studied electrontransporting polymer P(NDI2OD-T2) that also exhibits pronounced end-onorientation. The effects of various processing conditions on the thin-filmmicrostructure are studied with grazing-incidence wide-angle X-rayscattering. We find that end-on oriented form III crystallites directlyevolve from face-on oriented form I crystallites upon melting underthe specific conditions of high molecular weight, optimum annealingtemperature, and optimum film thickness. Furthermore, by studyingthe charge transport properties of P(NDI2OD-T2) thin films in electron-onlydiodes and field-effect transistors, we find that films with end-onoriented form III crystallites demonstrate a 5-fold increase in carriermobility in diodes (vertical transport direction) and a 1.5-fold increasein electron mobility in transistors (horizontal transport direction)compared to films with either face-on form I crystallites or edge-onform II crystallites.