Recrystallization upon Solvent Vapor Annealing and Impact of Polymer Crystallinity on Hole Transport in Poly(3-hexylthiophene):Small Molecule Blends

This work is a systematic study on charge carrier (hole) mobility and structure of blends of poly(3-hexylthiophene) (P3HT) with small-molecular aromatic diimides (ADIs): N,N'-di(n-hexyl)benzene-1,2,4,5-tetracarboxylic diimide, N,N'-di(n-butyl)- or N,N'-di(n-hexyl)naphthalene-1,4,5,8-tetracarboxylic diimide. The blends were spray coated to form films and kinetically stabilized by solvent vapor annealing that caused their recrystallization. The hole mobility determined in organic field-effect transistors based on the blends depended on molecular architecture of ADIs (their alkyl-to-aromatic ratios). Generally, the alkyl-dominated ADIs enhanced or were inert, whereas the aromatic-dominated ADIs worsened the hole mobility measured in the blends directly after the spray coating. Quantitative X-ray diffraction studies supported by electron microscopy indicated that the solvent vapor annealing caused the ADI crystals to grow, but reduced (typically by 35%) P3HT crystal sizes, and caused their reorientation from ‘face-on’ to ‘edge-on’. ADIs enhanced the crystallinity degree of P3HT in some blends, even up to as high as ~0.9. Flipping the orientation, reducing the sizes of P3HT crystals and increasing P3HT crystallinity degree in the blends all together contributed to ~2 to ~7-fold improvements in hole mobility. Reduction in P3HT crystal sizes and increasing the number of P3HT crystal domains exerted a stronger influence on hole mobility than the face-on to edge-on reorientation. ADIs’ molecular architecture exerted a sound influence on orientation and sizes of P3HT crystal domains.