Controlling the Crystal Growth and Morphology of Conjugated Polymers

Abstract

The semiconducting properties of conjugated polymers are finding use in optoelectronic applications, due to their low cost processing and readily-tailored optical and electrical properties. Many high-performance conjugated polymer semiconductors such as the poly(3-alkylthiophenes) (P3HT) derive their interesting optoelectronic properties from an ability to crystallize. Well-developed crystals of P3HT have a lamellar structure that originates from quasi-one-dimensional stacking of P3HTs induced by both an attractive pi-pi* interaction between polymer backbones and the crystallization of hexyl side chains. Energy transport is predicted to be improved with enhancing molecular order, which is highly sensitive to crystallization conditions. However, study of the dependence of electrical and optical properties on morphological characteristics in spin coated films is difficult because of extremely high nucleation density, such that macroscopic charge transport affected by an enormous number of randomly oriented fibrils whose density is neither well known or easily adjusted.

We have studied the formation P3HT single crystals in solution under systematically controlled conditions that dictate crystallization kinetics. By applying the self-seeding method, we were able to remove the nucleation barriers by first dissolving all but a few (thermodynamically most stable) crystals, which then act as nucleation centers (seed crystals) under subsequent recrystallization conditions. This method extends the possible condition for crystallization to lower degrees of supercooling, such that growth is extremely slow and promotes higher molecular order. Using such conditions, we could control crystal size in all three dimensions as well as aspect ratio. The obtained crystals were characterized by fully stretched polymer molecules and possessing a unique conformational orientation, regardless of how long they were. Interestingly, oligo- and poly(3-hexylthiophene) molecules made using different synthetic methods (differing in contour length, polydispersity and regioregularity), all form Form II P3HT single crystals. Although, crystal lattice parameters of crystals increased monotonically with polymer molecular weight, we concluded that having a precise control on nucleation and growth process of crystals, it is possible to organize all polymer molecules with long range order and unique molecular orientation. In order to elucidate the dependence of the morphological, optical, and electrical properties of the single crystals on crystallization conditions we used UV-vis absorption spectroscopy which gave us a direct knowledge about the optical energy gap of polymer molecules organized in a single crystal. A red shift up to 60 nm were observed in P3HT single crystals compared to the well-known absorption spectra of P3HT film. Since P3HT single crystals and polycrystalline films have significant difference in molecular order and crystal lattice parameters, we attributed such a redshift to the degree of order. Therefore, these results confirmed that the electronic properties of conjugated polymers could be highly improved with ordering of molecules.