Crystallization kinetics of semiconducting poly(2,5-bis(3-alkylthiophen-2-yl)-thieno-[3,2-b]thiophene) (PBTTT) from its different liquid phases

Abstract

Because the electronic properties of semiconducting polymers are inexorably linked to their solid-state microstructure, it is imperative to understand their complex crystallization processes fully. For example, poly(2,5-bis(3-alkylthiophen-2-yl)-thieno-[3,2-b]thiophene) (PBTTT), which is frequently considered a model system for highly ordered semicrystalline semiconducting polymers, can exhibit two distinct semi-crystalline thin film morphologies: the so-called terrace-phase, which features high charge carrier mobility (>1 cm² V⁻¹ s⁻¹), and the ribbon-phase with much poorer properties. The achievement of one or the other depends on the temperature at which the polymer is thermally annealed. Our results evidence that PBTTT is in the liquid state at those “annealing temperatures” and, therefore, the achievement of the terrace- or ribbon phases depends, in fact, on the distinct structural configuration of liquid PBTTT chains at each temperature (before crystallization). Motivated by this observation, we investigate the complex crystallization kinetics of spun cast PBTTT thin films crystallized from those liquid states. We achieve this using a methodology that combines fast scanning calorimetry, X-ray scattering, and optical microscopy. We demonstrate that a preexisting smectic order enhances crystal nucleation rate, speeding up the crystallization kinetics at the early stages of phase transformation. More interestingly, our analysis reveals a complex crystallization kinetics in PBTTT, which differs from the typical crystallization behavior of commodity polymers. These results evidence that the crystallization of semiconducting polymers occurs quite differently to that of most commodity polymers, highlighting (once again) the necessity to conduct more fundamental investigations on the structure development of this important class of polymers.