Systematic Control of Nucleation Density in Poly(3‐Hexylthiophene) Thin Films

Abstract

solution processing with interesting optoelectronic properties such as high charge transport mobility. Many of the best performing conjugated polymers, including the poly(alkylthiophene)s, derive their high charge mobility from an ability to crystallize. [ 1 , 2 ] While charge transport along an individual conjugated chain is predicted to be extremely rapid, [ 3 ] over longer distances charge must also pass between chains. [ 4 ] Crystallization aids interchain charge transfer by bringing planarized chains together in regular, more intimate contact. The importance of crystalline morphology has been established by previous studies reporting the sensitivity of measured charge transport to fi lm formation parameters that impose the kinetics of crystallization. [5][6][7] However, systematic study of transport-limiting morphological features and how one might optimize crystalline structure remain extremely challenging because of the diffi culty of incremental control over crystallization. In particular, typical solution casting conditions (even from high boiling point carrier solvents [ 5 ] ) lead to extremely high nucleation density, such that macroscopic charge transport probes average over an enormous number of randomly oriented grain boundaries whose density is neither well-known or easily adjusted. [ 8 , 9 ] Methodologies, such as self seeding, adapted from studies of classical semicrystalline polymers, exist that permit systematic control of important morphological characteristics such as nucleation density and lamellar width. We show how controlled solvent swelling and deswelling of a precast poly(3-hexylthiophene) (P3HT) fi lm is an extremely effective method for controlling crystalline morphology (independent of fi lm formation) by fully incremental control of nucleation density over many orders of magnitude.In P3HT and many other main chain conjugated polymers, crystallization is dominated by strong π − π interactions perpendicular to the thiophene ring, which drive a highly anisotropic growth of stacked aggregates. When confi ned to a thin fi lm, the π -stacking [010] direction lies in-plane, with the molecules adopting an edge-on orientation ([100] alkyl side chains aligned perpendicular to the substrate). Long crystalline lamellae separated by amorphous regions containing chain folds and ends provide effi cient in-plane transport channels along the π -stacking direction. Device transport characteristics, While molecular ordering via crystallization is responsible for many of the impressive optoelectronic properties of thin-fi lm semiconducting polymer devices, crystalline morphology and its crucial infl uence on performance remains poorly controlled and is usually studied as a passive result of the conditions imposed by fi lm deposition parameters. A method for systematic control over crystalline morphology in conjugated polymer thin fi lms by very precise control of nucleation density and crystal growth conditions is presented. A precast poly(3-hexylthiophene) fi lm is fi rst swollen into a solution-lik...

Structure and Morphology Control in Thin Films of Conjugated Polymers for an Improved Charge Transport

“…Secondly, the crystallization kinetics for growth after nucleation could also influence the crystalline region size. Crossland et al reported that a higher crystallization vapor pressure was helpful for the increase of crystalline region size and alignment order of nanofibrils due to the smaller supercooling and slow crystallization kinetics [159]. Xiao et al observed the gradually increasing length of P3HT needle-like crystals with longer treatment time in THF atmosphere [73].…”

“…are fixed in this regime influenced by factors such as solvent quality, vapor pressure and treatment time, etc. Firstly, the quality of solvent or vapor atmosphere is important due to its determining on C ∞ and nucleation [79,159,172]. For a bad solubility of solute arising from larger conjugation lengths, bad solvent or smaller vapor pressure, the value of C ∞ is small leading to a small size and larger density of crystallites or even no nucleates.…”

“…Literally, "insufficient" means that there is room to reach the optimal state and "excess" means that the optimal state is climbed over followed by a state no better. When tuning the structures and morphology in films, the insufficient driving force such as a fast coating rate [64], a low annealing temperature [38,39], a low annealing solvent vapor pressure [79,159] or a short treatment time [173] could result in the morphology not developing fully reflected by less fibrillar features or more fused shorter nanofibrils, etc. However, a excess treatment such as too slow drying rate [150], too long treatment time [156,173] or too high annealing temperature [38,39] lead to the destruction of the original optimal state reflected by reduced crystallinity, less ordered fibrillar features and fusing of nanofibrils again, etc.…”

“…Secondly, the growth of crystallites via inter-chain stacking undergoes a nucleation-growth mechanism. The nucleation stage plays a determining role in the final morphology according to the order, shape and density of nucleates [77,152,155,159]. An effective strategy to promote the fully crystallization of conjugated polymer chains based on nucleation is self-seeding [53,75,77,152,159].…”

“…The nucleation stage plays a determining role in the final morphology according to the order, shape and density of nucleates [77,152,155,159]. An effective strategy to promote the fully crystallization of conjugated polymer chains based on nucleation is self-seeding [53,75,77,152,159]. Self-seeding means the homogeneous formation of nucleates in the supersaturated solution at a relatively higher temperature (T ss ) preventing excessive aggregation followed by a further ordered growth of nucleates into crystals with a larger size at a constant lower temperature (T c ).…”