Solvent Based Hydrogen Bonding: Impact on Poly(3-hexylthiophene) Nanoscale Morphology and Charge Transport Characteristics

Abstract

We demonstrate that supramolecular assembly and subsequent enhancement of charge transport characteristics of conjugated polymers can be facilitated simply by adding small amounts of a more volatile poor solvent, which can hydrogen bond with the majority solvent. Addition of up to 2 vol % acetone to a precursor solution of poly(3-hexylthiophene) (P3HT) in chloroform leads to approximately a 4-fold increase in P3HT field-effect mobility. The improvement is associated with hydrogen bonding interactions between acetone and chloroform which decrease the evaporation rate of the mixed solvent. P3HT is less soluble in the binary solvent than in the more readily vaporized chloroform component, and this characteristic enables the supramolecular assembly of P3HT chains at the nanoscale. Two-dimensional molecular ordering of the polymer film was controlled by varying the quantity of poor solvent added to the precursor solution, and the correlation between field-effect mobility and molecular ordering was investigated. Hansen solubility parameters were used to systematically understand how the solvent mixture enhances the alignment and assembly of polymer chains and influences subsequent thin film properties. The value of the relative energy difference (RED) of the solvent with respect to P3HT increased from less than 1 to more than 1 during film formation, which indicates that the solvent characteristics are initially those of a good solvent but transform into those of a poor dissolution medium. A mechanistic illustration of the molecular ordering process during film formation is postulated.

Solving Materials’ Small Data Problem with Dynamic Experimental Databases

“…According to the one-dimensional analysis, the Hansen radius provides better discrimination between high and low performance devices compared with both the initial polymer concentration and the boiling point. The Hansen radius is a numerical descriptor of solvent-polymer interaction energy and can be used to reduce the number of design variables when solvent mixtures are utilized [11,[43][44][45]. As an example, the dissolution of P3HT in a good solvent, followed by the addition of a poor solvent, was utilized as a processing method by 3 of the 19 papers in the database [43,45,46].…”

“…The Hansen radius is a numerical descriptor of solvent-polymer interaction energy and can be used to reduce the number of design variables when solvent mixtures are utilized [11,[43][44][45]. As an example, the dissolution of P3HT in a good solvent, followed by the addition of a poor solvent, was utilized as a processing method by 3 of the 19 papers in the database [43,45,46]. To fully characterize this, two categorical variables (good solvent, poor solvent) and one numerical variable (volume fraction of poor solvent) need to be specified.…”

Control over the photophysical properties of nanoparticles of regioregular poly(3-octylthiophene) using various poor solvents

“…An increase of the hexane ratio to 80% v/v results in additional redshift bands at about 558 and 605 nm, indicating the formation of aggregates [27,29,[32][33][34][40][41][42]. At this condition, some rr-P3OT segments stack on top of each other, causing intersegment delocalization of p electrons and the increase of conjugation length (i.e., decrease of HOMO-LUMO energy gap).…”

“…3. It has been known that the absorption peak at 605 nm indicates to the presence of aggregates [27,29,[32][33][34]41]. Therefore, the absorbance ratio at 605/450 nm reflects aggregate fraction in the systems while the increase of PL intensity ratio at 650/575 nm indicates the formation of emissive aggregates.…”

“…The aggregation behaviors of conjugated polymers can be controlled by adjusting both molecular and experimental parameters including molecular weight [25][26][27], side chain [28][29][30][31], solvent [32][33][34][35][36][37] and preparation method [29,31,38,39]. The use of mixed solvent-nonsolvent provides an ability to control the number and nature of aggregates.…”