Upper and Apparent Lower Critical Solution Temperature Branches in the Phase Diagram of Polymer:Small Molecule Semiconducting Systems

Abstract

Solution-processable semiconducting materials are complex materials with a wide range of applications. Despite their extensive study and utility, their molecular interactions as manifested, for example, in phase behavior are poorly understood. Here, we aim to understand the phase behavior of conjugated systems by determining phase diagrams spanning extensive temperature ranges for various combinations of the highly disordered semiconducting polymer (PTB7-Th) with crystallizable (IT-M and PC61BM) and noncrystallizable (di-PDI) small molecule acceptors (SMAs), with polystyrene as an amorphous control, a nonsemiconducting commodity polymer. We discover that the apparent binodal of the studied blends frequently consists of an upper critical solution temperature (UCST) and lower critical solution temperature (LCST) branch, exhibiting a sharp kink where the branches join. Our work suggests that phase diagrams might be a probe in combination with sophisticated models to understand the complexity of semiconducting materials, including microstructure and molecular interactions.

Phase behavior of π-conjugated polymer and non-fullerene acceptor (PTB7-Th:ITIC) solutions and blends

“…7 a. Here, it is notable that the highly disordered PTB7-Th is known to be at the boundary of amorphous and semicrystalline 39 . However, in this study, according to the XRD data in Fig.…”

“…Accordingly, in this study, we constructed theoretically the phase diagrams of ternary solvent/polymer/NFA (or fullerene derivative) systems as a function of molecular weight, temperature, and electron-acceptor species 33 – 35 . For this purpose, we employed the Flory–Huggins lattice model 36 – 38 for the blend system composed of poly[4,8-bis(5-(2-ethylhexyl)-thiophen-2-yl)benzo[1,2-b;4,5-b0]-dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]-thiophene-)-2-carboxylate-2-6-diyl] (PTB7-Th) and 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2’,3’-d’]-s-indaceno[1,2-b:5,6-b’]dithiophene (ITIC) 19 , 39 – 42 . Here, PTB7-Th is a highly disordered polymer whereas ITIC is an easily crystallizable NFA.…”

Equilibration and thermal reversibility in mixtures of model OPV small-molecules and polymers

“…However, to fully understand the thermal behaviour of the system requires an approach beyond FH to account for volume changes in the system. Recent work by Peng et al 34 extending the temperature dependence of the FH χ parameter, based on theoretical considerations of compressibility, has enabled the fitting of binodal curves for polymer/small-molecule OPV mixtures that exhibit both UCST and lower-critical-solution-temperature (LCST) behaviour. However, to achieve predictive capabilities with regard to all of the behaviour that we observe (both layer composition and thickness as a function of temperature) requires a more fundamental approach, such as the locally-correlated-lattice (lcl) theory developed and implemented by Lipson and co-workers.…”

Renewed Prospects for Organic Photovoltaics

“…Such changes typically substantially alter device performance, and they can be characterized by PL measurements, which generate information on phase separation, and by X-ray diffraction techniques, which afford more detailed morphological data on the molecular spacing distances and level of crystallinity in a sample. In particular, detailed investigations of the phase diagram for organic semiconductor mixtures provide invaluable information about morphology formation and important guidelines for improving fabrication yield and reliability have been gained from studies of molecular interactions and kinetic quenching . Recently reported approaches for predicting morphological stability based on solubility calculations have provided key insights into active-layer stability. , However, these approaches were specifically developed for fullerene-containing composites, as fullerenes are prone to very rapid solid-state diffusion.…”

“…A stability improvement for thin active layers may compensate for any small absorption losses they exhibit. 370 invaluable information about morphology formation 371 and important guidelines for improving fabrication yield and reliability have been gained from studies of molecular interactions and kinetic quenching. 372 Recently reported approaches for predicting morphological stability based on solubility calculations have provided key insights into activelayer stability.…”