Phase Diagrams of Ternary π-Conjugated Polymer Solutions for Organic Photovoltaics.

Jung Yong Kim

doi:10.3390/polym13060983

Abstract

Phase diagrams of ternary conjugated polymer solutions were constructed based on Flory-Huggins lattice theory with a constant interaction parameter. For this purpose, the poly(3-hexylthiophene-2,5-diyl) (P3HT) solution as a model system was investigated as a function of temperature, molecular weight (or chain length), solvent species, processing additives, and electron-accepting small molecules. Then, other high-performance conjugated polymers such as PTB7 and PffBT4T-2OD were also studied in the same vein of demixing processes. Herein, the liquid-liquid phase transition is processed through the nucleation and growth of the metastable phase or the spontaneous spinodal decomposition of the unstable phase. Resultantly, the versatile binodal, spinodal, tie line, and critical point were calculated depending on the Flory-Huggins interaction parameter as well as the relative molar volume of each component. These findings may pave the way to rationally understand the phase behavior of solvent-polymer-fullerene (or nonfullerene) systems at the interface of organic photovoltaics and molecular thermodynamics.

Highlights

Since Flory-Huggins lattice theory was conceived in 1942, it has been widely used because of its capability of capturing the phase behavior of polymer solutions and blends [1,2,3].in 1949, Scott and Tompa applied the Flory-Huggins model to ternary systems, such as solvent-polymer-polymer and nonsolvent-solvent-polymer [4,5,6]
The phase diagrams of ternary π-conjugated polymer solutions were constructed as a function of temperature, molecular weight, solvent species, additive, and electron acceptor
The results indicate: (1) The miscibility gap decreases with increasing temperature, suggesting an upper critical solution temperature (UCST) phase behavior as expected from polymer solutions without specific interactions

Summary

Introduction

Since Flory-Huggins lattice theory was conceived in 1942, it has been widely used because of its capability of capturing the phase behavior of polymer solutions and blends [1,2,3]. The theoretical description for the ternary solvent-polymer-fullerene and solvent-polymer-NFA systems helps to rationally understand the morphology-generation mechanism for the active layer of solar cells, it is not as sophisticated, compared to the phase inversion membrane field [46,47,48,49,50,51,52,53]. The binary low bandgap conjugated polymer solutions and blends were investigated, resulting in various phase diagrams as a function of solvent species, polymer, and chain length [19]. Our interests were further extended to ternary polymer solutions such as solvent-polymer-fullerene and solvent-polymer-NFA systems, in which the composition effect on the phase behavior was examined through ternary phase diagrams. Flory-Huggins theory with a constant interaction parameter and the molar volume ratio of each component

Experimental Methods

Theoretical Methods

Results and Discussion

C94 H82 N4 O2 S4

H18 S2

Conclusions