Issue 11, 2024

Hysteresis in phase volumes, compositions and interfacial roughness in model OPV-small-molecule/polymer thin-films

Abstract

Domain morphology and composition, and the structure of interfaces between domains are key factors in the performance and stability of organic photovoltaics (OPVs) fabricated from polymer/small-molecule blends. In this study, we investigate the evolution of composition, phase-volume and interfacial roughness in model polymer/small-molecule bilayers, in response to thermal annealing. Polystyrene/fullerene mixing is studied as a function of annealing temperature, using in situ neutron reflectivity, in thin-film bilayer samples comprising pure component or mixed layers. Remarkably, we discover that thermal annealing at temperatures around or above the reported glass transition temperatures, Tg, of the components can result in extensive mass-transfer between layers, that has the superficial appearance of equilibration, but leaves the layer compositions, thicknesses, and/or the interfacial composition profile in a non-equilibrium state. This is not merely a case of slow kinetics near Tg, as subsequent heating to higher temperatures, followed by cooling, reveals pronounced hysteresis in these systems. This has important implications for the measurement of equilibrium compositions in polymer/small-molecule mixtures for OPV applications, and for device stability during operation.

Graphical abstract: Hysteresis in phase volumes, compositions and interfacial roughness in model OPV-small-molecule/polymer thin-films

Supplementary files

Article information

Article type
Paper
Submitted
11 Aug 2023
Accepted
16 Jan 2024
First published
17 Jan 2024
This article is Open Access
Creative Commons BY license

Soft Matter, 2024,20, 2532-2546

Hysteresis in phase volumes, compositions and interfacial roughness in model OPV-small-molecule/polymer thin-films

A. M. Higgins, P. Gutfreund, V. Italia, A. Nelson, J. T. Cabral and E. L. Hynes, Soft Matter, 2024, 20, 2532 DOI: 10.1039/D3SM01066J

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