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

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...

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Bibliographic Details
Published in:Soft matter 2024-03, Vol.2 (11), p.2532-2546
Main Authors: Higgins, A. M, Gutfreund, P, Italia, V, Nelson, A, Cabral, J. T, Hynes, E. L
Format: Article
Language:English
Subjects:
Annealing
Composition
Fullerenes
Glass transition temperature
High temperature
Hysteresis
Interface stability
Mass transfer
Photovoltaic cells
Photovoltaics
Polymer blends
Polymer films
Polymers
Polystyrene
Polystyrene resins
Roughness
Thickness
Thin films
Transition temperatures
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Summary: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, T g , 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 T g , 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. Non-equilibrium bilayer mixing on isothermal annealing near T g ; extensive mass-transfer occurs, but the top layer composition, layer thicknesses &/or interfacial width can become stuck in a local minimum.
ISSN:1744-683X
1744-6848
DOI:10.1039/d3sm01066j