Equilibrium and non-equilibrium behaviour in polymer/small-molecule mixtures for organic photovoltaic (OPV) applications

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Project: This project aims to understand fundamental aspects of the behaviour of organic photovoltaic (OPV) materials. The studentship will build on recent investigations into the mixing of polymer/small-molecule bilayers in a thin-film geometry. The objective of the project is to assess the degree to which model, amorphous, polymer/small-molecule blends equilibrate under thermal annealing. Whether or not thermodynamic equilibrium is reached will be assessed via two sets of measurements; the composition/phase volume of the two layers (in general, the layers can be pure or mixed at equilibrium) and the width of the interface between them. This will be investigated by the development and implementation of in-situ neutron reflectometry techniques to allow rapid measurements, and extensive annealing (including thermal cycling) of these materials at elevated temperatures. These neutron studies will be complimented by a range of laboratory investigations, include x-ray scattering (both wide and small angle to assess any crystal formation and larger scale structure, respectively), atomic force microscopy (to assess any in-plane surface structure), and various spectroscopic techniques (to ensure material integrity).Equilibrium will be assessed by; i) investigating the reversibility of composition, phase-volume and interfacial width changes, as a function of temperature; ii) investigating convergence (or otherwise) to an equilibrium state from different starting positions; iii) investigating changes to composition and interfacial width as a function of polymer molecular weight in comparison to existing theoretical predictions. Investigations to-date, in a model polymer/fullerene system, have uncovered situations in which equilibration occurs, and situations in which non-equilibrium (hysteresis) behaviour occurs. Two key aims of the studentship are i) to investigate whether novel non-fullerene-acceptor (NFA)/polymer systems evolve towards equilibrium and can be understood within a framework of existing theory, and ii) to further characterise the nature of the hysteresis in thermally annealed bilayers and try to understand why this occurs.Context: The outcomes of the project are potentially important in two aspects. Firstly, the equilibrium compositions of mixed domains have recently been linked to device efficiency. Correct determination of whether or not a system has properly equilibrated will have important implications for workers who wish to extract equilibrium parameters from intermixing experiments. Secondly, the proximity of optimised (but potentially non-equilibrated) domains within devices to equilibrium is of crucial importance with regard to device stability, as compositions may evolve towards equilibrium during operation (under thermal cycling).