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

项目：该项目旨在了解有机光伏(OPV)材料行为的基本方面。这项研究将建立在最近对薄膜几何中聚合物/小分子双层混合的研究基础上。该项目的目标是评估模型、无定形聚合物/小分子混合物在热退火下的平衡程度。是否达到热力学平衡将通过两组测量来评估；两层的组成/相体积(通常，两层在平衡时可以是纯的或混合的)以及它们之间的界面宽度。将通过开发和实施原位中子反射测量技术来研究这一点，以便能够进行快速测量，并在高温下对这些材料进行广泛的退火(包括热循环)。这些中子研究将得到一系列实验室调查的补充，包括X射线散射(分别用于评估任何晶体形成和较大尺度结构的广角和小角)、原子力显微镜(用于评估任何面内表面结构)和各种光谱技术(以确保材料完整性)。平衡将通过以下方式进行评估：i)调查组成、相体积和界面宽度随温度变化的可逆性；ii)调查从不同起始位置到平衡态的收敛(或其他)；iii)与现有理论预测相比，研究组成和界面宽度作为聚合物分子量的函数的变化。到目前为止，在模型聚合物/富勒烯系统中的研究已经发现了发生平衡的情况和发生非平衡(滞后)行为的情况。这项研究的两个关键目标是：i)研究新型非富勒烯-受体(NFA)/聚合物系统是否进化到平衡状态，是否可以在现有理论的框架内理解；ii)进一步表征热退火双层膜中滞后的性质，并试图理解为什么会发生这种情况。首先，混合磁区的平衡组成最近被认为与器件效率有关。对于希望从混合实验中提取平衡参数的工作者来说，正确确定系统是否达到适当的平衡将具有重要的意义。其次，器件内优化的(但潜在的非平衡的)结构域接近平衡对于器件的稳定性至关重要，因为在操作过程中(在热循环下)，成分可能向平衡演化。

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).