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。该学生奖学金将建立在最近的调查到混合的聚合物/小分子双层薄膜的几何形状。该项目的目的是评估模型，无定形，聚合物/小分子共混物在热退火下平衡的程度。是否达到热力学平衡将通过两组测量来评估;两个层的组成/相体积（通常，层可以是纯的或在平衡时混合的）和它们之间的界面的宽度。这将通过开发和实施原位中子反射技术进行研究，以允许快速测量，并在高温下对这些材料进行广泛退火（包括热循环）。这些中子研究将得到一系列实验室研究的补充，包括x射线散射（广角和小角度分别评估任何晶体形成和较大尺度结构），原子力显微镜（以评估任何平面内表面结构）和各种光谱技术（以确保材料的完整性）。平衡将通过以下方式进行评估：i）研究作为温度函数的组成、相体积和界面宽度变化的可逆性; ii）研究从不同起始位置到平衡状态的收敛（或以其他方式）; iii）与现有理论预测相比，研究作为聚合物分子量的函数的组成和界面宽度的变化。迄今为止，在一个模型聚合物/富勒烯系统的调查，已经发现的情况下，其中发生平衡，并在其中发生非平衡（滞后）行为的情况。该研究的两个主要目的是：i）研究新型非富勒烯受体（NFA）/聚合物体系是否向平衡方向发展，并在现有理论框架内得到理解; ii）进一步研究热退火双层膜中的滞后性质，并试图理解为什么会发生这种情况。背景：该项目的成果在两个方面具有潜在的重要性。首先，混合域的平衡组成最近已与器件效率相关联。正确地确定一个系统是否已经适当地平衡，对于希望从混合实验中提取平衡参数的工作者具有重要的意义。其次，器件内的优化（但可能未平衡）域与平衡的接近度对于器件稳定性至关重要，因为组合物在操作期间（在热循环下）可能朝向平衡演变。

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).