Convergent equilibration at model polymer/small-molecule OPV interfaces?

Solution-processed organic photovoltaics (OPVs) have steadily increased in efficiency over the last 2 decades. However, they face barriers to commercialisation due to efficiency reduction during use. One aspect of this is the evolution of morphology and interfacial structure during operation (under thermal cycling). The equilibrium behaviour of mixed, amorphous polymer/small-molecule phases (‘domains’) within the active-layers of OPVs has also recently been linked directly to device efficiency and stability. Our interest is to see how far we can understand changes in phase compositions and interfacial width (both key to OPV efficiency) in terms of fundamental thermodynamics, with the aim of providing a solid basis for interpretation of equilibration measurements in polymer/small-molecule thin-film OPV systems. In this proposed experiment we seek to understand the non-equilibrium and equilibrium mixing behaviour that can occur in polymer/small-molecule bilayers on thermal annealing. The key parameter is the width of the buried interface, which can display hysteresis as a function of annealing temperature. The proposed experiment will establish whether samples with different starting points (composition profiles) that mix on heating, but exhibit different interfacial widths, converge to a single minimum as a function of annealing temperature.

溶液加工型有机光伏（Solution-processed Organic Photovoltaics, OPVs）在近二十年间效率持续提升。然而，其在使用过程中出现的效率衰减问题，成为商业化推广的核心阻碍。其中一个关键诱因是器件在运行过程中（热循环条件下）的形貌与界面结构演变。近期研究还发现，有机光伏活性层内混合无定形聚合物/小分子相（即所谓“畴区（domains）”）的平衡行为，与器件效率及稳定性直接相关。本研究旨在从基础热力学角度出发，尽可能阐释相组成与界面宽度（二者均为影响有机光伏效率的关键因素）的变化规律，以期为聚合物/小分子薄膜型有机光伏体系的平衡测量解析提供坚实的理论基础。在本拟开展的实验中，我们将探究聚合物/小分子双层膜在热退火过程中可能出现的非平衡与平衡混合行为。核心表征参数为掩埋界面的宽度，该参数随退火温度变化可表现出滞后特性。本拟开展的实验将验证：不同初始状态（组成分布）的样品，虽在加热过程中发生混合且初始界面宽度各异，但最终是否会随退火温度变化收敛至单一极小值。