Carboxylate substitution is a common approach to tune the energy level of donor polymers for organic solar cells. However, the influence of carboxylate substitution on the morphological and electronic properties of donor polymers is not well understood. In this paper, we study two pairs of structurally similar terthiophene or quarterthiophene donor polymers with partial or complete carboxylate substitution on the alkyl side chains. It is found that the carboxylate substitution can enhance the crystallinity of the donor polymers and introduce larger and purer domains. Moreover, the polymers with the carboxylate substitution exhibit reduced bimolecular recombination due to the improved morphology. For device efficiencies, the terthiophene-based polymer, P3TEA (with 50% carboxylate substitution), exhibits the best performance. The alkyl side chains on P3TEA provide a typical temperature-dependent aggregation property, allowing for effective morphology control, while the carboxylate substitution deepens the HOMO level and enhances the crystallinity of the polymer. These benefits yield a near optimal morphology and high V_oc value, and thus the best device efficiency among the polymers studied.

中文翻译：

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了解羧酸酯取代对非富勒烯有机太阳能电池中高性能供体聚合物性能的影响†

羧酸盐取代是一种用于调节有机太阳能电池供体聚合物能级的常用方法。但是，羧酸盐取代对供体聚合物的形态和电子性能的影响尚不十分清楚。在本文中，我们研究了两对结构相似的叔噻吩或四噻吩供体聚合物，它们在烷基侧链上具有部分或完全的羧酸酯取代基。发现羧酸酯取代可以增强供体聚合物的结晶度并引入更大和更纯的结构域。而且，由于形态的改善，具有羧酸酯取代基的聚合物显示出降低的双分子重组。为了提高设备效率，基于噻吩基的聚合物P3TEA（具有50％的羧酸酯取代基）表现出最佳的性能。P3TEA上的烷基侧链具有典型的温度依赖性聚集特性，可进行有效的形态控制，而羧酸根取代则加深了HOMO的水平，并增强了聚合物的结晶度。这些好处产生了近乎最佳的形态并具有很高的V _oc值，因此是所研究的聚合物中最佳的器件效率。