Understanding the photodegradation mechanism of photoactive materials is critical for enhancing the long-term stability of organic photovoltaic cells (OPVs). However, definitive photodegradation mechanisms have not been reported yet. Here we report the comprehensive understanding of the photodegradation mechanism of the PTB7-Th polymer film. UV/vis absorption and photoluminescence spectra show that the π-conjugated backbone and the intermolecular π–π interactions are simultaneously broken under sunlight in air. Raman spectra reveal that the initial photooxidation begins at the thiophene ring in the benzo[1,2-b;3,3-b]dithiophene (BDT) unit, followed by the ring-opening of the thiophene and the break of the π-conjugated system. Infrared spectra indicate that −S–C═O and −COOH groups are formed as a result of the photooxidation. On the basis of these observations, we propose that the thiophene ring in the BDT unit reacts with oxygen to generate the BDT–O₂ adduct, which then produces thioester and carboxylic acid. The reaction sites in the BDT unit are consistent with the electrophilic attack positions of oxygen predicted by condensed Fukui functions. Furthermore, the DFT calculated spectrum of the proposed oxidation product agrees well with all the spectroscopic observations. Conclusively, the present work provides an important clue for understanding photodegradation of OPV materials.

中文翻译：

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PTB7-Th薄膜光降解机理的新见解：阳光照射下π共轭骨架的光氧化

了解光敏材料的光降解机理对于增强有机光伏电池（OPV）的长期稳定性至关重要。但是，尚未报道确定的光降解机理。在这里，我们报告对PTB7-Th聚合物薄膜的光降解机理的全面理解。UV / vis吸收和光致发光光谱表明，在空气中的阳光下，π共轭骨架和分子间π-π相互作用同时被破坏。拉曼光谱表明，初始光氧化作用始于苯并[1,2- b ; 3,3- b]中的噻吩环]二噻吩（BDT）单元，然后噻吩开环和π共轭体系断裂。红外光谱表明，由于光氧化作用，形成了-S–C═O和-COOH基团。根据这些观察结果，我们建议BDT单元中的噻吩环与氧气反应生成BDT-O ₂加合物，然后生成硫酯和羧酸。BDT单元中的反应位点与通过浓缩的Fukui函数预测的氧的亲电子进攻位置一致。此外，拟议的氧化产物的DFT计算光谱与所有光谱观察结果非常吻合。结论是，本工作为理解OPV材料的光降解提供了重要线索。