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–CO 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.
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