New Insights into the Photodegradation Mechanism of the PTB7-Th Film: Photooxidation of π-Conjugated Backbone upon Sunlight Illumination

Abstract

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.

Light‐Induced and Oxygen‐Mediated Degradation Processes in Photoactive Layers Based on PTB7‐Th

“…The peaks for the stretching modes of the conjugated backbone (1567 cm −1 ) and the CF bond connected to the TT unit (1400 cm −1 ) are shown in violet. For the polymer as well as the blend, they exhibit the most pronounced decay and virtually vanish in 8 h. Thus, as confirmed by other works on PTB7 and PTB7‐Th [ 18,21,25 ] and in contrast to polymers like P3HT, [ 12,53 ] the conjugated backbone is clearly the most sensitive reaction site within this process. Exemplary bending modes of the alkyl groups in the side chains (1460 and 1377 cm −1 ), both depicted in light pink, show a rather linear decrease until they almost vanish after 1 day.…”

“…However, the simultaneous, albeit slower, decrease in the maximum at higher wavelengths results from a concurrent breaking of conjugation occurring on longer time‐scales. [ 24,25 ]…”

“…Other groups have suggested that oxygen, as a dienophile, attacks the e − ‐rich regions within the thiophene rings of the BDT unit of the polymer [ 23 ] and creates intermediate cyclic peroxides, which in a later step are converted into carbonyl groups. [ 25,60 ]…”

“…[ 16–21 ] Chemical reactions, in contrast, were investigated using spectroscopy methods such as ultraviolet–visible (UV–vis) absorption or photoluminescence as well as Raman and Fourier transform infrared (FTIR) spectroscopy. [ 22–25 ] However, research so far has neglected potential interconnections between chemical and structural changes.…”

Photostability of benzodithiophene based polymer: effect of PC60BM and intermolecular interactions

“…This scission can lead to photooxidation that will finally produce a product with a different chemical property. However, PBDTI-DT did not reveal a change in the absorption profile after 70 h of irradiation that would indicate the change in its chemical structural which confimed it stability against photo-oxidation [21, 25,26]. The absorption of a similar copolymer, a thiophenealt-isoidigo, was found to be completely bleached in less than 40 h [6].…”